Fetal cells in the peripheral blood of pregnant women express thymidine kinase: a new marker for detection

Fetal cells occur in maternal blood in a substantial proportion of normal pregnancies. Several different approaches have been used to detect and enrich these cells for non-invasive prenatal diagnosis. However, before these fetal cells can routinely be used for prenatal diagnosis, perfectly reproducible procedures for detection and enrichment need to be established. We found that these fetal cells express high intracellular levels of the DNA precursor pathway enzyme thymidine kinase. Since normal adult peripheral blood cells do not exhibit any thymidine kinase activity, this enzyme is a potent new marker to detect and enrich fetal cells from maternal blood. We further describe the first successful application of a cytofluorometric thymidine kinase assay to detect fetal cells in the maternal circulation by virtue of their high thymidine kinase activity.