Large Amounts of Cell-Free Fetal DNA are Present in Amniotic Fluid (Technical Briefs‪)‬

Until recently, the focus of our research in the noninvasive diagnosis of Down syndrome was the genetic analysis of intact fetal nucleated erythrocytes from the circulation of pregnant women (1). The initial step in the physical isolation of these target cells involves the layering of diluted maternal blood on a density gradient (2). Before 1997, we routinely discarded the plasma layer of the density gradient, unaware of the fact that it could contain nucleic acids. In 1997, prompted by reports of large quantities of tumor-specific DNA sequences in the plasma and serum of cancer patients (3-5), Lo et al. (6) demonstrated the presence of male fetal DNA sequences in the serum and plasma of pregnant women. Subsequently, this same group extended their observations by quantifying the fetal DNA in maternal plasma (7) and studying its kinetics and physiology (8). The detection and/or quantification of fetal DNA sequences in maternal plasma have been used for a variety of clinical applications, including diagnosis of gender, Rhesus D genotype, single gene disorders, aneuploidy, and preeclampsia [reviewed in Refs. (9, 10)]. More recently, it has also been suggested that the genetic material in the plasma consists of a continuum of intact and apoptotic cells as well as cell-free DNA (11, 12). Fetal DNA, however, appears to exist predominantly in a cell-free form in the maternal plasma (13). Despite advances in the clinical applications of this technology, to date not much is known about the tissue of origin of the cell-free fetal DNA and how it is metabolized in the pregnant woman.