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A 6-week-old child presented with hypotonia, myopathy, and a rapidly worsening dilated cardiomyopathy with severe atrial and ventricular arrhythmias and pulmonary hypertension, which proved fatal at age 3 months. Biochemical analysis showed a combined deficiency of the enzymatic activities of complexes I and IV and molecular studies identified a T14709C mutation in the mitochondrial tRNA glutamic acid gene. A review of symptomatology in patients with this mutation shows that it mainly presents in childhood or young adults with mild myopathy and diabetes mellitus. Infants with a high, nearly homoplasmic mutant load can present with more severe symptoms including cardiomyopathy. Families with this mitochondrial DNA mutation should be aware that increased mutant load in a subsequent generation may result in severe and often fatal cardiac symptoms.

A 14-year-old boy had exercise intolerance, weakness, ataxia, and lactic acidosis. Because his muscle biopsy showed a mosaic pattern of fibers staining intensely with the succinate dehydrogenase reaction but not at all with the cytochrome c oxidase reaction, we sequenced his mitochondrial DNA and found a novel mutation (C14680A) in the gene for tRNAGlu. The mutation was present in accessible tissues from the asymptomatic mother but not from a brother with Asperger syndrome. These data expand the clinical heterogeneity of mutations in this mitochondrial gene.