A novel mitochondrial tRNA isoleucine gene mutation causing chronic progressive external ophthalmoplegia.

We report a sporadic case of chronic progressive external ophthalmoplegia that developed during childhood and was associated with ragged-red and cytochrome c oxidase (COX)-negative fibers in skeletal muscle. Sequencing of all the mitochondrial transfer RNA (tRNA) genes identified a single potentially pathogenic mutation--a T to C transition at position 4274 in the tRNA(Ile) gene. This mutation was not present in skeletal muscle from 79 controls, and the level of the mutation in COX-negative fibers was significantly greater than the level in COX-positive fibers.