Chloride channel myotonia: exon 8 hot-spot for dominant-negative interactions

Abstract

Myotonia congenita (MC) is the commonest genetic skeletal muscle ion channelopathy. It is caused by mutations in CLCN1 on chromosome 7q35, which alter the function of the major skeletal muscle voltage-gated chloride channel. Dominant and recessive forms of the disease exist. We have undertaken a clinical, genetic and molecular expression study based upon a large cohort of over 300 UK patients. In an initial cohort of 22 families, we sequenced the DNA of the entire coding region of CLCN1 and identified 11 novel and 11 known mutations allowing us to undertake a detailed genotype-phenotype correlation study. Generalized muscle hypertrophy, transient weakness and depressed tendon reflexes occurred more frequently in recessive than dominant MC. Mild cold exacerbation and significant muscle pain were equally common features in dominant and recessive cases. Dominant MC occurred in eight families. We noted that four newly identified dominant mutations clustered in exon 8, which codes for a highly conserved region of predicted interaction between the CLC-1 monomers. Expressed in Xenopus oocytes these mutations showed clear evidence of a dominant-negative effect. Based upon the analysis of mutations in this initial cohort as well as a review of published CLCN1 mutations, we devised an exon hierarchy analysis strategy for genetic screening. We applied this strategy to a second cohort of 303 UK cases with a suspected diagnosis of MC. In 23 individuals, we found two mutations and in 86 individuals we identified a single mutation. Interestingly, 40 of the cases with a single mutation had dominant exon 8 mutations. In total 48 individuals (from 34 families) in cohort 1 and 2 were found to harbour dominant mutations (37% of mutation positive individuals, 30% of mutation positive families). In total, we have identified 23 new disease causing mutations in MC, confirming the high degree of genetic heterogeneity associated with this disease. The DNA-based strategy we have devised achieved a genetic diagnosis in 36% of individuals referred to our centre. Based on these results, we propose that exon 8 of CLCN1 is a hot-spot for dominant mutations. Our molecular expression studies of the new exon 8 mutations indicate that this region of the chloride channel has an important role in dominant negative interactions between the two chloride channel monomers. Accurate genetic counselling in MC should be based not only upon clinical features and the inheritance pattern but also on molecular genetic analysis and ideally functional expression data.

The non-dystrophic myotonias: molecular pathogenesis, diagnosis and treatment

“…Patients with recessive myotonia congenita typically experience a peculiar transient weakness on initiating an action, which is only rarely seen in dominant myotonia congenita. Although Becker found that most patients with recessive myotonia congenita presented between the ages 4 and 12 years while the dominant form usually manifested before the age of 3 years (Becker, 1977), we found no difference in the age of onset (Fialho et al, 2007).…”

“…Some clinical findings are more common in the recessive than in the dominant form but considerable overlap exists. Recessive myotonia congenita tends to be more severe, is more frequently associated with muscle hypertrophy and with depressed tendon reflexes (Becker, 1977;Fialho et al, 2007). Patients with recessive myotonia congenita typically experience a peculiar transient weakness on initiating an action, which is only rarely seen in dominant myotonia congenita.…”

“…However, cases of myotonic dystrophy type II in which myotonia is the predominant complaint without any overt systemic features have been described (Fialho et al, 2007) and can lead to diagnostic difficulty.…”

“…Recent studies have revealed a wide range of clinical phenotypes which may present diagnostic difficulty. Importantly, it has also been observed that patients with myotonic dystrophy type II may present with a clinical phenotype that is difficult to distinguish from myotonia congenita (Fialho et al, 2007). It is now clear that the clinical severity of these disorders can range from a neonatal life threatening presentation through to mild late-onset symptoms.…”

Myotonia congenita: novel mutations in CLCN1 gene

“…2 Transient weakness, generalized muscle hypertrophy and depressed deep tendon reflexes were more common in recessive myotonia congenita. 2,3 The recessive form is believed to be caused by 2 loss-of-function mutations, whereas the dominant form is assumed to be a consequence of a dominant-negative effect. 4 CLCN1 is located on chromosome 7q35 and encompasses 35 kb in genomic DNA with 23 exons.…”