Erythroid-specific antibodies enhance detection of fetal nucleated erythrocytes in maternal blood

Noninvasive first-trimester screening for fetal aneuploidy

We reviewed all studies concerning noninvasive first trimester screening for fetal aneuploidy obtained from a MEDLINE search through June 1996 with additional sources identified through cross-referencing. Three screening and diagnostic modalities are of potential application in noninvasive first trimester testing for fetal aneuploidy: ultrasound, maternal biochemical markers, and analysis of fetal cells retrieved from maternal sources. Sensitivities of the sonographic finding of nuchal translucency thickness in combination with maternal age for trisomy 21, performed between 10 and 14 weeks of gestation in experienced hands, and maternal biochemical markers independently may be as high as 86 percent and 60 percent, respectively. Sensitivity, specificity, and predictive values of these diagnostic modalities alone, in combination with each other, or in conjunction with other predisposing factors such as maternal age, in large low risk populations have not currently been established. Analysis of fetal cells retrieved from maternal sources, although more complex, may offer definitive noninvasive prenatal diagnosis yet is not currently available in clinical practice. We conclude that noninvasive first trimester screening for fetal aneuploidy modalities including sonographic examination for nuchal translucency thickness and maternal biochemical markers, is feasible. Clinical feasibility; and all-encompassing clinical management paradigms of these and other early noninvasive first trimester screening methods for fetal aneuploidy, are not yet available.

Prenatal diagnosis using fetal cells isolated from maternal peripheral blood: a review

Many questions remain about the feasibility of using fetal cells from maternal blood for prenatal diagnosis. Although recently there has been more focus on clinically relevant methods, many studies have been performed using blood drawn after invasive procedures, and over a wide range of gestational ages. For methods to be applicable to clinical use, more work is needed on isolating cells early in pregnancy, when termination is still an option for parents who are found to have an affected pregnancy. It is generally agreed that fetal nucleated erythrocytes are the most efficacious cell type for prenatal diagnosis, but it has not yet been shown definitively whether there is an ideal gestational age for sampling, whether ABO incompatibility might limit availability of fetal cells, or whether the number of cells present might be different in normal versus abnormal pregnancies. PCR has been shown to be a powerful tool in allowing amplification and identification of very small amounts of fetal DNA. However, this is limited to cases in which a specific and unique gene from the father is sought. This means that there is the potential to diagnose many paternally inherited autosomal dominant diseases and some autosomal recessive diseases, in which the parents have different and identifiable mutations. However, when parents are both carriers of the same autosomal recessive mutations, or when the disease is X linked, PCR will not aid in prenatal diagnosis. Cytogenetic analysis of fetal cells by FISH after cell sorting is another potentially useful method of prenatal diagnosis, but requires relatively pure samples of fetal cells or an independent marker that allows easy microscopic identification. The latter might be accomplished by identifying fetal cells through their expression of embryonic hemoglobins or because they contain HLA-G mRNA. In addition, current techniques of cell sorting must be improved so that a higher percentage of fetal cells can be isolated. Currently, the best cell sorting techniques usually produce a maximum purity of 10% fetal cells. Commonly, in normal pregnancies, fewer than 0.1% of the cells isolated after sorting are fetal in origin. Improving the concentration and quantity of fetal cells will improve the accuracy of FISH. Methods such as immunophenotyping that allow the selective identification of fetal cells by microscopy, and can be used in conjunction with FISH, may be extremely valuable because they may allow the genetic analysis of only the few fetal cells within a background preponderance of maternal cells. Although the retrieval of fetal cells from maternal blood is an attractive concept, it must be clearly stated that presently it is only in the investigational phase because of the low sensitivity and specificity. There is no current application for these methods in clinical practice. It remains to be determined whether testing maternal blood for fetal cells or DNA will be used as a screening tool, similar to the maternal serum screening currently in use, or whether the accuracy can be improved to a level such that the techniques can be used diagnostically. Although there are many questions that remain unanswered at this time, the outlook for noninvasive prenatal genetic testing in the future is optimistic.

Prenatal diagnosis of sickle cell anaemia and thalassaemia by analysis of fetal cells in maternal blood

Currently, amniocentesis, chorionic villus sampling (CVS) and fetal blood sampling are used to obtain fetal cells for genetic diagnosis. These invasive procedures pose a small but not negligible risk for the fetus. Efforts have been directed towards the enrichment of fetal cells, such as erythroblasts, from maternal blood and progress has been made in the diagnosis of some chromosomal disorders and in sex determinations. We now report the detection of point mutations in single gene disorders using this method of prenatal diagnosis by enriching fetal cells from maternal blood by magnetic cell sorting followed by isolation of pure fetal cells by microdissection. In two pregnancies at risk for sickle cell anaemia and beta-thalassaemia, we successfully identified the fetal genotypes. Thus, prenatal diagnosis of single gene disorders by recovering fetal cells from maternal circulation appears to be a feasible approach.

Development of a rapid means of estimating the haemoglobin F content of candidate fetal cells isolated from maternal blood using HPLC

Prenatal diagnosis of genetic disorders in nucleated fetal red blood cells present in maternal blood requires methods to detect and enrich for such cells. Here we describe a rapid high performance liquid chromatography (HPLC) method that allows one to determine as few as 100 cells containing haemoglobin F (HbF) in the presence of a vast excess of haemoglobin A (HbA)-producing cells. The HPLC separations of haemoglobins were performed with a weak cation exchange column-silica gel-bound asparaginic acid-and ammonium phosphate buffer as the mobile phase. Separations were carried out in 7 min. When applied to estimation of the recovery of fetal cells from maternal blood, the HPLC method indicates in a timely manner whether or not to proceed with further techniques (i.e., FISH or PCR). Several current techniques such as Ficoll gradients and fluorescence (FACS) or magnetic (MACS) activated cell sorting were thus evaluated. Unexpectedly, our method indicates high cell losses with both single gradient and triple density Ficoll pre-enrichment methods. Less than 20 per cent of the nucleated red blood cells can be recovered in the most optimal setting. Lysis of erythrocytes may be an alternative technique that leaves nucleated red blood cells of all maturation stages intact. Thus, any further improvements in the technology for fetal cell recovery may be aided by monitoring the yield with HPLC.

Improved fetal nucleated erythrocyte sorting purity using intracellular antifetal hemoglobin and Hoechst 33342

Fetal nucleated erythrocytes (FNRBC) flow sorted from maternal peripheral blood, using monoclonal antibodies (mAb) that bind fetal cell surface antigens, are a noninvasive source of fetal DNA for prenatal diagnosis. These mAbs, however, also bind antigens shared by maternal cells. In sorted populations, this results in maternal cell contamination and low fetal cell purities, which complicates genetic analysis by fluorescence in situ hybridization (FISH) and polymerase chain reaction (PCR). Fetal hemoglobin, (alpha 2 gamma 2), has been proposed as a useful fetal marker. To improve fetal cell enrichment from maternal blood, we developed an intracellular staining protocol that combines anti-gamma mAb with Hoechst 33342 to identify and flow sort FNRBC. Artificial mixtures of male umbilical cord cells (as a source of fetal hemoglobin) and female adult, non-pregnant peripheral blood mononuclear cells were stained and flow sorted using this protocol. FISH analysis was performed using chromosome X and Y specific probes. Fetal cell purities were calculated by microscope confirmation of anti-gamma staining and counting the number of X and Y signals present after FISH. Results from microscope analyses showed a fetal cell yield of 39-100% and fetal cell purities of 59-73%. These purities are significantly higher than the .001-4.8% previously reported by us in maternal samples using cell surface staining protocols. FISH results demonstrated that 83-100% (mean = 98%) of anti-gamma positive cells were male, whereas 82-100% (mean = 92.5%) of anti-gamma negative cells were female. These results confirmed that the anti-gamma mAb is highly fetal specific. When applied to maternal blood samples, this protocol should lead to increased sensitivity for prenatal diagnosis.

Fetal RhD genotyping in fetal cells flow sorted from maternal blood

OBJECTIVE

The aim of this study was to determine the accuracy of noninvasive fetal RhD genotyping by fetal cell isolation from maternal blood.

STUDY DESIGN

Candidate fetal cells from 18 pregnant women (one twin gestation) were flow-sorted. Polymerase chain reaction amplification of a 261 bp fragment of the RhD gene was performed on sorted fetal cells. The presence of amplified product was considered predictive of the Rhd-positive genotype in the fetus.

RESULTS

Sixteen of the 19 fetal RhD genotypes were correctly predicted in fetal cells isolated from maternal blood (10 were Rh positive, 6 were Rh negative). In 3 cases no amplification products were detected in RhD-positive fetuses. The association between presence of the fragment and RhD-positive genotype was significant (p=0.003, Fisher's exact test).

CONCLUSIONS

Noninvasive prenatal diagnosis of the fetal RhD genotype is feasible. Absence of amplification products in the reaction requires confirmation that fetal material is present. Improvements in fetal cell purity and yield should increase diagnostic accuracy, although the current protocol has a positive predictive value of 100% and a negative predictive value of 67%.

Mental retardation: genetic findings, clinical implications and research agenda

The most important genetic advances in the field of mental retardation include the discovery of the novel genetic mechanism responsible for the Fragile X syndrome, and the imprinting involved in the Prader-Willi and Angelman syndromes, but there have also been advances in our understanding of the pathogenesis of Down syndrome and phenylketonuria. Genetic defects (both single gene Mendelizing disorders and cytogenetic abnormalities) are involved in a substantial proportion of cases of mild as well as severe mental retardation, indicating that the previous equating of severe mental retardation with pathology, and of mild retardation with normal variation, is a misleading over-simplication. Within the group in which no pathological cause can be detected, behaviour genetic studies indicate that genetic influences are important, but that their interplay with environmental factors, which are also important, is at present poorly understood. Research into the joint action of genetic and environmental influences in this group will be an important research area in the future.

A rapid combined immunocytochemical and fluorescence in situ hybridisation method for the identification of human fetal nucleated red blood cells

Fetal nucleated red blood cells are found in the maternal circulation during pregnancy. If a simple routine method of detection of these cells was developed, it could be used as the basis of non-invasive prenatal diagnosis of fetal genetic disorders. Fetal male and adult female blood were mixed to mimic maternal blood in pregnancy and used to establish a simple technique to unequivocally detect fetal nucleated red blood cells. These were identified by combined immunocytochemistry using a human fetal haemoglobin antibody and a rapid and simple-to-use fluorescence in situ hybridisation method using X and Y chromosome probes. Initial studies using the alkaline phosphatase anti-alkaline phosphatase technique as the first procedure showed that the stain was unstable and unsuitable for in situ hybridisation. An immunoperoxidase technique was found to produce a stable stain resistant to harsh fixation steps required in subsequent in situ hybridisation. This enabled the simultaneous visualisation of immunopositivity and in situ hybridisation signals on the same cell with neither procedure affecting the other's signal quality. We are currently using this procedure to detect a range of endoplasmic reticulum proteins in fetal nucleated red blood cells from maternal blood in an attempt to diagnose disorders of liver protein expression in early pregnancy.

Prenatal diagnosis by analysis of fetal cells in maternal blood

The data accumulated thus far indicate that fetal NRBCs are the target cell type of choice in maternal blood for most investigators, although some groups continue to work with the trophoblast. Reports of persistent circulation of hematopoietic stem cells, lymphoid/myeloid progenitors, and lymphocytes mandate that removal of these cell types must occur before clinical diagnosis of the current pregnancy can be made. In selected cases, accurate detection of fetal aneuploidy has been made from fetal cells in maternal blood; the clinical evaluation sponsored by the National Institute of Child Health and Human Development will determine the sensitivity and specificity of cytogenetic diagnosis in a larger group of pregnant women, but this information will not be available for several years. At present, detection of uniquely fetal, paternally inherited gene polymorphisms or mutations such as the Rh(D) antigen is possible only because the mother lacks these genes; hence, maternal cell contamination does not hinder diagnosis. Currently the presence of large numbers of maternal cells in enriched samples precludes single-gene diagnosis for conditions in which the mother carries a mutant gene, because her cells are preferentially amplified and difficult to distinguish from those of the fetus. It is likely, however, that as techniques of individual fetal cell isolation are perfected, maternal cell contamination will no longer be an issue, and the entire fetal genome will become available for diagnosis and therapy. Pediatricians need to be aware of the progress of research in this field, because fetal cell isolation from maternal blood not only could change prenatal diagnosis but would change the amount of genetic information that arrives with a newborn infant at birth. The ultimate goal of this work is to diagnose noninvasively, in the first trimester, the common fetal aneuploidies and single-gene disorders, to permit in utero treatment, or to allow low-risk pregnant women carrying an abnormal fetus an opportunity for reproductive choice.

Flow sorting of fetal erythroblasts using intracytoplasmic anti-fetal haemoglobin: preliminary observations on maternal samples

Monoclonal antibody to fetal haemoglobin (alpha 2 gamma 2) has been proposed as a fetal-specific reagent. We developed an intracellular staining protocol that combines fluorescein isothiocyanate or phycoerythrin conjugated anti-gamma with the DNA binding dye Hoechst 33342 to identify and flow sort fetal erythroblasts from maternal blood. Our preliminary observations on anti-gamma-positive cells sorted from four different pregnant women are described here, using fluorescence in situ hybridization (FISH) with chromosome-specific probes to identify fetal cells. Our data demonstrate that far fewer candidate fetal cells are sorted with this protocol than by current cell surface staining methods that employ the monoclonal antibody CD71. This results in increased fetal cell sorting purities. With this protocol, standard FISH techniques require modification due to the rigorous fixation with 4 per cent paraformaldehyde. Our initial data indicate the promise of this approach.

Recent advances in reproductive genetic technologies

New possibilities for the diagnosis and treatment of reproductive and genetic disorders are becoming available as a result of a series of recent technical advances. Intracytoplasmic sperm injection (ICSI) allows treatment of numerous infertile men whose sperm cannot penetrate the egg to initiate fertilization. Molecular genetic testing provides clients of reproductive age with additional information that permits prevention of genetic diseases such as fragile X syndrome, the leading cause of inherited mental retardation. Preimplantation genetic testing (PGT) offers couples who carry genetic disorders the prospect of having children with a greatly decreased risk of initiating a pregnancy involving an affected individual. Flow-cytometric sperm separation offers a new, effective approach for prevention of X-linked genetic disorders. Two major causes of recurrent pregnancy loss (RPL) involve recurrent trisomies and immunological disorders. Of the latter, 70% of studied populations of patients can attain live births with simple treatment protocols. Maternal serum assays involving multiple markers reduce both false positives and false negatives in detection of trisomies. Despite these advances in research, many safe and effective methods of diagnosis and treatment remain under-utilized in the clinical arena.

Detection of fetal HLA-DQa sequences in maternal blood: a gender-independent technique of fetal cell identification

The objective of this study was to detect fetal HLA-DQa gene sequences in maternal blood. HLA-DQa genotypes of 70 pregnant women and their partners were determined for type A1. We specifically sought couples where the father, but not the mother, had genotype A1. In 12 women, maternal blood samples were flow-sorted. Candidate fetal cells were isolated and amplified by using PCR primers specific for a paternal HLA-DQa A1 allele. Fetal HLA-DQa A1 genotype was predicted from sorted cells; amniocytes or cheek swabs were used for confirmation. Six of twelve sorted samples had amplification products indicating the presence of the HLA-DQa A1 allele; 6/12 did not. Prediction of the fetal genotype was 100 per cent correct, as determined by subsequent amplification of amniocytes or cheek swabs. We conclude that paternally inherited uniquely fetal HLA-DQa gene sequences can be identified in maternal blood. This system permits the identification of fetal cells independent of fetal gender, and has the potential for non-invasive prenatal diagnosis of paternally inherited conditions.

Development of non-invasive fetal DNA diagnosis from maternal blood

Several attempts have been made to detect and retrieve fetal nucleated cells including nucleated erythrocytes (NRBCs), leukocytes, and trophoblasts in maternal blood. We have recently developed a new method for non-invasive fetal DNA diagnosis from maternal blood. Peripheral blood granulocytes including NRBCs were isolated by a discontinuous density gradient method using Percoll (Pharmasia). NRBCs were found and retrieved at a single cell level using a micromanipulator under a microscope. To determine whether the origin of the NRBCs was maternal or fetal, the NRBCs were analysed by polymerase chain reaction (PCR) amplification to determine the presence of a Y-chromosome-specific repeat sequence in mothers carrying male fetuses. We were successful in predicting fetal sex accurately in 10 out of 11 samples taken from maternal blood. This new technique opens up fetal DNA diagnosis from maternal blood during the first trimester of pregnancy to the whole population because there is no risk to the fetus or the mother.

Mid-trimester fetal sex determination from maternal peripheral blood by fluorescence in situ hybridization without enrichment of fetal cells

To determine the fetal sex on 30 women who were 16-20 weeks pregnant, about 100,000 maternal blood nucleated cells were analysed by means of fluorescence in situ hybridization (FISH) with a Y-chromosome-specific DNA probe. Cells with the hybridization signal were detected in 12 of the 30 women. All the 12 mothers gave birth to a male child. Of the other 18 women who had no Y-positive cells in the peripheral blood, 14 gave birth to a female child and four gave birth to a male child. These false-negative results probably occurred because the number of cells examined was inadequate. The data obtained in this study suggest that fetal sex determination using maternal peripheral blood with FISH is possible and that this diagnostic method will be clinically useful when more cells are analysed.

Enrichment of fetal cells from maternal blood by high gradient magnetic cell sorting (double MACS) for PCR-based genetic analysis

For simple and effective isolation of fetal cells from peripheral maternal blood, we combined depletion of maternal cells and enrichment of fetal cells by high-gradient magnetic cell separation (MACS). First CD45+ and CD14+ cells were depleted from maternal peripheral blood mononuclear cells by MACS. From the depleted fraction, CD71+ erythroid cells were enriched up to 80 per cent by MACS. This double-MACS' procedure yielded an average depletion rate of 780-fold and an average enrichment rate of 500-fold, with approximate recovery rates of 40-55 per cent. For paternity testing, cells from unseparated blood and the various fractions were analysed for polymorphism of the HLA-DQ-A1 locus and D1S80 locus by the polymerase chain reaction (PCR). In CD45-/CD71+ sorted cells from maternal blood, but not in unfractionated cells from maternal blood or CD45-/CD14- cells, paternal alleles could be detected. In the CD45-/CD71+ fraction, the relative frequency of paternal alleles compared with maternal alleles ranged from 1 in 20 to 1 in 200 (determined by titration and depending on the quality of separation and biological variation). In 7 out of 11 cases, between weeks 12 and 25 of gestation, we could identify paternal alleles by PCR, either HLA-DQ-A1 or D1S80. This double-MACS procedure is simple, fast, efficient, and reliable for non-invasive prenatal diagnosis.

Prenatal sex determination by in situ hybridization on fetal nucleated cells in maternal whole venous blood

Our aim was to evaluate whether the sex of a fetus could be determined in maternal whole venous blood by in situ hybridization without enrichment of fetal cells. This procedure is virtually without risks to the fetus or the mother. Blood samples were obtained from 59 women at different stages of pregnancy. Twenty preparations were discarded because they were technically unfit for in situ hybridization. Of the remaining 39 pregnant women, 18 had a male fetus, one had male twins, and 20 had a female fetus. Y-positive cells were detected in 12 of the 19 pregnancies with male fetuses and in two of the 20 pregnancies with a female fetus. The frequencies of cells with Y-signals ranged from 1 in 100,000 to 1 in 639. Our results show that fetal cells in maternal blood cannot be reliably used for prenatal diagnosis without prior enrichment of fetal cells.

Fetal cells in maternal blood: determination of purity and yield by quantitative polymerase chain reaction

OBJECTIVE

The detection of fetal aneuploidy and gene mutations by analysis of fetal cells in maternal blood has demonstrated the feasibility of noninvasive prenatal diagnosis. Fetal cells are rare in the maternal circulation; all current methods used for their isolation also yield maternal cells. We developed a method that permits a quantitative assessment of the relative numbers of fetal and maternal cells.

STUDY DESIGN

Samples from 40 pregnant women were flow sorted with different monoclonal antibodies. Deoxyribonucleic acid was subsequently purified from candidate fetal cells; polymerase chain reaction was performed with synthetic primers specific for sequences on chromosomes Y and 7.

RESULTS

The maximum number of fetal cells detected was 52 in 1080 maternal cells. Fetal cell purity ranged from 0.001% to 4.8%. Fetal cells were detected with antibodies to CD71, CD36, and glycophorin A.

CONCLUSION

Quantitative polymerase chain reaction enables the determination of the purity and yield of fetal cells remaining after isolation from maternal blood, facilitating rapid comparisons between different cell separation techniques.

Clinical trials and experience: Boston

Our cumulative experience continues to validate the fetal nucleated erythrocyte as the target fetal cell type of choice, primarily because it reflects the cytogenetic status of the current pregnancy. Additional cell types, such as the granulocyte, await further study. Quantitative PCR is a sensitive and useful new method that can facilitate rapid comparisons between cell separation methods or different monoclonal antibodies. It can also be used on patient material to determine final purity of the enriched maternal samples. If the purity is too low, FISH studies will be complicated by the presence of thousands of maternal cells. Our planned studies include an analysis of why aneuploid pregnancies appear to have a higher number of fetal cells in the maternal circulation. We are also studying the timing of the fetomaternal transfer of cells with qPCR analysis of sorted maternal samples drawn weekly from well-dated women. We are continuously improving our methods (both in separations and antibodies) to reach a fetal cell purity of at least 20% for cytogenetic diagnosis by FISH studies. With the knowledge obtained thus far by us and by others, such a goal appears to be achievable within the near future.

Prenatal diagnosis in the 1990s

Prenatal diagnosis has become widely available and detects an increasing variety of birth defects and potentially harmful medical conditions. Many of the studies are complex and must be performed within a specific time period. Most prenatal diagnostic sampling techniques have some degree of risk for the mother or the fetus, and all produce at least transient anxiety. Nurses are involved in identifying families at risk, preparing women for the procedures, providing support, and counseling patients after the results are known; because of this role, nurses need updated information. This review describes current methods for early identification of a potential problem, discusses a variety of prenatal diagnostic procedures, reviews the most common types of laboratory studies, and introduces future trends in the field of prenatal diagnosis.

Screening of monoclonal antibodies recognizing oncofetal antigens for isolation of trophoblasts from maternal blood for prenatal diagnosis

Many monoclonal antibodies have been produced against tumour-associated cell surface antigens for cancer therapy. They have therefore been selected for minimal reactivity with normal tissues and in particular for lack of binding to blood cells or serum components. Many of the antigens recognized are of fetal origin. These monoclonal antibodies may therefore be ideal candidates to recognize and sort fetal trophoblasts from maternal blood for prenatal diagnosis of genetic abnormalities. A panel of 19 anti-tumour antibodies were therefore screened for reactivity with early trimester placenta and two, 340 and 154, were shown to stain trophoblasts. If MAb 340 is linked to magnetic beads, it can efficiently sort trophoblast cell lines from whole blood.

Techniques in current use in prenatal diagnosis

Prenatal diagnosis is now offered to the majority of pregnant women in Europe and the United States. Advances in obstetric and laboratory techniques mean that increasing numbers of conditions can be diagnosed prenatally; indeed, gene carriers can be identified before pregnancy in some cases. Current obstetric and laboratory techniques for prenatal screening and diagnosis of genetic disorders are discussed.

Transferrin receptor (CD71) expression on circulating mononuclear cells during pregnancy

OBJECTIVE

We studied transferrin receptor (CD71) expression in peripheral blood mononuclear cells from healthy pregnant women, to determine if a relationship existed between gestational age and circulating CD71+ mononuclear cells.

STUDY DESIGN

Cell suspensions were prepared from venous blood from 139 pregnant women (7 to 26 weeks of gestation), incubated with monoclonal anti-CD71 antibody, and analyzed by flow cytometry.

RESULTS

When only the first sample from each woman was analyzed, extensive biologic variation between women was shown. An apparent biphasic increase in the percentage of CD71+ cells with advancing gestation was suggested. A subgroup of 13 women studied on multiple occasions demonstrated linear increases in CD71+ cells as pregnancy progressed.

CONCLUSIONS

Pregnant women, when compared with each other, may have differences in the baseline number of circulating CD71+ cells. The increases seen in individuals studied repeatedly are likely to reflect maternal hematopoiesis and current fetomaternal transfusion.