Homozygote missense mutation, Thr221Met [c.662C>T], in two brothers from a consanguineous family. Mutation disrupts the protein's stability and impairs its normal function
Kinetic parameters
|
none
References:
Title: A novel mutation in PNLIP causes pancreatic triglyceride lipase deficiency through protein misfolding Szabo A, Xiao X, Haughney M, Spector A, Sahin-Toth M, Lowe ME Ref: Biochimica & Biophysica Acta, 1852:1372, 2015 : PubMed
Congenital pancreatic triglyceride lipase (PNLIP) deficiency is a rare disorder with uncertain genetic background as most cases were described before gene sequencing was readily available. Recently, two brothers with PNLIP deficiency were found to carry a homozygous missense mutation, c.662C>T (p.T221M) in the PNLIP gene (J. Lipid Res. 2014. 55:307-312). Molecular modeling suggested the substitution would change the orientation of residues in the catalytic site and disrupt the function of p.T221M PNLIP. To test the effect of the p.T221M mutation on PNLIP function, we expressed wild-type and p.T221M PNLIP in human embryonic kidney (HEK) 293A cells and dexamethasone-differentiated AR42J rat acinar cells. In both cellular models, wild-type PNLIP was secreted into the conditioned medium where it was readily detectable by protein staining, immunoblot or lipase activity assays. In contrast, mutant p.T221M was not secreted into the medium, but it was present in cell lysates where it accumulated in the insoluble fraction. Intracellular retention of mutant p.T221M resulted in endoplasmic reticulum (ER) stress as measured by elevated XBP1 splicing and increased levels of ER chaperones. Our results demonstrate that the presence of methionine at position 221 in the PNLIP protein sequence causes misfolding and aggregation of the p.T221M mutant inside the cell. The consequent loss of enzyme secretion adequately explains the clinical phenotype of PNLIP deficiency reported for homozygous carriers of p.T221M. Furthermore, the ability of mutant p.T221M to induce ER stress suggests that this form of PNLIP deficiency might cause acinar cell damage as well.
Congenital pancreatic lipase (PNLIP) deficiency is a rare monoenzymatic form of exocrine pancreatic failure characterized by decreased absorption of dietary fat and greasy voluminous stools, but apparent normal development and an overall good state of health. While considered to be an autosomal recessive state affecting a few dozens of individuals world-wide and involving the PNLIP gene, no causative mutations for this phenotype were so far reported. Here, we report the identification of the homozygote missense mutation, Thr221Met [c.662C>T], in two brothers from a consanguineous family of Arab ancestry. The observed genotypes among the family members were concordant with an autosomal recessive mode of inheritance but moreover a clear segregation between the genotype state and the serum PNLIP activity was evident. Based on biophysical computational tools, we suggest the mutation disrupts the protein's stability and impairs its normal function. Although the role of PNLIP is well established, our observations provide genetic evidence that PNLIP mutations are causative for this phenotype.