HER2 G776S Mutation Promotes Oncogenic Potential in Colorectal Cancer Cells when Accompanied by Loss of APC Function

Background Clinical cancer genome sequencing detects oncogenic variants that are potential targets for cancer treatment, but it also detects variants of unknown significance that may interact and affect the pathophysiology of the tumor; however, these interactions are not fully understood. In this study, we examined the interactions of a minor HER2 mutation (G776S) and APC mutations, which were detected by cancer genome sequencing of samples from a patient with colorectal cancer. Methods We transfected HER2-G776S mutant- or HER2 wild type- expressing vectors into several cell lines, HeLa, FHC, CACO-2 and COLO-320, to evaluate their effects on HER2 phosphorylation and kinase activity, HER2 downstream signaling (phosphorylation of AKT and MAPK), and anchorage-independent growth ability. APC- knockout cells and APC overexpressing cells were established to investigate the effect of APC function on the HER2 signaling pathway. We also evaluated the efficacy of a HER2 tyrosine kinase inhibitor on xenograft tumors derived from HER2-G776S transfected cells. Results HER2 G776S mutation increased the kinase activity and phosphorylation of HER2 protein, but these effects were weaker than those of the other HER2 driver mutation. HER2 G776S did not activate HER2-downstream signal pathways, such as ERK and AKT phosphorylation, in cells with wild-type APC (HeLa and FHC cells). By contrast, HER2 G776S increased the activation of HER2 downstream signaling, especially ERK phosphorylation, and anchorage-independent cell growth in cells with an APC mutation (CACO-2 and COLO-320) and APC-knockout HeLa cells. Wild-type APC overexpression in HER2 G776S-transfected COLO-320 cells neutralized ERK phosphorylation. Loss of APC function increased Wnt pathway activity but also increased RAS–GTP, which increased ERK phosphorylation triggered by HER2 G776S transfection. Afatinib, a pan-HER tyrosine kinase inhibitor, inhibited tumor growth of HER2 G776S-transfected COLO-320 xenografts Conclusions HER2 G776S mutation acts as a weak oncogenic driver, but it also increases HER2–ERK signaling activity by increasing RAS–GTP production when APC function is simultaneously impaired. These results suggest that even weakly active mutations may be therapeutic targets, and the use of this strategy may contribute to the development of HER2-targeted therapy for colorectal cancer.