Lack of sodium channel mutation in an Italian family with paramyotonia congenita

Objective: To conduct the genotype-phenotype correlation in a family in whi ch several individuals share clinical and electrophysiologic features of pa ramyotonia congenita (PC). Background: PC, hyperkalemic periodic paralysis (HyperPP), and potassium-aggravated myotonias form the group of hereditary sodium channelopathies. Each of these disorders is associated with differen t point mutations in SCN4A, the gene encoding the a-subunit of the adult hu man skeletal muscle sodium channel. However, in HyperPP families, evidence of a causative gene different from SCN4A has been found. Methods: We conduc ted direct clinical examination, electrophysiologic (EMG/electroneurographi c) and cardiologic studies, as well as laboratory screening in several affe cted and nonaffected members of the family. We performed the genotype-pheno type correlation by microsatellite linkage and cDNA-mutation analyses of th e SCN4A gene. Results: Affected members in this family showed clinical and electrophysiologic features typical of PC. The disease phenotype segregated with the chromosomal region that includes the SCN4A gene. Analysis of the entire cDNA sequence of the SCN4A gene in the index case disclosed a G3826A transition, which results in the Val1276Ile substitution. However, PCR-sin gle-stranded confirmation polymorphism and direct sequencing analysis of th e segment coding for Val-1276 on genomic DNA confirmed the G3826A transitio n in the index case but was negative in 11 affected members of the family; however, neither mutations nor aberrant splicings causative of the PC pheno type in this family were found on SCN4A. Conclusion: The existence of a sec ond gene different from SCN4A that can give rise to a clinical PC phenotype can be speculated upon.