554 Background: HER2+ breast cancer (BC) is particularly common in young women. Genomic features of HER2+ tumors before and after H-based therapy have not been described in a population of young women and may point to clinically targetable mechanisms of resistance. Methods: From a large prospective cohort of women diagnosed with BC age ≤40 years, we identified those with HER2+ BC and tumor tissue available for sequencing before and after chemo+H. Whole exome sequencing (WES) was performed on each tumor and on germline DNA from blood. Tumor-normal pairs were analyzed for mutations and copy number (CN) changes. Evolutionary analysis was performed for patients with both pre- and post-treatment (tx) samples. Results: 22 women had successful WES samples from at least one timepoint; 13 of these had paired sequencing results both before and after chemo+H. For the majority of women, post-tx sample was following neoadjuvant chemo + H, though post-tx timepoint for other women represented locoregional or distant metastasis (Table). TP53 was the only gene that was significantly recurrently mutated in both pre- and post-tx samples. Comparison of matched pre-tx and post-tx samples demonstrated that large changes in HER2 CN over the course of tx were uncommon, only 2/13 pts had > 2-fold change in HER2 CN. Other clonal and subclonal genomic alterations were found to be acquired in the post-tx sample compared to the pre-tx sample. One patient acquired a putative activating mutation in ERBB2. Another patient acquired a clonal hotpsot mutation in TP53. MYC gene amplification was observed in 4 post-tx tumors. NOTCH2 alterations were found in post-tx biopsies from 2 different patients, and mutations in STIL were also found in post-tx biopsies from 2 patients, though the function of these mutations is not known. Conclusions: HER2+ breast tumors in young women display genomic evolution following tx with chemo+H. HER2 CN changes are uncommon, but we identified several genes that warrant exploration as potential mechanisms of resistance to therapy in this population.[Table: see text]
Mechanisms of resistance and sensitivity to anti-HER2 therapies in HER2+ breast cancer
