RNA Editing of Tumor Suppressor Mir-26a Promotes Malignant Progenitor Propagation in Chronic Myeloid Leukemia

Recent studies demonstrate the importance of post-transcriptional adenosine-to-inosine (A-to-I) RNA editing mediated by adenosine deaminase acting on RNA1 (ADAR1) in normal fetal and adult hematopoiesis. RNA-sequencing studies have shown that elevated levels of the ADAR1 editase has emerged as a dominant driver of cancer progression and therapeutic resistance. Specifically, the deregulation of ADAR1 promotes the transformation of chronic myeloid leukemia (CML) from chronic phase (CP) to a therapy resistant blast crisis (BC) phase. Through the regulation of mRNA and microRNA (miRNA) stability, ADAR1 plays a pivotal role in embryonic development and stem cell regulation. We have previously shown that inflammation-responsive ADAR1 heavily contributes to stem cell differentiation and self-renewal in CML disease progression. Here, we describe a novel role of ADAR1 in cell cycle regulation of BC leukemia cells through regulation of miRNA biogenesis.

To investigate the role of ADAR1 in miRNA regulation, we performed miRNome miScript PCR array analysis of 1008 miRNAs in cord blood CD34+ expressing hematopoietic stem and progenitor cells (HSPCs) overexpressing ADAR1 wild type (WT) and A-to-I editing deficient ADAR1 mutant. Overall, a total of 112 miRNAs were significantly differentially expressed following ADAR1 expression with cell cycle identified as the top cellular pathway significantly targeted by miRNAs regulated by ADAR1. Notably, ADAR1 editase activity inhibits the expression of miR-26a-5p, a tumor suppressor miRNA that is frequently downregulated in BC CML. ADAR1 inhibits miR-26a-5p through direct editing of the DROSHA cleavage site of primary miR-26a-5p, preventing miR-26a-5p maturation and processing. In normal hematopoietic progenitors, ADAR1-mediated inhibition of miR-26a results in repression of cyclin-dependent kinase inhibitor 1A (CDKN1A) expression indirectly via suppression of the polycomb repressive complex, enhancer of zeste homolog 2 (EZH2), thereby accelerating cell cycle transit. However, in BC CML progenitors, decreased EZH2 and increased CDKN1A oppose the cell cycle accelerating effect of ADAR1. Moreover, we found that the miR-26a targets a different set of mRNA in BC CML compared to cord blood HSPC and has divergent roles in cell cycle regulation. Lentiviral miR-26a overexpression reduced BC leukemia stem cell (LSC) dormancy in the bone marrow and reverses the functional effects of ADAR1, including inhibition of BC cell proliferation in vivo and impaired LSC self-renewal capacity as measured by colony forming assays.

Our finding reveals the effects of ADAR1 in LSC generation through impairing biogenesis of cell cycle regulatory miRNAs. The deregulation of ADAR1 contributes to the malignant reprogramming of progenitors into dormant LSCs that are resistant to therapeutic treatments. Future development of ADAR1 inhibitors may be effective in the elimination of dormant BC CML LSCs that evade tyrosine kinase inhibitors.