A functional analysis of the role of IGF2 in IDDM2-encoded susceptibility to type 1 diabetes

Genetic studies have identified a number of loci demonstrating linkage to type 1 diabetes. One of the largest single contributors to genetic susceptibility, after the major histocompatability complex, is the IDDM2 locus, which maps to a nontranscribed variable number of tandem repeats (VNTR) minisatellite upstream of the insulin (INS) and insulin-like growth factor 2 (IGF2) genes. In a progression from population to functional studies, recent reports have shown that VNTR susceptibility alleles (class I) have different transcriptional effects on INS than protective VNTR alleles (class III) in thymus and pancreas, two tissues important in the pathogenesis of the disease. Similar VNTR transcriptional effects on IGF2 have also been proposed as a mechanism by which the IDDM2 locus confers susceptibility in addition to, or instead of, effects on INS. We evaluated this hypothesis by comparing IGF2 expression levels from chromosomes with the protective class III alleles to those with class I alleles in tissues relevant to type 1 diabetes pathogenesis. In thymus, class III alleles were associated with an IGF2 mRNA level of 4.7 +/- 0.9 (mean +/- SE, arbitrary units, n = 12) compared with 4.7 +/- 1.3 for class I alleles (n = 17). The same absence of a significant difference was found in pancreas, where class III alleles were associated with a level of 28.4 +/- 4.2 (n = 7) and class I alleles with a level of 29.5 +/- 5.2 (n = 6). There was a significant correlation between fetal age and IGF2 in both tissues, but fetal ages were not different in the genotype groups compared. We therefore did not detect any significant difference in IGF2 mRNA levels associated with the protective class of VNTR alleles as compared with the predisposing class. This is evidence against the hypotheses that have suggested IGF2 is a mediator of IDDM2-encoded susceptibility and corroborates previous studies suggesting insulin is the gene involved.