Characterization of a NaV1.4 Hypokalemic Periodic Paralysis Mutation in Domain I

Abstract

We characterized the clinical effects and biophysical properties of hypokalemic periodic paralysis mutation, R222W, initially identified in Korean patients. The heterozygous mutation was also found in a 52 year-old patient with weakness beginning at age 18 and associated with mild hypokalemia, and associated with myalgia. R222W muscle fibers exhibited action potentials with attenuated height, and slowed rate of rise compared to those from a control patient. Membrane potential recordings in normal potassium showed that R222W fibers exhibited a greater density of depolarized potentials (P2) compared to controls. R222W was functionally characterized with cut-open oocyte voltage clamp recordings at 20 °C. The mutation produced a depolarizing shift of the activation midpoint; this effect was shown to be indirect by comparing wild type and R222W channels in the IFM/QQQ inactivation deficient background. R222W produced several effects on inactivation consistent with hypoexcitability. Compared to wild type hNaV1.4, R222W produced a left-shift of the steady-state fast inactivation curve, accelerated entry, and slowed recovery. R222W also produced more complete slow inactivation and slowed its recovery. At hyperpolarized voltages, R222W promoted a cationic omega current, which may explain the paradoxical depolarization of muscle fibers. This work was supported by NIH 1R15NS093579-01A1 to JRG, NIH NIGMS P20GM103498 to ISU, the non-profit Else-Kroner-Fresenius Foundation, German DGM Muscle Disease Society, Taro Pharmaceuticals, and the German BMBF Ministry for IonNeurOnet project jointly to KJR and FLH.