RAS/RAF co-mutation and ERBB2 copy number modulates HER2 heterogeneity and responsiveness to HER2-directed therapy in colorectal cancer

Abstract Purpose: ERBB2 amplified colorectal cancer (CRC) is a distinct molecular subtype with expanding treatments. Implications of concurrent oncogenic RAS/RAF alterations are not known. Experimental Design: Dana-Farber and Foundation Medicine Inc. CRC cohorts with genomic profiling were used to identify ERBB2 amplified cases (Dana-Farber, n = 47/2,729 [1.7%]; FMI, n = 1857/49,839 [3.7%]). Outcomes of patients receiving HER2-directed therapies are reported (Dana-Farber, n = 9; Flatiron Health-Foundation Medicine clinicogenomic database, FH-FMI CGDB, n = 38). Multi-site HER2 immunohistochemistry and genomic profiling were performed to understand HER2 intratumoral and interlesional heterogeneity. The impact of concurrent RAS co-mutations on the effectiveness of HER2-directed therapies were studied in isogenic CRC cell lines and xenografts. Results: ERBB2 amplifications are enriched in left-sided CRC. 20% of ERBB2 amplified CRCs have co-occurring oncogenic RAS/RAF alterations. While RAS/RAF wild-type CRC typically have clonal ERBB2 amplification, CRCs with co-occurring RAS/RAF alterations have lower level ERRB2 amplification, higher intratumoral heterogeneity, and interlesional ERBB2 discordance. These distinct genomic patterns lead to differential responsiveness and patterns of resistance to HER2-directed therapy. ERBB2 amplified CRC with RAS/RAF alterations are resistant to trastuzumab-based combinations, such as trastuzumab/tucatinib, but retain sensitivity to trastuzumab deruxtecan in in vitro and murine models. Trastuzumab deruxtecan shows clinical efficacy in cases with high-level ERBB2 amplified RAS/RAF co-altered CRC. Conclusions: Co-occurring RAS/RAF alterations define a unique subtype of ERBB2 amplified CRC that has increased intratumoral heterogeneity, interlesional discordance and resistance to trastuzumab-based combinations. Further examination of trastuzumab deruxtecan in this previously understudied cohort of ERBB2 amplified CRC is warranted.