Research Article30 October 2018Open Access Source DataTransparent process A RAD51 assay feasible in routine tumor samples calls PARP inhibitor response beyond BRCA mutation Marta Castroviejo-Bermejo Marta Castroviejo-Bermejo Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Cristina Cruz Cristina Cruz Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain High Risk and Familial Cancer Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Alba Llop-Guevara Alba Llop-Guevara Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Sara Gutiérrez-Enríquez Sara Gutiérrez-Enríquez Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Mandy Ducy Mandy Ducy Genome Stability Laboratory, CHU de Québec Research Center, Québec City, QC, Canada Department of Molecular Biology, Medical Biochemistry and Pathology, Laval University Cancer Research Center, Québec City, QC, Canada CHU de Quebec - Université Laval Research Center, Genomics Center, CHUL, Québec City, QC, Canada Search for more papers by this author Yasir Hussein Ibrahim Yasir Hussein Ibrahim Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Albert Gris-Oliver Albert Gris-Oliver Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Benedetta Pellegrino Benedetta Pellegrino Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Department of Medical Oncology, University Hospital of Parma, Parma, Italy Search for more papers by this author Alejandra Bruna Alejandra Bruna Cancer Research UK Cambridge Institute and Department of Oncology, Li Ka Shing Centre, University of Cambridge, Cambridge, UK Search for more papers by this author Marta Guzmán Marta Guzmán Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Olga Rodríguez Olga Rodríguez Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Judit Grueso Judit Grueso Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Sandra Bonache Sandra Bonache Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Alejandro Moles-Fernández Alejandro Moles-Fernández Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Guillermo Villacampa Guillermo Villacampa Oncology Data Science (OdysSey Group), Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Cristina Viaplana Cristina Viaplana Oncology Data Science (OdysSey Group), Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Patricia Gómez Patricia Gómez Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Breast Cancer and Melanoma Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Maria Vidal Maria Vidal Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Breast Cancer and Melanoma Group, Vall d'Hebron Institute of Oncology, Barcelona, SpainCorrection added on 16 January 2019, after first online publication: the author name "Marc Vidal" was corrected to "Maria Vidal" Search for more papers by this author Vicente Peg Vicente Peg Pathology Department, Vall d'Hebron University Hospital, Barcelona, Spain CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Search for more papers by this author Xavier Serres-Créixams Xavier Serres-Créixams Department of Radiology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Graham Dellaire Graham Dellaire Department of Pathology, Dalhousie University, Halifax, NS, Canada Search for more papers by this author Jacques Simard Jacques Simard CHU de Quebec - Université Laval Research Center, Genomics Center, CHUL, Québec City, QC, Canada Search for more papers by this author Paolo Nuciforo Paolo Nuciforo CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Molecular Oncology Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Isabel T Rubio Isabel T Rubio CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Breast Surgical Unit, Breast Cancer Center, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Rodrigo Dienstmann Rodrigo Dienstmann Oncology Data Science (OdysSey Group), Vall d'Hebron Institute of Oncology, Barcelona, SpainCorrection added on 12 July 2019, after first online publication: the author name "Rodrigo Dientsmann" was corrected to "Rodrigo Dienstmann" Search for more papers by this author J Carl Barrett J Carl Barrett AstraZeneca, Waltham, MA, USA Search for more papers by this author Carlos Caldas Carlos Caldas Cancer Research UK Cambridge Institute and Department of Oncology, Li Ka Shing Centre, University of Cambridge, Cambridge, UK Breast Cancer Programme, Cancer Research UK (CRUK) Cambridge Cancer Centre, Cambridge, UK Search for more papers by this author José Baselga José Baselga Human Oncology and Pathogenesis Program (HOPP), Memorial Sloan Kettering Cancer Center, New York, NY, USA Department of Medicine, Memorial Sloan Kettering Cancer Center, New York, NY, USA Search for more papers by this author Cristina Saura Cristina Saura Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Breast Cancer and Melanoma Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Javier Cortés Javier Cortés CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Department of Oncology, Ramón y Cajal University Hospital, Madrid, Spain Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Olivier Déas Olivier Déas XenTech, Evry, France Search for more papers by this author Jos Jonkers Jos Jonkers orcid.org/0000-0002-9264-9792 Division of Molecular Pathology and Cancer Genomics, The Netherlands Cancer Institute, Amsterdam, The Netherlands Search for more papers by this author Jean-Yves Masson Jean-Yves Masson Genome Stability Laboratory, CHU de Québec Research Center, Québec City, QC, Canada Department of Molecular Biology, Medical Biochemistry and Pathology, Laval University Cancer Research Center, Québec City, QC, Canada Search for more papers by this author Stefano Cairo Stefano Cairo XenTech, Evry, France Search for more papers by this author Jean-Gabriel Judde Jean-Gabriel Judde XenTech, Evry, France Search for more papers by this author Mark J O'Connor Mark J O'Connor Oncology Innovative Medicines and Early Clinical Development Biotech Unit, AstraZeneca, Cambridge, UK Search for more papers by this author Orland Díez Orland Díez Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Clinical and Molecular Genetics Area, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Judith Balmaña Corresponding Author Judith Balmaña [email protected] orcid.org/0000-0002-0762-6415 High Risk and Familial Cancer Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Violeta Serra Corresponding Author Violeta Serra [email protected] orcid.org/0000-0001-6620-1065 Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Search for more papers by this author Marta Castroviejo-Bermejo Marta Castroviejo-Bermejo Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Cristina Cruz Cristina Cruz Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain High Risk and Familial Cancer Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Alba Llop-Guevara Alba Llop-Guevara Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Sara Gutiérrez-Enríquez Sara Gutiérrez-Enríquez Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Mandy Ducy Mandy Ducy Genome Stability Laboratory, CHU de Québec Research Center, Québec City, QC, Canada Department of Molecular Biology, Medical Biochemistry and Pathology, Laval University Cancer Research Center, Québec City, QC, Canada CHU de Quebec - Université Laval Research Center, Genomics Center, CHUL, Québec City, QC, Canada Search for more papers by this author Yasir Hussein Ibrahim Yasir Hussein Ibrahim Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Albert Gris-Oliver Albert Gris-Oliver Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Benedetta Pellegrino Benedetta Pellegrino Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Department of Medical Oncology, University Hospital of Parma, Parma, Italy Search for more papers by this author Alejandra Bruna Alejandra Bruna Cancer Research UK Cambridge Institute and Department of Oncology, Li Ka Shing Centre, University of Cambridge, Cambridge, UK Search for more papers by this author Marta Guzmán Marta Guzmán Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Olga Rodríguez Olga Rodríguez Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Judit Grueso Judit Grueso Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Sandra Bonache Sandra Bonache Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Alejandro Moles-Fernández Alejandro Moles-Fernández Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Guillermo Villacampa Guillermo Villacampa Oncology Data Science (OdysSey Group), Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Cristina Viaplana Cristina Viaplana Oncology Data Science (OdysSey Group), Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Patricia Gómez Patricia Gómez Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Breast Cancer and Melanoma Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Maria Vidal Maria Vidal Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Breast Cancer and Melanoma Group, Vall d'Hebron Institute of Oncology, Barcelona, SpainCorrection added on 16 January 2019, after first online publication: the author name "Marc Vidal" was corrected to "Maria Vidal" Search for more papers by this author Vicente Peg Vicente Peg Pathology Department, Vall d'Hebron University Hospital, Barcelona, Spain CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Search for more papers by this author Xavier Serres-Créixams Xavier Serres-Créixams Department of Radiology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Graham Dellaire Graham Dellaire Department of Pathology, Dalhousie University, Halifax, NS, Canada Search for more papers by this author Jacques Simard Jacques Simard CHU de Quebec - Université Laval Research Center, Genomics Center, CHUL, Québec City, QC, Canada Search for more papers by this author Paolo Nuciforo Paolo Nuciforo CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Molecular Oncology Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Isabel T Rubio Isabel T Rubio CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Breast Surgical Unit, Breast Cancer Center, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Rodrigo Dienstmann Rodrigo Dienstmann Oncology Data Science (OdysSey Group), Vall d'Hebron Institute of Oncology, Barcelona, SpainCorrection added on 12 July 2019, after first online publication: the author name "Rodrigo Dientsmann" was corrected to "Rodrigo Dienstmann" Search for more papers by this author J Carl Barrett J Carl Barrett AstraZeneca, Waltham, MA, USA Search for more papers by this author Carlos Caldas Carlos Caldas Cancer Research UK Cambridge Institute and Department of Oncology, Li Ka Shing Centre, University of Cambridge, Cambridge, UK Breast Cancer Programme, Cancer Research UK (CRUK) Cambridge Cancer Centre, Cambridge, UK Search for more papers by this author José Baselga José Baselga Human Oncology and Pathogenesis Program (HOPP), Memorial Sloan Kettering Cancer Center, New York, NY, USA Department of Medicine, Memorial Sloan Kettering Cancer Center, New York, NY, USA Search for more papers by this author Cristina Saura Cristina Saura Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Breast Cancer and Melanoma Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Javier Cortés Javier Cortés CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Department of Oncology, Ramón y Cajal University Hospital, Madrid, Spain Vall d'Hebron Institute of Oncology, Barcelona, Spain Search for more papers by this author Olivier Déas Olivier Déas XenTech, Evry, France Search for more papers by this author Jos Jonkers Jos Jonkers orcid.org/0000-0002-9264-9792 Division of Molecular Pathology and Cancer Genomics, The Netherlands Cancer Institute, Amsterdam, The Netherlands Search for more papers by this author Jean-Yves Masson Jean-Yves Masson Genome Stability Laboratory, CHU de Québec Research Center, Québec City, QC, Canada Department of Molecular Biology, Medical Biochemistry and Pathology, Laval University Cancer Research Center, Québec City, QC, Canada Search for more papers by this author Stefano Cairo Stefano Cairo XenTech, Evry, France Search for more papers by this author Jean-Gabriel Judde Jean-Gabriel Judde XenTech, Evry, France Search for more papers by this author Mark J O'Connor Mark J O'Connor Oncology Innovative Medicines and Early Clinical Development Biotech Unit, AstraZeneca, Cambridge, UK Search for more papers by this author Orland Díez Orland Díez Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Clinical and Molecular Genetics Area, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Judith Balmaña Corresponding Author Judith Balmaña [email protected] orcid.org/0000-0002-0762-6415 High Risk and Familial Cancer Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain Search for more papers by this author Violeta Serra Corresponding Author Violeta Serra [email protected] orcid.org/0000-0001-6620-1065 Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain CIBERONC, Instituto de Salud Carlos III, Madrid, Spain Search for more papers by this author Author Information Marta Castroviejo-Bermejo1, Cristina Cruz1,2,3, Alba Llop-Guevara1, Sara Gutiérrez-Enríquez4, Mandy Ducy5,6,7, Yasir Hussein Ibrahim1, Albert Gris-Oliver1, Benedetta Pellegrino1,8, Alejandra Bruna9, Marta Guzmán1, Olga Rodríguez1, Judit Grueso1, Sandra Bonache4, Alejandro Moles-Fernández4, Guillermo Villacampa10, Cristina Viaplana10, Patricia Gómez3,11, Maria Vidal3,11, Vicente Peg12,13, Xavier Serres-Créixams14, Graham Dellaire15, Jacques Simard7, Paolo Nuciforo13,16, Isabel T Rubio13,17, Rodrigo Dienstmann10, J Carl Barrett18, Carlos Caldas9,19, José Baselga20,21, Cristina Saura3,11, Javier Cortés13,22,23, Olivier Déas24, Jos Jonkers25, Jean-Yves Masson5,6, Stefano Cairo24, Jean-Gabriel Judde24, Mark J O'Connor26, Orland Díez4,27, Judith Balmaña *,2,3 and Violeta Serra *,1,13 1Experimental Therapeutics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain 2High Risk and Familial Cancer Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain 3Department of Medical Oncology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain 4Oncogenetics Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain 5Genome Stability Laboratory, CHU de Québec Research Center, Québec City, QC, Canada 6Department of Molecular Biology, Medical Biochemistry and Pathology, Laval University Cancer Research Center, Québec City, QC, Canada 7CHU de Quebec - Université Laval Research Center, Genomics Center, CHUL, Québec City, QC, Canada 8Department of Medical Oncology, University Hospital of Parma, Parma, Italy 9Cancer Research UK Cambridge Institute and Department of Oncology, Li Ka Shing Centre, University of Cambridge, Cambridge, UK 10Oncology Data Science (OdysSey Group), Vall d'Hebron Institute of Oncology, Barcelona, Spain 11Breast Cancer and Melanoma Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain 12Pathology Department, Vall d'Hebron University Hospital, Barcelona, Spain 13CIBERONC, Instituto de Salud Carlos III, Madrid, Spain 14Department of Radiology, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain 15Department of Pathology, Dalhousie University, Halifax, NS, Canada 16Molecular Oncology Group, Vall d'Hebron Institute of Oncology, Barcelona, Spain 17Breast Surgical Unit, Breast Cancer Center, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain 18AstraZeneca, Waltham, MA, USA 19Breast Cancer Programme, Cancer Research UK (CRUK) Cambridge Cancer Centre, Cambridge, UK 20Human Oncology and Pathogenesis Program (HOPP), Memorial Sloan Kettering Cancer Center, New York, NY, USA 21Department of Medicine, Memorial Sloan Kettering Cancer Center, New York, NY, USA 22Department of Oncology, Ramón y Cajal University Hospital, Madrid, Spain 23Vall d'Hebron Institute of Oncology, Barcelona, Spain 24XenTech, Evry, France 25Division of Molecular Pathology and Cancer Genomics, The Netherlands Cancer Institute, Amsterdam, The Netherlands 26Oncology Innovative Medicines and Early Clinical Development Biotech Unit, AstraZeneca, Cambridge, UK 27Clinical and Molecular Genetics Area, Hospital Vall d'Hebron, Universitat Autònoma de Barcelona, Barcelona, Spain ‡1–27 Affiliations can be found at the end of the article *Corresponding author. Tel: +34 93 2746000; Fax: +34 93 4894212; E-mail: [email protected] *Corresponding author. Tel: +34 93 2543450; Fax: +34 93 4894212; E-mail: [email protected] EMBO Mol Med (2018)10:e9172https://doi.org/10.15252/emmm.201809172 PDFDownload PDF of article text and main figures. Peer ReviewDownload a summary of the editorial decision process including editorial decision letters, reviewer comments and author responses to feedback. ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InMendeleyWechatReddit Figures & Info Abstract Poly(ADP-ribose) polymerase (PARP) inhibitors (PARPi) are effective in cancers with defective homologous recombination DNA repair (HRR), including BRCA1/2-related cancers. A test to identify additional HRR-deficient tumors will help to extend their use in new indications. We evaluated the activity of the PARPi olaparib in patient-derived tumor xenografts (PDXs) from breast cancer (BC) patients and investigated mechanisms of sensitivity through exome sequencing, BRCA1 promoter methylation analysis, and immunostaining of HRR proteins, including RAD51 nuclear foci. In an independent BC PDX panel, the predictive capacity of the RAD51 score and the homologous recombination deficiency (HRD) score were compared. To examine the clinical feasibility of the RAD51 assay, we scored archival breast tumor samples, including PALB2-related hereditary cancers. The RAD51 score was highly discriminative of PARPi sensitivity versus PARPi resistance in BC PDXs and outperformed the genomic test. In clinical samples, all PALB2-related tumors were classified as HRR-deficient by the RAD51 score. The functional biomarker RAD51 enables the identification of PARPi-sensitive BC and broadens the population who may benefit from this therapy beyond BRCA1/2-related cancers. Synopsis Sensitive and highly specific biomarkers usable in archived formalin fixed parafin embedded (FFPE) tumour samples are needed to extend the use of PARP inhibitors beyond BRCA1/2-related cancers. The RAD51 score may satisfy this clinical unmet need. The RAD51 score shows complete discriminative capacity in predicting PARP inhibitor response. The RAD51 score is feasible in routine breast tumor samples without prior exposure to DNA damaging agents. Carrying a PALB2 mutation is associated with a low RAD51score. Introduction Poly(ADP-ribose) polymerases (PARPs) are enzymes with diverse functions, including repair of DNA single-strand breaks (SSBs). PARP inhibition not only impairs SSB repair but also results in PARP trapping onto DNA with subsequent stalling of replication forks (Plummer, 2006; Helleday, 2011; Murai et al, 2012; Lord & Ashworth, 2017). Both effects contribute to the formation of DNA double-strand breaks (DSBs) that, in replicated areas of the genome, are repaired by homologous recombination repair (HRR), a conservative mechanism for error-free repair of DNA damage (Saredi et al, 2016; Pellegrino et al, 2017). Cells with defects in HRR including those with deleterious variants in BRCA1 or BRCA2 (BRCA1/2) genes are particularly sensitive to PARP inhibitors (PARPi; Bryant et al, 2005; Farmer et al, 2005; Rottenberg et al, 2008), which prompted the clinical development of PARPi as anticancer therapies (Fong et al, 2009; Audeh et al, 2010; Tutt et al, 2010). In breast cancer (BC), the efficacy results of the PARPi olaparib (Lynparza®) in metastatic patients carrying a germline BRCA1/2 (gBRCA) pathogenic variant have led to its recent approval by the Food and Drug Administration (Robson et al, 2017). PARPi have also shown preclinical and clinical activity beyond gBRCA in ovarian and prostate cancer (McCabe et al, 2006; Kaufman et al, 2014; Mateo et al, 2015; Mirza et al, 2016; Coleman et al, 2017; Pujade-Lauraine et al, 2017). Similarly, the use of PARPi could be extended beyond gBRCA to a wider group of BC patients harboring dysfunctional HRR. For example, the clinical and molecular similarities between BRCA1-associated tumors and a subset of triple negative BCs (TNBC) led to postulate that the latter may also have defects in HRR (Turner et al, 2004). In such cases, HRR deficiency can be explained by epigenetic silencing of BRCA1/2 or the genetic inactivation of several other HRR-related genes such as ATM, ATR, CHEK1, CHEK2, PALB2, and the FANC family genes (Konstantinopoulos et al, 2015; Shakeri et al, 2016). Specifically, PALB2 is essential for BRCA2 anchorage to nuclear structures and recruitment to DSBs, acting as the link between BRCA1 and BRCA2 (Buisson & Masson, 2012; Pauty et al, 2014). Despite the success of PARPi monotherapy in gBRCA BC, appropriate biomarkers are still needed for selection of non-gBRCA patients for PARPi therapy (Gelmon et al, 2011; Mutter et al, 2017). Some proposed approaches include the use of mRNA expression signatures, the analysis of genomic scars derived from defective HRR, or the individual analysis of genetic alterations in HRR-related genes (Konstantinopoulos et al, 2010; Abkevich et al, 2012; Wagle et al, 2012; Watkins et al, 2014; Davies et al, 2017; Polak et al, 2017). A potential limitation of these approaches is the lack of specificity in HRR-altered tumors that have restored the HRR function (Watkins et al, 2014; Konstantinopoulos et al, 2015). Other approaches entail the quantification of BRCA1 promoter hypermethylation, BRCA1 mRNA expression, or the detection of the HRR protein RAD51 forming nuclear foci after DNA damage, as surrogate of HRR functionality (Graeser et al, 2010; Naipal et al, 2014; ter Brugge et al, 2016). In these sense, we showed that, in gBRCA tumors, RAD51 foci could be detected in untreated samples and correlated with PARPi resistance regardless of the underlying mechanism restoring HRR function (Cruz et al, 2018). In this work, we analyzed five HRR biomarkers (genetic alterations in HRR-related genes, BRCA1 promoter methylation, BRCA1 expression, BRCA1 foci formation, and RAD51 foci formation) and tested which one performed better to predict PARPi response. Importantly, we further show that the RAD51 assay is feasible in routine formalin-fixed paraffin-embedded (FFPE) tumor samples without prior induction of DNA damage. Scoring RAD51 allowed the identification of non-gBRCA HRR-deficient BCs with high accuracy, which may help identify a wider BC population with intrinsic sensitivity to PARPi therapy. Results Olaparib antitumor activity in a non-gBRCA BC patient-derived tumor xenograft (PDX) panel distinguishes a subset of tumors highly sensitive to PARPi We assessed the antitumor activity of the PARPi olaparib in 18 PDX models derived from non-gBRCA BC patients (PDX cohort-1, Table EV1). Treatment with olaparib revealed antitumor activity in four PDX models as assessed by mRECIST (see Materials and Methods): complete response (CR, n = 2: PDX093 and PDX197) or partial response (PR, n = 2: PDX302 and STG201). The remaining eleven PDX models were olaparib-resistant (PD, progressive disease; Fig 1A and Appendix Table S1). Additional resistant models were generated from three of the four olaparib-sensitive PDXs after prolonged exposure and steep progression to olaparib (STG201OR, PDX093OR, and PDX302OR; Fig 1B). The fourth olaparib-sensitive model PDX197 did not grow after prolonged treatment (> 120 days). This PDX collection of 18 BC models was used to study clinically relevant mechanisms of PARPi sensitivity and acquired resistance in vivo. Figure 1. The antitumor activity of olaparib in PDXs identifies a subset of PARPi-sensitive tumors A. Waterfall plot showing the percentage of tumor volume change in olaparib-treated tumors compared to the tumor volume on day 1. +20% and −30% are marked by dotted lines to indicate the range of PR, SD, and PD. The box underneath summarizes different characteristics of each model and the clinical context at the moment of PDX implantation. Black boxes indicate the presence of the phenotype. TNBC, triple negative breast cancer; ER+BC, estrogen receptor positive breast cancer; P, primary; M, metastasis. Error bars indicate SEM from independent tumors (n ≥ 3). B. Graph showing the percentage of tumor volume change during olaparib treatment in PDXs from cohort-1. Olaparib-sensitive models are represented with discontinuous lines. Acquisition of PARPi resistance in PDX302, STG201, and PDX093 after prolonged exposure to olaparib is shown. Download figure Download PowerPoint HRR-related somatic alterations and PARPi sensitivity in PDX We next investigated the presence of alterations in HRR-related genes that could explain olaparib sensitivity. As aberrant BRCA1 promoter methylation is found in approximately 10% of sporadic breast cancers (Shakeri et al, 2016), we first measured the levels of epigenetic silencing of BRCA1 and analyzed BRCA1 expression and nuclear foci formation in PDX samples. Our approach validated previously reported BRCA1 promoter methylation and expression results from the STG139 and STG201 models (Bruna et al, 2016). Results showed that three out of four olaparib-sensitive models (PDX302, STG201, and PDX197) and one olaparib-resistant model (PDX270) presented BRCA1 promoter hypermethylation, while the remaining PDX models showed low levels of methylation (Fig 2A). In agreement, absence of BRCA1 mRNA expression and lack of BRCA1 nuclear foci were restricted to the four models that showed BRCA1 promoter hypermethylation (Fig 2A, larger views in Appendix Fig S1). Of note, the olaparib acquired-resistant models STG201OR and PDX302OR exhibited lower levels of BRCA1 promoter hypermethylation in comparison with the olaparib-sensitive counterparts, and displayed BRCA1 mRNA expression and BRCA1 nuclear foci formation (Fig 2A). Figure 2. HRR-related alterations in PDX cohort-1 and PARPi response A. Levels of BRCA1 promoter hypermethylation, levels of BRCA1 mRNA, and the presence of BRCA1 nuclear foci by immunofluorescence are shown (larger views and separate channels are shown in Appendix Fig S1). T127 and T162 were used as positive contr