circCDYL2 promotes trastuzumab resistance via sustaining HER2 downstream signaling in breast cancer

Background Approximate 25% HER2-positive (HER2 + ) breast cancer (BC) patients treated with trastuzumab recurred rapidly. However, the mechanisms underlying trastuzumab resistance remained largely unclear. Methods Trastuzumab-resistant associated circRNAs were identified by circRNAs high-throughput screen and qRT-PCR in HER2 + breast cancer tissues with different trastuzumab response. The biological roles of trastuzumab-resistant associated circRNAs were detected by cell vitality assay, colony formation assay, Edu assay, patient-derived xenograft (PDX) models and orthotopic animal models. For mechanisms research, the co-immunoprecipitation, Western blot, immunofluorescence, and pull down assays confirmed the relevant mechanisms of circRNA and binding proteins. Results We identified a circRNA circCDYL2, which was overexpressed in trastuzumab-resistant patients, which conferred trastuzumab resistance in breast cancer cells in vitro and in vivo. Mechanically, circCDYL2 stabilized GRB7 by preventing its ubiquitination degradation and enhanced its interaction with FAK, which thus sustained the activities of downstream AKT and ERK1/2. Trastuzumab-resistance of HER2 + BC cells with high circCDYL2 could be reversed by FAK or GRB7 inhibitor. Clinically, HER2 + BC patients with high levels of circCDYL2 developed rapid recurrence and had shorter disease-free survival (DFS) and overall survival (OS) following anti-HER2 therapy compared to those with low circCDYL2. Conclusions circCDYL2-GRB7-FAK complex plays a critical role in maintaining HER2 signaling, which contributes to trastuzumab resistance and circCDYL2 is a potential biomarker for trastuzumab-resistance in HER2 + BC patients.