Restoration of mitochondrial function through activation of hypomodified tRNAs with pathogenic mutations associated with mitochondrial diseases

Mutations in mitochondrial (mt-)tRNAs frequently cause mitochondrial dysfunction. Mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), and myoclonus epilepsy associated with ragged red fibers (MERRF) are major clinical subgroups of mitochondrial diseases caused by pathogenic point mutations in tRNA genes encoded in mtDNA. We previously reported a severe reduction in the frequency of 5-taurinomethyluridine (tau m(5)U) and its 2-thiouridine derivative (tau m(5)s(2)U) in the anticodons of mutant mt-tRNAs isolated from the cells of patients with MELAS and MERRF, respectively. The hypomodified tRNAs fail to decode cognate codons efficiently, resulting in defective translation of respiratory chain proteins in mitochondria. To restore the mitochondrial activity of MELAS patient cells, we overexpressed MTO1, a tau m(5)U-modifying enzyme, in patient-derived myoblasts. We used a newly developed primer extension method and showed that MTO1 overexpression almost completely restored the tau m(5)U modification of the MELAS mutant mt-tRNA(Leu(UUR)). An increase in mitochondrial protein synthesis and oxygen consumption rate suggested that the mitochondrial function of MELAS patient cells can be activated by restoring the tau m(5)U of the mutant tRNA. In addition, we confirmed that MTO1 expression restored the tau m(5)s(2)U of the mutant mt-tRNA(Lys) in MERRF patient cells. These findings pave the way for epitranscriptomic therapies for mitochondrial diseases.

Automated summary

Mutations in mitochondrial tRNA genes can lead to mitochondrial dysfunction. The diseases MELAS and MERRF are caused by pathogenic point mutations in mtDNA's tRNA genes. The modified tRNAs fail to efficiently decode codons, resulting in defective translation of respiratory chain proteins in mitochondria. Restoring the modified tRNAs can activate mitochondrial function in patient cells and pave the way for epitranscriptomic therapies for mitochondrial diseases.