Investigation Of p210 Bcr-Abl Rhogef Activity In The Progression Of Chronic Myelogenous Leukemia

The Philadelphia chromosome is formed by a balanced, reciprocal translocation that pairs sequences from BCR on Chromosome 22 with sequences from ABL on Chromosome 9 and results in the production of the constituitively active tyrosine kinase Bcr-Abl. Depending on the location of the breakpoint within BCR, three different sizes of Bcr-Abl can be produced (i.e., p190, p210 and p230) and they are associated with different clinical outcomes. The larger p210 form is observed in greater-than 95% of chronic myelogenous leukemia [CML], while the diminutive p190 is present in approximately 2/3 of Philadelphia-positive acute lymphoblastic leukemia [ALL]. Although both the p210 and p190 forms contain the same portion of Abl, importantly, they differ only in the amount of Bcr which is retained at the amino terminus. We previously identified a functional domain within the Bcr sequences preserved by p210, but not by p190, which demonstrates a constitutive Rho GTPase-specific guanine nucleotide exchange factor [RhoGEF] activity. To determine the contribution of this region to p210 Bcr-Abl-related disease progression in CML, we therefore introduced a single amino acid substitution [S509A] into this construct which abrogated its activity and then compared this mutant to the p210 and p190 variants in a murine bone marrow transplantation model. While all of the mice eventually developed myeloproliferative disease, those transplanted with either p210 Bcr-Abl S509A or p190 Bcr-Abl displayed a more rapid onset than the mice transplanted with p210 Bcr-Abl (within 12 vs. 23 days of transplantation, respectively). Interestingly, this reduced disease latency is associated with erythroid hyperplasia in the absence of anemia and expansion of megakaryocyte-erythrocyte progenitor, common myeloid progenitor and granulocyte-macrophage progenitor populations, which results in a phenotype that is similar to the M6 form of acute myeloid leukemia. This phenotype is also readily transplantable into secondary recipients, indicating that it is a true element of the malignancy and not a reactive process. Taken together, these results support a model wherein the RhoGEF activity of p210 Bcr-Abl actively regulates disease progression by downregulating the self-renewal of myeloid progenitors. While our animal studies indicate that the Bcr region plays a significant role in disease progression, to the best of our knowledge, this has yet to be evaluated using clinically derived mutations. Recently, the RhoGEF domain of p210 Bcr-Abl was reported to be mutated and/or partially deleted in tumors obtained from several CML blast crisis patients and a p210 Bcr-Abl-positive ALL patient. These findings suggest that the RhoGEF domain of p210 Bcr-Abl may in fact be actively involved in the aggressiveness of primary specimens as well. In order to determine the consequences of the reported mutations, we therefore examined their effects on disease progression using a murine bone marrow transplant model.