Novel Therapeutic Targets for Ph+ Chromosome Leukemia and Its Leukemia Stem Cells: A Dissertation

Abstract

The human Philadelphia chromosome (Ph) arises from a translocation between chromosomes 9 and 22 [t(9;22)(q34;q11)]. The resulting chimeric BCR-ABLoncogene encodes a constitutively activated, oncogenic tyrosine kinase that induces chronic myeloid leukemia (CML) and B-cell acute lymphoblastic leukemia (B-ALL). The BCR-ABL tyrosine kinase inhibitor (TKI), imatinib mesylate, induces a complete hematologic and cytogenetic response in the majority of CML patients, but is unable to completely eradicate BCR-ABL–expressing leukemic cells, suggesting that leukemia stem cells are not eliminated. Over time, patients frequently become drug resistant and develop progressive disease despite continued treatment. Two major reasons cause the imatinib resistance. The first one is the BCR-ABL kinase domain mutations which inhibit the interaction of BCR-ABL kinase domain with imatinib; the second one is the residual leukemia stem cells (LSCs) in the patients who are administrated with imatinib. To overcome these two major obstacles in CML treatment, new strategies need further investigation. As detailed in Chapter II, we evaluated the therapeutic effect of Hsp90 inhibition by using a novel water-soluble Hsp90 inhibitor, IPI-504, in our BCR-ABL retroviral transplantation mouse model. We found that BCR-ABL mutants relied more on the HSP90 function than WT BCR-ABL in CML. More interestingly, inhibition of HSP90 in CML leukemia stem cells with IPI-504 significantly decreases the survival and proliferation of CML leukemia stem cells in vitro and in vivo. Consistent with these findings, IPI-504 treatment achieved significant prolonged survival of CML and B-ALL mice. IPI-504 represents a novel therapeutic approach whereby inhibition of Hsp90 in CML patients and Ph+ ALL may significantly advance efforts to develop a cure for these diseases. The rationale underlying the use of IPI-504 for kinase inhibitor–resistant CML has implications for other cancers that display oncogene addiction to kinases that are Hsp90 client proteins. Although we proved that inhibition of Hsp90 could restrain LSCs in vitro and in vivo, it is still unclear how to define specific targets in LSCs and eradicate LSCs. In Chapter III, we took advantage of our CML mouse model and compared the global gene expression signature between normal HSCs and LSCs to identify the downregulation of Pten in CML LSCs. CML develops faster when Pten is deleted in Ptenfl/fl mice. On the other hand, Pten overexpression significantly delays the CML development and impairs leukemia stem cell function. mTOR is a major downstream of Pten-Akt pathway and it is always activated or overepxressed when Pten is mutated or deleted in human cancers. In our study, we found that inhibition of mTOR by rapamycin inhibited proliferation and induced apoptosis of LSCs. Notably, our study also confirmed a recent clinical report that Pten has been downregulated in human CML patient LSCs. In summary, our results proved the tumor suppressor role of Pten in CML mouse model. Although the mechanisms of Pten in leukemia stem cells still need further study, Pten and its downstream, such as Akt and mTOR, should be more attractive in LSCs study.