Functions of the RNA Editing Enzyme ADAR1 and Their Relevance to Human Diseases

The role of A-to-I RNA editing in cancer development

Adenosine-to-inosine (A-to-I) RNA editing is the most common type of post-transcriptional nucleotide modification in humans, which is catalyzed in ADAR enzymes. Recent genomic studies have revealed thousands of altered RNA editing events in various cancer tissues, leading to diverse functional consequences. A critical role of individual A-to-I RNA editing events in cancer has been reported. Here, we review the current state of our knowledge on key A-to-I RNA editing events in coding and non-coding regions for their roles in cancer development and discuss their potential clinical utility. A better understanding of A-to-I RNA editing and its oncogenic mechanisms may facilitate the development of novel cancer therapeutic strategies.

Use of E. coli Purine Nucleoside Phosphorylase in the Treatment of Solid Tumors

BACKGROUND

The selective expression of non-human genes in tumor tissue to activate non-toxic compounds (Gene Directed Prodrug Enzyme Therapy, GDEPT) is a novel strategy designed for killing tumor cells in patients with little or no systemic toxicity. Numerous non-human genes have been evaluated, but none have yet been successful in the clinic.

METHODS

Unlike human purine nucleoside phosphorylase (PNP), E. coli PNP accepts adenine containing nucleosides as substrates, and is therefore able to selectively activate non-toxic purine analogs in tumor tissue. Various in vitro and in vivo assays have been utilized to evaluate E. coli PNP as a potential activating enzyme.

RESULTS

We and others have demonstrated excellent in vitro and in vivo anti-tumor activity with various GDEPT strategies utilizing E. coli PNP to activate purine nucleoside analogs. A phase I clinical trial utilizing recombinant adenoviral vector for delivery of E. coli PNP to solid tumors followed by systemic administration of fludarabine phosphate (NCT01310179; IND# 14271) has recently been completed. In this trial, significant anti-tumor activity was demonstrated with negligible toxicity related to the therapy. The mechanism of cell kill (inhibition of RNA and protein synthesis) is distinct from all currently used anticancer drugs and all experimental compounds under development. The approach has demonstrated excellent ability to kill neighboring tumor cells that do not express E. coli PNP, is active against non-proliferating and proliferating tumors cells (as well as tumor stem cells, stroma), and is therefore very effective against solid tumors with a low growth fraction.

CONCLUSION

The unique attributes distinguish this approach from other GDEPT strategies and are precisely those required to mediate significant improvements in antitumor therapy.

Long non-coding RNA DLX6-AS1 aggravates hepatocellular carcinoma carcinogenesis by modulating miR-203a/MMP-2 pathway

Long non-coding RNAs (lncRNAs) have been wildly verified to modulate multiple tumorigenesis, especially hepatocellular carcinoma (HCC). In present study, our team aims to investigate the role of lncRNA DLX6-AS1 in the HCC carcinogenesis. Results of early-stage experiments found that DLX6-AS1 expression level was up-regulated in 60 cases of HCC tissue samples compared with adjacent normal tissue. Moreover, the aberrant overexpression of DLX6-AS1 indicated the poor prognosis of HCC patients. Loss-of-function experiments revealed that DLX6-AS1 knockdown inhibited the proliferation, migration and invasion of HCC cells in vitro, and decreased the tumor growth in vivo. Bioinformatics analysis predicted that miR-203a potentially targeted DLX6-AS1 3'-UTR, suggesting the interaction between miR-203a and DLX6-AS1. Furthermore, miR-203a also targeted MMP-2 mRNA 3'-UTR, which was validated by luciferase reporter assay. Taken together, our study discovered the oncogenic role of DLX6-AS1 in clinical specimens and cellular experiments, showing the potential DLX6-AS1/miR-203a/MMP-2 pathway. This results and findings provide a novel insight for HCC tumorigenesis.

RNA editing by ADAR1 regulates innate and antiviral immune functions in primary macrophages

ADAR1-dependent A-to-I editing has recently been recognized as a key process for marking dsRNA as self, therefore, preventing innate immune activation and affecting the development and resolution of immune-mediated diseases and infections. Here, we have determined the role of ADAR1 as a regulator of innate immune activation and modifier of viral susceptibility in primary myeloid and lymphoid cells. We show that ADAR1 knockdown significantly enhanced interferon, cytokine and chemokine production in primary macrophages that function as antiviral paracrine factors, rendering them resistant to HIV-1 infection. ADAR1 knockdown induced deregulation of the RLRs-MAVS signaling pathway, by increasing MDA5, RIG-I, IRF7 and phospho-STAT1 expression, an effect that was partially rescued by pharmacological blockade of the pathway. In summary, our results demonstrate a role of ADAR1 in regulating innate immune function in primary macrophages, suggesting that macrophages may play an essential role in disease associated to ADAR1 dysfunction. We also show that viral inhibition is exclusively dependent on innate immune activation consequence of ADAR1 knockdown, pointing towards ADAR1 as a potential target to boost antiviral immune response.

RNA-editing enzymes ADAR1 and ADAR2 coordinately regulate the editing and expression of Ctn RNA

Adenosine deaminases acting on RNA (ADARs) are proteins that catalyse widespread A-to-I editing within RNA sequences. We recently reported that ADAR2 edits and stabilizes nuclear-retained Cat2 transcribed nuclear RNA (Ctn RNA). Here, we report that ADAR1 coordinates with ADAR2 to regulate editing and stability of Ctn RNA. We observe an RNA-dependent interaction between ADAR1 and ADAR2. Furthermore, ADAR1 negatively regulates interaction of Ctn RNA with RNA-destabilizing proteins. We also show that breast cancer (BC) cells display elevated ADAR1 but not ADAR2 levels, compared to nontumourigenic cells. Additionally, BC patients with elevated levels of ADAR1 show low survival. Our findings provide insights into overlapping substrate preferences of ADARs and potential involvement of ADAR1 in BC.

Metabolomic and Lipidomic Profiling Identifies The Role of the RNA Editing Pathway in Endometrial Carcinogenesis

Endometrial cancer (EC) remains the most common malignancy of the genital tract among women in developed countries. Although much research has been performed at genomic, transcriptomic and proteomic level, there is still a significant gap in the metabolomic studies of EC. In order to gain insights into altered metabolic pathways in the onset and progression of EC carcinogenesis, we used high resolution mass spectrometry to characterize the metabolomic and lipidomic profile of 39 human EC and 17 healthy endometrial tissue samples. Several pathways including lipids, Kynurenine pathway, endocannabinoids signaling pathway and the RNA editing pathway were found to be dysregulated in EC. The dysregulation of the RNA editing pathway was further investigated in an independent set of 183 human EC tissues and matched controls, using orthogonal approaches. We found that ADAR2 is overexpressed in EC and that the increase in expression positively correlates with the aggressiveness of the tumor. Furthermore, silencing of ADAR2 in three EC cell lines resulted in a decreased proliferation rate, increased apoptosis, and reduced migration capabilities in vitro. Taken together, our results suggest that ADAR2 functions as an oncogene in endometrial carcinogenesis and could be a potential target for improving EC treatment strategies.

Restricting retrotransposons: ADAR1 is another guardian of the human genome

ADAR1 is an enzyme that belongs to the Adenosine Deaminases Acting on RNA (ADARs) family. These enzymes deaminate adenosines to inosines (RNA editing A-to-I) within double-stranded RNA regions in transcripts. Since inosines are recognized as guanosines by the cellular machinery, RNA editing mediated by ADARs can either lead to the formation of an altered protein (recoding) or affect different aspects of RNA metabolism. Recently, a proteomic analysis led to the identification of novel ADAR1-associated factors and found that a good fraction of them is shared with the Long Interspersed Element 1 (LINE-1 or L1) ribonucleoparticles (RNPs). This evidence suggested a possible role of ADAR1 in regulating the L1 life cycle. By taking advantage of the use of cell culture retrotransposition assays, a novel function of this deaminase as an inhibitor of L1 retrotransposition was demonstrated. These results pave the way toward a better comprehension of the mechanisms of restriction of retrotransposons.

Precancer Atlas to Drive Precision Prevention Trials

Cancer development is a complex process driven by inherited and acquired molecular and cellular alterations. Prevention is the holy grail of cancer elimination, but making this a reality will take a fundamental rethinking and deep understanding of premalignant biology. In this Perspective, we propose a national concerted effort to create a Precancer Atlas (PCA), integrating multi-omics and immunity - basic tenets of the neoplastic process. The biology of neoplasia caused by germline mutations has led to paradigm-changing precision prevention efforts, including: tumor testing for mismatch repair (MMR) deficiency in Lynch syndrome establishing a new paradigm, combinatorial chemoprevention efficacy in familial adenomatous polyposis (FAP), signal of benefit from imaging-based early detection research in high-germline risk for pancreatic neoplasia, elucidating early ontogeny in BRCA1-mutation carriers leading to an international breast cancer prevention trial, and insights into the intricate germline-somatic-immunity interaction landscape. Emerging genetic and pharmacologic (metformin) disruption of mitochondrial (mt) respiration increased autophagy to prevent cancer in a Li-Fraumeni mouse model (biology reproduced in clinical pilot) and revealed profound influences of subtle changes in mt DNA background variation on obesity, aging, and cancer risk. The elaborate communication between the immune system and neoplasia includes an increasingly complex cellular microenvironment and dynamic interactions between host genetics, environmental factors, and microbes in shaping the immune response. Cancer vaccines are in early murine and clinical precancer studies, building on the recent successes of immunotherapy and HPV vaccine immune prevention. Molecular monitoring in Barrett's esophagus to avoid overdiagnosis/treatment highlights an important PCA theme. Next generation sequencing (NGS) discovered age-related clonal hematopoiesis of indeterminate potential (CHIP). Ultra-deep NGS reports over the past year have redefined the premalignant landscape remarkably identifying tiny clones in the blood of up to 95% of women in their 50s, suggesting that potentially premalignant clones are ubiquitous. Similar data from eyelid skin and peritoneal and uterine lavage fluid provide unprecedented opportunities to dissect the earliest phases of stem/progenitor clonal (and microenvironment) evolution/diversity with new single-cell and liquid biopsy technologies. Cancer mutational signatures reflect exogenous or endogenous processes imprinted over time in precursors. Accelerating the prevention of cancer will require a large-scale, longitudinal effort, leveraging diverse disciplines (from genetics, biochemistry, and immunology to mathematics, computational biology, and engineering), initiatives, technologies, and models in developing an integrated multi-omics and immunity PCA - an immense national resource to interrogate, target, and intercept events that drive oncogenesis. Cancer Res; 77(7); 1510-41. ©2017 AACR.