Genotype-phenotype correlations in human skeletal muscle sodium channel diseases

Reduced muscle-fiber conduction but normal slowing after cold exposure in paramyotonia congenita

In this study we investigated a family with paramyotonia (PC) congenita caused by a Gly1306Val mutation in the voltage-gated sodium-channel gene SCN4A. A previous study showed that exposure to cold aggravates the muscle stiffness in patients with this mutation. However, the mechanism behind cold sensitivity and the sodium-channel defect remained unclear. In order to gain a better understanding of sarcolemmal propagation in these patients, we measured muscle-fiber conduction velocity (MFCV) invasively. We studied four PC patients and four healthy subjects at room temperature. After the muscle was cooled, MFCV was measured again in the two PC patients and four control subjects. MFCV was significantly lower in the PC patients at room temperature, compatible with dysfunctional sodium channels. After cooling, MFCV was significantly lower in both groups as compared with room temperature. The relative slowing was 1.4% per degrees C for PC patients and 1.5% per degrees C for healthy subjects. These results indicate that, in these PC patients, mutant and wild-type sodium channels respond equally to cold exposure. Thus, MFCV is abnormal in these patients, but the aggravation of muscle stiffness cannot be explained by an abnormal sarcolemmal response to cold.

Paramyotonia congenita: genotype to phenotype correlations in two families and report of a new mutation in the sodium channel gene

Sodium channel disorders include hyperkalemic periodic paralysis (hyperPP), paramyotonia congenita (PC) and potassium-aggravated myotonia (PAM). PC is a myotonic syndrome characterized by cold-induced muscle stiffness and weakness. In this paper, we report two families. The first is affected by PC with cold-induced stiffness and no weakness, in addition to hyperPP. This family displays the Arg1448Cys mutation in the sodium channel gene originally described in pure PC families. The fact that families with the same mutation present distinct phenotypes indicates that other factors, genetic or environmental, may modulate the expression of the disease in sodium channel disorders. The second family was unusual because patients presented cold-induced weakness without stiffness. A mutation was found in the sodium channel gene that changed an isoleucine into a threonine at position 693. These two families demonstrate that sodium channel mutations may cause either cold-induced stiffness or weakness.