Novel chloride channel mutations leading to mild myotonia among Chinese
Introduction
Patients with myotonia congenita present with muscle stiffness, manifested after a voluntary contraction, which typically decreases with repetitive movement. The disorder is inherited in an autosomal dominant (Thomsen’s disease) or in a recessive way (Becker’s disease) [1]. Patients with the recessive disorder often have a more severe presentation, with weakness, in addition, when muscles are activated after a period of rest. A number of different mutations have been found in the chloride channel gene, ClC-1, and cases from families with autosomal dominant myotonia congenita are usually heterozygous for a mutated and a normal allele, while those in families with recessive myotonia congenita are homozygous or compound heterozygous. However, some mutations have been found to lead to autosomal dominant myotonia congenita in some families and to a homozygous recessive form in others. For example, two mutations leading to a similar molecular defect, a fs793X truncated protein, have been found to result in different types of inheritance. A 2330delG mutation has been found in a family with autosomal dominant myotonia congenita [2] and a 2264delC in another family with recessive inheritance [3]. A clear understanding of the genotype–phenotype correlation is still lacking [4]. In general, investigations of channel properties have established a diminished chloride conductance at the muscle cell membrane, which leads to a decrease in the threshold to generate action potentials and, consequently, to repetitive firing. This can be assessed by characteristic EMG activity, usually described as “mytonic runs“ that occur immediately following electrode insertion or following stimulated or voluntary muscle activity. Most of the patients reported so far are from Caucasian origin, and myotonia congenita has only recently and rarely been reported among Chinese [5], [2], with a recent review of the Chinese literature describing 196 cases in 29 reports [6]. By contrast, thyrotoxic periodic paralysis, a disorder related to periodic paralysis due to mutations in the muscle sodium channel gene, SNC4A, is much more frequent among Chinese as compared to Caucasians [7]. The reasons for this variability remain completely unknown, and further data are needed to understand ethnic differences related to genotype and phenotype correlations.
In the present report, we provide additional evidence for the role of CLCN1 mutations in myotonia congenita by demonstrating novel mutations in two unrelated families of Chinese origin. Furthermore, physiological analysis of these mutated channels expressed in Xenopus oocytes has allowed us to examine the effect of the mutations on chloride conductance.
Section snippets
Mutation search
Blood was taken from the affected persons and other members of their families after informed consent. The study was approved by the local Ethics Committees. DNA isolation and PCR amplification for all exons of CLCN1 was performed as previously described[8]. Published primers were used for the CLCN1 gene [9], [8]. PCR products were sequenced in an ABI automated DNA Sequencing System (PE Applied Biosystems, Foster City, CA, USA).
Site directed mutagenesis
Single- and double-point mutations were introduced into the cDNA of
Family 1
A 19-year-old Chinese man was referred with complaints of lower-limb pain of 7 years’ duration. Initially mild, the pain progressed to impair sport activities. He had bilateral calf and thigh cramping with tightness occurring when the patient ran, ascended stairs, or participated in sports such as distance running and basketball. These episodes sometimes caused him to fall. He noticed that sudden bursts of movements triggered the attacks, which lessened with activity and were not followed by
Discussion
The prevalence of myotonia congenita is quite variable among different populations, with a reported range of 0.2–0.9 per 100,000 inhabitants in Caucasian populations[13], [14] and with clustering in some regions, like northern Finland where a figure of 7.3 per 100,000 inhabitants has been found [15]. Exact figures about prevalence in Chinese populations are still lacking. We have performed a search in the Chinese literature from 1980 to 2007 and found clinical description of 235 cases with
Acknowledgement
This work has been supported by Agency for Science, Technology and Research (A*STAR), Singapore.
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