P332: MIR-625-5P IS NOVEL CANDIDATE ONCOMIR IN T-CELL ACUTE LYMPHOBLASTIC LEUKEMIA IMPLICATED IN REGULATION OF APOPTOSIS VIA REPRESSION OF HARAKIRI

Abstract

Background: T-cell acute lymphoblastic leukemia (T-ALL) is an aggressive subtype of ALL, arising from T-cell precursors. miRNAs are non-coding RNAs, contributing to leukemogenesis by involvement in key cellular functions such as proliferation, apoptosis, and signaling. We previously found in miRNA transcriptome analysis that miR-625-5p is overexpressed in T-ALL patients, thus is a potential oncomiR in T-ALL. Here, we confirm the implication of miR-625-5p in the regulation of apoptosis in T-ALL cells in vitro. We show that it acts via repression of HARAKIRI (HRK). HRK was originally identified as a proapoptotic gene induced by reduced levels of cytokine in hematopoietic cells and repressed by the expression of death-repressor proteins. Although its proapoptotic function is well described, little is known about the post-transcriptional mechanisms that may participate in HRK inactivation. Aims: The aim of this study was to investigate the mechanism of miR-625-5p oncogenic action via unraveling its targetome and functional effects in T-ALL in vitro. Methods: T-ALL JURKAT cells (with high endogenous miR-625-5p level) were transduced for stable inhibition of this miRNA. To evaluate the influence of miR inhibition on growth of T-ALL cells, we used flow cytometry GFP competition assay and CCK8 proliferation assay. To identify the genes mediating the effect of miR-625-5p, we subjected JURKAT cells to Ago2-RIP-seq. We used magnetic beads coated with anti-AGO2 antibody to immunoprecipitate RNA bound to RISC complexes. Ago2-IP and total RNA fractions were sequenced (polyA RNA-seq, 150 bp reads, 60M PE reads/sample, Illumina NovaSeq 6000). Transcripts depleted in Ago2-IP fraction upon miR inhibition were screened for the presence of miR-625-5p binding sites. Luciferase assay was performed to confirm direct interaction between hsa-miR-625-5p and 3’UTR of HRK gene. The changes of HRK protein level upon miR-625-5p inhibition were evaluated via Western Blot. To estimate the apoptosis rate in T-ALL cells we used two flow cytometry assays based on Annexin V and Caspase 3/7 staining. Results: Inhibition of miR-625-5p decreased the growth of JURKAT cells. In RIP-seq analysis we identified 384 transcripts depleted in RISC by at least 20% upon miR-625-5p inhibition and having at least one putative 3’UTR binding site for this miRNA. Overrepresentation analysis performed for these genes indicated their involvement in the apoptotic process. Among transcripts depleted in RIP-seq with predicted binding sites for miR-625-5p, we found HRK gene, previously reported to have proapoptotic activity. We postulate that repression of this gene by overexpressed miR-625-5p contributes to decreased apoptosis and to growth advantage of T-ALL cells. To further confirm that miR-625-5p affects growth of T-ALL cells via negative regulation of apoptosis, we performed apoptotic assays in JURKAT cells and showed that inhibition of this miRNA increased apoptosis rate, in line with our hypothesis on the oncogenic potential of miR-625-5p. We confirmed the direct interaction between miR-625-5p and HRK 3’UTR by luciferase assay. Additionally, in Western blot a subtle (approaching statistical significance) increase of HRK protein upon miR-625-5p inhibition was observed, supporting our notion that this miRNA is indeed a negative regulator of HRK. Image:Summary/Conclusion: miR-625-5p is a novel candidate oncogenic miRNA in T-ALL, implicated in the negative regulation of apoptosis. Post-transcriptional repression of the proapoptotic HRK gene is a putative mechanism contributing to oncogenic properties of this miRNA when overexpressed in T-ALL cells.