Chronic myeloid leukemia in chronic phase (CML-CP) is induced by BCR-ABL1 oncogenic tyrosine kinase. Tyrosine kinase inhibitors eliminate the bulk of CML-CP cells, but fail to eradicate leukemia stem cells (LSCs) and leukemia progenitor cells (LPCs) displaying innate and acquired resistance, respectively. These cells may accumulate genomic instability, leading to disease relapse and/or malignant progression to a fatal blast phase. In the present study, we show that Rac2 GTPase alters mitochondrial membrane potential and electron flow through the mitochondrial respiratory chain complex III (MRC-cIII), thereby generating high levels of reactive oxygen species (ROS) in CML-CP LSCs and primitive LPCs. MRC-cIII–generated ROS promote oxidative DNA damage to trigger genomic instability, resulting in an accumulation of chromosomal aberrations and tyrosine kinase inhibitor–resistant BCR-ABL1 mutants. JAK2(V617F) and FLT3(ITD)–positive polycythemia vera cells and acute myeloid leukemia cells also produce ROS via MRC-cIII. In the present study, inhibition of Rac2 by genetic deletion or a small-molecule inhibitor and down-regulation of mitochondrial ROS by disruption of MRC-cIII, expression of mitochondria-targeted catalase, or addition of ROS-scavenging mitochondria-targeted peptide aptamer reduced genomic instability. We postulate that the Rac2-MRC-cIII pathway triggers ROS-mediated genomic instability in LSCs and primitive LPCs, which could be targeted to prevent the relapse and malignant progression of CML.

2736 BCR-ABL1 –positive chronic myeloid leukemia in chronic phase (CML-CP) is a leukemia stem cell (LSC)-derived but leukemia progenitor cell (LPC)-driven disease, which may eventually develop resistance to the tyrosine kinase inhibitors (TKIs) and progress to fatal CML blast phase (CML-BP). In CML-CP, LSCs and LPCs reside in the CD34+CD38- and CD34+CD38+ populations, respectively. In addition, majority of LSCs and LPCs belong to quiescent (CFSEmax) and proliferative (CFSElow) populations, respectively. Quiescent LSCs are intrinsically insensitive to TKIs, and LPCs can acquire resistance to TKIs. In the TKI era, these cells may eventually initiate the disease relapse and progression to CML-BP, which is associated with genomic instability manifested by accumulation of a new or additional TKI-resistant BCR-ABL1 kinase mutations and chromosomal aberrations. We reported that BCR-ABL1 –positive leukemia cells contain high levels of the reactive oxygen species (ROS)-induced oxidative DNA damage resulting in genomic instability (Nowicki et al., Blood, 2005; Koptyra et al., Blood, 2006; Koptyra et al., Leukemia, 2008). These studies highlighted the importance of identification of the origin of leukemia cell lineage accumulating genomic instability and the mechanisms responsible for generation of ROS-mediated oxidative DNA damage. Here we show that LSC-enriched CD34+CD38- cells and quiescent LSCs, and also LPC-enriched CD34+CD38+ and proliferating CML-CP cells contain higher levels of ROS (superoxide anion, hydrogen peroxide, and hydroxyl radical) and oxidative DNA lesions (8-oxoG and DNA double-strand breaks) than corresponding cells from healthy donors. Surprisingly, the most primitive quiescent LSCs accumulated the highest levels of ROS and oxidative DNA damage. On the basis of these observations and the studies in murine hematopoietic 32Dcl3 cells expressing TKI-resistant BCR-ABL1 kinase variants (Y253F, T315I, H396P), we would predict that primitive CML-CP cells carrying these mutations would also contain high levels of ROS and oxidative DNA damage. Moreover, inhibition of BCR-ABL1 kinase with imatinib exerted only modest, if any, effect on ROS and oxidative DNA damage in LSCs/LPCs in the presence of growth factors (GFs). Among numerous signaling proteins activated in CML cells, Rac GTPases were potential candidates to regulate production of ROS. Importantly, Rac was stimulated in leukemia cells expressing non-mutated BCR-ABL1 and TKI-resistant kinase mutants and it remained active in CML-CP cells treated with imatinib in the presence of GFs. We used Rac dominant-negative mutant (RacT17N), Rac specific inhibitors (NSC23766 and EHT1864) and Rac1, Rac2 and Rac3 knockout cells to document that Rac2 GTPase is responsible for elevation of ROS and oxidative DNA damage in LSC-enriched CD34+CD38- cells, quiescent LSCs, and also in LPCs. Active Rac2 reduced mitochondrial membrane potential (ΔΨm) and slowed the electron flow between mitochondrial respiratory chain (MRC) complexes I-II and I-III leading to overproduction of ROS. Using cells depleted of functional mitochondria (Rho0 cells), applying specific probes to measure mitochondrial ROS (MitosoxRed and mitochondria matrix-targeted circularly permuted yellow fluorescence protein = mt-cpYFP) and employing a specific inhibitor of mitochondrial ROS (MitoQ) we determined that mitochondria are the main source of ROS causing oxidative DNA damage in CD34+CD38- and quiescent LSCs and in LPCs. Furthermore, using selective inhibitors of various MRC complexes we pinpointed complex III as major producer of ROS in LSCs and LPCs. This conclusion is supported by the observation that BCR-ABL1 –positive cells with genetically inactivated complex III, but not complex I, displayed diminished capability to generate ROS. Targeting Rac2 GTPase by RacT17N and reduction of mitochondrial ROS by mitochondrial-targeted catalase and by mitochondrial-targeted ROS-scavenging peptide aptamers prevented genomic instability. Altogether, Rac2 - MRC-cIII pathway is a major source of ROS-mediated oxidative DNA damage resulting in genomic instability in LSCs and LPCs, which could be targeted to prevent the relapse and malignant progression of CML. We also postulate that similar mechanisms cause genomic instability in FLT3(ITD)-positive acute myeloid leukemia cells and in JAK2(V617F)-positive polycythemia vera cells. Disclosures: Holyoake: Novartis: Consultancy, Research Funding. Valent:Novartis: Consultancy, Honoraria, Research Funding. Hochhaus:Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Ariad: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding. Hughes:BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Ariad: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

BCR/ABL kinase is the founding member of a family of oncogenic tyrosine kinases (OTKs) also including TEL/JAK2, TEL/PDGFR, TEL/ABL, and JAK2V617F, which induce myeloproliferative disorders (MPDs). BCR/ABL transforms hematopoietic stem cells (HSCs) to induce chronic myelogenous leukemia in chronic phase (CML-CP), which eventually evolves into fatal blast crisis (CML-BC). CML is a stem cell-derived but progenitor-driven disease. In CML-CP, leukemia stem cells (LSCs) and leukemia progenitor cells (LPCs) reside in the CD34+CD38- and CD34+CD38+ populations, respectively, whereas in CML-BC, LSCs are also found in the CD34+CD38+ population. In addition, CD34+ CML cells belong to either proliferative or quiescent populations; the latter of which responds poorly to the ABL kinase inhibitors. BCR/ABL kinase stimulates genomic instability causing imatinib-resistant point mutations in the kinase domain and additional chromosomal aberrations associated with progression to CML-BC (Oncogene, 2007). Since genomic instability usually results from enhanced DNA damage, we investigated the mechanisms responsible for “spontaneous” DNA damage in cells transformed by BCR/ ABL and other OTKs. Much endogenous DNA damage arises from free radicals such as reactive oxygen species (ROS) and/or reactive nitrogen species (RNS). We showed that CD34+ stem/progenitor CML cells contain higher levels of ROS (superoxide anion = ·O2−, hydrogen peroxide = H2O2 and hydroxyl radical = ·OH) and RNS (nitric oxide = NO·) than CD34+ cells from normal donors (CML-BC>CML-CP>Normal). Moreover, ROS levels were elevated in CD34+CD38- and CD34+CD38+ sub-populations isolated from CML-BC and CML-CP patients in comparison to the corresponding cells from healthy donor. In addition, both proliferative and quiescent CD34+ CML cell sub-populations contained more ROS than their normal counterparts. Interaction with the stromal cells further elevated ROS levels in BCR/ABL-positive cells. Higher ROS/RNS levels induced more oxidative/nitrative DNA lesions, such as 8-oxoG and DNA double-strand breaks (DSBs), in CML-CP cells resulting in induction of point mutations in BCR/ABL kinase causing imatinib resistance and accumulation of chromosomal aberrations characteristic of CML-BC. In addition, cells transformed by other OTKs also displayed elevated ROS/ RNS and oxidative/nitrative DNA damage, implicating their role in malignant progression of MPDs. Our previous studies showed that elevated levels of oxidative DNA damage in OTK-transformed cells could be diminished by scavenging of ROS with N-acetyl-cysteine and vitamin E, which reduced the frequency of imatinib-resistant BCR/ABL point mutants and chromosomal aberrations in leukemia cells cultured in vitro and growing in SCID mice (Blood, 2006; Leukemia, 2008). These studies highlighted the importance of identification of the sources of free radicals in CML and other MPDs. We found that elevated levels of ROS in BCR/ABL-transformed cell lines and CD34+ CML cells were generated by three major mechanisms: NADPH oxidase (NOX) complexes containing NOX1 and/or NOX2, complex III of the mitochondrial respiratory chain (MRC), and 5-lipoxygenase (LOX). In addition, inducible nitric oxygen synthase (iNOS) produced RNS in leukemia cells. Using selective inhibitors of NOX, MRC, LOX and iNOS we estimated the contribution of these pathways to accumulation of free radicals causing oxidative/nitrative DNA damage in CML cells. In summary, BCR/ABL kinase-dependent elevation of ROS/RNS depends on several mechanisms, which are now targeted to determine their actual role in genomic instability in CML.

3268 Poster Board III-1 BCR/ABL kinase transforms hematopoietic stem cells to induce chronic myelogenous leukemia (CML). CML in chronic phase (CML-CP) is a leukemia stem cell (LSC)-derived but leukemia progenitor cell (LPC)-driven disease, which is, in most cases, sensitive to ABL tyrosine kinase inhibitors (TKIs) monotherapy. TKIs do not eradicate the leukemia but instead usually render the disease ‘inactive', since the residual quiescent LSCs are intrinsically insensitive to BCR-ABL inhibition and, in a significant cohort of CML patients, LPCs are also refractory or acquire resistance to TKIs due to mutations in BCR/ABL kinase. In the post-imatinib era, these cells may eventually undergo transformation and initiate fatal CML blast crisis (CML-BC). The malignant progression is usually associated with enhanced expression of BCR/ABL and accumulation of additional genetic aberrations, such as TKI-resistant mutations and chromosomal aberrations. In CML-CP, LSCs and LPCs reside in the CD34+CD38- and CD34+CD38+ populations, respectively, whereas in CML-BC, LSCs are also found in the CD34+CD38+ population. In addition, LSCs and LPCs usually belong to quiescent (CFSEmax) and proliferative (CFSElow) populations, respectively. However, the origin of CML-BC clone and the role of BCR/ABL “dosage” are not known. Since genomic instability usually results from DNA damage, we investigated the mechanisms responsible for enhanced DNA damage in CML cells. Much endogenous DNA damage arises from free radicals such as reactive oxygen species (ROS). Here we show that LSCs-enriched CD34+CD38- and quiescent (CFSEmax) CML cells and LPCs-enriched CD34+CD38+ cells contain higher levels of ROS (superoxide anion, hydrogen peroxide, and hydroxyl radical) than corresponding cells from normal donors (CML-BC>CML-CP>Normal). Interestingly, CFSEmax and CFSElow CML cells displayed similar elevation of ROS indicating that the presence of BCR/ABL and not the proliferative status enhances ROS. In addition, total cellular ROS and mitochondrial ROS levels were proportional to the expression of BCR/ABL kinase implicating the role of BCR/ABL kinase “dosage”. Higher levels of ROS caused more oxidative DNA lesions, such as 8-oxoG and DNA double-strand breaks (DSBs) in CD34+ and also in CD34+CD38- CML cells than in normal counterparts (CML-BC>CML-CP>Normal). Inhibition of BCR/ABL kinase with imatinib partially reduced ROS and oxidative DNA damage in CD34+ CML-CP cells, implicating BCR/ABL-dependent and -;independent mechanisms. Our previous studies showed that elevated levels of oxidative DNA damage in BCR/ABL-transformed cells were responsible for accumulation of TKI-resistant BCR/ABL mutants and chromosomal aberrations (Blood, 2006; Leukemia, 2008), highlighting the importance of identification of the sources of ROS in CML. Mitochondrial respiratory chain (MRC) is a major site of ATP production via oxidative phosphorylation, which is associated with electron flux through MRC. Some of the electrons may escape and react with molecular oxygen to form ROS. To shut down MRC, cells were depleted of mitochondrial DNA (mtDNA) by long-term exposure to ethidium bromide in the presence of uridine and pyruvate as confirmed by RT-PCR showing the absence/reduction of mtDNA-coded Cox II gene transcript. The absence of functional MRC reduced the level of ROS by 40% and 20% in CD34+ CML-CP cells and normal counterparts, respectively, suggesting that MRC is an important source of ROS in leukemia cells. Using selective inhibitors of various MRC complexes we identified complex III as major producer of ROS in LSCs and LPCs in CML-CP. The role of complex III in CML-BC cells is somehow diminished in concordance with the observation that prolonged exposure of MRC to elevated levels of ROS results in “mitochondrial injury” and reduction of MRC activity in advanced stages of cancer. In summary, we postulate that BCR/ABL kinase generates ROS and oxidative DNA damage in a dose-dependent manner not only in LPCs-enriched CD34+CD38+ and CFSElow cells, but also in LSCs-enriched CD34+CD38- and CFSEmax cells, and that MRC complex III generates significant amount of ROS in CML-CP cells. Thus, genomic instability causing TKI resistance and progression to CML-BC may originate in LSCs as well as in LPCs. Disclosures: No relevant conflicts of interest to declare.

1211 Background: BCR-ABL1 –positive chronic myeloid leukemia in chronic phase (CML-CP) is a leukemia stem cell (LSC)-derived but leukemia progenitor cell (LPC)-driven disease, which may eventually progress to fatal CML blast phase (CML-BP). In CML-CP, LSCs and LPCs reside in the CD34+CD38- and CD34+CD38+ populations, respectively. In addition, majority of LSCs and LPCs belong to quiescent (CFSEmax) and proliferative (CFSElow) populations, respectively. Tyrosine kinase inhibitors (TKIs) such as imatinib, dasatinib and nilotinib do not eradicate the leukemia but instead render the disease ‘dormant’. Residual quiescent LSCs are intrinsically insensitive to TKIs and, in a significant cohort of CML patients, LPCs acquire resistance to TKIs. In the TKI era, these cells may eventually initiate disease relapse and progression to CML-BP, which is associated with genomic instability manifested by accumulation of TKI-resistant BCR-ABL1 kinase mutations and chromosomal aberrations. Previously we showed that BCR-ABL1 kinase stimulates reactive oxygen species (ROS)-dependent oxidative DNA damage resulting in genomic instability (Nowicki et al., Blood, 2005; Koptyra et al., Blood, 2006; Koptyra et al., Leukemia, 2008). These studies highlighted the importance of identification of cellular lineage origin and mechanisms responsible for generation of ROS-mediated oxidative DNA damage in CML. Result: Here we show that LSC-enriched CD34+CD38- and quiescent (CFSEmax) CML-CP cells and LPC-enriched CD34+CD38+ and proliferating (CFSElow) CML-CP cells contain higher levels of ROS (superoxide anion, hydrogen peroxide, and hydroxyl radical) and oxidative DNA lesions (8-oxoG and DNA double-strand breaks) than corresponding cells from healthy donors. Non-mutated and TKI-resistant BCR-ABL1 kinase mutants (Y253F, T315I, H396P) stimulated ROS-induced oxidative DNA damage in a BCR-ABL1 dosage-dependent manner. Inhibition of BCR-ABL1 kinase with imatinib only partially reduced ROS and oxidative DNA damage in CD34+ CML-CP cells, implicating kinase-dependent and –independent mechanisms. Mitochondrial respiratory chain (MRC) is a major site of ATP production via oxidative phosphorylation, which is associated with electron flux through MRC. Some of the electrons may escape and react with molecular oxygen to form ROS. We detected that CD34+ CML-CP cells displayed lower mitochondrial potential than normal counterparts, which is indicative of enhanced ROS production. To determine the role of MRC in ROS-induced oxidative DNA damage, cells were depleted of mitochondrial DNA (mtDNA) by ethidium bromide, as confirmed by RT-PCR showing the absence/reduction of mtDNA-coded Cox II gene transcript (Rho0 cells). The absence of functional MRC reduced ROS and oxidative DNA damage in CD34+ CML-CP Rho0 cells and 32Dcl3 Rho0 cells transformed by non-mutated and TKI-resistant BCR-ABL1 kinase mutants, but not in normal counterparts, implicating a specific role of MRC in genomic instability in leukemia cells. In concordance, BCR-ABL1 –positive 32Dcl3 Rho0 cells accumulated fewer TKI-resistant BCR-ABL1 kinase mutants than cells with functional MRC. Using selective inhibitors of various MRC complexes we identified complex III as major producer of ROS and oxidative DNA damage in CD34+CD38- and quiescent LSCs and in CD34+CD38+ and proliferating LPCs in CML-CP. Moreover, BCR-ABL1 –positive cells in which complex III was inactive due to a single base substitution within the cytochrome b gene displayed diminished capability to generate ROS. In contrast, ROS was not affected in cells lacking complex I due to a mutation in the ND6 gene. In addition to BCR-ABL1 –positive CML-CP complex III also appeared to play a major role in generation of ROS in FLT3(ITD)-positive acute myeloid leukemia cells and in JAK2(V617F)-positive polycythemia vera cells. Conclusion: In summary, we postulate that enhanced production of ROS by MRC complex III induces genomic instability in LSC-enriched CD34+CD38- and quiescent cells, and also in LPC-enriched CD34+CD38+ and proliferating cells. Thus, genomic instability causing TKI resistance, disease relapse and progression to CML-BP may originate in LSCs as well as in LPCs. Disclosures: No relevant conflicts of interest to declare.