Abstract
Background
Myotonia congenita (MC) is mainly caused by variants in the CLCN1 Gene, which is characterized by having difficulty in relaxing the muscle after active contraction, known as myotonia. This study aims to investigate the clinical characteristics and gene mutations of myotonia congenita caused by CLCN1 mutation.
Case presentation
Five children with myotonia congenita due to CLCN1 mutations admitted to Nanjing Children’s Hospital were included. All five children had a juvenile onset of the disease (1 to 11 years of age). Two had onset before 2 years of age, and three had onset after 10 years of age. All patients experienced muscle stiffness (5/5, 100.0%), two reported delayed relaxation of the hand after forceful grasping (2/5, 40.0%), and three reported that the muscle stiffness worsened with changes in motor status (3/5, 60.0%). These symptoms improved with exercise (warm-up phenomenon) (5/5, 100.0%).Two children had elevated CK (2/5, 40.0%), and EMG showed muscle tonic potentials in all five children (5/5, 100.0%). Eight CLCN1 gene mutation sites were identified in five patients, including four unreported variants: c.688G > A (p.G230R), c.2653_c.2654insC (p.A885Afs*27), c.1938G > T (p.M646I) and c.1825 A > G (p.M609V). In this paper, we also summarized the Chinese CLCN1 mutation sites reported in the last 10 years, revealing that exons 8 and 15 may be the hotspot regions of mutation in Chinese children.
Conclusion
This study expands the clinical and genetic spectrum of Chinese children with myotonia congenita. The clinical manifestations observed in these children were similar with those previously reported in the literature. Additionally, exons 8 and 15 may be the hotspot regions for gene mutations in Chinese children with myotonia congentia.
Keywords: Myotonia congenita, CLCN1 gene, Thomsen, Becker
Background
Myotonia congenita (MC), also known as chloride channelopathic myotonia, is mainly caused by mutations in the chloride channel CLC-1 encoded by the CLCN1 gene located at 7q34. The chloride channel (CLC-1) is a regulator of membrane electrical excitability in skeletal muscle and plays a crucial role in the repolarisation and stabilisation of the membrane potential [1]. When the CLCN1 gene is mutated, the action potential generated during active muscle contraction is not able to revert back to the resting membrane potential in time via chloride-mediated transmembrane flow of chloride ions, leading to prolonged and sustained muscle contraction. This explains the basic feature of the disease: difficulty in relaxing the muscle after active contraction, known as myotonia. Myotonia is particularly noticeable at the beginning of exercise after a period of rest, and is reduced after repeated exercise or in warm environments, known as ‘warm-up phenomenon’ [2]. Patients may also exhibit significant muscle hypertrophy, giving them an ‘athlete’s’ appearance. In cold conditions, the symptoms tend to be more pronounced. Depending on the mode of inheritance, the disease can be classified into autosomal dominant-type Thomsen (TMC) (OMIM: 160800) and autosomal recessive-type Becker (BMC) (OMIM: 255700), which differ in their clinical phenotypes, such as age of onset and degree of myotonia. TMC typically has an earlier onset, often in infancy, with about 10% of patients presenting symptoms between 10–20 years of age. TMC starts earlier than BMC, mostly in infancy, and about 10% of patients start at 10–20 years of age. The clinical manifestations include widespread muscle tonus, usually involving the lower limbs. Compared with BMC, muscle stiffness is not obvious, and the prognosis is relatively good, with not obvious myotonia. BMC usually begins between 4–12 years of age, and does not progress until the age of 25–30 years. BMC usually manifests more severe phenotypes, particularly affecting the upper limbs and the face. Smooth muscle and cardiac muscle are not affected, but muscle tonus is more severe.
The prevalence of MC varies greatly between ethnic groups, ranging from 0.2–0.9/100,000 in Caucasian populations, to 7.5/100,000 in Finland [3]. Genetic studies of MC have shown a higher prevalence in BMC compared to TMC. In a German study that included 143 family lines, Thomsen patients accounted for 19% of the patients while Becker patients accounted for 73%. In another study it was also found that 20% of cases were dominantly inherited [3, 4].
There are 247 identified CLCN1 mutation loci, but the vast majority of studies have focused on Western white populations, with few reports of CLCN1 mutations in Chinese populations. In this study, we reported five Chinese patients with CLCN1-associated MC, revealing eight potentially pathogenic mutations (NM_000083), four of which are unreported and may be unique among Chinese (p.G230R, p.A885Afs*27, p.M646I, p.M609V). This further enriches the spectrum of CLCN1 mutations and refines the CLCN1 mutation information in the Chinese MC population, providing valuable reference for studying CLCN1 mutation types and inheritance modes in the Chinese population. It also highlights the need to explore the potential mechanisms of disease pathogenesis.
Case presentation
A total of five patients with MC, including three boys and two girls, with a median age of onset of myotonia of 10 years and a range of 1.0–11.0 years, were collected. The growth and development of the children were similar to that of children of the same age. The first symptom of all 5 children was muscle stiffness in the lower limbs, which manifested as limitation of movement when there was a change in motor status, such as difficulty in starting or standing up after sitting for a long period of time. Three patients reported that the muscle stiffness was aggravated by a change in motor status. Two children had difficulty in climbing stairs. Two children presented with sensory disturbances in the lower limbs, which could differentiate between hot and cold and relieved by several minutes of activity. One patient complained of lower limb weakness and abnormal gait. Two patients complained of lower limb weakness and gait abnormalities. Two patients reported that their lower limbs were not strong enough to move. Gait abnormalities were observed in two children. Two children presented with muscle stiffness in the upper limbs, delayed relaxation of the hand after grasping, inability to extend the fist immediately after clenching it with force, and maintenance of stiffness in the flexed position, which was alleviated by repetitive activity. Facial muscle involvement was observed in one child, with occasional jaw stiffness that resolved with activity. In all children, there was a ‘warm-up phenomenon’ and the symptoms were evident in a cold environment and diminished in a warm environment. All children had normal muscle strength and muscle tone, and one case had obvious muscle hypertrophy of both lower limbs. None of the five children had psychiatric symptoms, no eye diseases such as strabismus or diplopia, no urinary system diseases, and no digestive system diseases (Table 1).
Table 1.
Clinical data of the 5 patients
| Individual 1 | Individual 2 | Individual 3 | Individual 4 | Individual 5 | |
|---|---|---|---|---|---|
| Age at onset (gender) | 1y(female) | 10y(male) | 1y(male) | 11y(female) | 10y(male) |
| Warm-up | + | + | + | + | + |
| CK (U/L) | 254 | 397 | Normal | Normal | Normal |
| EMG | + | + | + | + | + |
| Medication | - | Oxcarbazepine | - | Oxcarbazepine | - |
Muscle MRI, electromyography (EMG), nerve conduction study (NCS) and serum creatine kinase (CK) were performed in all five children. No significant abnormality was found in the MRI of the lower limb muscles in all five children. The EMG examinations of the children all suggested myogenic damage, myoelectric changes with myotonic potentials. In child one, the NCS showed a mild decrease in CMAP wave amplitude of the left tibial motor nerve, and the EMG showed changes in muscle tonic potentials (Fig. 1). Two children had increased serum CK, 254 U/L and 397 U/L. The remaining three were normal.
Fig. 1.
1. NCS: Compared with the reference mean values of children of the same age, the CMAP wave amplitude of the left tibiomotor nerve was mildly reduced, and no obvious abnormality was found in the remaining motor and sensory nerves. 2. EMG: During relaxation of the examined muscles, the spontaneous potentials of some examined muscles were positive, and rhythmic discharges (positive-phase waves and short-range negative spikes) appeared in all of them when the needles were moved, and the amplitude and frequency of the waves showed a tendency to increase or decrease step by step. The wave amplitude and frequency tend to increase or decrease gradually. During light contraction, the MUP pattern of part of the examined muscle is narrow or partially narrow or individually narrow, with or without multiphase potentials and/or irregular waves increasing. Heavy contraction of the examined muscle is seen as mixed interference phase or interference mixed phase. Suggestion: myogenic damage myoelectric changes can be considered, accompanied by myotonic potentials
Child 2 and child 4 were treated with oxcarbazepine (at a dose of 150 mg bid), and the time taken to fully expand the fist after clenching (in seconds) was used as the standard for comparing pre- and post-treatment symptoms. It was found that both children’s symptoms were relieved by oxcarbazepine. The remaining three children were stabilised without medication. Through follow-up, we found that the symptoms of all five children were well-controlled, so we did not adjust their treatment.
After genetic testing, all five children had CLCN1 gene variants. Child 1 had c.1679T > C (p.M560T) and c.688G > A (p.G230R), with c.1679T > C being a missense mutation originating from her father and c.688G > A being a missense mutation originating from her mother. The parents were phenotypically normal. And, child 1 has a healthy elder brother. We have asked the brother for his birth and past history. There are no abnormalities or clinical manifestations associated with MC. Child 2 had c.2653_c.2654insC (p.A885Afs*27) and c.1938G > T (p.M646I). c.2653_c.2654insC was a missense mutation and originated from his father. c.1938G > T was also a missense mutation and originated from his mother, and both parents were phenotypically normal. Child 3 had c.1679T > C pure mutation (p.M560T), and both of his parents are carriers of this locus. Child 4 had c.1606G > A (p.V536I) and c.962T > A (p.V321E), but we were unable to determine genetic co-segregation because we did not have access to the DNA of the child’s parents. Child 5 had c.1012 C > T (p.R338*,651), c.1825 A > G (p.M609V). c.1825 A > G was a missense mutation originating from his father. c.1012 C > T was a nonsense mutation originating from his mother, and the parents were phenotypically normal (Fig. 2; Table 2). In family 3, the proband had a pure heterozygous mutation, and in family 1, 2, and 5, the proband was a compound heterozygous mutation. The parents of the probands in these four families were free of the disease, and they were carriers of the mutation (heterozygous), which was consistent with autosomal recessive inheritance. Therefore, these four families should be considered as having autosomal recessive Becker’s disease according to the inheritance pattern. The first witness in family 4 was compound heterozygous for the mutation. Although we were unable to obtain the DNA of the child 4’s parents, we hypothesised that the myotonia congenita suffered by this child was also autosomal recessive Becker disease, based on previous literature indicating that the two point mutations were recessive.
Fig. 2.
Pedigree and Sanger sequencing of four families. A Family 1, CLCN1 c.1679T > C (p.M560T) and c.688G > A (p.G230R) mutation. B Family 2, CLCN1 c.2653_c.2654insC (p.A885Afs*27) and c.1938G > T (p.M646I) mutation. C Family 3, CLCN1 c.1679T > C (p.M560T) mutation. D Family 5, CLCN1 c.1012 C > T (p.R338*,651) and c.1825 A > G (p.M609V) mutation
Table 2.
Mutation status and mode of inheritance of the five families
| Individual 1 | Individual 2 | Individual 3 | Individual 4 | Individual 5 | |
|---|---|---|---|---|---|
| Variant (GRCh37/hg19;NM_000083) |
c.688G > A (p.G230R); c.1679T > C (p.M560T) |
c.2653_c.2654insC(p.A885Afs* 27); c.1938G > T (p.M646I) |
c.1679T > C (p.M560T) |
c.1606G > A (p.V536I); c.962T > A (p.V321E) |
c.1012 C > T (p.R338*,651); c.1825 A > G (p.M609V) |
| Father | c.1679T > C (p.M560T) | c.2653_c.2654insC(p.A885Afs* 27); | c.1679T > C (p.M560T) | Unknown | c.1825 A > G (p.M609V) |
| Mother | c.688G > A (p.G230R); | c.1938G > T (p.M646I) | c.1679T > C (p.M560T) | Unknown | c.1012 C > T (p.R338*,651); |
| Inheritance | AR | AR | AR | Uncertain | AR |
AD: Autosomal dominant; AR: Autosomal recessive
CLCN1 mutation analysis revealed eight different mutations as six missense mutations, one nonsense mutation, and one shifted variant, four of which (p.G230R, p.A885Afs*27, p.M646I, p.M609V) were unreported. These four mutations were not reported in publications and public databases such as OMIM, UCSC, HGMD, dbSNP, 1000 Genome, ExAC, and gnomAD. We further analysed these four loci using UGENE (Fig. 3), showed that the new mutation sites are located in regions with high amino acid conservation, suggesting that mutations at these sites have a large impact on the function of the CLCN1 protein, and that children carrying this mutation have severe clinical symptoms, consistent with the phenotype of the children we found. We used Pymol software to predict the protein structure of the de novo point mutations p.G230R, p.M646I and p.M609V, and the results showed that p.G230R was mutated from Gly at position 230 to Arg, p.M646I was mutated from Met at position 646 to Ile, and p.M609V was mutated from Met at position 609 to Val. The mutations only changed the structure of individual amino acids and did not affect the hydrogen bonding strength of the corresponding sites (Fig. 4).
Fig. 3.
Conservativeness analysis of unreported mutant amino acids with surrounding sequences
Fig. 4.
Pymol predicts amino acid structure changes before and after mutations at p.G230R, p.M646I and p.M609V sites
Discussion and conclusions
Myotonia congenita is a rare skeletal muscle disorder caused by abnormalities of chloride channels, which was first reported by Bryant, Lipicky et al. in the 1960s [5, 6]. Currently, there are few reports on MC in China.
We summarized 63 case reports of CLCN1 published in the open literature base over the past 10 years. Combined with the five children reported in this article, a total of 68 Chinese children with MC were collected. There were 14 females and 54 males. According to the mode of inheritance and clinical manifestations, 29 cases could be classified as TMC children, and 39 cases could be classified as BMC children (Table 3). Most of the children had myotonia congenita as the first symptom, 92.6% (63/68) had lower limb involvement, 92.6% (63/68) had upper limb involvement, 25% (17/68) had facial muscle stiffness, 10.3% (7/68) had transient myotonia congenita, 2.9% (2/68) complained of myalgia, 2.9% (2/68) had sensory abnormalities. The ‘warm-up phenomenon’ was present in 97.1% (66/68) of the children, and symptoms were exacerbated by cold conditions in 48.5% (33/68) of the children. Creatine kinase was mostly normal or mildly elevated.
Table 3.
Summary of Chinese reported mutation loci in CLCN1 in the last 10 years
| Case | Sex | Age of onset | variant | Amino acid | Exon | Inheritance | Phenotype | References |
|---|---|---|---|---|---|---|---|---|
| 1 | M | 9 | c.1879 A > C | p.T627P | 16 | AD | Thomsen | He et al., 2024 [7] |
| 2 | M | 15 | c.-52 A > G | - | - | AD | Thomsen | |
| 3 | M | 13 | c.1205 C > T | p.A402V | 11 | AD | Thomsen | |
| 4 | M | 11 | c.364G > A | p.V122M | 3 | AD | Thomsen | |
| 5 | F | 15 | c.929 C > T | p.T310M | 8 | AD | Thomsen | |
| 6 | M | 13 |
c.962T > A; c.1505T > C |
p.V321E; p.M502T |
8; 14 |
AR | Becker | |
| 7 | M | 10 | c.2170G > T | p.E724* | 17 | AD | Thomsen | |
| 8 | M | 16 |
c.1258insC; c.1679T > C |
p.R421fs; p.M560T |
12; 15 |
AR | Becker | |
| 9 | M | 14 |
c.1205 C > T; c.1723 C > T. |
p.A402V; p.P575S. |
11; 15 |
AR | Becker | |
| 10 | M | 15 | c.1444 G > A | p.G482A | 13 | AD | Thomsen | |
| 11 | M | 8 | c.593T > C | p.L198P | 5 | AD | Thomsen | |
| 12 | F | 1.8 |
c.871G > A; c.1262insC |
p.E291K; p.R421Pfs*9 |
8; 12 |
AR | Becker | Li et al., 2022 [8] |
| 13 | F | 8 | c.1262insC | p.R421Pfs*9 | 12 | AD/AR | Becker | |
| 14 | M | 11 |
c.853 + 4 A > G; c.2527 C > T |
-; p.L843F |
-; 22 |
AR | Becker | |
| 15 | M | 5 | c.1657 A > T; c.2010_2014del | p.I553F; p.L671Rfs*41 | 15; 17 | AR | Becker | |
| 16 | M | 8 | c.1727 C > T | p.S576F | 15 | AD/AR | Becker | |
| 17 | M | 5 |
c.937G > A; c.2017G > C |
p.A313T; p.A673P | 8; 17 | AD/AR | Becker | |
| 18 | M | 9 | c.920T > C | p.F307S | 8 | AD | Thomsen | Wang et al., 2022 [9] |
| 19 | M | 3 | c.892G > A | p.A298T | 8 | AD | Thomsen | |
| 20 | M | 9 | c.1261dupC | p.R421fs | 12 | AD | Thomsen | |
| 21 | M | 9 | c.1261dupC | p.R421fs | 12 | AD | Thomsen | |
| 22 | M | 1 | c.1679T > C | p.M560T | 15 | AD | Thomsen | |
| 23 | M | 15 | c.214_215delAG | p.R72fs | 2 | AD | Thomsen | |
| 24 | M | 10 | c.2362 C > T | p.Q788X | 19 | AD | Thomsen | |
| 25 | F | 12 |
c.892G > A; c.2207 C > T |
p.A298T; p. T736I |
8; 18 |
AR | Becker | |
| 26 | M | 9 |
c.433G > T; c.1277 C > A |
p.A145S; p.T426N |
3; 12 |
AR | Becker | |
| 27 | F | 2 |
c.762 C > G; c.962T > A |
p.C254W; p.V321E | 6; 8 | AR | Becker | |
| 28 | M | 1 |
c.795T > G; c.1872G > T |
p.D265E; p.E624D | 7; 16 | AR | Becker | |
| 29 | M | 5 |
c.857T > A; c.1012 C > T |
p.V286E; p.R338* | 8; 9 | AR | Becker | |
| 30 | F | 7 |
c.857T > A; c.1012 C > T |
p.V286E; p.R338* | 8; 9 | AR | Becker | |
| 31 | M | 5 |
c.1389insT; c.2330del G |
p.F463fs; p.G777fs |
12; 19 |
AR | Becker | |
| 32 | M | 3 |
c.139 C > T; c.892G > A; c.1657 A > T |
p.R47W; p.A298T; p.I553F |
1; 8; 15 |
AR | Becker | Hu et al., 2021 [10] |
| 33 | F | 4 | c.1649 C > T | p.T550M | 15 | AD | Thomsen | |
| 34 | M | 5 |
c.962T > A; c.350 A > G |
p.V321E; p.D117G |
8; 3 |
AR | Becker | |
| 35 | M | 1 |
c.1250 A > T; c.1277 C > A |
p.E417V; p.T426N |
11; 12 |
AR | Becker | |
| 36 | M | 2.5 |
c.762 C > G; c.892G > A |
p.C254W; p.A298T |
6; 8 |
AR | Becker | |
| 37 | M | 11 |
c.1401 + 1G > A; c.1657 A > T |
-; p.I553F |
-; 15 |
AR | Becker | Jing et al., 2018 [11] |
| 38 | F | 12 |
c.937G > A; c.1205 C > T |
p.A313T; p.A402V |
8; 11 |
AR | Becker | Jing et al., 2018 [12] |
| 39 | M | 10 | c.1205 C > T | p.A402V | 11 | AD | Thomsen | |
| 40 | M | 12 |
c.2172 + 4 A > G; c.871G > A |
-; p.E291K |
-; 8 |
AD | Thomsen | Yang et al., 2016 [13] |
| 41 | M | 26 |
c.1013G > A; c.139 C > T |
p.R338Q; p. R47W |
9; 1 |
AR | Becker | |
| 42 | M | 1 | c.892G > A | p.A298T | 8 | AD | Thomsen | |
| 43 | M | 9 | c.892G > A | p.A298T | 8 | AD | Thomsen | |
| 44 | M | 15 | c.350 A > G | p.D117G | 3 | AD | Thomsen | |
| 45 | F | 4 |
c.782 A > G; c.2576G > A |
p.Y261C; p.G859D |
7; 22 |
AD | Thomsen | Liu et al., 2015 [14] |
| 46 | M | 2 | c.1568G > A | p.G523D | 14 | AD | Thomsen | |
| 47 | M | 4 | c.1679T > C | p.M560T | 15 | AR | Becker | |
| 48 | M | 1 |
c.1679T > C; c.2364 + 2T > C |
p.M560T; - |
15; - |
AR | Becker | |
| 49 | M | 9 |
c.139 C > T; c.685G > A |
p.R47W; p.V229M |
1; 5 |
AR | Becker | |
| 50 | M | 7 | c.673 A > C | p.S225R | 5 | AR | Becker | Li et al., 2022 [15] |
| 51 | M | 6 |
c.1205 C > T; c.1444G > A |
p.A402V; p.G482R |
11; 13 |
AR | Becker | |
| 52 | M | 9 |
c.1205 C > T; c.700 C > T |
p.A402V; p.P234S |
11; 5 |
AR | Becker | |
| 53 | F | 13 |
c.673 A > C; c.577G > A |
p.S225R; p.E193K |
5; 5 |
AR | Becker | |
| 54 | M | 8 | c.1879 A > C | p.T627P | 16 | AD | Thomsen | |
| 55 | F | childhood | c.907T > C | p.W303R | 8 | AD | Thomsen | Song et al., 2021 [16] |
| 56 | M | 13 | c.762 C > G | p.C254W | 6 | AD | Thomsen | |
| 57 | M | childhood |
c.1876 C > T; c.1408 A > G |
p.R626*; p.M470V |
16; 13 |
AR | Becker | |
| 58 | M | 8 | c.1879 A > C | p.T627P | 16 | AD | Thomsen | Cao et al., 2020 [17] |
| 59 | M | 14 | c.892G > A | p.A298T | 8 | AD | Thomsen | Yang et al., 2019 [18] |
| 60 | M | 9 | c.2169 C > A | p.S723R | 17 | AD | Thomsen | |
| 61 | M | 14 |
c.1205 C > T; c.896T > C |
p.A402V; p.V299Al |
11; 8 |
AR | Becker | Zhong et al., 2019 [19] |
| 62 | M | childhood |
c.1129 C > T; c.1887del C |
p.R377*; p.630 fs |
9; 16 |
AR | Becker | Gu et al.,2017 [20] |
| 63 | F | 4 |
c.782 A > G; c.2576G > A |
p.261Y > C; p.859G > D |
5; 22 |
AR | Becker | Liu et al.,2014 [21] |
| 64 | F | 1 |
c.688G > A; c.1679T > C |
p.G230R; p.M560T |
5; 15 |
AR | Becker | This article |
| 65 | M | 10 |
c.2653_c.2654insC; c.1938G > T |
p.A885Afs* 27; p.M646I |
22; 16 |
AR | Becker | |
| 66 | M | 1 | c.1679T > C | p.M560T | 15 | AR | Becker | |
| 67 | F | 11 |
c.1606G > A; c.962T > A |
p.V536I; p.V321E |
14; 8 |
Uncertain | Becker | |
| 68 | M | 10 |
c.1012 C > T; c.1825 A > G |
p.R338*,651; p.M609V |
9; 16 |
AR | Becker |
Among the 68 children with MC in China, 4 were pure heterozygous, 36 were compound heterozygous, and 28 were single heterozygous. There were 64 mutation sites in the CLCN1 gene, including frameshift mutations, nonsense mutations, missense mutations, and splice site mutations, were widely distributed in the coding sequence, among which 7 children had the c.892G > A mutation, 6 had the c.1205 C > T mutation, 5 had the c.1679T > C mutation, 4 had the c.962T > A mutation, 3 had the c.962T > A mutation, and 4 had the c.1679T > C mutation. mutation, c.1679T > C mutation in 5 cases, c.962T > A mutation in 4 cases, c.139 C > T mutation in 3 cases, c.762 C > G mutation in 3 cases, c.1012 C > T mutation in 3 cases, c.1657 A > T mutation in 3 cases, c.1879 A > C mutation in 3 cases. The frequency of these mutation sites was high. We analysed the distribution of mutation sites and found that the frequency of mutations in exon 8 and exon 15 was significantly higher than that in other exons (Fig. 5), which was basically consistent with the literature report that exon 8 was a hotspot for mutations.
Fig. 5.
Distribution of reported Chinese CLCN1 loci over the entire gene sequence in the last 10 years searched in the public repository, with red font indicating dominant inheritance and black font indicating recessive inheritance
The clinical manifestations of the patients in this study were similar to those reported in the previous literature, with all five children starting at a young age, and all of them had muscle ankylosis, which is the main feature of congenital myotonia congenita. However, the clinical presentation varied between individuals. The muscle groups involved in myotonia congenita varied, with two cases involving the upper limbs, one case involving the face, and two cases even showing sensory abnormalities in the lower limbs, suggesting that we should pay attention to these subtle changes during clinical evaluation. In addition, the improvement of symptoms in all patients after exercise (warm-up phenomenon) further supports the characterisation of myotonia congenita and emphasises the relationship between exercise and symptoms. The correlation between environment and symptoms is reflected by the fact that the children’s symptoms were pronounced in cold environments and diminished in warm environments, which may be instructive in the treatment of MC. Creatine kinase was normal or mildly elevated in five children. EMG performed on five children suggested myotonia congenita potentials in all five cases. This is in general agreement with creatine kinase and EMG findings reported in the literature [22].
We identified 8 CLCN1 mutation sites in 5 children, of which 4 were unreported de novo mutations, suggesting that the CLCN1 gene is genetically heterogeneous in Chinese children with MC. Combined with the literature review, a total of 64 mutations in the CLCN1 gene were identified, and these mutation sites were distributed in 23 exons. Further analysis revealed that exons 8 and 15 had the highest frequency of mutations, suggesting that they may be hotspot regions for mutations in Chinese children with MC, which provides important clues for future genetic screening and diagnosis.
Classifying children with MC according to clinical phenotype and genotype, we found that autosomal dominant Thomsen patients accounted for 42.6% and autosomal recessive Becker patients accounted for 57.4% of the Chinese MC population, and the proportion of autosomal dominant inheritance was significantly higher than that in Western countries, such as the United Kingdom, where the proportion of dominant inheritance accounted for 36% [23], which may be related to the population’s genetic background, environmental factors, and different screening methods. Our study provides a new perspective for understanding the characteristics of CLCN1 mutations in the Chinese population.
MC tends to be a benign process, with symptoms stabilising in adulthood, and early diagnosis can help to determine the prognosis of the patient at an early stage. Currently, the diagnosis is based on medical history, including family history, clinical signs and symptoms, electromyography, serum enzyme profiling and genetic testing. Children tend to have an early onset of the disease, with myotonia and a ‘warm-up phenomenon’. Serum muscle enzymes are usually normal or only slightly elevated between 3 and 4 times the upper reference limit [22]. Electromyography shows tonic discharges, and the diagnosis can eventually be confirmed by genetic testing.
When the symptoms of myotonia congenita significantly affect the life and study of the children, drug treatment can be used to relieve the symptoms of myotonia congenita. The principle of treatment for myotonia congenita is to reduce the excitability of the muscle membrane. Medicines such as mesylate, carbamazepine, phenytoin sodium can be used. These medicine are believed to reduce the permeability of sodium channels and the occurrence of repetitive action potentials, thus reducing myotonia [24]. Conventional drugs such as mesylate and carbamazepine are thought to be potentially effective, and phenytoin sodium and gabapentin are the most effective drugs for the treatment of cryptogenic CLCN1 myotonia [25]. Dantrolene sodium was previously mentioned in a case report for the treatment of myotonic phenomena, and it has now been shown that this drug, in combination with carbamazepine, reduces ankylosis [26]. Recent studies have shown that mesylate and lamotrigine are more effective [27–29]. In this paper, there were two cases of children who experienced symptomatic relief after oral administration of oxcarbazepine, as evidenced by improvement in the patient’s muscle ankylosis on activity initiation. The principle of action of this drug is similar to that of carbamazepine, which mainly stabilises cell membranes in an overexcited state by blocking sodium channels. It is also crucial to avoid aggravating factors such as cold, stress, and high-intensity exercise in daily life, as well as to strengthen psychological support for the affected child. With the continuous progress of molecular biology and gene technology, the diagnosis and treatment of myotonia congenita will be more precise and individualised, and in the future it may be expected that gene therapy can prevent the disease and delay its progression. Our study will also help to expand the genetic profile and provide support at the genetic level for prenatal counselling and diagnosis of this disease.
To further strengthen the patient-centered focus of this study, future research should incorporate qualitative data collection methods to capture patient and caregiver perspectives. By integrating patient-reported outcomes, we aim to better address the clinical and social impact of MC and inform more patient-centered care strategies.
In conclusion, our study reported five patients with CLCN1-associated MC, which expands the clinical and genetic spectrum of MC patients in China. All available reports on mutations associated with this disease in the Chinese population in the last 10 years were also reviewed and genetically distributed. Our findings may help to identify genetic determinants as well as a first look at genotype-phenotype relationships. We hope that in the future, we can expand our sample collection to further explore the relationship between CLCN1 mutations and clinical phenotypes, focusing on whether genotypic information can be used to predict the clinical performance and prognosis of patients. Additionally, expanding the sample size and conducting collaborative multicentre studies will contribute to a more comprehensive understanding of the clinical significance and genetic characteristics of CLCN1 mutations.
Acknowledgements
Not applicable.
Abbreviations
- MC
Myotonia congenita
- TMC
Dominant-type Thomsen
- BMC
Recessive-type Becker
- EMG
Electromyography
- NCS
Nerve conduction study
- CK
Creatine kinase
Author contributions
Xin Wang conducted data analysis and article writing. Shangyu Wang collected the data and wrote the article. Hongdan Qi and Bing Wu assisted in creating charts. Mingying He revised this article. Gang Zhang proposed the concept and supervised them. All authors revised and approved the article.
Funding
Not applicable.
Data availability
The data for this article are not publicly available because of privacy concerns. Requests to access these datasets should be directed to Gang Zhang(zhanggangnjmu@126.com).
Declarations
Ethics approval and consent to participate
The study involving human participants were reviewed and approved by the ethics committee of the Children’s hospital of Nanjing Medical University. Written informed consent was obtained from each participant in our study.
Consent for publication
Informed consent was obtained from all individual participants included in the study.
Competing interests
The authors declare that they have no conflicts of interest.
Footnotes
Publisher’s note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Xin Wang and Shangyu Wang contributed equally to this work.
References
- 1.Jentsch TJ, Pusch M. CLC chloride channels and transporters: structure, function, physiology, and disease. Physiol Rev. 2018;98(3):1493–590. 10.1152/physrev.00047.2017. [DOI] [PubMed] [Google Scholar]
- 2.Özgün N, Taşlıdere H. Congenital myotonia: a review of Twenty cases and a new splice-site mutation in the CLCN1 gene. Turk J Pediatr. 2020;62(3):450–60. 10.24953/turkjped.2020.03.012. [DOI] [PubMed] [Google Scholar]
- 3.Jou SB, Chang LI, Pan H, Chen PR, Hsiao KM. Novel CLCN1 mutations in Taiwanese patients with myotonia congenita. J Neurol. 2004;251(6):666–70. 10.1007/s00415-004-0383-6. [DOI] [PubMed] [Google Scholar]
- 4.Zhang Y, Zhang S, Shang H. Clinical characteristics of myo-tonia congenita in China literature analysis of the past 30 years. Neural Regenerat Res. 2008;3(2):216–20. [Google Scholar]
- 5.Bryant SH. Muscle membrane of normal and myotonic goats in normal and low external chloride. Fed Proc. 1962;21:312.
- 6.Bryant SH, Morales-Aguilera A. Chloride conductance in normal and myotonic muscle fibres and the action of Monocarboxylic aromatic acids. J Physiol. 1971;219(2):367–83. 10.1113/jphysiol.1971.sp009667. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.He Y, Qiu Y, Xiong Y, Shen Y, Jiang K, Yi H, Huang P, Zhu Y, Zhu M, Zhou M, Hong D, Tan D. Clinical and genetic characteristics of myotonia congenita in Chinese population. Channels (Austin). 2024;18(1):2349823. Epub 2024 May 8. PMID: 38720415; PMCID: PMC11086022. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Li Y, Li M, Wang Z, Yang F, Wang H, Bai X, Sun B, Chen S, Huang X. Clinical and molecular characteristics of myotonia congenita in china: case series and a literature review. Channels (Austin). 2022;16(1):35–46. PMID: 35170402; PMCID: PMC8855856. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Wang Q, Zhao Z, Shen H, Bing Q, Li N, Hu J. The clinical, myopathological, and genetic analysis of 20 patients with Non-dystrophic myotonia. Front Neurol. 2022;13:830707. 10.3389/fneur.2022.830707. PMID: 35350395; PMCID: PMC8957821. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Hu C, Shi Y, Zhao L, Zhou S, Li X. Myotonia congenita: clinical characteristic and mutation spectrum of CLCN1 in Chinese patients. Front Pediatr. 2021;9:759505. 10.3389/fped.2021.759505. PMID: 34790634; PMCID: PMC8591224. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Jing M, Xiao-Jing W, Xue-Mei L, et al. A case report: autosomal recessive myotonia congenita caused by a novel splice mutation (c.1401 + 1G > A) in CLCN1 gene of a Chinese Han patient. BMC Neurology. 2018;18(1):154. [DOI] [PMC free article] [PubMed]
- 12.Jing F, Li H, Yang D, Chen T, Liu Y, Yu L. Analysis of CLCN1 gene mutations in a family affected with myotonia congenita. Zhonghua Yi Xue Yi Chuan Xue Za Zhi. 2018;35(3):400–402. Chinese. 10.3760/cma.j.issn.1003-9406.2018.03.021. PMID: 29896741. [DOI] [PubMed]
- 13.Yang X, Jia H, An R, Xi J, Xu Y. Sequence CLCN1 and SCN4A in patients with nondystrophic myotonias in Chinese populations: genetic and pedigree analysis of 10 families and review of the literature. Channels (Austin). 2017;11(1):55–65. Epub 2016 Jul 14. PMID: 27415035; PMCID: PMC5279883. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Liu XL, Huang XJ, Shen JY, Zhou HY, Luan XH, Wang T, Chen SD, Wang Y, Tang HD, Cao L. Myotonia congenita: novel mutations in CLCN1 gene. Channels (Austin). 2015;9(5):292–8. Epub 2015 Aug 11. PMID: 26260254; PMCID: PMC4826137. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15.Li DZ. Clinical analysis and gene mutations screening of CLCN1in five families with myotonia congenita [D]. Guangxi Medical University; 2022. 10.27038/d.cnki.ggxyu.2022.000719.
- 16.Song J, Zhang JW, Li G, et al. Clinical and genetic features of three Chinese patients with myotonia congenita. J Stroke Neurol Dis. 2021;38(03):249–52. 10.19845/j.cnki.zfysjjbzz.2021.0062. [Google Scholar]
- 17.Cao XL, Qin QY, Huang W. Clinical and genetic analysis for a family with myotonia congenita of CLCN1 gene mutations. CJJ. 2020;23(23):2112–6. [Google Scholar]
- 18.Yang HJ, Zhao H, Yang XL. Clinical, electrophysiological and genetic studies of one family and one sporadic patient with congenital myotonia. J Int Neurol Neurosurg. 2019;46(04):364–7. 10.16636/j.cnki.jinn.2019.04.003. [Google Scholar]
- 19.Zhong J, Lin JF, Zhang C, et al. Treatment of Becker myotonia congenita with lamotrigine: one case report and review of literatures. CJJ. 2019;19(05):349–53. [Google Scholar]
- 20.Gu P, Sun ZQ, Wang WT, et al. Report of congenital myotonia in 1 case. Chin J Nerv Mental Dis. 2017;43(06):377–8. [Google Scholar]
- 21.Liu XL, Huang XJ, Luan XH. Clinical and genetic study of 1 case of congenital myotia [C]. In: Chinese Medical Association, Chinese Society of Neurology, editors. Proceedings of the 17th National Conference on Neurology of the Chinese Medical Association (II). Shanghai: Department of Neurology, Ruijin Hospital, Shanghai Jiao Tong University School of Medicine; 2014. p. 2.
- 22.Gutmann L, Phillips LH 2nd. Myotonia congenita. Semin Neurol. 1991;11(3):244-8. 10.1055/s-2008-1041228. PMID: 1947487. [DOI] [PubMed]
- 23.Fialho D, Schorge S, Pucovska U, Davies NP, Labrum R, Haworth A, et al. Chloride channel myotonia: exon 8 hot-spot for dominant-negative interactions. Brain. 2007;130(Pt 12):3265-74. 10.1093/brain/awm248. Epub 2007 Oct 11. PMID: 17932099. [DOI] [PubMed]
- 24.Richards S, Aziz N, Bale S, Bick D, Das S, Gastier-Foster J, Grody WW, Hegde M, Lyon E, Spector E, Voelkerding K, Rehm HL, ACMG Laboratory Quality Assurance Committee. Standards and guidelines for the interpretation of sequence variants: a joint consensus recommendation of the American college of medical genetics and genomics and the association for molecular pathology. Genet Med. 2015;17(5):405–24. 10.1038/gim.2015.30. Epub 2015 Mar 5. PMID: 25741868; PMCID: PMC4544753. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 25.Dupré N, Chrestian N, Bouchard JP, et al. Clinical, electrophysiologic, and genetic study of non-dystrophic myotonia in French-Canadians. Neuromuscul Disord. 2009;19(5):330–4. 10.1016/j.nmd.2008.01.007. [DOI] [PubMed] [Google Scholar]
- 26.Ohtaki E, Komori H, Yamaguchi Y, Matsuishi T. Successful Dantrolene sodium treatment of a patient with myotonia congenita (Thomsen’s disease). Acta Paediatr Jpn. 1991;33:668–71. 10.1111/j.1442-200x.1991.tb01884.x. [DOI] [PubMed] [Google Scholar]
- 27.Stunnenberg BC, Raaphorst J, Groenewoud HM, et al. Effect of mexiletine on muscle stiffness in patients with nondystrophic myotonia evaluated using aggregated N-of-1 trials. JAMA. 2018;320(22):2344–53. 10.1001/jama.2018.18020. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28.Andersen G, Hedermann G, Witting N, Duno M, Andersen H, Vissing J. The antimyotonic effect of lamotrigine in non-dystrophic myotonias: a double-blind randomized study. Brain. 2017;140(9):2295–305. 10.1093/brain/awx192. [DOI] [PubMed] [Google Scholar]
- 29.Modoni A, D’Amico A, Primiano G, Capozzoli F, Desaphy JF, Lo Monaco M. Long-Term safety and usefulness of mexiletine in a large cohort of patients affected by Non-dystrophic myotonias. Front Neurol. 2020;11:300. 10.3389/fneur.2020.00300. Published 2020 May 20. [DOI] [PMC free article] [PubMed] [Google Scholar]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data for this article are not publicly available because of privacy concerns. Requests to access these datasets should be directed to Gang Zhang(zhanggangnjmu@126.com).