The position of single-base deletions in the VNTR sequence of the carboxyl ester lipase (CEL) gene determines pathogenicity
Variable number of tandem repeat (VNTR) sequences present in the genome can have functional consequences that contribute to human disease. This is the case for the CEL gene, which encodes the digestive enzyme carboxyl ester lipase. CEL has a VNTR located in exon 11, and rare single-base deletions (DELs) within this region cause MODY8, an inherited disorder characterized by exocrine pancreatic dysfunction and diabetes. Here, we have studied how the position of single-base deletions within the CEL VNTR affects the proteins pathogenic properties. We investigated four naturally occurring CEL variants with single-base deletions in different VNTR segments (DEL1, DEL4, DEL9, DEL13), of which only DEL1 and DEL4 have been observed in MODY8 patients. When expressed in a cellular model system, only DEL1 and DEL4 exhibited significantly reduced secretion and increased intracellular aggregation compared to normal CEL. We found that all DEL variants had slightly decreased enzymatic activity and that their level of O-glycosylation was affected. Moreover, only DEL1 and DEL4 significantly increased endoplasmic reticulum (ER) stress. In conclusion, CEL single-base deletion variants have the highest pathogenic potential when the mutational event has taken place in the proximal VNTR part, resulting in the longest aberrant protein tails. Thus, DEL1 and DEL4 are pathogenic CEL variants, whereas we consider DEL13 as benign and DEL9 as likely benign. These findings have implications for our understanding of how CEL mutations cause pancreatic disease through protein misfolding and proteotoxicity, leading to ER stress and activation of the unfolded protein response.