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Mechanisms of Phenotypic Variability in Myotonia Congenita

Burge, JA; (2013) Mechanisms of Phenotypic Variability in Myotonia Congenita. Doctoral thesis (PhD), UCL (University College London). Green open access

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Abstract

The severity of Myotonia Congenita varies not only across individuals with different CLCN1 genotypes, but also within a pedigree, and can even fluctuate over time within a single individual in response to environmental circumstances. The functional consequences of eight naturally occurring sequence variants in the skeletal muscle chloride channel gene, CLCN1, were examined by whole cell patch-clamp of HEK293T cells expressing the gene product, ClC-1, in order to investigate potential differences in their mechanisms of pathogenicity. G276D and G523D caused complete loss of function, while S289G produced altered kinetics and a marked depolarizing shift of voltage dependence. H369P, A566T and M646T all tested normal in the HEK293T assay despite strong clinical support for pathogenicity. Their mechanism of pathogenicity may rely on muscle-specific processes that are not faithfully recapitulated in HEK293T cells. W118G and P744T were selected as examples of variants for which pathogenicity is unclear from the clinical evidence. The former is present in controls, but over-represented in the Myotonia Congenita population. The latter is present in an individual who also harbours a large deletion in CLCN1. Both variants tested normal in the HEK293T assay. A potent trigger for worsening of myotonia in some female patients is pregnancy. In order to clarify the role of sex hormones in non-genomic modulation of skeletal muscle excitability, the effects of progesterone and oestrogen on endogenous chloride currents through the wildtype ClC-1 of mouse skeletal muscle were tested by whole cell patch clamp. Progesterone and oestrogen rapidly reduced the chloride conductance and shifted its voltage dependence, thus a non-genomic mechanism exists in skeletal muscle linking sex hormones to ClC-1. However the effect was only significant at 500 times the highest physiological concentration encountered in pregnancy. The macroscopic chloride conductance of a membrane expressing wildtype ClC-1 was simulated in Matlab. The simulation improves on published models by recapitulating both time-dependence and voltage-dependence of the channel through a method based on independent representations of the fast and the slow gates. The applicability of the model for the purposes of exploring the effects of specific mutations was assessed by attempting to simulate the currents through S289G channels; the effects of S289G could be mimicked by slowing and inverting the kinetics of the fast gate and shifting the fast gate opening probability to more depolarized potentials. The mechanism of low chloride conductance myotonia and electrical factors likely to impact on its severity are discussed in the context of experiments conducted in a model of myotonic muscle. Slowing of ClC-1 kinetics alone did not produce myotonia, but could lower the threshold for myotonia caused by shifts in voltage dependence. Muscle fibre diameter is an important factor in the propensity to myotonia, which can be driven by asynchrony between surface and t-tubular action potentials in large muscle fibres. Increasing muscle fibre diameter could underly the age-dependence of symptom onset in Myotonia Congenita, and differences in diameter could contribute to phenotypic variability, including male-female differences.

Type: Thesis (Doctoral)
Qualification: PhD
Title: Mechanisms of Phenotypic Variability in Myotonia Congenita
Open access status: An open access version is available from UCL Discovery
Language: English
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Brain Sciences
UCL > Provost and Vice Provost Offices > School of Life and Medical Sciences > Faculty of Brain Sciences > UCL Queen Square Institute of Neurology
URI: https://discovery.ucl.ac.uk/id/eprint/1401157
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