Detecting and Characterizing A-To-I microRNA Editing in Cancer

Abstract

Adenosine to inosine (A-to-I) editing consists of an RNA modification where single adenosines along the RNA sequence are converted into inosines. Such a biochemical transformation is catalyzed by enzymes belonging to the family of adenosine deaminases acting on RNA (ADARs) and occurs either co- or post-transcriptionally. The employment of powerful, high-throughput detection methods has recently revealed that A-to-I editing widely occurs in non-coding RNAs, including microRNAs (miRNAs). MiRNAs are a class of small regulatory non-coding RNAs (ncRNAs) acting as translation inhibitors, known to exert relevant roles in controlling cell cycle, proliferation, and cancer development. Indeed, a growing number of recent researches have evidenced the importance of miRNA editing in cancer biology by exploiting various detection and validation methods. Herein, we briefly overview early and currently available A-to-I miRNA editing detection and validation methods and discuss the significance of A-to-I miRNA editing in human cancer.

Keywords: A-to-I RNA editing; ADAR; detection; functional characterization; microRNA targeting; microRNAs; quantification.

Figure 1

Illustration of canonical microRNA (miRNA) biogenesis. Once primary miRNA is transcribed, they are processed by the Drosha-DGCR8 complex and then exported to the cytoplasm. Here, pre-mRNA molecules are further processed by the Dicer-TRBP complex, usually generating two paired, partially complementary, mature miRNAs. One of the mature miRNA strands will be maintained while the other will be degraded. The selected miRNAs function as inhibitors of mRNA translation, usually by base-pairing of their seed region with complementary regions within the 3’ untranslated regions of the mRNAs.

Figure 2

Steps of miRNA biogenesis at which adenosine to inosine (A-to-I) RNA editing can prevent the processing of pri- or pre-miRNA. In certain cases, however, the A-to-I editing event does not impair miRNA biogenesis and is maintained in the mature miRNA.

Figure 3

Schematic illustration of miRNA retargeting caused by editing of mature miRNAs. RNA editing can affect the miRNA-mediated modulation of gene expression by modifying the primary structure of wild-type (Wt) mature miRNAs. Such an event generates edited (Ed) miRNAs, often destroying their base-pairing with several complementary regions in the 3′UTRs of their original targets and creating complementarity for new targets. However, both the Wt and Ed miRNAs seem to share a subset of their target mRNAs.

Figure 4

Possible effects of miRNA editing in the context of cancer. Editing of mature miRNAs can prevent the targeting of complementary regions in the 3′UTRs of original mRNAs. In case an editing event disrupts the base pairing of an oncomiR with its target mRNA (transcribed from a tumor suppressor gene), the translational process will lead to the expression of the tumor suppressor protein, increasing processes related to cell cycle arrest, differentiation, and/or apoptosis (top, left). In case an editing event disrupts the base pairing of a tsmiR with its target mRNA (transcribed from a proto-oncogene), the translational process will lead to the expression of the oncoprotein, increasing processes related to cell cycle progression, proliferation, and migration (top, right). However, editing of pri- or pre-miRNAs can cause the blockage of their biogenesis, significantly decreasing the expression of their related mature miRNAs. Thus, similar considerations can be done in the case of the underexpressed miRNA functions as an oncomiR (bottom, left) or a tsmiR (bottom, right).