Selective induction of chemotherapy resistance of mammary tumors in a conditional mouse model for hereditary breast cancer

Taxane resistance in breast cancer: mechanisms, predictive biomarkers and circumvention strategies

BACKGROUND

Taxanes are established in the treatment of metastatic breast cancer (MBC) and early breast cancer (EBC) as potent chemotherapy agents. However, their therapeutic usefulness is limited by de-novo refractoriness or acquired resistance, which are common drawbacks to most anti-cancer cytotoxics. Considering that the taxanes will remain principle chemotherapeutic agents for the treatment of breast cancer, we reviewed known mechanisms of resistance in with an outlook of optimizing their clinical use.

METHODS

We searched the PubMed and MEDLINE databases for articles (from inception through to 9th January 2012; last search 10/01/2012) and journals known to publish information relevant to taxane chemotherapy. We imposed no language restrictions. Search terms included: cancer, breast cancer, response, resistance, taxane, paclitaxel, docetaxel, taxol. Due to the possibility of alternative mechanisms of resistance all combination chemotherapy treated data sets were removed from our overview.

RESULTS

Over-expression of the MDR-1 gene product Pgp was extensively studied in vitro in association with taxane resistance, but data are conflicting. Similarly, the target components microtubules, which are thought to mediate refractoriness through alterations of the expression pattern of tubulins or microtubule associated proteins and the expression of alternative tubulin isoforms, failed to confirm such associations. Little consensus has been generated for reported associations between taxane-sensitivity and mutated p53, or taxane-resistance and overexpression of Bcl-2, Bcl-xL or NFkB. In contrary sufficient in vitro data support an association of spindle assembly checkpoint (SAC) defects with resistance. Clinical data have been limited and inconsistent, which relate to the variety of methods used, lack of standardization of cut-offs for quantitation, differences in clinical endpoints measured and in methods of tissue collection preparation and storage, and study/patient heterogeneity. The most prominent finding is that pharmaceutical down-regulation of HER-2 appears to reverse the taxane resistance.

CONCLUSIONS

Currently no valid practical biomarkers exist that can predict resistance to the taxanes in breast cancer supporting the principle of individualized cancer therapy. The incorporation of several biomarker analyses into prospectively designed studies in this setting are needed.

Impact of intertumoral heterogeneity on predicting chemotherapy response of BRCA1-deficient mammary tumors

The lack of markers to predict chemotherapy responses in patients poses a major handicap in cancer treatment. We searched for gene expression patterns that correlate with docetaxel or cisplatin response in a mouse model for breast cancer associated with BRCA1 deficiency. Array-based expression profiling did not identify a single marker gene predicting docetaxel response, despite an increase in Abcb1 (P-glycoprotein) expression that was sufficient to explain resistance in several poor responders. Intertumoral heterogeneity explained the inability to identify a predictive gene expression signature for docetaxel. To address this problem, we used a novel algorithm designed to detect differential gene expression in a subgroup of the poor responders that could identify tumors with increased Abcb1 transcript levels. In contrast, standard analytical tools, such as significance analysis of microarrays, detected a marker only if it correlated with response in a substantial fraction of tumors. For example, low expression of the Xist gene correlated with cisplatin hypersensitivity in most tumors, and it also predicted long recurrence-free survival of HER2-negative, stage III breast cancer patients treated with intensive platinum-based chemotherapy. Our findings may prove useful for selecting patients with high-risk breast cancer who could benefit from platinum-based therapy.

Predictive in vivo animal models and translation to clinical trials

Vast resources are expended during the development of new cancer therapeutics, and selection of optimal in vivo models should improve this process. Genetically engineered mouse models (GEMM) of cancer have progressively improved in technical sophistication and, accurately recapitulating the human cognate condition, have had a measureable impact on our knowledge of tumourigenesis. However, the application of GEMMs to facilitate the development of innovative therapeutic and diagnostic approaches has lagged behind. GEMMs that recapitulate human cancer offer an additional opportunity to accelerate drug development, and should complement the role of the widely used engraftment tumour models.

Proteomics of mouse BRCA1-deficient mammary tumors identifies DNA repair proteins with potential diagnostic and prognostic value in human breast cancer

Breast cancer 1, early onset (BRCA1) hereditary breast cancer, a type of cancer with defects in the homology-directed DNA repair pathway, would benefit from the identification of proteins for diagnosis, which might also be of potential use as screening, prognostic, or predictive markers. Sporadic breast cancers with defects in the BRCA1 pathway might also be diagnosed. We employed proteomics based on one-dimensional gel electrophoresis in combination with nano-LC-MS/MS and spectral counting to compare the protein profiles of mammary tumor tissues of genetic mouse models either deficient or proficient in BRCA1. We identified a total of 3,545 proteins, of which 801 were significantly differentially regulated between the BRCA1-deficient and -proficient breast tumors. Pathway and protein complex analysis identified DNA repair and related functions as the major processes associated with the up-regulated proteins in the BRCA1-deficient tumors. In addition, by selecting highly connected nodes, we identified a BRCA1 deficiency signature of 45 proteins that enriches for homology-directed DNA repair deficiency in human gene expression breast cancer data sets. This signature also exhibits prognostic power across multiple data sets, with optimal performance in a data set enriched in tumors deficient in homology-directed DNA repair. In conclusion, by comparing mouse proteomes from BRCA1-proficient and -deficient mammary tumors, we were able to identify several markers associated with BRCA1 deficiency and a prognostic signature for human breast cancer deficient in homology-directed DNA repair.

Epigenetic mechanisms in tumorigenesis, tumor cell heterogeneity and drug resistance

Resistance of cancer cells to chemotherapeutics and emerging targeted drugs is a devastating problem in the treatment of cancer patients. Multiple mechanisms contribute to drug resistance such as increased drug efflux, altered drug metabolism, secondary mutations in drug targets, and activation of downstream or parallel signal transduction pathways. The rapid kinetics, the reversibility of acquired drug resistance and the absence of genetic mutations suggest an epigenetic basis for drug insensitivity. Similar to the cellular variance seen in the human body, epigenetic mechanisms, through reversible histone modifications and DNA methylation patterns, generate a variety of transcriptional states resulting in a dynamic heterogeneous tumor cell population. Consequently, epigenomes favoring survival in the presence of a drug by aberrant transcription of drug transporters, DNA-repair enzymes and pro-apoptotic factors render cytotoxic and targeted drugs ineffective and allow selection of rare drug-resistant tumor cells. Recent advances in charting cancer genomes indeed strongly indicate a role for epigenetic regulators in driving cancer, which may result in the acquisition of additional (epi)genetic modifications leading to drug resistance. These observations have important clinical consequences as they provide an opportunity for "epigenetic drugs" to change reversible drug-resistance-associated epigenomes to prevent or reverse non-responsiveness to anti-cancer drugs.

Drug resistance in the mouse cancer clinic

Drug resistance is one of the most pressing problems in treating cancer patients today. Local and regional disease can usually be adequately treated, but patients eventually die from distant metastases that have become resistant to all available chemotherapy. Although work on cultured tumor cell lines has yielded a lot of information on potential drug resistance mechanisms, it has proven difficult to translate these results to clinical drug resistance in patients. The controversy regarding the contribution of ABC transporters to drug resistance in patients is one example. The study of genetically engineered mouse models (GEMMs), which closely resemble cancer in human patients, can help to bridge this gap. In models for BRCA1- or BRCA2-associated breast cancer, we observed a substantial synergy between the defect in homology-directed DNA repair and sensitivity to DNA-targeting drugs. Nevertheless, tumors are not easily eradicated and eventually drug resistance develops. In this review we will discuss the use of the new generation mouse models to address major clinical problems, such as mechanisms of drug resistance, predicting chemotherapy response or characterizing the nature of residual tumor cells that escape eradication. Moreover, we will address the contribution of ABC transporters to drug resistance in our model.

The right time, the right place: will targeting human cancer-associated mutations to the mouse provide the perfect preclinical model?

Over the past 10 years the realisation that genetic mouse models of cancer may play a key role in preclinical drug development has gained strong momentum. Moreover sequencing studies of human tumours have provided key insights into the mutational complexity of epithelial cancer, unleashing important clues for researchers to generate accurate genetically engineered mouse (GEM) models of cancer. Thus by targeting multiple cancer associated human mutations to the appropriate murine epithelia, mice develop tumours that more closely recapitulate the human disease. As a number of excellent models now exist, the next 5-10 years will ascertain whether these models will predict response of human cancer to intervention. If so they might become the 'gold standard' where all drugs are required to be tested in mouse models of disease before proceeding into the patient. However, although this principle is very attractive, it is relatively untested and here, using examples of prevalent human cancers, we will review the latest data on preclinical GEM studies and comment on what challenges are left to overcome.

Two-in-one: combined targeted chemo and gene therapy for tumor suppression and prevention of metastases

Aims: To develop an approach for the effective treatment of ovarian tumor, prevention of metastases and limitation of side effects. Materials & Methods: In order to combine chemotherapy with genotherapy, we constructed a nanoscale-based tumor-targeted system containing two anticancer drugs, two antisense oligonucleotides (suppressors of cellular resistance) and a ligand specific to receptors overexpressed in cancer cells. The system was tested in a mouse metastatic xenograft model using tumor isolates from patients with ovarian carcinoma. Results: Treatment with the proposed nanosystem decreased primary tumor, prevented intraperitoneal metastases and limited adverse side effects. Conclusions: The advantages of the proposed targeted complex treatment of primary aggressive ovarian tumor and prevention of metastases make the proposed approach promising for clinical applications.

Genomic instability in breast and ovarian cancers: translation into clinical predictive biomarkers

Breast and ovarian cancer are among the most common malignancies diagnosed in women worldwide. Together, they account for the majority of cancer-related deaths in women. These cancer types share a number of features, including their association with hereditary cancer syndromes caused by heterozygous germline mutations in BRCA1 or BRCA2. BRCA-associated breast and ovarian cancers are hallmarked by genomic instability and high sensitivity to DNA double-strand break (DSB) inducing agents due to loss of error-free DSB repair via homologous recombination (HR). Recently, poly(ADP-ribose) polymerase inhibitors, a new class of drugs that selectively target HR-deficient tumor cells, have been shown to be highly active in BRCA-associated breast and ovarian cancers. This finding has renewed interest in hallmarks of HR deficiency and the use of other DSB-inducing agents, such as platinum salts or bifunctional alkylators, in breast and ovarian cancer patients. In this review we discuss the similarities between breast and ovarian cancer, the hallmarks of genomic instability in BRCA-mutated and BRCA-like breast and ovarian cancers, and the efforts to search for predictive markers of HR deficiency in order to individualize therapy in breast and ovarian cancer.

Integrating Breast Cancer Genetics into Clinical Practice

Breast cancer prognosis and treatment is guided by traditional clinicopathological parameters and individual molecular markers. Despite the remarkable advances in our scientific understanding of breast cancer genetics, the impact of such information on medical care has, to date, been modest. Although the use of simple genetics is already in vogue in clinical practice, the concept of molecular profiling and multiparameter gene classifiers was raised after the introduction of the high-throughput gene expression microarrays. This technology, in addition to highlighting the molecular heterogeneity of breast cancer, has led to the development of prognostic and predictive gene signatures. Studies are underway to assess the clinical validity and clinical utility of these multigene assays and their incorporation into clinical practice. This article reviews the current status and projected future use of genetics and genomics in breast cancer management and their impact on the refinement of risk stratification to permit individualized and patient-tailored therapy. Limitations based on our current scientific understanding and realistic expectations are also explored.

Evolution of platinum resistance in high-grade serous ovarian cancer

High-grade serous ovarian cancers account for most ovarian-cancer mortality. Although this disease initially responds well to platinum-based chemotherapy, relapse and progression to chemotherapy resistance are frequently seen. Time to relapse after first-line therapy is a predictor of response to secondary platinum treatment: more than 12 months is associated with high chance of a secondary response, whereas relapses within 6 months generally indicate platinum resistance. In this Personal View we discuss whether patterns of response, relapse, and the development of drug resistance in high-grade serous ovarian cancers are related to distinct underlying molecular and cellular biological characteristics. In particular, we propose that rapid relapse with platinum-resistant disease is due to minor subpopulations of intrinsically resistant cancer cells at presentation.

BRCA1 RING function is essential for tumor suppression but dispensable for therapy resistance

Hereditary breast cancers are frequently caused by germline BRCA1 mutations. The BRCA1(C61G) mutation in the BRCA1 RING domain is a common pathogenic missense variant, which reduces BRCA1/BARD1 heterodimerization and abrogates its ubiquitin ligase activity. To investigate the role of BRCA1 RING function in tumor suppression and therapy response, we introduced the Brca1(C61G) mutation in a conditional mouse model for BRCA1-associated breast cancer. In contrast to BRCA1-deficient mammary carcinomas, tumors carrying the Brca1(C61G) mutation responded poorly to platinum drugs and PARP inhibition and rapidly developed resistance while retaining the Brca1(C61G) mutation. These findings point to hypomorphic activity of the BRCA1-C61G protein that, although unable to prevent tumor development, affects response to therapy.

Proteomic approach reveals FKBP4 and S100A9 as potential prediction markers of therapeutic response to neoadjuvant chemotherapy in patients with breast cancer

Although doxorubicin (Doxo) and docetaxel (Docet) in combination are widely used in treatment regimens for a broad spectrum of breast cancer patients, a major obstacle has emerged in that some patients are intrinsically resistant to these chemotherapeutics. Our study aimed to discover potential prediction markers of drug resistance in needle-biopsied tissues of breast cancer patients prior to neoadjuvant chemotherapy. Tissues collected before chemotherapy were analyzed by mass spectrometry. A total of 2,331 proteins were identified and comparatively quantified between drug sensitive (DS) and drug resistant (DR) patient groups by spectral count. Of them, 298 proteins were differentially expressed by more than 1.5-fold. Some of the differentially expressed proteins (DEPs) were further confirmed by Western blotting. Bioinformatic analysis revealed that the DEPs were largely associated with drug metabolism, acute phase response signaling, and fatty acid elongation in mitochondria. Clinical validation of two selected proteins by immunohistochemistry found that FKBP4 and S100A9 might be putative prediction markers in discriminating the DR group from the DS group of breast cancer patients. The results demonstrate that a quantitative proteomics/bioinformatics approach is useful for discovering prediction markers of drug resistance, and possibly for the development of a new therapeutic strategy.

Alterations in nuclear pore architecture allow cancer cell entry into or exit from drug-resistant dormancy

Phenotypic diversity arises in tumors just as it does in developing organisms, and tumor recurrence frequently manifests from the selective survival of divergent drug-resistant cells. Although the expanding tumor cell population may be successfully targeted, drug-resistant cells may persist and sustain the tumor or enter dormancy before igniting a future relapse. Herein, we show that partial knockdown of nucleoporin p62 (NUP62) by small-interfering RNA confers cisplatin resistance to cultured high-grade ovarian carcinoma cells. Treatment with NUP62 small-interfering RNA and cisplatin leaves resistant cells in a state of dormancy; some dormant cells can be induced to proliferate by transient induction of NUP62 expression from an ectopic expression construct. In addition to suggesting functional links between nuclear pore complex architecture and cancer cell survival, the culture system provides a novel experimental window into the dynamics of tumor cell drug resistance and dormancy.

ABCB1/MDR1 contributes to the anticancer drug-resistant phenotype of IPH-926 human lobular breast cancer cells

Contribution of the ABCB1/MDR1/P-glycoprotein drug transporter to breast cancer resistance has been controversial. One issue is that ABCB1-dependent drug-resistance has primarily been investigated in mammary epithelial cell models technically manipulated to overexpress ABCB1, either by gene transfer using appropriate expression vectors or by chronic anticancer drug-selection. However, an unmodified human breast cancer cell line with an endogenous overexpression of ABCB1 has not been described thus far. Using Affymetrix microarray analyses, we identified an endogenous overexpression of several tumor-biologically relevant transcripts including ABCB1, BCAR4, CCL28, SCGB2A2 and PIP in IPH-926, an anticancer drug-resistant human lobular breast cancer cell line derived from a chemo-refractory mammary carcinoma patient. In a panel of twenty breast cancer cell lines examined, overexpression of ABCB1 mRNA and protein was exclusively detected in IPH-926. This was further validated using chronically in vitro drug-selected KB-V-1 cells as a widely used reference model to accurately define an ABCB1 overexpression. IPH-926 and KB-V-1 displayed a similar overexpression of ABCB1. Flow cytometric analyses showed that IPH-926 but not ABCB1-negative breast cancer cells extruded the anticancer agent doxorubicin, a classical substrate of the ABCB1 drug transporter. PSC-833 (valspodar), a selective ABCB1 inhibitor, blocked this efflux, restored apoptotic PARP cleavage and increased doxorubicin sensitivity in IPH-926 and KB-V-1. To our knowledge, IPH-926 represents the first human breast cancer cell line with a genuine, endogenous overexpression of ABCB1. IPH-926 provides evidence that ABCB1 can occasionally cause anticancer drug-resistance in breast cancer patients and offers a new tool for the evaluation of compounds to overcome drug-resistance.

Drug therapy for hereditary cancers

Tumors arising in patients with hereditary cancer syndromes may have distinct drug sensitivity as compared to their sporadic counterparts. Breast and ovarian neoplasms from BRCA1 or BRCA2 mutation carriers are characterized by deficient homologous recombination (HR) of DNA, that makes them particularly sensitive to platinum compounds or inhibitors of poly (ADP-ribose) polymerase (PARP). Outstandingly durable complete responses to high dose chemotherapy have been observed in several cases of BRCA-related metastatic breast cancer (BC). Multiple lines of evidence indicate that women with BRCA1-related BC may derive less benefit from taxane-based treatment than other categories of BC patients. There is virtually no reports directly assessing drug response in hereditary colorectal cancer (CRC) patients; studies involving non-selected (i.e., both sporadic and hereditary) CRC with high-level microsatellite instability (MSI-H) suggest therapeutic advantage of irinotecan. Celecoxib has been approved for the treatment of familial adenomatous polyposis (FAP). Hereditary medullary thyroid cancers (MTC) have been shown to be highly responsive to a multitargeted tyrosine kinase inhibitor vandetanib, which exerts specific activity towards mutated RET receptor. Given the rapidly improving accessibility of DNA analysis, it is foreseen that the potential predictive value of cancer-associated germ-line mutations will be increasingly considered in the future studies.

An aCGH classifier derived from BRCA1-mutated breast cancer and benefit of high-dose platinum-based chemotherapy in HER2-negative breast cancer patients

BACKGROUND

Breast cancer cells deficient for BRCA1 are hypersensitive to agents inducing DNA double-strand breaks (DSB), such as bifunctional alkylators and platinum agents. Earlier, we had developed a comparative genomic hybridisation (CGH) classifier based on BRCA1-mutated breast cancers. We hypothesised that this BRCA1-like(CGH) classifier could also detect loss of function of BRCA1 due to other causes besides mutations and, consequently, might predict sensitivity to DSB-inducing agents.

PATIENTS AND METHODS

We evaluated this classifier in stage III breast cancer patients, who had been randomly assigned between adjuvant high-dose platinum-based (HD-PB) chemotherapy, a DSB-inducing regimen, and conventional anthracycline-based chemotherapy. Additionally, we assessed BRCA1 loss through mutation or promoter methylation and immunohistochemical basal-like status in the triple-negative subgroup (TN subgroup).

RESULTS

We observed greater benefit from HD-PB chemotherapy versus conventional chemotherapy among patients with BRCA1-like(CGH) tumours [41/230 = 18%, multivariate hazard ratio (HR) = 0.12, 95% confidence interval (CI) 0.04-0.43] compared with patients with non-BRCA1-like(CGH) tumours (189/230 = 82%, HR = 0.78, 95% CI 0.50-1.20), with a significant difference (test for interaction P = 0.006). Similar results were obtained for overall survival (P interaction = 0.04) and when analyses were restricted to the TN subgroup. Sixty-three percent (20/32) of assessable BRCA1-like(CGH) tumours harboured either a BRCA1 mutation (n = 8) or BRCA1 methylation (n = 12).

CONCLUSION

BRCA1 loss as assessed by CGH analysis can identify patients with substantially improved outcome after adjuvant DSB-inducing chemotherapy when compared with standard anthracycline-based chemotherapy in our series.

BRCA1 loss induces GADD153-mediated doxorubicin resistance in prostate cancer

BRCA1 plays numerous roles in the regulation of genome integrity and chemoresistance. Although BRCA1 interaction with key proteins involved in DNA repair is well known, its role as a coregulator in the transcriptional response to DNA damage remains poorly understood. In this study, we show that BRCA1 plays a central role in the transcriptional response to genotoxic stress in prostate cancer. BRCA1 expression mediates apoptosis, cell-cycle arrest, and decreased viability in response to doxorubicin treatment. Xenograft studies using human prostate carcinoma PC3 cells show that BRCA1 depletion results in increased tumor growth. A focused survey of BRCA1-regulated genes in prostate carcinoma reveals that multiple regulators of genome stability and cell-cycle control, including BLM, FEN1, DDB2, H3F3B, BRCA2, CCNB2, MAD2L1, and GADD153, are direct transcriptional targets of BRCA1. Furthermore, we show that BRCA1 targets GADD153 promoter to increase its transcription in response to DNA damage. Finally, GADD153 depletion significantly abrogates BRCA1 influence on cell-cycle progression and cell death in response to doxorubicin treatment. These findings define a novel transcriptional pathway through which BRCA1 orchestrates cell fate decisions in response to genotoxic insults, and suggest that BRCA1 status should be considered for new chemotherapeutic treatment strategies in prostate cancer.

BRCA1, PARP, and 53BP1: conditional synthetic lethality and synthetic viability

BRCA1 plays a critical role in the regulation of homologous recombination (HR)-mediated DNA double-strand break repair. BRCA1-deficient cancers have evolved to tolerate loss of BRCA1 function. This renders them vulnerable to agents, such as PARP inhibitors, that are conditionally 'synthetic lethal' with their underlying repair defect. Recent studies demonstrate that BRCA1-deficient cells may acquire resistance to these agents by partially correcting their defect in HR-mediated repair, either through reversion mutations in BRCA1 or through 'synthetic viable' loss of 53BP1. These findings and their clinical implications will be reviewed in this article.

miR-135a contributes to paclitaxel resistance in tumor cells both in vitro and in vivo

Cancer cell resistance to paclitaxel continues to be a major clinical problem. In this study, we utilized microRNA (miRNA) arrays to screen for differentially expressed miRNAs in paclitaxel-resistant cell lines established in vitro. We observed concordant upregulation of miR-135a in paclitaxel-resistant cell lines representing three human malignancies. Subsequently, the role of miRNA-135a was evaluated in an in vivo model of paclitaxel resistance. In this model, mice were inoculated subcutaneously with a non-small cell lung carcinoma cell line and treated with paclitaxel for a prolonged period. In paclitaxel-resistant cell lines, established either in vitro or in vivo, blockage of miR-135a sensitized resistant cell lines to paclitaxel-induced cell death. We further demonstrated a correlation between paclitaxel response and miR-135a expression in paclitaxel-resistant subclones that were established in vivo. The paclitaxel-resistant phenotype of these subclones was maintained upon retransplantation in new mice, as shown by decreased tumor response upon paclitaxel treatment compared with controls. Upregulation of miR-135a was associated with reduced expression of the adenomatous polyposis coli gene (APC). APC knockdown increased paclitaxel resistance in parental cell lines. Our results indicate that paclitaxel resistance is associated with upregulation of miR-135a, both in vitro and in vivo, and is in part determined by miR-135a-mediated downregulation of APC.

Preclinical strategies to define predictive biomarkers for therapeutically relevant cancer subtypes

Defining key driver mutations in cancer, the resulting aberrations in molecular mechanisms and the subsequent phenotype underpins the development and implementation of novel personalized medicine strategies. The literature is replete with biomarkers of prognosis and therapeutic responsiveness identified in single cohorts of patients that have not been independently validated and as a consequence, not developed. Integrating companion biomarker discovery with therapeutic development at the preclinical stage creates the opportunity to identify candidate biomarkers early, which would significantly facilitate both biomarker and therapeutic development. Advances in "-omic" technologies have led to large-scale efforts in characterizing and cataloguing the full range of aberrations in cancer. These include the International Cancer Genome Consortium and The Cancer Genome Atlas, which aim to comprehensively catalogue the range of genomic aberrations for large numbers of cancers for a progressively increasing range of cancer types and subtypes. The technical challenges associated with achieving these goals in some instances have required the generation of primary xenografts and cell lines. These extensively characterized model systems will provide an unprecedented resource for the discovery of biomarkers of therapeutic responsiveness for established therapies, and the development of companion biomarkers linked with preclinical novel therapeutic development in the future.

Metastatic triple-negative breast cancer

The triple-negative class (oestrogen receptor-negative, progesterone receptor-negative and human epidermal growth factor receptor 2 [HER2]-negative) comprises about 15% of breast cancer. It is associated with a poor prognosis compared with tumours that are positive for hormone receptors or HER2. Despite being sensitive to chemotherapy, many women with metastatic triple-negative breast cancer (TNBC) relapse quickly, and commonly develop visceral metastasis, including lung, liver and brain metastasis. TNBC has molecular features that overlap with breast cancer in BRCA1 germline mutation carriers and with those of the basal-like molecular class of tumours. Furthermore, tumours with the triple-negative phenotype have specific features and express markers that are potential therapeutic targets, for example an impaired DNA repair mechanism and increased expression of proliferation and basal-associated markers. The presence of these features has important implications for clinical practice and for the design of clinical trials looking at novel therapies. Targeted agents that are currently being investigated include poly (ADP-ribose) polymerase inhibitors, epidermal growth factor receptor inhibitors and anti-angiogenic compounds. Here we discuss the epidemiology, morphological and molecular spectrum of TNBC, the clinical significance of this important class of breast cancer and the current treatment options.

Secondary mutations of BRCA1/2 and drug resistance

Inherited mutations in the tumor suppressor genes BRCA1 and BRCA2 cause increased risk of developing various cancers, especially breast and ovarian cancers. Tumors that develop in patients with inherited BRCA1/2 mutations are generally believed to be BRCA1/2-deficient. Cancer cells with BRCA1/2 deficiency are defective in DNA repair by homologous recombination and sensitive to interstrand DNA crosslinking agents, such as cisplatin and carboplatin, and poly(ADP-ribose) polymerase inhibitors. Therefore, these agents are logical choices for the treatment for BRCA1/2-deficient tumors and have shown to be clinically effective. However, BRCA1/2-mutated tumors often develop resistance to these drugs. Restoration of BRCA1/2 functions due to secondary BRCA1/2 mutations has been recognized as a mechanism of acquired resistance to cisplatin and poly(ADP-ribose) polymerase inhibitors in BRCA1/2-mutated cancer cells. This indicates that even disease-causing inherited mutations of tumor suppressor genes can be genetically reverted in cancer cells, if the genetic reversion is advantageous for the cells' survival. In this review, we will discuss this drug resistance mechanism.

Studying therapy response and resistance in mouse models for BRCA1-deficient breast cancer

Worldwide, more than one million women are diagnosed with breast cancer every year, making it the most common malignancy of females in the developed world. Germline mutations in the breast cancer susceptibility genes BRCA1 and BRCA2 account for 4-6% of all breast cancer cases, and mutation carriers have a lifetime risk of 80% for developing breast cancer and 40% for developing ovarian cancer. Current treatment options are limited and often do not lead to cure. In the 17 years since the discovery of BRCA1, the generation of mouse models for BRCA1 deficiency has greatly aided our understanding of it's role in tumorigenesis. In contrast to human BRCA1 mutation carriers, mice carrying heterozygous mutations in Brca1 did not develop spontaneous tumors. This led to the generation of conditional mouse models in which tissue-specific Brca1 deletion induces formation of mammary tumors that closely resemble human BRCA1-mutated breast tumors. These models have proven useful for studying BRCA1-related tumor development, drug response and resistance. BRCA1-deficient cancer cells are defective in DNA repair mediated by homologous recombination (HR) and therefore highly sensitive to DNA-damaging agents such as platinum drugs and poly(ADP-ribose) polymerase (PARP) inhibitors. However, BRCA1-mutated tumors can develop resistance to these drugs; hence improved treatment strategies are critical. Existing mouse models have already proven useful for preclinical testing of (combinations of) therapeutic agents that may be beneficial for the treatment of patients with BRCA1-mutated tumors. In this review, we discuss the progress made towards modeling BRCA1-deficient breast cancer in mice and what we have learned from preclinical studies using these models.

BRCA genes: lessons learned from experimental and clinical cancer

Advances in the study of BRCA1 and BRCA2 gene functions have relied on the development of animal models for seeking to explore further what we have learned from the human disease. Specifically, mouse models of a 'triple-negative' breast cancer (utilizing conditional knockout of BRCA1 and p53 in the breast), of an endometrioid ovarian cancer (based on oncogenic kras and loss of function of pten), and of anatomic and functional consequences of BRCA1 mutations in granulosa cells, have led to further inquiry into the pathogenesis and therapeutic consequences of genetic alterations. A striking susceptibility of these murine malignancies to platinum drugs has emerged, providing further confidence in their relevance to the human disease. In addition to these models, the pathogenesis of high-grade serous disease derived from risk-reducing surgeries in mutation carriers has pointed to a role of mutations in p53 commonly encountered in tubal intraepithelial carcinomas.

Multidrug resistance in breast cancer: from in vitro models to clinical studies

The development of multidrug resistance (MDR) and subsequent relapse on therapy is a widespread problem in breast cancer, but our understanding of the underlying molecular mechanisms is incomplete. Numerous studies have aimed to establish the role of drug transporter pumps in MDR and to link their expression to response to chemotherapy. The ATP-binding cassette (ABC) transporters are central to breast cancer MDR, and increases in ABC expression levels have been shown to correlate with decreases in response to various chemotherapy drugs and a reduction in overall survival. But as there is a large degree of redundancy between different ABC transporters, this correlation has not been seen in all studies. This paper provides an introduction to the key molecules associated with breast cancer MDR and summarises evidence of their potential roles reported from model systems and clinical studies. We provide possible explanations for why despite several decades of research, the precise role of ABC transporters in breast cancer MDR remains elusive.

Pretreatment EGFR T790M mutation and BRCA1 mRNA expression in erlotinib-treated advanced non-small-cell lung cancer patients with EGFR mutations

PURPOSE

Advanced non-small-cell lung cancer (NSCLC) patients harboring epidermal growth factor receptor (EGFR) mutations (deletion in exon 19 or L858R) show an impressive progression-free survival of 14 months when treated with erlotinib. However, the presence of EGFR mutations can only imperfectly predict outcome. We hypothesized that progression-free survival could be influenced both by the pretreatment EGFR T790M mutation and by components of DNA repair pathways.

EXPERIMENTAL DESIGN

We assessed the T790M mutation in pretreatment diagnostic specimens from 129 erlotinib-treated advanced NSCLC patients with EGFR mutations. The expression of eight genes and two proteins involved in DNA repair and four receptor tyrosine kinases was also examined.

RESULTS

The EGFR T790M mutation was observed in 45 of 129 patients (35%). Progression-free survival was 12 months in patients with and 18 months in patients without the T790M mutation (P = 0.05). Progression-free survival was 27 months in patients with low BRCA1 mRNA levels, 18 months in those with intermediate levels, and 10 months in those with high levels (P = 0.02). In the multivariate analysis, the presence of the T790M mutation (HR, 4.35; P = 0.001), intermediate BRCA1 levels (HR, 8.19; P < 0.0001), and high BRCA1 levels (HR, 8.46; P < 0.0001) emerged as markers of shorter progression-free survival.

CONCLUSIONS

Low BRCA1 levels neutralized the negative effect of the T790M mutation and were associated with longer progression-free survival to erlotinib. We advocate baseline assessment of the T790M mutation and BRCA1 expression to predict outcome and provide alternative individualized treatment to patients based on T790M mutations and BRCA1 expression.

Questioning the value of (99m)Tc-HYNIC-annexin V based response monitoring after docetaxel treatment in a mouse model for hereditary breast cancer

Annexin V imaging is suggested to provide a good indication of cancer treatment efficacy. To study the accuracy of (99m)Tc-AnxV imaging, we monitored chemo-sensitive and chemo-resistant tumors in a mouse breast cancer model after treatment with docetaxel. Sensitive tumors showed a slight peak in (99m)Tc-AnxV uptake one day post-treatment, while uptake in resistant tumors remained constant. In contrast to immunohistochemical analysis, (99m)Tc-AnxV imaging could not be used to predict tumor response, due to large variation between animals.

Context-dependent bidirectional regulation of the MutS homolog 2 by transforming growth factor β contributes to chemoresistance in breast cancer cells

The TGF-β, a tumor suppressive cytokine in normal cells, is abused in cancer to promote the malignancy. In this study, we reported that TGF-β downregulated the mutS homolog 2 (MSH2), a central component of the DNA mismatch repair (MMR) system, in HER2-transformed MCF10A mammary epithelial cells and in breast cancer (BC) cells. This was mediated by a TGF-β-induced micro RNA (miRNA), miR-21, which targeted the 3' untranslated region of MSH2 mRNA and downregulated its expression. A negative correlation between the expression of TGF-β1 and MSH2 was also detected in primary breast tumors. In contrast, TGF-β upregulated MSH2 in nontransformed cells through Smad-mediated, p53-dependent promoter activation, which was absent in BC cells with impaired p53 function. Although this upregulating mechanism also existed in MCF10A/HER2 and p53-proficient BC cells, both basal and TGF-β-induced MSH2 promoter activities were significantly lower than those in MCF10A. Moreover, the basal and TGF-β-induced miR-21 levels were markedly higher in transformed cells, suggesting that the preset levels of miR-21 and MSH2 promoter activity, which is affected by the p53 status, determine the outputs of the bidirectional regulation of MSH2 by TGF-β in a certain cellular context. We further found that by downregulating MSH2, TGF-β contributed to resistance to DNA-damaging chemotherapy agents in cancer cells. Our results indicated a regulatory antagonism between promoter activation and miRNA-mediated posttranscriptional inhibition underlying a dual effect of TGF-β on the DNA repair machinery, which may influence the genomic stability in a context-dependent manner and contribute to chemoresistance in cancer.

Indicators of homologous recombination deficiency in breast cancer and association with response to neoadjuvant chemotherapy

BACKGROUND

Tumors with homologous recombination deficiency (HRD), such as BRCA1-associated breast cancers, are not able to reliably repair DNA double-strand breaks (DSBs) and are therefore highly sensitive to both DSB-inducing chemotherapy and poly (ADP-ribose) polymerase inhibitors. We have studied markers that may indicate the presence of HRD in HER2-negative breast cancers and related them to neoadjuvant chemotherapy response.

PATIENTS AND METHODS

Array comparative genomic hybridization (aCGH), BRCA1 promoter methylation, BRCA1 messenger RNA (mRNA) expression and EMSY amplification were assessed in 163 HER2-negative pretreatment biopsies from patients scheduled for neoadjuvant chemotherapy.

RESULTS

Features of BRCA1 dysfunction were frequent in triple-negative (TN) tumors: a BRCA1-like aCGH pattern, promoter methylation and reduced mRNA expression were observed in, respectively, 57%, 25% and 36% of the TN tumors. In ER+ tumors, a BRCA2-like aCGH pattern and the amplification of the BRCA2 inhibiting gene EMSY were frequently observed (43% and 13%, respectively) and this BRCA2-like profile was associated with a better response to neoadjuvant chemotherapy.

CONCLUSIONS

Abnormalities associated with BRCA1 inactivation are present in about half of the TN breast cancers but were not predictive of chemotherapy response. In ER+/HER2- tumors, a BRCA2-like aCGH pattern was predictive of chemotherapy response. These findings should be confirmed in independent series.

ABC transporters: unvalidated therapeutic targets in cancer and the CNS

The discovery of the multidrug transporter P-glycoprotein (Pgp) over 35 years ago in drug resistant cells prompted several decades of work attempting to overcome drug resistance by inhibition of drug efflux. Despite convincing laboratory data showing that drug transport can be inhibited in vitro, efforts to translate this discovery to the clinic have not succeeded. Since overexpression of Pgp and related transporters including ABCG2 and members of the ABCC family have been linked with poor outcome, it remains a reasonable hypothesis that this poor outcome is linked to reduction of drug exposure by efflux, and thus to drug resistance. In this review, we will discuss the question of whether ABC transporters mediate drug resistance in cancer through a reduction in drug accumulation in tumors, and whether the "Pgp inhibition hypothesis" might be wrong. The hypothesis, which holds that increased chemotherapy effectiveness can be achieved by inhibiting Pgp-mediated drug efflux has only been validated in model systems. Possible explanations for the failure to validate this clinically include the existence of other modulators of drug accumulation and uptake in tumors. Despite these difficulties, a potential role has emerged for drug transporters as therapeutic targets in the central nervous system (CNS). Both lines of investigation point to the need for imaging agents to facilitate the study of drug accumulation in human cancer. This is a critical need for targeted therapies where an important dose-response relationship is likely to exist, and where drug resistance renders many of the novel targeted agents ineffective in a subset of patients.

Tumor-initiating cells are not enriched in cisplatin-surviving BRCA1;p53-deficient mammary tumor cells in vivo

Although many breast cancers respond to chemotherapy or hormonal therapy, lack of tumor eradication is a central clinical problem preceding the development of drug resistant tumors. Using the K14cre;Brca1(F5-13/F5-13);p53(F2-10/F2-10) mouse model for hereditary breast cancer, we have previously studied responses of mammary tumors to clinically relevant anti-cancer drugs, including cisplatin. The BRCA1- and p53-deficient tumors generated in this model are hypersensitive to cisplatin and never become resistant to this agent due to the large, irreversible deletion in Brca1. We show here that even dose-dense treatment with a maximum tolerated dose of cisplatin does not result in complete tumor eradication. To explain this result we have addressed the hypothesis that the lack of eradication of drug-sensitive tumors is due to increased in vivo chemotherapy resistance of tumor-initiating cells (TICs). Using the CD24 and CD49f cell surface markers which detect normal mouse mammary stem cells, we have identified tumor-initiating cells in BRCA1- and p53-deficient tumors. In addition to the Lin⁻/CD24(+)/CD49f(+) subpopulation, we show that a larger population of Lin⁻/CD24(+)/CD49f-cells also has tumor-initiating capability in at least two serial orthotopic transplantations, suggesting that these are not more differentiated transit-amplifying cells. However, we did not find an enrichment of TICs in cisplatin-treated tumor remnants. We conclude that in this model the tolerance of the cisplatin-surviving cells cannot be attributed to special biochemical defense mechanisms of TICs.

Targeting homologous recombination repair defects in cancer

DNA repair is essential for cells to maintain genome stability in an environment that constantly produces DNA damage. There is a growing appreciation that defects in homologous recombination repair underlie hereditary and sporadic tumourigenesis, and that deficiency in this pathway may dictate the sensitivity of tumours to certain DNA-damaging agents. Homologous recombination deficiency (HRD) may therefore prove to be a diagnostic criterion per se if appropriate biomarkers become available to identify these tumours. In addition, homologous recombination-deficient tumours are more sensitive to inhibition of other DNA repair pathways through so-called 'synthetic lethal interactions', a principle that is currently being tested in clinical trials. Finally, homologous recombination repair-deficient cells may have an increased dependency on certain cell-cycle checkpoints, which can be therapeutically exploited. Here we describe recent advances in strategies to identify and target HRD tumours, approaches to overcome resistance, and combinatory strategies to optimize treatment outcome.

Non-germline genetically engineered mouse models for translational cancer research

Genetically engineered mouse models (GEMMs) of cancer have affected virtually all areas of cancer research. However, the accelerated discovery of new cancer genes emerging from large-scale cancer genomics and new chemical entities pouring from the drug discovery pipeline have strained the capacity of traditional germline mouse models to provide crucial insights. This Review introduces new approaches to modelling cancer, with emphasis on a growing collection of non-germline GEMMs (nGEMMs). These offer flexibility, speed and uniformity at reduced costs, thus paving the way for much needed throughput and practical preclinical therapeutic testing models.

53BP1 loss rescues BRCA1 deficiency and is associated with triple-negative and BRCA-mutated breast cancers

Germ-line mutations in breast cancer 1, early onset (BRCA1) result in predisposition to breast and ovarian cancer. BRCA1-mutated tumors show genomic instability, mainly as a consequence of impaired recombinatorial DNA repair. Here we identify p53-binding protein 1 (53BP1) as an essential factor for sustaining the growth arrest induced by Brca1 deletion. Depletion of 53BP1 abrogates the ATM-dependent checkpoint response and G2 cell-cycle arrest triggered by the accumulation of DNA breaks in Brca1-deleted cells. This effect of 53BP1 is specific to BRCA1 function, as 53BP1 depletion did not alleviate proliferation arrest or checkpoint responses in Brca2-deleted cells. Notably, loss of 53BP1 partially restores the homologous-recombination defect of Brca1-deleted cells and reverts their hypersensitivity to DNA-damaging agents. We find reduced 53BP1 expression in subsets of sporadic triple-negative and BRCA-associated breast cancers, indicating the potential clinical implications of our findings.

Potential value of color-coded dynamic breast-specific gamma-imaging; comparing (99m)Tc-(V)-DMSA, (99m)Tc-MIBI, and (99m)Tc-HDP in a mouse mammary tumor model

Using a mouse mammary tumor model based on orthotopic transplantation of luciferase-expressing mouse ILC cells (KEP1-Luc cells), we evaluated the diagnostic value of three clinically applied tracers: (99m)Tc-(V)-DMSA, (99m)Tc-MIBI, and (99m)Tc-HDP. Uptake of the tracers is compared using static and dynamic imaging procedures. We found that dynamic imaging in combination with pixel-by-pixel color coding has an added value over (high resolution) static imaging procedures. Such dynamic imaging procedures could enhance the potential of breast-specific gamma-imaging.

Nuclear factor (erythroid-derived 2)-like 2 regulates drug resistance in pancreatic cancer cells

OBJECTIVE

To investigate the molecular basis of drug resistance in pancreatic cancer.

METHODS

The expression of nuclear factor (erythroid-derived 2)-like 2 (Nrf2) levels in pancreatic cancer tissues and cell lines was analyzed. Clinical relevance between Nrf2 activation and drug resistance was demonstrated by measuring cell viability after Nrf2 and adenosine 5'-triphosphate-binding cassette, subfamily G member 2 (ABCG2) regulation by overexpression or knock-down of these genes. Activity of ABCG2 was measured by Hoechst 33342 staining.

RESULTS

Abnormally elevated Nrf2 protein levels were observed in pancreatic cancer tissues and cell lines relative to normal pancreatic tissues. Increasing Nrf2 protein levels either by overexpression of exogenous Nrf2 or by activating endogenous Nrf2 resulted in increased drug resistance. Conversely, a reduction in endogenous Nrf2 protein levels or inactivation of endogenous Nrf2 resulted in decreased drug resistance. These changes in drug resistance or sensitivity were also positively correlated to the expression levels of Nrf2 downstream genes. Similarly, the expression of ABCG2 was correlated with drug resistance.

CONCLUSIONS

Because the intrinsic drug resistance of pancreatic cancers is, in part, due to abnormally elevated Nrf2 protein levels, further research on regulating Nrf2 activity may result in the development of novel pancreatic cancer therapies.

Chronic cisplatin treatment promotes enhanced damage repair and tumor progression in a mouse model of lung cancer

Chemotherapy resistance is a major obstacle in cancer treatment, yet the mechanisms of response to specific therapies have been largely unexplored in vivo. Employing genetic, genomic, and imaging approaches, we examined the dynamics of response to a mainstay chemotherapeutic, cisplatin, in multiple mouse models of human non-small-cell lung cancer (NSCLC). We show that lung tumors initially respond to cisplatin by sensing DNA damage, undergoing cell cycle arrest, and inducing apoptosis-leading to a significant reduction in tumor burden. Importantly, we demonstrate that this response does not depend on the tumor suppressor p53 or its transcriptional target, p21. Prolonged cisplatin treatment promotes the emergence of resistant tumors with enhanced repair capacity that are cross-resistant to platinum analogs, exhibit advanced histopathology, and possess an increased frequency of genomic alterations. Cisplatin-resistant tumors express elevated levels of multiple DNA damage repair and cell cycle arrest-related genes, including p53-inducible protein with a death domain (Pidd). We demonstrate a novel role for PIDD as a regulator of chemotherapy response in human lung tumor cells.

Abstract A14: Lack of tumor eradication of chemotherapy-sensitive BRCA1;p53-deficient mouse mammary tumors

Many breast cancers respond to chemotherapy or hormonal therapy, however, the lack of tumor eradication is a central clinical problem preceding the development of drug resistant tumors. We have previously studied responses of mammary tumors generated in the K14cre;Brca1F5-13/F5-13;p53F2-10/F2-10 mouse model for hereditary breast cancer to clinically relevant anti-cancer drugs such as doxorubicin, topotecan, cisplatin and the poly(ADP-ribose) polymerase (PARP) inhibitor olaparib (1-4). The BRCA1- and p53-deficient tumors generated in this model are hypersensitive to these drugs and never become resistant to cisplatin due to the large, irreversible deletion in Brca1 (5).

We show here that even dose-dense treatment with a maximum tolerable dose of cisplatin does not result in tumor eradication. This is not due to the trivial possibility that the drug-resistant remnants are poorly accessible to drug, since we found a homogeneous distribution of the platinum-DNA adducts throughout the tumor. To explain this result we have addressed the hypothesis that the lack of eradication of drug-sensitive tumors is due to increased in vivo chemotherapy resistance of tumor-initiating cells (TICs). Using the CD24 and CD49f cell surface markers which detect normal mouse mammary stem cells, we have identified TICs in BRCA1- and p53-deficient tumors. In addition to the Lin-/CD24+/CD49f+ subpopulation, we show that a larger population of Lin-/CD24+/CD49f-cells also has tumor-initiating capability in at least two serial orthotopic transplantations, suggesting that these are not more differentiated transit-amplifying cells. However, we did not find an enrichment of TICs in cisplatin-treated tumor remnants.

We conclude that in this model the resistance of cisplatin-surviving cells cannot be attributed to special biochemical defense mechanisms of TICs. Instead we will present data supporting the hypothesis that surviving TICs arrest in their cell cycle and ‘hibernate’ until the drug is gone. Using Brca1−/−;p53−/− cell lines derived from the mouse tumors we found that only Brca1−/−;p53−/− cells that were in the G0/G1 phase of the cell cycle 24h after cisplatin treatment were capable of forming new colonies, whereas cells in G2 or M were not. Hence, a p53-independent arrest in G0/G1 appears to underlie the lack of eradication of residual Brca1−/−;p53−/− cells.

Citation Information: Clin Cancer Res 2010;16(7 Suppl):A14

Sensitivity and acquired resistance of BRCA1;p53-deficient mouse mammary tumors to the topoisomerase I inhibitor topotecan

There is no tailored therapy yet for human basal-like mammary carcinomas. However, BRCA1 dysfunction is frequently present in these malignancies, compromising homology-directed DNA repair. This defect may serve as the tumor's Achilles heel and make the tumor hypersensitive to DNA breaks. We have evaluated this putative synthetic lethality in a genetically engineered mouse model for BRCA1-associated breast cancer, using the topoisomerase I (Top1) poison topotecan as monotherapy and in combination with poly(ADP-ribose) polymerase inhibition by olaparib. All 20 tumors tested were topotecan sensitive, but response heterogeneity was substantial. Although topotecan increased mouse survival, all tumors eventually acquired resistance. As mechanisms of in vivo resistance, we identified overexpression of Abcg2/Bcrp and markedly reduced protein levels of the drug target Top1 (without altered mRNA levels). Tumor-specific genetic ablation of Abcg2 significantly increased overall survival of topotecan-treated animals (P < 0.001), confirming the in vivo relevance of ABCG2 for topotecan resistance in a novel approach. Despite the lack of ABCG2, a putative tumor-initiating cell marker, none of the 11 Abcg2(-/-);Brca1(-/-);p53(-/-) tumors were eradicated, not even by the combination topotecan-olaparib. We find that olaparib substantially increases topotecan toxicity in this model, and we suggest that this might also happen in humans.

Triple negative breast cancers: clinical and prognostic implications

Triple negative breast cancers are defined by the absence of oestrogen, progesterone and HER2 expression. Most triple negative cancers display distinct clinical and pathological characteristics with a high proportion of these tumours occurring at a younger age of onset and in African-American women. Triple negative tumours typically demonstrate high histological grade and are the most common breast cancer subtype in BRCA1 carriers. In addition, many of the features of triple negative cancers are similar to those identified in the basal-like molecular subtype which has recently been characterised by gene expression profiling. Although the two groups overlap, they are not synonymous. Triple negative breast cancers are of pivotal clinical importance given the lack of therapeutic options. The prognostic significance of triple negative tumours remains unclear since the group is heterogeneous and worst prognosis seems to be mostly confined to those that express basal cytokeratins or epidermal growth factor receptor (EGFR). This review focuses on outlining the pathological, molecular, and clinical features of triple negative breast cancers, discusses its prognostic value and summarises current therapeutic approaches and future directions of research.

Clinical relevance of the triple-negative breast cancer concept: genetic basis and clinical utility of the concept

The beginning of microarray technology in the 1990s and the sequencing of the human genome in 2000 paved the way for the seminal paper of the Stanford group on the molecular portraits of human breast tumours in the same year. They described four distinct breast cancer subtypes, which they called 'luminal', 'basal', 'HER2-positive', and 'normal breast-like', based on unique gene expression patterns. This paper caused a paradigm shift. Breast cancer was no longer hormone receptor-positive or -negative, but rather luminal, basal or HER2-positive. Since then, numerous papers have appeared, trying to further characterise these subtypes on the DNA, RNA and protein level. Other groups have focussed on the epidemiology, prognosis and outcome after therapy of breast cancer patients according to these molecular subtypes. A promising prognostic marker within the subgroup of basal-like breast cancer is an up-regulated immune response, which is associated with favourable outcome. In addition, the majority of basal-like breast cancers harbour traits of a DNA damage repair defect. This feature can be exploited by the use of DNA damaging agents, and first exciting clinical results of the combination of carboplatin, gemcitabine and a poly (ADP-ribose) polymerase 1 (PARP-1) inhibitor have recently been reported. In this review, the molecular characterisation of triple-negative breast cancer, a proxy for basal-like breast cancer, is described and findings have been put into clinical context.

Efficacy of neoadjuvant Cisplatin in triple-negative breast cancer

PURPOSE Cisplatin is a chemotherapeutic agent not used routinely for breast cancer treatment. As a DNA cross-linking agent, cisplatin may be effective treatment for hereditary BRCA1-mutated breast cancers. Because sporadic triple-negative breast cancer (TNBC) and BRCA1-associated breast cancer share features suggesting common pathogenesis, we conducted a neoadjuvant trial of cisplatin in TNBC and explored specific biomarkers to identify predictors of response. PATIENTS AND METHODS Twenty-eight women with stage II or III breast cancers lacking estrogen and progesterone receptors and HER2/Neu (TNBC) were enrolled and treated with four cycles of cisplatin at 75 mg/m(2) every 21 days. After definitive surgery, patients received standard adjuvant chemotherapy and radiation therapy per their treating physicians. Clinical and pathologic treatment response were assessed, and pretreatment tumor samples were evaluated for selected biomarkers. Results Six (22%) of 28 patients achieved pathologic complete responses, including both patients with BRCA1 germline mutations;18 (64%) patients had a clinical complete or partial response. Fourteen (50%) patients showed good pathologic responses (Miller-Payne score of 3, 4, or 5), 10 had minor responses (Miller-Payne score of 1 or 2), and four (14%) progressed. All TNBCs clustered with reference basal-like tumors by hierarchical clustering. Factors associated with good cisplatin response include young age (P = .001), low BRCA1 mRNA expression (P = .03), BRCA1 promoter methylation (P = .04), p53 nonsense or frameshift mutations (P = .01), and a gene expression signature of E2F3 activation (P = .03). CONCLUSION Single-agent cisplatin induced response in a subset of patients with TNBC. Decreased BRCA1 expression may identify subsets of TNBCs that are cisplatin sensitive. Other biomarkers show promise in predicting cisplatin response.

A high-throughput pharmaceutical screen identifies compounds with specific toxicity against BRCA2-deficient tumors

PURPOSE

Hereditary breast cancer is partly explained by germline mutations in BRCA1 and BRCA2. Although patients carry heterozygous mutations, their tumors have typically lost the remaining wild-type allele. Selectively targeting BRCA deficiency may therefore constitute an important therapeutic approach. Clinical trials applying this principle are underway, but it is unknown whether the compounds tested are optimal. It is therefore important to identify alternative compounds that specifically target BRCA deficiency and to test new combination therapies to establish optimal treatment strategies.

EXPERIMENTAL DESIGN

We did a high-throughput pharmaceutical screen on BRCA2-deficient mouse mammary tumor cells and isogenic controls with restored BRCA2 function. Subsequently, we validated positive hits in vitro and in vivo using mice carrying BRCA2-deficient mammary tumors.

RESULTS

Three alkylators-chlorambucil, melphalan, and nimustine-displayed strong and specific toxicity against BRCA2-deficient cells. In vivo, these showed heterogeneous but generally strong BRCA2-deficient antitumor activity, with melphalan and nimustine doing better than cisplatin and the poly-(ADP-ribose)-polymerase inhibitor olaparib (AZD2281) in this small study. In vitro drug combination experiments showed synergistic interactions between the alkylators and olaparib. Tumor intervention studies combining nimustine and olaparib resulted in recurrence-free survival exceeding 330 days in 3 of 5 animals tested.

CONCLUSIONS

We generated and validated a platform for identification of compounds with specific activity against BRCA2-deficient cells that translates well to the preclinical setting. Our data call for the re-evaluation of alkylators, especially melphalan and nimustine, alone or in combination with the poly-(ADP-ribose)-polymerase inhibitors, for the treatment of breast cancers with a defective BRCA pathway.

BRCA1 deficient mouse models to study pathogenesis and therapy of triple negative breast cancer

Genetically engineered mice along with allograft and xenograft models can be used to effectively model triple negative breast cancer both for studies of pathophysiology as well as preclinical prevention and therapeutic drug studies. In this review eight distinct genetically engineered mouse models of BRCA1 deficiency are discussed in relationship to the generation of triple negative mammary cancer. Allograft models derived from some of these genetically engineered mice are considered and xenograft models derived from breast cancers that developed from BRCA1 mutation are presented. Examples of the use of genetically engineered, allograft and xenografts models for preventive and therapeutic studies are presented.