Early-onset diabetes with low utilization of lipid as an energy source carrying a rare missense mutation in the CEL gene.

Summary: Carboxyl ester lipase (CEL) is a major component of pancreatic juice and is responsible for the duodenal hydrolysis of cholesteryl esters. Maturity-onset diabetes of the young (MODY) is a form of diabetes mellitus characterized by early onset and dominant inheritance of beta-cell dysfunction. CEL gene mutations cause the type of MODY denoted as MODY8. Herein, we describe a Japanese patient who harbored a heterozygous A689P mutation in the variable number of tandem repeats (VNTRs)-containing exon 11 of the CEL gene. The patient was not obese and his diabetes was characterized by onset in late adolescence, impaired insulin secretion and metabolic dysfunction-associated steatotic liver disease (MASLD). The C-terminal region of CEL has been postulated to be critical for its secretion and activity. Therefore, the A689P mutation may cause pancreatic exocrine insufficiency and eventually contribute to MASLD, which is associated with reduced lipid catabolism. MODY8 is also considered to be a protein-misfolding disease because a heterozygous single nucleotide deletion causes the production of mutant CEL protein leading to diabetes and exocrine dysfunction. In the present case, MASLD and diabetes characterized by impaired insulin secretion were observed. The CEL A689P missense mutation will expand the known genotype-phenotype correlation in diabetes if it can be demonstrated that the variant is pathogenic. Learning points: The CEL gene encodes the digestive enzyme carboxyl ester lipase, also known as bile salt-stimulated/dependent lipase. CEL is expressed in pancreatic acinar tissue but not in pancreatic β cells. MODY caused by mutations in the CEL gene (MODY8) is characterized by dominantly inherited diabetes mellitus manifesting in early adulthood. A classical feature of MODY8 is pancreatic exocrine dysfunction, often with onset in childhood. Known pathogenic mutations in the CEL gene affect the variable number tandem repeat (VNTR) region in exon 11. Our case suggests that some missense mutations of the CEL VNTR could have a phenotypic implication by being associated with impaired glucose-stimulated insulin secretion and reduced utilization of lipid as an energy source which leads to MASLD.