Recent Advances in Adenosine-to-Inosine RNA Editing in Cancer

RNA epigenetics, or epitranscriptome, is a growing group of RNA modifications historically classified into two categories: RNA editing and RNA modification. RNA editing is usually understood as post-transcriptional RNA processing (except capping, splicing and polyadenylation) that changes the RNA nucleotide sequence encoded by the genome. This processing can be achieved through the insertion or deletion of nucleotides or deamination of nucleobases, generating either standard nucleotides such as uridine (U) or the rare nucleotide inosine (I). Adenosine-to-inosine (A-to-I) RNA editing is the most prevalent type of RNA modification in mammals and is catalyzed by adenosine deaminase acting on the RNA (ADAR) family of enzymes that recognize double-stranded RNAs (dsRNAs). Inosine mimics guanosine (G) in base pairing with cytidine (C), thereby A-to-I RNA editing alters dsRNA secondary structure. Inosine is also recognized as guanosine by the splicing and translation machineries, resulting in mRNA alternative splicing and protein recoding. Therefore, A-to-I RNA editing is an important mechanism that causes and regulates "RNA mutations" in both normal physiology and diseases including cancer. In this chapter, we reviewed current paradigms and developments in the field of A-to-I RNA editing in the context of cancer.

Methods for recruiting endogenous and exogenous ADAR enzymes for site-specific RNA editing

Adenosine deaminases acting on RNA (ADARs) can be repurposed to achieve site-specific A-to-I RNA editing by recruiting them to a target of interest via an ADAR-recruiting guide RNA (adRNA). In this chapter, we present details towards experimental methods to enable this via two orthogonal strategies: one, via recruitment of endogenous ADARs (i.e. ADARs already natively expressed in cells); and two, via recruitment of exogenous ADARs (i.e. ADARs delivered into cells). Towards the former, we describe the use of circular adRNAs to recruit endogenous ADARs to a desired mRNA target. This results in robust, persistent and highly transcript specific editing both in vitro and in vivo. Towards the latter, we describe the use of a split-ADAR2 system, which allows for overexpression of ADAR2 variants that can be utilized to edit adenosines with high specificity, including at challenging to edit adenosines in non-preferred motifs such as those flanked by a 5' guanosine. We anticipate the described methods should facilitate RNA editing applications across research and biotechnology settings.

ADAR, the carcinogenesis mechanisms of ADAR and related clinical applications

Adenosine deaminases acting on RNA (ADARs) catalyze the conversion of adenosine (A) to inosine (I) in double-stranded RNA, which can change the codons after transcription. Abnormal ADAR editing is present in a variety of cancers. However, the study of the biological effects of ADARs in cancer is not very deep. Here, we review current important ADAR-mediated editing events, related carcinogenic mechanisms and applications in clinical medicine. Further exploration in ADARs can provide a new direction for cancer treatment.