Rac2 GTPase Activation Is Necessary for Development of p190-BCR-ABL-Induced B-Cell Acute Lymphoblastic Leukemia

The fusion gene BCR-ABL, resulting from t(9;22) reciprocal chromosomal translocations, encodes a constitutively active tyrosine kinase. Two different isoforms of BCR-ABL, p190 and p210, are associated to two completely different diseases. In the tyrosine kinase inhibitor (TKI) era, while p210-BCR-ABL-induced CML is highly responsive to TKI, p190-BCR-ABL still induces a poor prognosis B-cell acute lymphoblastic leukemia (B-ALL). The only difference between these two forms of BCR-ABL is the existence of a DH/Cdc24/PH domain in p210-BCR-ABL, which acts as a guanine nucleotide exchange factor (GEF) able to activate Rho GTPases. Rac is a subfamily of Rho GTPases with regulatory activity on hematopoietic stem cell and progenitor (HSC/P) functions. We have previously shown that Rac2 and further the combination of Rac1 and Rac2 mediate downstream signals in p210 BCR-ABL-induced myeloproliferation (Thomas EK, et al., Cancer Cell, 2007). Interestingly, despite the absence of a GEF domain in p190-BCR-ABL, Rac is activated, suggesting the activation of other GEF(s). Here we have analyzed whether Vav and Rac family members are involved in p190-BCR-ABL-induced B-ALL. We have used a combination of in vitro (Ba/F3 pro-B cells transduced with p190 or p210 BCR-ABL) and in vivo (murine transduction-transplantation model of p190 BCR-ABL-induced B-ALL) approaches. In Ba/F3 cells, both p190 BCR-ABL and p210 BCR-ABL activated Rac and the Rac effector p21 activated kinase (PAK), and their proliferation and survival appeared severely decreased in response to the Rac activation inhibitor NSC23766. Stat3, Stat5 and Jnk, but not ERK, p38 or NF-kB, were constitutively hyperactivated in p190 BCRABL-expressing Ba/F3 cells and primary murine B-ALL cells. Intracellular flow cytometry analysis demonstrated that Stat5 was specifically activated in the pro/pre-B leukemic cell population, compared to normal B cells. In the murine model of B-ALL, loss of Rac2, but not Rac3, prolonged survival and impaired leukemia development. Like in Ba/F3 cells, primary B-CFU and outgrowth in Witte-Whitlock assays of leukemic primary cells from mice was severely decreased by the addition of NSC23766 to the culture. Although Vav was activated by both p190- and p210-BCR-ABL, since NSC23766 does not block the activation by Vav1, we hypothesized that other GEFs were involved. Indeed, the loss of Vav1 or even combined loss of Vav1 and Vav2 did not impair BCR-ABL-mediated lymphoid leukemogenesis in vivo. Vav3, another member in the Vav family which uses a different mechanism of activation of Rac GTPases was a likely candidate. In fact, loss of Vav3 alone was able to significantly prolong the survival and attenuate development of p190 BCR-ABL-driven B-ALL. In conclusion, the results of this study indicate that Rac activation is necessary for the development of B-ALL induced by p190-BCR-ABL in vitro and in vivo, and validate a new signaling pathway as a therapeutic target for BCR-ABL-induced B-ALL.