The position of single-base deletions in the VNTR sequence of the carboxyl ester lipase (CEL) gene determines proteotoxicity.

Abstract Variable number of tandem repeat (VNTR) sequences in the genome can have functional consequences that contribute to human disease. This is the case for the CEL gene, which is specifically expressed in pancreatic acinar cells and encodes the digestive enzyme carboxyl ester lipase. Rare single-base deletions (DELs) within the first (DEL1) or fourth (DEL4) VNTR segment of CEL cause MODY8, an inherited disorder characterized by exocrine pancreatic dysfunction and diabetes. Studies on the DEL1 variant have suggested that MODY8 is initiated by CEL protein misfolding and aggregation. However, it is unclear how the position of single-base deletions within the CEL VNTR affects pathogenic properties of the protein. Here, we investigated four naturally occurring CEL variants, arising from single-base deletions in different VNTR segments (DEL1, DEL4, DEL9, DEL13). When the four variants were expressed in human embryonic kidney 293 (HEK293) cells, only DEL1 and DEL4 led to significantly reduced secretion, increased intracellular aggregation and increased endoplasmic reticulum (ER) stress compared to normal CEL protein. The level of O-glycosylation was affected in all deletion variants. Moreover, all variants had enzymatic activity comparable to that of normal CEL. We conclude that the longest aberrant protein tails, resulting from single-base deletions in the proximal VNTR segments, have highest pathogenic potential, explaining why DEL1 and DEL4 but not DEL9 and DEL13 have been observed in MODY8 patients. These findings further support the view that CEL mutations cause pancreatic disease through protein misfolding and proteotoxicity, leading to ER stress and activation of the unfolded protein response.