Circulating tumour cells: the evolving concept and the inadequacy of their enrichment by EpCAM-based methodology for basic and clinical cancer research
P. K. Grover
2
A. G. Cummins
1
T. J. Price
0
I. C. Roberts-Thomson
1
J. E. Hardingham
0
0
Haematology-Oncology, The Queen Elizabeth Hospital
,
Woodville South
,
Australia
1
Gastroenterology and Hepatology
2
Departments of Surgery
Increasing evidence suggests that circulating tumour cells (CTCs) are responsible for metastatic relapse and this has fuelled interest in their detection and quantification. Although numerous methods have been developed for the enrichment and detection of CTCs, none has yet reached the 'gold' standard. Since epithelial cell adhesion molecule (EpCAM)-based enrichment of CTCs offers several advantages, it is one of the most commonly used and has been adapted for high-throughput technology. However, emerging evidence suggests that CTCs are highly heterogeneous: they consist of epithelial tumour cells, epithelial-to-mesenchymal transition (EMT) cells, hybrid (epithelial/EMT+) tumour cells, irreversible EMT+ tumour cells, and circulating tumour stem cells (CTSCs). The EpCAM-based approach does not detect CTCs expressing low levels of EpCAM and non-epithelial phenotypes such as CTSCs and those that have undergone EMT and no longer express EpCAM. Thus, the approach may lead to underestimation of the significance of CTCs, in general, and CTSCs and EMT+ tumour cells, in particular, in cancer dissemination. Here, we provide a critical review of research literature on the evolving concept of CTCs and the inadequacy of their enrichment by EpCAM-based technology for basic and clinical cancer research. The review also outlines future perspectives in the field.
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introduction
Although the risk of cancer death is linked to the stage of
the disease at the time of diagnosis, >90% of cancer deaths result
from the development of disseminated metastases [14].
Currently, the mechanism of genesis of metastases from solid
tumours is not fully understood and remains one of the most
enigmatic aspects of cancer biology. New insights are fast
causing paradigm shifts in our understanding of metastatic
phenomena. For instance, recent evidence indicates that tumour
cells disseminate at a relatively early stage of the natural history
of tumour growth [5, 6]. Current tumour staging procedures
and high resolution imaging technologies are not sensitive
enough to detect micro-metastases or early tumour cell
dissemination, the key events in tumour progression. Therefore,
detection of disseminated tumour cells (DTCs) might be
complementary to the current imaging procedures used for tumour
staging and improve the identification of cancer patients at high
risk of metastatic relapse [710]. This view has spurred interest
in the development of sensitive assays that allow the specific
detection of single DTC in cancer patients. Since sampling for
the detection of DTCs is an invasive procedure, therefore, the
focus in recent years has shifted to circulating tumour cells
(CTCs) in the peripheral blood.
Over the years, many procedures have been developed for the
detection of CTCs in the peripheral blood, yet none has reached
the gold standard of sensitivity and more importantly, of
specificity. Epithelial cell adhesion molecule (EpCAM) is expressed
ubiquitously, albeit at variable levels, in epithelial cells and their
cancers, but is absent in blood cells [1113]. This galvanized our
group to pioneer the development of first-ever use of
EpCAMbased enrichment followed by reverse transcription-polymerase
chain reaction (RT-PCR) for detecting CTCs in cancer patients
[14]. The use of our anti-EpCAM-labelled microbeads for this
purpose was later commercialized as Epithelial Enrich (Dynal
Biotech, Norway). Among the current EpCAM-based
technologies, the automated CellSearch system (Veridex, Warren, NJ,
USA) has gained considerable attention over the past several
years [8]. The system has been cleared by the Food and Drug
Administration for monitoring CTCs in metastatic breast,
colon, and prostate cancers [1517]. Currently, the
EpCAMbased approach is one of the most commonly used in exploring
CTCs, and a great deal of effort and resources has been invested
in adapting the method for modern automated high-throughput
The Author 2014. Published by Oxford University Press on behalf of the European Society for Medical Oncology.
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technologies [1824]. Numerous studies from our laboratory [6,
25, 26] and others have shown that the presence of elevated
levels of CTCs, as determined by the EpCAM-based approach
including the CellSearch system, is negatively correlated with
prognosis in patients with various types of cancers. However,
the relatively low sensitivity and specificity of the approach is a
significant concern [2729]. Furthermore, increasing evidence
suggests that CTCs are highly heterogeneous [10, 3036]. They
consist of epithelial tumour cells, epithelial-to-mesenchymal
transition (EMT) cells, hybrid (epithelial/EMT+) tumour cells,
irreversible EMT+ tumour cells, and circulati (...truncated)