The advent of personalized medicine has created the need for simple, accurate and minimally invasive methods to repeatedly assess the genotype of patients’ tumors. Liquid biopsy, the use of body fluids as an alternative to tissue biopsy, has emerged as the next technological advance in cancer diagnosis and treatment. Three approaches to liquid biopsy have been explored: circulating tumor cells (CTCs), circulating tumor DNA (ctDNA) and exosomes. In this article, the potential utility for each source of material will be discussed.
CTCs are cells that have escaped from the primary tumor mass and entered the bloodstream. One of the biggest technical hurdles to the use of CTCs for molecular diagnostics is their relative rarity, ranging from 1 to 5 per ml of whole blood (1). Well-validated approaches for isolating CTCs typically rely on the expression of cell surface markers such as epithelial cell adhesion molecule (EpCAM), which may not be present depending on tumor type and stage. However, CTCs do have distinct advantages. Other than traditional biopsy, they are the only source of intact tumor cells with preserved morphology and cell surface proteins. Advances in whole genome amplification have enabled analysis of novel mutations and mRNA expression profiling in a single cell. In addition, CTC enumeration is currently FDA-approved for predicting progression-free survival and overall survival.
Although the exact mechanism of ctDNA release from the primary tumor is uncertain, ctDNA has great potential for early cancer detection, identification of residual disease, and monitoring treatment response. In a study of patients with primarily Stage I or Stage II breast cancer, ctDNA mutations in PIK3CA were identified with 96.7% accuracy, indicating that ctDNA may be useful in early stages of disease (2). The presence of ctDNA also predicts relapse after post‑surgery chemotherapy in patients with breast cancer (3). Multiple clinical trials are currently underway to evaluate the prognostic value of ctDNA, stratify patients, and correlate ctDNA mutations with treatment response or resistance to therapy.
Exosomes carry a variety of molecular cargo and have been implicated in metastatic progression. In addition to a functional role, exosomes and other microvesicles are particularly useful for studies of miRNA and lncRNA. In blood samples, exosomes are enriched in miRNA content compared to whole blood, cell-free plasma or serum (4). Because they can transport lipids, proteins and nucleic acids, exosomes present a rich source for molecular diagnostic analysis and are more abundant than CTCs. Current limitations are primarily technical, with a lack of consensus regarding exosome size and validated surface markers.
With many sources of material, including recent developments in the molecular characterization of tumor-educated platelets, the clinical application of liquid biopsy has only just begun. Interested in learning more about liquid biopsy? Download our webinar on CTC characterization here!
- 1. Gold, B. et al. (2015) Do circulating tumor cells, exosomes, and circulating tumor nucleic acids have clinical utility? A report of the association for molecular pathology. J. Mol. Diagn. 17, 209.
- 2. Beaver, J.A. et al. (2014) Detection of cancer DNA in plasma of patients with early-stage breast cancer. Clin .Cancer Res. 20, 2643.
- 3. Garcia-Murillas, I. et al. (2015) Mutation tracking in circulating tumor DNA predicts relapse in early breast cancer. Sci. Transl. Med. 7, 302ra133.
- 4. Cheng, L. et al. (2014) Exosomes provide a protective and enriched source of miRNA for biomarker profiling compared to intracellular and cell-free blood. J. Extracell. Vesicles 3.