Abstract PO3-14-08: Metaplastic Breast Cancer: Whole Genome Sequencing Analysis and Emerging Therapeutic Targets

Abstract Background: Metaplastic breast cancer (MpBC) is a rare histologic subtype of triple negative breast cancer (TNBC) with an aggressive behavior and poor outcome. Previous studies by whole-exome and targeted sequencing showed that MpBCs display a dominant mutational signature 3, associated to homologous recombination (HR) deficiency (HRD). Tumors displaying HRD are associated with response to therapies causing double strand breaks. Nonetheless, MpBCs consistently display significantly lower rates of response to neoadjuvant chemotherapy than non-metaplastic TNBCs and effective therapeutic alternatives are an unmet clinical need. Here we sought to assess the HR features of MpBCs using whole-genome sequencing (WGS), and to evaluate the expression in MpBCs of antigens that are targets of novel antibody drug conjugates (ADCs) in common forms of breast cancer. In addition, we sought to assess the efficacy of ADCs and drugs targeting the DNA damage response (DDR) using preclinical MpBC models. Materials and Methods: We subjected 25 primary MpBC to WGS. HRDetect, the ‘gold standard’ tool for assessment of HRD, was utilized. Mutational signatures were inferred using Signal. 31 MpBCs were subjected to RNA-sequencing to assess the gene expression levels ADC targets compared to non-MpBCs from The Cancer Genome Atlas (TCGA). Efficiency of ADCs and DDR drugs was evaluated using the MpBC cell models HCC1806, BT549 and Hs578T in vitroand/or using HCC1806 xenografts in athymic mice in vivo. Results: Although the majority of MpBCs (13/25) displayed a dominant SBS mutational signature 3, only a small subset of cases (3/25) were classified as HRD by HRDetect. These three bona fide HRD cases were found to harbor bi-allelic inactivation of BRCA1 (n=2) or RAD51C (n=1), and displayed various genomic features of HRD, including deletions with microhomology, enrichment in tandem duplications, rearrangement signatures 3 and 5, and high large scale state transitions (LST) and telomeric allelic imbalance (NtAI) scores. Most MpBCs (22/25), however, were not classified as HRD by HRDetect displaying an incomplete HRD phenotype, with only partial genomic features of HRD. 52% of MpBCs (13/25) displayed APOBEC mutagenesis exposure, which was present in the three bona fide HRD MpBCs. RNA-sequencing analyses revealed detectable ERBB2 and TROP2 expression in all MpBCs interrogated, although at lower levels than in non-MpBCs from TCGA. The HCC1806, BT549 and Hs578T MpBC cell models were found to be sensitive to Sacituzumab Govitecan and Trastuzumab Deruxtecan (T-DXd) in cell viability and colony formation assays, and to T-DXd in vivo. These MpBC cell models were also found to be sensitive to DDR compounds, such as ATR, WEE1 and PKMYT1 inhibitors in vitro, alone and in combination with T-DXd or Sacituzumab Govitecan. Conclusions: Most MpBCs display an incomplete HRD genomic scar, except for those cases harboring bi-allelic inactivation of HRD genes. APOBEC mutagenesis is operative in MpBCs and co-exists with HRD. MpBCs express ERBB2 and TROP2, and MpBC cell models are sensitive to T-DXd and Sacituzumab govitecan, as well as DDR compounds, targeting the incomplete HRD phenotype of MpBCs, alone and in combination. Taken together, these findings warrant further studies in patients with MpBCs testing the therapeutic efficacy of agents targeting the incomplete HRD phenotype, as well as the expression of ADC targets in these aggressive rare forms of TNBCs. Citation Format: Fresia Pareja, Andrea Gazzo, Higinio Dopeso, David Brown, Pier Selenica, Yingjie Zhu, Juan Blanco-Heredia, Laxmi Gusain, Xin Pei, Giacomo Montagna, Nour Abuhadra, Hanna Y Wen, Edi Brogi, Nadeem Riaz, Sarat Chandarlapaty, Maurizio Scaltriti, Larry Norton, Simon Powell, Jorge Reis-Filho, Britta Weigelt. Metaplastic Breast Cancer: Whole Genome Sequencing Analysis and Emerging Therapeutic Targets 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 PO3-14-08.