IMPRINTED AND GENOTYPE-SPECIFIC EXPRESSION OF GENES AT THE IDDM2 LOCUS IN PANCREAS AND LEUKOCYTES

One of the loci encoding susceptibility to insulin-dependent diabetes mellitus (IDDM) is IDDM2, mapped to a variable number of tandem repeat s (VNTR) polymorphism situated 596 bp upstream of the insulin gene (IN S). The shorter alleles (class I) predispose to IDDM, while the longer class III alleles are protective. Besides INS, it is possible that tr anscription levels of IGF2, the nearby gene encoding the insulin-like growth factor II, may be modulated by allelic forms of the VNTR. In an effort to define the pathophysiologic mechanism of the IDDM2 effect, we examined the effect, in cis, of VNTR genotype on steady-state mRNA levels of INS in samples of human fetal pancreas, and of IGF2 in leuco cytes of diabetic children. Relative levels of mRNA transcripts derive d from each chromosome carrying a defined VNTR allele were measured by RT-PCR, taking advantage of transcribed polymorphisms at the 3' untra nslated region of each gene. In 10 samples of human fetal pancreas, IN S transcripts from chromosomes carrying a class III VNTR were slightly but significantly (P=0.015) lower than those from class 1 (13% lower, 95% confidence limits 3-21%). In 10 leucocyte samples, mRNA from both IGF2 alleles was seen, indicating relaxation of the parental imprinti ng of IGF2 in these cells. However, this relaxation was incomplete as maternal allele mRNA was systematically at a lower level than paternal . The paternal/maternal ratio varied widely among individual subjects. Two of the most extreme cases, demonstrating almost complete repressi on of the maternal allele, were identical twins, suggesting that this variable relaxation of imprinting is genotype-dependent. However, this genotype-dependence cannot be accounted for by the maternal VNTR, as the mean ratios of paternal/maternal IGF2 mRNA levels were not statist ically different in individuals with a maternal VNTR of class I vs. cl ass III (3.2+/-1.5 vs. 3.89+/-0.94). Thus, we present evidence that: ( a) class III VNTR alleles are associated with lower INS mRNA in fetal pancreas than class I alleles. The biologic importance of this differe nce remains to be determined; and (b) the variable relaxation of IGF2 imprinting seen in human leucocytes is not dependent on the presence o f a class I vs. a class III VNTR. (C) 1996 Academic Press Limited