Abstract
BACKGROUND: While oncogene co-amplification is frequently observed, its clinical significance remains unclear, particularly with regard to its association with copy number. OBJECTIVE: The aim of this study was to elucidate the molecular traits of human epidermal growth factor receptor 2 (HER2)-positive gastric cancer, focusing on copy numbers of ERBB2 and other oncogenes, and to investigate their association with treatment efficacy. METHODS: This is a collaborative study associated with the HIGHSOX trial investigating the efficacy of trastuzumab combined with S-1 and oxaliplatin in HER2-positive gastric cancer, and next-generation sequencing was performed. Tumors in which ERBB2 was the highest copy number among co-amplified oncogenes were defined as "ERBB2 dominant". The others were defined as "non-ERBB2 dominant". RESULTS: A total of 32 patients were enrolled, and the median progression-free survival and overall survival were 11.8 and 28.8 months, respectively. ERBB2 amplification was present in 84.4%, with the majority (88%) exhibiting either co-amplification of ERBB2 with other oncogenes (75%) or amplification of other oncogenes alone (13%), including KRAS, MET, FGFR2, and CCNE1. A potential inverse relationship in copy number between ERBB2 and other oncogenes (ρ = - 0.345, p = 0.058) was observed. The ERBB2 copy number was significantly higher than that of other oncogenes in ERBB2-dominant tumors, whereas the opposite pattern was observed in non-ERBB2-dominant tumors (both p < 0.001). Non-ERBB2-dominant tumors showed significantly shorter progression-free survival (median progression-free survival: 6.9 vs 17.0 months, hazard ratio, 6.40, p = 0.001), overall survival (median overall survival 14.8 vs 35.5 months, hazard ratio, 2.72, p = 0.016), and a numerically lower response rate (63.6% vs 90.0%, p = 0.151). CONCLUSIONS: Copy number-based oncogene dominance may suggest tumor dependence and response to HER2-targeted therapy, highlighting a potential novel biomarker in HER2-positive gastric cancer.