Genetic Control of Diabetogenesis in NOD/Lt Mice: Development and Analysis of Congenic Stocks

Genetic outcross and backcross analysis of nonobese diabetic (NOD/Lt) mice with a related but diabetes-resistant strain, nonobese normal (NON/Lt), has demonstrated that susceptibility to insulin-dependent diabetes mellitus is controlled in a recessive fashion by multiple genetic loci, including one (Idd-1^s) associated with H-2 on chromosome 17 and another (Idd-2^s) associated with Thy-1^b/Apoa-1^b (formerly Alp-1) on chromosome 9. To analyze the separate pathogenic contributions of Idd-1^s and Idd-2^s, two distinct congenie stocks of NOD/Lt mice homozygous on chromosomes 17 and 9 for NON/Lt linkage markers for the respective resistance alleles (Idd-1^s and Idd-2^r) were developed. The recessive nature of Idd-1^s was confirmed at the fifth backcross generation in that 83% of females and 29% of males homozygous for NOD H-2 haplotype developed diabetes, whereas no diabetes occurred in any of the mice homozygous or heterozygous for the NON haplotype. However, codominant and recessive MHC-associated susceptibility genes in this congenie stock were indicated by the finding that at least one copy of the NOD/Lt MHC was required for insulitis development. Virtually no insulitis was detected in the pancreases of mice homozygous for NON haplotype at 42 wk of age, whereas heavy generalized insulitis was present in 3 of 19 H-2 heterozygotes and in 7 of 7 diabetic and 3 of 5 nondiabetic mice homozygous for NOD haplotype. Further indication of the presence of MHC-associated codominant and recessive MHC-associated susceptibility genes was the observation that the NOD MHC haplotype correlated in a codominant fashion with a relative increase in the percentage of splenic T-lymphocytes bearing the Ly-2 surface marker. Severe insulitis and concomitant high diabetes incidences occurred in all gentoypic classes of congenie mice carrying Thy-1/Apoa-1 linkage markers for either NOD or NON alleles at Idd-2. Molecular analysis indicated that the NON-derived Idd-2^r resistance allele had been replaced by recombination with Idd-2^s from NOD. Restriction-fragment–length polymorphism analysis of two polymorphic markers proximal to Thy-1, low-density lipoprotein receptor Ldlr and Ets-1, a protooncogene, confirmed a recombinant chromosome 9, because homozygosity for NOD genomie fragments was found centromeric to an NON congenie segment of at least 20 centiMorgans spanning the Thy-1 and Mod-1 loci. Of the polymorphic markers analyzed on chromosome 9 with a panel of first-backcross diabetic segregante, the strongest linkage association was with Thy-1^b. The ease with which the Idd-2^s susceptibility locus can apparently be uncoupled from the Thy-1 linkage marker underscores the difficulties of assessing gene action when a marker gene and the gene of interest recombine at a relatively high frequency. In summary, diabetogenesis in NOD/Lt mice requires a complex interaction between both recessive and dominant genes on multiple chromosomes, with both recessive and dominant expression indicated depending on the trait being assessed.