Effects of mutations causing hypokalaemic periodic paralysis on the skeletal muscle L-type Ca2+ channel expressed in Xenopus laevis oocytes.
1. A truncated form of the rabbit alpha1S Ca2+ channel subunit (alpha1SDeltaC) was expressed with the beta1b, alpha2delta and gamma auxiliary subunits in Xenopus laevis oocytes. After 5-7 days, skeletal muscle L-type currents were measured (469 +/- 48 nA in 10 mM Ba2+). All three of the auxiliary subunits were necessary to record significant L-type current. A rapidly inactivating, dihydropyridine-insensitive endogenous Ba2+ current was observed in oocytes expressing the auxiliary subunits without an exogenous alpha subunit. Expression of full-length alpha1S gave 10-fold smaller currents than the truncated form. 2. Three missense mutations causing hypokalaemic periodic paralysis (R528H in domain II S4 of the alpha1S subunit; R1239H and R1239G in domain IV S4) were introduced into alpha1SDeltaC and expressed in oocytes. L-type current was separated from the endogenous current by nimodipine subtraction. All three of the mutations reduced L-type current amplitude ( approximately 40 % for R528H, approximately 60-70 % for R1239H and R1239G). 3. The disease mutations altered the activation properties of L-type current. R528H shifted the G(V) curve approximately 5 mV to the left and modestly reduced the voltage dependence of the activation time constant, tauact. R1239H and R1239G shifted the G(V) curve approximately 5-10 mV to the right and dramatically slowed tauact at depolarized test potentials. 4. The voltage dependence of steady-state inactivation was not significantly altered by any of the disease mutations. 5. Wild-type and mutant L-type currents were also measured in the presence of (-)-Bay K8644, which boosted the amplitude approximately 5- to 7-fold. The effects of the mutations on the position of the G(V) curve and the voltage dependence of tauact were essentially the same as in the absence of agonist. Bay K-enhanced tail currents were slowed by R528H and accelerated by R1239H and R1239G. 6. We conclude that the domain IV mutations R1239H and R1239G have similar effects on the gating properties of the skeletal muscle L-type Ca2+ channel expressed in Xenopus oocytes, while the domain II mutation R528H has distinct effects. This result implies that the location of the substitutions is more important than their degree of conservation in determining their biophysical consequences. (+info)
Hypokalaemic paralysis is a relatively uncommon but potentially life-threatening clinical syndrome. If recognised and treated appropriately, patients recover without any clinical sequellae. The syndrome of hypokalaemic paralysis represents a heterogeneous group of disorders characterised clinically by hypokalaemia and acute systemic weakness. Most cases are due to familial or primary hypokalaemic periodic paralysis; sporadic cases are associated with numerous other conditions including barium poisoning, hyperthyroidism, renal disorders, certain endocrinopathies and gastrointestinal potassium losses. The age of onset, race, family history, medications, and underlying disease states can help in identifying the cause of hypokalaemic paralysis. Initial therapy of the patient with hypokalaemic paralysis includes potassium replacement and search for underlying aetiology. Further management depends on the aetiology of hypokalaemia, severity of symptoms, and duration of disease. This review presents the differential diagnosis for hypokalaemic paralysis and discusses management of the syndrome. (+info)
Voltage-sensor sodium channel mutations cause hypokalemic periodic paralysis type 2 by enhanced inactivation and reduced current.
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 alpha 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 alpha 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. (+info)
Clinical-molecular study of a family with essential tremor, late onset seizures and periodic paralysis.
We report the clinical features of, and the molecular study performed on, a Spanish family with essential tremor (ET), late onset epilepsy and autosomal dominant hypokalemic periodic paralysis (hypoPP). The presence of hypoPP in this kindred suggested an ion channel as a candidate gene for ET. Our study identified an Arg528His CACNL1A3 mutation in patients with hypoPP, and excluded this mutation as the cause of tremor or epilepsy in this kindred. (+info)
The human skeletal muscle Na channel mutation R669H associated with hypokalemic periodic paralysis enhances slow inactivation.
Missense mutations of the human skeletal muscle voltage-gated Na channel (hSkM1) underlie a variety of diseases, including hyperkalemic periodic paralysis (HyperPP), paramyotonia congenita, and potassium-aggravated myotonia. Another disorder of sarcolemmal excitability, hypokalemic periodic paralysis (HypoPP), which is usually caused by missense mutations of the S4 voltage sensors of the L-type Ca channel, was associated recently in one family with a mutation in the outermost arginine of the IIS4 voltage sensor (R669H) of hSkM1 (Bulman et al., 1999). Intriguingly, an arginine-to-histidine mutation at the homologous position in the L-type Ca(2+) channel (R528H) is a common cause of HypoPP. We have studied the gating properties of the hSkM1-R669H mutant Na channel experimentally in human embryonic kidney cells and found that it has no significant effects on activation or fast inactivation but does cause an enhancement of slow inactivation. R669H channels exhibit an approximately 10 mV hyperpolarized shift in the voltage dependence of slow inactivation and a twofold to fivefold prolongation of recovery after prolonged depolarization. In contrast, slow inactivation is often disrupted in HyperPP-associated Na channel mutants. These results demonstrate that, in R669H-associated HypoPP, enhanced slow inactivation does not preclude, and may contribute to, prolonged attacks of weakness and add support to previous evidence implicating the IIS4 voltage sensor in slow-inactivation gating. (+info)
Identification of mutations including de novo mutations in Korean patients with hypokalaemic periodic paralysis.
BACKGROUND: Hypokalaemic periodic paralysis (hypoPP) is an autosomal dominant disorder involving the abnormal function of ion channels and it is characterized by paralysis attacks of varying severity, accompanied by a fall in blood potassium levels. Linkage analysis showed that the candidate locus responsible for hypoPP was localized to chromosome 1q31-32, and this locus encoded the muscle dihydropyridine-sensitive calcium channel alpha(1)-subunit (CACNA1S). So far, three different mutations in CACNA1S gene have been identified in patients with hypoPP: Arg528His, Arg1239His and Arg1239Gly in Caucasian patients. However, there are few reports about the mutations of CACNA1S gene in other races. METHODS: In this study, four Korean families with five hypoPP patients were screened for mutations of CACNA1S gene with polymerase chain reaction-based restriction analysis and single-strand conformation polymorphism analysis. To determine the mode of inheritance, haplotype analysis was done with three microsatellite markers (D1S1726, CACNL1A3, and D1S1723). RESULTS: Arg528His mutation was detected in three families, and one family had no known mutations. Moreover, for the first time, we detected de novo Arg528His mutations in two out of three families with hypoPP. Haplotype analysis using three microsatellite markers (D1S1726, CACNL1A3, and D1S1723) suggested the occurrence of de novo Arg528His mutations in two of the three families with Arg528His mutation. CONCLUSIONS: Arg528His mutations of CACNA1S, including de novo Arg528His mutations, were found in Korean patients with hypoPP. These results imply that de novo mutation, in addition to non-penetrance, is one of the genetic mechanisms that can explain the previous clinical observation that hypoPP occurs sporadically without family history. (+info)
Hypokalaemic periodic paralysis type 2 caused by mutations at codon 672 in the muscle sodium channel gene SCN4A.
Hypokalaemic periodic paralysis (hypoPP) is an autosomal dominant muscle disorder characterized by episodic attacks of muscle weakness associated with a decrease in blood potassium levels. Mutations in the gene encoding the skeletal muscle voltage-gated calcium channel alpha-1 subunit (CACNL1A3) account for the majority of cases. Recently, mutations in the gene coding for the skeletal muscle voltage-gated sodium channel alpha subunit (SCN4A) have been reported in a small number of hypoPP families. In order to determine the relative frequency of the CANCL1A3 and SCN4A mutations in a large population of hypoPP patients, and to specify the clinical and pathological features associated with each of them, we searched for mutations in 58 independent hypoPP index cases. We detected the causative mutation in 45 cases: 40 were linked to the CACNL1A3 gene and five to the SCN4A gene. One mutation has not been described before. Some remarkable clinical features were observed in a large hypoPP family carrying an SCN4A mutation: a complete penetrance in men and women, an early age at onset, postcritic myalgias and an increased number and severity of attacks induced by acetazolamide. A muscle biopsy, performed in two members of this family, revealed a peculiar myopathy characterized by tubular aggregates. In contrast, vacuoles were predominant in muscles from hypoPP patients carrying CACNL1A3 mutations. Our findings point to the usefulness of a molecular characterization of hypoPP patients in clinical practice. They also provide new clues for understanding the mechanisms behind functional and structural alterations of the skeletal muscle in hypoPP. (+info)
Viral vector-mediated expression of K+ channels regulates electrical excitability in skeletal muscle.
Modification of K+ currents by exogenous gene expression may lead to therapeutic interventions in skeletal muscle diseases characterized by alterations in electrical excitability. In order to study the specific effects of increasing outward K+ currents, we expressed a modified voltage-dependent K+ channel in primary cultured rat skeletal muscle cells. The rat Kv1.4 channel was expressed as an N-terminal fusion protein containing a bioluminescent marker (green fluorescent protein). Transgene expression was carried out using the helper-dependent herpes simplex 1 amplicon system. Transduced myoballs, identified using fluorescein optics and studied electrophysiologically with single-cell patch clamp, exhibited a greater than two-fold increase in K+ conductance by 20-30 h after infection. This increase in K+ current led to a decrease in membrane resistance and a 10-fold increase in the current threshold for action potential generation. Electrical hyperexcitability induced by the Na+ channel toxin anemone toxin II (1 microM) was effectively counteracted by overexpression of Kv1.4 at 30-32 h after transduction. Thus, virally induced overexpression of a voltage-gated K+ channel in skeletal muscle has a powerful effect in reducing electrical excitability. (+info)