The detection of minimal numbers of contaminating epithelial tumor cells in blood or bone marrow: Use, limitations and future of RNA-based methods

Abstract

Many solid tumors commonly metastasize to the bone marrow and the presence of tumor cells in the bone marrow is associated with a poor prognosis. Detection of tumor cells in the bone marrow has been reported to be important to determine the prognosis of newly diagnosed patients and may be helpful in deciding whether or not systemic treatment is indicated. The majority of the studies focus on the detection of tumor cells in non-tumor tissue using immunocytochemistry and antibodies directed against epitopes of epithelial genes. Recently, the sensitive reverse-transcriptase polymerase chain reaction (RT-PCR) has been employed for the detection of tumor cells in bone marrow, using mRNA transcribed from epithelial genes as targets for RT-PCR. In some studies, encouraging results were reported when RT-PCR was used to detect expression of epithelial genes, but in many others frequent false-positive results were observed. These may results from the 'illegitimate expression' of epithelial genes in cells of non-epithelial tissues, such as bone marrow. Micrometastases in bone marrow can be detected with some sensitivity by antibodies directed against epithelial genes. RNA based methods, using epithelial genes as target for amplification, are less reliable. To improve these methods, a systematic approach is required to identify genes which are highly expressed in solid tumors and completely silent in blood and bone marrow of healthy individuals. Novel techniques, e.g., 'sequential analysis of gene expression (SAGE), are now available that allow such an endeavor.