Extraocular muscle hypertrophy in myotonia congenita

Sodium Channel Myotonia and a Novel Gly701Asp Mutation in the SCN4A Gene: From an Ophthalmological Symptom to a Familial Disease

A six-month-old female child came to an ophthalmology consultation because of a convergent strabismus, myotonia of the orbicularis muscles and difficulty walking in cold environments. Further investigation identified a family history of muscular myotonia in the father, grandmother and uncle. The father also presented with ocular myotonia. The child and family members underwent genetic testing, which was negative for CLCN1 mutations but was positive for a novel heterozygotic Gly701Asp mutation in the SCN4A gene, compatible with sodium channel myotonia. The non-dystrophic myotonias are caused by dysfunction of key skeletal muscle ion channels. Before the advent of DNA sequencing, non-dystrophic myotonias were differentiated based on clinical phenotypes. Sodium channel myotonia disorders are classically of dominant inheritance, in which eye closure myotonia is the most frequent manifestation. Over 40 different mutations have been reported in the SCN4A gene. The Gly701Asp mutation in exon 13 identified in this family has not been described before.

Mutations of SCN4A gene cause different diseases: 2 case reports and literature review

SCN4A encodes the Nav1.4 channel and mutations in SCN4A lead to different ionic channelopathies. In this study, one sporadic individual of periodic paralysis, one paramyotonia family and 200 normal healthy controls are enrolled. Genomic DNA was extracted from peripheral blood leukocytes, followed by polymerase chain reaction and DNA sequencing of candidate genes, including SCN4A and CACNA1S. As a result, heterozygous mutations c.2024G>A (R675Q) and c.1333G>A (V445M) of gene SCN4A were identified in the hypokalemic periodic paralysis patient and the paramyotonia congenita family respectively. Both mutations were not detected in healthy controls. Compared with reported cases, patients with mutation R675Q usually do not present hypokalemic periodic paralysis but hyperkalemic or normokalemic periodic paralysis. The mutation V445M was first reported in Chinese patients with nondystrophic myotonias. In addition, we carried out literature review by summarizing clinical features of the 2 mutations and establish the genotype-phenotype correlations to provide guidance for diagnosis.

A de novo Mutation in the SCN4A Gene Causing Sodium Channel Myotonia

We describe the case of a six year old boy with findings consistent with myotonia congenita: muscular hypertrophy, stiffness when commencing movements and typical warm-up signs. The most prominent symptom was myotonia of the eyelid muscles with apparent swelling around the eyes. Even though the pronounced warm-up phenomena in our patient suggested a chloride channel-associated myotonia congenita, the myotonia of his eyelid muscles indicated an involvement of sodium channels. Screening for mutations in the underlying CLCN1 gene was negative, however, in the SCN4A gene, we identified the missense mutation c.2108T>C; p.Leu703Pro for which there is strong evidence of pathogenicity because it arose de novo in the index patient.

Myotonia congenita with strabismus in a large family with a mutation in the SCN4A gene

BACKGROUND/AIMS

To determine the genetic basis of myotonia congenita (MC) and strabismus in a large Caucasian family.

METHODS

Seven patients making up four generations of a family with MC and strabismus were recruited. All patients had at least one standard ophthalmic examination, including best-corrected visual acuity, refraction, and ocular motility measurements. CLCN1 and SCN4A genes were sequenced and analysed for mutations.

RESULTS

Five out of the seven family members were diagnosed with MC by clinical history and electromyography. Ophthalmic history and exam revealed eyelid myotonia and strabismus. All patients with MC were diagnosed with strabismus between the ages of 3 and 6 and required surgical restoration of ocular alignment. Sequencing results revealed a c. 1333G>A; p. Val445Met mutation in the SCN4A gene.

CONCLUSION

There are few reports describing eyelid myotonia and strabismus in patients diagnosed with MC. We found significant ocular involvement in a family with a mutation in SCN4A. Future studies may confirm that MC with significant ocular involvement can be used to direct genetic analysis.

A sodium channel myotonia due to a novel SCN4A mutation accompanied by acquired autoimmune myasthenia gravis

Mutations of the voltage gated sodium channel gene (SCN4A) are responsible for non-dystrophic myotonia including hyperkalemic periodic paralysis, paramyotonia congenita, and sodium channel myotonia, as well as congenital myasthenic syndrome. In vitro functional analyses have demonstrated the non-dystrophic mutants to show a gain-of-function defect of the channel; a disruption of fast inactivation, an enhancement of activation, or both, while the myasthenic mutation presents a loss-of function defect. This report presents a case of non-dystrophic myotonia that is incidentally accompanied with acquired myasthenia. The patient presented a marked warm-up phenomenon of myotonia but the repeated short exercise test suggested mutations of the sodium channel. The genetic analysis identified a novel mutation, G1292D, of SCN4A. A functional study of the mutant channel revealed marked enhancement of activation and slight impairment of fast inactivation, which should induce muscle hyperexcitability. The effects of the alteration of channel function to the myasthenic symptoms were explored by using stimulation of repetitive depolarization pulses. A use-dependent channel inactivation was reduced in the mutant in comparison to normal channel, thus suggesting an opposing effect to myasthenia.

Proteomic profiling of naturally protected extraocular muscles from the dystrophin-deficient mdx mouse

Duchenne muscular dystrophy is the most frequent neuromuscular disorder of childhood. Although this x-linked muscle disease is extremely progressive, not all subtypes of skeletal muscles are affected in the same way. While extremities and trunk muscles are drastically weakened, extraocular muscles are usually spared in Duchenne patients. In order to determine the global protein expression pattern in these naturally protected muscles we have performed a comparative proteomic study of the established mdx mouse model of x-linked muscular dystrophy. Fluorescence difference in-gel electrophoretic analysis of 9-week-old dystrophin-deficient versus age-matched normal extraocular muscle, using a pH 4-7 gel range, identified out of 1088 recognized protein spots a moderate expression change in only seven protein species. Desmin, apolipoprotein A-I binding protein and perilipin-3 were found to be increased and gelsolin, gephyrin, transaldolase, and acyl-CoA dehydrogenase were shown to be decreased in mdx extraocular muscles. Immunoblotting revealed a drastic up-regulation of utrophin, comparable levels of beta-dystroglycan and key Ca(2+)-regulatory elements, and an elevated concentration of small stress proteins in mdx extraocular muscles. This suggests that despite the lack of dystrophin only a limited number of cellular systems are perturbed in mdx extraocular muscles, probably due to the substitution of dystrophin by its autosomal homolog. Utrophin appears to prevent the loss of dystrophin-associated proteins and Ca(2+)-handling elements in extraocular muscle tissue. Interestingly, the adaptive mechanisms that cause the sparing of extraocular fibers seem to be closely linked to an enhanced cellular stress response.

Myotonia congenita

Myotonia is a symptom of many different acquired and genetic muscular conditions that impair the relaxation phase of muscular contraction. Myotonia congenita is a specific inherited disorder of muscle membrane hyperexcitability caused by reduced sarcolemmal chloride conductance due to mutations in CLCN1, the gene coding for the main skeletal muscle chloride channel ClC-1. The disorder may be transmitted as either an autosomal-dominant or recessive trait with close to 130 currently known mutations. Although this is a rare disorder, elucidation of the pathophysiology underlying myotonia congenita established the importance of sarcolemmal chloride conductance in the control of muscle excitability and demonstrated the first example of human disease associated with the ClC family of chloride transporting proteins.