HER2 somatic mutation analysis in breast cancer: correlation with clinicopathological features.

HER2 mutations have been reported in approximately 2% of breast cancers. Regardless of HER2 overexpression or amplification status, breast cancer with HER2 mutations may respond to HER2-targeted therapy. As HER2 mutation is rare, the clinical and pathological features of HER2-mutated breast cancers, such as hormonal status, histological grade, and metastasis, remain poorly defined. Therefore, the identification of HER2-mutated breast cancer has clinical significance. We retrospectively screened patients with metastatic breast cancer in whom molecular profiling had been performed using next-generation sequencing from 2012 to 2015; we identified 18 patients with HER2 mutation. Mutations were found on next-generation sequencing-based panels, including Ion AmpliSeq Cancer Hotspot, Oncomine, FoundationOne, and Guardant360. HER2 mutations were identified in both the tyrosine kinase (n = 14) and extracellular (n = 4) domains. Of the 14 cases with tyrosine kinase domain mutations, 13 were estrogen receptor positive; the 4 cases with extracellular domain mutations were exclusively estrogen receptor negative. In addition, 11 of 14 patients with tyrosine kinase domain mutations had bone metastasis, whereas no patients with HER2 extracellular domain mutations had bone metastasis. Histologically, 13 patients had invasive ductal carcinoma, 1 had metaplastic carcinoma, and 4 had invasive lobular carcinoma (ILC). All 4 ILCs were high grade and pleomorphic, and not only had an HER2 mutation in the kinase domain but also had an HER2 mutation involving the L755 site. Specific mutation sites may be involved in the pathogenesis of nonclassic ILC.