DNA repair deficiency as a therapeutic target in cancer

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DNA repair deficiency as a therapeutic target in cancer Sarah A Martin, Christopher J Lord and Alan Ashworth Inhibitors of DNA repair proteins have been used in cancer therapy, mostly to potentiate the effects of cytotoxic agents. However, tumor cells frequently exhibit deficiencies in the signalling or repair of DNA damage. These deficiencies probably contribute to pathogenesis of the disease, but they also present an opportunity to target the tumor. Recently, inhibitors of poly(ADP-ribose) polymerase (PARP) have been shown to be highly selective for tumor cells with defects in the repair of double-strand DNA breaks (DSBs) by homologous recombination, particularly in the context of BRCA1 or BRCA2 mutation. It seems likely that other DNA repair processes can be targeted in a similar manner. These synthetic lethal approaches highlight how an understanding of DNA repair processes can be used in the development of novel cancer treatments. Address The Breakthrough Breast Cancer Research Centre, The Institute of Cancer Research, Fulham Road, London SW3 6JB, UK Corresponding author: Ashworth, Alan ([email protected])

Current Opinion in Genetics & Development 2008, 18:80–86 This review comes from a themed issue on Genetic and cellular mechanisms of oncogenesis Edited by Lisa Coussens and Tyler Jacks Available online 14th March 2008 0959-437X/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. DOI 10.1016/j.gde.2008.01.016

Introduction The genome is constantly exposed to agents, both exogenous and endogenous, that damage DNA. In the absence of DNA repair, the genome would be unable to survive the multitude of lesions that form throughout the cell cycle. Therefore, a range of molecular mechanisms has evolved that ensures that damaged DNA is effectively dealt with. In mammalian cells, more than 150 different proteins have been described that are involved in the response to DNA damage. These proteins coordinate the repair of DNA lesions and the stalling of the cell cycle to allow repair to occur [1,2]. Distinct DNA lesions are ostensibly repaired by different DNA repair pathways such as homologous recombination (HR), nonhomologous end joining (NHEJ), base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), and translesion synthesis (TLS). While these pathways apparently process different types of Current Opinion in Genetics & Development 2008, 18:80–86

DNA lesions (Figure 1), there is also considerable overlap and interaction between mechanisms. For example, the DNA lesions caused by oxidative damage are, in most cases, repaired by BER, but oxidative damage can also be repaired by NER [2]. Furthermore, single-strand DNA breaks (SSBs), if left unrepaired in the absence of BER, form double-strand breaks (DSBs), which are processed by HR [3]. The interaction between the pathways and the complexities of how these particular pathways interact is not yet fully understood. The crucial role DNA repair pathways play in limiting tumorigenesis is becoming better understood. DNA damage responses occur very early in the tumorigenic process. For example, early precursor lesions of human breast, colon, lung, and urinary bladder tumors are characterized by the phosphorylation of many DNA damage response proteins, including the kinases ATM and CHK2, the histone H2AX, and one of the master regulators of cell cycle arrest, P53 [4,5]. It has been suggested that this activation of the DNA damage response machinery is a consequence of oncogenic activity. This is proposed to lead to a hyper-proliferative phase, characterized by a significant increase in DNA replication that is kept in check by components of DNA damage response pathways [4,5]. Unsurprisingly, lossof-function mutations in a significant number of DNA damage response genes predispose to a variety of familial cancers [6]. Mutations in the HR proteins, BRCA1, and BRCA2 predispose to breast and ovarian cancer [7]. Also mutations in other double-strand break repair genes such as ATM predispose to the familial tumorigenic condition, ataxia-telangiectasia [8], and breast cancer [9] with mutations in NBS1 predisposing to Nijmegen breakage syndrome [10]. Somatic mutations in another DSB repair gene, ATR correlate with sporadic microsatellite instability (MSI)—positive stomach tumors [11]. PALB2, which encodes a BRCA2-interacting protein, has also been identified as a breast cancer susceptibility gene [12]. Mutations in multiple MMR components predispose to colorectal cancer [13]. Biallelic germ-line mutations of the BER gene MUTYH have been identified in patients with an autosomal recessive form of hereditary multiple colorectal adenoma and carcinoma [14,15], and defects in the NER pathway predispose to the rare autosomal-recessive syndromes, xeroderma pigmentosum (XP), Cockayne syndrome (CS), and trichothiodystrophy (TTD) [16]. In addition to gene mutation, epigenetic silencing of DNA repair genes has also been associated with tumorigenicity. For example, BRCA1, MLH1, MGMT, and the Werner syndrome gene, WRN have all been shown to be aberrantly methylated in human cancers [17]. In www.sciencedirect.com

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Figure 1

DNA damage, DNA repair pathways and mutation-associated disease (adapted from reference [2]). Common DNA damaging agents (top); examples of DNA lesions induced by these agents (top middle); most predominant DNA repair mechanism responsible for the removal of these lesions (lower middle) and cancer resulting from deficiency in particular DNA repair pathway (bottom). Abbreviations: cis-Pt and MMC, cisplatin and mitomycin C, respectively (both DNA-crosslinking agents); (6–4)PP and CPD, 6–4 photoproduct and cyclobutane pyrimidine dimer, respectively (both induced by UV light).

these cases, the methylation of promoter CpG islands presumably suppresses gene expression potentially leading to deficiency in specific DNA repair pathways [17]. Tumor defects in DNA damage responses may not only be causative of disease but can also be exploited therapeutically. For example, the sensitivity of tumor cells to agents that cause DNA damage, such as ionizing radiation and chemotherapeutics that bind DNA, may, partly, be explained by the inability of tumor cells to mount the appropriate DNA damage response. Furthermore, pharmacologically inhibiting DNA repair components (Table 1) may also be used to enhance chemosensitivity and radiosensitivity [18]. For example, the discovery that inhibition of DNA-PK (a crucial component of the NHEJ pathway that repairs DNA doublestrand DNA breaks) renders cells acutely sensitive to ionizing radiation and topoisomerase II inhibitors has stimulated efforts to synthesize DNA-PK inhibitors as www.sciencedirect.com

potential therapeutics. Currently, a number of potent and selective DNA-PK inhibitors are available including Vanillin, SU11752, IC87102, IC87361, NU7441, NU7026, and Salvicine [19]. Expression of MGMT in human cancers has been associated with resistance to therapies using alkylating agents. 06-(4 bromothenyl) guanine (06-BTG), a potent oral inhibitor of MGMT has been shown to have efficacy in combination with temozolomide, with Phase II trials ongoing in metastatic melanoma and colorectal cancer [20]. KU55933, a potent ATM inhibitor increases the cellular cytotoxicity of ionizing radiation and the chemotherapeutics camptothecin, etoposide, and doxorubicin [21]. ERCC1 is a key enzyme in NER, which eliminates platinum DNA adducts. Several independent studies demonstrate that high level of ERCC1 expression is correlated with low efficacy of platinum-based therapy; therefore, inhibitors of ERRC1 are being developed for use in concert with platinum therapies [22]. Current Opinion in Genetics & Development 2008, 18:80–86

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Table 1 Small molecule DNA repair inhibitors DNA repair inhibitors PARP inhibitors KU-0059436/AZD2281 (KuDos Pharma/AstraZeneca Plc) AG014699 (Pfizer) ABT-888 (Abbott) BSI-201 (BiPar) INO-1001 (Inotek)

Clinical trial Phase I and II; as monotherapy for BRCA1/2 mutant ovarian and breast cancers Phase I; treatment of solid tumors. Phase II; treatment of malignant melanoma. Both trials in combination with temolozide Phase I; as monotherapy for treatment of melanoma Phase I; as monotherapy for treatment of solid tumors Phase I; in combination with temozolomide for treatment of malignant; melanoma and glioblastoma multiforme

AG14361 3-amino-benzamide NU1025 MGMT Inhibitor 06-(4 bromothenyl) guanine

Phase II; in combination with temozolomide for treatment of metastatic melanoma and colorectal cancer

DNA-PK inhibitors Vanillin SU11752 IC87361 IC87102 NU7441 NU7026 Salvicine ATM inhibitor KU55933 (KuDos Pharma)

Targeting the specific DNA repair defects in tumors The development of DNA repair inhibitors initially focused mainly on their use as potentiators of cytotoxic agents. However, this approach can be extended to targetspecific DNA repair pathways that are deficient in particular tumors. For example, it is likely that germline BRCA1 and BRCA2 mutations strongly predispose to cancer because they cause a deficiency in a particular form of DNA repair, known as homologous recombination [3]. HR repairs DSBs and stalled replication forks by using the sister chromatid as a template to direct synthesis of DNA at the site of DNA damage (Figure 2). HR is a conservative form of DNA repair, in that the damaged DNA is restored to its original, pre-damaged sequence. However, in the absence of HR, non-conservative forms of DNA repair compensate, with the result that mutation and genomic instability occur, which probably contribute to tumorigenicity [3]. This information provides the opportunity to target the deficiency in HR that occurs as a result of BRCA1 or BRCA2 deficiency.

Targeting BRCA deficiency One of the major functions of HR is in the repair of stalled replication forks. Consistent with this, BRCA1 and BRCA2-deficient cells are highly sensitive to agents that caused replication forks to stall. In practice, agents that generate interstrand crosslinks and stall replication forks, Current Opinion in Genetics & Development 2008, 18:80–86

such as mitomycin C and the platinum salts, cisplatin and carboplatin are especially potent at killing BRCAdeficient cells [3,23]. This observation has now led to a clinical trial to assess the efficacy of carboplatin in treating tumors in BRCA1 and BRCA2 mutation carriers (http://www.geneticbreastcancertrial.usilu.net/Home.asp) [3]. There is a considerable body of evidence to suggest that single-strand DNA breaks (SSBs), if left unrepaired, also stall replication forks [3,24]. One of the sensors of SSBs, and also the enzyme that stimulates the recruitment of DNA repair proteins to repair these breaks, is poly(ADPRibose) polymerase (PARP). PARPs are a multigene family of enzymes; the best studied of these enzymes, PARP1 is a significant catalyst of poly(ADP-ribosyl)ation and plays a role in DNA base–excision repair [25]. This enzyme functions by detecting SSBs in the sugar–phosphate backbone via an N-teminal zinc finger domain. PARP then translates the occurrence of DNA breaks into a DNA repair response by covalently transfering ADPribose moieties to a variety of nuclear proteins including PARP itself that are then activated and/or recruited to the site of damage [26] (Figure 2). Using this information, we and others [27,28] demonstrated that inhibition of PARP1 by RNAi and both PARP1 and 2, by the use of potent small molecule inhibitors of PARP, was able to selectively kill BRCA1 and BRCA2-deficient cells. There were www.sciencedirect.com

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Figure 2

PARP inhibition and BRCA deficiency. A model for the selective effects of PARP inhibition on cells lacking wild-type BRCA1 or BRCA2. (a) PARP is important for the repair of DNA lesions, including DNA SSBs, by BER. When PARP activity is impaired, DNA SSBs persist. These SSBs encounter a DNA replication fork, arrest occurs resulting in fork collapse and/or a DSB. BRCA1 and BRCA2 in association with RAD51 are involved in the repair of such lesions by HR enabling restart of a collapsed replication fork. The excess number of replication fork arrests associated with loss of PARP function leads to an increase in sister chromatid recombination events and sister chromatid exchanges. (b) In the absence of functional BRCA1 or BRCA2, sister chromatid recombination and the formation of RAD51 foci are severely impaired. This leads to the utilization of error-prone RAD51-independent mechanisms such as non-homologous end joining (NHEJ) or single-strand annealing (SSA), and complex chromatid rearrangement results. Cells harboring these rearrangements may permanently arrest or undergo apoptosis. Confocal images of BRCA-deficient cells or wild-type BRCA cells are shown. RAD51 foci revealed by immunofluoresence are indicated in red. Nuclear staining with DAPI is indicated in blue.

relatively minimal effects on wild-type cells and cells that were heterozygous for either BRCA1 or BRCA2 mutations. Therefore, the therapeutic window is potentially quite large, as wild-type BRCA cells behave similarly to heterozygous BRCA mutant cells. This absence of significant cell death in heterozygous mutant cells treated with PARP inhibitor was noteworthy; normal, non-tumor cells in patients bearing BRCA mutations are heterozygous possessing one mutant allele and one wild-type allele. Tumors in these individuals predominantly show loss of heterozygosity and the wild-type allele, suggesting that PARP inhibition may have the necessary selectivity [27,28] that would be required of a therapeutic for use in patients with BRCA mutationassociated cancer. Subsequently, we have shown that PARP inhibition is also selective for cells with defects in HR components other than BRCA1 and BRCA2 [29]. This is a notable observation as there seems a subset of tumors that phenocopy BRCA-deficient tumors without actually bearing germline mutations in either of the BRCA genes, a property known as ‘BRCAness’ [30]. For example, the BRCA and Fanconi anemia (FA) pathways may be inacwww.sciencedirect.com

tivated by several mechanisms in a substantial proportion of sporadic cancers. Probable mechanisms of inactivation possibly include BRCA1 promoter methylation, methylation of the FA gene FANCF, and amplification of the gene, EMSY, the protein product of which interacts with BRCA2 [30]. The demonstration of the selectivity of PARP inhibitors for BRCA-deficient cells has now led to clinical trials of these agents in the treatment of BRCA-associated cancer (Table 1) [31]. Initial observations in patients with known BRCA-associated cancers, or those with a strong family history of disease suggestive of BRCA mutation, indicate the PARP inhibitor (KU-0059436/AZD2281) shows low toxicity, and there are suggestions of significant antitumor activity, as assessed by radiography and by measurement of tumor biomarkers [31]. This KuDOS PARP inhibitor is being tested for efficacy in BRCAassociated ovarian and breast cancer in Phase II trials [31,32]. A number of other PARP inhibitors are in development and included in clinical oncology trials [33]. The first Phase I trial of a PARP inhibitor (AG0146999) in cancer was in conjunction with temozolomide (Pfizer). Other inhibitors in development include ABT-888 Current Opinion in Genetics & Development 2008, 18:80–86

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(Abbott), BSI-201 (BiPar), and INO-1001 (Inotek), which is also being tested in combination with temozolomide for the treatment of malignant glioma [33]. Despite the promise of PARP inhibition in the treatment of BRCA-associated cancers, there are still contentious issues surrounding this approach. Some reports have suggested that the BRCA2-deficient CAPAN1 pancreatic cancer cell line and xenografts bearing BRCA mutations are insensitive to PARP inhibitors such as 3-amino-benzamide, AG14361, and NU1025 [34–36] This raised concerns about the uniformity of the specific cell killing of BRCA2-deficient cells upon PARP inhibition. However, these studies utilized older generation, less potent PARP inhibitors, in comparison to the studies using the more recent potent inhibitors AG0146999 (Pfizer GRD) and KU0058948 (KuDOS) [27,28]. McCabe et al. demonstrated that CAPAN-1 cells are indeed sensitive to the more potent PARP inhibitor, KU005894 [37], strongly suggesting that potency and specificity are crucial for the potential use of PARP inhibitors in the clinic. Furthermore, the initial in vivo modelling of PARP inhibition in the context of BRCA deficiency [27,28] has now been supported by the demonstration that PARP inhibition is also selective for Brca2-deficient intestinal crypt cells in a conditional Brca2 knockout mouse model [38]. Perhaps more debatable is the suggestion that the low toxicity of potent PARP inhibitors may enable their prophylactic use in women heterozygous for BRCA mutations [39]. Our initial in vivo modelling of PARP inhibition in mice showed that BRCA-deficient tumor outgrowth could be prevented by the use of a drug-like PARP inhibitor [27], but the increase in mammary tumorigenesis in Parp knockout mice suggests that prolonged suppression of PARP function could be detrimental [40]. This is not surprising considering that prolonged suppression of any DNA repair pathway could have a significant tumorigenic potential. Nevertheless, short periods of prophylactic use may have utility but must obviously be weighed up against the actual risk a BRCA mutation carrier has of developing malignancy [41]. Development of such prophylactic approval would include a randomized trial including existing surgical options. There are significant logistical, and ethical difficulties associated with such a trial particularly as the long-term toxicity of PARP inhibition is at present unknown.

Other approaches to the targeting of BRCAdeficient tumors As mentioned above, other approaches apart from PARP inhibition exist to target BRCA deficiency. Cells defective in BRCA1, BRCA2, or any of the Fanconi anemia proteins show a characteristic sensitivity to drugs that generate interstrand crosslinks, such as mitomycin C and the platinum analogs cisplatin and carboplatin. [3]. Similarly, the alkylating agent, Yondelis (ecteinascidin 743) can cause stalled replication forks and may also show Current Opinion in Genetics & Development 2008, 18:80–86

selectivity. This agent gives rise to DNA adducts that are recognized by NER proteins, which are recruited to and bind the DNA. Correcting these DNA lesions results in stalled replication forks at the sites of damage, which are recognized and repaired by the HR pathway. Therefore, patients exhibiting an intact NER pathway, but a deficiency in HR, may be more sensitive to treatment with this drug [42,43].

Synthetic lethality Given that loss-of-function mutations in many DNA repair proteins other than BRCA1 and BRCA2 predispose to cancer, there seems no reason why other therapeutic approaches cannot be designed using similar principles to the BRCA-PARP model. One way of viewing the BRCAPARP interaction is to see it as a synthetic lethal relationship [44]; loss or inhibition of either PARP or BRCA is still compatible with cellular viability but loss of both is lethal. With this in mind, identifying synthetic lethal partners for DNA repair proteins that are defective in tumors may aid the identification of better targets for cancer therapy [44]. Hartwell et al. originally suggested that synthetic lethality may exist between elements of the MMR pathway and proofreading DNA polymerases in yeast [44]. As an example of a synthetic lethal relationship that may be exploited therapeutically, this is particularly pertinent given the strong correlation between MMR deficiency and colorectal cancer. We have now shown that such a synthetic lethal relationship exists in human cells (Martin et al., submitted for publication). While synthetic lethal screening in human cells is still in its infancy [45], studies in yeast suggest that, for 74 genes known to be involved in maintaining genome integrity, 4956 unique pairs of synthetic lethal interactions exist, involving 875 genes [46]. The number of synthetic lethality relationships with DNA repair genes in yeast makes it statistically likely that many of these effects represent synthetic lethality between different DNA repair pathways, such that loss of one pathway is compensated for by the action of another therefore increasing the likelihood that novel lethal interactions may exist in humans that may not necessarily exist in yeast but involve compensatory DNA repair pathways. Other examples of synthetic lethality between DNA repair pathways have been demonstrated, especially in model systems such as chicken DT40 cells [47] and yeast. In principle, where synthetic lethal relationships exist and one of the components of this relationship is a tumor suppressor, the other becomes a therapeutic target. Given that many of the DNA repair deficiencies associated with tumorigenesis present as classical tumor suppressor effects [48], identifying synthetic lethal relationships with DNA repair proteins seems an attractive approach. With the development of techniques like high-throughput RNA interference screening [45], a number of synthetic lethality relationships involving DNA repair proteins are www.sciencedirect.com

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likely to be identified in human cells that can be exploited therapeutically.

Conclusions It now seems likely that an understanding of how DNA damage contributes to tumorigenesis and how this damage is repaired can be used to design novel therapeutic approaches to cancer. With the exploitation of synthetic lethal approaches it is possible that therapeutics can be identified that show strong selectivity for tumor cells, yield better response rates and lower toxicity.

damage-response genes ATR and CHK1 in sporadic stomach tumors with microsatellite instability. Cancer Res 2001, 61:7727-7730. 12. Rahman N, Seal S, Thompson D, Kelly P, Renwick A, Elliott A, Reid S, Spanova K, Barfoot R, Chagtai T et al.: PALB2, which encodes a BRCA2-interacting protein, is a breast cancer susceptibility gene. Nat Genet 2007, 39:165-167. 13. Jacob S, Praz F: DNA mismatch repair defects: role in colorectal carcinogenesis. Biochimie 2002, 84:27-47. 14. Al-Tassan N, Chmiel NH, Maynard J, Fleming N, Livingston AL, Williams GT, Hodges AK, Davies DR, David SS, Sampson JR et al.: Inherited variants of MYH associated with somatic G:C!T:A mutations in colorectal tumors. Nat Genet 2002, 30:227-232.

We thank Breakthrough Breast Cancer and Cancer Research UK for supporting our research.

15. Jones S, Emmerson P, Maynard J, Best JM, Jordan S, Williams GT, Sampson JR, Cheadle JP: Biallelic germline mutations in MYH predispose to multiple colorectal adenoma and somatic G:C!T:A mutations. Hum Mol Genet 2002, 11:2961-2967.

References and recommended reading

16. Leibeling D, Laspe P, Emmert S: Nucleotide excision repair and cancer. J Mol Histol 2006, 37:225-238.

Acknowledgement

Papers of particular interest, published within the annual period of review, have been highlighted as:  of special interest  of outstanding interest 1. 

Lord CJ, Garrett MD, Ashworth A: Targeting the double-strand DNA break repair pathway as a therapeutic strategy. Clin Cancer Res 2006, 12:4463-4468. A review on the double-strand break repair response as a therapeutic target.

2. Hoeijmakers JH: Genome maintenance mechanisms for  preventing cancer. Nature 2001, 411:366-374. A seminal review on DNA repair pathways. 3. 

Tutt AN, Lord CJ, McCabe N, Farmer H, Turner N, Martin NM, Jackson SP, Smith GC, Ashworth A: Exploiting the DNA repair defect in BRCA mutant cells in the design of new therapeutic strategies for cancer. Cold Spring Harb Symp Quant Biol 2005, 70:139-148. A paper demonstrating the concept behind a platinum trial in relation to BRCA deficiency.

4. 

Bartkova J, Horejsi Z, Koed K, Kramer A, Tort F, Zieger K, Guldberg P, Sehested M, Nesland JM, Lukas C et al.: DNA damage response as a candidate anti-cancer barrier in early human tumorigenesis. Nature 2005, 434:864-870. Two papers describing the importance of DNA repair proteins in limiting oncogene-induced transformation in the early stages of tumorigenesis.

5. 

Di Micco R, Fumagalli M, Cicalese A, Piccinin S, Gasparini P, Luise C, Schurra C, Garre M, Nuciforo PG, Bensimon A et al.: Oncogene-induced senescence is a DNA damage response triggered by DNA hyper-replication. Nature 2006, 444:638-642. See above. 6.

Spry M, Scott T, Pierce H, D’Orazio JA: DNA repair pathways and hereditary cancer susceptibility syndromes. Front Biosci 2007, 12:4191-4207.

7.

Bertwistle D, Ashworth A: BRCA1 and BRCA2. Curr Biol 2000, 10:R582.

8.

Lavin MF, Shiloh Y: Ataxia-telangiectasia: a multifaceted genetic disorder associated with defective signal transduction. Curr Opin Immunol 1996, 8:459-464.

9.

Renwick A, Thompson D, Seal S, Kelly P, Chagtai T, Ahmed M, North B, Jayatilake H, Barfoot R, Spanova K et al.: ATM mutations that cause ataxia-telangiectasia are breast cancer susceptibility alleles. Nat Genet 2006, 38:873-875.

10. Matsuura S, Kobayashi J, Tauchi H, Komatsu K: Nijmegen breakage syndrome and DNA double strand break repair by NBS1 complex. Adv Biophys 2004, 38:65-80. 11. Menoyo A, Alazzouzi H, Espin E, Armengol M, Yamamoto H, Schwartz S Jr: Somatic mutations in the DNA www.sciencedirect.com

17. Jacinto FV, Esteller M: Mutator pathways unleashed by epigenetic silencing in human cancer. Mutagenesis 2007, 22:247-253. 18. Ding J, Miao ZH, Meng LH, Geng MY: Emerging cancer therapeutic opportunities target DNA-repair systems. Trends Pharmacol Sci 2006, 27:338-344. 19. Salles B, Calsou P, Frit P, Muller C: The DNA repair complex DNA-PK, a pharmacological target in cancer chemotherapy and radiotherapy. Pathol Biol (Paris) 2006, 54:185-193. 20. Hegi ME, Diserens AC, Gorlia T, Hamou MF, de Tribolet N, Weller M, Kros JM, Hainfellner JA, Mason W, Mariani L et al.: MGMT gene silencing and benefit from temozolomide in glioblastoma. N Engl J Med 2005, 352:997-1003. 21. Hickson I, Zhao Y, Richardson CJ, Green SJ, Martin NM, Orr AI, Reaper PM, Jackson SP, Curtin NJ, Smith GC: Identification and characterization of a novel and specific inhibitor of the ataxiatelangiectasia mutated kinase ATM. Cancer Res 2004, 64:91529159. 22. Altaha R, Liang X, Yu JJ, Reed E: Excision repair cross complementing-group 1: gene expression and platinum resistance. Int J Mol Med 2004, 14:959-970. 23. Tutt A, Bertwistle D, Valentine J, Gabriel A, Swift S, Ross G, Griffin C, Thacker J, Ashworth A: Mutation in Brca2 stimulates error-prone homology-directed repair of DNA double-strand breaks occurring between repeated sequences. EMBO J 2001, 20:4704-4716. 24. Schultz N, Lopez E, Saleh-Gohari N, Helleday T: Poly(ADPribose) polymerase (PARP-1) has a controlling role in homologous recombination. Nucleic Acids Res 2003, 31:4959-4964. 25. Hassa PO, Hottiger MO: The diverse biological roles of mammalian PARPS, a small but powerful family of poly-ADP-ribose polymerases. Front Biosci 2008, 13:3046-3082. 26. Dantzer F, Ame JC, Schreiber V, Nakamura J, Menissier-de Murcia J, de Murcia G: Poly(ADP-ribose) polymerase-1 activation during DNA damage and repair. Methods Enzymol 2006, 409:493-510. 27. Farmer H, McCabe N, Lord CJ, Tutt AN, Johnson DA, Richardson TB, Santarosa M, Dillon KJ, Hickson I, Knights C et al.: Targeting the DNA repair defect in BRCA mutant cells as a therapeutic strategy. Nature 2005, 434:917-921. 28. Bryant HE, Schultz N, Thomas HD, Parker KM, Flower D, Lopez E, Kyle S, Meuth M, Curtin NJ, Helleday T: Specific killing of BRCA2-deficient tumours with inhibitors of poly(ADP-ribose) polymerase. Nature 2005, 434:913-917. 29. McCabe N, Turner NC, Lord CJ, Kluzek K, Bialkowska A, Swift S,  Giavara S, O’Connor MJ, Tutt AN, Zdzienicka MZ et al.: Deficiency in the repair of DNA damage by homologous recombination Current Opinion in Genetics & Development 2008, 18:80–86

86 Genetic and cellular mechanisms of oncogenesis

and sensitivity to poly(ADP-Ribose) polymerase inhibition. Cancer Res 2006, 66:8109-8115. A paper describing the targeting of defects in homologous recombination with PARP inhibition as a therapeutic strategy. 30. Turner N, Tutt A, Ashworth A: Hallmarks of ‘BRCAness’ in  sporadic cancers. Nat Rev Cancer 2004, 4:814-819. A review detailing the concept of ‘BRCAness’. 31. Yap TA, Boss DS, Fong PC, Roelvink M, Tutt A, Carmichael J, O’Connor MJ, Kaye SB, Schellens JH, Bono JSd: First in human phase I pharmacokinetic (PK) and pharmacodynamic (PD) study of KU-0059436 (Ku), a small molecule inhibitor of poly ADP-ribose polymerase (PARP) in cancer patients (p), including BRCA1/2 mutation carriers. J Clin Oncol 2007, 25(18S):3529. 32. Tuma RS: Combining carefully selected drug, patient genetics may lead to total tumor death. J Natl Cancer Inst 2007, 99: 1505–1506, 1509. 33. Ratnam K, Low JA: Current development of clinical inhibitors of  poly(ADP-ribose) polymerase in oncology. Clin Cancer Res 2007, 13:1383-1388. A recent review of the current PARP inhibitors. 34. De Soto JA, Deng CX: PARP-1 inhibitors: are they the longsought genetically specific drugs for BRCA1/2-associated breast cancers? Int J Med Sci 2006, 3:117-123. 35. De Soto JA, Wang X, Tominaga Y, Wang RH, Cao L, Qiao W, Li C, Xu X, Skoumbourdis AP, Prindiville SA et al.: The inhibition and treatment of breast cancer with poly(ADP-ribose) polymerase (PARP-1) inhibitors. Int J Biol Sci 2006, 2:179-185. 36. Gallmeier E, Kern SE: Absence of specific cell killing of the BRCA2-deficient human cancer cell line CAPAN1 by poly(ADP-ribose) polymerase inhibition. Cancer Biol Ther 2005, 4:703-706. 37. McCabe N, Lord CJ, Tutt AN, Martin NM, Smith GC, Ashworth A: BRCA2-deficient CAPAN-1 cells are extremely sensitive to the inhibition of poly(ADP-Ribose) polymerase: an issue of potency. Cancer Biol Ther 2005, 4:934-936. 38. Hay T, Jenkins H, Sansom OJ, Martin NM, Smith GC, Clarke AR:  Efficient deletion of normal Brca2-deficient intestinal epithelium by poly(ADP-ribose) polymerase inhibition models potential prophylactic therapy. Cancer Res 2005, 65:10145-10148.

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A paper describing the potential of PARP inhibition as a potential prophylactic therapy for BRCA deficiency. 39. Helleday T, Bryant HE, Schultz N: Poly(ADP-ribose) polymerase (PARP-1) in homologous recombination and as a target for cancer therapy. Cell Cycle 2005, 4:1176-1178. 40. Tong WM, Yang YG, Cao WH, Galendo D, Frappart L, Shen Y, Wang ZQ: Poly(ADP-ribose) polymerase-1 plays a role in suppressing mammary tumourigenesis in mice. Oncogene 2007, 26:3857-3867. 41. Roukos DH, Briasoulis E: Individualized preventive and therapeutic management of hereditary breast ovarian cancer syndrome. Nat Clin Pract Oncol 2007, 4:578-590. 42. Herrero AB, Martin-Castellanos C, Marco E, Gago F, Moreno S: Cross-talk between nucleotide excision and homologous recombination DNA repair pathways in the mechanism of action of antitumor trabectedin. Cancer Res 2006, 66:8155-8162. 43. Soares DG, Escargueil AE, Poindessous V, Sarasin A, de Gramont A, Bonatto D, Henriques JA, Larsen AK: Replication and homologous recombination repair regulate DNA doublestrand break formation by the antitumor alkylator ecteinascidin 743. Proc Natl Acad Sci U S A 2007, 104:13062-13067. 44. Hartwell LH, Szankasi P, Roberts CJ, Murray AW, Friend SH: Integrating genetic approaches into the discovery of anticancer drugs. Science 1997, 278:1064-1068. 45. Iorns E, Lord CJ, Turner N, Ashworth A: Utilizing RNA interference to enhance cancer drug discovery. Nat Rev Drug Discov 2007, 6:556-568. 46. Pan X, Ye P, Yuan DS, Wang X, Bader JS, Boeke JD: A DNA integrity network in the yeast Saccharomyces cerevisiae. Cell 2006, 124:1069-1081. 47. Yamashita YM, Okada T, Matsusaka T, Sonoda E, Zhao GY, Araki K, Tateishi S, Yamaizumi M, Takeda S: RAD18 and RAD54 cooperatively contribute to maintenance of genomic stability in vertebrate cells. EMBO J 2002, 21:5558-5566. 48. Vogelstein B, Kinzler KW: Cancer genes and the pathways they control. Nat Med 2004, 10:789-799.

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