Hypokalaemic periodic paralysis with a charge-retaining substitution in the voltage sensor.

Familial hypokalaemic periodic paralysis is a rare skeletal muscle disease caused by the dysregulation of sarcolemmal excitability. Hypokalaemic periodic paralysis is characterized by repeated episodes of paralytic attacks with hypokalaemia, and several variants in CACNA1S coding for CaV1.1 and SCN4A coding for NaV1.4 have been established as causative mutations. Most of the mutations are substitutions to a non-charged residue, from the positively charged arginine (R) in transmembrane segment 4 (S4) of a voltage sensor in either CaV1.1 or NaV1.4. Mutant channels have aberrant leak currents called 'gating pore currents', and the widely accepted consensus is that this current is the essential pathological mechanism that produces susceptibility to anomalous depolarization and failure of muscle excitability during a paralytic attack. Here, we have identified five hypokalaemic periodic paralysis cases from two different ethnic backgrounds, Japanese and French, with charge-preserving substitutions in S4 from arginine, R, to lysine, K. An R to K substitution has not previously been reported for any other hypokalaemic periodic paralysis families. One case is R219K in NaV1.4, which is located at the first charge in S4 of Domain I. The other four cases all have R897K in CaV1.1, which is located at the first charge in S4 of Domain III. Gating pore currents were not detected in expression studies of CaV1.1-R897K. NaV1.4-R219K mutant channels revealed a distinct, but small, gating pore current. Simulation studies indicated that the small-amplitude gating pore current conducted by NaV1.4-R219K is not likely to be sufficient to be a risk factor for depolarization-induced paralytic attacks. Our rare cases with typical hypokalaemic periodic paralysis phenotypes do not fit the canonical view that the essential defect in hypokalaemic periodic paralysis mutant channels is the gating pore current and raise the possibility that hypokalaemic periodic paralysis pathogenesis might be heterogeneous and diverse.

Mutation spectrum and health status in skeletal muscle channelopathies in Japan.

Skeletal muscle channelopathies, including non-dystrophic myotonia and periodic paralysis, are rare hereditary disorders caused by mutations of various ion channel genes. To define the frequency of associated mutations of skeletal muscle channelopathies in Japan, clinical and genetic data of two academic institutions, which provides genetic analysis service, were reviewed. Of 105 unrelated pedigrees genetically confirmed, 66 pedigrees were non-dystrophic myotonias [CLCN1 (n = 30) and SCN4A (n = 36)], 11 were hyperkalemic periodic paralysis (SCN4A), and 28 were hypokalemic periodic paralysis [CACNA1S (n = 16) and SCN4A (n = 12)]. Of the 30 families with myotonia congenita, dominant form (Thomsen type) consisted 67%, and unique mutations, A298T, P480T, T539A, and M560T, not found in Western countries, were commonly identified in CLCN1. Hypokalemic periodic paralysis caused by SCN4A mutations consisted 43% in Japan, which was much higher than previous reports. Furthermore, the quality of life of the patients was assessed using the patient-reported outcome measures, SF-36 and INQoL, for 41 patients. This study indicated that the etiology of skeletal muscle channelopathies in Japan was not identical to previous reports from Western countries, and provided crucial information for genetics as well as future therapeutic interventions.

Analysis of the genetic background associated with sporadic periodic paralysis in Japanese patients.

Periodic paralysis (PP) is a rare disease caused by abnormal excitability of the sarcolemma, resulting in the episodic weakness in extremities. Two major subtypes have been identified: primary/familial PP showing Mendelian inheritance of a mutation in the ion channel genes expressed in skeletal muscle, and secondary/sporadic PP which does not show Mendelian inheritance. Thyrotoxic periodic paralysis (TPP) contributes to the majority of secondary PP cases in Asians and Latin Americans, suggesting that genetic factors may underlie the pathogenesis. In contrast, sporadic periodic paralysis (SPP) has no familial history and no secondary factors. The genetic features associated with SPP in Japanese patients remain unexplored. Here, we investigate whether nine single nucleotide variants (SNVs), rs623011, rs312691, rs393743, rs312692, rs312736, rs992072, rs312732, rs723498, and rs312707, found in TPP and/or SPP in other Asian populations are also associated with Japanese SPP cases. The study cohort included 43 Japanese periodic paralysis patients with no mutations in causative genes (SCN4A, CACNA1S, and KCNJ2), no myotonia, and with euthyroid function. The results showed disease susceptibility for all nine SNVs in our Japanese SPP cohort. One of them, rs312691, was newly confirmed to show susceptibility to SPP. Our results suggest the genetic background underlies periodic paralysis.

The expanding phenotype of hypokalemic periodic paralysis in a Japanese family with p.Val876Glu mutation in CACNA1S.

BACKGROUND: Hypokalemic periodic paralysis (HypoPP) is an autosomal dominant disease characterized by the episodic weakness of skeletal muscles and hypokalemia. More than half patients with HypoPP carry mutations in CACNA1S, encoding alpha-1 subunit of calcium channel. Few reports have documented the non-neuromuscular phenotypes of HypoPP. METHODS: The proband is a Japanese woman who developed HypoPP at 6 years of age. An excessive insulin secretion with the oral glucose tolerance test rationalized that she had experienced frequent attacks of paralysis on high-carbohydrate diets. RESULTS: Voglibose and acetazolamide effectively controlled her paralytic episodes. Her 8-year-old son and 2-year-old daughter started showing the paralytic symptoms from 4 and 2 years of age, respectively. Laboratory tests revealed high concentrations of creatinine kinase in serum and elevated renin activities in plasma of these children. The targeted sequencing confirmed that these three patients had an identical heterozygous mutation (p.V876E) in CACNA1S. CONCLUSION: Our data indicate that the p.V876E mutation in CACNA1S contributes to the early onset of neuromuscular symptoms and unusual clinical phenotypes of HypoPP.

Functional analysis of a double-point mutation in the KCNJ2 gene identified in a family with Andersen-Tawil syndrome.

Andersen-Tawil syndrome (ATS) is a skeletal muscle channelopathy with autosomal dominant inheritance resulting in periodic paralysis, arrhythmia characterized by QT prolongation, and dysmorphic features. The KCNJ2 gene has been identified as the causative gene of ATS. Herein, we reported 2 cases of a 21-year-old man and his mother, with episodic paralytic attacks and/or arrhythmia, which are characteristic of ATS. Both G144A, a reported ATS mutation, and V296F, a novel mutation, were identified in the KCNJ2 gene on the same allele from the proband and his mother, but not from his father. In the present study, we investigated the functional effect of these variants on the potassium channel Kir2.1 and the significance of the double mutation. G144A, V296F, and G144A-V296F mutant channels expressed in cultured cells revealed a loss-of-function effect of these mutations on Kir2.1. The K+ currents of G144A and G144A-V296F channels were more suppressed than that of V296F channel alone, whereas was no difference between G144A and G144A-V296F. To our knowledge, a double mutation in the KCNJ2 gene has not been reported previously. While either of 2 mutations potentially causes ATS, the G144A mutation might cause the dominant effect on the patients' clinical presentation.