Prenatal detection of a cystic fibrosis mutation in fetal DNA from maternal plasma

The use of cell-free fetal nucleic acids in maternal blood for non-invasive prenatal diagnosis

BACKGROUND

Cell-free fetal nucleic acids (cffNA) can be detected in the maternal circulation during pregnancy, potentially offering an excellent method for early non-invasive prenatal diagnosis (NIPD) of the genetic status of a fetus. Using molecular techniques, fetal DNA and RNA can be detected from 5 weeks gestation and are rapidly cleared from the circulation following birth.

METHODS

We searched PubMed systematically using keywords free fetal DNA and NIPD. Reference lists from relevant papers were also searched to ensure comprehensive coverage of the area.

RESULTS

Cell-free fetal DNA comprises only 3-6% of the total circulating cell-free DNA, therefore diagnoses are primarily limited to those caused by paternally inherited sequences as well as conditions that can be inferred by the unique gene expression patterns in the fetus and placenta. Broadly, the potential applications of this technology fall into two categories: first, high genetic risk families with inheritable monogenic diseases, including sex determination in cases at risk of X-linked diseases and detection of specific paternally inherited single gene disorders; and second, routine antenatal care offered to all pregnant women, including prenatal screening/diagnosis for aneuploidy, particularly Down syndrome (DS), and diagnosis of Rhesus factor status in RhD negative women. Already sex determination and Rhesus factor diagnosis are nearing translation into clinical practice for high-risk individuals.

CONCLUSIONS

The analysis of cffNA may allow NIPD for a variety of genetic conditions and may in future form part of national antenatal screening programmes for DS and other common genetic disorders.

Improvement in strategies for the non-invasive prenatal diagnosis of Huntington disease

PURPOSE

We focused on the improvements of prenatal diagnosis by the analysis of DNA from maternal plasma, using Huntington disease as a model of disease.

METHODS

We studied plasma from a pregnancy at risk of having a fetus affected with Huntington disease by the use of two direct analysis of the mutation and polymorphic STRs.

RESULTS

Direct methods were not informative. Analysis with STRs revealed the presence of the allele that does not co-segregate with the disease, thus the fetus was healthy.

CONCLUSIONS

This strategy is very useful to face complex cases when the direct study is not informative not only for Huntington disease but also for many other disorders.

Prenatal diagnosis in maternal plasma of a fetal mutation causing propionic acidemia

Prenatal diagnosis (PD) is available to families affected with propionic acidemia (PA), however, it entails a risk of miscarriage. Fetal DNA circulating in maternal blood could allow performing a safe prenatal diagnosis of fetal mutations. Exclusion of the paternal mutation in maternal plasma may avoid conventional PD in cases of recessive disorders such us PA. In this work, we have correctly diagnosed in maternal plasma the status of a fetus at risk of PA for the paternal mutation.

New strategy for the prenatal detection/exclusion of paternal cystic fibrosis mutations in maternal plasma

BACKGROUND

Since the presence of fetal DNA was discovered in maternal blood, different investigations have focused on non-invasive prenatal diagnosis. The analysis of fetal DNA in maternal plasma may allow the diagnosis of fetuses at risk of cystic fibrosis (CF) without any risk of fetal loss. Here, we present a new strategy for the detection of fetal mutations causing CF in maternal plasma.

METHODS

We have used a mini-sequencing based method, the SNaPshot, for fetal genotyping of the paternal mutation in maternal blood from three pregnancies at risk of CF.

RESULTS

The paternal mutation was detected in the analysis of plasma samples from cases 1 and 3 but not in case 2. Results of a posterior conventional molecular analysis of chorionic biopsies were in full agreement with those obtained from analysis of the plasma samples.

CONCLUSIONS

The knowledge about the inheritance of the paternal mutation in a fetus may avoid the conventional prenatal diagnosis in some cases. The SNaPshot technique has been shown to be a sensitive and accurate method for the detection of fetal mutations in maternal plasma. Its ease handling, rapid and low cost makes it appropriate for a future routine clinical use in non-invasive prenatal diagnosis of cystic fibrosis.

Non-invasive prenatal diagnosis of single gene disorders: how close are we?

Analysis of cell free fetal DNA (cffDNA) in maternal plasma provides the opportunity for reliable, timely, safe and cost-effective diagnosis of single gene disorders. The detection of certain fetal loci using cffDNA and conventional molecular analytic approaches is possible from 4 weeks gestation. To date, non-invasive first-trimester analysis for single gene disorders has been limited by assay sensitivity and specificity, due to the background maternal DNA. The anticipated ability to enrich the fetal component of cell free DNA will increase the robustness of tests and permit semi-quantitative analysis, broadening the scope of testing to include recessive disorders such as cystic fibrosis. Testing for large-scale mutations might remain limited by the fragmented nature of cffDNA and, when testing very early in gestation, careful ultrasound examination will be needed to determine the number of gestational sacs, because of the risk of discordant twin pregnancies.

Foetal sex determination in maternal blood from the seventh week of gestation and its role in diagnosing haemophilia in the foetuses of female carriers

The existence of foetal DNA in maternal blood, discovered in 1997, opened new possibilities for noninvasive prenatal diagnosis. This includes foetal sex assessment by the detection of specific Y chromosome sequences in maternal blood, particularly important when a foetus may be affected by an X-linked disorder such as haemophilia. This study aims to validate this sex assessment method and to test its clinical utility in the diagnosis of 15 potentially affected pregnancies in female carriers of haemophilia. In the validation study, 316 maternal blood samples from 196 pregnant women at gestations ranging from 5 weeks to 12 weeks were analysed. In the clinical study, 15 pregnancies at risk of having a haemophilic foetus were tested. All pregnancies in the validation study were correctly diagnosed. The accuracy and specificity of the methodology from the seventh week of gestation was 100%. The sex of all 15 pregnancies identified as being at risk of bearing a haemophilic foetus was correctly diagnosed. Foetal sex assessment by detecting specific Y chromosome sequences in maternal blood is now routinely used in our hospital because of its high accuracy from the seventh week of gestation. Reliable foetal gender determination from maternal blood of pregnant women carriers of haemophilia in the first trimester of gestation can avoid more conventional, invasive methods of prenatal diagnosis.

Circulating nucleic acids in plasma/serum

Since the discovery of circulating nucleic acids in plasma in 1948, many diagnostic applications have emerged. For example, diagnostic and prognostic potentials of circulating tumour-derived DNA have been demonstrated for many types of cancer. The parallel development of fetal-derived DNA detection in maternal plasma has opened up the possibility of non-invasive prenatal diagnosis and monitoring of many pregnancy-associated disorders. In this regard, non-invasive fetal rhesus blood group genotyping has already been translated to clinical practice. Other applications of circulating DNA in traumatology and transplant monitoring have also been reported. The more recent discoveries of circulating tumour-derived RNA and fetal-derived RNA have proven to be equally important as their DNA counterparts. Successful prenatal diagnosis of Down's syndrome by fetal RNA analysis has recently been reported. However, the definite origin and release mechanisms of circulating nucleic acids have remained incompletely understood, with cell death being suggested to be associated with such nucleic acid release. Pre-analytical standardisation will become increasingly relevant when comparing data from different laboratories. In conclusion, studies of circulating nucleic acids have promised exciting developments in molecular diagnostics in the years to come.

Early detection of cell-free fetal DNA in maternal plasma

OBJECTIVES

We aimed to establish the earliest gestational age at which fetal DNA in maternal plasma could be detected and whether this was reliable at 12-13 weeks' gestation.

STUDY DESIGN

A prospective observational cohort study of 32 pregnancies either after IVF or before prenatal diagnosis by CVS. Maternal blood was taken and RT-PCR was carried out to detect the multi-copy Y chromosome associated DSY14 gene. The end point was gender as assessed at delivery or on karyotype.

RESULTS

Y signal was obtained as early as 14 days post conception (4 weeks' gestation) and has a good prediction rate by 12 weeks' gestation.

CONCLUSION

Free fetal DNA allows very early prediction of fetal sex in some cases and could be useful for clinical use for X-linked conditions by the end of the first trimester.

Free fetal DNA in maternal plasma in anembryonic pregnancies: confirmation that the origin is the trophoblast

OBJECTIVE

To test the hypothesis that free fetal DNA (ffDNA) circulating in maternal plasma originates mainly from the placenta we studied ffDNA levels in anembryonic pregnancies.

METHODS

Maternal blood samples were collected from 15 normal first-trimester pregnancies in which fetal sex was subsequently determined and nine patients with a diagnosis of anembryonic gestation (AG). The Y chromosome DYS14 gene was quantified by real-time quantitative PCR (RT-PCR) for the determination of fetal sex in both plasma and chorionic tissue samples. Fetal sex in chorionic tissue samples was also determined using quantitative fluorescence PCR (QF-PCR).

RESULTS

The correct sex result was obtained from maternal plasma in all. Four AG pregnancies were female (DYS14 negative) results. In five of the AG cases, the chorionic tissue was found to be male (by both QF-PCR and RT-PCR which agreed) and positive male signal was found in maternal plasma by RT-PCR. There was no statistical difference between median free fetal DNA concentration in plasma between the AG male cases (148.3 GE/mL) and controls (145.8 GE/mL).

CONCLUSION

Since ffDNA levels are normal in pregnancies without a fetus, the data support the hypothesis that the trophoblastic cells are the major source ffDNA in maternal plasma.

Prenatal diagnosis using cell-free nucleic acids in maternal body fluids: A decade of progress

The ability to detect cell-free fetal nucleic acids in pregnant women has greatly evolved over the past decade. Dozens of papers have explored the biology, kinetics, and clinical significance of both cell-free fetal DNA and mRNA in the maternal circulation. As a result, our overall understanding of fetal nucleic acid trafficking has expanded. To date, two applications, gender determination and fetal RhD status, have translated into clinical medicine. However, with advanced molecular techniques such as mass spectrometry, real-time quantitative polymerase chain reaction, and gene expression arrays, the ease with which fetal genes can be detected within the mother has greatly improved. Newly identified placental and fetal mRNA transcripts as well as an epigenetically modified placental DNA marker, maspin, have universal applicability. Global expression analyses of fetal mRNA in both amniotic fluid and blood provide new insights into fetal development and pathology. Prenatal diagnosis is poised to evolve from detection of aneuploidy to detection of deviation from normal development, which should provide novel opportunities for fetal treatment.

Recent Developments in the Detection of Fetal Single Gene Differences in Maternal Plasma and the Role of Size Fractionation

The presence of cell-free fetal DNA in maternal plasma allowed noninvasive prenatal diagnosis of fetal loci completely absent from the maternal genome, such as SRY gene and RhD gene. However, the detection of fetal point mutations is hindered by the predominance of maternal DNA sequences. Recent studies have shown that cell-free fetal DNA exists in maternal plasma in small fragments. Thus, cell-free fetal DNA can be enriched by size fractionation, which improves detection of fetal gene mutations. Furthermore, it has been shown that Matrix Assisted Laser Desorption Ionization Time-of-Flight (MALDI-TOF) mass spectrometry also permits the detection of fetal SNPs from maternal plasma. These two new developments are discussed.

Non-Invasive Prenatal Detection of Paternal Origin Hb Lepore in a Male Fetus at the 7th Week of Gestation

OBJECTIVE

To perform a reliable non-invasive prenatal detection of the Hb Lepore paternal mutation and determine the fetal gender in the first trimester of pregnancy.

METHODS

DNA was extracted from a serum sample obtained from a pregnant woman at the mid first trimester of gestation. Hb Lepore-specific, mutant and normal, primers as well as Y-chromosome-specific STSs were used to carry out the analysis.

RESULTS

Paternal Hb Lepore and the DYS14 and DYZ1 gene-specific sequences were detected in the serum sample obtained at the 7th week of pregnancy. None of the above sequences was detectable in the maternal peripheral blood cell DNA.

CONCLUSION

Conventional polymerase chain reaction analysis of cell-free fetal DNA can be used to determine fetal gender and paternal Hb Lepore as early as the 7th week of pregnancy.

Detection of a Paternally Inherited Fetal Mutation in Maternal Plasma by the Use of Automated Sequencing

The discovery of circulating fetal DNA in maternal blood has been an encouraging step forward in the prenatal diagnostic field. It has opened up the possibility of development of a noninvasive method for the genetic analysis of the fetus. Many techniques have been applied to the study of this fetal DNA, but automated sequencing has been seldom used. The intention of this study was to use the automated sequencing technique for the detection of a paternally inherited fetal mutation in maternal plasma. Maternal plasma samples from a pregnant woman, whose husband had a mutation (Q134X) in the RP2 gene, which is located in the X-chromosome, were collected at two different gestational ages (10th and 19th week of gestation) in order to determine whether the paternally inherited fetal mutation could be detected by automated sequencing. Restriction analysis was also performed to confirm the results. The fetal mutation was clearly detected in the maternal plasma by the use of automated sequencing. The automated sequencing enables the possibility of analyzing fetal sequences, at a nucleotide level, in order to detect mutations or polymorphisms which are distinguishable from maternal sequences.

Quantitation of fetal DNA in maternal serum during the first trimester of pregnancy by the use of a DAZ repetitive probe

Cell-free fetal DNA in maternal plasma or serum is at present widely investigated as a source of fetal genetic material, both in studies of pregnancy-related disorders and in planning strategies for non-invasive prenatal diagnosis. Despite the number of trials already performed on the quantitation of fetal DNA, data about the amount of DNA at the beginning of pregnancy, in particular in the first trimester, remain limited. A new probe mapping on the deleted in azoospermia (DAZ) repetitive region of the Yq chromosome was designed for an early assessment of fetal DNA concentration in maternal serum. Among 57 pregnant women prospectively studied in their first trimester, fetal DNA was detected already by the 5th gestational week, with the analysis becoming reliable by the 8th week of gestation when a 100% accuracy in fetal sex determination was achieved. Moreover, in the three cases of pregnancy ending in fetal loss, the amount of fetal DNA apparently decreased before the abortion was diagnosed, whereas it consistently showed an increasing trend in normal pregnancies. Real-time PCR with the use of DAZ multilocus probe can efficiently quantitate free fetal DNA in the maternal serum at the beginning of pregnancy.

Identification of fetal gender in maternal blood is a helpful tool in the prenatal diagnosis of haemophilia

Fetal DNA identification in maternal circulation has provided a new approach for non-invasive prenatal diagnosis. However, fetal DNA can persist in maternal blood long after the delivery, severely hampering this possibility. We addressed the issue of fetal DNA persistence in maternal blood. Thus, we investigated cell-free fetal DNA as a reliable approach in prenatal diagnosis of haemophilia. Forty non-pregnant women, who had had at least a male fetus, 29 control pregnant women, and 14 pregnant women, carriers of hemophilia A or B. The assessment of Y-chromosomal sequences was performed by analysing SRY and amelogenin genes using PCR-based techniques. A protocol consisting of double centrifugation at full speed followed by plasma filtration hampered the detection of Y chromosome-specific sequence in non-pregnant women. In 29 control pregnant women, blinded determination of fetal sex confirmed the specificity and sensitivity of the method applied. In 14 pregnant carriers of hemophilia, the investigation revealed a male fetus in nine pregnancies. Excluding the three cases in which a spontaneous miscarriage occurred, the sensitivity and specificity of fetal sex prediction by SRY and amelogenin gene analyses were both 100% as compared with the invasive approach and the fetal sex outcome at birth (six males and five females). Because of its high accuracy in prediction, fetal gender determination with cell-free fetal DNA in maternal plasma may be a useful tool in prenatal diagnosis of haemophilia allowing for the avoidance of invasive procedures for female fetuses.

Genetic characterisation of circulating fetal cells allows non-invasive prenatal diagnosis of cystic fibrosis

OBJECTIVES

Cystic fibrosis (CF) is an autosomal recessive disease due to mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. The purpose of this study was to develop a molecular method to characterise both paternal and maternal CFTR alleles in DNA from circulating fetal cells (CFCs) isolated by ISET (isolation by size of epithelial tumour/trophoblastic cells).

METHODS

The molecular protocol was defined by developing the F508del mutation analysis and addressing it both to single trophoblastic cells, isolated by ISET and identified by short tandem repeats (STR) genotyping, and to pooled trophoblastic genomes, thus avoiding the risk of allele drop out (ADO). This protocol was validated in 100 leucocytes from F508del carriers and subsequently blindly applied to the blood (5 mL) of 12 pregnant women, at 11 to 13 weeks of gestation, whose offspring had a 1/4 risk of CF. Ten couples were carriers of F508del mutation, while two were carriers of unknown CFTR mutations.

RESULTS

Results showed that one fetus was affected, seven were heterozygous carriers of a CFTR mutation, and four were healthy homozygotes. These findings were consistent with those obtained by chorionic villus sampling (CVS).

CONCLUSION

Our data show that the ISET-CF approach affords reliable prenatal diagnosis (PND) of cystic fibrosis and is potentially applicable to pregnant women at risk of having an affected child, thus avoiding the risk of iatrogenic miscarriage.

Prenatal fetal sex diagnosis by detecting amelogenin gene in maternal plasma

OBJECTIVES

To provide a new, reliable noninvasive method for fetal sex determination.

METHODS

Fetal sex was detected in 32 early pregnant women by identifying the amelogenin gene in maternal plasma using nested PCR analysis. First, the 122/128 bp of X-Y homologous region containing 6 bp deletions in the intron 3 of amelogenin gene in X chromosome was amplified, and then the nested PCR was carried out, whose 3' end of the upstream primer is just located in the deletion region. The fetus was male or female, depending on whether it had the 89-bp nested PCR product or not.

RESULTS

The 89 bp of nested PCR product was detected in 19 plasma samples obtained from pregnant women, deducing they bear the male fetus and the remaining pregnant women bear female. When compared with the birth outcome, two samples were pseudo-positive. The coincidence was 93.8%. This method had high sensitivity that even trace amount of target fetal DNA (10 pg) could be detected.

CONCLUSIONS

This conventional nested PCR analysis of amelogenin gene promises to be a reliable method for noninvasive fetal sex determination at early pregnancy using maternal plasma DNA.

Cell-free fetal DNA in the maternal circulation and its future uses in obstetrics

OBJECTIVE

To provide an introduction to new technologies involving maternal plasma cell-free fetal DNA for non-invasive prenatal diagnosis and screening in obstetrics.

OPTIONS

Limited to introductory discussion of maternal plasma cell-free fetal DNA.

EVIDENCE

MEDLINE was searched to identify publications related to the topic after 1996. This document represents an abstraction of the information.

VALUES

This update is a consensus of the Genetics Committee of the Society of Obstetricians and Gynaecologists of Canada (SOGC).

BENEFITS, HARMS, AND COSTS

This update educates about new technology and its future use in obstetrics. At present, there is no harm or cost (research with limited clinical application) identified.

CONCLUSIONS

1. Significant and measurable amounts of cell-free fetal DNA are present in the maternal circulation and increase throughout pregnancy. 2. Different fetal (trisomy 21, trisomy 13) and placental abnormalities can affect the levels of cell-free fetal DNA within the maternal plasma. 3. Diagnostic and screening techniques may be able to utilize this cell-free fetal DNA in the future to provide non-invasive screening and diagnosis opportunities. This DNA technique is already well established for fetal sexing in pregnancies at risk of an X-linked disorder and fetal rhesus-D evaluation. Other conditions with well-identified unique paternal mutations can also reliably apply this cell-free fetal DNA technology for prenatal diagnosis. 4. The overall use of this molecular technology is still limited and requires the identification of sex-independent DNA markers so that female fetal DNA can be distinguished from maternal DNA, allowing its use in the screening or diagnosis of fetal and placental disease in pregnancies of either fetal sex.

Fetal DNA detection in maternal plasma throughout gestation

The presence of fetal DNA in maternal plasma may represent a source of genetic material which can be obtained noninvasively. We wanted to assess whether fetal DNA is detectable in all pregnant women, to define the range and distribution of fetal DNA concentration at different gestational ages, to identify the optimal period to obtain a maternal blood sample yielding an adequate amount of fetal DNA for prenatal diagnosis, and to evaluate accuracy and predictive values of this approach. This information is crucial to develop safe and reliable non-invasive genetic testing in early pregnancy and monitoring of pregnancy complications in late gestation. Fetal DNA quantification in maternal plasma was carried out by real-time PCR on the SRY gene in male-bearing pregnancies to distinguish between maternal and fetal DNA. A cohort of 1,837 pregnant women was investigated. Fetal DNA could be detected from the sixth week and could be retrieved at any gestational week. No false-positive results were obtained in 163 women with previous embryo loss or previous male babies. Fetal DNA analysis performed blindly on a subset of 464 women displayed 99.4, 97.8 and 100% accuracy in fetal gender determination during the first, second, and third trimester of pregnancy, respectively. No SRY amplification was obtained in seven out of the 246 (2.8%) male-bearing pregnancies. Fetal DNA from maternal plasma seems to be an adequate and reliable source of genetic material for a noninvasive prenatal diagnostic approach.

Application of fetal DNA detection in maternal plasma: a prenatal diagnosis unit experience

Non-invasive prenatal diagnosis tests based on the analysis of fetal DNA in maternal plasma have potential to be a safer alternative to invasive methods. So far, different studies have shown mainly fetal sex, fetal RhD, and quantitative variations of fetal DNA during gestation with fetal chromosomal anomalies or gestations at risk for preeclampsia. The objective of our research was to evaluate the use of fetal DNA in maternal plasma for clinical application. In our study, we have established the methodology needed for the analysis of fetal DNA. Different methods were used, according to the requirements of the assay. We have used quantitative fluorescent polymerase chain reaction (QF-PCR) to perform fetal sex detection with 90% sensitivity. The same technique permitted the detection of fetal DNA from the 10th week of gestation to hours after delivery. We have successfully carried out the diagnosis of two inherited disorders, cystic fibrosis (conventional PCR and restriction analysis) and Huntington disease (QF-PCR). Ninety percent of the cases studied for fetal RhD by real-time PCR were correctly diagnosed. The detection of fetal DNA sequences is a reality and could reduce the risk of invasive techniques for certain fetal disorders in the near future.

Recent advances in fetal nucleic acids in maternal plasma

The discovery of cell-free fetal DNA in maternal plasma in 1997 has opened up new possibilities for noninvasive prenatal diagnosis. Circulating fetal DNA molecules have been detected in maternal plasma from the first trimester onwards and can be robustly detected using a variety of molecular methods. This approach has been used for the prenatal investigation of sex-linked diseases, fetal RhD status, and prenatal exclusion of beta-thalassemia major. Recently, fetal RNA has also been found in maternal plasma. Such fetal RNA has been shown to originate from the placenta and to be remarkably stable. The use of microarray-based approaches has made it feasible to rapidly generate new circulating RNA markers. It is hoped that further developments in this field will make the routine and widespread practice of noninvasive nucleic acid-based prenatal diagnosis for common pregnancy-associated disorders feasible in the near future.

Accurate and robust quantification of circulating fetal and total DNA in maternal plasma from 5 to 41 weeks of gestation

BACKGROUND

Detection of fetal DNA in maternal plasma is achievable at 5 weeks of gestation, but few large-scale studies have reported circulating fetal and maternal DNA across all trimesters.

METHODS

Blood samples were collected from 201 women between 5 and 41 weeks of pregnancy. Quantitative PCR was used to assess total and fetal DNA concentrations, and allelic discrimination analysis was investigated as a route to detecting specifically fetal DNA.

RESULTS

Male fetuses were detectable from 5 weeks amenorrhea with increasing fetal DNA concentrations across gestation. The sensitivity of fetal male gender determination in pregnancies with live birth confirmation was 99%, with 100% specificity. Total DNA concentrations did not correlate with gestational age, but appeared slightly higher in the first and third trimesters than in mid-pregnancy. Analysis of short tandem repeats demonstrated that significant improvements in the detection limit are required for specific detection of fetal DNA.

CONCLUSIONS

The high sensitivity of PCR-based detection, together with quantification provided by real-time DNA analysis, has clear potential for clinical application in noninvasive prenatal diagnosis. However, accurate quantification using best-fit data analysis, standardization of methods, and performance control indicators are necessary for robust routine noninvasive diagnostics.

Cell-free fetal DNA in maternal blood: kinetics, source and structure

The kinetics and structure of cell-free fetal DNA in maternal plasma is currently under investigation. Plasma fetal DNA seems quite stable albeit cleared rapidly following birth, suggesting continuous fetal DNA release into the maternal circulation during pregnancy. However, to understand better the kinetics of circulating DNA, studies to determine the biological (structural) form in which fetal and maternal DNA exist and the mechanisms underlying variation in plasma are warranted to ensure quantitative diagnostic reliability. It is likely that circulating fetal DNA is released from fetal and/or placental cells undergoing apoptosis. Thus, the majority of fetal DNA is proposed to circulate in membrane-bound vesicles (apoptotic bodies). This review summarizes the latest reports in this field.

What's new in prenatal screening and diagnosis?

This article provides clinicians with an overview of current methods for prenatal genetic screening and diagnosis. Topics include developments in prenatal screening procedures such as ethnicity-based carrier testing, maternal serum screening, and ultrasonography. Diagnostic alternatives to amniocentesis include chorionic villus sampling and preimplantation diagnosis. Future endeavors such as three-dimensional ultrasonography and fetal cell sorting are discussed.

MS analysis of single-nucleotide differences in circulating nucleic acids: Application to noninvasive prenatal diagnosis

The analysis of circulating nucleic acids has revealed applications in the noninvasive diagnosis, monitoring, and prognostication of many clinical conditions. Circulating fetal-specific sequences have been detected and constitute a fraction of the total DNA in maternal plasma. The diagnostic reliability of circulating DNA analysis depends on the fractional concentration of the targeted sequence, the analytical sensitivity, and the specificity. The robust discrimination of single-nucleotide differences between circulating DNA species is technically challenging and demands the adoption of highly sensitive and specific analytical systems. We have developed a method based on single-allele base extension reaction and MS, which allows for the reliable detection of fetal-specific alleles, including point mutations and single-nucleotide polymorphisms, in maternal plasma. The approach was applied to exclude the fetal inheritance of the four most common Southeast Asian beta-thalassemia mutations in at-risk pregnancies between weeks 7 and 21 of gestation. Fetal genotypes were correctly predicted in all cases studied. Fetal haplotype analysis based on a single-nucleotide polymorphism linked to the beta-globin locus, HBB, in maternal plasma also was achieved. Consequently, noninvasive prenatal diagnosis in a mother and father carrying identical beta-thalassemia mutations was accomplished. These advances will help in catalyzing the clinical applications of fetal nucleic acids in maternal plasma. This analytical approach also will have implications for many other applications of circulating nucleic acids in areas such as oncology and transplantation.

Improvement in Sensitivity of Allele-specific PCR Facilitates Reliable Noninvasive Prenatal Detection of Cystic Fibrosis

Background: Cell-free fetal DNA circulating in maternal blood has potential as a safer alternative to invasive methods of prenatal testing for paternally inherited genetic alterations, such as cystic fibrosis (CF) mutations.

Methods: We used allele-specific PCR to detect mutated CF D1152H DNA in the presence of an excess of the corresponding wild-type sequence. Pfx buffer (Invitrogen) containing replication accessory proteins and Taq polymerase with no proofreading activity was combined with TaqMaster PCR Enhancer (Eppendorf) to suppress nonspecific amplification of the wild-type allele. The procedure was tested on DNA isolated from plasma drawn from 11 pregnant women (gestational age, 11–19.2 weeks), with mutation confirmation by chorionic villus sampling.

Results: The method detected 5 copies of the CF D1152H mutant allele in the presence of up to ∼100 000 copies of wild-type allele without interference from the wild-type sequence. The D1152H mutation was correctly identified in one positive sample; the only false-positive result was seen in a mishandled sample.

Conclusions: This procedure allows for reliable detection of the paternally inherited D1152H mutation and has potential application for detection of other mutations, which may help reduce the need for invasive testing.

Circulating Fetal DNA: Its Origin and Diagnostic Potential—A Review

OBJECTIVE

In contrast to the traditional teaching that the placenta forms an impermeable barrier, multiple studies show that both intact fetal cells and cell-free nucleic acids circulate freely in maternal blood. Complications of pregnancy, such as pre-eclampsia, or fetal cytogenetic abnormalities, such as trisomy 21, increase transfusion of both intact fetal cells and cell-free fetal nucleic acids into the maternal circulation. The objective of our research is to show that abnormal feto-maternal trafficking of nucleic acids is associated with fetal and placental pathology, and that these observations may lead to novel non-invasive diagnostic and screening tests.

METHODS

Real-time quantitative PCR amplification of DYS1 is used to measure the levels of male fetal DNA in case-control sets of serum or plasma taken from pregnant women. In our laboratory, we use DYS1, a Y-chromosome specific gene, as a uniquely fetal DNA marker for the development of gestation-specific normal values and theoretical models.

RESULTS

Women carrying fetuses with trisomies 21 or 13 (but not 18) have increased levels of fetal DNA in their fresh or archived serum and/or plasma samples. Women destined to develop pre-eclampsia have a characteristic bi-phasic elevation of cell-free fetal DNA that precedes clinical symptoms. Data obtained from a variety of clinical scenarios suggest that the placenta is the predominant source of the circulating fetal nucleic acids, although apoptotic haematopoietic cells may contribute to the pool as well.

CONCLUSIONS

Fetal cell-free DNA is elevated in a number of conditions associated with placental pathology. Widespread clinical implementation of fetal DNA as a screening tool awaits discovery of a reliable gender-independent marker, which may be DNA polymorphisms, epigenetic markers, or mRNA. Fetal cell-free nucleic acids have potential for non-invasive monitoring of placental pathology.

Clinical applications of cell-free fetal DNA from maternal plasma

OBJECTIVE

To describe our clinical experience with detection and analysis of cell-free fetal DNA derived from maternal plasma for prenatal sexing and fetal rhesus-D typing.

METHODS

Real-time quantitative polymerase chain reactions (PCRs) of rhesus-D sequences and the SRY gene were validated and offered to patients with an enhanced risk for sex-linked fetal pathology and patients with rhesus-D antibodies.

RESULTS

In the validation group, 72 samples were analyzed. Sensitivity of the rhesus-D real-time quantitative PCR in maternal plasma was 100% (95% confidence interval [CI]91.8%, 100%) and specificity was 96.6% (95% CI 82.2%, 99.9%). Sensitivity of the SRY real-time quantitative PCR was 97.2% (95% CI 85.5%, 99.9%), and specificity was 100% (95% CI 88.1%, 100%). The technique was used successfully in a clinical setting in 24 women. Overall, invasive tests were avoided in 41.7% of these patients.

CONCLUSION

Detection of cell-free fetal DNA from maternal plasma is a reliable technique that can substantially reduce invasive prenatal tests.

Fetal cells and DNA in maternal blood

Although fetal cells have been known to escape to the maternal circulation for a number of years, research attempts to use them for prenatal diagnosis have not had any consistent success. This review attempts to trace the history of such attempts and to document their progress and reasons for success or failure. The opinions of recent conferences including that of the US National Institute of Child Health and Human Development, a sponsor of major US research in the field, are reported and discussed. It is concluded that although basic work has demonstrated the biologic availability of both fetal cells and their free DNA representatives in the maternal circulation at gestational ages relevant to prenatal diagnosis, much work remains to develop practical technology for their consistent recovery and assay.

Probing the fetal genome: progress in non-invasive prenatal diagnosis

Progress in our understanding of the molecular basis of heritable diseases, through identification of specific mutations, has provided a foundation for the development of DNA-based prenatal diagnosis. Genetic analysis of fetal DNA is now routinely performed from chorionic villus samples obtained as early as the tenth week of gestation or by amniocentesis from week 15 onwards. However, both of these approaches involve invasive procedures with increased risk of fetal loss. To avoid such complications, attempts have been made to develop non-invasive tests through the identification, characterization and isolation of fetal cells or free fetal DNA from the maternal circulation. Recently, progress has been made towards the development of novel strategies that are expected to provide non-invasive means for early prenatal diagnosis in pregnancy.

Prenatal detection of fetal hemoglobin E gene from maternal plasma

In order to provide a noninvasive prenatal diagnosis of the hemoglobin E (Hb E) related disorder, we have evaluated the possibility of identifying the fetal beta(E)-globin gene in maternal plasma. The analysis was performed during 8 to 18 weeks of gestation using DNA extracted from 200 micro L of plasma from pregnant women whose husbands carried Hb E. The beta(E)-globin mutation in maternal plasma was detected by a nested PCR amplification followed by the Mnl I restriction analysis. The result was compared with that of routine analysis of the CVS specimens. Among the five pregnant women examined, the fetal beta(E)-globin gene was identified in maternal plasma in three of them and the result was completely concordant with the conventional CVS analysis. This simple noninvasive prenatal detection of the fetal beta(E)-globin gene should prove useful in a prevention and control program of Hb E/beta-thalassemia in countries where the beta(E)-globin gene is prevalent.

Non-invasive prenatal diagnosis: on the horizon?

The launch of the genomics and postgenomics era has greatly expanded our understanding of the genetic basis of many diseases. In conjunction with the sociocultural trend to delay childbirth and to maintain smaller family units, extra demand may be placed on the existing prenatal diagnostic services. The inherent risk of fetal loss associated with current prenatal diagnostic procedures, such as amniocentesis and chorionic villus sampling, has spurred research into non-invasive prenatal diagnosis. Much research has been conducted on the exploitation of fetal genetic material present in the maternal circulation. The initial focus was on the isolation of intact fetal cells and subsequently, the existence of extracellular fetal DNA in maternal plasma was realized. Exciting developments have been achieved in recent years. A large-scale trial to evaluate the clinical utility of fetal cell isolation from maternal blood for fetal aneuploidy diagnosis was launched and data were recently published. Much has taken place in the research of fetal DNA analysis in maternal plasma and in one example, namely prenatal RhD determination, this type of analysis has been used in the clinical setting. This paper reviews the technological developments in non-invasive prenatal diagnosis.

Earliest gestational age for fetal sexing in cell-free maternal plasma

OBJECTIVES

To evaluate at what gestational age fetal DNA can reliably be detected at the earliest in maternal plasma.

METHODS

We performed consecutive blood sampling in the first trimester of pregnancy in 17 women who were pregnant after in vitro fertilization (IVF) or intrauterine insemination (IUI). DNA was isolated and the Y-chromosome specific SRY was amplified by real-time polymerase chain reaction (PCR). We likewise studied 31 women prior to invasive prenatal diagnosis procedures for test validation purposes. All test results were compared to cytogenetic sex or sex at birth.

RESULTS

The earliest SRY detection was at a gestational age of 5 weeks and 2 days. In none of 4 pregnancies ending in a miscarriage was SRY detected. We detected SRY in maternal plasma in 1 of 2 patients (50%) carrying a male fetus at a gestational age of 5 weeks, in 4 of 5 (80%) at a gestational age of 7 weeks, in 4 of 4 (100%) at a gestational age of 9 weeks. In all 7 women pregnant with a male fetus, the correct fetal sex was detected by 10 weeks. In none of the 6 patients who delivered a girl was SRY detected. In the validation group, SRY was detected in 13 of the 13 male, and none of the 18 female fetuses.

CONCLUSIONS

We conclude that real-time PCR of the SRY gene promises to be a reliable technique for early fetal sexing in maternal plasma.