OF1 |  CANCER DISCOVERYFEBRUARY 2013 www.aacrjournals.org Activating HER2 Mutations in HER2 Gene Amplifi cation Negative Breast Cancer

ABSTRACT Data from 8 breast cancer genome-sequencing projects identifi ed 25 patients with HER2 somatic mutations in cancers lacking HER2 gene amplifi cation. To determine the phenotype of these mutations, we functionally characterized 13 HER2 mutations using in vitro kinase assays, protein structure analysis, cell culture, and xenograft experiments. Seven of these muta-tions are activating mutations, including G309A, D769H, D769Y, V777L, P780ins, V842I, and R896C. HER2 in-frame deletion 755–759, which is homologous to EGF receptor (EGFR) exon 19 in-frame dele-tions, had a neomorphic phenotype with increased phosphorylation of EGFR or HER3. L755S produced lapatinib resistance, but was not an activating mutation in our experimental systems. All of these mutations were sensitive to the irreversible kinase inhibitor, neratinib. These fi ndings show that HER2 somatic mutation is an alternative mechanism to activate HER2 in breast cancer and they validate HER2 somatic mutations as drug targets for breast cancer treatment. SIGNIFICANCE: We show that the majority of HER2 somatic mutations in breast cancer patients are activating mutations that likely drive tumorigenesis. Several patients had mutations that are resistan