The CTRB1/2 locus affects diabetes susceptibility and treatment via the incretin pathway

Abstract

The incretin hormone glucagon-like peptide 1 (GLP-1) promotes glucose homeostasis and enhances β-cell function. GLP-1 receptor agonists (GLP-1 RAs) and dipeptidyl peptidase-4 (DPP-4) inhibitors, which inhibit the physiological inactivation of endogenous GLP-1, are used for the treatment of type 2 diabetes. Using the Metabochip, we identified three novel genetic loci with large effects (30-40%) on GLP-1-stimulated insulin secretion during hyperglycemic clamps in nondiabetic Caucasian individuals (TMEM114; CHST3 and CTRB1/2; n = 232; all P ≤ 8.8 × 10(-7)). rs7202877 near CTRB1/2, a known diabetes risk locus, also associated with an absolute 0.51 ± 0.16% (5.6 ± 1.7 mmol/mol) lower A1C response to DPP-4 inhibitor treatment in G-allele carriers, but there was no effect on GLP-1 RA treatment in type 2 diabetic patients (n = 527). Furthermore, in pancreatic tissue, we show that rs7202877 acts as expression quantitative trait locus for CTRB1 and CTRB2, encoding chymotrypsinogen, and increases fecal chymotrypsin activity in healthy carriers. Chymotrypsin is one of the most abundant digestive enzymes in the gut where it cleaves food proteins into smaller peptide fragments. Our data identify chymotrypsin in the regulation of the incretin pathway, development of diabetes, and response to DPP-4 inhibitor treatment.

FIG. 1.

Gene expression according to rs7202877 genotype in 35 whole pancreata and 45 human islet samples. Gene expression of CTRB1/2 and BCAR1 was measured using Taqman gene expression assays (pancreas) or RNAseq (islets). A: CTRB1/2 and BCAR1 gene expression in whole pancreas in TT (n = 26) and TG+GG (n = 9) carriers adjusted for age, sex, and BMI. CTRB1/2 (B) or BCAR1 (C) gene expression in islets from 27 TT carriers and 18 TG+GG carriers adjusted for age, sex, BMI, and A1C. A.U., arbitrary units.

FIG. 2.

Fecal chymotrypsin activity measurements according to rs7202877 genotype. Chymotrypsin activity in stool samples was measured with the method of Kaspar et al. (18). A: Mean chymotrypsin activity (Act. ± SEM) of healthy volunteers with TT (n = 40) or TG+GG genotype (n = 40) at rs7202877 near CTRB1/2. B: Relative chymotrypsin activity (% Act. ± SEM) in stool samples preincubated with or without human DPP-4 (0.1 units) for 30 min at 37°C. The samples without DPP-4 preincubation were used a reference. TT genotype, n = 15; TG+GG genotype, n = 14. A.U., arbitrary units.