Liquid Biopsy: Recent Advances in the Detection of Circulating Tumor Cells and Their Clinical Applications

Liquid biopsies using circulating tumor cells (CTCs) and cell-free DNA have tremendous potential for detecting, monitoring, and treating different cancers. The key rate-limiting factor of CTC-based technology has been the scarcity of recovery of CTCs hampering their downstream molecular analysis, as well as many different methods for detecting and capturing CTCs which lack standardization and clinical validation. We review different methods for capturing and evaluating CTCs, emphasizing a fluorescence in situ hybridization (FISH) antigen-independent approach which we have developed for accurate diagnosis of early lung cancer, as an adjunct to managing an indeterminate lung nodule. The increased sensitivity of our DNA FISH assay is based on identification of CTCs of all lineages that share similar genetic abnormalities, including the presence of EMT (epithelial-mesenchymal transitions) cells, stem cells, and cells co-expressing CD45 and cytokeratin. The molecular characterization of CTCs is a rapidly evolving field. An important development has been the ability to examine the genome of CTCs in real-time on a single-cell basis by whole-genome amplification followed by single-cell sequencing, providing unprecedented opportunities for targeted therapy. After clinical validation, we expect liquid biopsy testing for cancer to be incorporated routinely for cancer screening and prevention, the identification of tumors, and targeted therapy.