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Original Studies |
Departments of Human Genetics (S.C., N.J.C.) and Medicine (D.A.E.), University of Chicago, Chicago, Illinois 60637
Address all correspondence and requests for reprints to: David A. Ehrmann, M.D., Department of Medicine, Section of Endocrinology, University of Chicago Pritzker School of Medicine, 5841 South Maryland Avenue, MC 1027, Chicago, Illinois 60637. E-mail: dehrmann{at}medicine.bsd.uchicago.edu
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Abstract |
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 Top Abstract IntroductionSubjects and MethodsResultsDiscussionReferences |
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In the present study we used the frequently sampled iv glucose tolerance test to quantitate insulin secretion (AIRg), insulin action (Si), and their product (AIRg x Si) among women with PCOS (n = 33) and their nondiabetic first degree relatives (n = 48). We then quantitated the heritability of these measures from familial correlations estimated within a genetic model.
Familial (spousal, 
MF; parent-offspring, PO; and sibling, SS)
correlations were derived for log-transformed body mass index (BMI) as
well as for AIRg, Si, and AIRg x Si, the latter three of which
were adjusted for BMI. There was no evidence of significant
heritability for either lnBMI or lnSi in these families. In contrast,
the sibling correlation (SS = 0.74) for lnAIRg was highly
significant (
2 = 7.65; 1 df; P
= 0.006). In addition, the parameter quantitating insulin secretion in
relation to insulin sensitivity [i.e. ln(AIRg x
Si)] was significant among siblings (SS = 0.74;
2 = 4.32; 1 df; P = 0.04).
In summary, the results of the present study indicate that there is an heritable component to ß-cell dysfunction in families of women with PCOS. We conclude that heritability of ß-cell dysfunction is likely to be a significant factor in the predisposition to diabetes in PCOS.
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Insulin resistance plays a key role in the predisposition to diabetes in PCOS (6, 7), but although a substantial proportion of insulin-resistant women with PCOS develops either impaired glucose tolerance or diabetes, this is not the case for most. In our previous studies we sought to identify factors that distinguish insulin-resistant women with PCOS and glucose intolerance from those who are able to maintain normoglycemia. We (8) as well as others (9) found that a proportion of nondiabetic women with PCOS had defects in insulin secretion, particularly when analyzed in relation to the ambient level of insulin resistance. Further, such defects were most evident among those women who had a first degree relative with type 2 diabetes (8). This latter finding suggested that there was a genetic contribution to the reduction in the ability of the ß-cell to adequately compensate for insulin resistance, consistent with studies in nondiabetic family members of type 2 diabetics (10).
Given these findings, we hypothesized that heritability of defects in insulin secretion and/or insulin action would be evident within families of women with PCOS. We have tested this hypothesis in the present study using the frequently sampled iv glucose tolerance test (IVGTT) to simultaneously quantitate insulin secretion, insulin action, and their interrelationship among women with PCOS and their first degree relatives. The heritability of these measures was then determined from familial correlations estimated within a genetic model.
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Women with PCOS, 18–40 yr of age, were recruited from the Endocrinology Clinics of the University of Chicago between 1997 and 1999. All studies were approved by the institutional review board of the University of Chicago, and written informed consent was obtained from each subject.
A diagnosis of PCOS was assigned if subjects had historical, physical
examination, and hormonal evidence of androgen excess and met the most
commonly used diagnostic criteria for PCOS, often referred to as the
NIH consensus criteria (11). Specifically, all had a
history of oligo/amenorrhea, infertility, hirsutism, acne, or
androgenetic alopecia and hyperandrogenemia, defined by a supranormal
plasma free testosterone level (
34.7 pmol/L) (12).
Hormonal evidence of ovarian androgen overproduction was confirmed by
an abnormal 17-hydroxyprogesterone response to GnRH agonist
administration (12) or a supranormal plasma free
testosterone level after administration of dexamethasone
(12). Subjects with nonclassical 21-hydroxylase deficiency
congenital adrenal hyperplasia, Cushing’s syndrome, and
hyperprolactinemia were excluded from the study as were those known to
be diabetic. All steroid preparations (including oral contraceptives)
or medications known to alter insulin secretion and/or action had been
discontinued for at least 2 months before screening and enrollment.
First degree relatives of subjects with PCOS
All available nondiabetic first degree relatives of women with PCOS were contacted and invited to participate in the study. Relatives were recruited without regard to the glucose tolerance status of the proband.
Characterization of insulin secretion and insulin sensitivity: frequently sampled IVGTT
Subjects were admitted after an overnight fast. Two iv catheters were placed, one for the administration of glucose and tolbutamide, and the other for blood drawing. Blood samples were drawn for glucose and insulin at -20, -15, -10, and 0 min, at which time 300 mg/kg glucose was administered as an iv bolus. Blood samples for glucose and insulin were obtained at 2, 3, 4, 5, 6, 8, 10, 12, 15, and 19 min. At 20 min, tolbutamide (125 mg/m2; Orinase, Upjohn, Kalamazoo, MI) was given iv. Thereafter, blood was sampled at 21, 22, 24, 26, 28, 30, 35, 40, 45, 50, 55, 60, 70, 80, 100, 120, 140, 180, 210, and 240 min.
Summary measures derived from the IVGTT included 1) first phase insulin secretion (AIRg) in response to glucose, calculated as the mean increment above basal of insulin values measured at 2, 3, 4, 5, 6, 8, and 10 min; 2) insulin sensitivity index (Si), calculated using the MINMOD program, as previously described (8), provided by Dr. R. N. Bergman (the insulin sensitivity index represents the increase in net fractional glucose clearance rate per unit change in plasma insulin concentration after the iv glucose load); and 3) the relationship between the acute insulin response to glucose (AIRg) relative to the degree of insulin resistance (Si). This relationship, referred to as the disposition index, is calculated as the product of Si and AIRg and provides a measure of ß-cell secretory function adjusted for insulin sensitivity.
Assay methods
Plasma glucose was measured immediately using a glucose analyzer (model 2300 STAT, YSI, Inc., Yellow Springs, OH). The coefficient of variation of this method is less than 2%. Glycosylated hemoglobin was measured by boronate affinity chromatography with an intraassay coefficient of variation of 4% (Bio-Rad Laboratories, Inc., Hercules, CA). Serum insulin was assayed by a double antibody technique (4) with a lower limit of sensitivity of 20 pmol/L and an average intraassay coefficient of variation of 6%. The cross-reactivity of proinsulin in the RIA for insulin is approximately 40%.
Plasma testosterone was measured using a kit from Diagnostic Products (Los Angeles, CA). The free fraction of plasma testosterone and the concentration of sex hormone-binding globulin were measured by a competitive protein binding assay (4). The intra- and interassay coefficients of variation were 3.8% and 8.7%, respectively.
Data analysis/statistics
Phenotypic measures [body mass index (BMI), Si, AIRg, and AIRg x Si] were log-transformed to normalize their distributions and were also adjusted for any significant covariates found in this dataset. Covariates for each phenotype were tested for significance using a linear regression procedure in SAS statistical software (13).
Age, sex, and race were significant predictors for BMI in these families; thus, a residual was created adjusting for these factors and was used in all analyses. BMI, in turn, was the only covariate that was a significant predictor for lnAIRg, lnSi, and ln(AIRG x Si). These three measures were therefore adjusted for BMI in all analyses.
Spousal, sibling, and parent-offspring correlations were estimated from the covariate-adjusted residuals for each phenotype in the context of a genetic model provided by the REGC program in SAGE (14). Familial patterns of correlations were examined using class D regressive models, assuming no major gene effect (15, 16). In regressive models, genetic components of a trait can be estimated independently from related individuals because they successively condition each individual’s trait upon those of their ancestors. Class D regressive models are a specific type of regressive model that assumes that the sibling correlations within a family are equal and not necessarily due solely to common parentage. Because our families were ascertained through PCOS probands, who have an increased risk for diabetes (3, 4, 5), an ascertainment correlation was employed where each family’s likelihood was made conditional on the diabetes-related phenotype of the proband.
To test the significance of each familial correlation for a phenotype,
the likelihood scores between nested models were compared. First, a
general model that simultaneously estimated all three correlations
(spousal, MF; parent-offspring,
PO; and sibling, SS)
along with a population mean and variance was computed. Then a model
fixing the spousal correlation parameter at zero (no spousal
correlation model) was estimated and compared with the general model to
assess the significance of the spousal correlation. If the spousal
correlation was not significantly different from zero, a model fixing
the spousal and the parent-offspring correlation parameters at zero (no
parent-offspring model) was computed. This model was then compared with
the no spousal correlation model to assess the significance of the
parent-offspring correlation. Similarly, a model fixing the spousal and
sibling correlation parameters at zero (no sibling correlation model)
was generated and compared with the no spousal model to determine the
significance of the sibling correlation. Finally, a model
simultaneously fixing all three familial correlation parameters at zero
was computed (no correlation model) to test the significance of all
three correlations together.
Likelihood ratio tests (where twice the difference between ln
likelihoods for nested models is asymptotically distributed as a
2) were used to compute a
2 statistic for each correlation tested and
its corresponding P value. The number of degrees of freedom
for this 2 statistic is equal to the
difference in the number of independently estimated parameters between
the two models.
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Results |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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Baseline clinical and hormonal measures for PCOS subjects and
their first degree relatives are shown in Tables 1
and 2,
respectively. Of the 48 first degree relatives in this study, 31 (65%)
were Caucasian, 12 (25%) were African-American, 4 (8%) were Asian,
and 1 (2%) was Hispanic. Sixty-two percent of the first degree
relatives were female, and 38% were male. As expected, the PCOS
subjects had fasting hyperinsulinemia and substantially elevated levels
of total and free testosterone. The mean glycohemoglobin level was
normal in both PCOS subjects and relatives.
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Familial correlations were estimated for the natural log of BMI,
Si, AIRg, and AIRg x Si for 17 informative families with an
average family size of 2.5. Table 3 shows
the parameter estimates from a regressive model assessing the familial
correlations for the lnBMI residual. Even though the spousal
correlation (MF = 0.42) was the strongest
correlation estimated from the general model, this estimate was not
significantly different from zero (2 = 1.97; 1
df; P = 0.16). Both the parent-offspring and sibling
correlations for lnBMI were even weaker and therefore were not
significant (PO = 0.17;
2 = 0.98; 1 df; P = 0.32;
SS = 0.10; 2 = 0.25;
1 df; P = 0.62). These results suggest that BMI is not
highly familial in these PCOS families.
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). Both the spousal
correlation for lnSi (MF = -0.01) and the
parent-offspring correlation (PO = 0.08) from
the general model were not significantly different from zero
(2 = 0.002; 1 df; P = 0.96 and
2 = 0.09; 1 df; P = 0.76,
respectively). The sibling correlation for lnSi was estimated at its
lower bound of zero.
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shows the parameter estimates
from a regressive model assessing the familial correlations for lnAIRg
adjusted for BMI. From the general model, the spousal correlation
(MF) was -0.19, which was not significantly
different from zero (2 = 0.06; 1 df;
P = 0.81). The parent-offspring correlation
(PO) was 0.23, which was also not
significantly different from zero (2 = 0.95; 1
df; P = 0.33). In contrast, the sibling correlation
(SS = 0.74) was highly significant
(2 = 7.65; 1 df; P =
0.006).
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displays the familial
correlations for the log-transformed disposition index, ln(AIRg x
Si). The spousal correlation (MF = 0.41) was
not significantly different from zero (2 =
1.33; 1 df; P = 0.25), which was also the case for the
parent-offspring correlation (PO = 0.48;
2 = 1.84; 1 df; P = 0.18). The
sibling correlation (ss), however, was 0.74,
which was similar to that for lnAIRg alone and also statistically
significant (2 = 4.32; 1 df; P
= 0.04).
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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In the present study we used the rapidly sampled iv glucose tolerance test to quantitate insulin sensitivity and insulin secretion with the aim of determining whether either or both are heritable traits in PCOS families. Among the PCOS families studied, there was a significant familial (sibling) correlation for the acute insulin response to iv glucose and a lesser, but still significant, correlation when this measure was related to the degree of insulin resistance in the form of their product, the disposition index (AIRG x Si). These results provide evidence that ß-cell function is heritable in PCOS families and are consistent with recent studies by Elbein et al. (10), who found evidence for heritability of these measures in nondiabetic family members of type 2 diabetics. Our results also indicate that spousal correlations for AIRg and AIRg x Si were not significant. This implies that a shared environment does not have a significant role in predicting ß-cell function and, along with a significant sibling correlation, is consistent with a genetic model of inheritance.
Contrary to what was expected as well as previously reported (17), we did not find evidence for heritability of insulin resistance in PCOS families. Our ability to assess the heritability of insulin sensitivity in PCOS families, however, was limited. A relatively small number of subjects was studied, many of whom were both obese and profoundly insulin resistant, thus limiting the variability of this measure (18).
Given that BMI has shown reasonably strong heritabilities in other studies (19, 20), it is interesting to note that BMI did not appear familial in these kindreds. Because BMI largely determines and is highly correlated with insulin sensitivity, our results showing lack of heritability for BMI and the insulin sensitivity index even when adjusted for BMI, are consistent with one another.
In conclusion, the results of the present study indicate that there is an heritable component to ß-cell dysfunction in families of women with PCOS. This heritability of ß-cell dysfunction is likely a significant factor in the predisposition to diabetes in PCOS.
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Footnotes |
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Received October 3, 2000.
Revised January 18, 2001.
Accepted February 6, 2001.
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References |
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