Differential expression of carcinoembryonic antigen (CEA) splice variants in whole blood of colon cancer patients and healthy volunteers: implication for the detection of circulating colon cancer cells.

Quantification of circulating cancer cells in whole blood samples by real time quantitative RT-PCR might be of clinical value for monitoring therapeutic effectiveness. In colon cancer patients, carcinoembrynic antigen (CEA) and cytokeratin 20 (CK20) have been frequently used for RT-PCR based tumor cell detection, but the specificity in particular for CEA has been questioned. In this study, we compared real-time RT-PCR for CEA and CK20 and analysed patients with metastatic disease (n=32) and healthy volunteers (n=17). CK20 mean values were elevated in cancer patients (P<0.001) and defined a subgroup (38%) who showed CK20 levels at least 100-fold above the highest value of the healthy control group. In contrast, only two cancer patients (6%) showed elevated CEA levels. Samples of the healthy control group showed exclusively a CEA-PCR product of 79 degrees C melting temperature. Thirty per cent of the colon cancer patients showed an additional product of 82 degrees C melting temperature. The 82 degrees C product was identical with the amplification product of CEA-cDNA and cDNA from different colon cancer cell lines. Colon cancer cells were spiked into normal blood in 10-fold dilutions that resulted in a dose dependent shift of the melt curve from 79 degrees C to the 82 degrees C. Sequencing of the PCR products showed that white blood cells express a splice variant of CEA, which hinders detection of tumor cell cDNA in whole blood samples. Our findings have implications for the use of CEA as a diagnostic molecule (e.g. by RT-PCR). The discovery of a physiologically expressed CEA splice variant might lead to a better understanding of the biological function of CEA and its family members.