Gfi1b-A Novel Tumor Suppressor In Acute Myeloid Leukemia

The proper differentiation of hematopoietic stem cells is regulated by a concert of different so called transcription factors. Growth Factor Independence 1b (Gfi1b) is a repressing transcription factor, which is pivotal for the proper emergence and maturation of erythrocytes and platelets. Furthermore, Gfi1b controls quiescence as well as cell cycle progression of hematopoietic stem cells and early progenitor cells. It has been shown for other transcription factors that a disturbed function of these transcription factors can be the basis of malignant diseases such acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS). MDS is characterized by disturbed differentiation of one or several hematopoietic lineages. The accumulation of malignant blast cells, which are arrested in their development, is a key feature of AML.

Since transcription factors play a role in AML development, we sought to investigate whether Gfi1b might also play a role in the development and progression of AML. Based on published gene expression arrays and own patient samples, we observed that Gfi1b is expressed at a lower level in leukemic blasts and leukemic stem cells compared to the non-malignant control cells. We correlated Gfi1b expression level in blast cells of patients from Essen and we found out that patients with high Gfi1b levels had a poor prognosis and an increased risk of relapse. In contrast low levels of Gfi1b expression were associated with a good prognosis.

To test how different levels of Gfi1b might influence initiation of AML we have mouse strains available expressing Gfi1b at different expression levels. We have Gfi1b wt mice with one allele of Gfi1b deleted (Gfi1b het) and Gfi1b conditional mice, in which the expression of Gf1b (Mx Cre tg Gfi1b fl/fl) can be abrogated after injecting these mice with pIpC.

To explore the role of Gfi1b in leukemia development, we used different murine AML models, resembling human leukemia. First these mice were crossed with Nup98HoxD13 transgenic mice, a mouse strain that develops a disease similar to the human MDS. We observed that Gfi1b heterozygosity (n=15) accelerated AML development (p=0,03) compared to wt mice (n=16). More importantly, complete absence of Gfi1b (n=8) results in a substantially earlier onset of AML, with a median survival time of about 50 days (p=0.002).

To confirm our findings, we used a different murine AML model.

Recurrent so called oncofusion proteins such as AML1-Eto9a, CBFbeta-Myh11 or MLL-AF9 are characteristic for certain subtypes of AML. We transduced Lineage negative (Lin^-) bone marrow cells from wt, Gfi1b heterozygous (Gfi1b het) and Gfi1b deficient (MxCre Gfi1b fl/fl) mice with retroviruses encoding either AML1-Eto9a or MLL-AF9 oncofusion -proteins. Transduced Gfi1b heterozygous or Gfi1b deficient cells generated more colonies and higher cell number than wt transduced cells. We also used mice transgenically expressing CBFbeta-Myh11. Deletion of Gfi1b accelerated leukemia formation in these mice compared to mice in which Gfi1b was still expressed. On a molecular level, we found that loss of Gfi1b leads to increased levels of ROS level. It has been shown by other groups, that increased levels of Gfi1b contribute to leukemia development. In addition, Gfi1b represses the expression of Integrin beta 3 (ITGB3). Absence of Gfi1b leads to higher expression level of ITGB3. ITGB3 has been shown to promote growth and expansion of leukemic stem cells, which play an important role in AML development. Thus we report here that Gfi1b acts as a novel tumor suppressor in AML development.