LAUSR

masticatory and facial muscles, and mild dysarthria and To the Editor: Mutations in mitochondrial DNA (mtDNA) cause mitochondrial diseases with multisystem involvedysphagia. At the age of 32, he complained of distal ment and variable clinical phenotypes. Myoclonic epilepsy with ragged red fiber (MERRF) syndrome is characterized by myoclonus, generalized epilepsy, cerebellar ataxia, and mitochondrial myopathy with ragged red fibers (RRFs). Other features include hearing impairment, psychiatric disorders, and dysarthria. More than 20 heteroplasmic point mutations have been identified as responsible for MERRF, indicating the genetic heterogeneity of mitochondrial diseases. The m.8344A>G mutation in the mitochondrial tRNA (MT-TK) gene is the most common mutation and is responsible for approximately 80% of MERRF cases. Other MERRF-related mutations are relatively rare, but up to 10% of MERRF patients have no identifiable mutations. The m.5703G>A mutation in the mitochondrial tRNA (MT-TN) gene has previously been reported to cause mitochondrial myopathy (MM). To the best of our knowledge, no association has been reported between MERRF syndrome and the m.5703G>A mutation. The present report describes a patient with typical MERRF syndrome carrying a heteroplastic m.5703G>A mutation and expands not only the genotypic spectrum of MERRF but also the phenotypic spectrum of the m.5703G>A mutation.