[Show abstract][Hide abstract] ABSTRACT: Determination of lipoprotein lipase (LPL) activity is important for hyperchylomicronemia diagnosis, but remains both unreliable and cumbersome with current methods. Consequently by using human VLDL as substrate we developed a new LPL assay which does not require sonication, radioactive or fluorescent particles.
Post-heparin plasma was added to the VLDL substrate prepared by ultracentrifugation of heat inactivated normolipidemic human serums, diluted in buffer, pH 8.15. Following incubation at 37°c, the NEFA (non esterified fatty acids) produced were assayed hourly for 4 hours. LPL activity was expressed as µmol/l/min after subtraction of hepatic lipase (HL) activity, obtained following LPL inhibition with NaCl 1.5 mmol/l. Molecular analysis of LPL, GPIHBP1, APOA5, APOC2, APOE genes was available for 62 patients.
Our method was reproducible (coefficient of variation (CV): intra-assay 5.6%, inter-assay 7.1%), and tightly correlated with the conventional radiolabelled triolein emulsion method (n = 26, r = 0.88). Normal values were established at 34.8±12.8 µmol/l/min (mean±SD) from 20 control subjects. LPL activities obtained from 71 patients with documented history of major hypertriglyceridemia showed a trimodal distribution. Among the 11 patients with a very low LPL activity (<10 µmol/l/min), 5 were homozygous or compound heterozygous for LPL or GPIHBP1 deleterious mutations, 3 were compound heterozygous for APOA5 deleterious mutations and the p.S19W APOA5 susceptibility variant, and 2 were free of any mutations in the usual candidate genes. No homozygous gene alteration in LPL, GPIHBP1 and APOC2 genes was found in any of the patients with LPL activity >10 µmol/l/min.
This new reproducible method is a valuable tool for routine diagnosis and reliably identifies LPL activity defects.
[Show abstract][Hide abstract] ABSTRACT: GPIHBP1 is a new endothelial binding site for lipoprotein lipase (LPL), the key enzyme for intravascular lipolysis of triglyceride-rich lipoproteins (TGRL). We have identified two new missense mutations of the GPIHBP1 gene, C89F and G175R, by systematic sequencing in a cohort of 376 hyperchylomicronemic patients without mutations on the LPL, APOC2, or APOA5 gene.
Phenotypic expression and functional consequences of these two mutations were studied.
We performed clinical and genotypic studies of probands and their families. GPIHBP1 functional alterations were studied in CHO pgsA-745 transfected cells.
Probands are an adult with a homozygous G175R mutation and a child with a hemizygous C89F neomutation and a deletion of the second allele. C89F mutation was associated with a C14F signal peptide polymorphism on the same haplotype. Both patients had resistant hyperchylomicronemia, low LPL activity, and history of acute pancreatitis. In CHO pgsA-745 cells, both G175R and C14F variants reduce the expression of GPIHBP1 at the cell surface. C89F mutation is responsible for a drastic LPL-binding defect to GPIHBP1. C14F may further potentiate C89F effect.
The emergence of hyperchylomicronemia in the generation after a neomutation further establishes a critical role for GPIHBP1 in TGRL physiopathology in humans. Our results highlight the crucial role of C65-C89 disulfide bond in LPL binding by GPIHBP1 Ly6 domain. Furthermore, we first report a mutation of the hydrophobic C-terminal domain that impairs GPIHBP1 membrane targeting.
Full-text · Article · Aug 2011 · The Journal of Clinical Endocrinology and Metabolism
[Show abstract][Hide abstract] ABSTRACT: To provide phenotypic and functional data in new patients with APOA5 mutations and to identify genetic and metabolic factors influencing their phenotypic expression.
By sequencing APOA5 gene in a cohort of 286 hyperchylomicronemic subjects, free of LPL or APOC2 mutations, we identified 4 unrelated carriers of the Q97X mutation (3 heterozygotes and 1 homozygote) and one heterozygote with a new L242P mutation. Postheparin LPL activity level was reduced by about 50% in Q97X heterozygotes and more than 90% in the Q97X homozygote, but was normal in the L242P patient after resolution of hyperchylomicronemia. Plasma apoAV was undetectable in the Q97X homozygote and in the normal range in the L242P and Q97X heterozygous carriers. In Western blot studies, the association of apoAV with plasma lipoproteins was altered in Q97X heterozygous carriers but not in the L242P carrier. Hyperchylomicronemic heterozygotes for both mutations carried an additional APOA5 variant haplotype and/or APOE variant (E2 or E4). Type 2 diabetes or metabolic syndrome were not a major phenotypic determinant.
The L242P mutation was present in a hyperchylomicronemic proband but its causal involvement remains to be established. The Q97X mutation was clearly involved in hyperchylomicronemia with evidence of concomitant altered intravascular lipolysis, and a complete apoAV deficiency in the homozygote. The phenotypic expression variability of APOA5 mutations was mostly influenced by compound heterozygosity with APOA5 variant haplotypes plus additional genetic factors, and in a lesser extent by the metabolic environment.
[Show abstract][Hide abstract] ABSTRACT: Two minor apolipoprotein A5 (APOA5) gene haplotypes, represented by -1131T>C and S19W polymorphisms, are strong determinants of plasma triglyceride (TG) concentration variability across human populations. Hypertriglyceridemia is frequent in type 2 diabetes (T2D) and hyperchylomicronemia is not uncommon.
We investigated the association of -1131T>C and S19W polymorphisms with diabetic dyslipidemia in 400 Caucasian T2D patients divided in 2 groups: group N with 130 normotriglyceridemics (TG<90th percentile) and group M with 270 moderately hypertriglyceridemics. A third group of 51 diabetic patients (group H) with history of hyperchylomicronemia (TG>15 mM) was also studied.
The -1131C allele was more frequent in both mild and severe hypertriglyceridemia (20.6% vs 9.8% vs 5.0%, group H vs M vs N, p<0.001). The 19W allele was more frequent only in patients with hyperchylomicronemia (14.0% vs 6.5% vs 6.1%, group H vs M vs N, p=0.001). In group N+M, the -1131C allele was associated with higher TG (+13%, p=0.034) and lower HDLc (-10%, p=0.004). The 19W allele was only associated with lower HDLc (-9%, p=0.022).
These results suggest that in T2D APOA5 polymorphisms contribute to modulate dyslipidemia. Both -1131T>C and S19W polymorphisms are associated with hyperchylomicronemia and only -1131T>C polymorphism with mild hypertriglyceridemia.
Full-text · Article · Aug 2008 · Clinica Chimica Acta
[Show abstract][Hide abstract] ABSTRACT: While type 1 hyperlipidemia is associated with lipoprotein lipase or apoCII deficiencies, the etiology of type 5 hyperlipidemia remains largely unknown. We explored a new candidate gene, APOA5, for possible causative mutations in a pedigree of late-onset, vertically transmitted hyperchylomicronemia. A heterozygous Q139X mutation in APOA5 was present in both the proband and his affected son but was absent in 200 controls. It was subsequently found in 2 of 140 cases of hyperchylomicronemia. Haplotype analysis suggested the new Q139X as a founder mutation. Family studies showed that 5 of 9 total Q139X carriers had hyperchylomicronemia, 1 patient being homozygote. Severe hypertriglyceridemia in 8 heterozygotes was strictly associated with the presence on the second allele of 1 of 2 previously described triglyceride-raising minor APOA5 haplotypes. Furthermore, ultracentrifugation fraction analysis indicated in carriers an altered association of Apoa5 truncated and WT proteins to lipoproteins, whereas in normal plasma, Apoa5 associated with VLDL and HDL/LDL fractions. APOB100 kinetic studies in 3 severely dyslipidemic patients with Q139X revealed a major impairment of VLDL catabolism. Lipoprotein lipase activity and mass were dramatically reduced in dyslipidemic carriers, leading to severe lipolysis defect. Our observations strongly support in humans a role for APOA5 in lipolysis regulation and in familial hyperchylomicronemia.
Full-text · Article · Nov 2005 · Journal of Clinical Investigation
[Show abstract][Hide abstract] ABSTRACT: Hypertriglyceridaemia is common in Type II (non-insulin-dependent) diabetes mellitus. Only subgroups of patient however have type V hyperlipidaemia. To investigate the coordination between genetic factors in the modulation of hypertriglyceridaemia in Type II diabetes, we studied three major modifier loci: apoC-III (both Sst-I and insulin-responsive element polymorphisms), apolipoprotein E genotypes and lipoprotein-lipase mutations.
We studied apoCIII gene polymorphisms, apolipoprotein E genotypes and lipoprotein-lipase gene mutations in 176 patients with Type II (non-insulin-dependent) diabetes mellitus, either normolipaemic (group N, n = 116), mildly hypertriglyceridaemic (group T, n = 28) or with a history of severe hypertriglyceridaemia (triglyceride > 15 g/l) (group H, n = 32).
Mild hypertriglyceridaemia in Type II diabetes did not associate with any gene variants in this study. Severe hypertriglyceridaemia was, however, associated with the presence of the apoC-III S2 allele (50% of the patients in group H compared with 15.5 % in group N, p < 0.0001). Additionally this particular phenotype was associated with a low prevalence of the apo E3 allele (35.9% in group H vs 18.1 % in group N, p < 0.005) and a statistically significant over-representation of the E2E4 genotypes. Inactivating lipoprotein-lipase mutations were found in four patients (three heterozygotes, one homozygote), none was found in group N or T. Thus 68.7 % of group H patients (22/32) (vs 21.4 % in group T, p < 0.0005) were carriers of either S2 allele, lipoprotein-lipase mutants or E2E4 genotype with most lipoprotein-lipase mutants or E2E4 genotypes or both in the non-carriers for the S2 allele (6/7).
Our results strongly support the hypothesis that severe hyperlipaemia in Type II diabetes crucially depends on genetic factors which impair the clearance of triglyceride-rich lipoproteins.