Double methylation of tRNA-U54 to 2′-O-methylthymidine (Tm) synergistically decreases immune response by Toll-like receptor 7

Abstract

Sensing of nucleic acids for molecular discrimination between self and non-self is a challenging task for the innate immune system. RNA acts as a potent stimulus for pattern recognition receptors including in particular human Toll-like receptor 7 (TLR7). Certain RNA modifications limit potentially harmful self-recognition of endogenous RNA. Previous studies had identified the 2′-O-methylation of guanosine 18 (Gm18) within tRNAs as an antagonist of TLR7 leading to an impaired immune response. However, human tRNALys3 was non-stimulatory despite lacking Gm18. To identify the underlying molecular principle, interferon responses of human peripheral blood mononuclear cells to differentially modified tRNALys3 were determined. The investigation of synthetic modivariants allowed attributing a significant part of the immunosilencing effect to the 2′-O-methylthymidine (m5Um) modification at position 54. The effect was contingent upon the synergistic presence of both methyl groups at positions C5 and 2’O, as shown by the fact that neither Um54 nor m5U54 produced any effect alone. Testing permutations of the nucleobase at ribose-methylated position 54 suggested that the extent of silencing and antagonism of the TLR7 response was governed by hydrogen patterns and lipophilic interactions of the nucleobase. The results identify a new immune-modulatory endogenous RNA modification that limits TLR7 activation by RNA.

FICC-Seq: a method for enzyme-specified profiling of methyl-5-uridine in cellular RNA

“…Our findings reveal that TRMT2A is the major human m5U methyltransferase and that it commonly targets U54 of cytosolic tRNAs. Although the primary functions of m5U54 in cytosolic tRNA remain to be determined in detail, it has recently been shown to be involved in the Toll-like receptor 7-dependent immune response (30). By establishing TRMT2A as the human genetic factor likely solely responsible for cytosolic m5U54 formation, our study should aid further investigations into the regulation of the modification and its biological functions.…”

RNA Modifications Modulate Activation of Innate Toll-Like Receptors

“…Co-transfection experiments with otherwise immune stimulatory bacterial RNA elucidated that tRNA Lys 3 itself is indeed non-stimulatory without inhibiting TLR7 response against stimulatory RNA. A double-methylation of uridine to 2’- O -methyl thymidine at position 54 of human tRNA was identified as sufficient to partially silence immune recognition of RNA without dominant inhibition of TLR7 [ 86 ]. However, this effect was considerably weaker compared to Gm18 indicating that the collective engagement of all posttranscriptional modifications within human tRNA Lys 3 contribute to its immune silencing effect.…”

Transfer RNA Modification Enzymes from Thermophiles and Their Modified Nucleosides in tRNA

“…Furthermore, in some cases, the biosynthesis pathways of some modifications may differ between thermophiles and mesophiles. Moreover, in eukaryotes, tRNA modifications are related to higher biological processes such as cellular transport of tRNA [273,274,275,276,277,278], RNA quality control [274,279,280,281], infection [282,283,284,285,286], and the immune response [287,288,289,290]. As yet, modified nucleosides in tRNA from thermophilic eukaryotes have not been investigated, but it is possible that a relationship between modified nucleosides in tRNA and these biological phenomena may also be discovered in thermophilic eukaryotes.…”