Article

Characterization and Heritability of Obesity and Associated Risk Factors in Vervet Monkeys*

Wake Forest University School of Medicine, Medical Center Blvd., Winston-Salem, NC 27157, USA.
Obesity (Impact Factor: 3.73). 08/2007; 15(7):1666-74. DOI: 10.1038/oby.2007.199
Source: PubMed
ABSTRACT
The objective was to determine the prevalence and heritability of obesity and risk factors associated with metabolic syndrome (MS) in a pedigreed colony of vervet monkeys.
A cross-sectional study of plasma lipid and lipoprotein concentrations, glycemic indices, and morphometric measures with heritability calculated from pedigree analysis. A selected population of females was additionally assessed for insulin sensitivity and glucose tolerance.
All mature male (n=98), pregnant (n=40) and non-pregnant female (n=157) vervet monkeys were included in the study. Seven non-pregnant females were selected on the basis of high or average glycated hemoglobin (GHb) for further characterization of carbohydrate metabolism.
Morphometric measurements included body weight, length, waist circumference, and calculated BMI. Plasma lipids [total cholesterol (TC), triglycerides (TG), high-density lipoprotein cholesterol (HDL-C)] and glycemic measures (fasting blood glucose, insulin, and GHb) were measured. A homeostasis model assessment index was further reported. Glucose tolerance testing and hyperinsulinemic-euglycemic clamps were performed on 7 selected females.
Vervet monkeys demonstrate obesity, insulin resistance, and associated changes in plasma lipids even while consuming a low-fat (chow) diet. Furthermore, these parameters are heritable. Females are at particular risk for central obesity and an unfavorable lipid profile (higher TG, TC, and no estrogen-related increase in HDL-C). Selection of females by elevated GHb indicated impaired glucose tolerance and was associated with central obesity. This colony provides a unique opportunity to study the development of obesity-related disorders, including both genetic and environmental influences, across all life stages.

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Animal Physiology
Characterization and Heritability of Obesity
and Associated Risk Factors in Vervet Monkeys
Kylie Kavanagh,* Lynn A. Fairbanks,† Julia N. Bailey,† Matthew J. Jorgensen,† Martha Wilson,* Li Zhang,*
Lawrence L. Rudel,* and Janice D. Wagner*
Abstract
KAVANAGH, KYLIE, LYNN A. FAIRBANKS, JULIA
N. BAILEY, MATTHEW J. JORGENSEN, MARTHA
WILSON, LI ZHANG, LAWRENCE L. RUDEL, AND
JANICE D. WAGNER. Characterization and heritability of
obesity and associated risk factors in vervet monkeys.
Obesity. 2007;15:1666 –1674.
Objective: The objective was to determine the prevalence
and heritability of obesity and risk factors associated with
metabolic syndrome (MS) in a pedigreed colony of vervet
monkeys.
Design: A cross-sectional study of plasma lipid and lipopro-
tein concentrations, glycemic indices, and morphometric
measures with heritability calculated from pedigree analy-
sis. A selected population of females was additionally as-
sessed for insulin sensitivity and glucose tolerance.
Subjects: All mature male (n 98), pregnant (n 40) and
non-pregnant female (n 157) vervet monkeys were in-
cluded in the study. Seven non-pregnant females were se-
lected on the basis of high or average glycated hemoglobin
(GHb) for further characterization of carbohydrate metabo-
lism.
Measurements: Morphometric measurements included
body weight, length, waist circumference, and calculated
BMI. Plasma lipids [total cholesterol (TC), triglycerides
(TG), high-density lipoprotein cholesterol (HDL-C)] and
glycemic measures (fasting blood glucose, insulin, and
GHb) were measured. A homeostasis model assessment
index was further reported. Glucose tolerance testing and
hyperinsulinemic-euglycemic clamps were performed on 7
selected females.
Conclusion: Vervet monkeys demonstrate obesity, insulin
resistance, and associated changes in plasma lipids even
while consuming a low-fat (chow) diet. Furthermore, these
parameters are heritable. Females are at particular risk for
central obesity and an unfavorable lipid profile (higher TG,
TC, and no estrogen-related increase in HDL-C). Selection
of females by elevated GHb indicated impaired glucose
tolerance and was associated with central obesity. This
colony provides a unique opportunity to study the develop-
ment of obesity-related disorders, including both genetic
and environmental influences, across all life stages.
Key words: lipids, animal models, insulin resistance,
metabolic syndrome
Introduction
The prevalence of obesity is currently estimated at 20%
of the adult population. Overweight individuals add another
40%, such that two thirds of the adult population are at risk
of developing obesity-related disorders such as metabolic
syndrome (MS)
1
(1). MS, defined as a cluster of cardiovas-
cular disease (CVD) risk factors and insulin resistance (IR),
has been estimated to affect over 20% of the adult popula-
tion in the United States (2,3). MS comprises the combina-
tion of abdominal obesity, dyslipidemia, hypertension, and
impaired glucose tolerance (IGT), 3 of which must be
present to confirm the diagnosis (4). There is considerable
interest from both the public health sector and medical
economists as to the best predictors for MS stemming from
the fact that CVD is the number one cause of death and
hospitalization in the United States (5,6). There is abundant
evidence that IR and IGT precede diabetes mellitus (DM)
(7,8), and both IR and hyperglycemia are risk factors for
CVD in humans.
Received for review July 12, 2006.
Accepted in final form December 16, 2006.
The costs of publication of this article were defrayed, in part, by the payment of page
charges. This article must, therefore, be hereby marked “advertisement” in accordance with
18 U.S.C. Section 1734 solely to indicate this fact.
*Wake Forest University School of Medicine, Winston-Salem, North Carolina; and †Uni-
versity of California–Los Angeles Semel Institute for Neuroscience and Human Behavior,
Los Angeles, California.
Address correspondence to Kylie Kavanagh, Wake Forest University School of Medicine,
Medical Center Blvd., Winston-Salem, NC 27157.
E-mail: kkavanag@wfubmc.edu
Copyright © 2007 NAASO
1
Nonstandard abbreviations: MS, metabolic syndrome; CVD, cardiovascular disease; IR,
insulin resistance; IGT, impaired glucose tolerance; DM, diabetes mellitus; GHb, glycated
hemoglobin; HOMA, homeostasis model assessment; TC, total cholesterol; TG, triglyceride;
HDL-C, high-density lipoprotein cholesterol; IFG, impaired fasting glucose; h
2
, heritability.
1666 OBESITY Vol. 15 No. 7 July 2007
Page 1
Obesity prevalence increases with age (9,10), which is
presumed to reflect combinations of aging, consumption of
a westernized diet, and sedentary lifestyle. Genetic differ-
ences among individuals make some people more likely to
store excess fat in times of food abundance. Genetic sus-
ceptibilities for DM and obesity have been identified (11),
with a number of candidate genes characterized. However
as both the overweight and IGT phenotypes are multifacto-
rial, their associations and interactions with each other and
the environment become difficult etiologies to assess. Non-
human primates mirror these same changes in body com-
position and metabolic disturbances as they age or become
pregnant (9,12) and are prone to develop atherosclerosis
(13,14), which is augmented by DM (15). The purpose of
this study was to evaluate a closed population of vervet
monkeys (Chlorocebus aethiops) for the prevalence and
heritability of obesity and other parameters associated with
MS under relatively controlled circumstances that include a
uniform diet and equal opportunities for exercise. By as-
sessing heritability, we further validate this primate species
for investigations of MS pathophysiology, with the ability
to identify familial subsets at risk for obesity and investigate
genetic differences and effects of intervention across these
subsets and across their life stages.
Research Methods and Procedures
Subjects
The study population was a multigenerational pedigreed
colony (University of California-Los Angeles Vervet Re-
search Colony), of which all mature monkeys (age 4to22
years; n 295), including 98 males, 157 non-pregnant, and
40 pregnant female vervet monkeys were evaluated. Preg-
nancy was confirmed by parturition, and stage of pregnancy
at examination calculated back from actual parturition
dates. In captivity, vervet females reach puberty at 2.5 years
of age and achieve full adult size by the age of 4. Male
vervets reach puberty at 3 years of age and complete growth
by 5 years. The age range of the study population extends
from young adulthood (4 to 5 years) to old age (18 years).
Most females are kept in the colony for life, while the
majority of the males are culled by the age of 10 years.
Aged females demonstrate reproductive senescence; how-
ever, a true menopause has yet to be documented. All
subjects were descendants of 57 original founders imported
from St. Kitts, West Indies.
Animals were housed in outdoor corrals varying from
380 to 1482 square feet with elevated perches, platforms,
and climbing structures, and were fed commercial primate
laboratory chow (Laboratory Diet 5038; Purina, St. Louis,
MO) supplemented with fresh fruits and vegetables. All
animals had ad libitum access to food and opportunities to
exercise. Study procedures had been approved by Univer-
sity of California-Los Angeles, Veteran’s Administration,
and Wake Forest University Institutional Animal Care and
Use committees.
Sampling Methods
Monkeys were fasted overnight, before sedation with
intramuscular ketamine (8 to 10 mg/kg) to facilitate the
collection of blood samples and morphometric measure-
ments. Each monkey was weighed. A flexible tape measure
was placed around the monkey’s abdomen at the level of the
umbilicus to measure waist circumference. The distance
from the crown to the bottom of the pubic bone was re-
corded as length, which is the equivalent to sitting height.
An index of BMI was calculated from the weight (in kg)
divided by length (in meters) squared.
Glucose was measured on capillary blood sourced from a
finger stick (One Touch Ultra Glucometer; Lifescan, Inc.,
Milpitas, CA). Blood samples were collected from the fem-
oral vein and placed on ice until samples were processed.
Plasma and whole blood were stored at 80° C before
shipping to Wake Forest University.
Percent glycation of hemoglobin in whole blood (GHb)
was measured by high-performance liquid chromatography
borate affinity column (Primus PDQ, Kansas City, MO) to
assess long-term glycemic control. Fasting plasma insulin
concentrations were measured by enzyme-linked immu-
nosorbent assay (Mercodia, Uppsala, Sweden) (12). Ho-
meostasis model assessment (HOMA) was calculated from
the product of glucose (mM) and insulin (UI/L)/22.5, and
used as an indicator of IR (16). Plasma total cholesterol
(TC), triglycerides (TG), and cholesterol associated with
high-density lipoprotein (HDL-C) fractions were measured
enzymatically (15).
A small group of older females (15 to 18 years) were
characterized as IGT based on their elevated GHb (n 3)
or characterized as normal (n 4) based on normal GHb
and normal fasting glucose concentrations, as compared
with the study population (n 295). They were further
evaluated by intravenous glucose tolerance testing (17) and
insulin sensitivity assessment by hyperinsulinemic-euglyce-
mic clamp methodology (18). Briefly, glucose tolerance
testing involved the measurement of insulin and glucose
concentrations over an hour before and after a standard
glucose challenge (750 mg/kg). The rate of glucose disap-
pearance was calculated from the slope of the log trans-
formed glucose concentrations between minutes 5 to 20.
Areas under the curve for insulin and glucose were calcu-
lated between times 0 to 60 minutes (17). A hyperinsuline-
mic-euglycemic clamp was performed by peripheral infu-
sion of regular insulin (40 units/m
2
per min) and variable
administration of 20% dextrose solution to maintain glucose
levels at fasting value for 3 hours under sedation with
Obesity and Associated Risk Factors in Monkeys, Kavanagh et al.
OBESITY Vol. 15 No. 7 July 2007 1667
Page 2
ketamine. The M value is calculated from the required
dextrose infusion rate required over the final hour of the
clamp (18).
Data Analysis
Obesity was defined as a waist circumference 40.5 cm
(corresponding to the colony population upper 20th percen-
tile). This percentile cutoff was modeled on where the Adult
Treatment Panel III MS risk waist definition falls for males
and females in normal human populations (4,19,20) and was
rounded to 40.5 cm in both sexes. An elevated GHb was
defined as glycosylation of hemoglobin chain A1c being
5.7%, as has been described as predictive for MS in
related individuals (21) and a HOMA value 6 was used as
predictive for an insulin-resistant state (16).
The data are presented as the mean standard error for
each group. Quartiles of insulin ranges and waist circum-
ferences were based on even distribution of population
numbers. Data were analyzed for normality and logarithmi-
cally transformed where necessary (glucose, HOMA, TG)
before comparisons. Associations were evaluated by Pear-
son’s correlation coefficient excluding the pregnant fe-
males. Differences between groups were analyzed by anal-
ysis of covariance with age as a covariate and post hoc
testing for group and sex differences was performed with
Tukey’s honestly significant difference for multiple com-
parisons on detection of significant group and group by sex
interactions. The non-parametric Mann-Whitney U test was
utilized in comparing the parameters from the small subset
of females selected for further assessment of insulin sensi-
tivity. Statistical analysis was performed using Statistica 6
(StatSoft, Inc., Tulsa, OK), with significance set at
0.05.
Quantitative genetic analyses were performed to deter-
mine genetic contributions to variation in obesity and asso-
ciated traits using Sequential Oligogenic Linkage Analysis
Routines (SOLAR) (22). Variance component methods
were used to model the individual trait values as a function
of the mean trait value, covariates, genetic relatedness, and
unmeasured environmental effects. This is a pedigree-based
approach that accounts for the residual correlations among
relatives estimated through the kinship matrix. Additive
genetic effects are estimated from the covariance among
relatives. Estimation of the mean, variance, covariate, and
genetic effects are obtained simultaneously, using maxi-
mum likelihood methods. Heritability is a measure of the
total proportion of the variance explained by genetic simi-
larity among relatives. Components are tested using likeli-
hood testing, where the likelihood of the data is estimated
with and without the component to determine if the com-
ponent is significant. Traits were adjusted for sex, age, and
pregnancy before analysis. One female discovered to be
diabetic (fasting blood glucose 343) during the screening
was excluded from the genetic analysis.
Results
The baseline characteristics of the entire population are
presented in Table 1. As the population is a breeding col-
Table 1. Descriptive data for colony monkeys including breakdown by sex and pregnancy status
Parameter
Population
(n 295)
Males
(n 98)
Non-pregnant females
(n 157)
Pregnant females
(n 40) p
Age (yrs) 9.67 (0.26) 7.05 (0.26)
a
11.12 (0.38)
b
10.63 (0.62)
b
0.001
GHb (%) 5.48 (0.09) 5.53 (0.13) 5.48 (0.15) 5.36 (0.15) 0.25
Insulin (uIU/mL) 30.1 (1.35) 29.3 (2.01)
a
27.7 (1.72)
a
41.27 (5.18)
b
0.003
Weight (kg) 5.96 (0.075) 7.22 (0.10)
a
5.33 (0.07)
b
5.33 (0.11)
b
0.001
Length (cm) 45.94 (0.19) 49.76 (0.18)
a
43.95 (0.12)
b
43.62 (0.21)
b
0.001
Waist (cm) 37.59 (0.26) 37.14 (0.37) 37.8 (0.39) 38.16 (0.69) 0.09
Glucose (mg/dL) 61.047 (1.44) 62.16 (1.65) 60.31 (2.36) 61.23 (3.38) 0.05
BMI (kg/m
2
)
27.99 (0.21) 29.07 (0.29)
a
27.54 (0.31)
b
27.93 (0.49)
b
0.001
HOMA-IR 4.92 (0.36) 4.71 (0.41) 4.48 (0.55) 7.17 (1.23) 0.07
Triglycerides (mg/dL) 33.5 (1.19) 22.15 (1)
a
38.83 (1.81)
b
40.4 (3.07)
b
0.001
Total cholesterol (mg/dL) 141.24 (1.90) 133.03 (2.2)
a
153.21 (2.62)
b
114.32 (5.15)
c
0.002
HDL cholesterol (mg/dL) 65.89 (0.95) 71.27 (1.21)
a
67.87 (1.13)
a
44.92 (2.77)
b
0.001
GHb, glycated hemoglobin; HOMA-IR, homeostasis model assessment of insulin resistance; HDL, high-density lipoprotein. Comparison
is made between the groups after adjustment for age, with overall analysis of covariance p values shown. Group differences are noted by
different letter superscripts.
Obesity and Associated Risk Factors in Monkeys, Kavanagh et al.
1668 OBESITY Vol. 15 No. 7 July 2007
Page 3
ony, there were more females and those females were older
than the males (Figure 1A). Age was significantly different
between males and females and was significantly correlated
to all lipid parameters, waist size, BMI, glucose, and GHb
across the population. The uncorrected means for each of
these measures are shown in Table 1, but the statistical
analyses were performed with age as a covariate in the
ANOVA model. Despite their smaller size [length and
weight (Figure 1B), and the calculated BMI], waist circum-
ference in non-pregnant or pregnant (only first and second
trimester pregnancies were identified at screening) females
did not differ from males. Relatively high insulin concen-
trations were detected across the entire population as com-
pared with values reported in other primate species and
these values were further increased by pregnancy. The non-
pregnant females had elevated TG and TC, and tended
toward lower HDL-C concentrations, as compared with the
males, and these factors were not a function of age (Figure
2). In fact, when only the young adult population were
evaluated (age range 5 to 10 yrs, p 0.61 for age compar-
ison between sexes), females still had elevated TG concen-
trations by more than 50% (22.2 vs. 34.1 mg/dL, respec-
tively, p 0.001).
Heritability estimates for population characteristics
ranged between 20% to 45% (Table 2). Among the most
highly heritable phenotypes was waist circumference.
Plasma lipids and lipoprotein cholesterol were robustly her-
itable. Measures of carbohydrate parameters had much
weaker contributions of familial associations.
Twenty-five percent of the females and 16% of the males
were classified as abdominally obese, as determined by an
enlarged waist circumference (80th percentile). These
centrally obese animals were significantly hyperinsulinemic
(p 0.0003) and are classified as at-risk for type 2 diabetes
by HOMA scores (p 0.015; Figure 3) (16). Females that
were centrally obese had HOMA scores of nearly double
that of their lean counterparts (7.14 vs. 3.61, respectively),
as compared with the more modest increases seen in the
obese males (5.91 vs. 4.48, respectively). Consistent with
the MS classification in people, the monkeys with high
waist circumferences had 33% higher TG concentrations
(30 mg/dL vs. 40 mg/dL, p 0.03). Overall, triglyceride
concentrations were significantly associated with increasing
intra-abdominal fat as measured by waist circumference
Figure 1: Frequency histogram of (A) age and (B) weight of male and female monkeys included in the population analysis. Open bars
represent females, and black bars represent males.
Figure 2: Plasma lipids and lipoproteins of male and female vervet
monkeys. Females, on average, had significantly higher TC and
TG as compared with males, and did not have higher HDL-C
typically measured in premenopausal females. Open bars represent
females, and black bars represent males. TC, total cholesterol;
HDL-C, high-density lipoprotein cholesterol; TG, triglyceride.
* p 0.01.
Obesity and Associated Risk Factors in Monkeys, Kavanagh et al.
OBESITY Vol. 15 No. 7 July 2007 1669
Page 4
(r 0.31, p 0.001). Further, when waist circumference
was examined by quartiles, the more obese quartiles had
higher TG concentrations. Females had significantly higher
TG concentrations within every waist group (Figure 4).
Waist circumference was significantly and positively cor-
related with insulin concentrations (r 0.34, p 0.01) with
the obese animals having, on average, 40% higher insulin
concentrations (Figure 3A). Waist circumference was also
significantly and positively correlated with IR (HOMA; r
0.34, p 0.001) where the obese animals score for IR was
nearly double that of the normal population (Figure 3B).
Table 2. Heritability estimates (h
2
) for population
characteristics of obesity and associated risk factors
Parameter h
2
(SE)
p
BMI 0.44 (0.15) 0.0001
Waist circumference 0.37 (0.16) 0.02
TC 0.37 (0.13) 0.0006
TG 0.33 (0.13) 0.0001
HDL cholesterol 0.29 (0.13) 0.002
GHb 0.18 (0.10) 0.03
Insulin 0.21 (0.11) 0.02
Glucose 0.07 (0.11) 0.26
HOMA value 0.02 (0.10) 0.36
h
2
, heritability; SE, standard error; TC, total cholesterol; TG,
triglyceride; HDL, high-density lipoprotein; GHb, glycated hemo-
globin; HOMA, homeostasis model assessment.
Figure 3: Monkeys classified as abdominally obese (waist circumference 40.5 cm) were (A) hyperinsulinemic and (B) classified as at-risk
for type 2 diabetes by HOMA scores (6) (16). * p 0.05.
Figure 4: Waist circumference, when broken into quartiles, was
associated with increasing triglyceride concentrations. The mean
of each quartile is indicated by the horizontal line, and post hoc
significance between the mean of each quartile is denoted by
different letters (p 0.05). Females were at particular risk for
central obesity, and an unfavorable lipid profile as a significant
sex-by-waist quartile interaction indicated that females had higher
triglyceride concentrations than males at every waist group after
adjustment for age. Open circles represent females, and filled
circles represent males.
Obesity and Associated Risk Factors in Monkeys, Kavanagh et al.
1670 OBESITY Vol. 15 No. 7 July 2007
Page 5
Knowing waist size and insulin concentrations increase
together, insulin was then divided into quartiles and evalu-
ated for its relationship with glucose. Animals classified as
diabetic (n 5; fasting glucose 126 mg/dL) or with
impaired fasting glucose (IFG; n 4; fasting glucose 100
mg/dL) based on human criteria were present in all but the
lowest quartile of insulin (Figure 5). However, a significant
increase in mean glucose concentration was only seen in the
highest quartile of insulin (ANOVA post hoc testing p
0.01) and correlation between glucose and waist circumfer-
ence was modest but significant (r 0.17, p 0.004).
Diabetic animals all had enlarged waist circumferences (di-
abetic monkey average was 44 cm as compared with the
population average of 37.6 cm) and 40% higher TG
concentrations (mean of 47.3 mg/dL vs. 33.5 mg/dL for the
entire population). Together, these data demonstrate that
increasing abdominal obesity is associated with IR, hyper-
insulinemia, and glucose.
This proposed pathogenesis was born out when older
females selected for hyperglycemia (high GHb) were com-
pared with age-matched normal controls (Table 3). Despite
slightly higher fasting insulin concentrations, lack of insulin
response after the intravenous glucose challenge confirmed
abnormal pancreatic function (Figure 6). These animals had
high fasting glucose concentrations and were glucose intol-
erant based on delayed glucose clearance and lower calcu-
lated K
g
(Table 3; Figure 6). Associated with IFG status
were significant increases in waist circumference and trends
toward hypertriglyceridemia, which suggest these females
are at high risk for developing MS (Table 3). These animals
had HOMA scores indicative of IR, but unexpectedly had
normal insulin sensitivity based on M-values obtained from
hyperinsulinemic-euglycemic clamp.
Discussion
This colony of vervet monkeys demonstrates that without
the dietary and lifestyle influences seen in present-day hu-
man populations, obesity, IR, and associated changes in
plasma lipids are still observed and are heritable. In human
populations, abdominal obesity is considered the central
Figure 5: The population evaluated by fasting blood insulin quar-
tiles and their relationship to fasting blood glucose. The mean of
each quartile is indicated by the horizontal line, and post hoc
significance between the mean of each quartile is denoted by
different letters (p 0.05). Open circles represent females, and
filled circles represent males.
Table 3. Parameters (mean standard error) related to glucose tolerance and insulin sensitivity in age-matched
vervet females determined to be at-risk for metabolic syndrome based on elevated glycated hemoglobin (GHb; n
3) or normal based on low GHb % (n 4)
Unit Control Impaired glucose tolerant
Glycated hemoglobin % 5.27 (0.19) 8.3 (0.40)
Glucose mg/dL 60.5 (4.73) 104.7 (7.42)*
Insulin uIU/mL 20.81 (5.19) 26.52 (2.82)
HOMA 3.04 (0.61) 6.76 (0.41)*
K
g
hr 3.58 (0.33) 1.92 (0.30)*
M-value mg/kg per min 9.53 (1.06) 9.23 (0.45)
Waist circumference cm 38.37 (0.36) 43.33 (2.17)*
Triglycerides mg/dL 49.25 (10.5) 118 (40.15)
HOMA, homeostasis model assessment; K
g
, rate constant calculated for glucose disappearance following an intravenous glucose tolerance
test; M-value, glucose disposal rate calculated from a hyperinsulinemic-euglycemic clamp.
* p 0.05 for non-parametric comparison between groups.
Obesity and Associated Risk Factors in Monkeys, Kavanagh et al.
OBESITY Vol. 15 No. 7 July 2007 1671
Page 6
defining feature of MS because it is the best predictor of
incident MS in non-diabetic populations (20,23). Only a few
non-human primate species are well characterized for de-
veloping obesity without dietary intervention (12,24,25).
The vervet monkey has also been evaluated for atheroscle-
rosis development (14), making it an attractive model for
the study of interactions between MS and CVD.
In this colony, waist circumference and BMI were
strongly heritable, but estimates were lower than those made
from human populations (26). The heritability (h
2
)ofBMI
in this sample of vervet monkeys (h
2
0.44) was surpris
-
ingly close to that measured in a comparable sample of
pedigreed baboons (h
2
0.46) (27). As waist circumfer
-
ence has been described as the best predictor for incident
MS (20), subpopulations of vervets predicted at high risk for
obesity based on pedigree could be selected and evaluated
across all life stages. The abdominal location of adipose
tissue is specifically associated with development of the
MS, leading to the inclusion of waist circumference rather
than BMI or waist-hip ratio in the Adult Treatment Panel III
guidelines (28). Indeed, one study without a specific ab-
dominal measure found no relationship between generalized
adiposity and coronary artery disease (29).
A high waist circumference, indicating abdominal fat
accumulation, is linked to development of IR (30). In this
colony, higher insulin concentrations were associated with
greater waist size. Abdominal fat deposition often occurs
with aging, but it is the effect of an increase in waist size
independent of age that largely determines insulin sensitiv-
ity as lean aged people have been shown to be comparable
to lean youths (31). HOMA has been validated as a reliable
estimate of insulin sensitivity when compared with the
hyperinsulinemic-euglycemic clamp methodology, and is
correlated with visceral fat area (16,32). HOMA scores
were increased in animals with central adiposity and the
increase was most pronounced in females. The actual val-
ues, however, should be interpreted cautiously as glucose
concentrations were lower than in people, and insulin con-
centrations considerably higher than those in people
(20,33,34). Insulin concentrations in prior reports of vervet
monkeys (35) and other non-human primate species, includ-
ing the baboon and the rhesus macaque, are also reported to
be higher than in people when typically measured by radio-
immunoassay (9,12,27). The reasons for differences may
include species sensitivity to abundant food availability or
species differences in antibody reactivity and antibody
source where human assays are typically applied to measure
monkey samples (36,37). Lower fasting glucose concentra-
tions are also seen in other non-human primate species
(9,12) and may be a result of these generally higher insulin
concentrations. The reasons for the differences are not clear.
The MS criterion uses IFG (glucose 100 mg/dL) to
define pre-diabetes, which is less prevalent than IGT in
most populations (5,8) but is an easily measured parameter
for screening purposes. In the vervet colony, even females
selected for elevated GHb were not classified as IFG as their
fasting blood glucose averaged 20 mg/dL lower than
people (34). Thus, the prevalence of IGT or IFG is likely to
be underestimated by using human definitions in the vervet
population, as seen in other non-human primates (12). In-
cident diabetes risk increases with increasing fasting glu-
cose concentrations even within the normal range (34), so
establishment of species-specific reference values as de-
scribed here will allow detection of individuals with mar-
ginally elevated glucose values, along with other risk factors
for MS.
IR was prominent during pregnancy, demonstrated by
significant hyperinsulinemia, and is similar to the profile of
Figure 6: Comparison of mean (A) glucose and (B) insulin concentration-time curves generated from intravenous glucose tolerance testing
of age-matched females selected based on normal GHb (black squares) or elevated GHb (open squares).
Obesity and Associated Risk Factors in Monkeys, Kavanagh et al.
1672 OBESITY Vol. 15 No. 7 July 2007
Page 7
pregnant women. Insulin resistance and gestational DM are
seen in non-human primates (38,39) and are proposed to be
in response to sex steroids and prolactin, resulting in a
combination of
cell hyperplasia and enhanced insulin
secretion to combat IR in peripheral tissues (40). The same
group of pregnant females also had significantly lower
HDL-C than both male and non-pregnant female monkeys.
The reductions have been seen in other non-human primates
(39,41), and are in contrast to women who tend to develop
hypercholesterolemia and hypertriglyceridemia, and dem-
onstrate less of an effect on HDL-C (42). The reasons for
the differences are not clear.
Insulin, lipids, and lipoproteins were also heritable. Tri-
glyceride concentrations in the vervets had comparable her-
itability to triglyceride concentrations measured from adi-
pose tissue of baboons (h
2
0.33 vs. 0.2) (27). Our
estimates of heritability of insulin differed from baboons,
although both were significant (h
2
0.21 vs. 0.46, respec
-
tively) (27). Further, whereas HOMA values and glucose
were significantly heritable in the baboon sample (h
2
0.47
and 0.19, respectively) (27), neither achieved significance
in the vervet sample (h
2
0.02 and 0.07, respectively).
Differences between these values may reflect the greater
population heterogeneity or the more advanced average age
of the baboon colony (19 and 23 years in males and
females, respectively, compared with 7 and 11 years in
males and females in this report in vervets), which may have
allowed detection of a greater range of glucose abnormali-
ties. The failure to find significant genetic effects on fasting
blood glucose was related to the cross-sectional nature of
the sample and the colony management practices. Individ-
uals with hyperglycemia or other clinical signs of DM are
removed from the colony shortly after detection. If monkeys
were allowed to remain in the colony, the lifetime incidence
of hyperglycemia in this population would likely be herita-
ble.
Independent of age, females had a less favorable lipid
profile with triglyceride concentrations 70% higher than
their male counterparts and a greater fraction of their cho-
lesterol was carried on apolipoprotein-B-containing lipopro-
teins (the difference between the higher total cholesterol and
lower HDL-C seen in females). This fraction is known to
deliver cholesterol to the arterial wall in atherogenesis.
Although values seem low for TG and cholesterol fractions,
the diet is very low in cholesterol content and fat (13% of
energy supplied as fat as compared with the 40% consumed
in a typical western diet), and it would be expected that
dietary manipulation would exaggerate the sex differences
in lipid profile and further identify individuals prone to
weight gain and associated dyslipidemias.
In summary, we report that a significant proportion of
vervet monkeys have obesity and associated lipid changes,
of which all are heritable. The development of obesity was
associated with increasing insulin concentrations and lipid
changes analogous to the IR and dyslipidemia associated
with MS in people and was particularly apparent in females.
These monkeys may present a useful model of MS in a
species that is known to develop progression of atheroscle-
rosis with dietary intervention. Additionally, the aging fe-
male population within the colony appears to be at partic-
ular risk and may be a suitable species for evaluation of the
increased risk of CVD associated with menopause.
Acknowledgments
This study was supported by NIH Grants 1 P40
RR19963-01A1 and TR 5 T32 HL07115-28. The authors
thank Mary Jo Busa for editorial assistance.
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    • "The details of this interaction model are described in Mahaney et al. (1999) and Voruganti et al. (2013).Table 2 animals in both diets, but only abdominal circumference was significantly different between pregnant and non-pregnant vervets consuming TAD diet (P 5 0.0038). We also estimated phenotypic, genetic, and environmental correlations between vitamin D concentrations and both body weight and abdominal circumference (a better surrogate for obesity if 40.5cm; TABLE 1. Diet formulations for the " standard " Biscuit diet and the " typical " American diet (TAD) including the women's equivalent of daily vitamin D Kavanagh et al., 2007). Yet, body weight and abdominal circumference did not exhibit statistically significant correlations with other traits in the current study, indicating that they do not share genetic effects with traits including 25(OH)D 3 concentrations (data not shown). "
    [Show abstract] [Hide abstract] ABSTRACT: Objectives: The two objectives of the current study were to: 1) investigate the genetic contributions to variations in serum vitamin D concentrations under two dietary conditions (a standard monkey biscuit diet vs. a diet designed to model typical American consumption); and 2) explore the interaction of vitamin D with pregnancy status using a cohort of pedigreed female vervet/African green monkeys. Methods: This study includes 185 female (≥3.5 years) vervet/African green monkeys (Chlorocebus aethiops sabaeus) from a multi-generational, pedigreed breeding colony. The 25(OH)D3 concentrations were first measured seven to eight weeks after consuming a "typical American" diet (TAD), deriving 37, 18, and 45% of calories from fat, protein sources, and carbohydrates, and supplemented with vitamin D to a human equivalent of 1,000 IU/day. Vitamin D concentrations were assessed again when animals were switched to a low-fat, standard biscuit diet (LabDiet 5038) for 8 months, which provided a human equivalent of approximately 4,000 IU/day of vitamin D. All statistical analyses were implemented in SOLAR. Results: Pregnancy was associated with reduced 25(OH)D3 concentrations. Heritability analyses indicated a significant genetic contribution to 25(OH)D3 concentrations in the same monkeys consuming the biscuit diet (h(2) =0.66, P=0.0004) and TAD (h(2) =0.67, P=0.0078) diets, with higher 25(OH)D3 concentrations in animals consuming the biscuit diet. Additionally, there was a significant genotype-by-pregnancy status interaction on 25(OH)D3 concentrations (P<0.05) only among animals consuming the TAD diet. Discussion: These results support the existence of a genetic contribution to differences in serum 25(OH)D3 concentrations by pregnancy status and emphasize the role of diet (including vitamin D supplementation) in modifying genetic signals as well as vitamin D concentrations. Am J Phys Anthropol, 2015. © 2015 Wiley Periodicals, Inc.
    No preview · Article · Dec 2015 · American Journal of Physical Anthropology
  • Source
    • "In addition, genes affecting glycated haemoglobin levels are independent from genes inuencing glucose fasting levels (Snieder et al., 2001). The only other study that has investigated the heritability of glycated haemoglobin was conducted in a primate species, the vervet monkey, and it reported a heritability signicantly different from zero (h 2 = 18%), which is notably less than the value reported for humans (Kavanagh et al., 2007). Because glycated haemoglobin is a marker of glucose homeostasis, this parameter has nonetheless been used by eco-physiologists to test whether glycated haemoglobin levels may indicate individual variation in resource and nutrient accessibility. "
    [Show abstract] [Hide abstract] ABSTRACT: Ageing is characterized by a progressive deterioration of multiple physiological and molecular pathways, which impair organismal performance and increase risks of death with advancing age. Hence, ageing studies must identify physiological and molecular pathways that show signs of age-related deterioration, and test their association with the risk of death and longevity. This approach necessitates longitudinal sampling of the same individuals, and therefore requires a minimally invasive sampling technique that provides access to the larger spectrum of physiological and molecular pathways that are putatively associated with ageing. The present paper underlines the interest in using red blood cells (RBCs) as a promising target for longitudinal studies of ageing in vertebrates. RBCs provide valuable information on the following six pathways: cell maintenance and turnover (RBC number, size, and heterogeneity), glucose homeostasis (RBC glycated haemoglobin), oxidative stress parameters, membrane composition and integrity, mitochondrial functioning, and telomere dynamics. The last two pathways are specific to RBCs of non-mammalian species, which possess a nucleus and functional mitochondria. We present the current knowledge about RBCs and age-dependent changes in these pathways in non-model and wild species that are especially suitable to address questions related to ageing using longitudinal studies. We discuss how the different pathways relate with survival and lifespan and give information on their genetic and environmental determinants to appraise their evolutionary potential.
    Full-text · Article · Sep 2015 · Experimental Gerontology
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    • "Vervets are a small NHP species with a wellcharacterized menstrual cycle that mimics women in length and hormonal regulation25262728. Development of obesity in long-term vervet studies demonstrated an associated metabolic profile analogous to humans [30]. Our model allowed for invasive interventions, removal of the meticulously timed CL to collect tissue for analysis and paired design to maximize power in this costly setting. "
    [Show abstract] [Hide abstract] ABSTRACT: Obese women exhibit decreased fertility, high miscarriage rates and dysfunctional corpus luteum (CL), but molecular mechanisms are poorly defined. We hypothesized that weight gain induces alterations in CL gene expression. RNA sequencing was used to identify changes in the CL transcriptome in the vervet monkey ( Chlorocebus aethiops ) during weight gain. 10 months of high-fat, high-fructose diet (HFHF) resulted in a 20% weight gain for HFHF animals vs. 2% for controls (p = 0.03) and a 66% increase in percent fat mass for HFHF group. Ovulation was confirmed at baseline and after intervention in all animals. CL were collected on luteal day 7–9 based on follicular phase estradiol peak. 432 mRNAs and 9 miRNAs were differentially expressed in response to HFHF diet. Specifically, miR-28, miR-26, and let-7b previously shown to inhibit sex steroid production in human granulosa cells, were up-regulated. Using integrated miRNA and gene expression analysis, we demonstrated changes in 52 coordi
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