Myasthenic Syndrome, Congenital, 10

A number sign (#) is used with this entry because congenital myasthenic syndrome-10 (CMS10) is caused by homozygous or compound heterozygous mutation in the DOK7 gene (610285) on chromosome 4p.

Description

Congenital myasthenic syndromes (CMS) are a group of inherited disorders affecting the neuromuscular junction (NMJ). Patients present clinically with onset of variable muscle weakness between infancy and adulthood. These disorders have been classified according to the location of the defect: presynaptic, synaptic, and postsynaptic. CMS10 is an autosomal recessive CMS resulting from a postsynaptic defect affecting endplate maintenance of the NMJ. Patients present with limb-girdle weakness in the first decade. Treatment with ephedrine or salbutamol may be beneficial; cholinesterase inhibitors should be avoided (summary by Engel et al., 2015).

For a discussion of genetic heterogeneity of CMS, see CMS1A (601462).

Clinical Features

Azulay et al. (1994) reported a woman with proximal muscle weakness of the myasthenic type. Family history was not reported.

Shankar et al. (2002) reported 2 Indian sisters with classic features of limb-girdle myasthenia.

Beeson et al. (2006) summarized the clinical features of 19 of 21 index patients with limb-girdle myasthenia and the affected brother of case 11, all of whom had mutations in DOK7. All patients displayed electromyographic (EMG) evidence of a defect in neuromuscular transmission and all but 3 patients developed weakness within the first 5 years of life. The clinical onset of disease was generally characterized by difficulty in walking after initially achieving normal walking milestones. Features typically seen in patients with mutations in rapsyn (RAPSN; 601592), such as congenital joint deformity and squint, were not present. In adulthood, a proximal weakness of the affected patients' upper and lower extremities was evident, and most had weakness in the trunk and neck regions. All of the patients had weak facial muscles, and all but 2 patients had ptosis. Eye movements were generally unaffected. Anticholinesterase medication either had no effect or made the weakness worse, although a short-lived initial response was occasionally seen. Beeson et al. (2006) also undertook an analysis of motorpoint muscle biopsies, which showed that many features of the NMJ were normal, including the quantal release per unit area of synaptic content and the size and kinetics of the miniature endplate currents. However, 2 major abnormalities were identified: reduced size of the NMJs and reduced postsynaptic folding.

Beeson et al. (2006) noted that another genetic congenital myasthenic syndrome with a limb girdle-like pattern of muscle weakness is associated with tubular aggregates in muscle biopsies (CMSTA1; 610542). Tubular aggregates were present in the 1 patient from the study by Slater et al. (2006) in whom no DOK7 mutation was found, but were not present in patients with DOK7 mutations. The patient with tubular aggregates was later found by Belaya et al. (2012) to carry compound heterozygous mutations in the DPAGT1 gene (191350.0002 and 191350.0003; see CMSTA2, 614750). In contrast with patients harboring DOK7 mutations, patients with tubular aggregates tend to respond well to anticholinesterase medication, suggesting that the phenotypes constitute separate disorders.

Selcen et al. (2008) reported 16 patients with DOK7-related congenital myasthenia. The age at onset ranged from the first day of life to age 5 years, and there was great variability in disease severity and rate of progression. Some patients had mild static weakness limited to limb-girdle muscles, whereas others had severe generalized disease with marked muscle atrophy. Ten patients had intermittent worsenings lasting from days to weeks. All patients reported fatigue on exertion and proximal muscle weakness. Other common features included ptosis (14 patients), facial weakness (13), bulbar symptoms (11), and respiratory difficulties (13). Oculoparesis was less common (6), and only 3 showed decreased fetal movements. In general, there was a poor response to cholinesterase inhibitors and cholinergic agents. The authors found no consistent correlation between the clinical severity and expression of DOK7 at the endplate; in fact, some patients with severe disease showed almost normal expression. Electron microscopy of endplates showed variable changes, including degeneration of junctional folds, reduced nerve terminals, or degeneration of subsynaptic organelles, but some endplates appeared normal. In vitro microelectrode studies showed decreased numbers of released quanta and decreased synaptic response to acetylcholine. Acetylcholine receptors (AChR) were decreased in areas of degenerating junctional folds, but AChR kinetics were normal. Selcen et al. (2008) noted that the studies did not reveal a clear correlation between histologic or electrophysiologic findings and disease severity in individual patients, but indicated that changes in the structural integrity of the endplate likely contributed to the decreased safety margin of neuromuscular transmission.

Clinical Management

Lashley et al. (2010) reported that ephedrine therapy was a favorable and effective treatment for congenital myasthenic syndrome due to DOK7 mutations. A prospective trial of 12 patients showed that 10 tolerated ephedrine well. Over a 6 to 8 month treatment period, these 10 patients showed significant improvements in quantitative myasthenia gravis and mobility scores. Ephedrine was administered orally between 15 and 90 mg/day. Lashley et al. (2010) noted that these patients do not usually respond to acetylcholinesterase inhibitors, and postulated that the beneficial effect of ephedrine may be related to its action as a beta-2-adrenergic receptor (109690) agonist.

Molecular Genetics

Beeson et al. (2006) identified frameshift mutations in the DOK7 gene (610285) in 16 unrelated patients with limb-girdle type congenital myasthenic syndrome. In 3 additional patients, a frameshift mutation was identified in combination with a nonsense mutation, a splice site mutation, and a missense change of a conserved residue. These mutations were found in homozygosity or compound heterozygosity, consistent with recessive inheritance seen in this phenotype. Beeson et al. (2006) also identified C-terminal domain frameshift mutations in DOK7 in DNA available from 6 of 7 patients included in the study by Slater et al. (2006). The seventh patient reported by Slater et al. (2006) was found by Belaya et al. (2012) to carry compound heterozygous mutations in the DPAGT1 gene (191350.0002 and 191350.0003; see 614750).

Among 16 patients with limb-girdle congenital myasthenic syndrome, Selcen et al. (2008) identified 17 different mutations in the DOK7 gene, including 10 novel mutations (see, e.g., 610285.0009; 610285.0010). All of the mutations resulted in a termination codon or a frameshift, except for 3 that resulted in the in-frame deletion of 1 or more exons. In vitro functional expression studies in murine myotubes showed that many of the mutations resulted in decreased axial length and density of AChR clusters at the endplate. Selcen et al. (2008) concluded that the pathogenesis of the disorder results from destruction and simplification of synaptic structures with resultant decrease in neuromuscular transmission.

Animal Model

Okada et al. (2006) generated mice lacking Dok7 and observed that all homozygous Dok7-deficient mice were immobile at birth and died shortly thereafter. Alveoli of these mice were not expanded at birth, indicating a failure to breathe and suggesting a severe defect in neuromuscular transmission in skeletal muscles. Heterozygous-deficient littermates were normal. Okada et al. (2006) found that Dok7 homozygous mutants formed neither AChR clusters nor neuromuscular synapses. They concluded that neuromuscular synaptogenesis requires DOK7 within skeletal muscle.

Muller et al. (2010) reported that Dok7 deficiency led to motility defects in zebrafish embryos and larvae. The relative importance of Dok7 at different stages of neuromuscular junction development varied; it was crucial for the earliest step, the formation of acetylcholine receptor (AChR) clusters in the middle of the muscle fiber prior to motor neuron contact. At later stages, presence of Dok7 was not absolutely essential, as focal and nonfocal synapses did form when Dok7 expression was downregulated. However, these contacts were smaller than in the wildtype zebrafish, reminiscent of the neuromuscular endplate pathology seen in patients with DOK7 mutations. Changes in slow muscle fiber arrangement were also observed. The authors suggested an additional role for Dok7 in muscle that is independent of the muscle-specific tyrosine kinase MuSK (601296), the binding partner of Dok7 at the neuromuscular junction.

In a mouse model of DOK7 myasthenia, Arimura et al. (2014) showed that therapeutic administration of an adeno-associated virus (AAV) vector encoding human DOK7 resulted in an enlargement of neuromuscular junctions (NMJs) and substantial increases in muscle strength and life span. When applied to model mice of another neuromuscular disorder, autosomal dominant Emery-Dreifuss muscular dystrophy (EDMD2; 181350), DOK7 gene therapy likewise resulted in enlargement of NMJs as well as positive effects on motor activity and life span. Arimura et al. (2014) concluded that these results suggested that therapies aimed at enlarging the NMJ may be useful for a range of neuromuscular disorders.