Challenges and unanswered questions for the next decade of circulating tumour cell research in lung cancer

Abstract

Since blood borne circulating tumour cells (CTCs) initially shed from the primary tumour can seed and initiate metastasis at distant sites a better understanding of the biology of CTCs and their dissemination could provide valuable information that could guide therapeutic intervention and real time monitoring of disease progression. Although CTC enumeration has provided a reliable prognostic readout for a number of cancers, including lung cancer, the precise clinical utility of CTCs remains to be established. The rarity of CTCs together with the vanishingly small amounts of nucleic acids present in a single cell as well as cell to cell heterogeneity has stimulated the development of a wide range of powerful cellular and molecular methodologies applied to CTCs. These technical developments are now enabling researchers to focus on understanding the biology of CTCs and their clinical utility as a predictive and pharmacodynamics markers. This review summarises recent advances in the field of CTC research with focus on technical and biological challenges as well the progress made towards clinical utility of characterisation of CTCs with emphasis on studies in lung cancer.

Keywords: Circulating tumour cells (CTCs); biomarkers; liquid biopsy; lung cancer; metastasis.

Figure 1

Determining CTC copy number alterations (CNA). (A) Shows a schematic outline of the CTC NGS analysis workflow used. (B) Presents genome wide CNA profiles and accompanying images of single CTCs and WBCs isolated by epitope dependent and independent technologies from two patients with NSCLC. For all four cells the CNA pattern is presented in a linear format with marked copy number increases shown in red and losses in blue alongside the fluorescent stained images of the starting cells. The top panel presents the CNA patterns and images of a NSCLC cytokeratin (CK) positive, CD45 negative CTC and a CK negative, CD45 positive WBC obtained following CellSearch^® EpCAM enrichment and subsequent CK and CD45 staining with CK staining depicted as yellow, CD45 as green and nuclear staining (DAPI) as blue. The bottom panel presents the CNA patterns and images of a NSCLC vimentin (VIM) positive, CD45/CD31 negative CTC and a VIM positive, CD45/CD31 positive WBC obtained following Parsortix™ size enrichment and subsequent VIM plus combined CD45/CD31 staining with VIM staining depicted as red, CD45/CD31 as green and nuclear staining (DAPI) as blue. CTC, circulating tumour cell; NGS, next generation sequencing; NSCLC, non-small cell lung cancer.