Deletion of Rac2 inhibits Proliferation of Chronic Myelogenous Leukemia (CML) Stems Cells and Progenitors (HSC/P) In Vivo and Promotes Survival of Scl/p210-BCR-ABL Mice.

Abstract 3253

Poster Board III-1

Chronic myelogenous leukemia (CML) is a hematopoietic stem cell (HSC) malignancy induced by p210-BCR-ABL and is characterized by myeloproliferation in the bone marrow (BM) and egress of leukemic stem cells and progenitors (LSC/P) to extramedullary sites. Persistence of BCR-ABL⁺ HSCs in patients under imatinib suggests that inhibition of ABL-kinase alone is not sufficient to completely eliminate the LSC/P population. Rac GTPases represent integrative molecular switches for p210-BCR-ABL-induced HSC transformation and combined pharmacological and genetic attenuation of Rac GTPases significantly prolong survival in vivo, as reported in a retroviral transduction/transplantation model (Thomas EK & Cancelas JA et al, Cancer Cell 2008). Here, we analyzed the role of Rac2 GTPase in the leukemic maintenance and in the interaction of LSC/P with the leukemic microenvironment in vivo. We used a stem cell leukemia (Scl) promoter-driven, tet-off, Scl-tTA x TRE-BCR-ABL (Scl/p210-BCR-ABL) binary transgenic mouse model (Koschmieder S et al., Blood 2005), where expression of BCR-ABL is restricted to the HSC/P compartment, allowing the study of the intrinsic molecular changes in LSC/P during leukemogenesis. In these mice, Scl-driven expression of BCR-ABL is active in HSC (Lin^-/Sca1⁺/c-kit⁺; LSK) and progenitors (Lin^-/c-kit⁺/Sca-1^-; LK), and CML development is associated with the activation of downstream signaling effectors CrkL, p38-MAPK and JNK. Additionally, Scl/p210-BCR-ABL mice had increased cycling of LSK cells and expansion of circulating and splenic, but not BM, LSC/P, suggesting egress of LSC/Ps from the marrow. These mice share all the characteristics of HSC/P transformation in CML, including increased HSC/P proliferation and survival, severely reduced adhesion to fibronectin, increased migration towards CXCL12, increased cell surface expression of CD44 and decreased expression of L-selectin. Myeloproliferative disease (MPD) in these mice is transplantable into recipient mice, and CML splenocytes have a 10-fold increase in homing to the spleen than towards BM (P<0.05). Leukemic splenocytes are also enriched in endosteal lodging progenitors, compared to the BM-derived progenitors (1.9-fold, P≤0.05). In order to determine the contribution of Rac2 GTPase in the transformation phenotype of leukemic stem cells and progenitors, Scl/p210 mice were intercrossed with Rac2^-/- mice. Interestingly loss of Rac2 GTPase alone significantly prolongs survival of the leukemic mice (P≤0.001). Prolonged survival, as observed in Scl/p210 x Rac2^-/-, is associated with significantly reduced proliferation of leukemic LK (3-fold, P<0.05) and LSK (6-fold P<0.005) cells, both in BM as well as in spleen, in vivo. Scl/p210 x Rac2^-/- mice are also characterized by increased apoptosis (1.7-fold) and lower frequency of LSK cells (2-fold) compared to the Scl/p210 mice in vivo. However, deletion of Rac2 does not significantly reverse the adhesion and migration transformation phenotype of LSC/P. In summary, Rac2 deficiency induces a significant survival of CML mice in a HSC-initiated model of disease through decrease proliferation and survival but does not reverse the transformation phenotype affecting adhesion and migration. This murine model may represent an adequate in vivo system to dissect out the specific signaling pathways involved in p210-BCR-ABL-induced stem cell transformation.