Abstract 3433: Identification of a subtype specific molecular field across the mammary gland of breast cancer patients
Authors: Bedrosian Isabelle , Ju Zhenlin , Bhardwaj Anjana , Mistretta Brandon , Gunaratne Preethi H. , Wang Jing , El-zein Randa 
Published: September 2020

Abstract
Introduction: The role of field cancerization in sporadic breast cancer is not known. Clinical findings of multi-focality and the presence of molecular aberrations in the histologically normal tissue surrounding the primary tumor suggest that there is a field effect. We hypothesized that field cancerization is present in breast cancer and that a subtype specific molecular field can be identified.
Methods: RNA exome sequencing was performed using paraffin embedded sections obtained from breast cancer patients undergoing mastectomy; 25 cases were selected from each subtype: hormone receptor (HR) positive, Her2+, triple negative (TN) breast cancer. 4 sites were sampled from each patient: primary tumor (A), adjacent normal parenchyma within 2cm of the tumor (B) 2 separate, histologically normal sites at least 2cm away from the tumor(C &D) or, if available, tissue from the histologically normal contralateral breast (E). Normal breast tissue (N) from cancer free controls was obtained from reduction mammoplasty. Pathway analysis was performed by parametric analysis of gene set enrichment (PAGE). Differential genes and pathways were identified by LIMMA and a FDR adjusted q value of &lt; 0.05 was considered significant. To estimate tumor content across samples, we applied deconvolution analysis to tumor associated genes (TAG) to calculate indices which range from 0 (normal) to 1 (tumor). Genes or pathways that are associated with the field effects were identified by Pearson's correlation coefficient analysis.
Results: TAGs within each subtype of breast cancer were derived by identifying genes with at least 2-fold change between tumors (A) as compared to controls (N). Out of a total of 640, 470, and 829 TAGs identified for Her2+, HR, and TN, respectively, 283 TAGs, including 251 downregulated genes and 32 upregulated genes, were shared among all 3 subtypes. Using the tumor content indices calculated by deconvolution analysis, the degree to which the tumor content (A) was shared by histologically normal areas of breast (B,C, D,E) was determined. Overall, across all subtypes, shared tumor contents range from ~50% (E) to 80% (B) in contrast to ~15% of normal controls (N). Pathway scores were calculated for 3,195 gene sets/biological pathways downloaded from Molecular Signatures Database. 38, 18, and 21 deregulated pathways were identified that were shared across the breast parenchyma (samples A-E) in Her2+, HR, and TN, respectively. 3 shared pathways-Immune-, ILS- signaling and CP2_01- were deregulated across the breast parenchyma in all tumor subtypes.
Conclusions: Our study suggests the presence of a breast cancer molecular field effect that extends beyond adjacent normal breast tissue and includes the entire mammary gland. This field effect appears to be largely subtype specific. These findings provide new insight about the early changes across the mammary gland that may be contributing to subtype specific breast cancer risk.
Citation Format: Isabelle Bedrosian, Zhenlin Ju, Anjana Bhardwaj, Brandon Mistretta, Preethi H. Gunaratne, Jing Wang, Randa El-zein. Identification of a subtype specific molecular field across the mammary gland of breast cancer patients [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3433.