Clinical and cellular consequences of the mutation m.12300G>A in the mitochondrial tRNALeu(CUN) gene

► We describe a carrier patient of the mutation m.12300G>A in the mt-tRNALeu(CUN) gene. ► Clinical phenotype is characterized by an overlap MERRF-NARP syndrome. ► There is a correlation between muscle histochemistry and the levels of heteroplasmy. ► Respiratory chain activity is affected in muscle, fibroblasts and cybrid cell lines. ► A high mutant load is associated with a drop of ATP levels and the membrane potential.