Abstract PO3-23-11: Multiomics analysis of ER+ breast cancer with matched primaries, lymph node metastases and recurrent cancers on adjuvant endocrine therapy

Abstract Background: 80% of all breast cancers (BCs) are ER-positive (ER+). A significant number develop endocrine therapy resistance (ETR) . Clones develop in lymph nodes (LNs) which differ from the primary and these are implicated in ETR. A multiomics analysis of primary ER+ breast cancers, matched LN metastasis (LN+), if present, and matched recurrences (primary, nodal or distant) has been performed. Patients and Methods: The patients included are: Cohort A: 95 patients with matched primary and recurrent ER+ BC on different endocrine therapies. 35% had involved LNs at diagnosis. Recurrences: local in 59/95, concurrent local and nodal in 33/95, LN-only in 3/95. Median time to recurrence 4.5 years. Cohort B: 18 patients with ER+ LN+ BC who developed multiple recurrences (local and/or nodal, 3 consecutive recurrences/patient) on different lines of sequential endocrine therapy. Cohort C: 7 patients with ER+ breast cancer with multiple involved LNs treated with 3-6 months of neoadjuvant ET before surgery. Biopsies of primary and matched LNs performed at diagnosis and throughout treatment (3-4 timepoints/patient). Clinical response by serial ultrasound. Primary and LN samples (mean 12 samples/patient) were analysed together with normal DNA. Targeted RNAseq and DNA exome sequencing and whole-genome expression analysis was performed. Somatic mutations and copy number alterations (CNA) were determined, and differential gene expression analysis was performed. Validation of pathways implicated in ETR was by spatial NanoString GeoMx. All patients have long term follow-up. Results: Cohort A: A mean of 6 potential driver mutations identified by DNA sequencing in each primary BC (range 1-16). Most commonly mutated genes were PIK3CA, TP53, ERBB4, MAP3K1 and ERBB3. Multiple aberrations and a highly diverse mutational landscape were observed in all recurrences. In the majority of recurrences significant changes in variant allele frequency (VAF) were identified across several ETR-implicated genes and in over 50% new gains/losses were identified. Most frequently, losses were seen in ERBB3, ARID1A, HRAS and FOXA1 with gains in CDH1, NF1, PIK3CA, ESR1, TBX3 and GATA3 in recurrences vs primaries. Transcriptomic analysis revealed differentially expressed pathways including ER, HER2, GATA3, AKT, RAS and p63 signalling. Integrated analysis revealed several driver mutations with corresponding changes in upstream pathway activity. 33 patients had concurrent local and nodal recurrences and 52% had similar somatic mutation profiles in both with only minor variance in VAF. In 48%, 1 or more new potential driver mutations was identified in axillary nodes compared to recurrent primary cancers with losses in AKT1, CDH1 and TP53 being most common. Multiomics profiling of Cohorts B and C is complete and analysis underway. Discussion: This multiomics study is the largest cohort to-date of matched early and recurrent ER+/HER2- cancers. It sheds new light on the complex somatic and transcriptomic changes in local and nodal recurrent disease. The mechanisms of ETR are diverse and underlying mechanisms and clinically meaningful biomarkers including potentially actionable mutations and targets have been identified. Cohort C will uniquely capture and compare clonal evolution of breast cancer in primary and multiple matched lymph during neoadjuvant ET. Citation Format: Carlos Martinez-Perez, Charlene Kay, Rebecca Swan, Lorna Renshaw, J Michael Dixon, Arran K Turnbull. Multiomics analysis of ER+ breast cancer with matched primaries, lymph node metastases and recurrent cancers on adjuvant endocrine therapy abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-23-11.