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Departments of
1
Chemical Pathology and
2
Obstetrics and Gynecology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, New Territories, Hong Kong SAR
a address correspondence to this author at: Department of Chemical Pathology, The Chinese University of Hong Kong, Prince of Wales Hospital, Room 38023, 1/F Clinical Sciences Building, 30-32 Ngan Shing Street, Shatin, New Territories, Hong Kong SAR; fax 852-2194-6171, e-mail loym{at}cuhk.edu.hk
| Introduction |
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Pregnant women attending the Prenatal Diagnosis Unit at the Department of Obstetrics and Gynecology, Prince of Wales Hospital, Hong Kong were recruited with informed consent. The study was approved by the Clinical Research Ethics Committee. Women early and late in their pregnancies (n = 21 and 37, respectively) were recruited in this study. The mean gestational ages of the subjects in early and late pregnancies were 16 weeks (range, 1119 weeks) and 33 weeks (range, 2640 weeks), respectively. All early-pregnancy samples were obtained before any invasive procedure. On the other hand, late-pregnancy samples were collected either from women who had invasive procedures in early pregnancy (n = 21) or from women who did not have any prenatal invasive procedure (n = 16). All plasma samples were harvested within 30 min from EDTA-blood samples as described previously (1). Total RNA from plasma samples was isolated with the Trizol LS Reagent (Life Technologies) as instructed by the manufacturer. In general, RNA isolated from 1 mL of plasma was dissolved in 50 µL of RNase-free water.
In this study, we chose to detect fetal-derived, Y-chromosome-specific
zinc finger protein (ZFY) mRNA (6)(7)
in maternal plasma. As shown in Fig. 1
, RT-PCR products corresponding to ZFY mRNA were
observed only when male placental total RNA was used in the RT-PCR
assay (Fig. 1
, lane 1). By contrast, no positive signal was detected
when either reverse transcriptase was omitted (Fig. 1
, lane 2)
or female placental total RNA was used (Fig. 1
, lane 3) in the RT-PCR
assays. Among 20 women carrying male fetuses in late pregnancy,
ZFY-positive signals were detected (Fig. 1
, middle panel,
lanes 610) in 13 plasma samples. Positive signals were observed in
two of nine women carrying male fetuses in early pregnancy. The
identities of ZFY mRNA-specific RT-PCR products in the
positive cases were confirmed by DNA sequencing (data not shown). By
contrast, of 20 women carrying female fetuses either in early (n =
12) or in late (n = 8) pregnancy, all but 1 case were negative in
the assay (Fig. 1
, middle panel, lanes 1114). The only false-positive
case was presumably attributable to contamination during RNA
processing. As a control for the quality of the extracted RNA, we also
subjected all samples to a RT-PCR assay for HLA-G mRNA
(8). The HLA-G gene is expressed by both fetal
[e.g., trophoblasts (8)] and maternal [e.g., lymphocytes
(9)] tissues. As shown in the bottom panel of Fig. 1
, RT-PCR products specific for HLA-G mRNA were
detected in all tested plasma samples, demonstrating the presence of
amplifiable RNA in these samples.
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Recently, it has been demonstrated that a proportion of maternal plasma fetal DNA circulates in the form of intact fetal cells (10). Thus, theoretically it is possible that the fetal RNA that we detected in the current study could have originated from these "plasma-derived" cells. To conclusively test whether fetal RNA can be detected in the "cell-free" form in maternal circulation, maternal plasma samples were filtered by a 0.2 µm membrane (Nalgene), and the RNA extracted from these filtered plasma samples was tested by the ZFY RT-PCR assay. Of nine filtered plasma samples collected from women carrying male fetuses in late pregnancies, positive ZFY mRNA signals were detected in six samples (data not shown). These results indicate that at least a portion of fetal RNA in maternal plasma exists in the cell-free form. This observation is consistent with the recent finding that tumor-derived RNA can be detected in the circulation of cancer patients (11)(12).
Our data demonstrate that fetal RNA can be detected in maternal plasma. The detection rates of plasma fetal RNA in early and late pregnancies were 22% and 63%, respectively. The detection rate of fetal RNA in early pregnancy cases was lower than that in late pregnancy cases, suggesting that the concentration of plasma fetal RNA is lower in early pregnancy. This observation is similar to our previous finding that the concentration of fetal DNA in maternal plasma increases with gestation (13). We also realized that the detection rate of plasma fetal RNA in this study is lower than that of plasma fetal DNA (1). It is possible that fetal RNA is more susceptible to degradation in maternal blood. As a result, the amount of fetal RNA in plasma is much lower than plasma fetal DNA. This is supported by the fact that Y-specific DNA was detected in all plasma samples from women carrying male fetuses in this study (data not shown). To improve the sensitivity of maternal plasma fetal RNA detection, we are now developing a highly sensitive real-time quantitative RT-PCR assay for this purpose.
In conclusion, we have shown for the first time that fetal RNA can be detected in maternal plasma, and our data provide a novel means of noninvasive prenatal diagnosis. Plasma fetal DNA analysis can provide data on the presence and concentration of fetal genetic material in the maternal circulation. Plasma fetal RNA analysis, in addition, can provide valuable information regarding the gene expression patterns of fetal tissues. For example, abnormal pregnancies, such as those with preeclampsia, often are associated with abnormal gene expression patterns in fetal tissues (14). Thus, with the development of further RNA markers, maternal plasma RNA analysis may allow the noninvasive monitoring of fetal gene expression in a multitude of physiological and pathological conditions.
| Acknowledgments |
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| References |
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, interleukin 1 ß, and interleukin 10 is increased in preeclampsia. Am J Obstet Gynecol 1999;181:915-920.[ISI][Medline]
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