Poster Session II: Signal Transduction MUTANT FORMS OF BCR-ABL DEFICIENT IN INDUCING ABI DEGRADATION SHOW DIFFERENT LEUKEMOGENIC ACTIVITY

Bcr-Abl elicits the ubiquitin-dependent degradation of Abl interactor (Abi) proteins, a family of proteins that antagonize the oncogenic potential of Abl. Significantly, expression of the Abi proteins is lost in cell lines and bone marrow cells isolated from patients with aggressive Bcr-Abl-positive leukemias. To determine the role of Abi degradation in Bcr-Abl-induced leukemogenesis, we have mapped the sequences in Bcr-Abl that are required for inducing Abi degradation. The deletion of C-terminal proline-rich sequences (p185 Bcr-AblD924-1018 ) severely impairs Bcr-Abl-induced Abi degradation. The double deletions of the SH3 domain and C-terminal proline-rich sequences (p185 Bcr-AblD924-1018 ) completely abolish the degradation of Abi proteins. We then compared the leukemogenic activity of these mutant forms of Bcr-Abl to that of the wild type Bcr-Abl in a murine bone marrow transduction/transplantation model. Like wild type P185 Bcr-Abl , both p185 Bcr-AblD924-1018 and p185 Bcr-AblDSH3D924-1018 transform mouse bone marrow cells with similar potency, as judged by factor-independent growth in an agar colony assay. The mutant forms of Bcr-Abl, however, showed different leukemogenic activity in mice. Mice reconstituted with wild type P185 Bcr-Abl transduced marrow cells developed chronic myeloid leukemia (CML)-like disease and died in 5‐7 weeks after transplantation. The peripheral white blood cell (WBC) count of these mice was significantly elevated and the mice had enlarged spleens. Mice reconstituted with p185 Bcr-AblD918-1018 transduced marrow cells also died in 5‐7 weeks. The majority of these mice, however, did not show the elevated WBC count or the spenomegaly, suggesting that they died of a disease other than CML. Furthermore, mice reconstituted with p185 Bcr-AblDSH3D924-1018 transduced marrow cells survived much longer post-transplantation without the elevated WBC count. These results are consistent with the hypothesis that Abi degradation is involved in the development of Bcr-Abl positive CML. Studies are now in progress to define the pathology of diseases caused by mutant forms of Bcr-Abl.