The recurrent attacks of weakness in hypokalemic periodic paralysis (HypoPP) are caused by failure to maintain the resting potential, with paradoxical depolarization in low K+. Remarkably, 24 out of 25 HypoPP mutations are R/X substitutions in S4 segments of voltage-sensing domains of CaV1.1 (70% of cases) or NaV1.4 (10% of cases). Expression studies in oocytes and murine muscle show anomalous gating pore leakage currents (ω-pore) for six of eight CaV1.1-HypoPP mutations, with one exception being the charge-conserving R897K. The proposed consensus pathomechanism, whereby a gating pore leak predisposes to paradoxical depolarization in low K+, is now verified by continuous recording of Vm. Selective measurement of voltage-dependent Ca2+ release, in “healthy appearing” HypoPP fibers, shows only a modest decrease in the Ca2+-dependent peak fluorescence (Oregon green 488/EGTA), and supports the notion that stabilizing Vrest will be sufficient to prevent low-K+–induced loss of force. In our knockin mouse models of HypoPP (CaV1.1-R528H and NaV1.4-R669H), pretreatment with K+-channel openers protects against the loss of force with a 2 mM K+ challenge. Alternatively, gene editing offers the possibility of sustained protection from attacks of weakness, and may prevent the late-onset permanent myopathy. In a proof-of-principle study of cultured myoblasts and in vivo electroporation, we show selective editing of the mutant HypoPP allele, without compromise of the WT allele, using CRISPR/Cas-mediated indel formation to destroy the HypoPP allele or a CRISPR/Cas base editor to correct the missense mutation.
This work is supported by the National Institutes of Health (grants AR063182 and AR078198), and the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA Research Award Program.