Two new mutations in the CEL gene causing diabetes and hereditary pancreatitis: How to correctly identify MODY8 cases

Objective Maturity-onset diabetes of the young, type 8 (MODY8) is associated with mutations in the CEL gene, which encodes the digestive enzyme carboxyl ester lipase. Several diabetes cases and families have in recent years been attributed to mutations in CEL without any functional or clinical evidence provided. To facilitate correct MODY8 diagnostics, we screened two cohorts of diabetes patients and delineated the phenotype. Research design Young, lean Swedish and Finnish patients with a diagnosis of type 2 diabetes (352 cases, 406 controls) were screened for mutations in the CEL gene. We also screened 58 Czech MODY cases who had tested negative for common MODY genes. For CEL mutation-positive subjects, family history was recorded, and clinical investigations and pancreatic imaging performed. Results One Swedish and one Czech case with germline mutation in CEL were identified. Clinical and radiological investigations of these two probands and their families revealed dominantly inherited insulin-dependent diabetes, pancreatic exocrine dysfunction and atrophic pancreas with lipomatosis and cysts. Notably, hereditary pancreatitis was the predominant phenotype in one pedigree. Both families carried single-base pair deletions in the proximal part of the CEL variable number of tandem repeat (VNTR) region in exon 11. The mutations are predicted to lead to aberrant protein tails that make the CEL protein susceptible to aggregation. Conclusions The diagnosis of MODY8 requires a pancreatic exocrine phenotype and a deletion in the CEL VNTR in addition to dominantly inherited diabetes. CEL screening may be warranted also in families with hereditary pancreatitis of unknown genetic etiology.

Evolutionary expansion of poly-aspartate motif in the activation peptide of mouse cationic trypsinogen limits autoactivation and protects against pancreatitis

The activation peptide of mammalian trypsinogens typically contains a tetra-aspartate motif (positions P2-P5 in Schechter-Berger numbering) that inhibits autoactivation and facilitates activation by enteropeptidase. This evolutionary mechanism protects the pancreas from premature trypsinogen activation while allowing physiological activation in the gut lumen. Inborn mutations that disrupt the tetra-aspartate motif cause hereditary pancreatitis in humans. A subset of trypsinogen orthologs, including the mouse cationic trypsinogen (isoform T7), harbor an extended penta-aspartate motif (P2-P6) in their activation peptide. Here, we demonstrate that deletion of the extra P6 aspartate residue (D23del) increased autoactivation of T7 trypsinogen 3-fold. Mutagenesis of the P6 position in wild-type T7 trypsinogen revealed that bulky hydrophobic side-chains are preferred for maximal autoactivation and deletion-induced shift of the P7 Leu to P6 explains the autoactivation increase in the D23del mutant. Accordingly, removal of the P6 Leu by N-terminal truncation with chymotrypsin C reduced autoactivation of the D23del mutant. Homozygous T7D23del mice carrying the D23del mutation did not develop spontaneous pancreatitis and severity of cerulein-induced acute pancreatitis was comparable to that of C57BL/6N controls. However, sustained stimulation with cerulein resulted in markedly increased histological damage in T7D23del mice relative to C57BL/6N mice. Furthermore, when the T7D23del allele was crossed to a chymotrypsin-deficient background, the double-mutant mice developed spontaneous pancreatitis at an early age. Taken together, the observations argue that evolutionary expansion of the poly-aspartate motif in mouse cationic trypsinogen contributes to the natural defenses against pancreatitis and validate the role of the P6 position in autoactivation control of mammalian trypsinogens.