Growth Factor Independence 1 (Gfi1) Is An Essential Factor for the Development of Lymphoma

Hematopoietic differentiation is controlled to a large extent by a network of transcription factors and chromatin modifiers and disruption of this system can lead to leukemia or lymphoma. One of the transcription factor genes, which is aberrantly expressed in human T-cell lymphoma is Growth Factor Independence 1 (Gfi1). Since over expression of Gfi1 can accelerate experimentally induced T-cell tumors in mice, it is likely that Gfi1 plays a crucial role in establishing or maintaining lymphoid neoplasms. To test this hypothesis we have used, N-ethyl-N-nitrosourea (ENU) to induce T-cell tumors in WT mice (Gfi1^+/+), Gfi1-deficient mice (Gfi1^−/−) or mice transgenically over expressing Gfi1 under the control of the pan-hematopoietic vav-promoter (vav-Gfi1). As expected, most of Gfi1^+/+ mice (25/27) developed T-cell tumors and acute myeloid leukemia within 118 days. Similarly, vav-Gfi1 mice (10/10) developed T-cell lymphoma, but within a shorter latency period (88 days). In contrast, only 3/14 Gfi1^−/− mice developed hematopoietic neoplasia with a prolonged median latency period of 126 days. Other approaches using infection of newborn mice with Moloney Murine leukemia virus (MoMuLV) to induce T-cell lymphoma or co expression of an Eμ-myc transgene to induce B-cell lymphoma showed a similar dependency of tumor formation on the presence and expression of Gfi1. Closer analysis of tumors forming in Gfi1^−/− mice demonstrated that Gfi1 deficiency correlated with a smaller size of the tumors and a noticeably increased rate of cell death within the tumor samples. This pointed to a potential role of Gfi1 in the regulation of apoptosis. To explore this hypothesis, we exposed both thymocytes and hematopoietic stem cells (Lin^-, Sca1⁺, c-kit⁺, LSK) to ENU or gamma-irradiation in vitro. We could observe that Gfi1^−/− thymocytes and stem cells (LSK cells) have a higher rate of cell death following exposure to these DNA damage inducing agents in vitro than the WT controls. To validate these results, we recapitulated these experiments in vivo. Gfi1^−/− mice exhibited severe bone marrow failure and a more pronounced loss of hematopoietic stem cells (LSK) than Gfi1^+/+ mice after ENU treatment or gamma irradiation in vivo. To explore this mechanism on the molecular basis we evaluated expression of the different pro and antiapoptotic components in Gfi1^+/+ and Gfi1^−/− thymocytes after irradiation. Strikingly, Gfi1^−/− thymocytes expressed higher levels of the pro-apoptotic proteins such as Bax and Noxa and lower levels of the CDK inhibitor p21^WAF than WT thymocytes following induction of DNA damage. Our model would be that Gfi1 represents a new regulator in the cellular response to DNA damage in the hematopoietic system by inhibiting different proapoptotic factors. We propose that Gfi1 is essential for the development of lymphoid and potentially myeloid neoplasms by inhibiting apoptosis. We suggest that Gfi1 could represent a possible new target structure for therapeutic intervention.