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Scientists discover molecular mechanism for deafness induced by defects in modifications of mitochondrial tRNAs

2021-01-22

Hearing impairment may result from a multitude of factors, including heredity, environment and their interactions. Mitochondria are the hub of energy metabolism in eukaryotic cells and the control center of cellular signaling. tRNA plays a crucial role as a carrier of amino acid transport during protein synthesis. A lot of posttranscriptional modifications exist in mitochondrial tRNAs (8.7%). In human mitochondria, 18 types of nucleotide modifications are present in the 137 positions of 22 tRNA species, encoded by mitochondrial DNA (mtDNA) and they play an important role in the proper processing, stability, folding and decoding properties of tRNAs as well as the fidelity and efficiency of translation.The nucleotides at position 37 (A or G) of mammalian mitochondrial tRNAs contain a diverse range of complex modifications includingt6A, i6A, ms2i6A, and m1G.

The research team led by Prof. GUAN Minxin, Director of the Institute of Genetics at the Zhejiang University School of Medicine, offers a new insight into the mechanism for hearing impairment caused by deficient posttranscriptional modifications in the structure and function of tRNA, which was published in Nucleic Acids Research.

A total of 2651 genetically unrelated Han Chinese subjects with hearing impairment and 574 normal hearing Han Chinese control subjects participated in this study. Researchers investigated the molecular mechanism underlying a deafness-associated tRNAIle 4295A>G mutation affecting a highly conserved adenosine at position 37, 3′ adjacent to the tRNA’s anticodon. The Chinese pedigree exhibited the maternal inheritance of hearing loss and 9 of 14 matrilineal relatives displayed the variable degree of hearing impairment. The age-at-onset of hearing loss ranged from 23 to 50 years, with the average of 35 years old. There was no evidence that any of other members of this family had any other causes to account for hearing loss. 

Primer extension and methylation activity assays revealed that the m.4295A>G mutation introduced a tRNA methyltransferase 5 (TRMT5)-catalyzed m1G37 modification of tRNAIle. Molecular dynamics simulations suggested that the m.4295A>G mutation affected tRNAIle structure and function, supported by increased melting temperature, conformational changes and instability of mutated tRNA. An in vitro processing experiment revealed that the m.4295A>G mutation reduced the 5′ end processing efficiency of tRNAIle precursors, catalyzed by RNase P. 

This research demonstrated that cybrid cell lines carrying the m.4295A>G mutation exhibited significant alterations in aminoacylation and steady-state levels of tRNAIle. The aberrant tRNA metabolism resulted in the impairment of mitochondrial translation, respiratory deficiency, decreasing membrane potentials and ATP production, increasing production of reactive oxygen species and promoting autophagy. These revealed the pleiotropic effects of m.4295A>G mutation on tRNAIle and mitochondrial functions.

“These findings highlighted the essential role of deficient tRNA modifications in mitochondrial biogenesis and their pathogenic consequence of hearing loss,” said GUAN Minxin. “The approach utilized in this study also provides a paradigm for understanding the pathogenic effects of other mitochondrial tRNA mutations.”