Scapuloperoneal Myopathy, Myh7-Related

A number sign (#) is used with this entry because this form of scapuloperoneal myopathy is caused by mutation in the MYH7 gene (160760). Another form (300695) is caused by mutation in the FHL1 gene (300163).

Clinical Features

Scapuloperoneal syndrome was initially described by Jules Broussard (1886) as 'une forme hereditaire d'atrophie musculaire progressive' beginning in the lower legs and affecting the shoulder region earlier and more severely than distal arm.

Thomas et al. (1975) described 6 cases of adult-onset scapuloperoneal myopathy. Four were apparently sporadic. The other 2 cases occurred in mother and daughter. Progression was relatively slow. Electromyography and muscle biopsy showed myopathic changes in all. Facial involvement occurred in some. The authors considered that the disorder resembled that described by Ricker and Mertens (1968) and Serratrice et al. (1969). The latter group observed 9 cases in which autosomal dominant inheritance was suggested.

Tawil et al. (1995) described 4 individuals in 2 generations, 1 female and 3 males, affected with a scapuloperoneal myopathy. There was male-to-male transmission. Electromyography demonstrated small polyphasic units, and muscle biopsy demonstrated necrotic and regenerating fibers as well as an increase in endomesial connective tissue, demonstrating this to be a myopathy. Although the index case fulfilled the diagnostic criteria for facioscapulohumeral dystrophy (158900), none of the other 3 affected individuals demonstrated facial weakness. Furthermore, linkage to markers on 4q35 was excluded, demonstrating this to be a distinct genetic entity.

Molecular Genetics

In 2 patients diagnosed with myosin storage myopathy (608358) and 2 of 17 patients diagnosed with scapuloperoneal myopathy of unknown etiology, Pegoraro et al. (2007) detected a 5533C-T mutation in the MYH7 gene (160760.0028). Eleven other mutation carriers were identified through segregation analysis. The clinical spectrum in this cohort of patients included asymptomatic hyperCKemia (elevated serum creatine kinase), scapuloperoneal myopathy, and proximal and distal myopathy with muscle hypertrophy. Muscle MRI identified a unique pattern in the posterior compartment of the thigh, characterized by early involvement of the biceps femoris and semimembranosus, with relative sparing of the semitendinosus. Muscle biopsy revealed hyaline bodies characteristic of myosin storage myopathy in only half of biopsied patients (2 of 4). These patients without hyaline bodies had been diagnosed with scapuloperoneal myopathy prior to the identification of hyaline bodies in other family members, prompting MYH7 gene analysis. The authors pointed out that patients without hyaline bodies presented later onset and milder severity. Pegoraro et al. (2007) concluded that the phenotypic and histopathologic variability may underlie MYH7 gene mutation and the absence of hyaline bodies in muscle biopsies does not rule out MYH7 gene mutations.

Animal Model

De Repentigny et al. (2001) described a spontaneous autosomal recessive mutation in the mouse, which they named 'degenerative muscle' (dmu), that is characterized by skeletal and cardiac muscle degeneration. Dmu mice are weak and have great difficulty in moving due to muscle atrophy and wasting in the hindquarters. Histopathologic observations and ultrastructural analysis revealed muscle degeneration in both skeletal and cardiac muscle, but no abnormalities in sciatic nerves. It is noteworthy that SPM patients with associated cardiomyopathy have been described. Using linkage analysis, the authors mapped the dmu locus to the distal portion of mouse chromosome 15 in a region syntenic to human chromosome 12q13. Intact transcripts for Scn8a (600702), the gene encoding the sodium channel 8a subunit, were present in dmu mice but their levels were dramatically reduced. Furthermore, genetic complementation crosses between dmu and med (mutation in Scn8a) mice revealed that they are allelic. The authors concluded that at least a portion of the dmu phenotype may be caused by a downregulation of Scn8a, and that SCN8A is a candidate gene for human SPM.