Abstract: Introduction: Genome wide association studies of pancreatic ductal adenocarcinoma (PDAC) have identified a CTRB2 exon 6 deletion variant associated with increased PDAC risk. It has been proposed that this variant results in a truncated CTRB2 protein that accumulates in the endoplasmic reticulum (ER), leading to ER stress, inflammation, and cancer.
Methods: Using CRISPR/Cas9, we developed a new mouse strain recapitulating the human CTRB2 variant on its mouse ortholog, Ctrb1 (Ctrb1 exon6-del). We have extensively profiled the pancreas of mice carrying one or two mutant alleles (e14.5 to 18 months), leveraging histology, electron microscopy, protein analysis, RNA-seq, and spatial transcriptomics. We have imputed the variant in the GTEx transcriptome dataset to compare findings in human and mouse pancreas.
Results: Ctrb1 exon6-del mice express a truncated CTRB1 that accumulates in the ER as biomolecular condensates and in cytoplasmic inclusions, associated with reduced chymotrypsin activity and total protein synthesis. We find dramatic ER dilation (i.e., ER stress) and transcriptomic changes starting at 3 months. Contrary to expectations, we observe an organ adaptation to the effects of the mutant allele along the time course. This adaptation is evidenced by mild histological alterations upon aging and a modest evolution of ER stress and inflammation: activity of ER stress pathways peaks at 6 months and subsequently declines while inflammatory pathways peak at 3 months. These changes are associated with sustained attenuation of the acinar differentiation program. We also find a mosaic pattern of ER stress and an increased activity of pathways related to cellular plasticity. The findings in Ctrb1 exon6-del mice recapitulate the genetic defect occurring in humans: the pancreas of subjects from the GTEx dataset carrying imputed CTRB2 exon6-del alleles shows an upregulation of ER stress-related pathways and an enrichment in the signature specific for Ctrb1 exon6-delpancreata.
Conclusions: This Ctrb1 exon6-del strain provides strong evidence supporting a causal nature of the exon6 deletion identified as a functional PDAC GWAS risk variant and reveals novel mechanisms of homeostatic acinar adaptation. These studies should contribute to translate the knowledge from the mouse model to humans and offer insights into PDAC pathogenesis, as well as potential preventive approaches.
