Early enrichment of ESR1 mutations and the impact on gene expression in pre-surgical primary breast cancer treated with aromatase inhibitors

Purpose: To investigate the presence of ESR1 mutation in primary oestrogen-receptorpositive breast cancer (ER+BC) treated with extended (>4 weeks) neoadjuvant (pre-surgical) aromatase inhibitor(NAI) therapy and to identify patients who may gain less benefit from aromataseinhibition (AI) alone based upon on-treatment changes in gene expression. Experimental Design: We evaluated ER, progesterone receptor and Ki67 by immunostaining, ESR1 mutations by droplet-digital-PCR and expression of over 800 key BC genes in paired pre- and post-NAI tumour samples from 87 ER+BC patients. Results: Cell proliferation and oestrogen-regulated genes ( ERGs) remained suppressed in most tumours indicative of persistent response to NAI. Enrichment of ESR1 mutations was found in five tumours and predominantly in patients receiving therapy for >6 months. ESR1 mutant tumours showed increased expression of ESR1-transcript and limited suppression of ERGsand proliferation associated genes in response to NAI. ESR1 wild-type tumours with high residual proliferation (Ki67r≥10%; 15/87 tumours) showed lower ESR1/ER expression pre- and post-therapy and lower ERGs. Tumours with ESR1 mutations or Ki67r≥10% showed less inhibition of oestrogen-response, cell-cycle and E2F-target genes. Conclusion: Ligand-independent ER-signalling, as a result of ESR1 mutation or reduced ER-dependence, identified after extended NAI therapy, can guide early selection of patients who would benefit from combination therapy.