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Invited critical review
Chylomicrons: Advances in biology, pathology, laboratory testing,
, Jesús M. Martín-Campos
, Joan Carles Escolà-Gil
, Francisco Blanco-Vaca
Institut de Recerca de l'HSCSP –Institut d'Investigacions Biomèdiques (IIB) Sant Pau, Barcelona, Spain
Departament de Bioquímica i Biologia Molecular, Universitat Autònoma de Barcelona, Barcelona, Spain
CIBER de Diabetes y Enfermedades Metabólicas Asociadas, Barcelona, Spain
Hospital de la Santa Creu i Sant Pau, Servei de Bioquímica, Barcelona, Spain
Received 29 December 2015
Received in revised form 1 February 2016
Accepted 6 February 2016
Available online 8 February 2016
The adequate absorption of lipids is essentialfor all mammalian species due to their inability to synthesize some
essential fatty acids and fat-soluble vitamins. Chylomicrons (CMs) are large, triglyceride-rich lipoproteins that
are producedin intestinal enterocytes in responseto fat ingestion, whichfunction to transportthe ingested lipids
to different tissues. In addition to the contribution of CMs to postprandial lipemia, their remnants, the degrada-
tion products following lipolysis by lipoprotein lipase, are linked to cardiovascular disease. In this review, we will
focus on the structure–function and metabolism of CMs. Second, we will analyze the impact of gene defects re-
ported to affect CM metabolism and, also, the role of CMs in other pathologies, such as atherothrombotic cardio-
vascular disease and diabetes mellitus. Third, we will provide an overview of the laboratory tests currently used
to study CM disorders, and, ﬁnally, we will highlight current treatments in diseases affecting CMs.
© 2016 Elsevier B.V. All rights reserved.
Type I hyperlipidemi a
Type V hyper lipidemi a
Chylomicron retention disease
1. Structureandfunctionofchylomicrons.................................................. 135
1.1. Physico-chemicalandfunctionalcharacteristicsofCMs ....................................... 135
1.2. LipidcompositionofCMs..................................................... 136
1.3. CMapolipoproteins ....................................................... 136
1.3.1. ApolipoproteinB-48................................................... 136
1.3.2. ApolipoproteinA-I.................................................... 136
1.3.3. ApolipoproteinA-II ................................................... 136
1.3.4. ApolipoproteinA-IV ................................................... 136
1.3.5. ApolipoproteinA-V ................................................... 137
1.3.6. ApolipoproteinsC.................................................... 138
1.3.7. ApolipoproteinE .................................................... 138
Clinica Chimica Acta 455 (2016) 134–148
Abbreviations: ACAT, acyl CoA acyl transferase; Angptl3, angiopoietin-like protein 3; Angptl4, angiopoietin-like protein 4; Angptl8, angiopoietin-like protein 8; apo, apolipopr otein;
apobec 1, apoB mRNA editing enzyme 1; CM, chylomicron; CMs, chylomicrons; COPII, coat protein complex II; DGAT, diglyceride acyltransferase; FABP, fatty acid binding protein; GLP-
1, glucagon-like peptide-1;GPIHBP1, glycosylphosphatidylinositol-anchored high-density lipoproteinbinding protein 1; HDL,high-density lipoproteins; HSPG,heparan sulfate proteogly-
can; LDL, low-density lipoproteins; LDLR, LDL receptor; LR11/SorLA1, LDL receptor relative with 11 ligand binding repeats/sorting protein-related receptors containing LDLR class A
repeats; LRP, LDL receptor-related protein; LMF1, lipase maturation factor 1; MGAT, monoglyce ride acyltransferase; MTTP, mi crosomal triglyceride transfer protein; NPC1L1,
Niemann–Pick C1-like 1; PCV, preCM transport vesicles; PPAR-alpha, peroxisome proliferator-activated receptor alpha; PUFAs, polyunsaturated fatty acids; Sar1b, secretion-associated,
Ras-related GTPase 1B; SORT1, sortilin-1; SR-BI,scavenger receptor class B type 1; TRL, triglyceride-rich lipoprotein; TRLs, triglyceride-richlipoproteins; VLDL, very low-density lipopro-
teins; VLDLR, VLDL receptor.
⁎Corresponding authors at: Grup bases metabòliques del risc cardiovascular, pavelló 17. Institut de Recerca de l'HSCSP-IIB Sant Pau, C/ Antoni M. Claret 167, 08025 Barcelona, Spain.
E-mail addresses: firstname.lastname@example.org (J. Julve), email@example.com (J.M. Martín-Campos).
Both authors contributed equally.
0009-8981/© 2016 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Clinica Chimica Acta
journal homepage: www.elsevier.com/locate/clinchim
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2. MetabolismofCM........................................................... 139
2.1. IntracellularmetabolicdeterminantsofsynthesisandsecretionofCM ................................ 139
2.1.1. Fattyacidtransporters.................................................. 139
2.1.2. Fattyacidbindingproteins(FABPs)............................................ 139
2.1.3. Enzymesinvolvedintriglyceridesynthesis ........................................ 139
2.1.4. MTTP ......................................................... 139
2.1.5. CoatproteincomplexII(COPII).............................................. 139
2.1.6. Sortilin-1........................................................ 139
2.2. ExtracellularmetabolicdeterminantsofcatabolismandclearanceofCMsandtheirremnants...................... 140
2.2.1. Lipoproteinlipase.................................................... 140
2.2.2. Hepaticlipase ..................................................... 140
2.2.3. Glycosylphosphatidylinositol-anchoredhigh-densitylipoprotein-bindingprotein1(GPIHBP1)................. 140
2.2.4. Lipasematurationfactor(LMF)1............................................. 140
2.3. ReceptorsforCMremnantclearance ............................................... 140
2.3.1. LDLR.......................................................... 140
2.3.2. LDLreceptor-relatedproteins(LRPs) ........................................... 140
3. Pathology............................................................... 141
3.1. Hyperchylomicronemia ..................................................... 141
3.1.1. Monogenichyperchylomicronemia............................................ 141
3.1.2. Polygenichyperchylomicronemia............................................. 141
3.1.3. Autoimmunehyperchylomicronemia........................................... 142
3.2. Hypochylomicronemia...................................................... 142
3.2.1. Chylomicronretentiondisease.............................................. 142
3.2.2. Abetalipoproteinemia.................................................. 142
3.2.3. Familialhypobetalipoproteinemia(FHBL)......................................... 142
3.2.4. Familialcombinedhypolipidemia............................................. 142
3.3. Chylomicronremnantsandcardiovasculardisease ......................................... 143
3.4. Regulationofintestinalchylomicronproductionintype2diabetes.................................. 143
4. LaboratorytestsforCManalysis..................................................... 143
4.1. Biochemicaltesting ....................................................... 144
4.2. Genetictesting ......................................................... 144
5. Treatmentofchylomicrondisorders................................................... 144
5.1. Hyperchylomicronemia ..................................................... 144
5.1.1. Dietarytreatmentofhyperchylomicronemia........................................ 144
5.1.2. Drugtreatmentofhyperchylomicronemia......................................... 145
5.1.3. Genetherapyofhyperchylomicronemia.......................................... 145
5.2. Hypochylomicronemia...................................................... 145
5.2.1. Dietarytreatmentofhypochilomicronemia ........................................ 145
Acknowledgments ............................................................. 145
1. Structure and function of chylomicrons
In healthy humans, dietary fat, including lipids and fat-soluble vita-
mins, is efﬁciently absorbed by the small intestine. Thus, postprandial
lipemia refers to the systemic elevation of triglyceride-rich lipoprotein
(TRL) levels after a meal. The induction of postprandial lipemia is due
in part to an elevation in the serum concentration of chylomicrons
CMs transport most of the dietary fatty acids (mainly long-chain
fatty acids) and their remnants, resulting from the triglyceride hydroly-
sis of these TRLs by the lipolytic enzyme lipoprotein lipase, and are also
present in circulation during the postabsorptive state and cleared main-
ly by liver receptors .
1.1. Physico-chemical and functional characteristics of CMs
CMs are the largest lipoproteins found in circulation . Their size
has been found to be signiﬁcantly dependent on the fed/fasted state
, the rate of fat absorption, and the type and amount of fat absorbed
[5,6]. Conversely, the number of TRL particles produced during the post-
prandial state is only modestly increased compared with those released
under fasting conditions .
The increase in postprandial lipids is inﬂuenced by gender, genetics,
age, body size, exercise, weight loss, and metabolic syndrome .
Although postprandial concentrations of triglycerides may not be
found to be signiﬁcantly inﬂuenced by the type of meal, the extent of
postprandial CM response is highly dependent on the meal-fat mixtures
and, therefore, the nature and amount of different dietary components,
especially fatty acids . For instance, meal mixtures enriched in n-3
polyunsaturated fatty acids (PUFAs) generally decrease the postprandi-
al triglyceride response compared with those rich in saturated and
monounsaturated fatty acids [8–10]. Independent studies assessing
the impact on the serum levels of CMs after chronic consumption of iso-
caloric diets rich in saturated fatty acids, n-3 PUFAs, and n-6 PUFAs re-
vealed that the serum levels of CMs increased with saturated fatty
acids compared with n-6 PUFAs and n-3 PUFAs, respectively. Although
CM production rates were not assessed, subjects on the n-6 PUFA or
n-3 PUFA diets displayed enhanced lipolysis compared with those on
a saturated fatty acid diet, suggesting an increased in CM clearance.
Recent evidence suggests that enterocyte-triglyceride processing also
inﬂuences postprandial lipemia . Firstly, some evidence supports the
notion that the initial rise of lipids (within 10 to 30 min) secreted right
after a fatty meal—and preceding the primary postprandial peak of
serum triglycerides that occurs 3 to 4 h after the start of a meal—actually
correspond to those consumed in the previous meal. Of note, this strong-
ly suggests the existence of triglyceride storage within enterocytes. Sec-
ondly, the release of CMs in response to the cephalic phase is induced by
an oral stimulation, which occurs when fat is yet not consumed. This last
135J. Julve et al. / Clinica Chimica Acta 455 (2016) 134–148
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ﬁnding has led to the hypothesis of a taste-gut-brain axis, which may
also be involved in regulating the serum levels of triglycerides in re-
sponse to oral taste.
1.2. Lipid composition of CMs
Quantitative analysis shows that CMs have a central lipid core main-
ly composed of triglycerides and monoglycerides (85–92%), free and es-
teriﬁed cholesterol (1–3%), phospholipids (6–12%), and traces of fatty
acids . Ingeneral, the fatty acid composition of postprandial CM tri-
glycerides fairly closely reﬂects that of ingestedfat [5,13]. However, sev-
eral studies report that the fatty acid composition of a fatty meal is not
always reﬂected in the fatty acid composition of CM triglycerides. For
instance, low- and medium-chain fatty acid species (mainly C10:0,
C12:0, and C14:0) in the diet are frequently poorer substrates for re-
esteriﬁcation into triglycerides within the enterocyte; therefore, they
are not transported to a signiﬁcant extent into CMs but are rather
absorbed and rapidly delivered via the portal system to the liver, [5,14].
1.3. CM apolipoproteins
Apolipoproteins are the key protein constituents of CMs. Although
these proteins generally represent only less than 2% of the CMs' total
mass, they determine the intra- and extra-cellular metabolic fate of
these lipoproteins .
1.3.1. Apolipoprotein B-48
The apolipoprotein (apo) B-48 is the main, nonexchangeable CM pro-
tein and plays a critical role in CMs' synthesis, assembly, and secretion
. Under fasting conditions, the serum levels of apoB-48 are very low
or undetectable in most individuals. In human beings, apoB-48 is exclu-
sively produced in the small intestine in response to dietary fat through a
unique mRNA-editing event that converts codon 2153 (CAA, encoding a
glutamine) into a premature stop codon (UAA). This stop codon induces
the translation of a truncated apoB, apoB-48, which contains 48% of the
mature apoB primary sequence. The C-to-U site-speciﬁc editing of apoB
mRNA is accomplished by a large multiprotein complex, which includes
a deaminase, the apoB mRNA editing enzyme (apobec)-1, and the
apobec-1 complementation factor, among other factors. Importantly,
APOBEC1 is highly expressed in the small intestine but is absent in the
liver in humans. Consequently, all the apoB produced in the small intes-
tine essentially corresponds to apoB-48. The absence of the C-terminal
portion of apoB in apoB-48 signiﬁcantly determines its metabolic role.
For instance, the C-terminal domain of apoB-48 does not contain the
LDL receptor (LDLR) binding domain , and it is therefore unable to
mediate the clearance of this type of lipoproteins via LDLR.
1.3.2. Apolipoprotein A-I
ApoA-I is the main protein constituent of circulating HDL . Intes-
tinal CMs are also a source of signiﬁcant amounts of circulating apoA-I.
ApoA-I is generated in the endoplasmic reticulum of enterocytes but is
transported to the Golgi separately from CM vesicles and added to the
CMs before the mature particle is secreted into the mesenteric lymph
 (Fig. 1). Its suppression does not reduce triglyceride release from
cells , thereby indicating that the assembly of apoA-I may not be re-
quired for an appropriate lipidation of CMs.
1.3.3. Apolipoprotein A-II
ApoA-II is mainly synthesized in the liver [20,21] and, to a lesser ex-
tent, in theintestine. The role of apoA-II as a determinant of triglyceride
metabolism is supported by several observations in studies on humans
and experimental models [20,21], even though whether this is a result
of a function over CMs, VLDL, or both is unclear. For instance, there is
asigniﬁcant association of the −265C allele in the APOA2 gene with de-
creased serum apoA-II concentration and the enhanced postprandial
metabolism of large TRL (i.e., VLDL). The postprandial serum levels of
apoA-II have been found to be directly correlated with those of triglyc-
erides in healthy individuals. The deﬁciency of apoA-II in mice is associ-
ated with thereduced serum levels of TRLs and theincreased catabolism
of TRL remnants . The overexpression of human apoA-II in mice pro-
motes postprandial hypertriglyceridemia in independently transgenic
mice [23–25], this being mainly attributed to a defective catabolism of
circulating postprandial CMs. Because excess apoA-II associates with
CMs in circulation , it was ﬁrst suggested that its accumulation in
these particles might directly convert them in poorer lipolytic sub-
strates [23,26]. More recently, it has been proposed that an altered pro-
teome of the HDL from mice overexpressing apoA-II [23,25] might also
contribute to an impaired trafﬁcking of apoC-II, C-III, and E from HDL to
1.3.4. Apolipoprotein A-IV
ApoA-IV is the largest member of the exchangeable apolipoprotein
family . It is most highly produced by enterocytes in response to
lipid absorption , with minor amounts made in the liver .Itisse-
creted in association with nascent CM particles into the mesenteric
lymph . During the intestinal assembly of CMs, several lines of evi-
dence support that apoA-IV is incorporated into CMs at an early stage,
Fig. 1. Biology and pathology of the formation and packaging of CMs in the enterocyte.
Graphic symbolo gy: open, double-surrounded ell ipsoids indicate gene targets. Re d
circles indicate gene targets with known causal loss-of-fu nction mutations: MTTP in
abetalipoproteinemia (1) and Sar1 GTPase in chylomicron retention disease (2). A curly
black line was used to indicate the apoB-48 protein in nascent CM. Abbreviations used
include ACAT-2, acyl CoA acyltransferase; apo, apolipoprotein; CE, cholesteryl ester; CM,
chylomicron; COPII, coat protein complex II; DGAT, diglyceride acyltransferase; FA, fatty
acids; FC, free cholesterol; FABP, fatty acid binding protein; MG, monoglyceride; MGAT,
monoglyceride acyltransferase; MTTP, microsomal triglyceride transfer protein; NPC1L1,
Niemann–Pick C1-like 1; PCV, pre-CM transport vesicle; PLIN, perilipin; TG, triglycerides.
(For interpretation of the references to color in this ﬁgure legend, the reader is referred
to the web version of this article.)
136 J. Julve etal. / Clinica Chimica Acta 455(2016) 134–148
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and within the endoplasmic reticulum, and participate in the intestinal
assembly and secretion of these lipoproteins [31,32] (Fig. 1). First, apoA-
IV intestinal synthesis is increased during the absorption of long-chain
fatty acids, which require CM assembly, but not during the absorption
of short-chain fatty acids, which are directly transported into portal circu-
lation rather than packaged into CMs. Second, apoA-IV secretion into
lymph declines when intestinal CM formation is inhibited, and it increa ses
together with CM secretion when the inhibitor is removed. Third, the in-
hibition of CM formation has also been found to result in the accumula-
tion of intracellular lipid droplets containing both apoB-48 and apoA-IV,
thus showing that these apoliproteins associate with nascent primordial
CMs. Fourth, apoA-IV overexpression induced triglyceride packaging in
CMs in vitro. This indicates that apoA-IV has a role in the stabilization of
the expanding lipid interfaces during the formation of primordial CMs in-
side the enterocyte, enabling the enlargement of these particles. Fifth and
last, dietary fat absorption results in an increase in the serum concentra-
tion of apoA-IV, whereas it remains unchanged in patients with defective
intestinal lipid absorption in response to a fatty meal.
Despite this solid body of evidence, neither the overexpression nor
deﬁciency of apoA-IV in experimental animals has been shown to inﬂu-
ence fat absorption or production [31,32], thus suggesting that this pro-
tein exerts an auxiliary role in these processes or, at least, one function
that can be substituted by other proteins. On the other hand, it has also
been proposed that this protein inﬂuences serum CM clearance. For in-
stance, apoA-IV modulates the activation of lipoprotein lipase activity
by apoC-II , whereas its absence causes a delay in the CM clearance
in mice . During the lipolysis of the CM triglycerides, most apoA-IV
detaches from the CMs. It has been reported that approximately 25%
transfers to serum HDL and the remaining percentage is found in the
lipoprotein-depleted serum . Recent reports show that apoA-IV
also exerts an effect on the regulation of satiety and appetite ,there-
by suggesting a unique role for this protein in integrating feeding be-
havior and intestinal lipid absorption.
1.3.5. Apolipoprotein A-V
ApoA-V is mainly expressed in the liver and is found mainly bound
to TRLs and HDL at a very low serum concentration (20–500 ng/mL)
. This protein is recognized as a potent regulator of triglycerideme-
tabolism and leads to the enhanced lipolysis of circulating TRL and the
clearance of their remnants . This role has been, in part, attributed
to its ability to promote a greater interaction between both the TRL
and lipoprotein lipase, which might enhance the catabolism of TRLs by
stimulating the lipoprotein lipase-mediated lipolysis of these lipopro-
teins. Several factors might be involved inthe apoA-V-mediated modu-
lation of lipoprotein lipase action . First, it has been suggested that
the interaction between apoA-V and glycosylphosphatidylinositol-
anchored high-density lipoprotein binding protein 1 (GPIHBP1) might
favor the triglyceride hydrolysis of TRLs in mediating the interaction of
apoA-V with lipoprotein lipase (Fig. 2).
ApoA-V has also been shown to facilitate the binding of TRL rem-
nants to LDLR family members and thereby the clearanceof these parti-
cles from circulation (Fig. 3). In particular, sortilin-1 (SORT1), which is a
multiligand receptor in this family, has been reported to bind lipopro-
tein lipase and apoA-V. Lastly, apoA-V has also been described to medi-
ate the hepatic heparan-sulfate-mediated clearance of TRL remnants.
Few studies have analyzed the effect of apoA-V on the serum levels
of triglycerides after a fatty meal. The serum levels of apoA-V appear
to be closely related to the magnitude of postprandial lipemia and in-
crease after a meal challenge, this ﬁnding reﬂecting an association
with CMs , possibly as part of a regulatory mechanism to cover the
increasing need of postprandial lipolysis . The increase in the
serum levels of apoA-V after a meal challenge might be, in part,
accounted for by the action of PPAR-alpha. For instance, PPAR-alpha is
activated after a fat load ,and this transcription factor is involved
in the upregulation of APOA5 [38,39].
The potential role of apoA-V on the control of the absorption of
dietary fats by the gut still remains enigmatic. Interestingly, apoA-V
Fig. 2. Biology and pathology of chylomicron triglyceride catabolism. Graphic symbology: open, double-surrounded ellipsoids indicate gene targets. Red circles indicate gene targets with
known causal loss-of-function mutationsin familial hyperchylomicronemia: LPL (1), apoC-II(2), GPIHBP1 (3), LMF1 (4), apoA-V (5). FA, fatty acids; GPIHBP1, glycophosphatidylinositol
HDL-binding protein-1; HSPG, heparan sulfate proteoglycans; LMF-1, lipase maturation factor-1; LPL, lipoprotein lipase; TG, triglycerides. (For interpretation of the references to color in
this ﬁgure legend, the reader is referred to the web version of this article.)
137J. Julve et al. / Clinica Chimica Acta 455 (2016) 134–148
Author's personal copy
deﬁciency in mice has been experimentally demonstrated to increase
the production rate of CMs by the small intestine . This could be
due to the presence of apoA-V in the bile .
1.3.6. Apolipoproteins C
The human apoCs (i.e., C-I, C-II, and C-III) are distributed among all
lipoprotein classes, especially TRL and HDL . Nascent apoCs are
largely released in their lipid-poor form and rapidly associate with cir-
culating lipoproteins. Under fasting conditions, apoCs are mainly associ-
ated with HDL, whereas in the fed state they preferentially redistribute
to serum CMs and VLDL.
18.104.22.168. Apolipoprotein C-I. ApoC-I is the smallest exchangeable protein of
the family and is synthesized in the liver [15,43,44]. Several experimen-
tal studies support a role of apoC-I in TRL metabolism . For instance,
apoC-I inhibits the lipoprotein lipase activity in vitro, and transgenic
mice overexpressing the human protein display hypertriglyceridemia.
Like apoC-III (see Section 22.214.171.124), apoC-I also inhibits the clearance of
TRLs and their remnants by impairing interactions with their receptors,
this action being in part related to the apoC-I-induced displacement of
TRLs are known to stabilize and protect lipoprotein lipase from
inactivating factors, including the angiopoietin-like protein 4 (Angptl4).
The addition of apoC-I to TRLs (either CM or synthetic lipid emulsion)
signiﬁcantly interferes with their protective effect on lipoprotein lipase,
which becomes more prone to inactivation by Angptl4 in vitro .
126.96.36.199. Apolipoprotein C-II. This protein is mainly synthesized in the liver
and to a lesser extent in the small intestine . Apart from HDL, apoC-II
is also bound to CMs and VLDL, becoming one of the key components in
the metabolism of TRLs . The importance of apoC-II as an activator of
lipoprotein lipase has been unequivocally demonstrated in patients
with genetic defects in their structure or production and in transgenic
animals [42,46]. ApoC-II is therefore required for maximal rates of TRL
lipolysis  (Fig.2). However, the mechanism whereby this protein ac-
tivates lipoprotein lipase still remains elusive. For instance, it is under
discussion whether apoC-II binds or not directly to lipoprotein lipase
The hepatic expression of APOC2 is upregulated by a high-fat diet
and by agonists for PPAR-alpha. Similar to the concept described for
apoA-V (see Section 3.2.4), oleoylethanolamide, a potent PPAR-alpha li-
gand, which is overproduced by enterocytes in response to food intake
, might activate PPAR-alpha and induce the transcription rate of
APOC2. Consistent with this, an increase in the serum levels of apoC-II
has been reported under postprandial conditions .
188.8.131.52. Apolipoprotein C-III. ApoC-III is a component and one of the most
important markers of TRL . It is the most abundant apoC in human
serum, and its concentration is positively associated with that of triglyc-
erides. Apart from TRLs, it is also found to be associated with HDL and,
though to a lesser degree, with LDL. This protein is synthesized by the
liver and to a lesser extent by the intestine and, along with the APOA1,
APOA4,andAPOA5 genes, belongs to a gene cluster that has been impli-
cated as a potential genetic determinant for variations in triglycerides.
Several lines of evidence involve apoC-III as a major player contributing
to the development of hypertriglyceridemia [42,45,48]. First, apoC-III
inhibits lipoprotein lipase activity (Fig. 2). Although the mechanism of
this action is not yet understood, it may act byinterfering with the bind-
ing of lipoprotein lipase to lipids, which would result in a decreased ac-
tivity and enhanced inactivation of the enzyme. Second, apoC-III also
interferes with the binding of apoB-100 and apoE to hepatic receptors,
leading to a decreased lipoprotein remnant clearance by receptor-
mediated endocytosis. ApoC-III also impairs the binding of lipoproteins
APOC3 is also a PPAR gene target . In contrast to its action on the
expression of APOC2 and APOA5 (see Sections 184.108.40.206 and 1.3.5,respec-
tively), activated PPAR-alpha inhibits the transcription of the gene,
which results in a concomitant reduction of both the serum levels of
this protein and those of triglycerides.
1.3.7. Apolipoprotein E
In the circulation, TRLs (i.e., CMs and VLDL) are immediately hydro-
lyzed by lipoprotein lipase, leading to the generation of TRL remnants.
During lipolysis, these remnants recruit HDL-derived apoE, which will
eventually facilitate their rapid clearance through speciﬁc endocytic re-
ceptors  (Fig. 3). Human apoE serves as a ligand for the LDL receptor
and the LDL receptor-related protein , promoting the efﬁcient clear-
ance of TRL from circulation. The LDLRpreferentially mediates the clear-
ance of apoB-containing lipoproteins, which include the LDL and
different types of TRLs containing apoB-100, or combinations of apoB-
48 and apoE , while LRP preferentially interacts with apoE .
ApoE also binds to speciﬁc subsets of the sulfate groups of the heparan
sulfate chains and mediates the clearance of TRLs by hepatic HSPGs
[50,51] even though this property is not exclusive for apoE. Other pro-
teins associated with TRLs (i.e., apoB-48 , apoB-100 ,and
apoA-V  as well as the lipolytic enzymes lipoprotein lipase and he-
patic lipase ) also interact with HSPGs. This interaction may
Fig. 3. Biology and pathology of the hepatic clearance of remnant chylomicrons. Graphic
symbology: open, double-surrounded ellipsoids indicate gene targets. Red circles indicate
gene targets with known causal loss-of-function mutations: apoE (1) and apoA-V (2) in
dysbetalipoproteinemia, and HL and/or LPL (3), and LRP1 (4). Abbreviations usedinclude
CE, cholesteryl ester; CM, chylomicron; HL, hepatic lipase; HSPG, heparan sulfate
proteoglycans; LDLR, LDL receptor; LRP1, LDLR-related protein-1; LPL, lipoprotein lipase;
TG, triglycerides; TRL, triglyceride-rich lipoprotein. (For interpretation of the references to
color in this ﬁgure legend, the reader is referred to the web version of this article.)
138 J. Julve etal. / Clinica Chimica Acta 455(2016) 134–148
Author's personal copy
contribute to sequestrating the TRL remnants in the space of Disse be-
fore they are rapidly internalized by hepatocytes either directly or indi-
rectly via binding to endocytic receptors (i.e., LDLR and LRP).
The endocytic uptake of TRLs mediated by the LRP is more complex
than that of the classical LDLR-mediated endocytosis. It has been sug-
gested that LRP-mediated internalization might prevail in the liver during
the postprandial state . During the LRP-mediated internalization,
TRLs are thought to be hydrolyzed in peripheral endosomes, where
core lipids and apoB-48 are targeted to lysosomes. Conversely, most of
the TRL-derived apoE content is maintained in peripheral recycling
endosomes, thus maintaining the production and release to serum of
apoE-containing HDL, which in turn accelerate CM remnant enrichment
2. Metabolism of CM
Dietary lipid trafﬁcking compressed from enterocyte absorption
until their delivery into systemic cells is a complex, multistep process
involving the participation of several key molecular players. We will
mainly focus on proteins involved in clinically relevant alterations
inﬂuencing CMs and will distinguish between the intracellular (see
Section 2.1) and extracellular (see Section 2.2) determinants.
2.1. Intracellular metabolic determinants of synthesis and secretion of CM
During CM assembly and secretion, dietary free fatty acids (mainly
long-chain fatty acids) are internalized across the luminal membrane of
enterocytes via passive diffusion and, to a lesser extent, by different
fatty acid transporters . Fatty acids within the enterocyte are trans-
ferred to the endoplasmic reticulum by intracellular fatty acid binding
proteins (FABP), L-FABP (FABP-1), and I-FABP (FABP-2), where fatty
acids are re-esteriﬁed into triglycerides (Fig. 1). In parallel, the transport
of dietary cholesterol occurs through the brush border by speciﬁc choles-
terol transporters (i.e., Niemann–Pick C1-like 1 –NPC1L1 –and Scavenger
receptor class B type 1—SR-BI). Free cholesterol may be either transported
back to the intestinal lumen by ABCG5/G8 or esteriﬁed by the enzyme
acyl CoA acyl transferase (ACAT)-2. Inside the endoplasmic reticulum,
newly synthesized apoB-48 acquires triglycerides, cholesteryl esters,
and phospholipids in a process catalyzed by the enzyme microsomal tri-
glyceride transfer protein (MTTP) to produce the primordial CMs 
(Fig. 1). Further lipidation of these newly produced primordial CMs by
MTTP form preCMs. During this process, these lipoproteins further ac-
quire apoA-IV (see Fig. 1). PreCMs are then exported to the Golgi in
preCM transport vesicles (PCV) from the endoplasmic reticulum in a pro-
cess modulated by CD36 and L-FABP. When PCVs reach the Golgi, they
fuse with the cis-Golgi in a process controlled by both the secretion-
associated, Ras-related GTPase 1B (Sar1b) and the soluble N-ethyl
maleimide sensitive-factor attachment protein receptor proteins
(SNARE). In the Golgi, preCMs further acquire apoA-I and, to a lesser ex-
tent, apoA-II to form mature CMs. Subsequently, mature CMs are
exported as vesicles from the Golgi to the basolateral surface of the
enterocyte and are released into the lymph, from where they will reach
2.1.1. Fatty acid transporters
As previously stated (see Section 2.1), the fatty acid absorption by
enterocytes is mainly produced via passive diffusion (Fig. 1). Albeit
minor, fatty acids are alsotransported via certain fatty acid transporters
located at the luminal membrane of enterocytes . One of these
transporters is CD36, which is abundantly expressed in the small intes-
tine and where it is thought to exert a role in fatabsorption. This is con-
sistent with its expression pattern, which is abundant in the proximal
intestine, and its apical localization within the luminal surface of
enterocytes. CD36 deﬁciency in humans wasnot found to be associated
with altered fatty acid absorption but, rather, with higher postprandial
serum levels of triglycerides and apoB-48-containing lipoproteins in
respect to non-deﬁcient subjects . Of note, however, the size of post-
prandial CMs from CD36-deﬁcient patients is smaller than those from
non-deﬁcient subjects, thereby indicating a potential role of CD36 in
postprandial CM formation rather than in fatty acid uptake .
2.1.2. Fatty acid binding proteins (FABPs)
Within the enterocyte, cytosolic FABPs (L-FABP and I-FABP) promote
the targeted delivery of fatty acids to speciﬁc metabolic sites during post-
prandial lipemic response . Different ﬁndings suggest a role of L-FABP
in intracellular lipid trafﬁcking  (Fig. 1). This protein displays an aux-
iliary role, along with CD36, in the exportation of nascent CMs from the
endoplasmic reticulum to the Golgi via PCV. Consistent with this, L-
FABP-deﬁcient mice accumulate triglycerides within the enterocytes
concomitantly with a reduction in triglyceride release into circulation.
However, there are no reports on human mutations or variants within
the FABP1 gene leading to a dysfunctional L-FABP. Regarding other
FABP, patients bearing a genetic sequence variant in the human gene
(FABP2) present elevated serum triglyceride levels . Although the im-
pact on postprandial lipemia was not studied in these patients, there
were no alterations in intestinal fatty acid absorption, thus showing
that FABP2 may not be essential in this process .
2.1.3. Enzymes involved in triglyceride synthesis
Dietary fatty acids and monoglycerides are intracellularly esteriﬁed
within theenterocytes by monoglyceride acyltransferases(MGATs, cod-
iﬁed by MGAT2 and MGAT3) to form diglycerides. Subsequently, newly
synthesized diglycerides are acylated by diglyceride acyltransferases
(DGATs) to form triglycerides  (Fig. 1).
Functional studies conducted in MGAT2-deﬁcient mice, the main
isoform in mice, reveal that the absence of this enzyme results in a di-
minished postprandial triglyceridemia . In relation with the other
enzyme involved in triglyceride synthesis during CM packaging, an im-
paired DGAT function attributed to mutations within the DGAT1  has
been found in a family with congenital diarrheal disorder .Incon-
trast, mice lacking DGAT1show only delayed fat absorption. This appar-
ent disparity could possibly be explained by the fact that mice also
express DGAT2 in the intestine, which may compensate, at least in
part, for the deﬁciency in DGAT1.
MTTP controls apoB lipidation in the endoplasmic reticulum in both
the liver and small intestine and therefore participates in the formation
of VLDL and CMs, respectively . MTTP acts as a chaperone to assistin
apoB folding and catalyzing the addition of triglycerides, esteriﬁed cho-
lesterol, and phospholipids to the apoB-48 to produce primordial CMs
[8,59] (Fig. 1).
2.1.5. Coat protein complex II (COPII)
Newly synthesized and properly folded proteins are transported
from the endoplasmic reticulum to the Golgi via COPII vesicles .
The intracellular trafﬁc of apoB-48-containing CMs is dependent on
the COPII complex, which allows the vesicle budding from the endo-
plasmic reticulum membrane to transport newly synthesized proteins
to the Golgi. Secretion-associated, Ras-related GTPase 1B (Sar1b),
which is encoded by the gene SARA2, is one of the main components
of this multicomplex protein and plays a pivotal role in the assembly, or-
ganization, and function of the COPII. In the context of CM packaging, it
has been proposed that Sar1b might gather COPII, which in turn assem-
bles the lipid vesicles to transport primordial CM particles to the Golgi
(pre-CM transport vesicle, PCV in ﬁg. 1). Sar1b overexpression enhances
intestinal lipoprotein trafﬁcking and sorting through the stimulation of
CM assembly and release (extensively reviewed in ).
Sortilin-1 (SORT1) is one of the members of the multi-ligand vacuo-
lar protein sorting 10 receptor family. It is primarily localized in the
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trans Golgi and early endosomes. Although it mainly functions as a traf-
ﬁcking receptor to sort lysosomal hydrolases to the lysosome, a small
fraction of this receptor is translocated to the cell membrane, where it
may act as an uptake receptor, mediate the signal transduction of
neurotrophins, or be cleaved by a speciﬁc secretase to release a soluble
extracellular domain of still unknown function  SORT1 has been re-
ported to bind LDL and facilitate its uptake and degradation. Also, SORT1
has been reported to bind lipoprotein lipase and receptor-associated
protein (RAP) , apoA-V , and apoE . Beyond its role in the
clearance of LDL particles from circulation, it has been proposed that
SORT1 may also modulate hepatic VLDL secretion .Sincethisrecep-
tor binds apoB-100, but not apoB-48 , it could be speculated that it
would not display any role in CM synthesis.
2.2. Extracellular metabolic determinants of catabolism and clearance of
CMs and their remnants
In circulation, CMs acquire apoC-II, C-III, and E mainly from HDL and
are subjected to triglyceride hydrolysis by lipoprotein lipase(Fig. 2). The
function of this enzyme is modulated by apoC-II, which enhances the
catalytic rate of the enzymeand is inhibited by apoC-III. During this hy-
drolytic process, a substantial portion of phospholipid, apoAs, and Cs is
removed from the delipidated TRLs and is transferred to the HDL
The physiological importance of other determinants of CM clear-
ance has been revealed from genetic defects in humans in recent
years  (Fig. 2), including the lipase maturation factor (LMF) 1,
glycosylphosphatidylinositol-anchored high-density lipoprotein-
binding protein 1 (GPIHBP1), and apoA-V. In addition, the hydrolysis
of CM triglycerides originates remnant CMs, which are removed
from circulation by the liver. This is, in part, due to the enrichment
of these CM remnants with apoE, which promotes their binding to
HSPG on the luminal membrane of hepatocytes, and it is the protein
ligand recognized by LDLR and LRP , a process which is facilitated
by hepatic lipase [67,68].
2.2.1. Lipoprotein lipase
Lipoprotein lipase is a main determinant for postprandial lipemia
. This lipase is primarily produced by the parenchymal cells of adi-
pose tissue, skeletal muscle, and myocardium. It is anchored to heparan
sulfate chains (i.e., HSPGs) on the luminal surface of vascular endotheli-
um, where it hydrolyzes the triglycerides of postprandial CMs and large
VLDL (Fig. 2).
The expression of lipoprotein lipase is regulated in a tissue/cell-
speciﬁc manner and in response to metabolic demands in different tis-
sues. During triglyceride hydrolysis, the released fatty acids are then
taken up and reesteriﬁed for storage or oxidized to provide energy to
adipose tissue and muscle, while some fatty acids remain in the circula-
tion bound to albumin. The active form of lipoprotein lipase is a homo-
dimer, which may eventually dissociate toproduce inactive monomers.
The function of lipoprotein lipase is modulated by several factors
(depicted in Fig. 2). First, apoC-II enhances its action, while apoC-III is
the endogenous inhibitor of this enzyme  (see Sections 220.127.116.11 and
18.104.22.168). Second, genetic mutations (see Section 3.1.1)havealsore-
vealed the physiological importance of the lipase maturation factor
(LMF1), GPIHBP1, and apoA-V (encoded by APOA5) in regulating the ca-
tabolism of CMs .
Lipoprotein lipase is inhibited by the angiopoietin-like proteins 3, 4,
and 8 (Angptl3, Angptl4, and Angptl8, respectively) . The mecha-
nisms of action of both Angptl3 and Angptl4 in disabling lipoprotein li-
pase are distinct . Angptl3 reduces the lipoprotein lipase activity
without inducing its inactivation, whereas Angptl4 irreversibly sup-
presses lipoprotein lipase activity. Angptl8 inhibits lipoprotein lipase ei-
ther directly or indirectly by promoting the activation of Angptl3 .
2.2.2. Hepatic lipase
Hepatic lipase is a member of the lipase gene family that is mainly
synthesized and secreted by hepatocytes . This enzyme may hydro-
lyze triglycerides and phospholipids in different lipoproteins, but its
function is critical in the conversion of intermediate-density lipopro-
teins (IDLs) to low-density lipoproteins (LDL) and in the conversion of
triglyceride-rich HDL into triglyceride-poor HDL. Apart from its catalytic
action, it has also been described to act as a ligand of lipoproteins, there-
by facilitating the removal of remnant lipoproteins (Fig. 3). In this re-
gard, it has been suggested that hepatic lipase may mediate the
interaction between remnant lipoproteins and cell surface receptors
2.2.3. Glycosylphosphatidylinositol-anchored high-density lipoprotein-
binding protein 1 (GPIHBP1)
Human GPIHBP1 is expressed in the same tissues that also express
lipoprotein lipase . Although it was ﬁrst identiﬁed as an HDL binding
protein, it directs the transendothelial transport of lipoprotein lipase to
help anchor CMs to the endothelium. The acidic domain of GPIHBP1 ap-
pears to be critical for lipoprotein lipase binding, since its replacement
leads to an impaired ability of GPIHBP1 to bind both lipoprotein lipase
and CMs (Fig. 2). Interestingly, mutations in the heparin-binding do-
mains of both LPL and APOA5 abrogate the binding of these proteins to
GPIHBP1. GPIHBP1 also prevents lipoprotein lipase inhibition by
Angptl3 and Angptl4 , thereby providing another mechanism to
preserve the lipoprotein lipase function.
2.2.4. Lipase maturation factor (LMF) 1
LMF1 is a chaperone required for the proper posttranslationally fold-
ing and processing of lipoprotein lipase and hepatic lipase and, therefore,
is critical for their function . LMF1 is a membrane-bound protein that
is localized in the lumen of endoplasmic reticulum. Although mutations
in this gene are rare, they are associated with a reduced expression of
both lipases in patients with severe hypertriglyceridemia (Fig. 2). The
fact that the tissue distribution of LMF1 is not restricted only to the tis-
sues synthesizing these two lipases suggests that LMF1 may exert
broader biological functions.
2.3. Receptors for CM remnant clearance
The liver-dependent clearance from the circulation of CMs and VLDL
remnants via LDLR and LRP receptors is apoE-dependent . In contrast,
apoB-48 does not contribute to receptor recognition. Triglyceride-rich
VLDL produced by the liver is also cleared from circulation competing
with CMs. The delayed clearance of CMs is concomitantly accompanied
by a delay in the clearance of VLDL, since the LDLR preferentially takes
up CM remnants. In this regard, another additional potential participant
in the clearance of postprandial VLDL is the VLDL receptor (VLDLR).
HSPGs and, in particular, transmembrane syndecans have also been
proposed to mediate the binding and uptake of CM remnants by the
LDLR binds apoB-100 and/or apoE-containing lipoproteins. There-
fore, it accounts for the bulk clearance of serum lipoproteins into the
liver, including the apoE-containing CM remnants  (Fig. 3). LDLR de-
ﬁciency in both patients and animal models does not result in a defec-
tive clearance of CM remnants, indicating the existence of alternative
apoE-speciﬁc remnant receptors for their clearance .
2.3.2. LDL receptor-related proteins (LRPs)
One of the main candidates for apoE-speciﬁc CM remnant receptors
is the LRP1 . LRP1 belongs to the LRP family that represents a group
of structurally related transmembrane proteins involved in a diverse
range of biological activities . In this regard, the discovery that
LRP1 binds apoE led to the notion that it acts as a remnant receptor
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 (Fig. 3). Consistently, LRP1 deﬁciency or inhibition does result in a
decreased clearance of CM remnants.
Aside from LRP1, the LRP family also includes other members, such
as LRP1b, LRP2 (also called megalin), LRP4 (also called multiple epider-
mal growthfactor-like domains protein 7 or MEGF7), LRP5/6,LRP8 (also
called apo E receptor 2), the VLDL receptor (VLDLR), and LR11/SorLA1
(LDL receptor relative with 11 ligand binding repeats/sorting protein-
related receptors containing LDLR class A repeats). Of note, it has been
recently proposed that apoA-V might mediate in the binding of this
last sortilin class to the HSPG of endothelial cell surfaces .
There are some disorders in which CMs are involved. CMs are
considered to be present in blood serum when triglyceride concen-
tration is above 1000 mg/dL (N11.3 mmol/L) . Severe hypertri-
glyceridemia is also deﬁned by the same criteria. The prevalence of
hyperchylomicronemia in North America is between 1, 5 and 4 per
1000 individuals . Hyperchylomicronemia is a well known cause
of acute pancreatitis. Although the exact pathophysiological mechanism
remains unclear , pancreatic ischemia and acidosis—due to an im-
pairment in the blood ﬂow due to increased viscosity—have been pro-
posed. Hypertriglyceridemia was considered causative of the attack in
up to 10% of all pancreatitis episodes and in more than a half of gesta-
tional pancreatitis patients . It is noteworthy that ﬁve of the six clas-
sical hyperlipidemic phenotypes show HTG, but only those deﬁned
with high CM concentrations (types I and V) may trigger acute pancre-
Also, there is epidemiological and clinical evidence that fasting, and
to a greater extent non-fasting, hypertriglyceridemia are important risk
factors for cardiovascular disease (CVD) [11,80,81]. Although CMs are
too large to cross the endothelium, their smaller remnant particles can
penetrate the arterial wall, contributing to plaque formation and induc-
ing endothelial dysfunction . The atherogenic potential role of TRL
remnants is clearly demonstrated in the type III hyperlipidemia pheno-
type, a remnant hyperlipidemia where unequivocal accelerated athero-
sclerosis and CVD have been described. CMs and VLDL share the same
clearance pathway in a competitive manner , but CMs seem to be
the preferred substrate of lipoprotein lipase .
The hyperchylomicronemia syndrome is a disorder characterized by
extreme hypertriglyceridemia, the presence of chylomicrons, and one
or more of the following clinical manifestations: eruptive xanthomas,
lipemia retinalis, hepatosplenomegaly, recurrent abdominal pain, and/
or acute pancreatitis . Although CMs are usually considered to be
present when serum triglyceride concentrations are above 1000 mg/dL,
the symptoms associated with the chylomicronemia syndrome almost
always occur at higher triglyceride levels . Early diagnosis of severe
chylomicronemia syndrome is crucial to avoid pancreatitis-derived con-
sequences, including abdominal pain and diabetes mellitus.
There are genetic causes of hyperchylomicronemia, but the ma-
jority of patients seem to have a combination of a common genetic
predisposition and an acquired cause of hypertriglyceridemia. Famil-
ial combined hyperlipidemia, familial hypertriglyceridemia and, oc-
casionally dysbetalipoproteinemia, in combination with untreated
diabetes mellitus, high-saturated fat/high carbohydrate diet, certain
drug therapies (e.g., estrogen therapy, antiretroviral drugs), or ex-
cessive alcohol consumption account for nearly 90% of the cases of
hyperchylomicronemia in adults [85,86].
As mentioned above, ﬁve of the six classical HLP phenotypes initially
classiﬁed by Fredrickson have HTG as a deﬁning component, and
the chylomicronemia syndrome is a feature of two of them—type I
and type V HLP [87,88]—which have a partially common phenotype
consisting of fasting serum triglyceride N1000 mg/dL, concomitant
with the presence of CMs.
3.1.1. Monogenic hyperchylomicronemia
Type I hyperlipidemia, or familial hyperchylomicronemia, is a rare
disease with a prevalence of ~ 1 case in 1,000,000inhabitants ,char-
acterized by the early onset of severe hypertriglyceridemia associated
with chylomicronemia but without an increase in VLDL. It is mainly
due to rare homozygous or double heterozygous, loss-of-function LPL
gene variants (HLP1A, OMIM 238600) (Fig. 2). Also, mutations in sever-
al additional loci, which encode for proteins involved in the activity, as-
sembly, or transport of lipoprotein lipase, have been identiﬁed in
patients with familial hyperchylomicronemia without LPL mutations.
According to the OMIM database, less common causes of familial
chylomicronemia (also represented in Fig. 2), are rare, loss-of-function
variants in APOC2 (HLP1B, OMIM 207750), GPIHBP1 (HLP1D, OMIM
615947), LMF1 (combined lipase deﬁciency, OMIM 246650), or APOA5
(late on-set hyperchylomicronemia, OMIM 144650), as well as thepres-
ence in blood serum of circulating lipoprotein lipase inhibitors (HLP1C,
OMIM 118830). However, in the latter, only one affected family has
been described  as presenting an autosomal dominant mode of
Recently, two studies of the molecular diagnosis of hyperchylo-
micronemia in a clinicallaboratory setting have been reported. In a sam-
ple of 29 patients with severe hypertriglyceridemia from Spain ,
44.8% were homozygous or compound heterozygous for rare, loss-of-
function variants in the LPL gene and could be classiﬁed as HLP1A, and
10.3% were heterozygous or compound heterozygous with a rare and
common variant. Additionally, 6.9% of the patients were homozygous
or compound heterozygous for loss-of-function variants in APOA5.In
the other study, in a sample of 149 severe HTG patients from Italy
, the proportion of homozygous or compound heterozygous for
loss-of-function variants in the LPL gene was 20.1%, while 14.1% were
heterozygous for rare LPL gene variants. The higher number of HLP1A
in the ﬁrst study could be explained by the higher proportion of new-
borns in the Spanish sample, as HLP1 has an earlier onset than other
forms of hyperchylomicronemia.
3.1.2. Polygenic hyperchylomicronemia
22.214.171.124. Type V hyperlipidemia. The presence of fasting CMs is also a char-
acteristic of type V hyperlipidemia (HLP5, OMIM 144650), but this form
also presents with a concomitant increase in VLDL and is usually ob-
served in adults rather than children. Type V hyperlipidemia is thought
to be the result of complex interactions between genetics and environ-
mental factors, with the former including heterozygous, loss-of-func-
tion mutations in LPL (in approximately 10% of affected individuals)
together with contributions from more common variants in APOA5,
APOE,orANGPTL3 [93,94]. In a study of 86 Dutch patients with severe
HTG, 26% of the cases were carriers of only common variants in LPL
and APOA5 . In a Spanish population, the common LPL variants
p.Asp36Asn and p.Asn318Ser, APOA5 p.Ser19Trp, and APOE4 were inde-
pendently associated with an increase in serum triglyceride levels ,
although an additive effect was observed among the LPL p.Asp36Asn,
APOA5 p.Ser19Trp, and APOE4 variants. Therefore, type V hyperlipid-
emia is thought to be of a polygenic nature in most cases, where the
individual's environment is obviously relevant and patients with genet-
ic susceptibility (e.g., those with heterozygous mutations in the LPL
gene) in combination with pregnancy, excessive alcohol intake, obesity,
uncontrolled type 1 or 2 diabetes mellitus, or different medications
(i.e., estrogens, glucocorticoids, tamoxifen, 13-cis-retinoic acid, antire-
troviral therapies) cooperate in developing the disease [89,97].
In this context, it is known that an accumulation of rare variants
contributes to the heritability of complex traits among individuals at
the extreme of a lipid phenotype . It has been estimated that ap-
proximately 53% of new missense variants in humans have mildly
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deleterious effects and give rise to many low-frequency deleterious alle-
lic variants , the accumulationof which can produce a burden of var-
iants with small effect size. In a recent study performed in healthy male
adults, 42 SNPs in 23 genes explained 88% of the variance in chylomi-
cron response to dietary fat , some of them affecting fatty acid up-
take and triglyceride synthesis (e.g., CD36,ELOVL5,SLC27A5,and
SLC27A6), CM clearance (LPL,APOB,APOA5,andLIPC), insulin and carbo-
hydrate metabolism (IRS1,INSIG2, and TCF7L2), HDL function (APOA1
and ABCA1), and appetite and body weight regulation (MC4R). Further
investigations are needed to test whether common variants in these
genes accumulate in chylomicronemia,but in the study by Martín-Cam-
pos et al. , 28% subjects with severe HTG were heterozygous for loss-
of-function variants in LPL or a combination of common, small effect
variants in LPL,APOA5,APOE,andGPIHBP1.
126.96.36.199. Dysbetalipoproteinemia. Another polygenic disorder involving
TRL remnants is dysbetalipoproteinemia (HLP3, OMIM 107741).
Type III hyperlipidemia is caused by the conjunction of a defect in the
clearance of remnant particles, mainly due to homozygosity for the
binding-defective apoE2 isoform or other rare APOE variants, with
other environmental and/or genetic factors that cause the overproduc-
tion of triglyceride-rich particles or a reduction in LDL receptor activity
 (Fig. 3). This would be the reason why only 10–20% of E2/E2 sub-
jects develop type III hyperlipidemia and 30–40% of them have a muta-
tion in APOA5 as a secondary factor [88,102]. In contrast, less than 5% of
type III hyperlipidemia patients have a monogenic, dominant form due
to a mutation in APOE, which would not require other factors to develop
the disease. Type III hyperlipidemia patients usually exhibit fasting HTG
between 300 and 1000 mg/dL due to the impairment in hepatic rem-
nant clearance and a putative impairment in lipoprotein lipase activa-
tion (i.e., from the presence of reduced-expression APOA5 mutants),
which could induce hyperchylomicronemia frequently, at least during
the postprandial state  (Fig. 3).
3.1.3. Autoimmune hyperchylomicronemia
Some autoimmune diseases, such as systemic lupus erythematosus,
systemic sclerosis, polymyositis, and rheumatoid arthritis, have shown
the presence of anti-lipoprotein lipase antibodies, which could explain
some hyperlipidemias observed during rheumatic diseases . Circu-
lating anti-lipoprotein lipase antibodies could bind the enzyme
and have an inhibitory effect on activity and, therefore, impair triglycer-
ide degradation and lead to hypertriglyceridemia. Anti-lipoprotein li-
pase antibodies could be present both in normotriglyceridemic and
hyperchylomicronemic subjects without autoimmune disease ;
however, although the proportion of anti-lipoprotein lipase antibodies
was signiﬁcantly higher in hyperchylomicronemic subjects, it would ex-
plain only a minority of sporadic cases. In a study of 63 HLP5 patients,
27% presented an anti-lipoprotein lipase serum level above the 95th
percentile of a control population, but less than 10% displayed a sub-
stantial inhibition of serum lipolysis . In another study of 44 pa-
tients with HTG, anti-lipoprotein lipase antibodies were present in
only 2 of them . It is interestingto point out that in the ﬁrst descrip-
tion of an autoimmune hyperchylomicronemia due to a defect of lipo-
protein lipase activity, in a 35-year-old woman with severe HLP1
, the circulating anti-lipoprotein lipase antibodies were bound to
CMs, thus emphasizing their ability to transport lipoprotein lipase.
Hypochylomicronemia is deﬁned as the low level or absence of post-
prandial CMs, and it can result from genetic or acquired causes.
As hypolipidemia is frequently in association with different diseases,
especially those that are serious and consumptive, the diagnosis of
hypochylomicronemia is usually considered in the presence of very
low serum lipids either in healthy subjects or in concomitance with
malabsorption, as this suggests a primary defect in chylomicron
Inherited causes of hypochylomicronemia are caused by very rare
mutations in the genes involved in the packaging and secretion of
apoB-containing lipoprotein particles, resulting in either a strong de-
crease or the absence of CMs, VLDL, and LDL in blood serum. Three
inherited diseases affecting chylomicrons can be distinguished: CM re-
tention disease (CMRD, OMIM 246700), abetalipoproteinemia (ABL,
OMIM 200100), and familial hypobetalipoproteinemia (FHBL1, OMIM
3.2.1. Chylomicron retention disease
This disease, also called Anderson disease, is an autosomal recessive
disorder characterized by an intestinal defect in lipid transport due to a
failure of CM formation in enterocytes, which results in severe malab-
sorption with steatorrhea, fat-soluble vitamin deﬁciency, low blood
cholesterol levels, and failure to thrive in infancy. Chylomicron reten-
tion disease is caused by homozigous and compound heterozygous mu-
tations in SAR1B, a gene encoding Sar1 homolog B GTPase  (Fig. 1).
Sar1b is one of the subunits of the coat protein (COPII) complex, which
has been found to be critical for the vesicular transport of apoB-48-
containing particles from endoplasmic reticulum to the Golgi.
Familial abetalipoproteinemia is an autosomal recessive disorder
involving the improper packaging and secretion of apoB-containing
particles as a root cause. It is caused by homozygous and compound het-
erozygous mutations in the gene encoding the large subunit of the
MTTP  (Fig. 1). MTTP is a heterodimer composed of the multifunc-
tional protein disulﬁde isomerase (P4HB), which is related with Cole-
Carpenter syndrome and is a unique, large subunit of MTTP. This sub-
unit binds to amino acids 1–264 and 512–721 or 270–570 of apoB—an
interaction important for the initiation of translocation of the nascent
apoB chain to the endoplasmic reticulum and for the addition of lipids
to this chain, causing the defective cellular secretion of the apoB. The in-
cidence of familial abetalipoproteinemia is less than 1 in 1 million .
3.2.3. Familial hypobetalipoproteinemia (FHBL)
FHBL1 is, in contrast to chylomicron retention disease and abeta-
lipoproteinemia, an autosomal co-dominant disorder. The majority of
familial hypobetalipoproteinemic patients are heterozygotes, carriers
of a truncating mutation in APOB that results low serum lipids, especial-
ly LDL cholesterol and intestinal and hepatic triglyceride accumulation
as a result of the decreased export of triglycerides . Homozygous
subjects are rare and present, as in ABL, a near absence of apoB-
containing lipoproteins, with more severe clinical manifestations in
childhood, such as failure to thrive, intestinal fat malabsorption causing
liposoluble vitamin deﬁciency, and nonalcoholic liver steatosis.
3.2.4. Familial combined hypolipidemia
Familial combined hypolipidemia (FHBL2, OMIM 605019) is caused
by homozigous and compound heterozygous mutations in the ANGPTL3
gene and, in addition to low apoB-containing lipoproteins, includes low
HDL cholesterol [112–115]. Angptl3 is secreted and expressed almost
exclusively in the liver and in mice plays a role in the inhibition of lipo-
protein lipase and endothelial lipase (encoded by LIPG)—two key
enzymes in the metabolism of triglyceride-rich particles and HDL. In a
population study, common loss-of-function mutations in the ANGPTL3
gene either interfered with the synthesis/secretion of the protein or
failed to inhibit lipoprotein lipase activity in vitro but did not reduce
HDL cholesterol (HDL-C) . Serum Angptl3 has also been strongly
correlated with hepatic lipase (encoded by LIPC) activity and serum
HDL cholesterol in healthy Japanese subjects  but not with serum
triglycerides [118–120] or lipoprotein lipase activity . However,
the complete absence of Angptl3 results in an increased lipoprotein li-
pase activity, and decreased serum free fatty acids .Thus, the effect
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of Angptl3 on lipid metabolism seems to involve different, as of yet not
completely understood, mechanisms.
3.3. Chylomicron remnants and cardiovascular disease
Postprandial hyperlipidemia is considered an important risk factor
for cardiovascular disease. Indeed, three prospective epidemiologic
studies found highly signiﬁcant associations between non-fasting tri-
glycerides and the risk of adverse cardiovascular events [80,81,122],
and, also, strong evidence supports the role of cholesterol in remnant li-
poproteins as a clinically signiﬁcant cardiovascular risk factor for CVD
[123,124]. The non-fasting triglycerides and remnant cholesterol are
mainly associated to CMs synthesized in the intestine and the liver-
derived VLDLs and also their corresponding remnant lipoproteins. As
commented above, both VLDL and CMs may compete for LPL—being
the CMs' preferred substrate—thereby prolonging the blood residence
time of VLDL particles . For this reason, although 80% of the triglyc-
eride increase after a fat-load meal is associated to CMs , the main
increase in particle number is due to an increase in VLDL particles [125,
127]. However, CMs and large VLDLs are too large to penetrate into the
arterialintima, and only their remnants penetrate and, eventually, accu-
mulate in the atherosclerotic lesions . This is in line with the fact
that hyperchylomicronemia syndromes display increased risk of pan-
creatitis but in most cases no increase in cardiovascular disease .
It should also be noted that postprandial hyperlipidemia may promote
atherosclerosis indirectly by increasing the small, dense LDL and reduc-
ing HDL .
Fasting serum apoB-48 levels were signiﬁcantly higher in coronary
disease patients compared with those without [129,130]. Importantly,
apoB-48 is markedly increased in patients with early atherosclerosis
having a new onset of lesion progression . Consistent with these
ﬁndings, apoB-48 levels under fasting conditions have also been signiﬁ-
cantly correlated with carotid intima-media thickness in normolipidemic
subjects . Furthermore, both type 1 and type 2 diabetic patients ex-
hibited a greater total serum apoB-48 compared with that of controls
[132–135], and the severity of angiographically coronary artery disease
correlated with the postprandial response of apoB-48 in type 2 diabetic
patients . Importantly, this alteration was prevented in type 2 dia-
betic patients by improving their metabolic control . However, in-
ternationally standardized assays and normal reference ranges are
needed for extending the use of apoB-48 determinations to clinical
The potential atherogenic effects of remnant CMs have also been
demonstrated in vivo. Therefore, early studies with rabbits demonstrat-
ed that carotid-perfused remnant CMs and LDL were retained within
the subendothelial space . The retention of cholesterol was partic-
ularly high within the intimaof heritable hyperlipidemic Watanabe rab-
bits, particularly that of apoB-48-containing lipoproteins .This
ﬁnding, together with the presence of triglyceride-containing remnant
lipoproteins in human atherosclerotic plaques [138–140],stronglyindi-
cate that remnant CMs may promote the initiation and progression of
lesion development by itself. The mechanism whereby the CM particles
may promote atherosclerosis has also been extensively investigated
in vitro. Indeed, remnant CMs contribute to macrophage foam cell for-
mation after being taken up by multiple receptor-mediated routes
[141–143]. Remnant CMs may also promote an increase in the inﬂam-
matory state of monocytes, thereby enhancing their susceptibility to en-
dothelium adhesion and invasion of the artery wall (reviewed in ).
Finally, some evidence suggests that remnant CMs may promote rodent
vascular smooth muscle cell proliferation [145,146], but the importance
of these changes in human cells is unknown.
In summary, the current consensus view supports the notion that
CM remnant particles are critical cardiovascular risk factors. Strong evi-
dence indicates that remnant CMs are proatherogenic lipoproteins,
most likely by promoting macrophage foam cell formation and mono-
cyte dysfunction. It is also recognized that these actions are likely
exacerbated in some metabolic diseases, such as type 1 and 2 diabetes.
However, the potential therapeutic effects of remnant lipoproteins-
lowering measures on cardiovascular outcomes remain largely un-
known, and further studies are needed in this ﬁeld.
3.4. Regulation of intestinal chylomicron production in type 2 diabetes
The most common causes of elevated cholesterol and triglycerides in
remnant lipoproteins are obesity and poorly controlled diabetes
mellitus . The increased secretion of liver-derived VLDL in insulin
resistance states has been extensively investigated, and this explains,
at least in part, the dyslipidemic proﬁle in type 2 diabetic patients
. However, other studies have also shown an abnormal metabolism
of CMs in type 2 diabetic patients . More importantly, insulin-
resistance is strongly associated with intestinal-derived apoB-48 pro-
duction rate in insulin-resistant subjects and type 2 diabetic patients
[132,147]. It should be noted that NPC1L1 and MTTP expression were
found to be increased in duodenal biopsies of type 2 diabetic patients,
whereas ABCG5 and ABCG8 were downregulated —thereby indi-
cating that diabetic patients could have increased intestinal cholesterol
absorption and increased levels of enterocyte cholesterol for CM assem-
bly. These ﬁndings are consistent with the positive effects of ezetimibe,
the main drug targeting intestinal NPC1L1, on postprandial apoB-48
levels in dyslipidemic type 2 diabetic patients . Furthermore, rem-
nant CMs were highly retained in arteries from diabetic rats, and this ef-
fect was reversed by ezetimibe . As described in Section 2.1.5,
Sar1b plays a pivotal role in the routing of apoB-48-containing CMs
from the endoplasmic reticulum to the Golgi apparatus. Sar1b expres-
sion has been recently associated to obesity and insulin-resistance in
mice fed a high-fat diet by promoting CM production  (Fig. 1).
One of the main incretins in humans is the glucagon-like peptide-1
(GLP-1), which is produced by intestinal enteroendocrine K-cells and L-
cells located in the intestinal mucosa next to the enterocytes .The
ability of GLP-1 to prevent postprandial triglyceride increase has been
demonstrated in healthy humans given GLP-1 infusions . Further-
more, fat intake is a strong stimulus of GLP-1 secretion, whereas type 2
diabetic patients displayed a reduced and delayed postprandial response
. The importance GLP-1 receptor stimulation for normalizing post-
prandial CM levels has been supported by the positive effects of the
GLP-1 receptor agonist exenatide on intestinal CM secretion in healthy
subjects . Exenatide also reduced postprandial apoB-48 levels in
type 2 diabetic patients . The mechanisms whereby GLP-1 prevents
CM production remain poorly understood. However, the positive thera-
peutic effects may involve a GLP-1-mediated increase in insulin secretion
and impaired gastric emptying or a direct effect of the GLP-1 on
enterocytes . Importantly, insulin inhibits apoC-III expression,
which may overcome the unrestrained apoC-III production in the
insulin-resistance state , thereby enhancing the lipoprotein lipase-
mediated lipolysis of CMs and promoting their liver uptake and clearance.
Recent evidence also supports the importance of GLP-1 neural accessibil-
ity for controlling intestinal CM production, since the intracerebroven-
tricular injection of the GLP-1 receptor agonist exendin 4 prevented
apoB-48 accumulation in hamsters via central melanocortin-4 receptors
4. Laboratory tests for CM analysis
The potentially fatal risk of acute pancreatitis associated with
hyperchylomicronemia makes their diagnosis and treatment very im-
portant. Secondary causes of chylomicronemia include hypothyroidism,
unhealthy diet, excessive ethanol use, renal disease, HIV infection,
Cushing's syndrome, sarcoidosis andmieloma as well as differenttreat-
ments such as antiretroviral therapy, estrogens, tamoxifen, 13-cis-
retinoic acid, and some antihypertensive or antipsychotic drugs .
143J. Julve et al. / Clinica Chimica Acta 455 (2016) 134–148
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4.1. Biochemical testing
A laboratory diagnosis of hyperchylomicronemia is assumed when
triglyceride levels exceed 1000 mg/dL. Ten to twelve hours of fast is usu-
ally recommended for the optimal assessment of serum triglyceride
The method to directly demonstrate and quantitate hyperchylo-
micronemia requires the isolation of CMs by ultracentrifugation, typi-
cally at 10,000 g, for 30 min in saline solution at a density of
1.006 g/mL. After this time period, CMs' ﬂoat can be isolated and its
composition measured. This allows for the determination of the amount
of triglycerides or cholesterol associated with the CMs. This method of
isolating chylomicrons is also useful for measuring (in the serum with-
out chylomicrons) other clinical chemistry components that can be in-
terfered with by hyperchylomicronemia.
When hyperchylomicronemia is suspected in a serumsample, a clas-
sical way of conﬁrming its existence is to keep the sample overnight at
4 °C and then look for CMs as they will ﬂoat due to lower density with
respect to the whole serum. As most measurements in clinical chemis-
try testing are performed in fasted patients, the presence of CMs may
be unexpected; in these cases, hyperchylomicronemic patients should
ﬁrst be asked about their period of fasting prior to blood sampling.
Only serious impairments of chylomicron metabolism will be de-
tected by traditional fasting testing, and this has been the basis for pro-
posing studies during a postprandial period. However, and contrary to
what was thought for many years, the VLDL remnants rather than the
CM remnantsare the major componentof the TRLs during the postpran-
dial state. This should be keptin mind when using parameters that
are informative of the postprandial state such as non-fasting triglycer-
ides or calculated, non-fasting remnant cholesterol . At least two
methods to quantitate remnant-like cholesterol and triglycerides are
available. One is based on the antibody-speciﬁc separation of lipopro-
teins containing apoB and A-I coupled to Sepharose 4B. All apoB-48-
and apoB-100-containing lipoproteins are recovered in the remnant
fraction, where cholesterol and triglycerides can be measured .
The other system uses a speciﬁc detergent to modify CMs and VLDL
remnants to allow cholesterol determination in these particles .Al-
though remnant-like particles have been reported to predict cardiovas-
cular events in patients with coronary artery disease , available
data based on these two methods is limited .
Oral fat tolerance tests have also been used to follow postprandial
lipemia, measuring mainly triglycerides as a marker. However, the
lack of a standardized meal, sampling times, and reference ranges has
limited the extension of these tests in a clinical setting .
The determination of the main protein marker of chylomicrons,
apoB-48, is possible by electrophoresis of isolated TRL , especially
if followed by the Western blot. However, this methodology is imprac-
tical for clinical laboratory use, and enzyme-linked assays have been de-
veloped. The low concentration of apoB-48 with respect to that of apoB-
100 and the lack of internationally recognized standardized assays have
prevented stronger clinical evaluation of the assays .
4.2. Genetic testing
Genetic testing may be used to conﬁrm clinical diagnosis, establish
the carriers within families, and carry out genetic counseling for mono-
genic diseases affecting chylomicrons. Genetic testing requires previous
in-depth clinical, familial, and biochemical data to allow a diagnostic hy-
pothesis. This will be the basis for selectingwhich gene or genes will be
studied and in which sequential order and to predict how many patho-
genic mutations will be expected depending on whether a recessive ora
codominant or a dominant disorder is foreseen (Table 1). In most cases,
the method used will be the PCR of exons and exon–intron boundaries
and Sanger sequencing. It is possible, and even likely, that in the near fu-
ture some assays could result from simultaneous gene testing using
next-generation sequencing methods. Gross deletion/insertions are in-
frequent in monogenic disorders affecting chylomicrons, but methods
to detect them should be considered if point mutations are either unde-
tected or insufﬁcient to explain the phenotype.
5. Treatment of chylomicron disorders
Hyperchylomicronemia is present mainly in type I and type V hyper-
lipidemia. While the former is usually present in children and is charac-
terized by a decrease in VLDL, the latter is usually seen in adults and is
also characterized by a concomitant increase in VLDL. In both cases,
the main risk is that of acute pancreatitis, which is especially high
with serum triglycerides N1000 mg/dL.
Agents known to increase endogenous triglyceride concentration
are to be avoided, including alcohol, oral estrogens, diuretics, isotreti-
noin, glucocorticoids, sertraline, and beta-adrenergic blocking agents.
5.1.1. Dietary treatment of hyperchylomicronemia
Treatment for type I hyperlipidemia includes chronic dietary fat re-
striction (fat b15% of total energy intake or 20 g/day) together with
medium-chain fatty acids that are thought to arrive directly to the
Genetic diagnosis of monogenic diseases affecting chylomicrons.
Disease OMIM Gene or genes
Inheritance Pathogenic Other lipoproteins affected
by the disease
Familial hyperchylomicronemia HLP1A, #238600 LPL
HLP1B, #207750 APOC2
Recessive Homozygous or compound heterozygous No
Abetalipoproteinemia ABL, #200100 MTTP Recessive Homozygous or compound heterozygous Mainly all apoB-containing lipoproteins
Anderson disease or chylomicron
CMRD, #246700 SARA2 Recessive Homozygous or compound heterozygous Mainly all apoB-containing lipoproteins
Familial hypobetalipoproteinemia FHBL1 #615558 APOB Codominant Homozygous and heterozygous forms All apoB-containing lipoproteins
Familial combined hypolipidemia FHBL2, #605019 ANGPTL3⁎⁎ Recessive Homozygous or compound heterozygous Mainly all apoB-containing lipoproteins,
but HDL cholesterol may be decreased
⁎In monogenic disorders only one gene containsthe mutations; if mutations in more than one gene are the potential cause of the disease, those are usually studied depending on the
frequency (where the ﬁrst studied is the one whose mutations are the most frequent cause of the disease).
⁎⁎ Mutations in these genes also directly affect HDL metabolism.
144 J. Julve etal. / Clinica Chimica Acta 455(2016) 134–148
Author's personal copy
liver independently of chylomicron transport. Fish oil supplements are
contraindicated in familial deﬁciency of lipoprotein lipase because
they contribute to CM levels .
Treatment for type V hyperlipidemia depends on the level of triglyc-
erides. When fasting triglycerides N500 mg/dL patient need to be sub-
jected to therapeutic lifestyle change and be considered for drug
treatment . In hyperchylomicronemic patients with fasting triglyc-
erides N500 mg/dL and abdominal pain, or fasting triglycerides
N1000 mg/dL, the patients need to be hospitalized without receiving al-
iments by mouth and be hydrated with intravenously administered
ﬂuids to decrease serum triglycerides. Low-doses of insulin should
also be administered (with glucose in non diabetic patients to prevent
5.1.2. Drug treatment of hyperchylomicronemia
Drug options for hyperchylomicronemia are the same as those for
hypertriglyceridemia treatment, even though maximal doses or drug
combination could be tried.
Omega-3 fatty acids, at pharmaceutical doses (3–4g/day)de-
crease serum triglycerides by about 30% and, therefore, have been
used in hypertriglyceridemia . The mechanism of action in-
cludes the suppression of adipose tissue inﬂammation, which is
thought to increase insulin sensitivity and decrease the lipolysis me-
diated by the lipase-sensitive hormone. Also, omega-3 fatty acids in-
crease lipoprotein lipase activity and fatty acid oxidation in adipose
tissue, skeletal muscle, and the heart, thus reducing fatty acid arrival
to the liver, which is a major input for triglyceride and VLDL synthe-
sis and secretion. If omega-3 fatty acids are insufﬁcient for control-
ling hypertriglyceridemia, hypotriglyceridemic drugs can be added
to the treatment. Fibrates and nicotinic acid are medications
approved to treat hypertriglyceridemia in the context of a type V
hyperlipidemia . Fibrates may decrease serum triglyceride up
tion. Their use is contraindicated in those with hepatic or renal
dysfunction. Major side effects include lithogenicity and potentia-
tion of the effects of oral anticoagulants. Nicotinic acid can decrease
triglycerides by up to 35%, but it is not used as a primary agent for
chylomicronemic patients. Chylomicronemia in the setting of diabe-
tes can be improved by adequate insulinization.
There is poor or absent response to drugs in patients with type I
5.1.3. Gene therapy of hyperchylomicronemia
The ﬁrst gene therapy treatment was recently approved by the
European Union for the treatment of lipoprotein lipase deﬁciency in
cases of severe or multiple acute pancreatitis despite diet restriction,
under the name of alipogene tiparvovec (Glybera®). It is based on a
human LPL gene-replacement approach that uses adenovirus as a carri-
er of the human gene . Thus, this treatment requires previous ge-
netic diagnosis that proves the existence of mutations that inactive
Glybera® is administered once by multiple intramuscular injec-
tions, and, although triglyceride levels returned to pre-treatment
levels within 16–26 weeks after administration, a 6-year follow-up
demonstrated a clinically relevant reduction in the incidence of doc-
umented pancreatitis and acute abdominal pain events consistent
with pancreatitis .
A different approach was followed recently in three patients with li-
poprotein lipase deﬁciency and recurrent pancreatitis who were treated
with an inhibitor of APOC3 mRNA (ISIS 304801), which was injected sub-
cutaneously once weekly for 13 weeks. Serum triglycerides were reduced
up to 86%, and all patients had triglycerides b500 mg/dL —an ap-
proach that could also be of interest in other patients with hypertriglyc-
5.2.1. Dietary treatment of hypochilomicronemia
Monogenic diseases altering CM synthesis or secretion have in
common lipid malabsorption, which can lead to liposoluble vitamin
deﬁciency by insufﬁcient arrival to the tissues and steatorrhea,
reﬂecting intestinal lipid malabsorption . It is thus important to
consider decreases in dietary fat intake, to supplement with medium-
change fatty acids, and to add complements of liposoluble vitamins to
prevent complications—especially neurological ones, as these may be
The authors declare that there is no duality of interest associated
with this manuscript.
Conﬂicts of interest
All authors have read the journal's policy on disclosure of potential
conﬂicts of interest. The authors declare that there is no duality of inter-
est associated with this article.
This work was supported by Ministerio de Sanidad y Consumo,
Instituto de Salud Carlos III, and FEDER “Una manera de hacer Europa”
(grant numbers: CP13-00070 to JJ; PI1401648 to FBV and JMMC; PI12-
0291 to JC-E). JJ is the recipient of a Miguel Servet Type 1 contract
(CP13-00070). CIBER de Diabetes y Enfermedades Metabólicas Asociadas
(CIBERDEM) is a project of the Instituto de Salud Carlos III.
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