Background: Circulating tumor cells (CTCs) are associated with prognosis in a variety of human cancers and have been proposed as a liquid biopsy for follow-up examinations. We show that tumor suppressor and metastasis suppressor genes are epigenetically silenced in CTCs isolated from peripheral blood of breast cancer patients.
Methods: We obtained peripheral blood from 56 patients with operable breast cancer, 27 patients with verified metastasis, and 23 healthy individuals. We tested DNA extracted from the EpCAM-positive immunomagnetically selected CTC fraction for the presence of methylated and unmethylated CST6, BRMS1, and SOX17 promoter sequences by methylation-specific PCR (MSP). All samples were checked for KRT19 (keratin 19, formerly CK-19) expression by reverse-transcription quantitative PCR.
Results: In CTCs of patients with operable breast cancer, promoter methylation of CST6 was observed in 17.9%, BRMS1 in 32.1%, and SOX17 in 53.6% of patients. In CTCs of patients with verified metastasis, promoter methylation of CST6 was observed in 37.0%, BRMS1 in 44.4%, and SOX17 in 74.1%. In healthy individuals, promoter methylation of CST6 was observed in 4.3%, BRMS1 in 8.7%, and SOX17 in 4.3%. DNA methylation of these genes for both operable and metastatic breast cancer was significantly different from that of the control population.
Conclusions: DNA methylation of tumor suppressor and metastasis suppressor genes is a hallmark of CTCs and confirms their heterogeneity. Our findings add a new dimension to the molecular characterization of CTCs and may underlie the acquisition of malignant properties, including their stem-like phenotype.