Chromosome 17 polysomy in circulating tumor cells in patients with metastatic breast cancer: a case series

The human epidermal growth factor receptor 2 (HER2) gene is located on the long arm of chromosome 17 (Chr-17). While primary tumors with Chr-17 polysomy (polysomy 17) are histopathologically similar to HER2-negative tumors, the role of polysomy 17 in circulating tumor cells (CTCs) is still unknown. We report the detection rate of polysomy 17 in CTCs in patients with metastatic breast cancer (MBC). We determined the CTC count per 7.5 ml blood and polysomy 17 in CTCs at 3- to 4-week intervals up to 12 weeks in 52 patients. Polysomy was defined as Chr-17 ≥2.2. CTCs were detected in 40 of 52 patients (76.9%) during the study period, in 32 of the 52 patients (61.5%) at baseline, and in 21 of 49 patients (42.9%) at 3–4 weeks. Polysomy 17 in CTCs was present in 10 of 52 patients (19.2%) during the study period, in 5 of 52 patients (9.6%) at baseline, and in 7 of 49 patients (14.3%) at 3–4 weeks. The individual patient counts of polysomy 17 in CTCs/total count of CTCs examined for polysomy 17 at 3–4 weeks were 1/1, 1/7, 1/7, 2/27, 2/30, 2/50, and 3/50. Six of the 7 patients with polysomy 17 in CTCs had HER2-negative primary tumors. None of the CTCs displaying polysomy 17 themselves had HER2 amplification by FISH. In summary, polysomy 17 in CTCs was observed in only a small population of patients with MBC. We should prospectively evaluate its prognostic value in both HER2-positive and -negative metastatic breast cancer.