Major susceptibility locus for nephropathy in type 1 diabetes on chromosome 3q: results of novel discordant sib-pair analysis.

Diabetic nephropathy (DN) clusters in families with type 1 diabetes and the degree of clustering suggests that a major gene having a common disease allele may be responsible. To investigate the chromosomal regions containing genes for the renin-angiotensin system, we performed a linkage study using pairs of siblings with type 1 diabetes who were discordant for DN. Theoretical considerations supported by simulation studies indicated that such discordant pairs, rather than the usual concordant pairs, would be more effective in detecting a major susceptibility gene for DN. We applied this novel strategy to test for linkage between DN and chromosomal regions containing genes for the ACE, angiotensinogen (AGT), and angiotensin II type 1 receptor (AT1). Two polymorphic markers were genotyped in the vicinity of each of the three loci in 66 discordant sib pairs and were analyzed with multipoint methods. The regions containing ACE and AGT loci were not linked with DN, while the region containing the AT1 locus showed linkage with DN. As a result of these positive findings, eight additional polymorphic markers spanning a 63-cM region around AT1 locus were genotyped. Linkage was demonstrated between DN and a 20-cM region that includes AT1 (P = 7.7 x 10(-5)), an obvious candidate gene for DN. To investigate whether AT1 could account for the observed linkage, we sequenced all exons, splicing junctions, and the promoter region and examined the identified polymorphisms/mutations for association with DN using the transmission disequilibrium test. Four new polymorphisms in the gene were found, but neither these nor previously described polymorphisms were associated with DN. Thus, while our study does not implicate AT1 itself in the etiology of DN, it provides very strong evidence that a 20-cM region around AT1 contains a major locus for susceptibility to DN.