First, it should be noted that the majority of circulating breast tumor cells lack the phenotypic characteristics necessary to establish a metastatic disease; furthermore, most of the circulating tumor cells have been shown to be apoptotic [2] and this could explain the observation that after surgery the released cells are cleared from the circulation [3
]. Only a small proportion of these circulating cells, identified as those that express CD44 but not CD24, may be capable of clonogenic growth in vitro. Furthermore, molecular detection of cytokeratin 19 in circulating tumor cells is unable to distinguish between non-viable and clonogenic cancer cells.
Secondly, although the sensitivity of molecular techniques has greatly improved the potential for detecting circulating tumor cells, it has also resulted in the possibility of false-positives, as was also experienced by us. Some of the reasons for such spurious positive results can be summarized as follows: illegitimate expression of CK19 and expression of pseudogenes (CK19 and CK19ß).
Furthermore, several investigations have shown that the addition of anticoagulants to blood samples affects leucocyte viability and causes ex vivo changes in cytokine production, which significantly alters the ability to detect accurately the presence of a particular target gene.
Finally, the expression of cytokeratin 19 can be induced in peripheral blood mononuclear cells by cytokines and growth factors. These factors circulate at higher concentrations in inflammatory conditions and neutropenia, so false-positive results may be more likely under these circumstances.
Department of Medical Oncology, University of Messina, Messina, Italy
* E-mail: pitini{at}ciaoweb.it
References
1. Stathopoulos EN, Sanidas E, Kafousi M et al. Detection of CK19 mRNA-positive cells in the peripheral blood of breast cancer patients with histologically and immunohistochemically negative axillary lymph nodes. Ann Oncol 2005; 16: 240246.
2. Mehes G, Witt A, Kubista E, Ambros PF. Circulating breast cancer cells are frequently apoptotic. Am J Pathol 2001; 159: 1720.
3. Choy A, McCulloch P. Induction of tumor cell shedding into effluent venous blood breast cancer surgery. Br J Cancer 1996; 73: 7982.[ISI][Medline]
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