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Other entities represented in this entry:
SNOMEDCT: 57938005, 726051002; ICD10CM: G71.12; ORPHA: 614; DO: 0081336;
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
Gene/Locus |
Gene/Locus MIM number |
|---|---|---|---|---|---|---|
| 7q34 | Myotonia levior | 160800 | Autosomal dominant | 3 | CLCN1 | 118425 |
| 7q34 | Myotonia congenita, dominant | 160800 | Autosomal dominant | 3 | CLCN1 | 118425 |
A number sign (#) is used with this entry because of evidence that autosomal dominant myotonia congenita (Thomsen disease) is caused by heterozygous mutation in the gene encoding skeletal muscle chloride channel-1 (CLCN1; 118425) on chromosome 7q34.
Autosomal recessive myotonia congenita, or Becker disease (255700), is also caused by mutation in the CLCN1 gene.
Autosomal dominant myotonia congenita is a nondystrophic skeletal muscle disorder characterized by muscle stiffness and an inability of the muscle to relax after voluntary contraction (Sun et al., 2001). Thomsen disease is less common and less severe than Becker disease.
See also paramyotonia congenita (PMC; 168300) and potassium-aggravated myotonia (608390), overlapping phenotypes caused by mutations in the SCN4A gene (603967).
Myotonia congenita was first described by the Danish physician Julius Thomsen (1876) in his own family. A follow-up report (Thomasen, 1948) identified 64 affected persons in 7 consecutive generations. The pedigree of Birt (1908), who, like Thomsen, was himself affected, showed skipped generations.
Isaacs (1959) reported a mother and son with myotonia congenita. Quinine, local procaine, procainamide, insulin, injections of 50% magnesium sulfate, curarization, sodium loading and sodium depletion had no effect on the mother's myotonia. However, marked improvement occurred when potassium depletion was achieved with cortisone and chlorothiazide. The son improved when treated with chlorothiazide only. Pasternack and Lindqvist (1962) described 6 cases in 3 generations, and personally examined 4. Celesia et al. (1967) reported monomelic myotonia congenita, which may have been due to somatic mutation.
Sanders (1976) reported a family with dominant inheritance of myotonia congenita. Two affected family members had painful muscle contractions and hypothyroidism; they showed improvement after thyroid replacement therapy.
Becker (1977) provided a classification of the myotonias, and suggested 3, and perhaps 5, different varieties of dominant myotonia. Type I was classic Thomsen disease. Type II, represented by 4 families in Becker's series, was characterized by muscle pain and a fluctuating course. In type III, a marked relationship of myotonia to cold was noted, especially in the muscles around the eyes, nose, and mouth. It differed from paramyotonia congenita (168300) by the lack of cold-induced paralysis. Types IV and V, although not clearly distinct, were characterized by lack of involvement of facial muscles and isolated percussion myotonia of the tongue, respectively. Lehmann-Horn et al. (1995) commented that Becker had found that many forms of autosomal dominant myotonia exhibited a clinical picture that did not fit the classic form of Thomsen disease. These disorders were later found to be caused by mutations in the gene encoding the alpha subunit of the muscle sodium channel (SCN4A; 603967). These atypical Thomsen cases, now classified as potassium-aggravated myotonias (608390), are more common than Thomsen disease.
Dupre et al. (2009) reported 9 French Canadian patients from 4 unrelated families with autosomal dominant myotonia caused by heterozygous CLCN1 mutations (see, e.g., S189F; 118425.0018). The mean age of onset was 13 years (range, 2 to 20). The most common clinical features included percussion myotonia (44%), handgrip myotonia (56%), warm-up phenomenon (100%), generalized hypertrophy (78%), generalized muscle stiffness (78%), and exacerbation with cold temperatures (56%). Less common features included lid lag (11%), lid myotonia (22%), tongue myotonia (22%), and muscle pain (11%). None had weakness, and none had sought to use medications to alleviate their symptoms. About half of affected females reported aggravation of symptoms during menstruation or pregnancy, and alleviation of symptoms after menopause. Some also reported symptom improvement with alcohol. Electrophysiologic studies showed less severe myotonia and less severe CMAP decrements compared to patients with recessive CLCN1 mutations, but similar results compared to patients with dominant SCN4A (603967) mutations.
Myotonia Levior
Myotonia levior, a mild form of autosomal dominant myotonia, was first described by de Jong (1966). Siciliano et al. (1988) reported 2 families with myotonia levior. Affected individuals had isolated myotonia without muscle weakness, hypotrophy, or hypertrophy. They suggested that myotonia levior was a 'low expressivity variant' of Thomsen disease.
Lehmann-Horn et al. (1995) reported a family in which 2 brothers and their mother had myotonia levior. The brothers had onset at age 5 years of impeded muscle relaxation which was pronounced during exercise. Physical examination showed normotrophic skeletal muscles, lid lag, percussion myotonia, mild myotonia most pronounced in the forearm muscles, 'warm-up' phenomenon, and no muscle weakness. EMG showed myotonic runs. Muscle biopsy and CT scans of thigh and leg muscles were normal.
The transmission pattern of myotonia congenita in the families reported by George et al. (1993) was consistent with autosomal dominant inheritance.
Among 22 patients with paramyotonia congenita (PMC; 168300), 14 with sodium channel myotonia (608390), and 18 myotonia patients with mutations in the CLCN1 gene, Fournier et al. (2006) found that cold temperature was able to exaggerate electromyographic findings in a way that enabled a clear correlation between EMG findings and genetic defects. Those with PMC showed a clear worsening of compound muscle action potential with cold temperature. Those with sodium channel myotonia tended not to show a decline in compound action muscle potentials, whereas those with myotonia due to CLCN1 mutations tended to show improvement of the muscle potential with exercise, concomitant with the clinical warm-up phenomenon.
Hughes and Wilson (1991) reported apparent benefit from antihistaminics, specifically antazoline and trimeprazine, in myotonia congenita.
Lipicky and Bryant (1973) found that sarcolemmal chloride conductance was significantly reduced in intercostal muscle biopsies of patients with myotonia congenita, suggesting a defect in a chloride channel.
Ptacek et al. (1993) discussed the genetics and physiology of the myotonic muscular disorders: the sodium-channel disorders resulting from mutations in the SCN4A gene (603967) on chromosome 17; disorders of the chloride channel; and myotonic dystrophy (DM1; 160900) caused by mutation in the DMPK gene (605377) on 19q13.
Because of the similarities between myotonia congenita and the mouse disorder Adr, which maps to mouse chromosome 7, Abdalla et al. (1992) looked for linkage to the human TCRB gene (see 186930) on the homologous region 7q35. In 4 pedigrees, they found a maximum cumulative lod score of 3.963 at a recombination fraction of 0.10 (1-lod support interval = 0.048-0.275). Abdalla et al. (1992) excluded linkage of myotonia congenita from at least 24 cM on either side of the CFTR gene (602421) on 7q31, and from the SCN4A and GH1 genes (139250) on 17q.
In 4 families with Thomsen disease, Koch et al. (1992) found linkage to the CLCN1 gene on 7q35 (maximum multipoint lod score of 4.58 at theta = 0.0).
In affected members of 3 unrelated families with autosomal dominant myotonia congenita, George et al. (1993) identified a heterozygous mutation in the CLCN1 gene (G230E; 118425.0002). The findings indicated that Thomsen disease and Becker disease are allelic.
In affected members of Thomsen's own family (Thomasen, 1948) with autosomal dominant myotonia congenita, Steinmeyer et al. (1994) identified a heterozygous mutation in the CLCN1 gene (P480L; 118425.0006).
After identifying mutations in the CLCN1 gene in patients with autosomal recessive myotonia congenita, Koch et al. (1992) concluded that mutations in the CLCN1 gene can cause either dominant or recessive myotonia congenita. A recessive form was explicable on the basis of total loss of function. A mutation acting dominantly in producing Thomsen disease could be explained by a homomultimeric structure of the channel, whereby the channel subunit encoded by the mutated gene associates with and inactivates the functional subunits encoded by the normal allele.
In affected members of 18 unrelated families from Norway and Sweden with both autosomal dominant (5 families) and autosomal recessive (13 families) inheritance of myotonia congenita, Sun et al. (2001) identified 8 different mutations, including 3 novel mutations, in the CLCN1 gene. Fifteen probands had mutations in both alleles; 2 had mutations in a single allele, and 2 had no CLCN1 mutations. The majority of the patients were compound heterozygous with all possible mutational combinations, even in families with a dominant pattern of inheritance. Families with apparently dominant segregation of myotonia congenita may actually represent recessive inheritance with undetected heterozygous individuals married-in as a consequence of a high population carrier frequency of some mutations. The findings, together with the very variable clinical presentation, challenged the classification into dominant Thomsen or recessive Becker disease. Sun et al. (2001) suggested that most cases of myotonia congenita show recessive inheritance with some modifying factors or genetic heterogeneity.
In 2 Costa Rican families (family 1 and family 4) in which the proband had Thomsen disease, Vindas-Smith et al. (2016) identified heterozygous mutations in the CLCN1 gene (F167L; Q412P, 118425.0022, respectively) by bidirectional sequencing of the CLCN1 gene, with confirmation by RFLP-PCR. Functional studies of CLCN1 with the F167L mutation did not show alterations of gating parameters or channel conductance. Functional studies of CLCN1 with the Q412P mutation expressed in Xenopus oocytes showed reduced surface expression and reduced current density. Vindas-Smith et al. (2016) concluded that the Q412P mutation induces a severe folding defect that leads to its degradation before it can dimerize with the wildtype subunit.
Altamura et al. (2018) evaluated the functional significance of 7 mutations in the C-terminal region of the CLCN1 gene associated with either autosomal dominant Thomsen disease or autosomal recessive Becker disease. CLCN1 with each mutation was transfected into HEK293 cells and analyzed with patch-clamp analysis. Five of the mutations were in the CBS2 domain (V829M, T832I, V851M, G859V, L861P) and 2 of the mutations were in the C-terminal peptide (P883T, V947E). Mutations located between residues 829 and 835 and in residue 883 resulted in alteration of voltage dependence. Mutations between residues 851 and 859 and in residue 947 resulted in a reduction of chloride currents. The results were consistent with a role for CBS2 in protein channel gating and demonstrated the importance of the C-peptide region in protein function and expression.
Suetterlin et al. (2022) evaluated the functional significance of 95 CLCN1 mutations, including 34 novel mutations, identified in 233 patients with myotonia congenita. Mutations that altered voltage dependence of activation clustered in the first half of the transmembrane domains and mutations resulting in absent currents clustered in the second half of the transmembrane domains. Mutations that resulted in dominant functional features clustered in the TM1 domain and variants associated with recessive functional features and without a shift in voltage dependence of activation were clustered in the TM2 domain. Mutations in the intracellular domain were not associated with a dominant inheritance pattern.
Myotonia Levior
In 2 brothers with myotonia levior, Lehmann-Horn et al. (1995) identified a heterozygous mutation in the CLNC1 gene (118425.0007). The findings indicated the myotonia levior is a variant or allelic form of Thomsen disease due to a mutation leading to low clinical expressivity.
Sun et al. (2001) stated that the worldwide prevalence of myotonic congenita, both dominant and recessive forms, is 1:100,000. In the northern Norwegian population, Sun et al. (2001) found a prevalence of about 9:100,000, which was comparable to the Finnish experience.
Beck et al. (1996) noted that the current hypotheses regarding the pathophysiology of autosomal dominant myotonia congenita, or Thomsen disease, were initially formulated from studies of the myotonic goat, an unusual breed afflicted with severe autosomal dominant congenital myotonia that closely resembles the human disease clinically and in its mode of inheritance. Beck et al. (1996) demonstrated that the phenotype of the myotonic goat was due to a heterozygous mutation in the Clcn1 gene, confirming that it is an animal model for Thomsen disease.
Myotonia with muscular hypertrophy and hyperirritability was described in 3 generations, with male-to-male transmission, by Torbergsen (1975), who maintained that the disorder was distinct from Thomsen myotonia congenita; see rippling muscle disease (606072).
Koch et al. (1989) excluded myotonia congenita from a distance within 9 cM of the myotonic dystrophy locus (160900) on chromosome 19, indicating that the 2 disorders are not allelic. Ptacek et al. (1992) excluded linkage of myotonia congenita to the SCN4A sodium channel gene (603967) on 17q.
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