Abstract 1458: Single cell resolution chromatin accessibility and transcriptomic atlas of genetic ancestry mapped healthy breast tissues
Authors: Bhat-Nakshatri Poornima , Chen Duojiao , Gao Hongyu , Liu Yunlong , Storniolo Anna Maria , Nakshatri Harikrishna 
Published: June 2022

Abstract
Single cell mRNA atlases of multiple organs have been reported using scRNA-seq. We had previously reported single cell mRNA atlas of the breast. In this study, we performed scATAC-seq and scRNA-seq of breast tissues from 22 women of Ashkenazi (A) ancestry (19,935 nuclei), 20 women of European (non-A) ancestry (19,090 nuclei), six BRCA1 (6,261 nuclei) and five BRCA2 (3,400 nuclei) mutation carriers to correlate cell type-specific gene expression with chromatin accessibility. An integrated analysis revealed 17 cell clusters including basal, luminal progenitor, and mature luminal epithelial cells, two endothelial subtypes and two fibroblast/mesenchymal stem-like cell types. Basal cells could be further subdivided into two subclusters that differentially expressed genes such as TSHZ2, POSTN and ETV6. Similarly, there were four luminal progenitor subclusters that differentially expressed genes such as MALAT1, SOX13, and MDM4. Although ESR1 expression was observed predominantly in mature luminal cells, binding sites for ESR1 were accessible in multiple epithelial and non-epithelial cells with the exception of basal cells. FOXA1 expression and binding site enrichment were observed mostly in mature luminal cells. GATA3 expression was observed in all epithelial cell types with highest expression in mature luminal cells but binding sites for GATA3 were accessible in multiple cell types. Expression and binding site enrichment of alveolar differentiation-associated EHF were predominant in luminal progenitor cells. TP63 expression and binding site enrichment were restricted to basal cells, which is consistent with the proposed role of TP63 in maintaining stem cells among the basal cell populations. EMT-associated ZEB1 showed an intriguing pattern. Similar to our published immunohistochemistry-based analysis of breast tissues, ZEB1 expression was detected in fibroblasts/mesenchymal cells, endothelial cells and adipocytes but not in epithelial cells. However, binding sites for this transcription factor were enriched in epithelial cell types and in T-cells. ZEB1 mRNA stability is tightly controlled by the epithelial cell-enriched miR-200 family of microRNAs, and it is possible that ZEB1 transcribed in epithelial cells is immediately targeted for degradation through miR-200. To gain insight into this possibility, we examined the chromatin accessibility status of ZEB1 regulatory regions in fibroblasts/mesenchymal cells and in epithelial cells. ZEB1 regulatory regions displayed open chromatin status in both epithelial cells and fibroblasts. With the exception of a few genes such as SIGLEC1 in macrophages and IL7R in T cells, cell type-specific expression of many genes is not controlled at the level of chromatin accessibility of their regulatory regions. Collectively, these results reveal complexities in gene expression in different cell types of the breast.
Citation Format: Poornima Bhat-Nakshatri, Duojiao Chen, Hongyu Gao, Yunlong Liu, Anna Maria Storniolo, Harikrishna Nakshatri. Single cell resolution chromatin accessibility and transcriptomic atlas of genetic ancestry mapped healthy breast tissues [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1458.