Fetal cells in maternal blood. An update from Basel

Noninvasive prenatal diagnosis of fetal aneuploidies and Mendelian disorders: new innovative strategies

The application of recent technical developments, such as digital PCR or shot-gun sequencing, for the analysis of cell-free fetal DNA, have indicated that the long-sought goal of the noninvasive detection of Down syndrome may finally be attained. Although these methods are still cumbersome and not high throughput, they provide a paradigm shift in prenatal diagnosis, as they could effectively pronounce the end of invasive procedures, such as amniocentesis or chorionic villous sampling for the detection of such fetal anomalies. However, it remains to be determined how suitable these approaches are for the detection of more subtle fetal genetic alterations, such as those involved in hereditary Mendelian disorders (e.g., thalassemia and cystic fibrosis). New technical developments, such as microfluidics and reliable automated scanning microscopes, have indicated that it may be possible to efficiently retrieve and examine circulating fetal cells. As these contain the entire genomic complement of the fetus, future developments may include the noninvasive determination of the fetal karyotype.

Quantitative analysis of intact fetal cells in maternal plasma by real-time PCR

OBJECTIVE

Fetal genetic materials in maternal circulation might be potentially used for non-invasive prenatal diagnosis (NIPD). In this study, we quantitatively analysed the intact fetal cells existing in maternal plasma, which would be a special method of NIPD.

STUDY DESIGN

Eleven samples from preeclamptic patients, two samples from patients with RhD incompatibilities, and 30 samples from normal pregnancies as controls were collected. The intact cells in the plasma fraction were separated by centrifugation at high speed. The intact cell pellets were washed with PBS to remove any cell-free DNA.

RESULTS

We were able to detect intact fetal cells in 3 out of 11 preeclamptic plasma samples from women carrying male fetuses, and from both plasma samples of RhD incompatibility affected pregnancies. However, intact fetal cells were not detected in all the 16 normal pregnancies with a male fetus, although cell-free fetal DNA was detected in all these cases.

CONCLUSIONS

Intact fetal cells might be present in maternal plasma. However, the rarity of these cells limits their use for reliable, non-invasive prenatal diagnosis. The detection of such cells was successful in some preeclamptic and RhD incompatibility samples, not in the normal controls. Therefore, as opposed to free fetal DNA in maternal blood the use of intact fetal cells does not provide a reliable mode for NIPD.

Prenatal diagnosis using fetal cells and free fetal DNA in maternal blood

The presence of fetal cells and free fetal DNA in maternal blood offers an exciting opportunity for the development of safe noninvasive forms of prenatal diagnosis. Research in this field has, however, also indicated that their levels in the maternal circulation are increased in certain pregnancy-related disorders, such as preeclampsia. Their closer examination may shed new light on the underlying etiology of this enigmatic disorder.

Optimization of nucleated red blood cell (NRBC) recovery from maternal blood collected using both layers of a double density gradient

The isolation of fetal nucleated red blood cells (NRBC) from maternal blood represents a promising approach to non-invasive prenatal diagnosis. However, the number of fetal NRBC in maternal circulation is quite low and therefore difficult to isolate. An enrichment procedure in which both layers from a double density 1.077/1.107 g/ml gradient are collected was optimized, followed by MACS selection using non-commercial monoclonal antibodies. The influence of the delay in processing maternal blood on the NRBC distribution in both interfaces of the gradient was also studied in cord blood and peripheral maternal blood samples. A significant increase in the number of NRBC isolated from maternal blood was achieved by collecting both layers of the double density gradient compared with the previous protocol in which only the lower layer was recovered. Cord blood samples showed significant differences in the number of NRBC recovered when processed at 24 instead of within 3 h. This effect was also observed in the number of NRBC collected only from the upper layer of peripheral maternal blood samples. Therefore, in order to minimize the target cell losses, it is advisable to process the maternal blood samples as soon as possible.

Elevation in erythroblast count in maternal blood before the onset of preeclampsia

OBJECTIVE

We recently showed that both maternal and fetal erythroblast counts are elevated in the peripheral blood of pregnant women with preeclampsia. The purpose of this study was to examine whether this elevation actually occurs before the clinical onset of the disorder.

STUDY DESIGN

Erythroblasts were enriched and enumerated in 97 maternal blood samples obtained in the second trimester, and results were subsequently correlated with pregnancy outcomes.

RESULTS

Significantly higher quantities of erythroblasts (mean, 6041.7 vs 928.9; P =.008) were detected in blood samples obtained from women who later acquired preeclampsia (n = 15) than in blood samples from the control cohort (n = 72). Intrauterine growth restriction (n = 10) was not accompanied by a similar rise in erythroblast count.

CONCLUSION

Because a large proportion of the erythroblasts in maternal blood are fetal, our data suggest that fetal-maternal cell traffic is affected early in pregnancies that are later complicated by preeclampsia but not in those affected only by intrauterine growth restriction.

[Fetal cells in the maternal blood and prenatal diagnosis]

Fetal loss after amniocentesis or chorionic villus sampling is a limit to prenatal diagnosis practice and to its generalization. The existence of fetal cells in the blood of pregnant women is now well established. Recognizing these cells with specific antibodies and isolating them with fluorescent or magnetic systems are the subject of numerous studies. However, to date, neither the sensitivity nor the specificity of these methods are sufficient to allow a non invasive prenatal diagnosis.

Fetal DNA in maternal plasma is elevated in pregnancies with aneuploid fetuses

Current non-invasive screening methods for the prenatal diagnosis of fetal aneuploidies are hampered by low sensitivities and high false positive rates. Attempts to redress this situation include the enrichment of fetal cells from maternal blood, or the use of fetal DNA in the plasma of pregnant women. By the use of real-time quantitative polymerase chain reaction (PCR) it has recently been shown that circulatory male fetal DNA in maternal plasma is elevated in pregnancies with trisomy 21 fetuses. In this independent study we confirm and extend upon these results by showing that the levels of fetal DNA are also elevated in pregnancies with other chromosomal aneuploidies (mean=185.8 genome equivalents/ml; range=62.2-471.7) when compared to pregnancies with normal male fetuses (mean=81.9 genome equivalents/ml; range=28.8-328.9), p=0.005. This elevation was greatest for fetuses with trisomy 21, whereas it was not significant for fetuses with trisomy 18, p=0.356. These data suggest that a quantitative analysis of such fetal DNA levels may serve as an additional marker for certain fetal chromosomal abnormalities, in particular for trisomy 21.

High levels of fetal erythroblasts and fetal extracellular DNA in the peripheral blood of a pregnant woman with idiopathic polyhydramnios: case report

Abnormal amniotic fluid volume can be associated with increased maternal risk as well as perinatal morbidity and mortality. Polyhydramnios is often indicative of fetal, placental or maternal problems. In a large proportion of patients the aetiology of the disorder is unclear. Here we report on a case in which numerous fetal erythroblasts and large quantities of extracellular fetal DNA were found in the peripheral blood of a pregnant woman with idiopathic polyhydramnios bearing a male fetus. Following enrichment of erythroblasts by magnetic separation (MACS) and anti-CD71 antibodies, approximately 45-fold more erythroblasts were determined per ml peripheral maternal blood than in matched controls (231 versus 5). Single cell multiplex polymerase chain reaction (PCR) of individually micromanipulated erythroblasts showed that approximately 122 of these were of fetal origin. The concentration of extracellular fetal circulatory DNA in maternal plasma was determined by real-time quantitative PCR and shown to be almost double that of the control group (749.2 versus 404 fetal genome equivalents per ml maternal plasma). It can be speculated that the increased intrauterine pressure in polyhydramnios leads to an enhanced influx of fetal cells and free extracellular fetal DNA into the maternal circulation. This hypothesis will have to be tested with further cases.

Fetal cells in cervical mucus and maternal blood

Research in developing effective and accurate methods for non-invasive prenatal diagnosis has focused on two main techniques: the retrieval of trophoblast cells from the cervix and the enrichment of fetal erythroblasts from the blood of pregnant women. The isolation of fetal cells by both approaches has permitted the identification of fetal aneuploidies by the use of fluorescence in-situ hybridization (FISH) with appropriate probes, as well as fetal single gene disorders by polymerase chain reaction (PCR). In the latter instance, it has been shown that in order to attain the high degree of specificity required for prenatal diagnosis, it is necessary to analyse single fetal cells isolated by micromanipulation. This practice has permitted the successful characterization of fetal rhesus status, haemoglobinopathies, Duchenné's muscular dystrophy and spinal muscular atrophy, amongst others.Further developments include investigations into whether the diagnostic potential of fetal cells retrieved by either method can be expanded by the possible culturing of such cells, as well as the possibility of performing successive rounds of FISH and PCR by the recycling of isolated fetal cells.A novel observation that our group has made is that the traffic of fetal cells is enhanced in pregnancies affected by the pregnancy related disorder, pre-eclampsia. Our subsequent investigations have shown that this elevation in fetal cell traffic may serve as an early marker for those pregnancies at risk for this disorder.A very recent exciting discovery has been that free extracellular fetal DNA can be detected in the plasma and serum of pregnant women, which may permit the rapid and accurate detection of uniquely fetal loci, such as the fetal rhesus D gene in rhesus D negative pregnant women.

Detection of fetal Rhesus D and sex using fetal DNA from maternal plasma by multiplex polymerase chain reaction

OBJECTIVE

To test the sensitivity, specificity and reproducibility using fetal DNA obtained from plasma of pregnant women by polymerase chain reaction for the simultaneous detection of both fetal sex and Rhesus D genotype.

METHOD

Blood samples were obtained from 22 Rhesus D negative pregnant women about to undergo an invasive procedure. DNA was extracted from the plasma fraction and analysed by a multiplex nested polymerase chain reaction using Y chromosome-and Rhesus D-specific primers. The results of this experimental procedure were compared with those obtained from the analysis performed on material gained by the invasive procedure.

RESULTS

The sensitivity of the plasma polymerase chain reaction-based method was surprisingly high, with both fetal genotypes being correctly determined in almost 100% of the cases examined. In only one instance was a false positive result for the detection of Rhesus D recorded, which on subsequent analysis was negative.

CONCLUSIONS

The ease and rapidity with which the plasma polymerase chain reaction-based method can be performed makes it a promising method for the analysis of multiple fetal loci, such as Rhesus D and sex.