The role of genetic susceptibility in T2DM is well established. In the previous funding period, we mapped T2DM to chromosome 1q21-q23 by linkage in multigenerational Utah families. These results were confirmed by parallel studies in Pima Indians, Amish and French families, and English sib pairs. Several markers within this region show promising associations with T2DM. Three hypotheses are proposed: 1) a single locus or haplotype on 1q21-q23 increases diabetes susceptibility in multiple populations; 2) additional susceptibility loci will be apparent by conditioning on the 1q21 locus; 3) the 1q21-q23 susceptibility locus alters gene expression in fat and muscle. A comprehensive, complementary strategy is proposed to address these hypotheses and to compensate for the weaknesses inherent in any single approach. This strategy incorporates a close collaboration with other laboratories with evidence for linkage. First, a dense marker map will be constructed to test linkage disequilibrium by placing 1000 single nucleotide polymorphisms (SNPs) and 75 microsatellites over a 15 Mb region. Initial typing will use a pooled typing strategy, followed by confirmation in individual case control samples and typing of additional SNPs. Second, a positional candidate strategy will be pursued by identifying candidate genes on 1q21-q23 both by known function and by expression in diabetes-related tissues. Candidates will be sought also through differential expression in sib pairs discordant for diabetes. Each candidate gene will be screened extensively for variation in exons, flanking regions, and intronic sequences proximal to exons. Variants identified will be tested in case control studies, first using a pooled strategy with subsequent confirmation in individual samples. For variants that are associated with T2DM in either linkage disequilibrium or candidate gene studies, all closely spaced markers will be typed in multigenerational families to establish haplotypes. Markers will be tested for family-based association, haplotype sharing, decay of haplotype sharing, and association with evidence for linkage. Finally, using the associated variant or haplotype for 1q21-q23, 2-locus conditional search methods will be used to identify additional linked regions. Regions of suggestive linkage will be identified from all Caucasian genome scans, and microsatellite marker density increased to 3-5 cM in Utah families. Functional candidates at each region will be identified and screened for variation. Multiple conditional search methods will be tested and compared. The proposed studies have a strong likelihood of finding the 1q21-q23 susceptibility locus and begin to explore other loci.
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