Positional Cloning of a Type 2 Diabetes Quantitative Trait Locus; Tomosyn-2, a Negative Regulator of Insulin Secretion

Abstract

We previously mapped a type 2 diabetes (T2D) locus on chromosome 16 (Chr 16) in an F2 intercross from the BTBR T (+) tf (BTBR) Lepob/ob and C57BL/6 (B6) Lepob/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 (B6.16BT36–38) replicated the phenotypes of the consomic mice. Pancreatic islets from the B6.16BT36–38 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.

CTSH regulates β-cell function and disease progression in newly diagnosed type 1 diabetes patients

“…It is conceivable that the differences between the two tests are explained by rs3825932 primarily affecting the second-phase insulin secretion response. Importantly, a recent study identified a type 2 diabetes candidate gene that only affects second-phase insulin secretion (29).…”

Contrasting genetic architectures in different mouse reference populations used for studying complex traits

“…Mapping a QTL to a chromosome will likely require higher resolution mapping to reach the point at which candidate gene analysis is feasible, either with additional crosses or by generating congenic and subcongenic panels. Analysis of subcongenic panels are most likely to lead to gene identification when the interval they span is just a few Mb (Shao et al 2008;Millward et al 2009;Bhatnagar et al 2011;Yazbek et al 2011;Buchner et al 2012). Generating and phenotyping subcongenic panels at this level of resolution requires approximately 1500 mice per panel (Yazbek et al 2011;Buchner et al 2012).…”

Tsc2, a positional candidate gene underlying a quantitative trait locus for hepatic steatosis

“…1B ), as measured by real-time PCR, indicating that a potential increase in de novo lipogenesis contributes to the higher amount of TG in B6 livers compared with BTBR livers. Based on this large strain difference in liver TG content, we mapped the gene loci contributing to these differences in an F2 cross (sample size ‫ف‬ 550) and detected strong linkage on chromosome 17 with a LOD severe diabetes when made obese due to a failure to expand ␤ -cell mass and to a defect in insulin secretion ( 14,23,24 ). In addition to diabetes, the B6 and BTBR strains differ in their susceptibility to develop hepatic steatosis ( 25 ).…”