Common variants in genes encoding adiponectin (ADIPOQ) and its receptors (ADIPOR1/2), adiponectin concentrations, and diabetes incidence in the Diabetes Prevention Program

AIMS

Baseline adiponectin concentrations predict incident Type 2 diabetes mellitus in the Diabetes Prevention Program. We tested the hypothesis that common variants in the genes encoding adiponectin (ADIPOQ) and its receptors (ADIPOR1, ADIPOR2) would associate with circulating adiponectin concentrations and/or with diabetes incidence in the Diabetes Prevention Program population.

METHODS

Seventy-seven tagging single-nucleotide polymorphisms (SNPs) in ADIPOQ (24), ADIPOR1 (22) and ADIPOR2 (31) were genotyped. Associations of SNPs with baseline adiponectin concentrations were evaluated using linear modelling. Associations of SNPs with diabetes incidence were evaluated using Cox proportional hazards modelling.

RESULTS

Thirteen of 24 ADIPOQ SNPs were significantly associated with baseline adiponectin concentrations. Multivariable analysis including these 13 SNPs revealed strong independent contributions of rs17366568, rs1648707, rs17373414 and rs1403696 with adiponectin concentrations. However, no ADIPOQ SNPs were directly associated with diabetes incidence. Two ADIPOR1 SNPs (rs1342387 and rs12733285) were associated with ∼18% increased diabetes incidence for carriers of the minor allele without differences across treatment groups, and without any relationship with adiponectin concentrations.

CONCLUSIONS

  ADIPOQ SNPs are significantly associated with adiponectin concentrations in the Diabetes Prevention Program cohort. This observation extends prior observations from unselected populations of European descent into a broader multi-ethnic population, and confirms the relevance of these variants in an obese/dysglycaemic population. Despite the robust relationship between adiponectin concentrations and diabetes risk in this cohort, variants in ADIPOQ that relate to adiponectin concentrations do not relate to diabetes risk in this population. ADIPOR1 variants exerted significant effects on diabetes risk distinct from any effect of adiponectin concentrations.

Alpha-5/alpha-3 nicotinic receptor subunit alleles increase risk for heavy smoking

Twin studies indicate that additive genetic effects explain most of the variance in nicotine dependence (ND), a construct emphasizing habitual heavy smoking despite adverse consequences, tolerance and withdrawal. To detect ND alleles, we assessed cigarettes per day (CPD) regularly smoked, in two European populations via whole genome association techniques. In these approximately 7500 persons, a common haplotype in the CHRNA3-CHRNA5 nicotinic receptor subunit gene cluster was associated with CPD (nominal P=6.9 x 10(-5)). In a third set of European populations (n= approximately 7500) which had been genotyped for approximately 6000 SNPs in approximately 2000 genes, an allele in the same haplotype was associated with CPD (nominal P=2.6 x 10(-6)). These results (in three independent populations of European origin, totaling approximately 15 000 individuals) suggest that a common haplotype in the CHRNA5/CHRNA3 gene cluster on chromosome 15 contains alleles, which predispose to ND.

Genetics of polycystic ovary syndrome

We have found evidence for the involvement of two major genes in the aetiology of PCOS. The results of both linkage and association studies suggest that CYP11a (coding for P450 cholesterol side chain cleavage) and the insulin VNTR regulatory polymorphism are important genes in the aetiology of PCOS and may explain, in part, the heterogeneity of the syndrome. Differences in expression of CYP11a could account for variation in androgen production in women who have polycystic ovaries and those subjects who are homozygous for III alleles at the insulin gene VNTR locus are more likely to be hyperinsulinaemic. It is likely that other genes are involved in the aetiology of PCOS. Recent results lend weight to the idea that PCOS represents a complex trait in which several genes--but perhaps a relatively small number of key genes--contribute, in conjunction with nutritional factors, to the observed clinical and biochemical heterogeneity.

Current developments in the molecular genetics of the polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is most likely a complex trait with an oligogenic basis. In this article, we present evidence from molecular genetic studies for involvement of the steroid synthesis gene CYP11a (coding for P450 cholesterol side-chain cleavage) in the aetiology of hyperandrogenism. Variation in the regulatory region of the insulin gene also appears to contribute to the development of PCOS.

The genetic basis of polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is the most common endocrinopathy in women of reproductive age. Familial clustering of cases suggests that genetic factors play an important part in its aetiology. A number of studies of families with several cases of PCOS have produced results suggesting an autosomal dominant trait. Detailed analysis of a large number of affected families has, however, cast some doubt about the mode of inheritance. An autosomal dominant trait remains possible but a more complex aetiology seems more likely. The results of our recent studies support the concept of an oligogenic disorder in which genes affecting metabolic pathways in glucose homeostasis and steroid biosynthesis are both involved. We review evidence for an important role for the insulin gene minisatellite in the aetiology of anovulatory PCOS and for the gene coding for P450 cholesterol side chain cleavage (CYP11a) in the mechanism of excessive androgen secretion in women with polycystic ovaries. We propose that the heterogeneity of clinical and biochemical features in PCOS can be explained by the interaction of a small number of key genes with environmental, particularly nutritional, factors.

Hypersecretion of androgens by polycystic ovaries: the role of genetic factors in the regulation of cytochrome P450c17 alpha

A single base change has been found in the promoter region of CYP17, the gene encoding P450c17 alpha, which appears to be a significant factor in the expression of hyperandrogenism in PCO but which can be excluded as the primary genetic defect. These findings are consistent with the biochemical data from the studies of patients with PCOS reported above, in which the production of ovarian 17 hydroxyprogesterone and androstenedione were observed to be greatly increased but the generation of progesterone was also exaggerated in PCO theca. Thus, genetic factors may well be involved in the observed dysregulation of 17 hydroxylase/17,20 lyase, but this does not appear to be the whole story. It remains a tenable hypothesis that a single-gene effect is the major cause of PCOS and that a gene involved in the expression of androgen production will be implicated. On the other hand, increased androgen production may be a reflection of an 'upstream' abnormality in the ovary, perhaps involving the fundamental processes of proliferation, differentiation and atresia in ovarian follicles. It is also possible that PCOS is truly polygenic and that CYP17 is one of several genes-including those related to insulin secretion and action-that contribute to the PCOS phenotype. Further candidate genes will need to be investigated using well-characterized, large families, but if several predisposing genes are involved, other approaches may be applicable, for example analysis of shared alleles by affected sibling pairs, which has proved valuable in understanding the genetics of type 1 diabetes (Davies et al, 1994). In conclusion, PCOS--one of the most common endocrinopathies--remains an enigmatic condition but one which may prove to be an important model for understanding the interaction of genetic and environmental factors in the aetiology of endocrine disorders.

Polycystic ovaries and premature male pattern baldness are associated with one allele of the steroid metabolism gene CYP17

Fourteen Caucasian families with 81 affected individuals have been assessed in which polycystic ovaries/male pattern baldness (PCO/MPB) segregates as an autosomal dominant phenotype (1). The gene CYP17, coding for P450c17 alpha (17 alpha-hydroxylase; 17/20 lyase) on chromosome 10q24.3 is the rate-limiting step in androgen biosynthesis. We have identified a new single base change in the 5' promoter region of CYP17 by heteroduplex analysis. This creates an additional SP1-type (CCACC box) promoter site, which may cause increased expression. This base change also creates a recognition site for the restriction enzyme MspA1 allowing a simple screening procedure. There is a significant association between the presence of this base change (A2) and the affected state for consecutively identified Caucasian women with PCO as compared either to consecutively matched controls (P = 0.03) with an odds ratio for those with at least one A2 allele of 3.57, or to a random population (P = 0.02) with an odds ratio of 2.50. Within the fourteen families, members with PCO or MPB have a significant association with the occurrence of at least one A2 allele compared to their normal relatives, with an odds ratio of 2.20 (P = 0.05). The base change does not cosegregate with the affected phenotype within the families showing association, demonstrating that this mutation of CYP17 does not cause PCO/MPB. Variation in the A2 allele of the CYP17 gene is a significant factor modifying the expression of PCO/MPB in families where it has been demonstrated to segregate as a single gene disorder, but it is excluded as the primary genetic defect.