Positional cloning of a type 2 diabetes quantitative trait locus; Tomosyn-2,a negative regulator of insulin secretion

We previously mapped a type 2 diabetes (t2d) locus on chromosome 16 (chr 16) in an f2 intercross from the btbr t (+) tf (btbr) lep ob/ob and c57bl/6 (b6) lep ob/ob mouse strains. introgression of btbr chr 16 into b6 mice resulted in a consomic mouse with reduced fasting plasma insulin and elevated glucose levels. we derived a panel of sub-congenic mice and narrowed the diabetes susceptibility locus to a 1.6 mb region. introgression of this 1.6 mb fragment of the btbr chr 16 into lean b6 mice (b6.16 bt36-38) replicated the phenotypes of the consomic mice. pancreatic islets from the b6.16 bt36-38 mice were defective in the second phase of the insulin secretion,suggesting that the 1.6 mb region encodes a regulator of insulin secretion. within this region,syntaxin-binding protein 5-like (stxbp5l) or tomosyn-2 was the only gene with an expression difference and a non-synonymous coding single nucleotide polymorphism (snp) between the b6 and btbr alleles. overexpression of the b-tomosyn-2 isoform in the pancreatic β-cell line,ins1 (832/13),resulted in an inhibition of insulin secretion in response to 3 mm 8-bromo camp at 7 mm glucose. in vitro binding experiments showed that tomosyn-2 binds recombinant syntaxin-1a and syntaxin-4,key proteins that are involved in insulin secretion via formation of the snare complex. the b6 form of tomosyn-2 is more susceptible to proteasomal degradation than the btbr form,establishing a functional role for the coding snp in tomosyn-2. we conclude that tomosyn-2 is the major gene responsible for the t2d chr 16 quantitative trait locus (qtl) we mapped in our mouse cross. our findings suggest that tomosyn-2 is a key negative regulator of insulin secretion. © 2011 bhatnagar et al.