HER2 mutation status in Japanese HER2-positive breast cancer patients

Human epidermal growth factor receptor 2 (HER2) gene amplification/overexpression is a major therapeutic target in breast cancer, and has been introduced as a predictive biomarker to identify patients who may benefit from therapy with anti-HER2 agents. HER2 somatic mutations have been reported, and these may influence the effect of HER2-targeted drugs. Here, we sought HER2 mutations in a group of 135 Japanese breast cancer patients with HER2-positive tumors. We analyzed HER2 mutations by direct Sanger sequencing of two major areas, the extracellular domain at position 309-310 and the kinase domain between 755 and 781. Two patients with the HER2 somatic mutation S310F in the extracellular domain were found in this series. One patient with the S310F mutation had a node-negative invasive ductal carcinoma classified as HER2 2+ by the HercepTest and fluorescence in situ hybridization (FISH) positive, and which was estrogen receptor (ER)-negative and progesterone receptor (PgR)-negative. Another patient with the S310F mutation had an apocrine carcinoma with seven lymph nodes positive for metastasis, classified as HER2 3+ by the HercepTest, but which was FISH-negative, as well as ER-negative and PgR-negative. Both patients had received adjuvant single-agent trastuzumab therapy, and had no local recurrence or distant metastasis for five and three years after surgery, respectively. Our data show that HER2 mutations are rare in HER2-positive Japanese breast cancer patients. The two mutations found in this study were identical, S310F. We suggest that in vitro experiments to determine whether the S310F mutation could be involved in resistance to anti-HER2 drugs are worthwhile in future.