Neurology India Medknow Publications on behalf of the Neurological Society of India ISSN: 0028-3886 EISSN: 1998-4022 Vol. 58, Num. 5, 2010, pp. 743-746

Neurology India, Vol. 58, No. 5, September-October, 2010, pp. 743-746

Brief Report

Novel chloride channel gene mutations in two unrelated Chinese families with myotonia congenita

Feng Gao, Fu Chan Ma, Zhe Feng Yuan, Cui Wei Yang, Hai Feng Li, Zhe Zhi Xia, Quan Xiang Shui, Ke Wen Jiang

Department of Neurology, The Children's Hospital of Zhejiang University School of Medicine, Hangzhou, China

Correspondence Address:
Zhe Feng Yuan
57 Zhugan Xiang, Hangzhou 310003, Zhejiang Province
China
yuanzhefeng@sina.com

Date of Acceptance: 13-Jul-2010

Code Number: ni10203

PMID: 21045501

DOI: 10.4103/0028-3886.72163

Abstract

Keywords: Chinese, chloride channel gene mutations, myotonia congenita

Introduction

Myotonia congenita (MC) is a genetic myopathy and includes Thomsen disease (autosomal dominant disorder) and Becker disease (autosomal recessive disorder). ^[1] Intial studies on the pathogenesis of MC suggested that the skeletal muscle chloride channel gene (CLCN1) as a strong candidate gene. Subsequent studies proved that mutations in the CLCN1 gene responsible for the two types of human MC. ^[2],[3] In humans, the CLCN1 gene has been localized to chromosome 7q35, and it contains 23 exons that encode 988 amino acids. ^[1] Human skeletal muscle chloride channel (CLC-1) is a homodimer with a unique double-barreled molecular structure, it is composed of 18 transmembrane helices. To date, nearly 130 different CLCN1 mutations have been reported, ^[1] and the mutation spectrum is further expanding. ^[4] These mutations involved 23 exons and most of them are recessive. Thomsen disease is caused by heterozygous mutations, whereas Becker disease by homozygous or compound heterozygous mutations. There is no clear information regarding the relationship between the genotype and clinical phenotype. ^[5] Fialho et al.^[6] obtained information on more than 300 MC patients and found that exon 8 was a mutational hotspot in Thomsen disease. Most of the MC mutations reported to date have been identified in Caucasian families, and very few mutations have been identified in the Chinese population. ^[7],[8],[9] In this study, we identified two new CLCN1 mutation sites (A298T and M560T) and one known mutation site (Y261C) in two Chinese families with MC, thus adding further data to the CLCN1 mutation spectrum.

Materials and Methods

Clinical data

Family 1: Proband 1, a 6.5-year-old girl had been experiencing muscle myotonia and stiffness since five years. These symptoms used to get aggravated under cold conditions and relieved on warming up or after repeated movement. She had mild hypertrophy of the limb muscles, especially in the forearm and gastrocnemius muscle and also percussion myotonia. Serum creatine kinase (CK), electrolytes, and calcium were normal and acetylcholine receptor antibodies were negative. Electromyograpy (EMG) revealed bursts of myotonia. Similar symptoms were observed in the family, grandfather, father, and aunt. They were subsequently proven genetically to have the same type of MC [Figure - 1]a.

Family 2: Proband 2 a 8-year-old girl had been experiencing stiffness since six months. The stiffness was characterized by a slow pace at the commencement of motor activities. The symptoms used to be aggravated under cold conditions or when the patient experienced nervousness, and used to get relieved on warming up or after repeated movement. She had mild limb-muscle hypertrophy, especially in the muscles of the forearm, gastrocnemius, and triangular muscle and also percussion myotonia. Serum CK, electrolytes, and calcium were normal and acetylcholine receptor antibodies were negative. The EMG showed myotonic potentials. The patient's father had mild symptoms of myotonia, which gradually disappeared as he grew older. At present, his neurological examination is normal. Her mother, however, had no similar history, [Figure - 2]a.

Control:

To assess the possible occurrence of polymorphisms in any detected nucleotide substitutions, we analysed 100 healthy children with the same ethnic background as control, coming from the Children's Hospital of Zhejiang University School of Medicine. The study was approved by the Ethics Committee of the hospital.

Genetic analysis

After obtaining informed consent, a standard DNA extraction method was used to extract genomic DNA from vein blood samples of members of families 1 and 2 and 100 normal controls. Polymerase chain reaction (PCR) and DNA sequencing were used to analyze the mutations in the CLCN1 encoding region. The primers used in the PCR and DNA sequencing of all the 23 exons of the CLCN1 gene were designed by appropriately modifying known primers mentioned in other studies. ^[10],[11] The PCR reaction mixture (total volume, 50 μl) contained 100 ng genomic DNA, 1.5 mM MgCl₂ , 200 μM dNTPs, 20 pmol each of the forward and reverse primers, and 2.5UEx Taq enzyme (TaKaRa) in the reaction buffer. After amplification, the PCR products were separated on 1.5% agarose gel for one hour at 100 V and sequenced by using an ABI3730 system (Bio Basic Inc, Shanghai, China). Exons that showed a mutation in the first round of sequencing were amplified and sequenced again to exclude the mismatching caused by the low fidelity of Taq DNA polymerase. Repeated sequencing was performed to confirm the mutations.

Results

Direct sequencing of the 23 exons of the CLCN1 gene revealed a heterozygous G-to-A transition at nucleotide 892 of exon 8 in affected members of family 1; this mutation resulted in the conversion of alanine to threonine at amino acid position 298 (A298T). However, no such mutations were observed in the controls [Figure - 1]b. DNA sequencing of the members of family 2 revealed compound heterozygous mutations in the genome of the proband 2; these mutations included a heterozygous A-to-G transition at nucleotide 782 in exon 7 that resulted in the conversion of tyrosine to cysteine at amino acid position 261 (Y261C) for one allele, and a heterozygous T-to-C transition at nucleotide 1679 in exon 15 that resulted in the conversion of methionine to threonine at amino acid position 560 (M560T) for another allele. The proband's father and mother had the heterozygous mutations M560T and Y261C. No such mutations were found in the 100 controls. In addition, a known polymorphic mutation D718D was also identified [Figure - 2]b.

Discussion

Incidence studies show a higher incidence of the recessive-type MC disease than the dominant-type. In a study of 142 families of MC in West Germany, dominant-type accounted for 19% and recessive-type for 73%. ^[6] Similarly, a dominant inheritance pattern was found in only 20% in a study of approximately 130 different mutations of CLCN1. ^[1] In contrast, in the Chinese literature most of the reported cases (86.1%) had autosomal dominant inheritance pattern. ^[12] The differences in the incidence rates of the two types of MC in the Chinese and Caucasians may be attributable to the racial differences.

Reports on gene mutations in MC families of Chinese population are rare and even not known in mainland China. We identified two new CLCN1 mutation sites. In the studies on family 1, we found a new heterozygous mutation site A298T in proband 1 and other members of the family. This mutation is a heterozygous G-to-A transition at nucleotide 892 in exon 8, which leads to the conversion of alanine to threonine at amino acid position 298. A298T is located at the junction of H-I helix of the CLC-1, which is the interface of the chloride channel dimer; this junction is recognized as a predilection site of dominant mutations ^[13] and a hotspot region of dominant MC mutations. ^[6] A dominant mutation F297S has been found in the region adjacent to A298T. ^[6] The mutation can drastically shift the channel voltage dependence to more positive voltages through a dominant negative effect (the negative impact of the mutated subunit on the WT subunit), thereby increasing the half-activated voltage (V1/2) in the heteromeric mutant/WT complexes; this phenomenon can significantly reduce CLC-1's open probability and chloride ions conductance at physiological voltages, and thus increase the excitability of the membrane. All three generations of family 1 showed a dominant inheritance pattern of the disease, and genetic studies confirmed that all the patients had an A298T mutation; however, the 100 normal controls did not show similar changes. Thus, we suggest that the A298T mutation is similar to the F297S mutation and can cause myotonia by causing CLC-1 dysfunction through a dominant negative effect. However, the exact impact of the mutation needs to be confirmed by future studies on function.

In family 2, we found a compound heterozygous mutations (Y261C and M560T) in proband 2. The proband's father and mother had the heterozygous mutations M560T and Y261C. The M560T mutation, which has not been reported in China or in any developed countires, is located on the Q helix of the CLC-1. The neighboring mutations include I556N, V563I, and A566T; these mutations have different degrees of impact on channel function. All these mutations exhibit recessive inheritance. The I556N mutation is characterized by incomplete penetrance. ^[14] The V563I mutation is located in a highly conserved amino acid region, and has an important effect on the pore and the fast gate. ^[15] Patients with the A566T mutation show mild to moderate myotonia. ^[6] The Y261C mutation, located on the G helix of CLC-1, was first reported by Mailander et al.^[16] The patient had a compound heterozygous mutations (Y150C and Y261C) and typical symptoms of myotonia. The patient's mother (Y261C) had no clinical symptoms, and a muscle EMG did not indicate myotonia. An EMG examination of the patient's father (Y150C) indicated myotonia, although there were no clinical symptoms; these finding suggested potential myotonia. Wollnik et al. ^[17] used the Xenopus expression system to study these mutations and found that the Y261C mutation had little impact on CLC-1 function, thereby indicating the presence of other mechanisms that are not completely represented by the Xenopus expression system. Proband 2 (Y261C and M560T) showed typical myotonia; the proband's father (M560T) had shown mild symptoms of myotonia in his teens and the mother (Y261C) had no clinical manifestations. This finding suggested that the MC in this family was characterized by a recessive pattern of inheritance, and that the M560T mutation showed incomplete penetrance. Besides the mutation of Y261C, the proband 2 had mutation in M560T. The combined effect of these mutations had a major impact on the functioning of CLC-1, thus resulting in the typical clinical symptoms of myotonia. However, the exact mechanism underlying the occurrence of myotonia needs to be investigated future. In addition, a known polymorphic site D718D was also found in family 2.

References

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