Résumé
The pathomechanism of familial hypokalemic periodic paralysis
(HypoPP) is a mystery, despite knowledge of the underlying
dominant point mutations in the dihydropyridine receptor (DHPR) voltage
sensor. In five HypoPP families without DHPR gene defects, we
identified two mutations, Arg-672→His and →Gly, in the voltage
sensor of domain 2 of a different protein: the skeletal muscle sodium
channel α subunit, known to be responsible for hereditary muscle
diseases associated with myotonia. Excised skeletal muscle fibers from
a patient heterozygous for Arg-672→Gly displayed depolarization and
weakness in low-potassium extracellular solution. Slowing and smaller
size of action potentials were suggestive of excitability of the
wild-type channel population only. Heterologous expression of the two
sodium channel mutations revealed a 10-mV left shift of the
steady-state fast inactivation curve enhancing inactivation and a
sodium current density that was reduced even at potentials at which
inactivation was removed. Decreased current and small action potentials
suggested a low channel protein density. The alterations are decisive
for the pathogenesis of episodic muscle weakness by reducing the number
of excitable sodium channels particularly at sustained membrane
depolarization. The results prove that SCN4A, the gene encoding the
sodium channel α subunit of skeletal muscle is responsible for
HypoPP-2 which does not differ clinically from DHPR-HypoPP. HypoPP-2
represents a disease caused by enhanced channel inactivation and
current reduction showing no myotonia.