Sir—We have repeated the Genehunter analysis for linkage of 11p15·5 to PCOS. Irrespective of whether HUMTH01, the INS VNTR, or both loci are excluded from the analysis, we obtain the same support for linkage at D11S922 (single point non-parametric linkage value [NPL]=2·77, p=0·005; multipoint [excluding HUMTH01] NPL=3·2, p=0·002).
Previous studies have reported associations of the INS VNTR class III/III genotype with type 2 diabetes and related disorders.1
Women with PCOS have increased risk of developing type 2 diabetes, and anovulatory PCOS is associated with hyperinsulinaemia. Therefore, our hypothesis was that the INS VNTR class III allele frequency would be higher in anovulatory PCOS patients than in controls. This prediction was confirmed in the casecontrol data set: the exact p-value obtained is 0·0059 (Statxact, Cytel Corporation, Cambridge, MA). The value cited by McKeigue and Wild (p=0·011) is conservative. We use the more conventional mid-point value. Analysis of the entire 3×3 table (our table 2) shows striking departure from independent assortment (p=0·008 for genotypes, Kruskal-Wallis test). The source of this departure lies predominantly in the comparison of normal versus anovulatory PCOS data sets (p=0·0027).
We did a family-based association analysis with an independent family data set. The constructed AFBAC control genotypes are not in Hardy-Weinberg equilibrium, as pointed out by McKeigue and Wild. We believe this to be a chance event. Nevertheless, the trend is positive and consistent with the independent case-control analysis. Furthermore, we have analysed these families for evidence of linkage of PCOS to the insulin VNTR in the presence of association with the transmission disequilibrium test, in which allele transmission from heterozygous parents to affected offspring is measured and is not dependent on Hardy-Weinberg equilibrium.3
Class III alleles were transmitted more often than class I VNTR alleles from class I/III heterozygous parents to offspring with anovulatory PCOS (27 transmissions, 17 non-transmissions; 61% transmission; χ2
-test, p=0·132). Because parent-of-origin effects have been observed at this locus in type 1 diabetes4
and there is evidence for gametic imprinting of this region of 11p15·5,5
we evaluated allele transmission from mothers and fathers, separately. Maternal transmissions did not depart from that expected for random transmission (12 transmissions, 12 nontransmissions; 50%). However, class III alleles were transmitted significantly more often than class I alleles from I/III fathers (15 transmissions, five nontransmissions; 75%; p=0·025). This analysis was extended to include transmissions of VNTR class III alleles in the 17 PCOS pedigrees used for the linkage study. In the combined data, class III alleles had a 47% transmission from mothers (14 transmissions, 16 non-transmissions; p=0·715), and a 77% transmission from fathers (20 transmissions, six non-transmissions; p=0·006). The inter-parental difference was also significant (14/16 vs 20/6; p=0·021).
The results from three independent data sets are entirely consistent with the insulin gene region on 11p15·5 harbouring a susceptibility locus for anovulatory PCOS associated with hyperinsulinaemia. Moreover, the activity of this susceptibility locus, designated PCOS1, may be influenced by parent-of-origin effects.