Linkage and Association Studies in Insulin-Dependent Diabetes with a New Dinucleotide Repeat Polymorphism at the Gad₆₅ Locus

Abstract

Glutamic acid decarboxylase (GAD) is an important autoantigen in insulin-dependent diabetes mellitus (IDDM). The islet cell specific, 65 kDa form of GAD (GAD₆₅) is encoded by a gene on chromosome 10p. Recently, a putative IDDM susceptibility gene has been localized to the same general region based on allele sharing for the anonymous marker D10S193. To determine whether variation in the GAD₆₅ gene plays a role in genetic susceptibility to IDDM, possibly explaining the reported evidence for linkage on 10p, we isolated eosmid clones containing GAD₆₅, and identified a highly polymorphic dinuclcotide repeat physically linked to the gene. This GAD65 microsatellite marker, along with the other 10p markers D10S193 and D10S211. were used to genotype the members of 186 multiplex IDDM families with 2 or more affected siblings. Linkage analysis localized the GAD65 marker 5.6 cM from D10S193. Sharing of alleles identical by descent (IBD) in affected sib pairs for each of the markers was determined and compared to the expected 50: 50 distribution under an assumption of no linkage. Analyses were also carried out after stratification of the data for sharing of HLA class II alleles. The family data for GAD65 were further assessed for allelic association with IDDM using the transmission/disequilibrium test. No significant deviations from expected values were observed in any of these tests, suggesting that variation in the GAD₆₅ gene docs not play a significant role in genetic susceptibility to IDDM.

Key Words:

• polymerase chain reaction
• genetics
• DNA polymorphism
• glutamic acid decarboxylase
• chromosome 10p