Defective fast inactivation recovery of Na_v1.4 in congenital myasthenic syndrome

Abstract

Objective To describe the unique phenotype and genetic findings in a 57-year-old female with a rare form of congenital myasthenic syndrome (CMS) associated with longstanding muscle fatigability, and to investigate the underlying pathophysiology. Methods We used whole-cell voltage clamping to compare the biophysical parameters of wild-type and Arg1457His-mutant Nav1.4. Results Clinical and neurophysiological evaluation revealed features consistent with CMS. Sequencing of candidate genes indicated no abnormalities. However, analysis of SCN4A, the gene encoding the skeletal muscle sodium channel Nav1.4, revealed a homozygous mutation predicting an arginine-to-histidine substitution at position 1457 (Arg1457His), which maps to the channel’s voltage sensor, specifically D4/S4. Whole-cell patch clamp studies revealed that the mutant required longer hyperpolarization to recover from fast inactivation, which produced a profound use-dependent current attenuation not seen in the wild type. The mutant channel also had a marked hyperpolarizing shift in its voltage dependence of inactivation as well as slowed inactivation kinetics. Interpretation We conclude that Arg1457His compromises muscle fiber excitability. The mutant fast-inactivates with significantly less depolarization, and it recovers only after extended hyperpolarization. The resulting enhancement in its use dependence reduces channel availability, which explains the patient’s muscle fatigability. Arg1457His offers molecular insight into a rare form of CMS precipitated by sodium channel inactivation defects. Given this channel’s involvement in other muscle disorders such as paramyotonia congenita and hyperkalemic periodic paralysis, our study exemplifies how variations within the same gene can give rise to multiple distinct dysfunctions and phenotypes, revealing residues important in basic channel function.

Congenital myopathy with “corona” fibres, selective muscle atrophy, and craniosynostosis associated with novel recessive mutations in SCN4A

“…Recessive mutations of SCN4A causing loss of function have been associated with rare cases of congenital myasthenic syndrome [3,6]. Recessive mutations which completely abolish channel function were reported to cause a neonatal lethal condition, however the combination of full and partial loss of function mutations were associated with a classical congenital myopathy phenotype [7].…”

“…These mutations in SCN4A can result in sustained muscle contraction, causing myotonia, or a prolonged refractory state with fast inactivation of the channel, yielding a periodic paralysis phenotype [4,5]. In rare cases recessive loss of function SCN4A mutations have been shown to cause a congenital myasthenic syndrome [3,6]. Recently, Zaharieva et al (2016) described a spectrum of phenotypes ranging from severe foetal hypokinesia to congenital myopathy, associated with loss of function of the channel due to recessive mutations in SCN4A in 11 patients from six unrelated families.…”

Channelopathies of Skeletal Muscle Excitability

“…At a resting potential of −85 mV, greater than 90% of the mutant channels would be inactivated, thereby effectively causing haploinsufficiency and a 50% reduction of the Na + current. In the second reported case, the proband was homozygous for R1457H and heterozygous carriers in the family were asymptomatic (8). Expression studies in HEK cells revealed −15 mV left shift of inactivation and a 10-fold slower rate of recovery from inactivation.…”

“…Two families have been described for which de novo mutations of Na V 1.4 caused a myasthenic pattern of weakness (8, 260). Patients had generalized weakness during brief exercise, attacks of weakness involving the face and soft palate, and a decremental CMAP response during repetitive stimulation at 10 Hz and 50 Hz.…”

Sodium Channelopathies of Skeletal Muscle

“…This combination of effects suggested a defect in muscle excitability downstream from the neuromuscular junction, and sequencing of SCN4A revealed a missense mutation in the voltage sensor of domain IV. Increased awareness that mutations of Na V 1.4 may occur in patients with symptomatic overlap between myasthenia and periodic paralysis led to the identification of additional isolated CMS cases, each of which was associated with recessive inheritance and homozygous missense mutations at arginines in the S4 of domain IV (Arnold et al, 2015; Habbout et al, 2016). …”