Phenotypic variation of a Thr704Met mutation in skeletal sodium channel gene in a family with paralysis periodica paramyotonica

Abstract

Objectives-Patients with paralysis periodica paramyotonica exhibit st clinical syndrome with characteristics of both hyperkalaemic periodic paralysis and paramyotonia congenita. In several types of periodic paralysis associated with. hyperkalaemia. mutations in the skeletal muscle sodium channel (SCN4A) gene la ve been previously reported, Phenotypic variations of mutations in SCN4A, however, have not been described Set. The present study aimed to evaluate genetic variations in a family with clinical and electrosphysiological characteristics of paralysis periodica paramyotonia. Methods-Seven members of a family affected with symptoms of paralysis periodica paramyotonia were studied by electrophysiological and genetic analyses, There were increased serum potassium concentrations in four members during paralytic attacks induced by hyperkalaemic periodic paralysis provocation tests. Short exercise tests before and after cold immersion were carried out in four patients to distinguish electrophysiological characteristics of hyperkalaemic periodic paralysis and paramyotonia. Sequencing analyses of SCN4A were performed an one patient and a normal control to identify polymorphisms, Restriction fragment length polymorphism (RFLP) analysis was then performed at the identified polymorphic sites. Results-Electrophysiological studies showed both exercise sensitivity and temperature sensitivity. Compound motor action potential (CMAP) amplitudes were decreased (7.3%-25.6%) after short exercise tests. The CMAP amplitudes were even more severely decreased (21.7%-56.5%) in short exercise tests after cold exposure. Three polymorphic sites, Gln371Glu, Thr704Met, and Asp1376Asn were identified in SCN4A. BP;LP analyses showed that all affected patients carried the Thr704Met mutation, whereas unaffected family members and a normal control did not. Conclusion- Phenotypic variation of the Thr784Met mutation, which was previously reported in patients with hyperkalaemic periodic paralysis, is described in a family affected with paralysis periodica paramyotonia.