A family with an unusual myotonic and myopathic phenotype and no CTG expansion (proximal myotonic myopathy syndrome): a challenge for future molecular studies

Abstract

Myotonic dystrophy (DM) is a well-defined autosomal dominant disorder characterized by myotonia, muscle weakness, cardiac conduction defects, cataracts, and endocrine abnormalities. Recently a newly recognized disorder, similar to but distinct from DM, has been observed with multisystem findings including intermittent myotonia, proximal myopathy, and occasional cardiac conduction disturbances. This disorder has been called proximal myotonic myopathy (PROMM). No history of anticipation is present and there is no linkage to the gene locus for DM or to the loci for the muscle sodium or chloride channels. This report describes a family with a normal size of the CTG trinucleotide repeat expansion of the DM gene in which affected individuals have myotonia (intermittent, exacerbated by cold), bilateral cataracts, mild hypogonadism and mild temporal atrophy. Affected individuals also have proximal muscle weakness, facial involvement, nonspecific abnormalities on muscle biopsy, normal cardiac conduction, and no glucose intolerance. The absence of trinucleotide repeat expansion in the DM gene is consistent with this family being affected by a disorder distinct from DM, possibly a form of PROMM.