The IDDM2 type 1 diabetes susceptibility locus was mapped to and identified as allelic variation at the insulin gene (INS) VNTR regulatory polymorphism. In Caucasians; INS VNTR alleles divide into two discrete size classes. Class I alleles (26 to 63 repeats) predispose in a recessive way to type 1 diabetes, while class III alleles (140 to more than 200 repeats) are dominantly protective. The protective effect may be explained by higher levels of class III VNTR-associated INS mRNA in thymus such that elevated levels of preproinsulin protein enhance immune tolerance to preproinsulin, a key autoantigen in type 1 diabetes pathogenesis. The mode of action of IDDM2 is complicated, however, by parent-of-origin effects and possible allelic heterogeneity within the two defined allele classes. We have now analysed transmission of specific VNTR alleles in 1,316 families and demonstrate that a particular class I allele does not predispose to disease when paternally inherited, suggestive of polymorphic imprinting. But this paternal effect is observed only when the father's untransmitted allele is a class III. This allelic interaction is reminiscent of epigenetic phenomena observed in plants (for example, paramutation; ref. 17) and in yeast (for example, trans- inactivation; ref. 18). If untransmitted chromosomes can have functional effects on the biological properties of transmitted chromosomes, the implications for human genetics and disease are potentially considerable.
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