Fine Mapping, Gene Content, Comparative Sequencing, and Expression Analyses SupportCtla4andNramp1as Candidates forIdd5.1andIdd5.2in the Nonobese Diabetic Mouse

Abstract

At least two loci that determine susceptibility to type 1 diabetes in the NOD mouse have been mapped to chromosome 1, Idd5.1 (insulin-dependent diabetes 5.1) and Idd5.2. In this study, using a series of novel NOD.B10 congenic strains, Idd5.1 has been defined to a 2.1-Mb region containing only four genes, Ctla4, Icos, Als2cr19, and Nrp2 (neuropilin-2), thereby excluding a major candidate gene, Cd28. Genomic sequence comparison of the two functional candidate genes, Ctla4 and Icos, from the B6 (resistant at Idd5.1) and the NOD (susceptible at Idd5.1) strains revealed 62 single nucleotide polymorphisms (SNPs), only two of which were in coding regions. One of these coding SNPs, base 77 of Ctla4 exon 2, is a synonymous SNP and has been correlated previously with type 1 diabetes susceptibility and differential expression of a CTLA-4 isoform. Additional expression studies in this work support the hypothesis that this SNP in exon 2 is the genetic variation causing the biological effects of Idd5.1. Analysis of additional congenic strains has also localized Idd5.2 to a small region (1.52 Mb) of chromosome 1, but in contrast to the Idd5.1 interval, Idd5.2 contains at least 45 genes. Notably, the Idd5.2 region still includes the functionally polymorphic Nramp1 gene. Future experiments to test the identity of Idd5.1 and Idd5.2 as Ctla4 and Nramp1, respectively, can now be justified using approaches to specifically alter or mimic the candidate causative SNPs.

Allelic variant inCTLA4alters T cell phosphorylation patterns

“…In the nonobese diabetic (NOD) mouse, the orthologous CTLA4 region also is associated with autoimmune diabetes, and, similar to humans, an allelic variant causes the increased expression of a major splice isoform. In the NOD model, the isoform is ligandindependent CTLA-4 (11) and results in strongly inhibited T cell responses, as well as increased susceptibility to disease (12). Together these investigations in both the experimental model and in humans suggest that genetic variation in the CTLA4 gene region may have an important effect on T cell function because of altered T cell signaling.…”

Molecular Genetic Analysis of the Idd4 Locus Implicates the IFN Response in Type 1 Diabetes Susceptibility in Nonobese Diabetic Mice

“…A functional variant of ␤2 microglobulin has been demonstrated to function as a susceptibility allele in the murine Idd13.1 region, suggesting that variants affecting MHC class I function also are important in disease (7). A negative regulator of T cell activation, Ctla4, has been shown to be a strong candidate T1D susceptibility gene for the murine Idd5.1 and human IDDM12 loci (8,9). The variable-length terminal repeat in the insulin gene promoter is strongly associated with T1D susceptibility in humans, although it was not detected in genetic linkage studies (10 -12).…”

Sex-Specific Effect of Insulin-Dependent Diabetes 4 on Regulation of Diabetes Pathogenesis in the Nonobese Diabetic Mouse

“…Analysis of recently developed Idd5 congenic strains in which portions of the C57BL/10 (B10) genome were introgressed into the NOD background suggested presence of two loci, Idd5.1 and Idd5.2, each conferring protection from insulitis and T1D ( (43,44); Fig. 9).…”

“…One attractive candidate is the natural resistance-associated macrophage protein (Nramp1) involved in the clearance of intracellular pathogens (46). This gene lies within the Idd5.2 region and is shown to be polymorphic between NOD and B10 strains (44). There are a number of genes without an obvious link to diabetes pathogenesis that map to the Idd5.3, including those for the nicotinic acetylcholine receptor, Acrg (47); collagen type VI, Col6a3 (48); the high density lipoprotein-binding protein, Hdlbp (49); and the receptor activity-modifying protein 1, Ramp1 (50).…”