A Japanese family of autosomal dominant hypokalemic periodic paralysis with a CACNL1A3 gene mutation

Autonomously functioning thyroid nodule associated with thyrotoxic periodic paralysis

Thyrotoxic periodic paralysis (TPP) is mainly associated with Graves' disease but rarely with autonomously functioning thyroid nodule (AFTN). We herein report a case of AFTN associated with TPP in which the latter resolved after (131) I therapy for the former. We analyzed the genes encoding thyrotropin receptor (TSHR), the alpha-subunit of the stimulatory G protein (Gsalpha), calcium channel CACNA1S and potassium channel KCNE3, and found that the patient does not carry the known mutations in these genes. Whereas the pathogenesis of TPP and AFTN remains to be understood, the present case suggests that ion channel defects responsible for familial hypokalemic periodic paralysis may not be associated with TPP, and that mutations in TSHR and Gs alpha genes may be less frequent in AFTN patients in the Japanese population.

Periodic paralysis

Periodic paralyses are rare diseases characterized by severe episodes of muscle weakness concomitant to variations in blood potassium levels. It is thus usual to differentiate hypokalemic, normokalemic, and hyperkalemic periodic paralysis. Except for thyrotoxic hypokalemic periodic paralysis and periodic paralyses secondary to permanent changes of blood potassium levels, all of these diseases are of genetic origin, transmitted with an autosomal-dominant mode of inheritance. Periodic paralyses are channelopathies, that is, diseases caused by mutations in genes encoding ion channels. The culprit genes encode for potassium, calcium, and sodium channels. Mutations of the potassium and calcium channel genes cause periodic paralysis of the same type (Andersen-Tawil syndrome or hypokalemic periodic paralysis). In contrast, distinct mutations in the muscle sodium channel gene are responsible for all different types of periodic paralyses (hyper-, normo-, and hypokalemic). The physiological consequences of the mutations have been studied by patch-clamp techniques and electromyography (EMG). Globally speaking, ion channel mutations modify the cycle of muscle membrane excitability which results in a loss of function (paralysis). Clinical physiological studies using EMG have shown a good correlation between symptoms and EMG parameters, enabling the description of patterns that greatly enhance molecular diagnosis accuracy. The understanding of the genetics and pathophysiology of periodic paralysis has contributed to refine and rationalize therapeutic intervention and will be without doubts the basis of further advances.

Hypokalemic periodic paralysis: a model for a clinical and research approach to a rare disorder

Rare diseases have attracted little attention in the past from physicians and researchers. The situation has recently changed for several reasons. First, patient associations have successfully advocated their cause to institutions and governments. They were able to argue that, taken together, rare diseases affect approximately 10% of the population in developed countries. Second, almost 80% of rare diseases are of genetic origin. Advances in genetics have enabled the identification of the causative genes. Unprecedented financial support has been dedicated to research on rare diseases, as well as to the development of referral centers aimed at improving the quality of care. This expenditure of resources is justified by the experience in cystic fibrosis, which demonstrated that improved care delivered by specialized referral centers resulted in a dramatic increase of life expectancy. Moreover, clinical referral centers offer the unique possibility of developing high quality clinical research studies, not otherwise possible because of the geographic dispersion of patients. This is the case in France where national referral centers for rare diseases were created, including one for muscle channelopathies. The aim of this center is to develop appropriate care, clinical research, and teaching on periodic paralysis and myotonia. In this review, we plan to demonstrate how research has improved our knowledge of hypokalemic periodic paralysis and the way we evaluate, advise, and treat patients. We also advocate for the establishment of international collaborations, which are mandatory for the follow-up of cohorts and conduct of definitive therapeutic trials in rare diseases.

A family of hypokalemic periodic paralysis with CACNA1S gene mutation showing incomplete penetrance in women

Familial hypokalemic periodic paralysis is an autosomal dominant genetic muscle disease characterized by periodic attacks of muscle weakness associated with a decrease in serum potassium. There are two major missense mutation sites in the calcium channel alpha1 subunit (CACNA1S) gene in these patients. We recently encountered a 13-year-old Japanese boy who had collapsed following exercise and was found to have a low serum potassium level. Clinical and genetic studies including exploration of his family tree proved that he and his maternal relatives had the disease with the missense mutation, Arg528His (CGC --> CAC). However, his mother and grandmother had no symptoms of the disease, indicating reduced penetrance in female carriers. Sexual difference in the penetrance of this disease and the association between the clinical symptoms and the types of genetic defects are discussed.

Muscle weakness in a Japanese family of Arg1239His mutation hypokalemic periodic paralysis

Familial hypokalemic periodic paralysis is an autosomal dominant disorder. Recently, three predominant mutations were found in the muscle dihydropyridine-sensitive calcium channel alpha 1-subunit gene. We present a Japanese family that displays one of these, the Arg1239His mutation. All the affected individuals of this family displayed this mutation. Two cases had a history of recurrent episodes of muscle weakness and difficulty in running before their first paralytic attack. It is suggested that there are no racial differences with this mutation, and that mild muscle weakness occurs not only after but also preceding the first attack.