Claire Seedhouse

Polymorphisms in Genes Involved in Homologous Recombination Repair Interact to Increase the Risk of Developing Acute Myeloid Leukemia

Purpose: Double-strand break repair via homologous recombination is essential in maintaining genetic integrity. RAD51 and XRCC3 are involved in the repair of DNA by this pathway, and polymorphisms have been identified in both the RAD51 (RAD51-G135C) and XRCC3 (XRCC3-Thr241Met) genes. The object of this study was to examine whether these polymorphisms may modulate susceptibility to the development of acute myeloid leukemia (AML), a disease that is characterized by genetic instability. Experimental Design: We studied the distribution of polymorphisms in RAD51 and XRCC3 in 216 cases of de novo AML, 51 cases of therapy-related AML (t-AML), and 186 control subjects using PCR followed by restriction enzyme digestion. The polymorphic deletion of the detoxification gene glutathione S-transferase M1 (GSTM1) was also examined by PCR. Results: The risk of the development of AML was found to be significantly increased when both variant RAD51-135C and XRCC3-241Met alleles are present [odds ratio (OR), 3.77; 95% confidence interval (CI), 1.39–10.24], whereas the risk of t-AML development is even higher (OR, 8.11; 95% CI, 2.22–29.68), presumably because of the large genotoxic insult these patients receive after their exposure to radiotherapy or chemotherapy. If we further divide the AML group into patients in which the burden of DNA damage is increased, because of the deletion of the GSTM1 gene, the risk of development of AML is further increased (OR, 15.26; 95% CI, 1.83–127.27). Conclusions: These results strongly suggest that DNA double-strand breaks and their repair are important in the pathogenesis of both de novo and t-AML.

Advances in the understanding of susceptibility to treatment-related acute myeloid leukaemia

Treatment‐related acute myeloid leukaemia (t‐AML) is a devastating complication following exposure to the cytotoxic and genotoxic agents used to treat a primary malignancy. Whilst the incidence of t‐AML is rising, it still only occurs in a minority of patients who have received chemotherapy and/or radiotherapy treatment and hence it is important to identify factors that may confer susceptibility to the development of the condition. This paper reviews the literature and discusses the advances and limitations in our understanding of susceptibility factors to t‐AML. In particular, it concentrates upon genetic polymorphisms in detoxification genes and in genes belonging to the major DNA repair pathways. This review also considers more novel susceptibility factors, such as those proposed to determine stem cell number. Increased understanding of t‐AML susceptibility may enable steps to be taken to prevent its development and increase the effectiveness of treatment of the disease.

Synthetic lethal targeting of DNA double-strand break repair deficient cells by human apurinic/apyrimidinic endonuclease inhibitors

An apurinic/apyrimidinic (AP) site is an obligatory cytotoxic intermediate in DNA Base Excision Repair (BER) that is processed by human AP endonuclease 1 (APE1). APE1 is essential for BER and an emerging drug target in cancer. We have isolated novel small molecule inhibitors of APE1. In this study, we have investigated the ability of APE1 inhibitors to induce synthetic lethality (SL) in a panel of DNA double‐strand break (DSB) repair deficient and proficient cells; i) Chinese hamster (CH) cells: BRCA2 deficient (V‐C8), ATM deficient (V‐E5), wild type (V79) and BRCA2 revertant [V‐C8(Rev1)]. ii) Human cancer cells: BRCA1 deficient (MDA‐MB‐436), BRCA1 proficient (MCF‐7), BRCA2 deficient (CAPAN‐1 and HeLa SilenciX cells), BRCA2 proficient (PANC1 and control SilenciX cells). We also tested SL in CH ovary cells expressing a dominant‐negative form of APE1 (E8 cells) using ATM inhibitors and DNA‐PKcs inhibitors (DSB inhibitors). APE1 inhibitors are synthetically lethal in BRCA and ATM deficient cells. APE1 inhibition resulted in accumulation of DNA DSBs and G2/M cell cycle arrest. SL was also demonstrated in CH cells expressing a dominant‐negative form of APE1 treated with ATM or DNA‐PKcs inhibitors. We conclude that APE1 is a promising SL target in cancer.

DNA repair contributes to the drug-resistant phenotype of primary acute myeloid leukaemia cells with FLT3 internal tandem duplications and is reversed by the FLT3 inhibitor PKC412

The presence of internal tandem duplications (ITD) mutations in the FMS-like tyrosine kinase 3 (FLT3) receptor influences the risk of relapse in acute myeloid leukaemia (AML). We have investigated DNA repair in FLT3-ITD and wild-type (WT) cells. Using the comet assay, we have demonstrated that the FLT3 inhibitor PKC412 significantly inhibits repair of DNA damage in the MV4-11-FLT3-ITD cell line and FLT3-ITD patient samples but not in the HL-60-FLT3-WT cell line or FLT3-WT patient samples. Following the discovery that transcript levels of the DNA repair gene RAD51 are significantly correlated with FLT3 transcript levels in FLT3-ITD patients, we further investigated the role of RAD51 in FLT3-ITD-AML. The reduction in DNA repair in PKC412-treated FLT3-ITD cells was shown to be associated with downregulation of RAD51 mRNA and protein expression and correlates with the maintenance of phosphorylated H2AX levels, implying that PKC412 inhibits the homologous recombination double-strand break repair pathway in FLT3-ITD cells. Using FLT3-short interfering RNA (siRNA), we also demonstrated that genetic silencing of FLT3 results in RAD51 downregulation in FLT3-ITD cells but not in FLT3-WT cells. This work suggests that the use of FLT3 inhibitors such as PKC412 may reverse the drug-resistant phenotype of FLT3-ITD-AML cells by inhibiting repair of chemotherapy-induced genotoxic damage and thereby reduce the risk of disease relapse.

Ataxia Telangiectasia Mutated and Rad3 Related (ATR) Protein Kinase Inhibition Is Synthetically Lethal in XRCC1 Deficient Ovarian Cancer Cells

Introduction Ataxia telangiectasia mutated and Rad3 Related (ATR) protein kinase is a key sensor of single-stranded DNA associated with stalled replication forks and repair intermediates generated during DNA repair. XRCC1 is a critical enzyme in single strand break repair and base excision repair. XRCC1-LIG3 complex is also an important contributor to the ligation step of the nucleotide excision repair response. Methods In the current study, we investigated synthetic lethality in XRCC1 deficient and XRCC1 proficient Chinese Hamster ovary (CHO) and human ovarian cancer cells using ATR inhibitors (NU6027). In addition, we also investigated the ability of ATR inhibitors to potentiate cisplatin cytotoxicity in XRCC1 deficient and XRCC1 proficient CHO and human cancer cells. Clonogenic assays, alkaline COMET assays, γH2AX immunocytochemistry, FACS for cell cycle as well as FITC-annexin V flow cytometric analysis were performed. Results ATR inhibition is synthetically lethal in XRCC1 deficient cells as evidenced by increased cytotoxicity, accumulation of double strand DNA breaks, G2/M cell cycle arrest and increased apoptosis. Compared to cisplatin alone, combination of cisplatin and ATR inhibitor results in enhanced cytotoxicity in XRCC1 deficient cells compared to XRCC1 proficient cells. Conclusions Our data provides evidence that ATR inhibition is suitable for synthetic lethality application and cisplatin chemopotentiation in XRCC1 deficient ovarian cancer cells.

Targeting XRCC1 deficiency in breast cancer for personalized therapy

XRCC1 is a key component of DNA base excision repair, single strand break repair, and backup nonhomologous end-joining pathway. XRCC1 (X-ray repair cross-complementing gene 1) deficiency promotes genomic instability, increases cancer risk, and may have clinical application in breast cancer. We investigated XRCC1 expression in early breast cancers (n = 1,297) and validated in an independent cohort of estrogen receptor (ER)-α-negative breast cancers (n = 281). Preclinically, we evaluated XRCC1-deficient and -proficient Chinese hamster and human cancer cells for synthetic lethality application using double-strand break (DSB) repair inhibitors [KU55933 (ataxia telangectasia-mutated; ATM inhibitor) and NU7441 (DNA-PKcs inhibitor)]. In breast cancer, loss of XRCC1 (16%) was associated with high grade (P < 0.0001), loss of hormone receptors (P < 0.0001), triple-negative (P < 0.0001), and basal-like phenotypes (P = 0.001). Loss of XRCC1 was associated with a two-fold increase in risk of death (P < 0.0001) and independently with poor outcome (P < 0.0001). Preclinically, KU55933 [2-(4-Morpholinyl)-6-(1-thianthrenyl)-4H-pyran-4-one] and NU7441 [8-(4-Dibenzothienyl)-2-(4-morpholinyl)-4H-1-benzopyran-4-one] were synthetically lethal in XRCC1-deficient compared with proficient cells as evidenced by hypersensitivity to DSB repair inhibitors, accumulation of DNA DSBs, G2-M cell-cycle arrest, and induction of apoptosis. This is the first study to show that XRCC1 deficiency in breast cancer results in an aggressive phenotype and that XRCC1 deficiency could also be exploited for a novel synthetic lethality application using DSB repair inhibitors. Cancer Res; 73(5); 1621-34. ©2012 AACR.

Microsatellite instability occurs in defined subsets of patients with acute myeloblastic leukaemia

Using a sensitive fluorescent‐polymerase chain reaction technique we looked for microsatellite instability (MSI) as functional evidence of mismatch repair defects in 71 cases of acute myeloblastic leukaemia (AML). MSI was assessed at 11 loci in matched leukaemic and constitutional DNA. Nine out of 71 patients (13%) were found to have MSI. Four of these patients had therapy‐related leukaemia and the remaining five were all over the age of 60 years. There was a high incidence of adverse‐risk cytogenetics in the patients with MSI, including abnormalities of chromosomes 5 and/or 7. Of the nine cases of t‐AML included in this study, four (44%) had MSI. MSI was also seen in five of 51 cases (10%) over the age of 60 years but not in any cases under the age of 60 years with de novo AML. Using a sensitive assay, our results suggest that MSI occurs in two subgroups of patients with AML: those with t‐AML and the elderly (> 60 years), but is rare in younger patients.

Methylation of the hMLH1 promoter and its association with microsatellite instability in acute myeloid leukemia.

The hMLH1 and hMSH2 genes are involved in the DNA mismatch repair (MMR) pathway. Defects in either of these genes have been associated with genetic instability in a wide variety of malignancies. A molecular mechanism involved in aberrant MMR gene expression is the epigenetic silencing of transcription by promoter methylation. The importance of MMR promoter methylation in leukemia is presently unclear and we have therefore undertaken a detailed analysis of the promoter regions of hMLH1 and hMSH2 using the technique of bisulfite genomic sequencing. DNA from 55 patients with acute myeloid leukemia (AML) including 23 patients with therapy-related AML (t-AML) have been analyzed. Two patients with de novo AML demonstrated extensive methylation throughout the whole hMLH1 region sequenced, one of whom had previously shown widespread genetic instability, measured as microsatellite instability (MSI). However methylation of hMLH1 was not found in t-AML which has previously been associated with MSI. In addition, methylation was seen at a restricted region of the hMLH1 promoter in both AML patients and healthy controls. The significance of this methylated region of the hMLH1 promoter is uncertain, however, our results confirm that in some patients with AML extensive methylation of hMLH1, but not of hMSH2 may occur, and as is the case in solid tumors this can be associated with the presence of a defective DNA mismatch repair pathway resulting in MSI.

Analysis of factors that affect in vitro chemosensitivity of leukaemic stem and progenitor cells to gemtuzumab ozogamicin (Mylotarg) in acute myeloid leukaemia.

Relapse in acute myeloid leukaemia (AML) is considered to result from the persistence of drug-resistant leukaemic stem and progenitor cells (LSPC) within a bone marrow ‘niche’ microenvironment. Identifying novel agents that have the potential to target these LSPC in their niche microenvironment will aid in the characterization of candidate agents for post-remission chemotherapy. Using an in vitro model, we found that 48-h culture with gemtuzumab ozogamicin (Mylotarg) resulted in a 34% reduction in CD34+CD38−CD123+ LSPC number, whereas normal CD34+CD38− haemapoietic stem cells were insensitive to this agent. As there was considerable heterogeneity in LSPC response to Mylotarg treatment, various factors potentially underpinning the differential response were assessed. LSPC that overexpressed CD33 (P=0.01), which were P-glycoprotein-negative (P=0.008) and with internal tandem duplication (ITD) of the FLT3 gene (FLT3/ITD) status (P=0.006) responded better to Mylotarg treatment. LSPC from patient samples that have these combined characteristics as well as low LSPC burden showed significantly more chemosensitivity to Mylotarg compared with all other cases (P=0.002). In multivariate analysis, LSPC burden and FLT3 status were found to be predictors of LSPC chemosensitivity to Mylotarg treatment (P<0.0001). In conclusion, we have shown heterogeneity in the LSPC compartment of AML patients underpinning differential in vitro sensitivity to Mylotarg.

Flow cytometric measurement of phosphorylated STAT5 in AML: lack of specific association with FLT3 internal tandem duplications

STAT5 phosphorylation has been noted in 69-95% of AML cases by Western blotting. We used flow cytometry to measure phosphorylated STAT5 on a semi-quantitative scale. The method was validated on K562 cells, which constitutively express phosphorylated STAT5, but lose this when BCR-abl tyrosine kinase activity is blocked by STI571. Phosphorylated STAT5 was found to measure 2.22+/-0.09 relative fluorescence units (RFU) falling to 0.925+/-0.005RFU in the presence of STI571. Phosphorylated STAT5 expression was 0.99 to 2.09RFU in 28 primary AML samples. There was no logical cut-off point between positive and negative fluorescence. FLT3 internal tandem duplications, found in 11/28 samples, were not significantly associated with the level of phosphorylated STAT5 expression. We conclude that STAT5 phosphorylation can be measured sensitively by flow cytometry in AML and that its expression should not be dichotomised as present or absent.