An atypical phenotype of hypokalemic periodic paralysis caused by a mutation in the sodium channel gene SCN4A

Novel compound heterozygous mutations in SCN4A as a potential genetic cause contributing to myopathic manifestations: A case report and literature review

Background

SCN4A mutations account for a diverse array of clinical manifestations, encompassing periodic paralysis, myotonia, and newly recognized symptoms like classical congenital myopathy or congenital myasthenic syndromes. We describe the initial occurrence of myopathic features mimic with recessive classical CM in a Korean infant presenting with novel compound heterozygous SCN4A mutations. The infant exhibited profound hypotonia after birth, thereby expanding the spectrum of SCN4A-related channelopathy.

Methods

The genetic analyses comprised targeted exome sequencing, employing a Celemics G-Mendeliome DES Panel, along with Sanger sequencing.

Results

Considering the clinical manifestations observed in the proband, SCN4A variants emerged as the primary contenders for autosomal recessive (AR) congenital myopathy 22a, classic (#620351). Sanger sequencing validated the association of SCN4A variants with the phenotype, affirming the AR nature of the compound heterozygous variants in both the carrier mother (c.3533G > T/p.Gly1178Val) and the father (c.4216G > A/p.Ala1406Thr).

Conclusion

Our report emphasizes the association of novel compound heterozygous mutations in SCN4A with myopathic features resembling CM, as supporting by muscle biopsy. It is essential to note that pathogenic SCN4A LoF mutations are exceedingly rare. This study contributes to our understanding of SCN4A mutations and their role in myopathic features mimic with classical CM.

Ramen noodle neuropathy: an atypical case of partial paralysis from malnutrition

INTRODUCTION

Due to a COVID-related job loss resulting in financial and food insecurity, a 28-year-old woman initiated a diet consisting solely of one cup of ramen noodles daily for twenty-two months, leading to 27 kg of weight loss. Ramen noodles are low in calories and lack key nutrients, including potassium, chloride, and vitamin B12.

CASE DESCRIPTION

The patient presented to the emergency department with acute, worsening weakness and paresthesias in her left wrist and hand. Exam revealed no other abnormalities aside from a cachectic appearance. Labs revealed marked hypokalemia, hypochloremia, lactic acidosis, a mixed metabolic alkalosis with respiratory acidosis, and low levels of zinc and copper. An EKG revealed a prolonged QT interval. After a neurology and psychiatry consult, the patient was admitted for failure to thrive with malnutrition, peripheral neuropathy, hypokalemia, and an acid-base disorder. An MRI of the brain was unremarkable. Studies of other nutritional deficiencies, autoimmune conditions, and sexually transmitted infections were unremarkable. The patient received food and vitamin supplementation, was monitored for re-feeding syndrome, and had a significant recovery.

DISCUSSION

After stroke, spinal injury, multiple sclerosis, and the most common focal mononeuropathies were ruled out, the clinical focus turned to nutritional deficiencies, the most significant of which was hypokalemia. Prior research has shown that severe hypokalemia can lead to weakness. It has also shown that chronically insufficient dietary intake is a common cause of hypokalemia. This case, with its partial paralysis of a unilateral upper extremity, may add to the known clinical manifestations of hypokalemia. We review the role of hypokalemia and hypochloremia in acid-base dynamics. Etiologies and clinical manifestations of cobalamin, thiamine, pyridoxine, and copper deficiencies, along with lead toxicity, are also discussed. Diagnostic clarity of mononeuropathies in the context of malnutrition and hypokalemia can be aided by urine potassium levels prior to repletion, neuroimaging that includes the cervical spine, and follow-up electromyography.

Prevalence and risk factors of low vitamin D levels in children and adolescents with familial hypokalemic periodic paralysis

Patients with familial hypokalemic periodic paralysis (HOKPP) experience episodes of reversible immobility and are at an increased risk of limited sunlight exposure, potentially leading to vitamin D deficiency. However, there is a lack of data on vitamin D levels in this population. We investigated serum vitamin D levels and their associated factors in children with HOKPP. This study included 170 genetically-confirmed children with HOKPP, aged 3-18 years, and 170 age-, sex-, and body mass index (BMI)-matched healthy controls from the Korean Channelopathy Study, a prospective controlled investigation. Anthropometric and clinical characteristics were recorded, and serum levels of calcium, ionized calcium, phosphorus, alkaline phosphatase, 25-hydroxyvitamin D, and intact parathyroid hormone (PTH) were analyzed. Vitamin D deficiency (< 20 ng/mL) was observed in 87.0% of the patients compared to 45.5% of the controls (P < 0.05) during the summer-fall season. During the winter-spring season, 91.7% of the patients and 73.4% of the controls were deficient (P < 0.05). A strong positive correlation was found between onset age of the first paralytic attack and vitamin D levels (r = 0.78, P < 0.01). Conversely, the frequency and duration of paralytic attacks were negatively correlated with vitamin D levels (r = -0.82 and r = -0.65, P < 0.01, respectively). Age, BMI, age at onset, frequency and duration of attacks, and PTH levels were independently associated with vitamin D levels (ß = -0.10, -0.12, 0.19, -0.27, -0.21, and -0.13, P < 0.05, respectively).

CONCLUSIONS

Vitamin D deficiency was highly prevalent in children with HOKPP, and vitamin D levels correlated with various disease characteristics. We recommend routine screening for vitamin D levels in these patients to address this prevalent deficiency. Considering the high prevalence of vitamin D deficiency observed, further research on other diseases characterized by reversible immobility is warranted.

WHAT IS KNOWN

• A correlation between immobility and low serum vitamin D levels has been established. However, the vitamin D status of patients with familial hypokalemic periodic paralysis (HOKPP) who experience periods of reversible immobility remains unknown.

WHAT IS NEW

• Vitamin D deficiency was highly prevalent in children with HOKPP, and vitamin D levels correlated with various disease characteristics.

NaV1.4 DI-S4 periodic paralysis mutation R222W enhances inactivation and promotes leak current to attenuate action potentials and depolarize muscle fibers

Hypokalemic periodic paralysis is a skeletal muscle disease characterized by episodic weakness associated with low serum potassium. We compared clinical and biophysical effects of R222W, the first hNaV1.4 domain I mutation linked to this disease. R222W patients exhibited a higher density of fibers with depolarized resting membrane potentials and produced action potentials that were attenuated compared to controls. Functional characterization of the R222W mutation in heterologous expression included the inactivation deficient IFM/QQQ background to isolate activation. R222W decreased sodium current and slowed activation without affecting probability. Consistent with the phenotype of muscle weakness, R222W shifted fast inactivation to hyperpolarized potentials, promoted more rapid entry, and slowed recovery. R222W increased the extent of slow inactivation and slowed its recovery. A two-compartment skeletal muscle fiber model revealed that defects in fast inactivation sufficiently explain action potential attenuation in patients. Molecular dynamics simulations showed that R222W disrupted electrostatic interactions within the gating pore, supporting the observation that R222W promotes omega current at hyperpolarized potentials. Sodium channel inactivation defects produced by R222W are the primary driver of skeletal muscle fiber action potential attenuation, while hyperpolarization-induced omega current produced by that mutation promotes muscle fiber depolarization.

N1366S mutation of human skeletal muscle sodium channel causes paramyotonia congenita

KEY POINTS

Paramyotonia congenita is a hereditary channelopathy caused by missense mutations in the SCN4A gene, which encodes the α subunit of the human skeletal muscle voltage-gated sodium channel NaV1.4. Affected individuals suffered from myotonia and paralysis of muscles, which were aggravated by exposure to cold. We report a three-generation Chinese family with patients presenting paramyotonia congenita and identify a novel N1366S mutation of NaV1.4. Whole-cell electrophysiological recordings of the N1366S channel reveal a gain-of-function change of gating in response to cold. Modelling and molecular dynamic simulation data suggest that an arginine-to-serine substitution at position 1366 increases the distance from N1366 to R1454 and disrupts the hydrogen bond formed between them at low temperature. We demonstrate that N1366S is a disease-causing mutation and that the temperature-sensitive alteration of N1366S channel activity may be responsible for the pronounced paramyotonia congenita symptoms of these patients.

ABSTRACT

Paramyotonia congenita is an autosomal dominant skeletal muscle channelopathy caused by missense mutations in SCN4A, the gene encoding the α subunit of the human skeletal muscle voltage-gated sodium channel NaV1.4. We report a three-generation family in which six members present clinical symptoms of paramyotonia congenita characterized by a marked worsening of myotonia by cold and by the presence of clear episodes of paralysis. We identified a novel mutation in SCN4A (Asn1366Ser, N1366S) in all patients in the family but not in healthy relatives or in 500 normal control subjects. Functional analysis of the channel protein expressed in HEK293 cells by whole-cell patch clamp recording revealed that the N1366S mutation led to significant alterations in the gating process of the NaV1.4 channel. The N1366S mutant displayed a cold-induced hyperpolarizing shift in the voltage dependence of activation and a depolarizing shift in fast inactivation, as well as a reduced rate of fast inactivation and accelerated recovery from fast inactivation. In addition, homology modelling and molecular dynamic simulation of N1366S and wild-type NaV1.4 channels indicated that the arginine-to-serine substitution disrupted the hydrogen bond formed between N1366 and R1454. Together, our results suggest that N1366S is a gain-of-function mutation of NaV1.4 at low temperature and the mutation may be responsible for the clinical symptoms of paramyotonia congenita in the affected family and constitute a basis for studies into its pathogenesis.

Gating pore currents are defects in common with two Nav1.5 mutations in patients with mixed arrhythmias and dilated cardiomyopathy

Na_v1.5 channels bearing voltage-sensor domain mutations associated with atypical cardiac arrhythmias and dilated cardiomyopathy generate gating pore currents.

The gating pore current, also called omega current, consists of a cation leak through the typically nonconductive voltage-sensor domain (VSD) of voltage-gated ion channels. Although the study of gating pore currents has refined our knowledge of the structure and the function of voltage-gated ion channels, their implication in cardiac disorders has not been established. Two Na_v1.5 mutations (R222Q and R225W) located in the VSD are associated with atypical clinical phenotypes involving complex arrhythmias and dilated cardiomyopathy. Using the patch-clamp technique, in silico mutagenesis, and molecular dynamic simulations, we tested the hypothesis that these two mutations may generate gating pore currents, potentially accounting for their clinical phenotypes. Our findings suggest that the gating pore current generated by the R222Q and R225W mutations could constitute the underlying pathological mechanism that links Na_v1.5 VSD mutations with human cardiac arrhythmias and dilatation of cardiac chambers.

Mutation analysis of CACNA1S and SCN4A in patients with hypokalemic periodic paralysis

Mutations in CACNA1S (calcium channel, voltage‑dependent, L type, alpha 1S subunit) and SCN4A (sodium channel, voltage‑gated, type IV, alpha subunit) are associated with hypokalemic periodic paralysis (HPP). The aim of the current study was to investigate CACNA1S and SCN4A mutations in patients with HPP. Mutations in CACNA1S and SCN4A were detected in three familial hypokalemic periodic paralysis (FHPP) pedigrees and in two thyrotoxic hypokalemic periodic paralysis (THPP) pedigrees using polymerase chain reaction, DNA sequencing and sequence alignment with GenBank data. A single base mutation from cytosine to guanine at site 1582 was identified in exon 11 of CACNA1S in one FHPP pedigree, resulting in an arginine to glycine (R528G) substitution. A single base mutation from thymine to cytosine at site 2012 was identified in exon 12 of SCN4A in one THPP pedigree, resulting in a phenylalanine to serine (F671S) substitution. No mutations in CACNA1S or SCN4A were identified in the remaining three pedigrees. The present study indicated that CACNA1S and SCN4A mutations are relatively rare in patients with HPP, and further studies are required to determine whether these mutation‑associated substitutions are representative of patients with HPP.

The large-conductance calcium-activated potassium channel holds the key to the conundrum of familial hypokalemic periodic paralysis

Purpose

Familial hypokalemic periodic paralysis (HOKPP) is an autosomal dominant channelopathy characterized by episodic attacks of muscle weakness and hypokalemia. Mutations in the calcium channel gene, CACNA1S, or the sodium channel gene, SCN4A, have been found to be responsible for HOKPP; however, the mechanism that causes hypokalemia remains to be determined. The aim of this study was to improve the understanding of this mechanism by investigating the expression of calcium-activated potassium (K_Ca) channel genes in HOKPP patients.

Methods

We measured the intracellular calcium concentration with fura-2-acetoxymethyl ester in skeletal muscle cells of HOKPP patients and healthy individuals. We examined the mRNA and protein expression of KCa channel genes (KCNMA1, KCNN1, KCNN2, KCNN3, and KCNN4) in both cell types.

Results

Patient cells exhibited higher cytosolic calcium levels than normal cells. Quantitative reverse transcription polymerase chain reaction analysis showed that the mRNA levels of the K_Ca channel genes did not significantly differ between patient and normal cells. However, western blot analysis showed that protein levels of the KCNMA1 gene, which encodes K_Ca1.1 channels (also called big potassium channels), were significantly lower in the membrane fraction and higher in the cytosolic fraction of patient cells than normal cells. When patient cells were exposed to 50 mM potassium buffer, which was used to induce depolarization, the altered subcellular distribution of BK channels remained unchanged.

Conclusion

These findings suggest a novel mechanism for the development of hypokalemia and paralysis in HOKPP and demonstrate a connection between disease-associated mutations in calcium/sodium channels and pathogenic changes in nonmutant potassium channels.

Normokalemic periodic paralysis is not a distinct disease

INTRODUCTION

Recent molecular studies of the original cases of normokalemic periodic paralysis (normoKPP) have raised suspicions that these families actually had hyperkalemic periodic paralysis (hyperKPP) due to mutations in the skeletal muscle sodium channel gene SCN4A. However, there is still a debate about the existence of normoKPP.

METHODS

We screened 230 individuals with primary periodic paralysis for mutations in the SCN4A, CACNA1S, and KCNJ2 genes. All patients had either a hyperKPP or a hypoKPP phenotype, and none had a normoKPP phenotype.

RESULTS

In 4 hyperKPP patients from 2 families, molecular analyses revealed Arg675Gly and Arg675Gln mutations of SCN4A, which were previously reported to cause normoKPP. Each patient exhibited the characteristic clinical and laboratory features (including hyperkalemia during spontaneous attacks) of hyperKPP.

CONCLUSION

Our findings support the notion that normoKPP is not a distinct disease.

Cloning and expression of the two new variants of Nav1.5/SCN5A in rat brain

The α-subunit of tetrodotoxin-resistant (TTX-R) voltage-gated sodium channel (VSGC, Nav1.5/SCN5A) has been found from the rat heart and human neuroblastoma cell line NB-1, but its expression in rat brain has not been identified radically. In this study, a reverse transcriptase-polymerase chain reaction was used to clone the full sequence of Nav1.5 (designated as rN1) α-subunit in rat brain and compared the distribution in different lobe of brain in different developmental stages. The open reading frame of rN1 encodes 2,016 amino acid residues and sequence analysis indicated that rN1 is highly homologous with 96.53% amino acids identity to rat cardiac Nav1.5 (rH1) and 96.13% amino acids identity to human neuroblastoma Nav1.5 (hNbR1). It has all the structural features of a VSGC and the presence of a cysteine residue (C373) in the pore loop region of domain I suggests that this channel is resistant to TTX. A new exon (exon6A) that is distinct from rH1 was found in DI-S3-S4, meanwhile an isomer of alternative splicing that deleted 53 amino acids (exon18) was found for the first time in domain DII-III in rN1. (designated as rN1-2). Distribution results demonstrated that rN1 expressed discrepancy in different ages and lobe in brain. The expression level of rN1 was gradually more stable in adult than in neonatal; these results suggest that rN1 has a newly identified exon for alternative splicing that is differentfrom rat heart and is more widely expressed in rat brain than previously thought.

Familial hyperkalemic periodic paralysis caused by a de novo mutation in the sodium channel gene SCN4A

Familial hyperkalemic periodic paralysis (HYPP) is an autosomaldominant channelopathy characterized by transient and recurrent episodes of paralysis with concomitant hyperkalemia. Mutations in the skeletal muscle voltage-gated sodium channel gene SCN4A have been reported to be responsible for this disease. Here, we report the case of a 16-year-old girl with HYPP whose mutational analysis revealed a heterozygous c.2111C>T substitution in the SCN4A gene leading to a Thr704Met mutation in the protein sequence. The parents were clinically unaffected and did not have a mutation in the SCN4A gene. A de novo SCN4A mutation for familial HYPP has not previously been reported. The patient did not respond to acetazolamide, but showed a marked improvement in paralytic symptoms upon treatment with hydrochlorothiazide. The findings in this case indicate that a de novo mutation needs to be considered when an isolated family member is found to have a HYPP phenotype.

Skeletal muscle na channel disorders

Five inherited human disorders affecting skeletal muscle contraction have been traced to mutations in the gene encoding the voltage-gated sodium channel Na_v1.4. The main symptoms of these disorders are myotonia or periodic paralysis caused by changes in skeletal muscle fiber excitability. Symptoms of these disorders vary from mild or latent disease to incapacitating or even death in severe cases. As new human sodium channel mutations corresponding to disease states become discovered, the importance of understanding the role of the sodium channel in skeletal muscle function and disease state grows.