How cancer cells hijack DNA double-strand break repair pathways to gain genomic instability

DNA double-strand breaks (DSBs) are a significant threat to the viability of a normal cell, since they can result in loss of genetic material if mitosis or replication is attempted in their presence. Consequently, evolutionary pressure has resulted in multiple pathways and responses to enable DSBs to be repaired efficiently and faithfully. Cancer cells, which are under pressure to gain genomic instability, have a striking ability to avoid the elegant mechanisms by which normal cells maintain genomic stability. Current models suggest that in normal cells DSB repair occurs in a hierarchical manner that promotes rapid and efficient rejoining first, with the utilisation of additional steps or pathways of diminished accuracy if rejoining is unsuccessful or delayed. We evaluate the fidelity of DSB repair pathways and discuss how cancer cells promote the utilisation of less accurate processes. Homologous recombination serves to promote accuracy and stability during replication, providing a battlefield for cancer to gain instability. Non-homologous end-joining, a major DSB repair pathway in mammalian cells, usually operates with high fidelity and only switches to less faithful modes if timely repair fails. The transition step is finely tuned and provides another point of attack during tumour progression. In addition to DSB repair, a DSB signalling response activates processes such as cell cycle checkpoint arrest, which enhance the possibility of accurate DSB repair. We will consider the ways by which cancers modify and accost these processes to gain genomic instabilit

Single Agent Activity of the Cyclin-Dependent Kinase (CDK) Inhibitor Dinaciclib (SCH 727965) In Acute Myeloid and Lymphoid Leukemia Cells

Abstract Abstract 3981 Dinaciclib (SCH 727965) is a selective and potent inhibitor of CDK 1, 2, 5 and 9 (IC50 &lt; 5 nM) that has demonstrated in vitro and in vivo anti-tumor activity against a variety of tumor cell lines and human tumor xenograft models. The concentration of dinaciclib required to achieve these effects (&lt; 100 nM) is achieved in clinical trials, and dinaciclib was found to have a more favorable therapeutic index, in preclinical murine models, than other CDK inhibitors. We have studied the effect of dinaciclib on human acute myelogenous (AML) and acute lymphoblastic (ALL) leukemia cell lines, including HL-60, K562 and Molt-4, and primary leukemia cells. Dose response curves (0.0004-10 μM) were generated for different exposure times (2, 24 and 72 h), and data from cell proliferation assay (WST-1) were used to calculate the IC50 values. Short 2 h exposure to dinaciclib followed by 24 h culture without drug resulted in different responses between the cell lines (IC50 values of 0.13 μM, 2.17 μM and ND; and viability at 10 μM 62%, 76% and 95%, for HL-60, Molt-4, and K562, respectively). With longer exposure times (24 and 72 h), the IC50 was similar between the cell lines (IC50 24 h values of 0.017, 0.015, and 0.019 μM for HL-60, Molt-4, and K562, respectively). However, even in the presence of the highest drug concentration tested (10 μM), approximately 5–25% of cells remained metabolically active after 24 h culture, and in a colony forming assay were able to proliferate and form colonies after removal of the drug. Longer 72 h exposure to dinaciclib (0.2-10 μM) completely inhibited cell proliferation in all cell lines and prevented colony formation. Next, we examined the effect of dinaciclib (2-200 nM) on cell cycle in HL-60 and K562 cells (2, 6, 9, 24 h). While lower drug concentrations and shorter exposures resulted in a minor increase in the proportion of cells in the G2 phase, a considerable increase of cells in the sub-G1 phase was observed with prolonged exposures and higher drug concentrations, most prominently in HL-60 cells (4h 200 nM 38%; 6h 20 nM 53% or 200 nM 71%, and 24 h 20 nM 84%), which is consistent with cell viability assay data. These findings were also confirmed by Annexin V/PI staining. To characterize the molecular mechanisms behind the induction of cell cycle arrest and apoptosis by dinaciclib, we measured the changes in protein expression of Mcl-1, phosphorylation of retinoblastoma (Rb) protein, and cleavage of PARP by Western blotting. Dinaciclib treatment in a dose- and time-dependent manner (6 and 24 h; 10–500 nM) significantly decreased the expression of anti-apoptotic protein Mcl-1, Rb phosphorylation at Ser 811/817, and induced cleavage of the PARP protein in the three cell lines tested. For HL-60 cells, even 2 h exposure to dinaciclib was able to induce these effects when cells were examined 4 h after treatment; however, both Mcl-1 and p-Rb returned to baseline 24 h later, suggesting that the cells were able to recover. Using HL-60 cells, we were also able to demonstrate that a decrease in Mcl-1 correlates with the decrease in phosphorylation of the carboxy-terminal domain of RNA polymerase II, suggesting that dinaciclib successfully inhibits CDK9 which may lead to transcriptional down-regulation of Mcl-1. Dinaciclib treatment also down-regulated the expression of XIAP, Bcl-xl, and phosphorylation of Bad at Ser 112 (the pro-survival form of Bad), while Bak and Bax levels remained unaffected. The cleavage of PARP correlated with the activation of the caspase-3 and -9, suggesting the involvement of the intrinsic pathway of apoptosis. We confirmed our findings in primary leukemia cells. Dinaciclib was able to induce growth inhibition in all 7 primary AML samples (IC50 for 24 h exposure ranging from 0.008 to 0.017 μM) and apoptosis (Annexin V/PI staining). Treatment with dinaciclib also resulted in down-regulation of Mcl-1, cleavage of PARP, and dephosphorylation of Rb in all primary leukemia cells examined. In summary, dinaciclib potently inhibits the growth and induces apoptosis of human leukemia cells in vitro. Prolonged exposure times may be required for its maximum efficacy, and given its short half-life in humans (1.5 to 3.3 hours), this should be considered when designing the clinical studies for patients with acute leukemias. Disclosures: Sadowska: Merck &amp; Co: Research Funding. Muvarak:Merck &amp; Co: Research Funding. Lapidus:Merck &amp; Co: Equity Ownership, Research Funding. Bannerji:Merck &amp; Co: Employment, Equity Ownership. Gojo:Merck &amp; Co.: Research Funding. </jats:sec

Demethylating Agents Reprogram Myelodysplastic Syndrome and Leukemia Cells, Sensitizing Them To Poly-(ADP)-Ribose Polymerase Inhibitors

Abstract Several lines of evidence suggest that genomic instability in myeloid malignancies is promoted by increased endogenous DNA damage and error-prone repair that lead to disease progression and resistance to therapy. We recently reported increased levels of Poly-(ADP)-ribose polymerase (PARP) and DNA Ligase IIIα as well as increased activity of a highly error-prone pathway for repair of DNA double-strand breaks in myeloid leukemias. Importantly, these leukemia cells are sensitive to inhibitors of DNA Ligase IIIα and PARP, suggesting their dependence on these factors for survival. Parallel studies have shown that transient exposure to DNA demethylating agents at low nM concentrations reprograms cancer cells, altering heritable gene expression patterns in key cellular pathways, including DNA repair pathways, suggesting that pre-treatment of leukemia cells with demethylating agents may further sensitize them to PARP inhibitors. Thus, established cell lines from acute myeloid leukemia (AML; MV411, KASUMI-1), myelodysplastic syndrome transformed to AML (MDS; P39) and bone marrow mononuclear cells obtained from AML patient samples (N=7) were exposed to non-cytotoxic doses of DNA methyltransferase inhibitors (DNMTis, decitabine, DAC), followed by four days without drug treatment and subsequent treatment with low doses of PARP inhibitors (PARPis; ABT888, or BMN673) alone or in combination with DNMTis. Clonogenicity, apoptosis, DNA repair efficiency, and activity and expression level of DNA repair and DNA methyltransferase proteins were then studied. In all the cell lines tested, treatment with DAC (5-10nM) followed by ABT888 (500nM) induced a significant decrease in colony survival compared to control or single treatment. The use of a more potent PARPi, BMN673 (0.1-10nM), confirmed that treatment with DNMTis followed by PARPis induces a robust inhibition of AML and MDS cell line colony forming capacity. Interestingly, the same schedule treatment of decitabine followed by PARPis significantly decreases the clonogenic capacity in 4 out of 7 (57%) of bone marrow mononuclear cells from AML patient tested so far, suggesting that DNMTis and PARPis sequential treatment could be a valuable therapeutic option for AML and MDS patient. We next initiated studies to elucidate the mechanism by which DAC may sensitize myeloid malignancies to PARPis. As expected, DAC treatment alone was sufficient to decrease DNMT1 expression levels and increase caspase 3 cleavage in AML cell lines, compared to control treated cells. But surprisingly, DAC treatment alone also induced a decrease in PARP protein expression, with a further decrease in cells treated with DAC followed by PARPis, suggesting that both methylation and DNA repair signaling alter PARP1 steady-state levels. Moreover, preliminary results show that the presence of PARP on chromatin is decreased with DAC treatment and further decreased following PARPis. In conclusion, our results suggest that DNMTis reprogram cells, sensitizing them to PARP inhibition in AML/MDS patient and cell line models, paving the way for testing the therapeutic potential of sequential treatment with these agents in clinical trials. We are exploring one hypothesis that decreased levels of PARP on chromatin following DAC treatment may lead to more effective trapping of PARP1 at sites of DNA damage by PARPis, leading to abrogation of DNA repair. Understanding how these proteins interact may explain the mechanisms underlying the sensitization of epigenetically reprogrammed cells to PARPis, and may define the molecular subsets of AML patients that may respond to this novel therapeutic strategy. Disclosures: No relevant conflicts of interest to declare. </jats:sec

C-MYC and C-MYC-Regulated Micrornas Increase The Activity Of The Error-Prone ALT NHEJ Pathway Through Upregulation Of LIG3 and PARP1 In Tyrosine Kinase-Activated Leukemias

Abstract Constitutively activated tyrosine kinases (TK) BCR-ABL1 and FLT3/ITD not only increase cell survival and proliferation, but also increase levels of endogenous DNA damage and activity of an error-prone DNA double-strand break (DSB) repair pathway. This genomic instability leads to acquisition of genomic alterations that can result in disease progression and/or resistance to therapy. We have previously demonstrated that, in TK-activated leukemias, activity of the classic non homologous end-joining (C-NHEJ) pathway that repairs DSBs is decreased, and, as a consequence, an alternative, highly error-prone form of NHEJ (ALT NHEJ) predominates, evidenced by increased expression of DNA ligase IIIα (LIG3) and PARP1 (components of ALT NHEJ), increased frequency of large DNA deletions, and repair using DNA microhomologies. In this study, we sought to elucidate the role of a key downstream target of TKs, c-MYC, in upregulating LIG3 and PARP1 expression and consequently increasing ALT NHEJ and genomic instability. We demonstrated that MYC increases the expression of LIG3 and PARP1 through two mechanisms: 1) Increased binding to the promoters of LIG3 and PARP1, leading to increased transcription, and 2) Repression of microRNAs (miRs) that putatively regulate LIG3 and PARP1. Chemical and siRNA-mediated knockdown of MYC in MO7e-BCR/ABL and FLT3/ITD(+) MOLM14 cells results in significant reduction (p&lt;0.05) in LIG3 and PARP1 mRNA and protein compared to controls. Chromatin immunoprecipitation assays revealed MYC binding to the promoters of LIG3 and PARP1 in AML (MOLM14) and CML (K562 and MO7e-BCR/ABL) cell lines. Additionally, transfection of PARP1 and LIG3 promoter-luciferase constructs into TK-activated (32D-FLT3/ITD, MO7e-BCR/ABL) cells showed significantly (p&lt;0.01) increased LIG3 and PARP1 promoter activity compared to parental controls (32D, MO7e). Moreover, knockdown of MYC in 32D-FLT3/ITD and MO7e-BCR/ABL cells resulted in a significant reduction of promoter activity in luciferase assays (p&lt;0.05). Conversely, overexpression of c-MYC in 293T cells caused an increase (p&lt;0.05) in LIG3 and PARP1 promoter activity. We next determined whether MYC-repressed miRs that have predicted binding sites in the 3’-UTR and coding regions of LIG3 and PARP1 are involved in regulating expression of LIG3 and PARP1. We found that there was a significant inverse correlation between LIG3 expression and miR-22, miR-23a, and miR-150 (Pearson’s r ≤ -0.3, p&lt;0.05). Similarly, there was a significant inverse correlation between PARP1 expression and miR-22, miR-23a, miR-27a, and miR-150 (Pearson r ≤ -0.3, p&lt;0.05). Over-expression of miR-22 in the CML cell line K562 decreased both LIG3 and PARP1 protein levels by 52% and 63% respectively. Similar results were seen upon over-expression of miR-34a (59% and 45%) and miR-150 (46% and 62%) for LIG3 and PARP1. This indicates that MYC-regulated miRs may function coordinately to regulate NHEJ repair. Importantly, our functional NHEJ assays demonstrate an overall significant (p&lt;0.05) reduction in the average size of deletions at the sites of DSB repair when MYC is knocked down, indicating a reduction in ALT NHEJ activity. To determine whether increased expression of LIG3 and PARP1 correlated with MYC expression in primary leukemia samples, we examined mRNA levels from bone marrow of 21 CML patients (12 chronic phase, 1 accelerated, 7 blast crisis, and 1 unknown). Twelve patients were resistant to Imatinib, 7 were responsive, and 2 undetermined. There was a strong positive correlation between levels of MYC and PARP1 (Pearson’s r= 0.75, p=0.001), as well as MYC and LIG3 (Pearson’s r =0.45, p=0.03). While there was no correlation between levels of gene expression and disease phase, we found that the majority of samples with elevated levels of MYC, LIG3 and PARP1 were from Imatinib-resistant patients (64%), compared to samples from Imatinib-sensitive patients (36%) (p=0.03). Additionally, 2 patient samples with TKI-resistant T315I mutation in BCR-ABL1 exhibited elevated levels of MYC, LIG3 and PARP1. Thus, increased MYC expression, and repression of miRs 22, 150 and 34a augment expression of LIG3 and PARP1, generating DSB repair errors that may lead to resistance to TKI therapy. Altered expression of MYC, LIG3, PARP1 and miRs 22, 150 and 34a may be biomarkers for those patients likely to become resistant to TKI therapy. Disclosures: No relevant conflicts of interest to declare. </jats:sec

Phenotypic Correction of Adamts13-Deficient Mice by Early Intra-Amniotic Gene Transfer of Lentiviral Vector Encoding ADAMTS13 Genes.

Abstract ADAMTS13, a member of A Disintegrin and Metalloprotease with ThromboSpondin type 1 repeats (ADAMTS) family, is mainly synthesized in the hepatic stellate cells, endothelial cells and megakaryocytes or platelets. It controls the sizes of von Willebrand factor (VWF) multimers by cleaving VWF at the Tyr1605-Met1606 bond. Genetic deficiency of plasma ADAMTS13 activity results in hereditary thrombotic thrombocytopenic purpura (TTP), also named Upshaw-Schülman syndrome. To develop a potential gene therapy approach and to determine the domains of ADAMTS13 required for recognition and cleavage of VWF in vivo, a self-inactivating lentiviral vector encoding human wild-type ADAMTS13 or variant truncated after the spacer domain (construct MDTCS) was administrated by intra-amniotic injection on embryonic day 8. Direct stereomicroscopy and immunofluorescent microscopic analysis revealed that the green fluorescent protein (GFP) reporter, ADAMTS13 and MDTCS were predominantly expressed in the heart, kidneys and skin. The synthesized ADAMTS13 and truncated variant were detectable in mouse plasma by immunoprecipitation and Western blot, as well as by proteolytic cleavage of FRETS-VWF73 substrate. The levels of proteolytic activity in plasma of mice expressing ADAMTS13 and MDTCS were 5 ± 7% and 60 ± 70%, respectively using normal human plasma as a standard, and this proteolytic activity persisted for at least 24 weeks in Adamts13−/−mice and 42 weeks in wild-type mice tested (the duration of observation). The mice expressing both recombinant ADAMTS13 and MDTCS showed a significantly decreased ratio of plasma VWF collagen-binding activity to antigen and a reduction in VWF multimer sizes as compared to those in the controls. Moreover, the mice expressing ADAMTS13 and MDTCS showed a significant prolongation of ferric chloride-induced carotid arterial occlusion time (9.0 ± 0.6 and 25.2 ± 3.2 min, respectively) as compared to the Adamts13−/− mice expressing GFP alone (5.6 ± 0.5 min) (p&amp;lt;0.01). The ferric chloride-induced carotid occlusion time in Adamts13−/− mice expressing ADAMTS13 was almost identical to that in wild type mice with same genetic background (C56BL/6) (8.0 ± 0.2 min) (p&amp;gt;0.05). The data demonstrate the correction of the prothrombotic phenotype in Adamts13−/−mice by gene transfer to the fetus by viral vectors encoding human wild type ADAMTS13 and the carboxyl terminal truncated variant (MDTCS), supporting the feasibility of developing a gene therapy based treatment for hereditary TTP. The discrepancy in the proteolytic activity of MDTCS between in vitro (Zhang P et al. Blood, 2007 in press) and in vivo in the present study suggests the potential cofactors in murine circulation that may rescue the defective proteolytic activity of the carboxyl-terminal truncated ADAMTS13 protease seen in vitro.</jats:p