An RNA editing/binding-independent gene regulatory mechanism of ADARs and its clinical implication in cancer

Abstract

Adenosine-to-inosine (A-to-I) editing, catalysed by Adenosine DeAminases acting on double-stranded RNA (dsRNA) (ADAR), occurs predominantly in the 3' untranslated regions (3'UTRs). Here we uncover an unanticipated link between ADARs (ADAR1 and ADAR2) and the expression of target genes undergoing extensive 3'UTR editing. Using METTL7A (Methyltransferase Like 7A), a novel tumor suppressor as an exemplary target gene, we demonstrate that its expression could be repressed by ADARs beyond their RNA editing and dsRNA binding functions. ADARs interact with Dicer to augment the processing of pre-miR-27a to mature miR-27a. Consequently, mature miR-27a targets the METTL7A 3'UTR to repress its expression level. In sum, our study unveils that the extensive 3'UTR editing is merely a footprint of ADAR binding, and is dispensable for the regulation of at least a subset of target genes. Instead, ADARs contribute to cancer progression by regulating cancer-related gene expression through their non-canonical functions independent of RNA editing and dsRNA binding.The functional significance of ADARs is much more diverse than previously appreciated and this gene regulatory function of ADARs is most likely to be of higher importance than the best-studied editing function. This novel non-editing side of ADARs opens another door to target cancer. This study is timely and represents a major break-through in the field of ADAR gene regulation and cancer biology. (215 words)peer-reviewed) is the author/funder. All rights reserved. No reuse allowed without permission.The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/062778 doi: bioRxiv preprint first posted online Qi, L., et al., Page 3 Adenosine DeAminases acting on dsRNA (ADAR) are highly conserved family of enzymes catalysing adenosine to inosine deamination (A-to-I editing) (Bass and Weintraub 1988;Wagner et al. 1989).There are 3 ADAR proteins (ADAR1, ADAR2 and ADAR3) in humans which all share a common modular structure characterized by 2 to 3 N-terminal dsRNA binding domains (dsRBDs) and a conserved C-terminal catalytic deaminase domain (Nishikura 2010;Qi et al. 2014). Being the beststudied function associated with ADAR1 and ADAR2 (ADARs), A-to-I RNA editing contributes to multi-level gene regulation depending on where it occurs. ADAR3, which has no documented deaminase activity, is only expressed in central nervous system (Melcher et al. 1996). In coding regions, A-to-I RNA editing can lead to a codon change and the consequent alterations of proteincoding sequences since inosine is interpreted by the ribosome as guanosine (Nishikura 2010). The differential editing frequencies of these recoding sites are found to impact on human diseases such as neurological diseases and cancer (Martinez et al. 2008;Chen et al. 2013;Galeano et al. 2013;Han et al. 2014;Paz-Yaacov et al. 2015;Villa et al. 2015). In non-coding regions, the vast majority of A-to-I RNA editing sites are in repetitive Alu elements embedded in 3' untranslated regions (3'UTRs) (Fa...