N6-methyladenosine (m6A), the most abundant internal modification in eukaryotic messenger RNAs (mRNAs) has been shown to play important roles in diverse cellular and pathological processes (Deng X, et al. Cell Res. 2018;28:507-517). ALKBH5, recently identified as a m6A demethylase, was reported to promote tumorigenesis and proliferation in glioblastoma stem-like cells (GSCs) (Zhang, S. et al. Cancer Cell. 2017;31:591-606) and breast cancer stem cells (BCSCs) (Zhang, C et al. PNAS. 2016;113: E2047-E2056). While ALKBH5 is well-recognized to function as an oncogene in solid tumors, it was reported that shallow/deep deletion of ALKBH5 is associated poor prognosis in patients with acute myeloid leukemia (AML), and is frequently co-existing with TP53 mutation (Kwok, C. T et al. J Hematol Oncol. 2017; 10(1): 39), implying that ALKBH5 may function as a tumor suppressor in AML. Thus, a systematic investigation of the definitive role of ALKBH5 in AML is warranted.
To this end, we performed series of in vitro and in vivo experiments to determine the function of ALKBH5 in AML. For the in vitro experiments, we used three lentiviral shRNAs (shALKBH5-A, shALKBH5-D and shALKBH5-E) to deplete ALKBH5 expression in three human AML cell lines with different TP53 mutation status: NOMO-1 (TP53-mutant), MV4;11 (TP53-WT) and MA9.3 cells (TP53-WT). Somewhat surprisingly, ALKBH5 depletion significantly (p<0.05) inhibited AML cell proliferation/growth in all three AML cells lines, regardless of the status of TP53 mutation. We next conducted colony forming assays and found that ALKBH5 knockdown significantly (p<0.01) impaired the colony forming ability to 18% ~45% of the control group level in all three AML cell lines. We further showed that ALKBH5 depletion caused a significant increase in apoptosis (with a 1.5 ~ 4 fold increase; p<0.001) in all three AML cell lines, which is consistent with the previous report that knockout of ALKBH5 caused severe apoptosis of mouse testis cells (Zheng G et al. Mol Cell. 2013; 49:18-29). In contrast, ALKBH5 knockdown did not significantly affect cell cycles.
To further confirm ALKBH5's role in AML development in vivo. We utilized Xenografted AML model as well as mouse bone marrow transplantation (BMT) model. Consistent with the in vitro results, we found that NSGS mice xeno-transplanted with MV4;11-ALKBH5-knockdown cells survived significantly longer than those with MV4;11 control cells (p<0.001). Moreover, we have also conducted mouse bone marrow transplantation (BMT) assays with MLL-AF9-transduced mouse bone marrow lineage negative (Lin-) progenitor cells collected from mice carrying Alkbh5 wild-type (Alkbh5+/+), or heterozygous (Alkbh5+/-) or homozygous (Alkbh5-/-) deletion. Consistent with the xeno-transplanted mouse model results, our BMT assays also showed that Alkbh5 depletion significantly inhibited leukemogenesis and prolonged survival in BMT recipient mice (median survival of ALKBH5wt/wt +MA9 vs. ALKBH5+/- +MA9 or ALKBH5-/- +MA9: 32 days vs. 64 days or 68 days; p<0.005).
Taken together, our in vitro and in vivo functional studies data indicate ALKBH5 also functions as an oncogene in AML regardless of TP53 mutation status, similar to its role in solid tumors. We are currently conducting as series of studies to reveal the molecular mechanism(s) underlying the oncogenic role of ALKBH5 in AML.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.