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Exercise intolerance due to a nonsense mutation in the mtDNA ND4 gene

We report the first molecular defect in an NADH‐dehydrogenase gene presenting as isolated myopathy. The proband had lifelong exercise intolerance but no weakness. A muscle biopsy showed cytochrome c oxidase (COX)–positive ragged‐red fibers (RRFs), and analysis of the mitochondrial enzymes revealed complex I deficiency. Sequence analysis of the mitochondrial genes encoding the seven NADH‐dehydrogenase subunits showed a G‐to‐A transition at nucleotide 11832 in the subunit 4 (ND4) gene, which changed an encoded tryptophan to a stop codon. The mutation was heteroplasmic (54%) in muscle DNA. Defects in mitochondrially encoded complex I subunits should be added to the differential diagnosis of mitochondrial myopathies. Ann Neurol 1999;45:820–823