Contributions of the RhoGEF activity of p210 BCR/ABL to disease progression.

We have previously identified a tyrosine kinase-independent, guanine nucleotide exchange factor (GEF) activity, which is contained within the region of p210 no expansion BCR/ABL that distinguishes it from p190 BCR/ABL. In the current study, we have compared the transforming activity of p190 BCR/ABL, p210 BCR/ABL and a mutant that lacks GEF activity (p210 BCR/ABL(S509A)). In cell-based, ex vivo, and murine bone marrow transplantation (BMT) assays the transforming activity of p210 BCR/ABL(S509A) mimics p190 BCR/ABL, and is distinct from p210 BCR/ABL. Thus, in the BMT assay, the p190 BCR/ABL- and p210 BCR/ABL(S509A)-transplanted mice exhibit a more rapid onset of disease than mice transplanted with p210 BCR/ABL. The reduced disease latency is associated with erythroid hyperplasia in the absence of anemia, and expansion of the megakaryocyte-erythrocyte progenitor (MEP), common myeloid progenitor (CMP) and granulocyte-macrophage progenitor (GMP) populations, producing a phenotype that is similar to acute myeloid leukemia (AML-M6). The disease phenotype is readily transplantable into secondary recipients. This is consistent with ex vivo clonogenicity assays, where p210 BCR/ABL preferentially supports the growth of colony forming unit (CFU)-granulocyte-macrophage (GM), whereas p190 BCR/ABL and the mutant preferentially support the growth of burst forming unit-erythroid (BFU-E). These results suggest that the GEF activity that distinguishes p210 BCR/ABL from p190 BCR/ABL actively regulates disease progression.

Loss of the xeroderma pigmentosum group B protein binding site impairs p210 BCR/ABL1 leukemogenic activity.

Previous studies have demonstrated that p210 BCR/ABL1 interacts directly with the xeroderma pigmentosum group B (XPB) protein, and that XPB is phosphorylated on tyrosine in cells that express p210 BCR/ABL1. In the current study, we have constructed a p210 BCR/ABL1 mutant that can no longer bind to XPB. The mutant has normal kinase activity and interacts with GRB2, but can no longer phosphorylate XPB. Loss of XPB binding is associated with reduced expression of c-MYC and reduced transforming potential in ex-vivo clonogenicity assays, but does not affect nucleotide excision repair in lymphoid or myeloid cells. When examined in a bone marrow transplantation (BMT) model for chronic myelogenous leukemia, mice that express the mutant exhibit attenuated myeloproliferation and lymphoproliferation when compared with mice that express unmodified p210 BCR/ABL1. Thus, the mutant-transplanted mice show predominantly neutrophilic expansion and altered progenitor expansion, and have significantly extended lifespans. This was confirmed in a BMT model for B-cell acute lymphoblastic leukemia, wherein the majority of the mutant-transplanted mice remain disease free. These results suggest that the interaction between p210 BCR/ABL1 and XPB can contribute to disease progression by influencing the lineage commitment of lymphoid and myeloid progenitors.