BRCA1-deficient Mammary Tumors Research Articles

Abstract PS07-07: The RXR agonist, IRX4204, delays the formation of Brca1 mutant mammary tumors via modulation of the anti-tumor immune response

Abstract Background: Women with germline BRCA1 mutations are at an increased risk for developing breast cancer in their lifetime, often at a young age with more aggressive tumors. At present, prophylactic bilateral mastectomy is the most effective strategy for reducing breast cancer risk. However, this invasive procedure is irreversible and associated with potential complications. We and others have found that PARP inhibitors can delay Brca1-mutant tumor formation in mice and could be beneficial for the prevention of breast cancer. However, currently available PARP inhibitors are associated with modest toxicities that may not be acceptable to women without cancer. Thus, there remains an urgent need for the development of safe and effective therapies for the prevention of breast cancer. Here, we present data demonstrating the activity of IRX4204, a minimally toxic and highly specific agonist of the nuclear retinoid X receptor (RXR), to delay the formation of mammary tumors in a Brca1-deficient mouse model. This inhibitory effect on tumor growth is due, in part, to the role of IRX4204 in stimulating the anti-tumor immune response. Methods: We used the established MMTV-Cre, conditional Brca1 gene knockout, p53 heterozygous loss mouse model (BRCA1co/co; MMTV-Cre+/+; p53+/-) and selected for mutant female pups using PCR genotyping. At 16 weeks of age, all mice were separated into 4 treatment groups (n=10 per group): (1) sesame oil control; (2) the novel RXR agonist, IRX-4204 (10mg/kg); (3) high-dose IRX4204 (20 mg/kg); and (4) the RXR agonist, 9-cis-UAB-30 (5 mg/kg). All treatments were given by oral gavage five days per week, and mice were observed daily for tumor formation and toxicity. At the study endpoint, tumors and normal mammary glands were collected for additional analyses. Immunohistochemical staining was used to quantify CD8a, Ki-67 and cleaved caspase 3 expression in Brca1-deficient tumors. Oil Red O staining was used to measure changes in lipid accumulation in Brca1-deficient cell lines treated with IRX4204. qPCR was used to quantify the changes in gene expression of lipid metabolism-associated genes upon treatment with IRX4204 in vitro. Results: Vehicle-treated Brca1-deficient mice had a median time-to-tumor formation (TTF) of 211 days, with 100% developing tumors by 330 days. Mice treated with UAB 5 mg/kg had an improved median TTF of 261 days, whereas mice treated with IRX4204 10mg/kg or 20mg/kg had a median TTF of 347 and 304 days, respectively (p < 0.01). In addition, 60% of mice treated with IRX4204 10 mg/kg remained tumor-free at 330 days. IRX4204-treated tumors showed an increased infiltration of CD8-positive T-cells over vehicle-treated tumors (p < 0.05). Treatment of Brca1-deficient cell lines with IRX4204 in vitro resulted in a significant increase in lipid accumulation accompanied by a 2-fold increase of Srebf1 expression (a key transcription factor that regulates lipid homeostasis) within 24 hours of treatment (p < 0.05). Conclusion: These data demonstrate a novel use of the RXR agonist, IRX4204, to delay the formation of Brca1-deficient mammary tumors. We have found that IRX4204 treatment modulates the tumor immune response through increased infiltration of cytotoxic CD8-positive T-cells in Brca1-deficient mammary tumors in vivo. We have also determined that IRX4204 modulates lipid metabolism in breast cancer cell lines in vitro. It is known that lipid-derived antigens can stimulate T-cell activity. Our findings suggest that RXR agonists may alter lipid antigen production to activate an anti-tumor response. Additional immune and lipidomic studies are on-going. This work was supported by NCI-PREVENT grant (to PB and AM HHSN26100008) and CFP Foundation (Odyssey Fellowship to CM). Citation Format: Cassandra Moyer, Darian Coleman, Jamal Hill, Lana A. Vornik, Michelle Savage, Martin Sanders, Sei Shizuko, Altaf Mohammed, Powel Brown, Abhijit Mazumdar. The RXR agonist, IRX4204, delays the formation of Brca1 mutant mammary tumors via modulation of the anti-tumor immune response [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS07-07.

Combined inhibition of EZH2 and ATM is synthetic lethal in BRCA1-deficient breast cancer

BackgroundThe majority of BRCA1-mutant breast cancers are characterized by a triple-negative phenotype and a basal-like molecular subtype, associated with aggressive clinical behavior. Current treatment options are limited, highlighting the need for the development of novel targeted therapies for this tumor subtype.MethodsOur group previously showed that EZH2 is functionally relevant in BRCA1-deficient breast tumors and blocking EZH2 enzymatic activity could be a potent treatment strategy. To validate the role of EZH2 as a therapeutic target and to identify new synergistic drug combinations, we performed a high-throughput drug combination screen in various cell lines derived from BRCA1-deficient and -proficient mouse mammary tumors.ResultsWe identified the combined inhibition of EZH2 and the proximal DNA damage response kinase ATM as a novel synthetic lethality-based therapy for the treatment of BRCA1-deficient breast tumors. We show that the combined treatment with the EZH2 inhibitor GSK126 and the ATM inhibitor AZD1390 led to reduced colony formation, increased genotoxic stress, and apoptosis-mediated cell death in BRCA1-deficient mammary tumor cells in vitro. These findings were corroborated by in vivo experiments showing that simultaneous inhibition of EZH2 and ATM significantly increased anti-tumor activity in mice bearing BRCA1-deficient mammary tumors.ConclusionTaken together, we identified a synthetic lethal interaction between EZH2 and ATM and propose this synergistic interaction as a novel molecular combination for the treatment of BRCA1-mutant breast cancer.

Loss of nuclear DNA ligase III reverts PARP inhibitor resistance in BRCA1/53BP1 double-deficient cells by exposing ssDNA gaps

Inhibitors of poly(ADP-ribose) (PAR) polymerase (PARPi) have entered the clinic for the treatment of homologous recombination (HR)-deficient cancers. Despite the success of this approach, preclinical and clinical research with PARPi has revealed multiple resistance mechanisms, highlighting the need for identification of novel functional biomarkers and combination treatment strategies. Functional genetic screens performed in cells and organoids that acquired resistance to PARPi by loss of 53BP1 identified loss of LIG3 as an enhancer of PARPi toxicity in BRCA1-deficient cells. Enhancement of PARPi toxicity by LIG3 depletion is dependent on BRCA1 deficiency but independent of the loss of 53BP1 pathway. Mechanistically, we show that LIG3 loss promotes formation of MRE11-mediated post-replicative ssDNA gaps in BRCA1-deficient and BRCA1/53BP1 double-deficient cells exposed to PARPi, leading to an accumulation of chromosomal abnormalities. LIG3 depletion also enhances efficacy of PARPi against BRCA1-deficient mammary tumors in mice, suggesting LIG3 as a potential therapeutic target.

PDGFR\u03b2 is an essential therapeutic target for BRCA1-deficient mammary tumors

BackgroundBasal-like breast cancers (BLBCs) are a leading cause of cancer death due to their capacity to metastasize and lack of effective therapies. More than half of BLBCs have a dysfunctional BRCA1. Although most BRCA1-deficient cancers respond to DNA-damaging agents, resistance and tumor recurrence remain a challenge to survival outcomes for BLBC patients. Additional therapies targeting the pathways aberrantly activated by BRCA1 deficiency are urgently needed.MethodsMost BRCA1-deficient BLBCs carry a dysfunctional INK4-RB pathway. Thus, we created genetically engineered mice with Brca1 loss and deletion of p16INK4A, or separately p18INK4C, to model the deficient INK4-RB signaling in human BLBC. By using these mutant mice and human BRCA1-deficient and proficient breast cancer tissues and cells, we tested if there exists a druggable target in BRCA1-deficient breast cancers.ResultsHeterozygous germline or epithelium-specific deletion of Brca1 in p18INK4C- or p16INK4A-deficient mice activated Pdgfrβ signaling, induced epithelial-to-mesenchymal transition, and led to BLBCs. Confirming this role, targeted deletion of Pdgfrβ in Brca1-deficient tumor cells promoted cell death, induced mesenchymal-to-epithelial transition, and suppressed tumorigenesis. Importantly, we also found that pharmaceutical inhibition of Pdgfrβ and its downstream target Pkcα suppressed Brca1-deficient tumor initiation and progression and effectively killed BRCA1-deficient cancer cells.ConclusionsOur work offers the first genetic and biochemical evidence that PDGFRβ-PKCα signaling is repressed by BRCA1, which establishes PDGFRβ-PKCα signaling as a therapeutic target for BRCA1-deficient breast cancers.

Dissecting the heterogeneity and tumorigenesis of BRCA1 deficient mammary tumors via single cell RNA sequencing.

Background: BRCA1 plays critical roles in mammary gland development and mammary tumorigenesis. And loss of BRCA1 induces mammary tumors in a stochastic manner. These tumors present great heterogeneity at both intertumor and intratumor levels.Methods: To comprehensively elucidate the heterogeneity of BRCA1 deficient mammary tumors and the underlying mechanisms for tumor initiation and progression, we conducted bulk and single cell RNA sequencing (scRNA-seq) on both mammary gland cells and mammary tumor cells isolated from Brca1 knockout mice.Results: We found the BRCA1 deficient tumors could be classified into four subtypes with distinct molecular features and different sensitivities to anti-cancer drugs at the intertumor level. Whereas within the tumors, heterogeneous subgroups were classified mainly due to the different activities of cell proliferation, DNA damage response/repair and epithelial-to-mesenchymal transition (EMT). Besides, we reconstructed the BRCA1 related mammary tumorigenesis to uncover the transcriptomes alterations during this process via pseudo-temporal analysis of the scRNA-seq data. Furthermore, from candidate markers for BRCA1 mutant tumors, we discovered and validated one oncogene Mrc2, whose loss could reduce mammary tumor growth in vitro and in vivo.Conclusion: Our study provides a useful resource for better understanding of mammary tumorigenesis induced by BRCA1 deficiency.

Abstract IA18: Genetic dissection of breast cancer development and therapy resistance in mouse models

Abstract Genetically engineered mouse models (GEMMs) of human cancer not only permit us to gain a detailed insight into the specific genetic changes that drive tumor development and metastasis but also provide powerful tools to study the mechanisms underlying drug response and acquired resistance. Once these processes are understood in sufficient detail, it may be possible to design combination therapies that not only cause complete remissions but also eliminate remnant cells that elicit recurrent disease. We have developed GEMMs of E-cadherin mutated lobular breast cancer. These mice develop mammary tumors that closely resemble the lobular morphology and the metastatic spectrum of the cognate tumors in humans. Using Sleeping Beauty (SB) transposon-based insertional mutagenesis in conditional E-cadherin mutant mice, we have found that heterozygous loss of MYH9 or overexpression of hyperactive truncated forms of MYPT1/2 and ASPP2 reduces actomyosin contractility and thereby promotes malignant transformation of E-cadherin-deficient mammary epithelial cells, resulting in ILC formation in mice. This work highlights actomyosin hypercontractility induced by E-cadherin loss as a critical barrier to ILC development. We have also developed several GEMMs for somatic modeling of BRCA1-mutated triple-negative breast cancer using intraductal injection of lentiviral vectors for stable overexpression of exogenous genes and CAS9-mediated disruption or APOBEC-CAS9n-UGI (BE3)-mediated base editing of endogenous genes. We have used these GEMMs to validate RB, PTEN, PIK3CA, MYC, and MCL1 as bona fide driver genes in BRCA1-associated breast cancer. Moreover, MCL1 inhibition potentiated the in vivo efficacy of the PARP inhibitor (PARPi) olaparib, underscoring the therapeutic potential of this combination for treatment of BRCA1-associated cancer patients with poor response to PARPi monotherapy. The BRCA1-deficient mammary tumors from our mouse models are characterized by genomic instability and hypersensitivity to DNA-damaging agents, including platinum drugs and PARP inhibitors. Nevertheless, none of these drugs are curative: tumors grow back after drug treatment and eventually become resistant. We found that PARPi resistance of BRCA1-deficient GEMM tumors can be induced by several mechanisms, including activation of drug efflux transporters, type of BRCA1 mutation, and loss of components of the 53BP1-RIF1-shieldin and CST complexes that govern end-protection of DNA double-strand breaks. Citation Format: Jos Jonkers. Genetic dissection of breast cancer development and therapy resistance in mouse models [abstract]. In: Proceedings of the AACR Special Conference on the Evolving Landscape of Cancer Modeling; 2020 Mar 2-5; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2020;80(11 Suppl):Abstract nr IA18.

EZH2 Is Overexpressed in BRCA1-like Breast Tumors and Predictive for Sensitivity to High-Dose Platinum-Based Chemotherapy.

BRCA1-deficient breast cancers carry a specific DNA copy-number signature ("BRCA1-like") and are hypersensitive to DNA double-strand break (DSB) inducing compounds. Here, we explored whether (i) EZH2 is overexpressed in human BRCA1-deficient breast tumors and might predict sensitivity to DSB-inducing drugs; (ii) EZH2 inhibition potentiates cisplatin efficacy in Brca1-deficient murine mammary tumors. EZH2 expression was analyzed in 497 breast cancers using IHC or RNA sequencing. We classified 370 tumors by copy-number profiles as BRCA1-like or non-BRCA1-like and examined its association with EZH2 expression. Additionally, we assessed BRCA1 loss through mutation or promoter methylation status and investigated the predictive value of EZH2 expression in a study population of breast cancer patients treated with adjuvant high-dose platinum-based chemotherapy compared with standard anthracycline-based chemotherapy. To explore whether EZH2 inhibition by GSK126 enhances sensitivity to platinum drugs in EZH2-overexpressing breast cancers we used a Brca1-deficient mouse model. The highest EZH2 expression was found in BRCA1-associated tumors harboring a BRCA1 mutation, BRCA1-promoter methylation or were classified as BRCA1 like. We observed a greater benefit from high-dose platinum-based chemotherapy in BRCA1-like and non-BRCA1-like patients with high EZH2 expression. Combined treatment with the EZH2 inhibitor GSK126 and cisplatin decreased cell proliferation and improved survival in Brca1-deficient mice in comparison with single agents. Our findings demonstrate that EZH2 is expressed at significantly higher levels in BRCA1-deficient breast cancers. EZH2 overexpression can identify patients with breast cancer who benefit significantly from intensified DSB-inducing platinum-based chemotherapy independent of BRCA1-like status. EZH2 inhibition improves the antitumor effect of platinum drugs in Brca1-deficient breast tumors in vivo.

Comparative oncogenomics identifies combinations of driver genes and drug targets in BRCA1-mutated breast cancer

BRCA1-mutated breast cancer is primarily driven by DNA copy-number alterations (CNAs) containing large numbers of candidate driver genes. Validation of these candidates requires novel approaches for high-throughput in vivo perturbation of gene function. Here we develop genetically engineered mouse models (GEMMs) of BRCA1-deficient breast cancer that permit rapid introduction of putative drivers by either retargeting of GEMM-derived embryonic stem cells, lentivirus-mediated somatic overexpression or in situ CRISPR/Cas9-mediated gene disruption. We use these approaches to validate Myc, Met, Pten and Rb1 as bona fide drivers in BRCA1-associated mammary tumorigenesis. Iterative mouse modeling and comparative oncogenomics analysis show that MYC-overexpression strongly reshapes the CNA landscape of BRCA1-deficient mammary tumors and identify MCL1 as a collaborating driver in these tumors. Moreover, MCL1 inhibition potentiates the in vivo efficacy of PARP inhibition (PARPi), underscoring the therapeutic potential of this combination for treatment of BRCA1-mutated cancer patients with poor response to PARPi monotherapy.

Abstract B21: Improved efficacy of mitochondrial disrupting agents upon inhibition of autophagy in a mouse model of BRCA1-deficient breast cancer

Abstract Breast cancer is a heterogeneous disease, and stratification of patients is fundamental to the success of treatment modalities. Breast tumors deficient in BRCA1 are mostly associated with basal-like breast cancers, and targeted therapeutics for this disease subtype are still lacking. In order to address whether autophagy inhibition will be effective in BRCA1-deficient mammary tumors, we generated mice with conditional deletion of an essential autophagy gene FIP200 along with Brca1 and Trp53, through utilization of the K14-Cre transgene. We found that FIP200 deletion suppressed tumorigenesis in the BRCA1-deficient model when compared to wild-type and heterozygous FIP200 controls. However, in contrast to previous studies in the MMTV-PyMT model, tumor growth and the distribution of histologic subtypes were not affected by loss of FIP200. Interestingly, loss of FIP200 decreased mitochondrial mass and oxidative respiratory capacity of these tumor cells, along with a decrease in the phosphorylation of mTOR substrates and transcript levels of genes involved in mitochondrial biogenesis. Importantly, we observed an increased sensitivity to mitochondrial disrupting agents upon loss of FIP200. Consequently, our data showed that combination of an autophagy inhibitor, Spautin-1, along with a mitochondrial complex I inhibitor, metformin, was more effective in limiting oxidative respiratory capacity, colony-forming ability, and tumor growth. Altogether, our results indicate that inhibition of autophagy can increase the benefits of metformin treatment in BRCA1-deficient breast cancers. Citation Format: Syn Kok Yeo, Ritama Paul, Michael Haas, Chenran Wang, Jun-Lin Guan. Improved efficacy of mitochondrial disrupting agents upon inhibition of autophagy in a mouse model of BRCA1-deficient breast cancer [abstract]. In: Proceedings of the AACR Special Conference: Advances in Breast Cancer Research; 2017 Oct 7-10; Hollywood, CA. Philadelphia (PA): AACR; Mol Cancer Res 2018;16(8_Suppl):Abstract nr B21.

Estrogen promotes estrogen receptor negative BRCA1-deficient tumor initiation and progression

BackgroundEstrogen promotes breast cancer development and progression mainly through estrogen receptor (ER). However, blockage of estrogen production or action prevents development of and suppresses progression of ER-negative breast cancers. How estrogen promotes ER-negative breast cancer development and progression is poorly understood. We previously discovered that deletion of cell cycle inhibitors p16Ink4a (p16) or p18Ink4c (p18) is required for development of Brca1-deficient basal-like mammary tumors, and that mice lacking p18 develop luminal-type mammary tumors.MethodsA genetic model system with three mouse strains, one that develops ER-positive mammary tumors (p18 single deletion) and the others that develop ER-negative tumors (p16;Brca1 and p18;Brca1 compound deletion), human BRCA1 mutant breast cancer patient-derived xenografts, and human BRCA1-deficient and BRCA1-proficient breast cancer cells were used to determine the role of estrogen in activating epithelial-mesenchymal transition (EMT), stimulating cell proliferation, and promoting ER-negative mammary tumor initiation and metastasis.ResultsEstrogen stimulated the proliferation and tumor-initiating potential of both ER-positive Brca1-proficient and ER-negative Brca1-deficient tumor cells. Estrogen activated EMT in a subset of Brca1-deficient mammary tumor cells that maintained epithelial features, and enhanced the number of cancer stem cells, promoting tumor progression and metastasis. Estrogen activated EMT independent of ER in Brca1-deficient, but not Brca1-proficient, tumor cells. Estrogen activated the AKT pathway in BRCA1-deficient tumor cells independent of ER, and pharmaceutical inhibition of AKT activity suppressed EMT and cell proliferation preventing BRCA1 deficient tumor progression.ConclusionsThis study reveals for the first time that estrogen promotes BRCA1-deficient tumor initiation and progression by stimulation of cell proliferation and activation of EMT, which are dependent on AKT activation and independent of ER.

Improved efficacy of mitochondrial disrupting agents upon inhibition of autophagy in a mouse model of BRCA1-deficient breast cancer

ABSTRACTBreast cancer is a heterogeneous disease, and stratification of patients is fundamental to the success of treatment modalities. Breast tumors deficient in BRCA1 are mostly associated with basal-like breast cancers and targeted therapeutics for this disease subtype are still lacking. In order to address whether macroautophagy/autophagy inhibition will be effective in BRCA1-deficient mammary tumors, we generated mice with conditional deletion of an essential autophagy gene, Rb1cc1, along with Brca1 and Trp53, through utilization of the K14-Cre transgene. We found that Rb1cc1 deletion suppressed tumorigenesis in the BRCA1-deficient model when compared to wild type and heterozygous Rb1cc1 controls. However, in contrast to previous studies in the mouse mammary tumor virus (MMTV)-polyoma middle T antigen (PyMT) model, tumor growth and the distribution of histological subtypes were not affected by loss of RB1CC1. Interestingly, loss of RB1CC1 decreased mitochondrial mass and oxidative respiratory capacity of these tumor cells, along with a decrease in the phosphorylation of MTOR substrates and transcript levels of genes involved in mitochondrial biogenesis. Importantly, we observed an increased sensitivity to mitochondrial disrupting agents upon loss of RB1CC1. Consequently, our data showed that combination of an autophagy inhibitor, spautin-1, along with a mitochondrial complex I inhibitor, metformin, was more effective in limiting oxidative respiratory capacity, colony-forming ability and tumor growth. Altogether, our results indicate that inhibition of autophagy can increase the benefits of metformin treatment in BRCA1-deficient breast cancers.

Inhibition of AKT suppresses the initiation and progression of BRCA1-associated mammary tumors.

Despite the high incidence of BRCA1-mutant breast cancer, few substantial improvements in preventing or treating such cancers have been made. Using a Brca1-mutant mouse model, we examined the contribution of AKT to the incidence and growth of Brca1-mutated mammary tumors. A haploinsufficiency of Akt1 in Brca1-mutant mouse model significantly decreased mammary tumor formation from 54% in Brca1co/coMMTV-Cre mice to 22% in Brca1 co/coMMTV-Cre Akt1+/- mice. Notably, treatment of tumor-bearing Brca1-mutant mice with the AKT-inhibitor, MK-2206, yielded partial response or stable disease up to 91% of mice in maximum response. MK-2206 treatment also significantly reduced tumor volume and delayed recurrence in allograft and adjuvant studies, respectively. A correlation analysis of MK-2206 responses with gene expression profiles of tumors at baseline identified seven genes that were differentially expressed between tumors that did and did not respond to MK-2206 treatment. Our findings enhance our understanding of the involvement of AKT signaling in BRCA1-deficient mammary tumors and provide preclinical evidence that targeted AKT inhibition is a potential strategy for the prevention and therapeutic management of BRCA1-associated breast cancer.

Inhibition of Estrogen Signaling Reduces the Incidence of BRCA1-associated Mammary Tumor Formation.

BRCA1-deficient breast cancer is a very well-known hereditary cancer. However, except for resection of normal mammary glands and ovaries, there is no acceptable measure for proactively preventing tumor development. Importantly, inherited BRCA1 mutations are closely associated with tumors in hormone-responsive tissues. Here, we examined the effects of estrogen on the accumulation of genetic instabilities upon loss of BRCA1, and assessed the contribution of estrogen signaling to the incidence and progression of Brca1-mutated mammary tumors. Our in vitro studies showed that treatment of BRCA1-depleted breast cancer cells with estrogen induced proliferation. Additionally, estrogen reduced the ability of these BRCA1-knockdown cells to sense radiation-induced DNA damage and also facilitated G1/S progression. Moreover, long-term treatment of Brca1-mutant (Brca1co/coMMTV-Cre) mice with the selective estrogen receptor (ER)-α degrader, fulvestrant, decreased the tumor formation rate from 64% to 36%, and also significantly reduced mammary gland density in non-tumor-bearing mice. However, in vivo experiments showed that fulvestrant treatment did not alter the progression of ER-positive Brca1-mutant tumors, which were frequently identified in the aged population and showed less aggressive tendencies. These findings enhance our understanding of how ER-α signaling contributes to BRCA1-deficient mammary tumors and provide evidence suggesting that targeted inhibition of ER-α signaling may be useful for the prevention of BRCA1-mutated breast cancer.

Selected Alkylating Agents Can Overcome Drug Tolerance of G0-like Tumor Cells and Eradicate BRCA1-Deficient Mammary Tumors in Mice.

Purpose: We aimed to characterize and target drug-tolerant BRCA1-deficient tumor cells that cause residual disease and subsequent tumor relapse.Experimental Design: We studied responses to various mono- and bifunctional alkylating agents in a genetically engineered mouse model for BRCA1/p53-mutant breast cancer. Because of the large intragenic deletion of the Brca1 gene, no restoration of BRCA1 function is possible, and therefore, no BRCA1-dependent acquired resistance occurs. To characterize the cell-cycle stage from which Brca1-/-;p53-/- mammary tumors arise after cisplatin treatment, we introduced the fluorescent ubiquitination-based cell-cycle indicator (FUCCI) construct into the tumor cells.Results: Despite repeated sensitivity to the MTD of platinum drugs, the Brca1-mutated mammary tumors are not eradicated, not even by a frequent dosing schedule. We show that relapse comes from single-nucleated cells delaying entry into the S-phase. Such slowly cycling cells, which are present within the drug-naïve tumors, are enriched in tumor remnants. Using the FUCCI construct, we identified nonfluorescent G0-like cells as the population most tolerant to platinum drugs. Intriguingly, these cells are more sensitive to the DNA-crosslinking agent nimustine, resulting in an increased number of multinucleated cells that lack clonogenicity. This is consistent with our in vivo finding that the nimustine MTD, among several alkylating agents, is the most effective in eradicating Brca1-mutated mouse mammary tumors.Conclusions: Our data show that targeting G0-like cells is crucial for the eradication of BRCA1/p53-deficient tumor cells. This can be achieved with selected alkylating agents such as nimustine. Clin Cancer Res; 23(22); 7020-33. ©2017 AACR.

Abstract IA07: Genetic determinants of tumor development, therapy response and resistance in mouse models of BRCA-deficient breast cancer

Abstract Heterozygous germline mutations in BRCA1 or BRCA2 strongly predispose to development of breast and ovarian cancer (as well as other cancer types) via loss of the remaining wildtype allele. BRCA1/2-deficient cancers are defective in DNA double-strand break (DSB) repair via homologous recombination (HR) and therefore hypersensitive to DNA-damaging agents, including platinum drugs and poly(ADP-ribose) polymerase (PARP) inhibitors. However, these treatments do not result in tumor eradication and eventually resistance develops. To maximize therapeutic efficacy of these drugs and achieve durable remissions, it is important to unravel the mechanisms by which these tumors acquire resistance to platinum drugs and PARP inhibitors, in order to develop combination therapies that prevent development of resistance or re-sensitize resistant tumors. To study therapy response and resistance in a realistic in vivo setting, we have established several genetically engineered mouse models (GEMMs) and patient-derived tumor xenograft (PDX) models for BRCA-deficient breast cancer. These mice develop mammary tumors that are characterized by genomic instability and hypersensitivity to DNA-damaging agents, including platinum drugs and PARP inhibitors. Using cross-species oncogenomics and reverse genetics, we have identified several cancer genes including p53, MYC and RB as critical drivers in BRCA1-associated breast cancer. In addition, we have used these mammary tumor models for preclinical evaluation of therapy response and elucidation of mechanisms of acquired drug resistance. Using functional genetic screens, reverse genetics and genomic analysis of therapy-resistant tumors, we found that therapy response and resistance of BRCA1-deficient mammary tumors to cisplatin and the clinical PARP inhibitor olaparib is affected by several factors, including drug efflux transporter activity, type of BRCA1 founder mutation and restoration of HR repair via loss of 53BP1 or REV7. Also BRCA1 re-activation via genetic or epigenetic mechanisms contributes to therapy resistance in PDX models of BRCA1-deficient breast cancer. Importantly, pharmacokinetic or HR-related mechanisms underlie olaparib-resistance in only a fraction of BRCA1/2-deficient mammary tumors, indicating the existence of additional, unknown mechanisms. Citation Format: Jos Jonkers. Genetic determinants of tumor development, therapy response and resistance in mouse models of BRCA-deficient breast cancer [abstract]. In: Proceedings of the AACR Special Conference on DNA Repair: Tumor Development and Therapeutic Response; 2016 Nov 2-5; Montreal, QC, Canada. Philadelphia (PA): AACR; Mol Cancer Res 2017;15(4_Suppl):Abstract nr IA07.

Abstract A24: Characterization of a novel p53; BRCA1-deficient claudin-low mammary tumor cell line

Abstract Triple-negative breast cancer (TNBC) is a highly diverse and typically aggressive group of cancers. Consequently, elucidating the biology of the various subtypes is necessary for more effective diagnosis and better targeted treatment of this disease. Recently, a novel claudin-low TNBC subtype has been identified. Patients with this subtype have a very poor prognosis, specifically young African-American females. The major focus of our study is to characterize a novel claudin-low cell line cultured from a “humanized” mouse model that is p53 and BRCA1 deficient (K14TRT cell line). Procedures: This cell line has been maintained for more than 40 months and for over 25 subcultures, and was characterized by microscopy, histopathology, genotyping, cytogenetics, and bioinformatics based techniques. Results: Histological analysis supports that the primary K14TRT tumors were highly penetrant, with some having spindloid morphology while others were undifferentiated adenocarcinoams, with large areas of necrosis. As compared to normal mammary cells, K14TRT cells possessed larger variable shaped nuclei with a higher nuclear-to-cytoplasmic ratio. The tumor cells also demonstrated a highly aneuploidy karyotype. When isolated via single cell suspension and subsequently passaged, the K14TRT cells were able to form colonies in soft agar. Tumorigenicity was additionally supported by induction of solid tumors following serial injections of the K14TRT cells into the flanks of nude mice. The primary K14TRT tumor overwhelmingly possessed a basal-like breast tumor profile, whereas the transplanted and serially passaged K14TRT cells diverged to a claudin-low profile, as demonstrated by microarray analysis. These serially passaged cells were highly proliferative, with low to absent expression of tight junction proteins (claudin 3, 4, & 7 and E-cadherin), and typically negative for estrogen receptors (ER-), progesterone receptors (PR-), and HER2/neu (HER2-). Since this cell line did demonstrate an ability to form new tumors in recipient mice we speculated they could possess tumor stem cell properties. There competence for self-renewal and capacity to reestablish tumor heterogeneity was supported by fluorescence-activating cell sorting (FACS) analysis of the primary BRCA1/p53 deficient mammary tumor cells in which two populations were identified. Cells that possess low levels of epithelial marker CD24 and high levels of the B1 integrin marker CD29 have been shown to regenerate entire mammary glands indicative of mammary STEM cells. Strikingly, FACS analysis of the K14TRT primary cell line identified two cell populations, one which expressed markers that were CD24high/CD29high (differentiated) and the other population CD24low/CD29high (undifferentiated stem cell-like profile). The sorted undifferentiated mammosphere population when analyzed by microarray analysis clustered with transplanted and serially passaged K14TRT cells identified as being claudin-low, whereas the differentiated sorted population clustered with the primary basal-like cells. Conclusions: Our current data has demonstrated that the K14TRT cell line is a good candidate for a “humanized” TNBC claudin-low cell line with stem cell properties, and could play a significant role in devising better diagnostic tools and chemotherapeutic strategies for this aggressive tumor subtype. Citation Format: Stephanie T. Dance-Barnes, Ebone Evans, Shanderys Steward, Xiaping He, Jerry Usary, Charles Perou. Characterization of a novel p53; BRCA1-deficient claudin-low mammary tumor cell line. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr A24.

Abstract 736: Combining PD1- and CTLA4-inhibiting antibodies with cisplatin or PARP inhibition in an attempt to eradicate BRCA1-deficient mouse mammary tumors

Abstract Solid tumors are usually not eradicated by conventional chemotherapy resulting in disease relapse and mortality. An intriguing example is BRCA-associated breast cancer. A genetically engineered mouse model for BRCA1-associated breast cancer (K14Cre;Brca1F/F;p53F/F), which highly resembles disease in human patients, can be used to study therapy escape and residual disease in BRCA-deficient tumors. Due to the BRCA inactivation, the tumors that arise in this model lack homologous recombination-directed DNA repair, an Achilles heel that has provided a therapeutic opportunity to eradicate tumors with DNA damage-inducing agents. Despite repeated sensitivity some residual cancer cells escape the deadly effect of anticancer therapy and lead to disease relapse. A special histopathological feature of BRCA1-mutated tumors in both humans and mice is the presence of infiltrating lymphocytes. This may be explained by the high frequency of genomic alterations of BRCA1-mutated tumors resulting in the generation of many neo-antigens. Our hypothesis is that blocking inhibitory T-cell signaling, using antibodies directed against CTLA4 and PD1, may increase the activity of the T-cell compartment towards a diverse pool of antigens, and successfully eliminate residual tumor cells in combination with DNA damage-inducing drugs. In particular, we will present data of combining CTLA4- and PD1- targeting antibodies with PARP inhibition or cisplatin to eliminate residual tumor cells in this mouse model for BRCA1-mutated breast cancer. Citation Format: Sohvi Blatter, Charlotte Guyader, Aslı Küçükosmanoğlu, Stephan Freriks, Karin de Visser, Piet Borst, Sven Rottenberg. Combining PD1- and CTLA4-inhibiting antibodies with cisplatin or PARP inhibition in an attempt to eradicate BRCA1-deficient mouse mammary tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 736. doi:10.1158/1538-7445.AM2015-736

Abstract IA9: Studying therapy response and resistance in mouse models of human breast cancer

Abstract Mouse models of human cancer not only permit us to gain a detailed insight into the specific genetic changes that drive tumor development and metastasis but also provide powerful tools to study the mechanisms underlying drug response and acquired resistance. Once these processes are understood in sufficient detail it may be possible to design combination therapies that not only cause complete remissions but also eliminate remnant cells that might elicit recurrent disease. We have developed genetically engineered mouse models (GEMMs) of E-cadherin mutated lobular breast cancer. These mice develop mammary tumors that closely resemble the lobular morphology and the metastatic spectrum of the cognate tumors in humans. We have used Sleeping Beauty (SB) based insertional mutagenesis (IM) screens in conditional E-cadherin mutant mice to identify cancer genes that collaborate with E-cadherin loss in mammary tumorigenesis. Approximately 50% of all tumors carry activating SB insertions in Fgfr2. These tumors are highly sensitive to FGFR inhibitors but eventually become resistant. We are currently analyzing SB insertions in FGFRi-resistant tumors to identify genes that are causal to the resistance phenotype. We have also established GEMMs and patient-derived xenograft (PDX) models for BRCA1-deficient triple-negative breast cancer. These mice develop mammary tumors that are characterized by genomic instability and hypersensitivity to DNA-damaging agents, including platinum drugs and PARP inhibitors. Nevertheless, none of these drugs are curative: tumors grow back after drug treatment and eventually become resistant. We have used DNA methylation analysis and focused or whole-exome sequencing to identify resistance mechanisms in matched pairs of therapy-sensitive and –resistant BRCA1-deficient mammary tumors from our GEMMs and PDX models. We found that resistance of BRCA1-deficient GEMM tumors to the PARP inhibitor olaparib can be induced by several mechanisms, including activation of P-glycoprotein drug efflux transporters, type of BRCA1 founder mutation and 53BP1 loss. Therapy resistance of BRCA1-mutated PDX tumors is caused by additional intragenic deletions that lead to restoration of the open reading frame. Therapy resistance of BRCA1-methylated PDX tumors is driven by loss of BRCA1 promoter methylation or by a novel resistance mechanism involving de novo BRCA1 gene fusions created by intrachromosomal genomic rearrangements. Citation Format: Jos Jonkers. Studying therapy response and resistance in mouse models of human breast cancer. [abstract]. In: Proceedings of the AACR Special Conference: The Translational Impact of Model Organisms in Cancer; Nov 5-8, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(11 Suppl):Abstract nr IA9.

Abstract 3141: Epithelial-to-mesenchymal transition and therapy resistance in BRCA1-associated breast cancer

Abstract Despite progress in the treatment of breast cancer, patients with triple-negative breast cancer (TNBC) have limited means for therapy. Since TNBCs do not express HER2, estrogen and progesterone receptors, TNBC patients cannot be treated with Herceptin or endocrine therapy. TNBCs frequently harbor defects in homologous recombination (HR) due to (epi)genetic inactivation of BRCA1, making them sensitive to the clinical PARP1 inhibitor olaparib. However, long-term treatment of BRCA1-deficient mouse mammary tumors with olaparib results in drug resistance. The growing awareness that epithelial-to-mesenchymal transition (EMT) may influence therapy response prompted us to investigate whether olaparib sensitivity of BRCA1-deficient TNBCs is affected by MET, a known EMT inducer that is over-expressed in TNBC and associated with poor survival. For this purpose we have developed a breast cancer GEMM-ESC pipeline, using female GEMM-ESCs derived from our well-validated K14cre;Brca1F/F;Trp53F/F (KB1P) mouse model, of which females spontaneously develop BRCA1-deficient TNBC due to cre-mediated deletion of Brca1 and Trp53 in epithelial tissues. The MET proto-oncogene, was introduced in the Col1a1 locus and resulting KB1P-MET ESCs were injected into blastocysts and good quality chimeras were monitored for mammary tumor formation. Our preliminary data show that BRCA1-deficient mouse mammary tumors with engineered overexpression of MET show accelerated growth, undergo EMT and show intrinsic resistance to olaparib. The underlying mechanisms are, however, unclear. Currently we are studying the impact of EMT-regulators on BRCA1-associated mammary tumorigenesis and assess the contribution of EMT-regulators to olaparib resistance in genetically engineered mouse models of BRCA1-associated breast cancer. Together, these studies will provide novel insights into the roles of EMT regulators in BRCA1-associated breast tumor development, metastasis and therapy resistance. Citation Format: Martine Van Miltenburg, Jos Jonkers. Epithelial-to-mesenchymal transition and therapy resistance in BRCA1-associated breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3141. doi:10.1158/1538-7445.AM2014-3141

Tissue-specific tumor suppression by BRCA1

Germ-line mutations in the tumor suppressor gene BRCA1 increase the lifetime risk for breast cancer and ovarian cancer by up to ∼80% and ∼50%, respectively. Population-based studies also support a sex- and tissue-specific tumor suppressor function of BRCA1, but the mechanisms of this specificity are not fully understood. Somatic loss of the normal functioning allele in BRCA1 carriers is common in cancer development and additional somatic events, including mutations of PTEN and TP53, occur at high frequencies (detailed below). Research over the last two decades has described the role of the ubiquitously expressed BRCA1 in maintaining genomic stability through its function in DNA repair and cell cycle checkpoint control, in addition to its activity as a transcriptional regulator. In PNAS, Gorrini et al. shed light on the specificity of BRCA1’s tumor suppressor function in breast tissue. The authors identify an estrogen-induced pathway that promotes the survival of BRCA1-deficient mammary epithelial cells (MECs), as well as Brca1-deficient mammary tumor cells from oxidative stress-induced cell death (1).