Circulating Tumor Cells (CTC) in Cancer: a Great Leap Forward

Abstract

The major cause of cancer-associated mortality is tumor metastasis, but our understanding of this process is far from complete. During successful dissemination, tumor cells invade the surrounding tissue of the primary tumor, intravasate into blood and lymphatic vessels, translocate to distant tissues, extravasate, adapt to the new microenvironment, and eventually seed, proliferate, and colonize to form metastases. Because dissemination mostly occurs through the blood, circulating tumor cells (CTCs) that have been shed into the vasculature and may be on their way to potential metastatic sites are of obvious interest. Despite the presence of CTCs in cancer patients was first detected in 1869, just in the past decade numerous studies have shown that CTCs may be used as a marker to predict disease progression and survival in metastatic and possibly even in early-stage cancer patients. High CTC numbers correlate with aggressive disease, increased metastasis, and decreased time to relapse. Moreover, CTCs can initiate metastasis in a xenograft model. If CTCs are indeed a source of metastatic cells, they could become an integral part of tumor staging criteria, which are currently focused on the primary tumor. Because blood collection is simple and minimally invasive, CTCs could be used as a real-time marker for disease progression and survival. CTCs also have the potential to guide therapeutic management, indicate therapy effectiveness or necessity, even in the absence of detectable metastases, and offer insights into mechanisms of drug resistance. They could be used as a surrogate endpoint marker in clinical trials, and could also become a treatment target. Despite this great potential, the use of CTCs faces many hurdles. CTCs may be shed from different locations within tumors, which are heterogeneous in nature, and even from metastases. Indeed, frequently there is a clear discrepancy in gene expression between primary tumors and CTCs, as well as heterogeneity within the CTC population. It may become possible to identify the tissue of origin of CTCs by using expression profiling to detect organ-specific metastatic signatures. This would help to localize small metastatic lesions and to guide further diagnostic and therapeutic strategies. Cancer-associated traits in some cancers have been traced to so-called cancer stem cells (CSCs). The traits that define CSCs, i.e., self-renewal, tumor-initiating, motile, invasive, and heightened resistance to apoptosis, are also instrumental for metastasis, implying that, in cancers that follow the cancer stem cell model, CTCs with high metastatic potential might be CSCs. There is an ongoing discussion on whether tumor cells undergo epithelial-mesenchymal transition (EMT) during dissemination, resulting in a more mesenchymal or even more stem cell–like phenotype. However, current CTC detection methods mostly use the epithelial cell adhesion molecule (EpCAM), which may under-estimate CTC number and potentially miss a critical subpopulation. EMT can induce non-CSCs to enter a CSC-like state, generating CSCs de novo. However, the role of EMT in enabling metastatic dissemination is yet to be fully proven and may only operate in a fraction of cancer cells that are in close contact with adjacent reactive stroma. Indeed, in many tumors, carcinoma cells exhibit a partial mesenchymal state, a phenotype absent from normal tissues. Still, it remains to be determined what fraction of CTCs lose some or all EpCAM expression and undergo (partial) EMT and whether these (or any) CTCs have increased metastatic seeding potential or heightened resistance to systemic therapy and therefore greater prognostic value. That some CTCs are undetectable and not all detected CTCs have metastatic potential may falsely indicate that CTC enumeration is not a good marker for disease staging and prognosis. Advanced CTC analysis is being made possible by constant technical improvements in CTC detection and isolation, although there are still unresolved issues, specifically the need to standardize detection assays. The development of single-cell analyses that can detect gene expression in individual CTCs has revealed interesting data, such as the finding of human epidermal growth factor receptor 2 (HER2)–positive CTCs in HER2-negative breast cancers. Such studies have induced clinical trials on CTCs and indeed, the numerous ongoing trials are indicative of the vast interest in these cells. However, CTC genomics is still in its infancy, mainly due to a lack of technologies capable of isolating sufficient numbers of CTCs to analyze somatic mutations, and the lack of suitable material with which to compare results due to CTC heterogeneity. The potential clinical value of CTCs is clear: Early detection and treatment of metastatic spread are keys for disease outcome, and CTCs offer the ability to target metastasis in real time. Elucidating CTC biology will also help standardize detection and isolation of the potentially metastatic subpopulation of CTCs. The next frontier in the CTC field is their characterization using the constantly improving single-cell characterization techniques. Considerable challenges still exist however, and there is no one technique that is obviously superior. It is clear that every technique has some weaknesses and the methodologies being used are being refined continuously. Antigen-dependent immunological techniques may sacrifice sensitivity for the sake of specificity and vice versa for non-immunological techniques. The heterogeneity of CTCs is now better understood and it is clear that in future investigations, simple enumeration of CTCs may be overly simplistic, and techniques to identify particular subtypes of CTCs may be more relevant. Circulating Tumour Microemboli, CTCs with stem like features and CTCs associated with platelets all may have more metastasis-forming potential than others and techniques to isolate these forms specifically may be informative. The possibility of “anti-CTC” therapeutic targets may also be a fruitful strategy for the future, thereby preventing the formation of metastases that are so often the cause of relapse after seemingly effective initial treatment. For this to occur, it will be necessary to first identify the subpopulation of CTCs that are capable of forming metastases. The identification and phenotypic and genotypic characterization of this subgroup of CTCs may ultimately provide potential therapeutic targets. Antiplatelet therapy, targeting CTC–endothelium interactions or inhibiting the process of EMT may all be worthwhile future endeavors. Much remains to be learned about CTCs and their clinical potential as biomarkers and therapeutic targets.