Chronic myeloid leukemia (CML) results from transformation of a primitive hematopoietic cell by the BCR-ABL gene. The BCR-ABL tyrosine kinase (TK) inhibitor imatinib is highly effective in inducing remissions and improving survival in CML patients but does not eradicate primitive malignant hematopoietic stem and progenitor cells (HSPC), which persist in CML patients in remission on imatinib. Therefore patients need continued treatment to maintain remission and may be at risk for relapse. In the preceding grant period we have extensively investigated mechanisms of response and resistance of CML progenitors to imatinib. Our results indicate that imatinib and BCR-ABL TK inhibitors, including dasatinib and nilotinib, effectively inhibit BCR-ABL TK activity in CML progenitors and markedly inhibit progenitor proliferation, but induce only modest increase in apoptosis. Quiescent CML progenitors are particularly resistant to apoptosis following imatinib treatment. Persistence of CML HSPC despite effective BCR-ABL TK inhibition indicates the need for additional strategies to enhance elimination of malignant stem cells. We have shown that CML progenitors-treated with imatinib and other BCR-ABL TK inhibitors continue to be responsive to microenvironmental growth signals that help maintain their viability. In Specific Aim 1 will further investigate the role of microenvironmental interactions in resistance of CML hematopoietic stem cells to apoptosis following imatinib treatment using in vitro stromal co-coculture and in vivo mouse models of CML. We will investigate whether the a4b1 integrins and N-cadherin adhesion receptors contribute to protection of CML HSPC by the microenvironment. We will also investigate the role of Wnt-?-catenin signaling in CML stem cell maintenance following imatinib treatment. We have found that treatment with the histone deacetylase (HDAC) inhibitor LAQ824 in combination with imatinib results in significantly increased apoptosis of CML CD34+ cells, which is associated with markedly reduced expression of the anti-apoptotic protein Mcl-1. In Specific Aim 2 we will further investigate whether HDAC inhibitors can selectively and effectively induce apoptosis and eliminate CML HSPC in combination with imatinib, and will investigate mechanisms of apoptotic targeting of CML HSPC. We will investigate the mechanisms of Mcl-1 inhibition and the role of Mcl-1 downregulation in mediating HDAC effects on CML HSPC. We will also identify and investigate additional HDAC-inhibitor induced alterations in gene regulation that may lead to enhanced apoptotic targeting of CML HSPC using chromatin immunoprecipitation and gene expression array based approaches. Successful completion of these studies is expected to lead to future clinical application of these approaches to eliminate leukemia stem cells and potentially achieve cure in CML patients. PUBLIC HEALTH RELEVANCE: The BCR-ABL kinase inhibitor imatinib is highly effective in inducing remissions and improving survival in chronic myeloid leukemia (CML) patients but does not eradicate primitive leukemia stem and progenitor cells. Therefore patients need continued treatment to maintain remission and may be at risk for relapse and additional methods to enhance elimination of CML stem cells are required. The studies proposed here will investigate mechanisms of protection of leukemia stem cells from elimination by imatinib and examine whether blocking these mechanisms will enhance their elimination. The results if these studies will have direct application to clinical approaches to eliminate leukemia stem cells and potentially achieve cure in CML patients.