Abstract 463: MicroRNA-7 suppresses RB1 expression leading to chromosomal instability in leukemia cells harboring c-Kit mutation
Authors: Kurata Keiji , Kawamoto Shinichiro , Masutani Ryota , Yakushijin Kimikazu , Yamamoto Katsuya , Matsuoka Hiroshi , Takubo Takayuki , Minami Hironobu 
Published: July 2017

Abstract
C-Kit mutation D816V is a well-known indicator of poor prognosis in acute myeloid leukemia (AML) harboring t(8;21) chromosomal translocations. However, the mechanism the D816V mutation promotes therapeutic resistance is still under investigation. C-Kit V814 mutation is a murine counterpart of the human D816V. We utilized a murine IL-3 dependent cell line Ba/F3 with or without the c-Kit V814 mutation to investigate its downstream signaling pathway and the effect on the expression of bcl-2 family members and critical granulocytic transcription factors. The c-Kit V814 and wild type (WT) c-Kit were retrovirally transduced into Ba/F3 cells. Unexpectedly, Gata1 mRNA was significantly downregulated to one-tenth in V814+Ba/F3 cells (p &lt; 0.01), contrary to the previous reports that WT c-Kit signaling induces Gata1 expression. When V814+Ba/F3 cells were treated with several c-Kit signaling pathway inhibitors, mRNA and protein expression levels of Gata1 recovered to normal levels only with MEK 1/2 inhibitor (PD325901) treatment. We then hypothesized that miRNAs might control Gata1 transcription. We used an array to identify differences in miRNA expression between Ba/F3 cells and V814+Ba/F3 cells, and between V814+Ba/F3 cells treated with or without PD325901. Only miR-7a-5p and miR-706 were significantly upregulated in V814+Ba/F3 cells compared with Ba/F3 without the V814 mutation, and downregulated (&gt; 2-fold) in cells treated with PD325901 compared with controls. We focused our further analysis on miR-7 as it is highly conserved in vertebrates. The differences in miR-7 expression were confirmed using qRT-PCR (10.5-fold change, p &lt; 0.01). In silico analysis implicated Rb1 as a candidate miR-7 target that may regulate Gata1 transcription. Luciferase reporter constructs containing murine and human Rb1 miR-7 target sequence exhibited 0.51 and 0.49 times lower luciferase activity than the controls, respectively (p &lt; 0.01). To further determine the biological significance of the c-Kit mutation that would transform the cells to more aggressive phenotype, we focused on DNA and chromosomal instability caused by RB1 deterioration. We exposed WT c-Kit Ba/F3 and V814+Ba/F3 cells to 4 Gy irradiation and then used immunohistochemistry to analyze the frequency of γH2AX foci in the cells. The percentage of cells with more than 10 γH2AX foci was significantly higher in V814+Ba/F3 cells than WT Ba/F3 cells (27% vs. 14%, p &lt; 0.05), Here, we clearly show that a single mutation of c-Kit is sufficient to regulate miR-7 expression leading to RB1 translational suppression. Since c-Kit mutation requires only a single step to inhibit RB1, this mechanism is more likely to facilitate oncogenesis than bi-allelic RB1 chromosomal and/or gene alterations in leukemia cells. Taken together, we identified miR-7 as a suppressor of RB1 in V814 mutation-positive cells, which might reflect the responsibility of D816V to refractory feature in AML.
Citation Format: Keiji Kurata, Shinichiro Kawamoto, Ryota Masutani, Kimikazu Yakushijin, Katsuya Yamamoto, Hiroshi Matsuoka, Takayuki Takubo, Hironobu Minami. MicroRNA-7 suppresses RB1 expression leading to chromosomal instability in leukemia cells harboring c-Kit mutation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 463. doi:10.1158/1538-7445.AM2017-463