Carboxypeptidase A6 mutations and epilepsy

Abstract

Carboxypeptidase A6 (CPA6) is a protease that removes C-terminal hydrophobic residues from peptides and proteins. The CPA6 gene is expressed in the central nervous system of vertebrates, including humans. It is translated with a prodomain, which is removed by a furin-like cleavage before secretion. The active form of CPA6 binds tightly to the extracellular matrix (ECM) In the present studies, mutations in the CPA6 gene have been identified in patients with temporal lobe epilepsy, febrile seizures and juvenile myoclonic epilepsy with generalized epilepsy. The present studies identified 6 rare deleterious mutations in CPA6 in 10 patients with febrile seizures and/or epilepsy, and are the first association between CPA6 mutations and these disorders. To assess structural consequences of the amino acid substitutions, these mutants and others found in the SNP database were modeled within the predicted structure of the enzyme. To examine the effects of these mutations on enzyme expression and activity we expressed the mutated enzymes in human embryonic kidney 293T cells. These analyses revealed that the mutations associated with epilepsy generally had deleterious effects on the amount or activity of CPA6 in the ECM, indicating that ECM CPA6 activity is required for protection from epilepsy. Further, these mutations are most likely loss of function. All but one of the mutant genes identified in the heterozygous condition produced no active CPA6 when expressed in human embryonic kidney 293T cells, while the mutation found in the homozygous condition reduced ECM levels of CPA6 to 40% of WT. Taken together, these results confirm that these mutations are deleterious to the protein and provide a substantial body of evidence for the involvement of CPA6 mutations in the predisposition to several types of epilepsy, as well as febrile seizures. In order to model CPA6 deficiency in animals, attempted to create a knockout mouse. However, this animal model was not successful. CPA6 is a novel epilepsy gene, unlike other genes studied in relation to seizures. Future studies should be aimed at identifying CPA6 substrates and their effects on excitability in the central nervous system.