Raf1 Is Required for Induction of a Bcr-Abl Positive CML Like Myeloproliferative Disease in Mice

Introduction: Chronic myelogenous leukemia (CML) results from neoplastic transformation of hematopoietic stem cells (HSC), characterized by a chromosomal translocation t(9;22)(q34;q11). This aberration leads to the expression of the oncogenic tyrosine kinase Bcr-Abl, which mediates signals for proliferation, transformation and anti-apoptosis via various different pathways including the Raf/MEK/ERK cascade. The cytoplasmic protein Raf1 is a key molecule within this cascade. Recent studies have revealed an additional function of the Raf-1 kinase that is independent of the activation of the MAPK cascade and whose effect is to increase resistance to apoptosis. Therefore Raf1 is an interesting target for molecular therapies and more effective Raf1 inhibitors have recently been developed by the pharmaceutical industry. Here we report the impact of Raf1 signalling for Bcr-Abl mediated transformation.

Methods: We exerted a siRNA based approach in combination with a murine bone marrow transplantation model. To this end we designed a MSCV based retrovirus encoding both the Raf1 microRNA and the Bcr-Abl oncogene on a single construct. This approach ensured knockdowns of more than 90% of Raf1 in every Bcr-Abl transformed cell.

Results: Methylcellulose assays demonstrated that bone marrow coexpressing Raf1 microRNA and Bcr-Abl had a 2 fold decreased colony forming ability compared to control cells. We then transduced bone marrow (BM) with retrovirus coexpressing Raf1 microRNA and p185 Bcr-Abl and transplanted lethally irradiated recipient Balb/C mice. The onset and progression of leukemia was significantly delayed in mice transplanted with Raf1 microRNA and Bcr-Abl compared with the Bcr- Abl transduced control microRNA group. Raf1 knockdown mice showed only a moderate rise of white blood cell (WBC) counts and prolonged overall survival (median survival 39 ± 7.1 days) in comparison to control mice (23.3 ± 2.4 days). However, we were not able to completely avoid the development of leukemia by Raf1 knockdown.

Conclusion: Taken together our data demonstrate that Raf1 is important for the development of a myeloproliferative disease by Bcr-Abl in mice. Therefore Raf1 inhibition in combination with Bcr-Abl kinase inhibition depicts an interesting approach towards eradication of Bcr- Abl positive leukemia. In addition, this study describes a novel and versatile approach to express an oncogene and a microRNA using a single retroviral construct. Thus this powerful tool can be used to systematically screen drugable signalling targets involved in oncogenesis.