Class III alleles of the variable number of tandem repeat insulin polymorphism associated with silencing of thymic insulin predispose to type 1 diabetes

Type 1 diabetes results from autoimmune destruction of the insulin-producin g pancreatic beta cells. The insulin gene (INS) is also expressed in human thymus, an ectopic expression site likely involved in immune tolerance. The IDDM2 diabetes susceptibility locus maps to a minisatellite composed of a variable number of tandem repeats situated 0.5 kb upstream of INS. Chromoso mes carrying the protective long INS variable number of tandem repeats alle les (class III) produce higher levels of thymic INS mRNA than those with th e predisposing, short class I alleles. However, complete silencing of thymi c INS transcripts from the class III chromosome was found in a small propor tion of heterozygous human thymus samples. We hypothesized that the specifi c class III alleles found on these chromosomes silence rather than enhance thymic insulin expression. To test the prediction that these alleles are pr edisposing, we developed a DNA fingerprinting method for detecting two puta tive "silencing" alleles found in two thymus samples (S1, S2). In a set of 287 diabetic children and their parents we found 13 alleles matching the fi ngerprint of the S1 or S2 alleles. Of 18 possible transmissions, 12 of the S1-S2 alleles were transmitted to the diabetic offspring, a frequency of 0. 67, significantly higher than the 0.38 seen in the remaining 142 class III alleles; P = 0.025. This confirms our prediction and represents an addition al level of correlation between thymic insulin and diabetes susceptibility, which supports a thymic enhancer effect of the INS variable number of tand em repeats as the mechanism of IDDM2 and refines the contribution of IDDM2 genotyping to diabetes risk assessment.