Molecular Mechanism On The Mitochondrial TRNA^Asp 7551A>G Mutation Associated With Deafness

Mutations in mitochondrial tRNA genes are associated with a wide spectrum of human diseases. The defects in mitochondrial protein synthesis have been associated with both syndromic deafness (hearing loss with other medical problems such as diabetes) and non-syndromic deafness (hearing loss is the only obvious medical problem). Human mitochondrial DNA (mtDNA) encodes 2 rRNAs and 22 tRNAs required for the mitochondrial protein synthesis and 13 subunits of oxidative phosphorylation system. The m.7551 mutation in the mitochondrial tRNAAsp gene was associated with maternally transmitted non-syndromic deafness in a large Han Chinese pedigree. The m.7551A>G mutation is localized at a highly conserved nucleotide (A37), adjacent (3’) to the anticodon of the tRNAsp. The adenine at this position (A37) of tRNAsP is extraordinarily conserved from bacteria to human mitochondria. Modification at position A37 was shown to contribute to the high fidelity of codon recognition and to the structural formation and stabilization of functional tRNAs.To investigate the pathogenic mechanism of the m.tRNAAsp 7551A>G mutation, cybrid cell lines were constructed by transferring mitochondria from lymphoblastoid cell lines derived from an affected matrilineal relative in a Chinese family carrying the mtDNA mutation and from a control individual lacking the mtDNA mutation (the same haplotype), into human mtDNA-less (p°) cells. These cybrid cell lines were first examined for the presence and degree of the mtDNA mutation. These cell lines were then assessed for the effects of the mtDNA mutation on the aminoacylation capacity and stability of this tRNA, mitochondrial translation, respiration, and the production of ATP and oxidative reactive species (ROS). Three cybrid cell lines derived from an affected matrilineal relative carrying the m.7551A>G mutation exhibited-20% decreased efficiency of aminoacylated tRNAAsp in mutant cell lines, compared with three control cybrid cell lines. Mutant cybrid cell lines also displayed-42% decrease in the steady-state level of tRNAAsp, as compared with control cybrid cell lines. Altered metabolism of tRNAsP was responsible for the variable reductions in 7 mtDNA-encoded polypeptides in mutant cells, which ranged from 24% to 61%, with an average of 35%. The impaired mitochondrial translation caused defects in respiratory capacity in mutant cells. As a result, this respiratory deficiency reduced mitochondrial ATP production and increased the production of oxidative reactive species. While, our results showed that the m.tRNAAsp 7551A>G mutation did not make the significant influence on the mitochondrial membrane potential and autophagy. However, mild mitochondrial dysfunction caused by the m.tRNAAsp 7551A>G mutation suggested that deafness expression could be involved in other genetic, epigenetic and environmental modifier factors. Thus, our findings may provide the new insights into the understanding of pathophysiology of maternally inherited deafness.The protein of TRIT1 and TRMT5 is the very important modification enzymes for the tRNA 37 site. The localizations of TRIT1 and TRMT5 are in the mitochondria. Moreover, three cybrid cell lines carrying the m.7551 A>G mutation exhibited obvious decreased the mRNA expression of TRIT1 and TRMT5 in mutant cell lines, compared with three control cybrids. Therefore, the m.7551A>G mutation may affect the modification of tRNA 37 site by decreasing the expression of TRIT1 and TRMT5.