The ETS Transcription Factor ETV7 Exhausts Hematopoietic Stem Cells By Enhancing The Cell Cycle Entry and Cell Proliferation

Although ETS-transcription factors play a role in normal and malignant hematopoiesis, their function in hematopoietic stem cells (HSCs) and leukemia initiating cells (LICs) is largely unknown. We originally identified the novel ETS transcription factor ETV7, which is highly homologous to ETV6/TEL, a frequent target of chromosomal translocation in human leukemia. Previously we have shown that ETV7 is a hematopoietic oncoprotein that requires cooperating mutations to induce leukemic transformation. Microarray analysis revealed that ETV7 expression is upregulated in 70% of pediatric ALL and AML samples. This indicates a possible oncogenic function of ETV7 in a variety of leukemias, although the molecular mechanism of ETV7-mediated leukemogenesis remains to be elucidated. ETV7 is widely but not abundantly expressed in various human tissues. Recently we found that overexpression of ETV7 in human cord blood-derived CD34⁺ cells depletes the number of CD34⁺CD38^- HSCs. In addition, ETV7-transduced cells slightly accerelated cell proliferation. These results suggest that overexpression of ETV7 activates cell proliferation in primary human CD34⁺cells and depletes the number of HSCs. Here, by using a mouse model, we show that ectopic expression of ETV7 in quiescent HSCs accelerates their cell cycle entry and proliferation, leading to the exhaust of HSCs in mice.

The ETV7 gene locus is deleted in part of the rodents including the mouse despite its high level of conservation among vertebrates. To circumvent this limitation, we have generated an ETV7 BAC transgenic mouse that carries a single copy of a human BAC DNA containing the ETV7 gene locus. In flow cytometry (FCM) analysis of wild type (WT) and ETV7 bone marrow (BM)-derived Lin^-Sca1⁺cKit⁺(LSK) cells, the size and frequency of LT(long term)-HSCs (CD48^-CD150⁺LSK) in ETV7 LSK was 2-fold lower than that in WT LSK, while the frequency of LSK and hematopoietic common progenitor cells in WT and ETV7 BM are similar. As compared with WT-LSK, ETV7-LSK showed a significantly decreased number of myeloid progenitor colonies in both the initial plating (MC1) and replating of MC1 colonies (MC2) in methylcellulose colony formation assay in vitro. To assess the ETV7 HSC function contributing to blood cell generation in vivo, we performed competitive repopulation assays. In agreement with the in vitro results, the repopulation ability of HSC is significantly compromised in ETV7 mice as measured 7 weeks post transplantation. This defect was even more pronounced 16 weeks post transplantation. Since enhanced cell cycle entry is known to cause loss of hematopoietic stem/progenitor cells (HSPCs) through the activation of a tumor suppressor response, we quantified p19^ARF, p16^INK4a, and p21^CDKN1A gene expression in LSK cells by qRT-PCR. At day 6 and day 9 of in vitro culture, ETV7 LSK cells activated the p19^ARF, p16^INK4a, and p21^CDKN1A genes about 2-fold greater than WT LSK cells. To measure the de novo DNA replication of HSPCs in vivo, BrdU-pulse labeled BM cells were harvested and BrdU incorporation was quantified by FCM analysis. ETV7 LSK cells showed elevated BrdU incorporation compared with that of WT. In addition, Hoechst33342/Pyronin Y staining revealed that ETV7 LSK enhanced transition from G₀ to the G₁ phase of the cell cycle, suggesting that ETV7 forced cell cycle entry of quiescent HSCs. Finally to clarify the involvement of the CDKN2A tumor suppressor in ETV7-associated HSC exhaustion, we examined the frequency of HSPCs in CDKN2A^-/- and ETV7^+/-CDKN2A^-/- LSK cells in vivo by FCM analysis. Loss of CDKN2A but not ARF restored the depletion of ETV7 LT-HSCs. Moreover, loss of CDKN2A rescued the defect of repopulation ability in vivo and self-renewal activity in vitro of ETV7 HSPCs. These results indicate that exhaustion of HSC in ETV7 BM occurred through ETV7-induced activation of cell proliferation and the CDKN2A tumor suppressor pathway in mice.