A recent discovery, once again inspired by oncological observations concerning tumor-derived mRNA transcripts, is that placenta-derived mRNA species are present in maternal plasma samples. Unlike their cell-free fetal DNA counterparts, which are truly acellular, these mRNA molecules are contained in small membrane particles. This difference becomes apparent when filtering the plasma sample, in that the mRNA containing particles are retained by the filter, whereas the cell-free DNA passes through. Disruption of the protective membrane particles renders the mRNA highly labile, and it is rapidly degraded by nucleases in the plasma sample. Consequently, stored or frozen samples are not suitable for these studies. Current analyses have focused on mRNA transcripts expressed in the placenta such as human placental lactogen (hPL), human chorionic gonadotropin (phCG) or cortico-tropin-releasing hormone (CRH). In the latter instance, quantitative PCR analyses have indicated that the level of CRH mRNA species is elevated in pregnancies with manifest preeclampsia.1-19-1 The immediate advantage of studying mRNA species is that it permits fetal sex-independent analysis in that the study is not restricted to the detection of Y chromosome-specific DNA sequences.
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