133 Neoadjuvant HER2 inhibition induces <i>ESR1</i> DNA methylation alterations resulting in clinically relevant ER expression changes in breast cancer

Abstract Background Divergent estrogen receptor (ER) and HER2 status with breast cancer disease progression has important consequences for clinical management and long-term survival. Molecular subtype expression is dynamic and influenced by therapeutic intervention and the metastatic environment. There remains however, a lack of data regarding the mechanism of subtype switching and subsequent clinical consequences. Methods We undertook a clinical study of neoadjuvant HER2 inhibitor treated patients (n=161) to determine the consequence of treatment and metastatic environment on receptor subtype. HER2 status, ER expression levels and global DNA methylation was assessed in pre-treatment biopsies, in post-treatment samples in patients with residual disease and, where relevant, in metastatic tumour samples (total, n = 22). Results Overall gains in ER tumour expression were significantly associated with improved disease-free survival (n=72, P &amp;lt; 0.05) post-treatment. Mapping of the methylation landscape revealed global gains in hypomethylation following treatment (n=7 matched tumours, 16 samples) in contrast to hypermethylation on metastasis (n=5 matched tumours, 14 samples). Differential methylation of key signalling pathways, including estrogen response, epithelial to mesenchymal transition and PI3K/AKT/mTOR, is conserved between post-treatment and metastasis. However, where core pathway genes were hypomethylated following treatment, there was a shift to hypermethylation on metastasis, facilitating alterations in the tumour phenotype. Conclusion This study unlocks DNA methylation as a key process in breast cancer progression providing vital insights into the effects of targeted HER2 treatment. This work provides a clear rationale to develop combined HER2 inhibitor and endocrine therapeutic strategies to enhance long-term survival in HER2 positive patients.