Abstract B059: Epigenetically defined sub-clonal heterogeneity drives therapy resistance in triple-negative breast cancer
Authors: Ahmadimoghari Fatemeh , Zhang Yu , Mavrommatis Ioanna , Thakur Shafali , Starling Chris , Roxanis Ioannis , Haider Syed , Natrajan Rachael 
Published: February 2024

Abstract
Background: Triple-negative breast cancer (TNBC) is a highly aggressive and heterogeneous disease with an average survival of less than 50%. Although immunotherapy is showing promising results, many patients do not respond, and chemotherapy remains the main treatment. High TNBC recurrence rates are in part due to its molecular heterogeneity. Here, we sought to characterise the sub-clonal and spatial diversity of chemotherapy resistant TNBC to identify the epigenetic hallmarks of lethal sub-populations of cells that may induce cell state transition, therapy resistance and ultimately metastatic progression. Methods: Sixteen primary untreated triple-negative breast tumours (eight chemotherapy sensitive (ChemoS) and eight chemotherapy resistant (ChemoR, who died of their disease within three years)) were subjected to cell-matched high-depth multiome single nuclei RNA- and ATAC-sequencing. Four tumours were subject to spatial RNA and ATAC profiling. Fourteen TNBC PDX models generated from patients with residual disease post NAC were subjected to single nuclei sequencing and used for validation. Meta-clustering of epithelial cells from the RNA profiles was used to identify sub-populations of cells harbouring distinct epigenetic and transcriptomic features enriched in ChemoR patients. Findings were validated in the BrighTNess TNBC NAC clinical trial and in chemotherapy treated TNBC patients from the SCAN-B study. Results: Unsupervised meta-clustering (MC) of epithelial cells in G1 phase of the cell cycle identified nine MC’s across the dataset, including a distinct immune-like MC. Two MCs were dominated by cells from ChemoR patients and were defined by hypoxia, EMT, and extra-cellular matrix signalling. Sub-populations of cells in the ChemoR dominant MCs were underpinned by both promoter and distal differential chromatin accessibility, that mapped to H3K27ac enhancer sites, suggesting a role of enhancer regulation of MC transcription. Spatial ATAC and RNA profiling of a subset of tumours identified spatially epigenetically defined regions that showed distinct chromatin profiles, suggesting that the epigenetic regulation of genes that define the MCs are spatially distinct. ChemoR clusters were independently reproduced in a cohort of fourteen residual disease TNBC PDXs. Genes pertaining to the immune-like MC were associated with a good prognosis in independent cohorts of contemporary treated TNBC (SCAN-B) and with response to NAC in the BrighTNess trial. Marker genes from ChemoR dominant MCs were associated with poor prognosis and significant levels of residual disease following NAC. Conclusions: Our multi-modal integrative analysis reveals unprecedented insight into the role of epigenetically defined spatially distinct, sub-clonal populations that pre-exist in treatment naïve TNBC and predict therapy response. This suggests pre-existing populations of cells have already acquired the epigenetic footprints and hence transcriptomic features to allow them to evade therapy.
Citation Format: Fatemeh Ahmadimoghari, Yu Zhang, Ioanna Mavrommatis, Shafali Thakur, Chris Starling, Ioannis Roxanis, Syed Haider, Rachael Natrajan. Epigenetically defined sub-clonal heterogeneity drives therapy resistance in triple-negative breast cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr B059.