Developing targeted strategies for precision breast cancer prevention

Background: Breast cancer (BC) is the leading cause of death in UK women aged 35-64. Women at increased risk are offered tamoxifen as prevention therapy, which reduces BC incidence. However, tamoxifen may not reduce BC mortality, and does not prevent oestrogen receptor negative cancers.

Our Biomarkers of Breast Cancer Prevention (BBCP) study has identified differential epithelial responses to preventive tamoxifen therapy, with some women responding to tamoxifen with reduced proliferation, whilst others are resistant.

We want to define the biological mechanisms behind these different responses and find novel prevention targets for women who are resistant to tamoxifen. Aims 1. Identify the biological features of resistance to preventive tamoxifen. 2. Determine how these features can be identified by mammographic imaging and genomic biomarkers. 3.

Translate tamoxifen resistant biology into novel preventive approaches Methods: We will use existing samples and ongoing recruitment from BBCP.

Premenopausal women at increased BC risk have a baseline vacuum assisted biopsy (VAB) prior to commencing preventive tamoxifen followed by contralateral VAB after 3 months.

We will characterise 12 matched VAB pairs, 6 that respond (EResponse) and 6 resistant (EResist), by 2D X-ray projection, 3D X-ray microtomography (µCT), spatial proteomics, multiplexed imaging mass cytometry, and spatial transcriptomics. We will use machine learning to identify features within the breast that predict tamoxifen response.

We will use Machine Learning to identify mammographic features that predict BC risk and response to preventive tamoxifen, including for BC subtypes. We will use training and validation datasets (PROCAS n=57,000 and Grampian n=140,000).

Subgroup analyses will be performed at points of hormonal fluctuation; menopause and HRT use before evaluation in a dataset of n=240 premenopausal participants receiving tamoxifen in our prior studies. Oncoarray data will be used to explore associations between SNPs, mammogram features and tissue response.

We will examine the transcriptional regulation of tamoxifen response in newly recruited participants to BBCP.

Fresh samples will be subjected to ChIP-seq, qPLEX-RIME and single-cell ATAC-seq and retrospectively classified as EResponse or EResist.

Potential gene regulatory networks will be verified on tissue samples using tissue microarray and RNAScope-based RNA-FISH analyses. Key transcriptional programmes, differentially regulated in EResist will be validated in explant culture in vitro.

How this research will be used: Identification of biological markers of tamoxifen response will allow us to: - design a prospective cohort study to validate predictive biomarkers of tamoxifen response - develop novel strategies for BC prevention in those women resistant to tamoxifen.