A mutant Stat5b with weaker DNA binding affinity defines a key defective pathway in nonobese diabetic mice

A number of cytokines that finely regulate immune response have been implicated in the pathogenesis or protection of type 1 diabetes and other autoimmune diseases. It is, therefore, of pivotal importance to examine a family of proteins that serve as signal transducers and activators of transcription (STATs), which regulate the transcription of a variety of cytokines. We report here a defective gene (Stat5b) located on chromosome 11 within a previously mapped T1D susceptibility interval (Idd4) in the nonobese diabetic ( NOD) mice. Our sequencing analysis revealed a unique mutation C1462A that results in a leucine to methionine (L327M) in Stat5b of NOD mice. Leu(327), the first residue in the DNA binding domain of STAT proteins, is conserved in all identified mammalian STAT proteins. Homology modeling predicted that the mutant Stat5b has a weaker DNA binding, which was confirmed by DNA-protein binding assays. The inapt transcriptional regulation ability of the mutated Stat5b is proved by decreased levels of RNA of Stat5b-regulated genes (IL-2Rbeta and Pim1). Consequently, IL-2Rbeta and Pim1 proteins were shown by Western blotting to have lower levels in NOD compared with normal B6 mice. These proteins have been implicated in immune regulation, apoptosis, activation-induced cell death, and control of autoimmunity. Therefore, the Stat5b pathway is a key molecular defect in NOD mice.