Mutations in skeletal muscle

Abstract

Mutations in skeletal muscle sodium channels (NaV1.4) cause periodic paralysis. Paramyotonia congenita and hyperkalemic periodic paralysis are caused by gain-of-function mutations spread widely through the protein, which increase channel activity and lead to repetitive firing or depolarization block. In contrast, mutations that cause hypokalemic (HypoPP) and normokalemic (NormoPP) periodic paralysis are localized in the outermost three gating-charge-carrying arginine residues (R1-R3) in the S4 segment in domain II, and they do not have major effects on sodium channel function as typically measured. Site-directed mutations of these residues cause gating pore current, a voltage-gated leak current through the voltage sensor (Sokolov et al., 2005); mutations of R1 and R2 cause gating pore current in the resting state, whereas mutation of R3 causes gating pore current in the activated state. Similar studies of the HypoPP mutant R2G revealed gating pore current of approximately 1% of peak current at the resting membrane potential, which was decreased by depolarization (Sokolov et al., 2007). This gating pore current was selective for Cs>K>Na and blocked by mM concentrations of divalent cations, Zn>Ba>Ca. A gating pore current of similar size was observed in the resting state for the HypoPP mutants R1H and R2H, but this current is selective for protons. In contrast to HypoPP, the mutations that cause NormoPP are in R3 (R3G/Q/W). All of these mutations cause gating pore conductance for sodium in the activated and slow-inactivated states, in which the voltage sensors are in their outward position. The common pathogenic feature of these mutations is likely to be depolarization and sodium overload, which are observed in patient biopsies. Dominant gain-of-function pathogenic effects may arise directly from excess sodium entry for R2G and R3G/Q/W and indirectly from excessive Na-H exchange for R1H and R2H.