Aberrant methylation of tRNAs links cellular stress to neuro‐developmental disorders

Abstract

Mutations in the cytosine-5 RNA methyltransferase NSun2 cause microcephaly and other neurological abnormalities in mice and human. How post-transcriptional methylation contributes to the human disease is currently unknown. By comparing gene expression data with global cytosine-5 RNA methylomes in patient fibroblasts and NSun2-deficient mice, we find that loss of cytosine-5 RNA methylation increases the angiogenin-mediated endonucleolytic cleavage of transfer RNAs (tRNA) leading to an accumulation of 5′ tRNA-derived small RNA fragments. Accumulation of 5′ tRNA fragments in the absence of NSun2 reduces protein translation rates and activates stress pathways leading to reduced cell size and increased apoptosis of cortical, hippocampal and striatal neurons. Mechanistically, we demonstrate that angiogenin binds with higher affinity to tRNAs lacking site-specific NSun2-mediated methylation and that the presence of 5′ tRNA fragments is sufficient and required to trigger cellular stress responses. Furthermore, the enhanced sensitivity of NSun2-deficient brains to oxidative stress can be rescued through inhibition of angiogenin during embryogenesis. In conclusion, failure in NSun2-mediated tRNA methylation contributes to human diseases via stress-induced RNA cleavage.

NSUN 3 and ABH 1 modify the wobble position of mt‐t RNA Met to expand codon recognition in mitochondrial translation

“…m 5 C modifications can be installed by any of the seven proteins of the Nol1/Nop2/SUN domain (NSUN) family and by an enzyme named DNA methyltransferase 2 (DNMT2). DNMT2 mainly catalyses the m 5 C modification in position 38 of tRNA Asp in human cells (Goll et al , 2006), while the so far characterised NSUN proteins show specificity for tRNAs (NSUN2, NSUN6; Schaefer et al , 2010; Tuorto et al , 2012; Blanco et al , 2014; Haag et al , 2015a) or rRNA (NSUN1/NOP2, NSUN5; Sloan et al , 2013; Tafforeau et al , 2013; Schosserer et al , 2015). NSUN2 can also modify vault RNAs and mRNAs (Hussain et al , 2013), and NSUN4 was described to localise to mitochondria where it was shown to methylate the mitochondrial 12S rRNA in mice (Cámara et al , 2011; Metodiev et al , 2014).…”

Sex hormone-dependent tRNA halves enhance cell proliferation in breast and prostate cancers

“…Supporting important roles of tRNA halves in pathogenesis, the accumulation of tRNA halves triggers cellular stress responses and apoptosis in NSun2-mutated fibroblasts of patients from neurodevelopmental disorders or Nsun2-deficient mice (14).…”

“…Ribonuclease/angiogenin inhibitor 1 (RNH1), an ANG inhibitor, was shown to be a negative regulatory factor for ANG cleavage (10). As other regulatory factors, DNA methyltransferase 2 (Dnmt2) and NOP2/Sun RNA methyltransferase 2 (NSun2) modify many tRNAs to generate the 5-methylcytidine (m 5 C) modification, which protects tRNAs from stress-induced cleavage (13,14).…”

Publisher's Note

“…This process is also influenced by modifications -yeast Trm9p, and mammalian NSUN2 and DNMT2 have all been implicated in regulating cleavage through their methyltransferase activity. 32,[63][64][65] The resulting fragments have been demonstrated to bind and inhibit translation machinery, 66 as well as bind and destabilize mRNAs directly. 67 In mouse epidermal stem cells, the expression level of the RNA m 5 C methyltransferase NSUN2 is low but increases over the course of differentiation.…”

“…How tRNA fragments influence cell biology more broadly remains to be worked out. However, the fact that their accumulation has already been connected to neuronal loss, 65 cancer progression, 67 and stem cell differentiation 68 suggests that this may represent a general mechanism by which tRNA modifications can influence cell signaling processes in both normal and disease states.…”