Targeting the DNA repair defect in BRCA mutant cells as a therapeutic strategy

Links between DNA double strand break repair and breast cancer: Accumulating evidence from both familial and nonfamilial cases

DNA double strand break (DSB) repair dysfunction increases the risk of familial and sporadic breast cancer. Advances in the understanding of genetic predisposition to breast cancer have also been made by screening naturally occurring polymorphisms. These studies revealed that subtle defects in DNA repair capacity arising from low-penetrance genes, or combinations thereof, are modified by other genetically determined or environmental risk factors and correlate to breast cancer risk. Overexpression of DSB repair enzymes, absence of surveillance factors and mutation or loss of heterozygosity in any of these genes contributes to the pathogenesis of sporadic breast cancers. The results identifying DSB repair defects as a common denominator for breast cancerogenesis focus attention on functional assays in order to assess DSB repair capacity as a diagnostic tool to detect increased breast cancer risk and to enable therapeutic strategies specifically targeting the tumor.

Electron microscopy reconstructions of DNA repair complexes

Lesions in DNA compromise the integrity of the genome; their consequences can range from cell malfunction to malignant transformation. DNA damage is repaired by huge multisubunit macromolecular complexes of dynamic composition and conformation. Hence, single-particle electron microscopy has started to contribute significantly to resolving the DNA repair machinery. In many cases, the complexity of the task means that the work requires laborious purification, well-designed strategies for image processing and meticulous labelling of subunits; often, only negative staining is feasible. Recent electron microscopy studies have revealed that the association of DNA-PKcs with Ku70/Ku80 and DNA during non-homologous end joining induces conformational changes that activate the kinase and direct the formation of a synaptic complex. Also, rearrangements of Rad51 filaments and their association with Brca2 were found to regulate homologous recombination.

Triple-negative breast cancer: therapeutic options

Triple-negative breast cancers are defined by a lack of expression of oestrogen, progesterone, and ERBB2 receptors. This subgroup accounts for 15% of all types of breast cancer and for a higher percentage of breast cancer arising in African and African-American women who are premenopausal. Because of the absence of specific treatment guidelines for this subgroup, triple-negative breast cancers are managed with standard treatment; however, such treatment leaves them associated with a high rate of local and systemic relapse. Histologically, such cancers are poorly differentiated, and most fall into the basal subgroup of breast cancers, characterised by staining for basal markers (ie, cytokeratin 5/6). Analyses of microarray gene-expression profiling data show that they form a homogeneous group (or so-called cluster) in transcriptional terms and, increasingly, research studies are identifying basal cancers on the basis of exhibiting this distinctive transcriptional profile. Histologically and transcriptionally, triple-negative breast cancers have many similarities to BRCA1-associated breast cancers, which suggests that dysfunction in BRCA1 or related pathways occurs in this subset of sporadic cancers. In this review, we discuss the molecular features of triple-negative breast cancers and consider how the use of existing cytotoxic agents can be optimised for this patient group. We discuss the implications of a possible underlying BRCA1-pathway dysfunction in this subgroup in terms of treatment and we also investigate the predominant proliferative signals and the on-going research addressing the suitability of these signals as therapeutic targets.

New targets and challenges in the molecular therapeutics of cancer

The past 20 years have seen an explosion of information on the molecular changes that lead to cancer. The pathways that have been uncovered include many targets for the development of novel therapeutics. Several such drugs have been approved for clinical use and many additional drugs and targets are now being evaluated in preclinical studies. These new drugs may exhibit impressive therapeutic activity, but this is often restricted to a subpopulation of cancers with a particular molecular change. Moreover, toxicity or even antagonism may result from off-target effects of the drugs. Accordingly, it will be critical to stratify patients for treatment based on the propensity of their tumours to respond. In addition, defining the appropriate dose of targeted agents to administer is challenging; early clinical trial designs must include assays to define the effective biological dose, in addition to more traditional end-points such as the maximum tolerable dose. These and many other challenges exist in the preclinical and clinical development of these drugs.

Minding the gap: the underground functions of BRCA1 and BRCA2 at stalled replication forks

The hereditary breast and ovarian cancer predisposition genes, BRCA1 and BRCA2, participate in the repair of DNA double strand breaks by homologous recombination. Circumstantial evidence implicates these genes in recombinational responses to DNA polymerase stalling during the S phase of the cell cycle. These responses play a key role in preventing genomic instability and cancer. Here, we review the current literature implicating the BRCA pathway in HR at stalled replication forks and explore the hypothesis that BRCA1 and BRCA2 participate in the recombinational resolution of single stranded DNA lesions termed "daughter strand gaps", generated during replication across a damaged DNA template.

Methylating agents and DNA repair responses: Methylated bases and sources of strand breaks

The chemical methylating agents methylmethane sulfonate (MMS) and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) have been used for decades as classical DNA damaging agents. These agents have been utilized to uncover and explore pathways of DNA repair, DNA damage response, and mutagenesis. MMS and MNNG modify DNA by adding methyl groups to a number of nucleophilic sites on the DNA bases, although MNNG produces a greater percentage of O-methyl adducts. There has been substantial progress elucidating direct reversal proteins that remove methyl groups and base excision repair (BER), which removes and replaces methylated bases. Direct reversal proteins and BER, thus, counteract the toxic, mutagenic, and clastogenic effects of methylating agents. Despite recent progress, the complexity of DNA damage responses to methylating agents is still being discovered. In particular, there is growing understanding of pathways such as homologous recombination, lesion bypass, and mismatch repair that react when the response of direct reversal proteins and BER is insufficient. Furthermore, the importance of proper balance within the steps in BER has been uncovered with the knowledge that DNA structural intermediates during BER are deleterious. A number of issues complicate the elucidation of the downstream responses when direct reversal is insufficient or BER is imbalanced. These include inter-species differences, cell-type-specific differences within mammals and between cancer cell lines, and the type of methyl damage or BER intermediate encountered. MMS also carries a misleading reputation of being a radiomimetic, that is, capable of directly producing strand breaks. This review focuses on the DNA methyl damage caused by MMS and MNNG for each site of potential methylation to summarize what is known about the repair of such damage and the downstream responses and consequences if the damage is not repaired.

Why we will need to learn new skills to control cancer

As a society and as specialists involved in the diagnosis and management of cancer, we must begin to find new cost-effective ways to provide equitable access to the innovative, effective and expensive drugs that may begin to make cancer a chronic rather than rapidly lethal disease. Drugs such as trastuzumab and gefitinib are safer 'targeted therapies' that only attract government subsidies after the pathologist identifies the target present in a minor subset of patients. Nonetheless, funding for pathological identification of these targets remains a challenge. To illustrate, gefitinib may produce 'Lazarus' responses and prolonged survival among patients with epidermal growth factor receptor mutation-positive non-small-cell lung cancer. Many such examples will enter the clinical domain in the coming years. As we enter this era of personalized medicine, we argue that the use of expensive targeted therapies should be limited to pathologically proven indications because truly effective drugs are best applied to those individuals who would most benefit. It follows that medical oncologists should be trained properly to use targeted therapies. Then a new generation of oncologists would be empowered to participate in the iterative cycles of research between bench and bedside that are necessary for optimal use of biotherapies and their integration into multimodality cancer treatment programmes. We propose that cancer pathology be made available as a training option in the postgraduate education of medical oncologists. Oncologists and pathologists may jointly administer and mutually accredit the training module, which may also contribute towards the award of a higher degree.

Haploinsufficiency of Parp1 accelerates Brca1-associated centrosome amplification, telomere shortening, genetic instability, apoptosis, and embryonic lethality

The breast tumor associated gene-1 (BRCA1) and poly(ADP-ribose) polymerase-1 (PARP1) are both involved in DNA-damage response and DNA-damage repair. Recent investigations have suggested that inhibition of PARP1 represents a promising chemopreventive/therapeutic approach for specifically treating BRCA1- and BRCA2-associated breast cancer. However, studies in mouse models reveal that Parp1-null mutation results in genetic instability and mammary tumor formation, casting significant doubt on the safety of PARP1 inhibition as a therapy for the breast cancer. To study the genetic interactions between Brca1 and Parp1, we interbred mice carrying a heterozygous deletion of full-length Brca1 (Brca1(+/Delta11)) with Parp1-null mice. We show that Brca1(Delta11/Delta11);Parp1(-/-) embryos die before embryonic (E) day 6.5, whereas Brca1(Delta11/Delta11) embryos die after E12.5, indicating that absence of Parp1 dramatically accelerates lethality caused by Brca1 deficiency. Surprisingly, haploinsufficiency of Parp1 in Brca1(Delta11/Delta11) embryos induces a severe chromosome aberrations, centrosome amplification, and telomere dysfunction, leading to apoptosis and accelerated embryonic lethality. Notably, telomere shortening in Brca1(Delta11/Delta11);Parp1(+/-) MEFs was correlated with decreased expression of Ku70, which plays an important role in telomere maintenance. Thus, haploid loss of Parp1 is sufficient to induce lethality of Brca1-deficient cells, suggesting that partial inhibition of PARP1 may represent a practical chemopreventive/therapeutic approach for BRCA1-associated breast cancer.

ATR signaling mediates an S-phase checkpoint after inhibition of poly(ADP-ribose) polymerase activity

Human fibroblasts, capable of expressing a kinase-dead form of ATR (ATRkd), can be sensitized to the cytotoxic effects of methyl methanesulfonate (MMS) by the PARP inhibitor 4-amino-1,8-naphthalimide (4-AN). The combination of MMS+4-AN results in accumulation of cells in S-phase of the cell cycle and activation of Chk1. Inhibition of ATR activity by expression of ATRkd suppresses the S-phase accumulation and partially reverses the Chk1 phosphorylation. The results confirm involvement of an ATR-mediated damage response pathway in the MMS+4-AN-induced S-phase cell cycle checkpoint in human fibroblasts. Consistent with this hypothesis, the inhibitors caffeine and UCN-01 also abrogate the ATR- and Chk1-mediated delay in progression through S-phase. In the absence of ATR-mediated signaling, MMS+4-AN exposure results in a G(2)/M arrest, rather than an S-phase checkpoint. Thus, whereas ATR mediates the S-phase response, it is not critical for arrest of cells in G(2)/M.

BRCA1 , a Potential Predictive Biomarker in the Treatment of Breast Cancer

To date, estrogen receptor, progestogen receptor, and HER2/neu represent molecular biomarkers currently used in routine clinical practice to aid treatment decisions. Over the last few years, a large body of preclinical and retrospective clinical data has accumulated that suggests that BRCA1 mutation functions as a novel predictive marker of response to chemotherapy. This article reviews the role of BRCA1 as a predictive marker of chemotherapy response in breast cancer and examines the link between BRCA1 deficiency and the basal-like phenotype. Search strategy. Data for this article were identified through MEDLINE and PubMed searches for published reports using the terms BRCA1, breast cancer, basal-like, chemotherapy, prognosis, and predictive markers. In some cases, due to the restriction of space, readers are referred to review articles to allow further reading. Only articles published in English were included.

Gene-expression analysis and the basal-like breast cancer subtype

Analysis of the patterns of gene expression in breast cancer suggests that it is not a single entity, but is comprised of several biologically distinct subtypes with characteristic molecular profiles. These molecular profiles confirm the clinical impression that estrogen receptor (ER)-negative differs from ER-positive, and expands our understanding by identifying breast cancer subtypes, including the basal-like and human epidermal growth factor receptor (HER)2/ER subtypes within the ER-negative subset, and the luminal A and B subtypes within ER-positive disease. The basal-like subtype is characterized by the low expression levels of the ER-related and the HER2-related group of genes, and therefore is often ‘triple negative’ on clinical assays for these proteins. This review discusses the molecular profiles of breast cancer with a focus on the clinical characteristics of, and treatment options for, the basal-like subtype.

p53, BRCA1 and breast Cancer chemoresistance

The tumor suppressor genes p53 and BRCA1 are involved in hereditary as well as sporadic breast cancer development and therapeutic responses. While p53 mutations contribute to resistance to chemo- and radiotherapy, BRCA1 dysfunction leads to enhanced sensitivity to DNA damaging therapeutic agents. The biochemical pathways used by p53 and BRCA1 for signaling tumor suppression involve some cross-talk including repression of BRCA1 transcription by p53 and altered selectivity of p53-dependent gene activation by BRCA1. In this chapter we review clinical and preclinical data implicating p53 and BRCA1 in breast cancer chemosensitivity. We discuss the known signaling pathways downstream of p53 or BRCA1 that contribute to their modulation of therapeutic responses, and we discuss the implications of p53 or BRCA1 mutation in therapeutic design.

New approaches to molecular cancer therapeutics

Cancer drug development is leading the way in exploiting molecular biological and genetic information to develop "personalized" medicine. The new paradigm is to develop agents that target the precise molecular pathology driving the progression of individual cancers. Drug developers have benefited from decades of academic cancer research and from investment in genomics, genetics and automation; their success is exemplified by high-profile drugs such as Herceptin (trastuzumab), Gleevec (imatinib), Tarceva (erlotinib) and Avastin (bevacizumab). However, only 5% of cancer drugs entering clinical trials reach marketing approval. Cancer remains a high unmet medical need, and many potential cancer targets remain undrugged. In this review we assess the status of the discovery and development of small-molecule cancer therapeutics. We show how chemical biology approaches offer techniques for interconnecting elements of the traditional linear progression from gene to drug, thereby providing a basis for increasing speed and success in cancer drug discovery.

Poly(ADP-ribose) polymerase-1 plays a role in suppressing mammary tumourigenesis in mice

The DNA strand break-binding molecule, poly(ADP-ribose) polymerase-1 (PARP-1), plays a role in DNA repair, chromosomal stability, transcription and cell death. Accumulating evidence suggests that dysfunction of PARP-1 contributes to tumorigenesis. Here, we report that PARP-1 deficiency causes mammary carcinoma formation in female mice, and that the introduction of Trp53 mutations accelerates the onset and shortens the latency of mammary tumorigenesis. We show that PARP-1 deficiency results in chromosomal aneuploidy and centrosome amplification, which are substantiated by the inactivation of Trp53 in primary mammary epithelial (PME) cells. In addition, PARP-1 deficiency compromises p53 activation and impairs BRCA1 recruitment to the sites of DNA damage in PME cells. PARP-1 complementation partly rescues the defective DNA damage response mediated by p53 and BRCA1. The present study thus identifies a role of PARP-1 in suppressing mammary tumorigenesis in vivo and suggests that dysfunction of PARP-1 may be a risk factor for breast cancer in humans.

Clinical management of BRCA1 and BRCA2 mutation carriers

The cancer susceptibility genes BRCA1 and BRCA2 appear to be responsible for virtually all hereditary breast ovarian families, and a smaller subset of hereditary site-specific breast cancer families. Fortunately, effective strategies have been developed to reduce the risk for the development of breast and ovarian cancer in women with BRCA1/2 mutations, making genetic testing for these mutations an important part of the management at women with a strong family history of these diseases. Here, we review the current evidence for risk reduction strategies and outline future research directions.

PARP-1 and Ku compete for repair of DNA double strand breaks by distinct NHEJ pathways

Poly(ADP-ribose)polymerase 1 (PARP-1) recognizes DNA strand interruptions in vivo and triggers its own modification as well as that of other proteins by the sequential addition of ADP-ribose to form polymers. This modification causes a release of PARP-1 from DNA ends and initiates a variety of responses including DNA repair. While PARP-1 has been firmly implicated in base excision and single strand break repair, its role in the repair of DNA double strand breaks (DSBs) remains unclear. Here, we show that PARP-1, probably together with DNA ligase III, operates in an alternative pathway of non-homologous end joining (NHEJ) that functions as backup to the classical pathway of NHEJ that utilizes DNA-PKcs, Ku, DNA ligase IV, XRCC4, XLF/Cernunnos and Artemis. PARP-1 binds to DNA ends in direct competition with Ku. However, in irradiated cells the higher affinity of Ku for DSBs and an excessive number of other forms of competing DNA lesions limit its contribution to DSB repair. When essential components of the classical pathway of NHEJ are absent, PARP-1 is recruited for DSB repair, particularly in the absence of Ku and non-DSB lesions. This form of DSB repair is sensitive to PARP-1 inhibitors. The results define the function of PARP-1 in DSB repair and characterize a candidate pathway responsible for joining errors causing genomic instability and cancer.

Deconstructing the molecular portrait of basal-like breast cancer

Gene-expression profiling has revealed several molecular subtypes of breast cancer, which differ in their pathobiology and clinical outcomes. Basal-like tumors are a newly recognized subtype of breast cancer, which express genes that are characteristic of basal epithelial cells, such as the basal cytokeratins, and are associated with poor relapse-free and overall survival. However, the genetic and epigenetic alterations that are responsible for the biologically aggressive phenotype of these estrogen receptor-negative and HER2/ErbB2-negative tumors are not well understood, thereby hindering efforts to develop targeted therapies. Here, we focus on new insights into the molecular pathogenesis of basal-like breast cancer and explore how these discoveries might impact the treatment of these poor-prognosis tumors.

Novel poly(ADP-ribose) polymerase-1 inhibitors

Synthesis and activity of a series of 4-thiazol-yl substituted analogs of novel pyrrolocarbazole 1 as poly(ADP-ribose) polymerase-1 (PARP-1) inhibitors have been disclosed.

Poly(ADP-ribose)polymerase inhibition decreases angiogenesis

Inhibitors of poly(ADP-ribose)polymerase (PARP), a nuclear enzyme involved in regulating cell death and cellular responses to DNA repair, show considerable promise in the treatment of cancer both in monotherapy as well as in combination with chemotherapeutic agents and radiation. We have recently demonstrated that PARP inhibition with 3-aminobenzamide or PJ-34 reduced vascular endothelial growth factor (VEGF)-induced proliferation, migration, and tube formation of human umbilical vein endothelial cells (HUVECs) in vitro. Here, we show dose-dependent reduction of VEGF- and basic fibroblast growth factor (bFGF)-induced proliferation, migration, and tube formation of HUVECs in vitro by two potent PARP inhibitors 5-aminoisoquinolinone-hydrochloride (5-AIQ) and 1,5-isoquinolinediol (IQD). Moreover, PARP inhibitors prevented the sprouting of rat aortic ring explants in an ex vivo assay of angiogenesis. These results establish the novel concept that PARP inhibitors have antiangiogenic effects, which may have tremendous clinical implications for the treatment of various cancers, tumor metastases, and certain retinopathies.

BRCA1 at the crossroad of multiple cellular pathways: approaches for therapeutic interventions

Approximately 10% of the cases of breast cancer and invasive ovarian cancer are hereditary, occurring predominantly in women with germ-line mutations in the BRCA1 or BRCA2 genes. Low expression of these genes in sporadic tumors extends their significance to sporadic breast and ovarian cancers as well. For over a decade since its identification, extensive research has been directed toward understanding the function of the breast and ovarian tumor suppressor gene BRCA1. The long-term goal has been to identify the biochemical pathways reliant on BRCA1 that can be exploited for developing targeted therapies and benefit mutation carriers. To date, no one specific role has been identified, but rather it is clear that BRCA1 has significant roles in multiple fundamental cellular processes, including control of gene expression, chromatin remodeling, DNA repair, cell cycle checkpoint control, and ubiquitination, and overall is important for maintenance of genomic stability. Major findings and potential BRCA1-dependent therapies will be discussed.

BRCA1 dysfunction in sporadic basal-like breast cancer

Basal-like breast cancers form a distinct subtype of breast cancer characterized by the expression of markers expressed in normal basal/myoepithelial cells. Breast cancers arising in carriers of germline BRCA1 mutations are predominately of basal-like type, suggesting that BRCA1 dysfunction may play a role in the pathogenesis of sporadic basal-like cancers. We analysed 37 sporadic breast cancers expressing the basal marker cytokeratin 5/6, and age- and grade-matched controls, for downregulation of BRCA1. Although BRCA1 promoter methylation was no more common in basal-like cancers (basal 14% vs controls 11%, P=0.72), BRCA1 messenger RNA expression was twofold lower in basal-like breast cancers compared to matched controls (P=0.008). ID4, a negative regulator of BRCA1, was expressed at 9.1-fold higher levels in basal-like breast cancer (P<0.0001), suggesting a potential mechanism of BRCA1 downregulation. BRCA1 downregulation correlated with the presence of multiple basal markers, revealing heterogeneity in the basal-like phenotype. Finally, we found that 63% of metaplastic breast cancers, a rare type of basal-like cancers, had BRCA1 methylation, in comparison to 12% of controls (P<0.0001). The high prevalence of BRCA1 dysfunction identified in this study could be exploited in the development of novel approaches to targeted treatment of basal-like breast cancer.

Topoisomerase I inhibitors: camptothecins and beyond

Nuclear DNA topoisomerase I (TOP1) is an essential human enzyme. It is the only known target of the alkaloid camptothecin, from which the potent anticancer agents irinotecan and topotecan are derived. As camptothecins bind at the interface of the TOP1-DNA complex, they represent a paradigm for interfacial inhibitors that reversibly trap macromolecular complexes. Several camptothecin and non-camptothecin derivatives are being developed to further increase anti-tumour activity and reduce side effects. The mechanisms and molecular determinants of tumour response to TOP1 inhibitors are reviewed, and rational combinations of TOP1 inhibitors with other drugs are considered based on current knowledge of repair and checkpoint pathways that are associated with TOP1-mediated DNA damage.

Chemotherapeutic approaches for targeting cell death pathways

For several decades, apoptosis has taken center stage as the principal mechanism of programmed cell death in mammalian tissues. It also has been increasingly noted that conventional chemotherapeutic agents not only elicit apoptosis but other forms of nonapoptotic death such as necrosis, autophagy, mitotic catastrophe, and senescence. This review presents background on the signaling pathways involved in the different cell death outcomes. A re-examination of what we know about chemotherapy-induced death is vitally important in light of new understanding of nonapoptotic cell death signaling pathways. If we can precisely activate or inhibit molecules that mediate the diversity of cell death outcomes, perhaps we can succeed in more effective and less toxic chemotherapeutic regimens.

Small interfering RNA screens reveal enhanced cisplatin cytotoxicity in tumor cells having both BRCA network and TP53 disruptions

RNA interference technology allows the systematic genetic analysis of the molecular alterations in cancer cells and how these alterations affect response to therapies. Here we used small interfering RNA (siRNA) screens to identify genes that enhance the cytotoxicity (enhancers) of established anticancer chemotherapeutics. Hits identified in drug enhancer screens of cisplatin, gemcitabine, and paclitaxel were largely unique to the drug being tested and could be linked to the drug's mechanism of action. Hits identified by screening of a genome-scale siRNA library for cisplatin enhancers in TP53-deficient HeLa cells were significantly enriched for genes with annotated functions in DNA damage repair as well as poorly characterized genes likely having novel functions in this process. We followed up on a subset of the hits from the cisplatin enhancer screen and validated a number of enhancers whose products interact with BRCA1 and/or BRCA2. TP53(+/-) matched-pair cell lines were used to determine if knockdown of BRCA1, BRCA2, or validated hits that associate with BRCA1 and BRCA2 selectively enhances cisplatin cytotoxicity in TP53-deficient cells. Silencing of BRCA1, BRCA2, or BRCA1/2-associated genes enhanced cisplatin cytotoxicity approximately 4- to 7-fold more in TP53-deficient cells than in matched TP53 wild-type cells. Thus, tumor cells having disruptions in BRCA1/2 network genes and TP53 together are more sensitive to cisplatin than cells with either disruption alone.

Repression of RAD51 gene expression by E2F4/p130 complexes in hypoxia

We and others have shown that the dysregulation of DNA repair pathways can contribute to the phenomenon of hypoxia-induced genetic instability within the tumor microenvironment. Several studies have revealed that the recombinational repair genes, RAD51 and BRCA1, and the DNA mismatch repair genes, MLH1 and MSH2, are decreased in expression in response to hypoxic stress, prompting interest in elucidating the mechanistic basis for these responses. Here we report that the downregulation of RAD51 by hypoxia is specifically mediated by repressive E2F4/p130 complexes that bind to a single E2F site in the proximal promoter of the gene. Intriguingly, this E2F site is conserved in the promoter of the BRCA1 gene, which is also regulated by a similar mechanism in hypoxia. Mechanistically, we have found that hypoxia induces substantial p130 dephosphorylation and nuclear accumulation, leading to the formation of E2F4/p130 complexes and increased occupancy of E2F4 and p130 at the RAD51 and BRCA1 promoters. These findings reveal a coordinated transcriptional program mediated by the formation of repressive E2F4/p130 complexes that represents an integral response to hypoxic stress. In addition, this co-regulation of key factors within the homology-dependent DNA repair pathway provides a further basis for understanding genetic instability in tumors and may guide the design of new therapeutic strategies for cancer.

Fanconi anaemia genes and susceptibility to cancer

Fanconi anaemia (FA) is a rare recessive disorder associated with chromosomal fragility, aplastic anaemia, congenital abnormalities and a high risk of cancer, including acute myeloid leukaemia and squamous cell carcinomas. The identification of 11 different FA genes has revealed a complex web of interacting proteins that are involved in the recognition or repair of DNA interstrand crosslinks and perhaps other forms of DNA damage. Bi-allelic mutations in BRCA2 are associated with a rare and highly cancer-prone form of FA, and the DNA helicase BRIP1 (formerly BACH1) is mutated in FA group J. There is little convincing evidence that FA heterozygotes are at increased risk of cancer, but larger studies are needed to address the possibility of modest risk effects. Somatic inactivation of the FA pathway by mutation or epigenetic silencing has been observed in several different types of sporadic cancer, and this may have important implications for targeted chemotherapy. Inhibition of this pathway represents a possible route to sensitization of tumours to DNA crosslinking drugs such as cisplatin.

Mouse models of BRCA1 and BRCA2 deficiency: past lessons, current understanding and future prospects

Germline mutations in BRCA1 and BRCA2 are responsible for a large proportion of hereditary breast and ovarian cancers. Soon after the identification of both genes in the mid-1990s, investigators set out to develop mouse models for the associated disease. Whereas conventional Brca1 and Brca2 mouse mutants did not reveal a strong phenotype in a heterozygous setting, most homozygous mutations caused embryonic lethality. Consequently, development of mouse models for BRCA-associated tumorigenesis required the generation of tissue-specific conditional knockout animals. In this review, we give an overview of the conventional and the conditional mouse models of BRCA1 and BRCA2 deficiency generated over the last decade, as well as the contribution of these models to our understanding of the biological and molecular functions of BRCA1 and BRCA2. The most advanced mouse models for BRCA1- and BRCA2-associated tumorigenesis mimic human disease to the extent that they can be used in studies addressing clinically relevant questions. These models will help to resolve yet unanswered questions and to translate our increasing knowledge of BRCA1 and BRCA2 biology into clinical practice.

The roles of BRCA1 and BRCA2 and associated proteins in the maintenance of genomic stability

The BRCA1 and BRCA2 proteins are important in maintaining genomic stability by promoting efficient and precise repair of double-strand breaks. The main role of BRCA2 appears to involve regulating the function of RAD51 in the repair by homologous recombination. BRCA1 has a broader role upstream of BRCA2, participating in various cellular processes in response to DNA damage. The DNA repair defect associated with mutations in BRCA1 or BRCA2 could be exploited to develop new targeted therapeutic approaches for cancer occurring in mutation carriers.

Targeting the ubiquitin-proteasome pathway in breast cancer therapy

The ubiquitin-proteasome pathway (UPP) is the major eukaryotic mechanism for regulated intracellular proteolysis. Targeting this pathway with proteasome inhibitors has been validated as a rational strategy against hematologic malignancies, but for most solid tumor populations, including breast cancer, such agents have not shown encouraging activity. However, there is an increasing body of evidence showing that UPP dysregulation plays an important role in mammary tumorigenesis. Moreover, modulation of ubiquitin-proteasome function is emerging as a rational strategy to enhance chemosensitivity and overcome chemoresistance. Taken together, these facts suggest that we are only beginning to appreciate the relevance of this pathway for the current and future therapy of patients with breast cancer. This review provides an overview of the biology of the UPP, its role in the malignant process, the current state of knowledge regarding clinical heat shock protein and proteasome inhibition, and some likely future directions that may enhance our ability to exploit this pathway therapeutically.

Pharmacological inhibition of poly(ADP-ribose) polymerase inhibits angiogenesis

Poly(ADP-ribose) polymerase (PARP) is a nuclear enzyme which plays an important role in regulating cell death and cellular responses to DNA repair. Pharmacological inhibitors of PARP are being considered as treatment for cancer both in monotherapy as well as in combination with chemotherapeutic agents and radiation, and were also reported to be protective against untoward effects exerted by certain anticancer drugs. Here we show that pharmacological inhibition of PARP with 3-aminobenzamide or PJ-34 dose-dependently reduces VEGF-induced proliferation, migration, and tube formation of human umbilical vein endothelial cells in vitro. These results suggest that treatment with PARP inhibitors may exert additional benefits in various cancers and retinopathies by decreasing angiogenesis.

Exploiting the DNA repair defect in BRCA mutant cells in the design of new therapeutic strategies for cancer

Individuals harboring germ-line mutations in the BRCA1 or BRCA2 genes are at highly elevated risk of a variety of cancers. Ten years of research has revealed roles for BRCA1 and BRCA2 in a wide variety of cellular processes. However, it seems likely that the function of these proteins in DNA repair is critically important in maintaining genome stability. Despite this increasing knowledge of the defects present in BRCA-deficient cells, BRCA mutation carriers developing cancer are still treated similarly to sporadic cases. Here we describe our efforts, based on understanding the DNA repair defects in BRCAdeficient cells, to define the optimal existing treatment for cancers arising in BRCA mutation carriers and, additionally, the development of novel therapeutic approaches. Finally, we discuss how therapies developed to treat BRCA mutant tumors might be applied to some sporadic cancers sharing similar specific defects in DNA repair.

Strategies for optimizing combinations of molecularly targeted anticancer agents

The rapid emergence of hundreds of new agents that modulate an ever-growing list of cancer-specific molecular targets offers tremendous hope for cancer patients. However, evaluating targeted agents individually, in combination with standard treatments, and in combination with other targeted agents presents significant development challenges. Because the number of possible drug combinations is essentially limitless, a strategy for determining the most promising combinations and prioritizing their evaluation is crucial. Here, we consider the crucial elements of a development strategy for targeted-agent combinations. Issues that pose challenges to the rational preclinical and clinical evaluation of such combinations will be described, and possible approaches to overcoming these challenges will be discussed.

Chinese hamster cell mutant, V-C8, a model for analysis of Brca2 function

The previously described Chinese hamster cell mutant V-C8 that is defective in Brca2 shows a very complex phenotype, including increased sensitivity towards a wide variety of DNA damaging agents, chromosomal instability, abnormal centrosomes and impaired formation of Rad51 foci in response to DNA damage. Here, we demonstrate that V-C8 cells display biallelic nonsense mutations in Brca2, one in exon 15 and the other in exon 16, both resulting in truncated Brca2 proteins. We generated several independent mitomycin C (MMC)-resistant clones from V-C8 cells that had acquired an additional mutation leading to the restoration of the open reading frame of one of the Brca2 alleles. In two of these revertants, V-C8-Rev 1 and V-C8-Rev 6, the reversions lead to the wild-type Brca2 sequence. The V-C8 revertants did not gain the entire wild-type phenotype and still show a 2.5-fold increased sensitivity to mitomycin C (MMC), higher levels of spontaneous and MMC-induced chromosomal aberrations, as well as abnormal centrosomes when compared to wild-type cells. Our results suggest that Brca2 heterozygosity in hamster cells primarily gives rise to sensitivity to DNA cross-linking agents, especially chromosomal instability, a feature that might also be displayed in BRCA2 heterozygous mutation carriers.

Poly(ADP-ribose): novel functions for an old molecule

The addition to proteins of the negatively charged polymer of ADP-ribose (PAR), which is synthesized by PAR polymerases (PARPs) from NAD(+), is a unique post-translational modification. It regulates not only cell survival and cell-death programmes, but also an increasing number of other biological functions with which novel members of the PARP family have been associated. These functions include transcriptional regulation, telomere cohesion and mitotic spindle formation during cell division, intracellular trafficking and energy metabolism.

DNA repair pathways in clinical practice: lessons from pediatric cancer susceptibility syndromes

Human cancers exhibit genomic instability and an increased mutation rate due to underlying defects in DNA repair. Cancer cells are often defective in one of six major DNA repair pathways, namely: mismatch repair, base excision repair, nucleotide excision repair, homologous recombination, nonhomologous endjoining and translesion synthesis. The specific DNA repair pathway affected is predictive of the kinds of mutations, the tumor drug sensitivity, and the treatment outcome. The study of rare inherited DNA repair disorders, such as Fanconi anemia, has yielded new insights to drug sensitivity and treatment of sporadic cancers, such as breast or ovarian epithelial tumors, in the general population. The Fanconi anemia pathway is an example of how DNA repair pathways can be deregulated in cancer cells and how biomarkers of the integrity of these pathways could be useful as a guide to cancer management and may be used in the development of novel therapeutic agents.

BRCA1 and BRCA2: chemosensitivity, treatment outcomes and prognosis

BRCA1 and BRCA2 are important breast and ovarian cancer susceptibility genes, and mutations in these two genes confer lifetime risks of breast cancer of up to 80% and ovarian cancer risks of up to 40%. Clinico-pathological studies have identified features that are specific to BRCA1-related breast cancer, but this has been more difficult for BRCA2-related breast cancer. Ovarian cancers due to BRCA1 or BRCA2 mutations cannot usually be distinguished from their non-hereditary counterparts on morphological grounds, but micro-array data suggest that differences do exist. Prognostic studies have shown that breast cancer in a BRCA1 mutation carrier is likely to have a similar, or worse, outcome than that occurring in a BRCA2- or non-carrier of the same age. By contrast, most studies indicate that women developing a BRCA1/2-related ovarian cancer have an improved survival compared with non-carriers, particularly if they receive platinum-based therapy. In support of this, in vitro chemo-sensitivity studies have found that human cells lacking BRCA1 may be particularly sensitive to cisplatinum and to other drugs that cause double-strand breaks in DNA. Nevertheless, in breast cancer, little is known regarding clinically important differences in response to chemotherapy between BRCA1/2 mutation carriers and non-carriers, and between different chemotherapeutic regimens within existing series of BRCA1/2 mutation carriers. There are no published prospective studies. It is hoped that, in the near future, randomised controlled trials will be started with the aim of answering these important clinical questions.

Deficiency in the Repair of DNA Damage by Homologous Recombination and Sensitivity to Poly(ADP-Ribose) Polymerase Inhibition

Deficiency in either of the breast cancer susceptibility proteins BRCA1 or BRCA2 induces profound cellular sensitivity to the inhibition of poly(ADP-ribose) polymerase (PARP) activity. We hypothesized that the critical role of BRCA1 and BRCA2 in the repair of double-strand breaks by homologous recombination (HR) was the underlying reason for this sensitivity. Here, we examine the effects of deficiency of several proteins involved in HR on sensitivity to PARP inhibition. We show that deficiency of RAD51, RAD54, DSS1, RPA1, NBS1, ATR, ATM, CHK1, CHK2, FANCD2, FANCA, or FANCC induces such sensitivity. This suggests that BRCA-deficient cells are, at least in part, sensitive to PARP inhibition because of HR deficiency. These results indicate that PARP inhibition might be a useful therapeutic strategy not only for the treatment of BRCA mutation-associated tumors but also for the treatment of a wider range of tumors bearing a variety of deficiencies in the HR pathway or displaying properties of 'BRCAness.'

Targeting the Double-Strand DNA Break Repair Pathway as a Therapeutic Strategy: Fig. 1

DNA repair pathways are crucial for the maintenance of genome integrity. The pathway that repairs DNA double-strand breaks (DSB) has components involved in both signaling and repairing DNA damage. Impairing DSB repair using specific inhibitors of signaling or repair might, in principle, sensitize tumor cells to particular DNA-damaging agents. Moreover, the existence of specific defects in DNA repair pathways in tumors provides the rationale for the use of "synthetic lethal" approaches targeting this cellular "Achilles' heel." Here, we discuss the mechanisms involved in DSB repair and detail potential therapeutic approaches based on targeting this pathway.

Inhibition of poly(ADP-ribose) polymerase modulates tumor-related gene expression, including hypoxia-inducible factor-1 activation, during skin carcinogenesis

Poly(ADP-ribose) polymerase (PARP)-1, an enzyme that catalyzes the attachment of ADP ribose to target proteins, acts as a component of enhancer/promoter regulatory complexes. In the present study, we show that pharmacologic inhibition of PARP-1 with 3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinolinone (DPQ) results in a strong delay in tumor formation and in a dramatic reduction in tumor size and multiplicity during 7,12-dimethylbenz(a)anthracene plus 12-O-tetradecanoylphorbol-13-acetate-induced skin carcinogenesis. This observation was parallel with a reduction in the skin inflammatory infiltrate in DPQ-treated mice and tumor vasculogenesis. Inhibition of PARP also affected activator protein-1 (AP-1) activation but not nuclear factor-kappaB (NF-kappaB). Using cDNA expression array analysis, a substantial difference in key tumor-related gene expression was found between chemically induced mice treated or not with PARP inhibitor and also between wild-type and parp-1 knockout mice. Most important differences were found in gene expression for Nfkbiz, S100a9, Hif-1alpha, and other genes involved in carcinogenesis and inflammation. These results were corroborated by real-time PCR. Moreover, the transcriptional activity of hypoxia-inducible factor-1alpha (HIF-1alpha) was compromised by PARP inhibition or in PARP-1-deficient cells, as measured by gene reporter assays and the expression of key target genes for HIF-1alpha. Tumor vasculature was also strongly inhibited in PARP-1-deficient mice and by DPQ. In summary, this study shows that inhibition of PARP on itself is able to control tumor growth, and PARP inhibition or genetic deletion of PARP-1 prevents from tumor promotion through their ability to cooperate with the activation AP-1, NF-kappaB, and HIF-1alpha.

New tricks for old drugs: the anticarcinogenic potential of DNA repair inhibitors

Defective or abortive repair of DNA lesions has been associated with carcinogenesis. Therefore it is imperative for a cell to accurately repair its DNA after damage if it is to return to a normal cellular phenotype. In certain circumstances, if DNA damage cannot be repaired completely and with high fidelity, it is more advantageous for an organism to have some of its more severely damaged cells die rather than survive as neoplastic transformants. A number of DNA repair inhibitors have the potential to act as anticarcinogenic compounds. These drugs are capable of modulating DNA repair, thus promoting cell death rather than repair of potentially carcinogenic DNA damage mediated by error-prone DNA repair processes. In theory, exposure to a DNA repair inhibitor during, or immediately after, carcinogenic exposure should decrease or prevent tumorigenesis. However, the ability of DNA repair inhibitors to prevent cancer development is difficult to interpret depending upon the system used and the type of genotoxic stress. Inhibitors may act on multiple aspects of DNA repair as well as the cellular signaling pathways activated in response to the initial damage. In this review, we summarize basic DNA repair mechanisms and explore the effects of a number of DNA repair inhibitors that not only potentiate DNA-damaging agents but also decrease carcinogenicity. In particular, we focus on a novel anti-tumor agent, beta-lapachone, and its potential to block transformation by modulating poly(ADP-ribose) polymerase-1.

Promyelocytic leukemia nuclear bodies are predetermined processing sites for damaged DNA

The promyelocytic leukemia protein (PML) participates in several cellular functions, including transcriptional regulation, apoptosis and maintenance of genomic stability. A key feature of this protein is its ability to induce the assembly of nuclear compartments termed PML-nuclear bodies (PML-NBs). Here we show that these nuclear structures recruit single-stranded DNA (ssDNA) molecules in response to exogenous DNA damage. ssDNA was readily detected in PML-NBs within 1 hour following exposure of cells to UV light. Confocal real-time imaging of cells expressing YFP-tagged PML did not reveal de novo formation of new PML-NBs following UV-irradiation, which shows that ssDNA focus formation occurred within pre-existing PML-NBs. Moreover, siRNA-mediated depletion of PML prevented ssDNA focus formation and sensitized cells to UV-induced apoptosis. PML-dependent ssDNA focus formation was found to be particularly efficient during S-phase of the cell cycle, and PML-depleted cells became retarded in S-phase upon growth in the presence of etoposide. In addition, we found that caffeine and the poly(ADP-ribose) polymerase (PARP) inhibitor NU1027 enhanced UV-induced recruitment of ssDNA to PML-NBs. Together, our results show that PML-NBs have the capacity to accommodate DNA metabolic activities that are associated with processing of damaged DNA.

Orbital metastasis from ovarian carcinoma in a patient with BRCA-2 mutation

A 45-year-old woman with BRCA-2 mutation-positive ovarian carcinoma had pain, swelling, and proptosis of her left eye. CT demonstrated a mass in the left orbit. Fine-needle aspiration and lateral orbitotomy were used to obtain tissue for diagnosis. Pathology confirmed poorly differentiated tumor cells consistent with ovarian carcinoma. Cytopathologic testing confirmed receptor and hormonal markers within the orbital specimen that were identical to specimens removed 4 years prior.

PARP-1 inhibitors: are they the long-sought genetically specific drugs for BRCA1/2-associated breast cancers?

Recent studies demonstrated that PARP-1 [poly(ADP-ribose) polymerase-1] inhibitors kill breast cancer associated gene-1 and -2 (BRCA1/2) deficient cells with extremely high efficiency while BRCA+/- and BRCA+/+ cells are relatively non-responsive to the treatment. It was therefore proposed that PARP-1 inhibitors might be the long-sought genetically specific drugs that are both safe and effective for treating BRCA1/2-associated breast cancers. However, a report published in a recent issue of the International Journal of Biological Sciences revealed that PARP-1 inhibitors, although able to kill naïve BRCA1 mutant cells with high specificity both in vitro and in vivo, exhibit minimal specificity in inhibiting the growth of mouse mammary tumor cells irrespective of their BRCA1 status in allograft nude mice. Non-specific inhibition in human BRCA1+/+, BRCA1+/-, and BRCA1-/- breast cancer cells by PARP-1 inhibitors was also observed. Additional mutations occurring during cancer progression may be a culprit, although the exact cause for the resistance of BRCA1-/- breast cancer cells to PARP-1 inhibitors remains elusive. These findings suggest that PARP inhibition may serve as an approach for the prevention of BRCA related breast cancer and may be useful in combination with other chemotherapeutic agents in the treatment of breast cancer.

Update on hereditary breast cancer

Women with BRCA1 or BRCA2 mutations are at substantial risk for breast and ovarian cancer. This review describes recent developments in the approach to hereditary breast cancer. Risk-reducing surgeries remain the most effective means of preventing breast cancer in mutation carriers. For women with breast tissue at risk, magnetic resonance imaging is rapidly becoming incorporated into screening programs. For affected women, management does not currently differ from that of women with sporadic breast cancer, although women may choose to undergo bilateral mastectomy. Preclinical data suggest that BRCA mutation-associated breast cancers may benefit from specific targeted therapeutic approaches.

BRCA1--a good predictive marker of drug sensitivity in breast cancer treatment?

There are currently only two predictive markers of response to chemotherapy for breast cancer in routine clinical use, namely the Estrogen receptor-alpha and the HER2 receptor. The breast and ovarian cancer susceptibility gene BRCA1 is an important genetic factor in hereditary breast and ovarian cancer and there is increasing evidence of an important role for BRCA1 in the sporadic forms of both cancer types. Our group and numerous others have shown in both preclinical and clinical studies that BRCA1 is an important determinant of chemotherapy responses in breast cancer. In this review we will outline the current understanding of the role of BRCA1 as a determinant of response to DNA damaging and microtubule damaging chemotherapy. We will then discuss how the known functions of this multifaceted protein may provide mechanistic explanations for its role in chemotherapy responses.