Clinical data has increasingly shown that there is a correlation between the presence of tumor cells circulating in the peripheral blood, and a poor prognosis in solid tumors, such as prostate, breast and colorectal cancer. However, further characterization of CTCs remains pivotal not only to help identifying diagnostically and therapeutically relevant targets; but to assist in stratifying cancer patients for individual therapies. Furthermore, while only a small subpopulation of CTCs might lead to a poor patient outcome, the genetic and phenotypic heterogeneity of CTCs and its link with clinical course remains poorly understood. In recent published work, we have revealed that CTC populations of individual colon cancer patients detected with the CellSearch System consist of subclones diverging in their protein expression and genetic background (1, 2).
To follow up this study, we established a novel protocol for RNA-in situ hybridisation (RNA-ISH) that enables a direct analysis of individual CTCs for the expression of specific microRNAs (miRNAs). miRNAs are increasingly important markers and potential future therapy targets. Increased miR-10b expression has been suggested to be associated with a more malignant and invasive phenotype in breast cancer. In first experiments using breast cancer cell line cells (MCF-7 and MDA-MB-231) we could show that miR-10b expression can be investigated on single cell level after the CellSearch run. Moreover, we could detect a heterogenous expression pattern of miR-10b in CTCs of patients with solid tumors.
In conclusion, determining the genetic and phenotypic heterogeneity of CTCs, and link to patient outcome will be key to the development and rational application of future effective anticancer therapies.