The only calcium channel mutation reported to date is a deletion in the gene for the DHP-receptor alpha 1-subunit resulting in neonatal death in muscular dysgenesis mice (1). In humans, this gene maps to chromosome 1q31-32. An autosomal dominant muscle disease, hypokalemic periodic paralysis (HypoPP), has been mapped to the same region (2). Sequencing of cDNA of two patients revealed a G-to-A base exchange of nucleotide 1583 predicting a substitution of histidine for arginine528. This affects the outermost positive charge in the transmembrane segment IIS4 that is considered to participate in voltage sensing. By restriction fragment analysis, the mutation was detected in the affected members of 9 out of 25 HypoPP families. The results indicate that the DHP-receptor alpha 1-subunit mutation causes HypoPP. An altered excitation-contraction coupling may explain the occurrence of muscle weakness.

SUMMARY1. Three families with a form of myotonia (muscle stiffness due to membrane hyperexcitability) clinically distinct from previously classified myotonias were examined. The severity of the disease greatly differed among the families.2. Three dominant point mutations were discovered at the same nucleotide position of the SCN4A gene encoding the adult skeletal muscle Na+ channel asubunit. They predict the substitution of either glutamic acid, valine or alanine for glycine1306, a highly conserved residue within the supposed inactivation gate.Additional SCN4A mutations were excluded.3. Electrophysiological studies were performed on biopsied muscle specimens obtained for each mutation. Patch clamp recordings on sarcolemmal blebs revealed an increase in the time constant of fast Na+ channel inactivation, Th, and in late channel openings as compared to normal controls. Th was increased from 1-2 to 1-6-2-1 ms and the average late currents from 04 to 1-6 % of the peak early current.4. Intracellular recordings on resealed fibre segments revealed an abnormal tetrodotoxin-sensitive steady-state inward current, and repetitive action potentials. Since K+ and Cl-conductances were normal, only the increase in the number of non-inactivating Na+ channels has to be responsible for the membrane hyperexcitability.5. Length, ramification and charge of the side-chains of the substitutions correlated well with the Na+ channel dysfunction and the severity of myotonia, with alanine as the most benign and glutamic acid as the substitution with a major steric effect.6. Our electrophysiological and molecular genetic studies strongly suggest that these Na+ channel mutations cause myotonia. The naturally occurring mutants allowed us to gain further insight into the mechanism of Na+ channel inactivation. ¶ To whom correspondence should be addressed.MS 2411

We describe three families with a dominantly inherited disorder. Affected individuals have myotonia, proximal muscle weakness, and cataracts. There was no abnormal CTG repeat expansion of the myotonic dystrophy (DM) gene in DNA from blood and muscle. The structure of the three families permitted linkage analysis, and there is no linkage to the gene loci for DM or to the loci for the muscle chloride channel disorders or muscle sodium channel disorders. The collection of symptoms in these three families seems to represent a new disorder.

Hypokalaemic periodic paralysis (HypoPP) is an autosomal dominant muscle disease thought to arise from an abnormal function of ion channels. Performing a genome-wide search using polymorphic dinucleotide repeats, we have localized the HypoPP locus in three families of different geographic origin to chromosome 1q31-32, by linkage analysis. Using an intragenic microsatellite, we also demonstrate that the gene encoding the muscle DHP-sensitive calcium channel alpha 1 subunit (CACNL1A3) maps to the same region, sharing a 5 centiMorgan (cM) interval with the HypoPP locus. Moreover, CACNL1A3 co-segregates with HypoPP without recombinants in the two informative families, and is therefore a good candidate for the HypoPP gene.

1. Wild type (WT) and V1589M channels were expressed in human embryonic kidney (HEK293) cells for the study of the pathophysiology of the V1589M muscle Na+ channel mutation leading to K+-aggravated myotonia. 2. In comparison to WI, whole-cell recordings with V1589M channels showed an increased Na+ steady-state to peak current ratio (I/Ipeak) (3-15 + 0 70 vs. 0-87 + 0410%, at -15 mV) and a significantly faster recovery from inactivation. The recovery time constants, Trl and Tr2, were decreased from 1-28 + 0-12 to 0-92 + 0-08 ms and from 4-74 + 0 94 to 2-66 + 0-51 ms for the WT and mutant channels, respectively.3. Single-channel recordings with mutant channels showed higher probability of short isolated late openings (0 40 + 0 09 vs. 0-06 + 0-02, at -30 mV) and bursts of late openings (0-011 + 0 003 vs. 0 003 + 0 001, at -30 mV) compared to VVW.4. These results suggest that the mutation increases the probabilities for channel transitions from the inactivated to the closed and the opened states. 5. Increased extracellular concentrations of K+ had no effects on either V1589M or WI currents in HEK293 cells. The aggravation of myotonia seen in patients during increased serum K+ may arise from the associated membrane depolarization which favours the occurence of late openings in the mutant channel.