Impact of HER2 low status on genomic signatures in triple negative breast cancer (TNBC).

1092 Background: HER2 status has become more important with new treatment regimens subcategorizing HER2 to into low (1+or 2+ FISH-) or absent HER2 expression. Through comprehensive genomic profiling (CGP), we aim to identify distinctive molecular features associated with various immunohistochemistry (IHC) categories of HER2 low tumors. Methods: Retrospective review of pathology specimens submitted to Foundation Medicine were done and 514 TNBC (ER-/PR-/HER20,1+,2+FISH-) and 191 3+/HER2 amplified clinically advanced BC underwent hybrid capture-based CGP to evaluate genomic alterations (GA) in 324 genes, MSI status and tumor mutational burden (TMB, mutations/Mb). PD-L1 expression was determined by IHC (Dako 22C3 TPS). Chi-square test and Mann Whitney U test were used in the statistical comparisons of the 2 groups. Statistical significance was defined as p = 10 mutations/Mb was significantly higher in the HER2 3+/amplified group (p=.004). Conclusions: We found that TNBC HER2 2+ positive FISH show key molecular differences from their HER2 low (1+) or absent (0+) counterparts. The most substantial difference was an increase in GA in PIK3CA. While this is targetable in the hormone positive setting, its potential role as a target in the hormone negative setting is uncertain. Also noted but did not reach significance was a decrease in GA within BRCA1/2 and a positive linear relationship with CDH1 and HER2 IHC expression. These GA differences may indicate potential treatment options for different HER2 subgroups and warrant further investigation. Limitations of our study include the retrospective nature and the lack of clinical data. Table: see text