A common CTRB misfolding variant associated with pancreatic cancer risk  causes ER stress and inflammation in mice
Objective: Genome wide association studies have identified an exon 6 CTRB2 deletion variant that is associated with increased PDAC risk. To acquire evidence on its causal role, we developed a new mouse strain carrying an equivalent variant in Ctrb1, the mouse orthologue of CTRB2.  Design: We used CRISPR/Cas9 to introduce a 707bp deletion in Ctrb1 encompassing exon 6 (Ctrb1Dexon6). This mutation closely mimics the human deletion variant. Mice carrying the mutant allele were extensively profiled at 3 months to assess their phenotype.   Results: Ctrb1Dexon6 mutant mice express a truncated CTRB1 that accumulates in the ER. The pancreas of homozygous mutant mice displays reduced chymotrypsin activity and total protein synthesis. The histological aspect of the pancreas is inconspicuous but ultrastructural analysis shows evidence of dramatic ER stress, and cytoplasmic and nuclear inclusions. Transcriptomic analyses of the pancreas of mutant mice reveals acinar program down-regulation and increased activity of ER stress-related and inflammatory pathways. Heterozygous mice have an intermediate phenotype. Agr2 is one of the most up-regulated genes in mutant pancreata. Ctrb1Dexon6 mice exhibit impaired recovery from acute caerulein-induced pancreatitis. Administration of TUDCA or sulindac partially alleviates the phenotype. A transcriptomic signature derived from the mutant pancreata is significantly enriched in normal human pancreas of CTRB2 exon 6 deletion variant carriers from the GTEx cohort.  Conclusions: This mouse strain provides formal evidence that the exon 6 deletion variant causes ER stress and inflammation in vivo, providing an excellent model to understand its contribution to pancreatic ductal adenocarcinoma and to identify preventive strategies. Overall design: To gain a comprehensive understanding of the molecular mechanisms underlying the phenotype of the pancreas of 3- month -old Ctrb1?/? mice, we performed genome wide transcriptomic analysis using RNA-seq. N=6 mice/genotype sequenced as pool of two mice of the same genotype.