Evidence of multiple causal sites affecting weight in the IGF2-INS-TH region of human chromosome 11

The subtelomeric region of 11p harbours three closely linked genes, TH, INS and IGF2, that have been associated with obesity, size at birth, type I diabetes, polycystic ovary syndrome, overgrowth in Beckwith-Wiedemann syndrome and possibly hypertension. We have previously shown that the IGF2 ApaI single nucleotide polymorphism (SNP) associates with weight and body mass index in middle-aged Caucasian males but that there is no such association with the INS -23/HphI site that marks INS 5' variable number of tandem, repeats (VNTR) class I vs class III VNTR alleles. We report here the examination of three SNP markers in IGF2: 6815 A/T in the P1 promoter, AluI in exon 3 and ApaI in the 3' untranslated region (UTR), INS 5'VNTR class I alleles and the TH01 tetranucleotide microsatellite in a population sample. The analysis has taken into account the possibility that typing failure and the number of parameters required to model multiallelic loci could create spurious significance. We have exercised Hardy-Weinberg equilibrium tests, dichotomised multiallelic series to impose parsimony, and examined the data with failures modelled or excluded. Regression analysis infers that three markers, IGF2 ApaI, TH01 and subclasses of INS VNTR class I independently predict derived weight indices (combined P<10(-8) and accounting up to 2% of population weight variance), with no evidence of interaction. This establishes that there must be multiple causal sites impacting on weight in this genomic region.