Key result
Four different SCN4A mutations, including a novel p.R225W mutation, were identified in six Korean patients exhibiting diverse clinical phenotypes such as pure myotonia, paramyotonia congenita, and hyperkalemic periodic paralysis.
Observational (n=6)
SCN4A mutations exhibit significant clinical diversity, and mutational analysis of both SCN4A and CLCN1 is essential for differentiating sodium-channel myotonia from myotonia congenita.
May support SCN4A testing with CLCN1 for myotonia differentiation; extends mutation spectrum in Koreans but remains hypothesis-generating.
BACKGROUND AND PURPOSE: Mutations of the skeletal muscle sodium channel gene SCN4A, which is located on chromosome 17q23-25, are associated with various neuromuscular disorders that are labeled collectively as skeletal muscle sodium channelopathy. These disorders include hyperkalemic periodic paralysis (HYPP), hypokalemic periodic paralysis, paramyotonia congenita (PMC), potassium-aggravated myotonia, and congenital myasthenic syndrome. This study analyzed the clinical and mutational spectra of skeletal muscle sodium channelopathy in Korean subjects. METHODS: Six unrelated Korean patients with periodic paralysis or nondystrophic myotonia associated with SCN4A mutations were included in the study. For the mutational analysis of SCN4A, we performed a full sequence analysis of the gene using the patients' DNA. We also analyzed the patients' clinical history, physical findings, laboratory tests, and responses to treatment. RESULTS: We identified four different mutations (one of which was novel) in all of the patients examined. The novel heterozygous missense mutation, p.R225W, was found in one patient with mild nonpainful myotonia. Our patients exhibited various clinical phenotypes: pure myotonia in four, and PMC in one, and HYPP in one. The four patients with pure myotonia were initially diagnosed as having myotonia congenita (MC), but a previous analysis revealed no CLCN1 mutation. CONCLUSIONS: Clinical differentiating between sodium-channel myotonia (SCM) and MC is not easy, and it is suggested that a mutational analysis of both SCN4A and CLCN1 is essential for the differential diagnosis of SCM and MC.
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Lee et al. (2009) conducted an observational in Skeletal muscle sodium channelopathy (n=6). SCN4A mutations was evaluated on SCN4A mutation spectrum and clinical phenotypes. Four different SCN4A mutations, including a novel p.R225W mutation, were identified in six Korean patients exhibiting diverse clinical phenotypes such as pure myotonia, paramyotonia congenita, and hyperkalemic periodic paralysis.
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