tRNAs are the core molecule of mRNA translation. tRNAs harbor approximately 80% of the RNA modification types, with the majority located in the anticodon loop, for stabilizing codon-anticodon interactions, and regulating the translation efficiency and fidelity.
In a study published in Nucleic Acids Res, teams led by Prof. ZHOU Xiao-Long from the Center for Excellence in Molecular Cell Science, Shanghai Institute of Biochemistry and Cell Biology of the Chinese Academy of Sciences, in collaboration with Prof. SHEN Bin from Nanjing Medical University and Prof. YAN Chuanzhu from Qilu Hospital, Shandong University revealed that taurine hypomodification underlies mitochondrial tRNATrp-related genetic diseases.
Mitochondrion is one of the most crucial organelles in eukaryotes, regulating energy production and other multiple cellular functions. Human mitochondrion harbors a separate protein synthesis system. Human mitochondrial genome encodes 22 tRNAs (mtRNAs), responsible for generating 13 proteins in respiratory chain complexes.
Notably, mtRNAs contain 18 types of post-transcriptional modification, among which five mtRNAs harbor 5-taurinomethyluridine (τm5U) or its derivative 5-taurinomethyl-2-thiouridine (τm5s2U) on wobble position U34, which are mediated by nuclear DNA encoded tRNA modifying enzymes GTPBP3 and MTO1. In bacteria, structurally equivalent 5-carboxymethylaminomethyluridine (cmnm5U) and its derivative 5-carboxymethylaminomethyl-2-thiouridine (cmnm5s2U) are introduced by homologous MnmE and MnmG.
Previous studies found that m.3243A>G in mtRNALeu(UUR) and m.8344A>G in mtRNALys lead to τm5U defect and inability to efficiently decode the cognate codons, resulting in mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS) and myoclonic epilepsy with ragged red fibers (MERRF), respectively. Besides the two best-studied mtRNAs, it has been clinically reported that there are also several mutations on the other three tRNAs harboring τm5U and τm5s2U (mtRNAGln, mtRNAGlu, mtRNATrp) leading to mitochondrial diseases.
However, whether mtDNA variants in these mtRNAs are associated with τm5U modification defect remains unknown. Meanwhile, the underlying mechanism for τm5U modification defects induced by mtDNA mutations has remained unclear for several decades due to difficulty in efficiently reconstituting τm5U modification.
In this work, the researchers purified naturally assembled EcMnmEG complex and confirmed its high activity in mediating cmnm5U of EctRNAArg(UCU). Notably, EcMnmEG showed the plasticity of amino acid and tRNA substrates, which could introduce both cmnm5U and τm5U in mtRNATrp.
They revealed that multiple clinical mtRNATrp pathogenic mutations caused U34-hypomodification in vitro. A MELAS patient-derived cell line harboring m.5541C>T exhibited mtRNATrp τm5U-hypomodification. Furthermore, via mitochondrial gene editing, the researchers constructed two cell lines carrying m.5532G>A or m.5545C>T mutation, which likewise exhibited mtRNATrp τm5U-hypomodification. They verified that selected mutations in the anticodon and D-stem of the mtRNATrp locus abolished τm5U biogenesis in vivo and impaired mitochondrial translation and function. Finally, they demonstrated that taurine supplementation could ameliorate τm5U modification level and mitochondrial translation in patient-derived cells. Taurine supplementation as a potential intervention strategy should be further explored in mtRNATrp-related diseases.
Taken together, this study, utilizing the plasticity of EcMnmEG for amino acid and tRNA substrates, provides the third example of an mtRNA species exhibiting mutation-related τm5U hypomodification, and sheds light into pathogenesis and intervention of specific mtDNA variants.
Reference: https://academic.oup.com/nar/advance-article/doi/10.1093/nar/gkae854/7815982
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