thiouridine and Mitochondrial-Diseases

Mitochondria are eukaryotic organelles that generate most of the energy in the cell by oxidative phosphorylation (OXPHOS). Each mitochondrion contains multiple copies of a closed circular double-stranded DNA genome (mtDNA). Human (mammalian) mtDNA encodes 13 essential subunits of the inner membrane complex responsible for OXPHOS. These mRNAs are translated by the mitochondrial protein synthesis machinery, which uses the 22 species of mitochondrial tRNAs (mt tRNAs) encoded by mtDNA. The unique structural features of mt tRNAs distinguish them from cytoplasmic tRNAs bearing the canonical cloverleaf structure. The genes encoding mt tRNAs are highly susceptible to point mutations, which are a primary cause of mitochondrial dysfunction and are associated with a wide range of pathologies. A large number of nuclear factors involved in the biogenesis and function of mt tRNAs have been identified and characterized, including processing endonucleases, tRNA-modifying enzymes, and aminoacyl-tRNA synthetases. These nuclear factors are also targets of pathogenic mutations linked to various diseases, indicating the functional importance of mt tRNAs for mitochondrial activity.

Topics: Amino Acyl-tRNA Synthetases; Aminoacylation; Animals; Humans; Mammals; MELAS Syndrome; Mitochondria; Mitochondrial Diseases; Mutation; Oxidative Phosphorylation; Protein Conformation; RNA; RNA Processing, Post-Transcriptional; RNA, Mitochondrial; RNA, Transfer; Thiouridine; Transcription, Genetic; Uridine

Childhood-onset mitochondrial encephalomyopathies are severe, relentlessly progressive conditions. However, reversible infantile respiratory chain deficiency (RIRCD), due to a homoplasmic mt-tRNA(Glu) mutation, and reversible infantile hepatopathy, due to tRNA 5-methylaminomethyl-2-thiouridylate methyltransferase (TRMU) deficiency, stand out by showing spontaneous recovery, and provide the key to treatments of potential broader relevance. Modification of mt-tRNA(Glu) is a possible functional link between these two conditions, since TRMU is responsible for 2-thiouridylation of mt-tRNA(Glu), mt-tRNA(Lys) and mt-tRNA(Gln). Here we show that down-regulation of TRMU in RIRCD impairs 2-thiouridylation and exacerbates the effect of the mt-tRNA(Glu) mutation by triggering a mitochondrial translation defect in vitro. Skeletal muscle of RIRCD patients in the symptomatic phase showed significantly reduced 2-thiouridylation. Supplementation with l-cysteine, which is required for optimal TRMU function, rescued respiratory chain enzyme activities in human cell lines of patients with RIRCD as well as deficient TRMU. Our results show that l-cysteine supplementation is a potential treatment for RIRCD and for TRMU deficiency, and is likely to have broader application for the growing group of intra-mitochondrial translation disorders.

Topics: Cell Line; Cysteine; Gene Expression Regulation; Humans; Mitochondria; Mitochondrial Diseases; Mitochondrial Encephalomyopathies; Mitochondrial Proteins; Muscle, Skeletal; Mutation; Myoblasts; Oxidative Phosphorylation; Protein Biosynthesis; RNA, Transfer; Thiouridine; tRNA Methyltransferases