HER2 heterogeneous breast cancer models reveal novel therapeutic targets and subclonal dynamics during evolution to resistance to HER2-targeted therapies

Intratumor heterogeneity for HER2 expression and amplification, observed in up to 40% of HER2-positive breast cancer is a driver of resistance to HER2-targeted therapies. The advancement of treatment for HER2 heterogeneous tumors has been hindered by the lack of preclinical models that accurately replicate the human disease. Here we describe human HER2 heterogeneous breast cancer cell models composed of ERBB2 amplified (HER2hi) and non-amplified (HER2lo) cell populations derived from the same tumor. Utilizing these models and cellular barcoding technology, we provide evidence for subclonal cooperation between HER2hi and HER2lo subpopulations and that HER2lo cells drive resistance to HER2-targeting antibody-drug conjugates (ADC) such as T-DXd but are sensitive to HER2 kinase inhibitors. CRISPR screens in heterogeneous co-cultures identified sensitizers of HER2lo cells to T-DXd including the USP9X deubiquitinating enzyme. USP9X inhibition enhances the lysosomal targeting of HER2 to potentiate ADC payload release, diminishing tumor recurrence after T-DXd treatment. Our results elucidate the functional relevance of HER2 heterogeneity and propose improved therapies for these tumors.