A novel mitochondrial tRNA^Leu(UUR) mutation in a patient with features of MERRF and Kearns-Sayre syndrome

In a patient with clinical features of both myoclonus epilepsy ragged-red fibers (MERRF) and Kearns-Sayre syndrome (KSS), we identified a novel guanine-to-adenine mitochondrial DNA (mtDNA) mutation at nucleotide 3255 (G3255A) of the tRNA^Leu(UUR) gene. Approximately 5% of the skeletal muscle fibers had excessive mitochondria by succinate dehydrogenase histochemistry while a smaller proportion showed cytochrome c oxidase (COX) deficiency. In skeletal muscle, activities of mitochondrial respiratory chain complexes I, I+III, II+III, and IV were reduced. The G3255A transition was heteroplasmic in all tissues tested: muscle (53%), urine sediment (67%), peripheral leukocytes (22%), and cultured skin fibroblasts (<2%). The mutation was absent in 50 control DNA samples. Single-fiber analysis revealed a higher proportion of mutation in COX-deficient RRF (94%±5, n=25) compared to COX-positive non-RRF (18%±9, n=21). The identification of yet another tRNA^Leu(UUR) mutation reinforces the concept that this gene is a hot-spot for pathogenic mtDNA mutations.