Lessons (to be) learned from liquid biopsies: assessment of circulating cells and cell-free DNA in cancer and pregnancy-acquired microchimerism

Abstract

Tumors constantly shed cancer cells that are considered the mediators of metastasis via the blood stream. Analysis of circulating cells and circulating cell-free DNA (cfDNA) in liquid biopsies, mostly taken from peripheral blood, have emerged as powerful biomarkers in oncology, as they enable the detection of genomic aberrations. Similarly, liquid biopsies taken from pregnant women serve as prenatal screening test for an abnormal number of chromosomes in the fetus, e.g., via the analysis of microchimeric fetal cells and cfDNA circulating in maternal blood. Liquid biopsies are minimally invasive and, consequently, associated with reduced risks for the patients. However, different challenges arise in oncology and pregnancy-acquired liquid biopsies with regard to the analyte concentration and biological (background) noise among other factors. In this review, we highlight the unique biological properties of circulating tumor cells (CTC), summarize the various techniques that have been developed for the enrichment, detection and analysis of CTCs as well as for analysis of genetic and epigenetic aberrations in cfDNA and highlight the range of possible clinical applications. Lastly, the potential, but also the challenges of liquid biopsies in oncology as well as their translational value for the analysis of pregnancy-acquired microchimerism are discussed.

Keywords: Biomarkers; Cell-free DNA; Circulating tumor DNA; Circulating tumor cells; Liquid biopsy; Pregnancy-acquired microchimerism.

Fig. 1

Schematic overview of CTC capture methods with label-dependent enrichment (enrichment & immunofluorescent staining, enrichment & sorting, enrichment & molecular characterization), the combination of negative enrichment and differential centrifugation (combinatorial approach), label-independent enrichment (size & deformability) and enrichment-free methods (immunofluorescent staining). Label-dependent methods are highlighted in green, the label-independent method is highlighted in blue and the enrichment-free method is highlighted in gray. CD45 – cluster of differentiation 45; CTCs – circulating tumor cells; EGFR – epidermal growth factor receptor; EpCAM – epithelial cell adhesion molecule; HER2 – human epidermal growth factor receptor 2; MACS – magnetic-activated cell sorting. The Figure was created using BioRender.com

Fig. 2

Diagram depicting differences and similarities between cell-based (surrounded by blue border) and cell-free (surrounded by green border) approaches in cancer and feto-maternal medicine. Cancer-specific statements are displayed in light blue, shared statements in cyan and feto-maternal-specific statements in light green. ccfDNA – circulating cell-free fetal cell; CFC – circulating fetal cells; cfDNA – circulating cell-free DNA; ctDNA – circulating tumor DNA; FDA – US Food and Drug Administration. The Figure was created using BioRender.com