A novel heteroplasmic tRNAleu(CUN) mtDNA point mutation in a sporadic patient with mitochondrial encephalomyopathy segregates rapidly in skeletal muscle and suggests an approach to therapy

A novel mtDNA point mutation was detected in the tRNAleu(CUN) gene (G to Aat position 12315) in a sporadic patient with chronic progressive externalophthalmoplegia, ptosis, limb weakness, sensorineural hearing loss and apigmentary retinopathy. The mutation disrupts base pairing in the T psi Cstem at a site which has been conserved throughout evolution. Although theother mtDNA tRNAleu gene (UUR) is a hotspot for mutation, this is the firstpathogenic mutation to be reported in the gene coding for tRNAleu(CUN).MtDNAs carrying the mutation constituted 94% of total mtDNAs in twoseparate muscle biopsies. Single fibre analysis showed that skeletal musclefibres without detectable cytochrome c oxidase activity (COX-ve fibres)contained predominantly mutant mtDNAs (93-98%) while fibres with apparentlynormal COX activity had up to 90% mutant mtDNAs, demonstrating that theG12315A mutation is functionally recessive. Immunofluorescence studies withspecific antibodies to mtDNA- or nuclear-encoded subunits of COX wereconsistent with a defect in mitochondrial protein translation. The mutationwas not present in blood cells or cultured fibroblasts and surprisingly, itcould not be detected in satellite cells cultured from the patient'smuscle. This pattern, which may by typical of patients who have inheritednew germline pathogenic mtDNA mutations, possibly reflects loss of themutation by random genetic drift in mitotic tissues and proliferation ofmitochondria containing the mutant mtDNA in post- mitotic cells. Theabsence of mtDNA carrying the mutation in satellite cells suggests thatregeneration of skeletal muscle fibres from satellite cells could restore awild-type mtDNA genotype and normal muscle function.