Frequent ESR1 and CDK Pathway Copy Number Alterations in Metastatic Breast Cancer

Abstract

DNA sequencing has identified a limited number of driver mutations in metastatic breast cancer beyond single base-pair mutations in the estrogen receptor (ESR1). However, our previous studies and others have observed that structural variants, such as ESR1 fusions, may also play a role. Therefore, we expanded upon these observations by performing a comprehensive and highly sensitive characterization of copy number (CN) alterations in a large clinical cohort of metastatic specimens. NanoString DNA hybridization was utilized to measure CN gains, amplifications and deletions of 67 genes in 108 breast cancer metastases, and in 26 cases, the patient-matched primary tumor. For ESR1, a copyshift algorithm was applied to identify CN imbalances at exon-specific resolution and queried large datasets (>15,000 tumors) that had previously undergone next-generation sequencing (NGS). Interestingly, a subset of ER+ tumors showed increased ESR1 CN (11/82, 13); three had CN amplifications (4) and eight had gains (10). Increased ESR1 CN was enriched in metastatic specimens versus primary tumors and this was orthogonally confirmed in a large NGS dataset. ESR1-amplified tumors showed a site-specific enrichment for bone metastases and worse outcomes than non-amplified tumors. No ESR1 CN amplifications and only one gain was identified in ER- tumors. ESR1 copyshift was present in five out of the 11 ESR1-amplified tumors. Other frequent amplifications included ERBB2, GRB7 and cell cycle pathway members CCND1 and CDK4/6, which showed mutually exclusivity with deletions of CDKN2A, CDKN2B, and CDKN1B. Implications: Copy number alterations of ESR1 and key CDK pathway genes are frequent in metastatic breast cancers, and their clinical relevance should be tested further.