Biopsy of human preimplantation embryos and sexing by DNA amplification

The polymerase chain reaction: a tool for molecular medicine

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Hatching rate: an optimal discriminator for the assessment of single-blastomere biopsy

PURPOSE

In order to determine an optimal marker to discriminate embryo injury following single-blastomere embryo biopsy, mouse embryos were examined for rates of blastocyst formation, hatching, implantation, and fetal development following single-blastomere biopsy.

RESULTS

Early studies of single-blastomere biopsy (1-8 series) resulted in similar rates of blastocyst formation (P > 0.05) but a lower rate of hatching of biopsied (n = 140) versus control (nonbiopsied) (n = 145) embryos (78.6 vs 95.2%; p < 0.01). Subsequent experience (9-13 series) eliminated this difference between biopsied (n = 145) and control embryos (n = 133) (95.9 vs 94.0%; P > 0.05). Embryo transfer of hatching blastocysts of biopsied (n = 100) and nonbiopsied control (n = 100) groups resulted in equivalent rate of fetal development (70.0 vs 68.0%; p > 0.05).

CONCLUSIONS

The hatching rate appeared to be a simple, sensitive, and reliable method to evaluate the single-blastomere biopsy technique.

Preimplantation embryo sexing by polymerase chain reaction amplification of the sry gene on single mouse blastomeres

Accurate and rapid sex determination of preimplantation embryos has great potential both in animal breeding and in human pathology. In the past, sex determination has been accomplished by cytogenetic or immunologic means and by polymerase chain reaction amplification of Y-chromosome-specific repetitive sequences. More recently, amplification of the Y-specific single-copy ZFY gene has been used in humans for sex determination of preimplantation embryos. The experiments reported here indicate that another Y-chromosome-specific single-copy gene, the sex-determining region gene (sry) can be successfully amplified from single mouse blastomeres. Blastocysts positive for sry amplification were reimplanted to foster mothers, and six of six newborns were male. We conclude that sry gene amplification can represent a good marker for embryo sex determination.

A fast and efficient method for simultaneous X and Y in situ hybridization of human blastomeres

PURPOSE

To use a 6-hr fluorescence in situ hybridization (FISH) procedure involving fluorochrome-labeled probes to determine the gender of blastomeres from arrested biopsied human embryos.

RESULTS

Simultaneous detection of X and Y chromosomes was performed on 68 blastomeres with this technique. The FISH efficiency for gender determination was 95.5% (65/68). In addition, rehybridization with chromosome 8-specific probes was performed to determine the ploidy of blastomeres with more than two sex chromosomes.

CONCLUSION

This technique offers an alternative to polymerase chain reaction for the preimplantation diagnosis of X-linked diseases and can also be used for ploidy assessment.

Human embryo biopsy on the 2nd day after insemination for preimplantation diagnosis: removal of a quarter of embryo retards cleavage

OBJECTIVE

To assess any reduction in viability and development in vitro after biopsy of a quarter of the cells of human embryos on day 2 after insemination.

DESIGN

A prospective study in which normally fertilized surplus embryos of good morphology with two to eight cells approximately 48 hours after insemination were randomly allocated to a control or biopsied group, respectively.

SETTING

In vitro fertilization (IVF) unit and laboratories of the Hammersmith Hospital, Institute of Obstetrics and Gynaecology, London University.

PATIENTS, PARTICIPANTS

One hundred twenty-nine embryos from 28 infertile IVF patients.

INTERVENTIONS

Follicular aspiration by ultrasound-guided transvaginal puncture and embryo biopsy by micromanipulative procedures.

MAIN OUTCOME MEASURE(S)

Pyruvate uptake and cell number at the blastocyst stage.

RESULTS

Embryo biopsy did not have an adverse effect on either the proportion developing to the blastocyst stage (50% [32 of 64] and 47.7% [31 of 65] for the control and biopsied groups, respectively) or embryo viability, measured indirectly through pyruvate uptake. However, the proportion of embryos that reached the morula stage after day 4 (retarded embryos) was significantly higher (44%, 11 of 25 versus 8.7%, 2 of 23) in the biopsied group. The total number of cells (29.6 +/- 3.1 versus 62.4 +/- 4.7), numbers of inner cell mass (7.7 +/- 2.2 versus 24.5 +/- 1.4) and trophectoderm (24.0 +/- 5.2 versus 45.0 +/- 6.4) cells, and the inner cell mass:trophectoderm ratio (34.7 +/- 7.9 versus 59.5 +/- 11.7) were strikingly reduced at the blastocyst stage in the biopsied group. This reduction was greater in embryos that reached the morula stage after day 4.

CONCLUSIONS

More investigation is needed to assess whether the detrimental effects observed were because of the biopsy method used in this study or to a high sensitivity of human embryos at early stages to manipulation in vitro.

Rapid determination of fetal sex using amniotic fluid cells and the polymerase chain reaction

We determined the sex of 50 fetuses by an amplification of the Y-chromosome specific fragment using the polymerase chain reaction (PCR). Amniotic fluid cells were collected by amniocentesis from pregnant women at 14 to 17 weeks of gestation. Total DNA was purified from cells in 1 ml of amniotic fluid. When only the expected 130 base pair X-chromosome specific fragment was detected, we identified the fetus as female, while when both the expected 170 base pair Y-chromosome specific and X-chromosome specific fragments were detected, we identified it as male. In all cases, identification was confirmed either by chromosome analysis or post partum.

Whole genome amplification from a single cell: implications for genetic analysis

We have developed an in vitro method for amplifying a large fraction of the DNA sequences present in a single haploid cell by repeated primer extensions using a mixture of 15-base random oligonucleotides. We studied 12 genetic loci and estimate that the probability of amplifying any sequence in the genome to a minimum of 30 copies is not less than 0.78 (95% confidence). Whole genome amplification beginning with a single cell, or other samples with very small amounts of DNA, has significant implications for multipoint mapping by sperm or oocyte typing and possibly for genetic disease diagnosis, forensics, and the analysis of ancient DNA samples.

Efficiency and accuracy of polymerase-chain-reaction assay for cystic fibrosis allele delta F508 in single cell

Diagnosis of genetic disorders in the embryo before implantation, though possible by removal of one or two blastomeres at the eight-cell stage, is still experimental because the procedures of gene analysis of DNA from a single cell are not yet reliable enough for clinical application. We have evaluated the efficiency and accuracy of polymerase-chain-reaction (PCR) amplification of a single-copy gene, wild-type or cystic fibrosis delta F508 allele, on single sperm cells from a donor known to be a heterozygous carrier of the delta F508 mutation. DNA from single spermatozoa was decontaminated by restriction-enzyme treatment, then the region around the delta F508 site was amplified by nested PCR. The distribution of the wild-type and mutant alleles (59 [55%] and 48 [45%, respectively]) in the 107 single spermatozoa did not differ from that expected (50% each). 1 sample did not provide an amplified signal. To check that the two alleles would be amplified with equal efficiency when they were both present within a cell, we did PCR for 51 two-sperm samples. Again the distribution did not deviate from that expected (17 [33%] both wild-type; 21 [41%] one wild-type, one delta F508; 13 [26%] both delta F508 vs 25%; 50%; 25% expected). None of the 74 blanks in these experiments was contaminated. We conclude that our delta F508 single-cell assay is efficient and accurate and can be used for analysis of blastomere DNA to diagnose cystic fibrosis before embryo implantation.

Prevention of pre-PCR mis-priming and primer dimerization improves low-copy-number amplifications

A Hot Start Polymerase Chain Reaction (PCR) entails the withholding of at least one reagent from the reaction mixture until the reaction tube temperature has reached 60-80 degrees C. Hot Start amplification with an AmpliWax vapor barrier uses a layer of solid wax to separate the retained reagent(s) and the test sample from the bulk of the reagents until the first heating step of automated thermal cycling melts the wax and convectively mixes the two aqueous layers. Wax-mediated Hot Start PCR greatly increases the specificity, yield, and precision of amplifying low copy numbers of three HIV targets. In the presence of 1 microgram of human placental DNA (1.6 x 10(5) diploid genomes) the specificity improvement entails considerable to complete reduction in the amplification of mis-primed sequences and putative primer oligomers. When mis-priming is negligible, the procedural improvement still suppresses putative primer oligomerization. Hot Start PCR with an AmpliWax vapor barrier permits routine amplification of a single target molecule with detection by ethidium stained gel electrophoresis; nonisotopically visualized probing suffices for confirmation. The improved amplification performance is evident for target copy numbers below approximately 10(3).

Dual fluorescent in situ hybridisation for simultaneous detection of X and Y chromosome-specific probes for the sexing of human preimplantation embryonic nuclei

Dual fluorescent in situ hybridisation has been used for the simultaneous detection of X and Y chromosome-specific probes in single cleavage nuclei from disaggregated 4- to 7-cell human embryos. Based on the presence of a Y signal or 2 X signals in the absence of a Y, 89% of poor quality metaphases and 72% of interphase nuclei could be classified as male or female. With further refinements, this technique will offer a credible alternative to the polymerase chain reaction for the diagnosis of sex in human preimplantation embryos in families segregating for X-linked genetic disease.

Analysis of peripheral blood of pregnant women for the presence of fetal Y chromosome-specific ZFY gene deoxyribonucleic acid sequences

Deoxyribonucleic acid sequences of human ZFY (zinc-finger-Y) gene, a Y-chromosome-specific gene and candidate for the testis-determining factor, has been identified by an in vitro enzymatic deoxyribonucleic acid amplification method in peripheral blood specimens of women pregnant with male fetuses. This technique permits detection of ZFY gene deoxyribonucleic acid sequences in as few as a single male cell among 1,000,000 female cells. Maternal blood results were confirmed by amplification of ZFY gene deoxyribonucleic acid sequences in chorionic villus cells and by karyotyping in 33 of 36 pregnant women. There was no false-positive male result, and two of the three blood specimens with false-negative results were obtained from pregnant women at a very early gestational age. With properly designed guidelines, this deoxyribonucleic acid amplification method may be an alternative to determine the fetal sex for those pregnancies at risk for X-linked genetic disorders.

Molecular genetic approaches to the analysis and diagnosis of human inherited disease: an overview

The advent of recombinant DNA technology has contributed enormously to our understanding of human genome pathology. In this review, current approaches to the analysis and diagnosis of human genetic disease are presented and their contribution to diagnostic medicine assessed. At the level of the gene, the nonrandom nature of human gene mutation is described and the role of the local DNA sequence environment explored.

Human embryos and genetic testing: a private policy model

The practice of medicine, which traditionally falls outside the sphere of substantive governmental control, poses challenges to political scientists who perceive the need for oversight regarding problematic new technologies. The challenge is even greater when the technologies involve reproduction, in which constitutional liberties are at stake. This article suggests a private policy model for overseeing one problematic emerging reproductive technology—the diagnosis of chromosomal and genetic disorders in human embryos. It bids political scientists to examine and recommend creative private sector policies appropriate to biomedicine. Rules developed in the private sector, if regarded as obligatory by clinicians, offer a theoretically interesting bridge between voluntary ethical principles and mandatory public sector rules.

Preclinical models for human pre-embryo biopsy and genetic diagnosis. I. Efficiency and normalcy of mouse pre-embryo development after different biopsy techniques

OBJECTIVE

To compare the usefulness of three micromanipulative methods at two different stages of pre-embryo development and to assess possible effects on postbiopsy survival and development.

DESIGN

Four-cell and eight-cell mouse pre-embryos were biopsied using enucleation, aspiration, or extrusion of single blastomeres. After biopsy, pre-embryos were observed for in vitro and in vivo development.

SETTING

Laboratories of The Jones Institute for Reproductive Medicine, Department of Obstetrics and Gynecology, Eastern Virginia Medical School.

PATIENTS, PARTICIPANTS

Only mice were used.

INTERVENTIONS

Pre-embryo biopsy, developmental normalcy and pre-embryo transfer were studied.

MAIN OUTCOME MEASURE(S)

Few pre-embryos died as a result of biopsy trauma. High postbiopsy survival rates were associated with normal intrauterine and postnatal development.

RESULTS

Expanded blastocyst formation rates from four-cell and eight-cell pre-embryos were 94.6%, 96.7% (controls); 80.7%, 89.1% (enucleation); 90.1%, 91.7% (aspiration); 83.1%, 91.5% (extrusion), respectively. Live birth rates at the four-cell stage were slightly lower in the enucleation group than in the blastomere aspiration and extrusion groups or controls (49.2% versus 58.8%, 56.3% and 66.7%, respectively). For the eight-cell stage, there were no differences between the groups. No developmental abnormalities were found in body or organ weights, in neonates or at 3 weeks of age, or in their subsequent ability to reproduce a second generation.

CONCLUSIONS

Biopsy of mouse pre-embryos produces only a small loss of viability because of trauma and permits normal prenatal and postnatal development among surviving pre-embryos.

Preconception and preimplantation diagnosis for cystic fibrosis

Preimplantation diagnosis provides couples at high genetic risk the possibility of avoiding genetic disease without the need for prenatal diagnosis and selective abortion of the affected pregnancy. Following extensive background work on the reliability of genetic diagnosis in a single cell, we offered on a research basis preimplantation diagnosis to five couples at risk for offspring with the delta-F508 mutation (the major mutation causing cystic fibrosis). There was no detrimental effect from polar body removal on either fertilization or preimplantation development. Genetic analysis, undertaken in 22 polar bodies and 15 corresponding blastomeres, identified 21 embryos of which ten were transferred.

Germ-line gene therapy: a new stage of debate

Ethical debate on human germ-line gene therapy is in a new stage. After an era when only individual convictions could be examined, technology is on a threshold of real possibilities. Germ-line gene therapy can conceivably be carried out in either of two practical ways: 1) insertion of a gene into a pre-embryo, which is the subject of this paper, or 2) insertion of a gene into the germ cells of an individual.

Transgenic animal research and pre-implantation embryo diagnosis have implications for human embryonic germ-line experiments to correct single gene disorders. When would such experiments be feasible and ethically acceptable? If further animal research supports it, we argue for a moral obligation to learn if human germ-line experiments are feasible and safe to attempt. The obligation is grounded in several social-ethical principles that lead society and researchers to set gods for studies that promisc to relieve and to prevent human suffering and premature death. These principles also shape the practices and restrictions of biomedical research.

Carrier detection and prenatal diagnosis of hemophilia A: 5-years experience at a hemophilia center

The identification of carriers and the prenatal diagnosis of hemophilia A have greatly improved with knowledge of the structure of the factor VIII gene. This has permitted the defect to be traced in families by using restriction fragment length polymorphisms. This study summarizes 5 years' experience at A. Bianchi Bonomi Hemophilia and Thrombosis Center and the problems encountered. One hundred and forty-four women at risk of hemophilia A from 93 families were referred to our center for DNA analysis. Carrier detection was performed in 95 women, including 11 who were pregnant. Prenatal diagnosis was performed at 7-14 weeks' gestation in 56 pregnant women. We employed two intragenic restriction fragment length polymorphisms (XbaI and BclI) and the two extragenic restriction fragment length polymorphisms (TaqI and BglII). Combining the results of intra and extragenic restriction fragment length polymorphisms with pedigree analysis, a diagnosis was reached in approximately 90% of cases. Of the 33 male fetuses, 11 were affected and 19 not affected. Two cases of recombination between extragenic restriction fragment length polymorphisms and the factor VIII locus were found.

The polymerase chain reaction and its potential role in clinical diagnostics and research

In-vitro amplification of deoxyribonucleic acid molecules by means of the polymerase chain reaction (PCR) must be regarded as the most important advance in the life sciences to take place during the last decade. Originally applied to the identification of mutations in well-known and fully sequenced human genes, its applications have now been extended to a wide variety of biological and medical disciplines, accompanied by significant technical improvements and sophisticated variations of the basic principle. Specialized molecular genetics laboratories were the first to employ this new method, and they still are in the process of extending its potential. Due to its unique properties, applications of PCR quickly spread to other areas of research, and numerous clinical studies have already employed PCR. The field is currently still expanding rapidly.

Nucleotide sequence of p49a, a genomic Y-specific probe with potential utilization in sex determination

The nucleotide sequence of probe 49a, a genomic polymorphic probe located on the long arm of the Y chromosome, is described. It is an untranslated sequence of about 750 bp with a high A + T content. This sort of selectively neutral sequence can be used with the polymerase chain reaction in sex determination.

Absence of fetal cells in maternal circulation at a level of 1 in 25,000

A dual polymerase chain reaction (PCR) technique is developed which enables the detection of one part male DNA in 25,000 parts female DNA. The technique amplifies a part of the X-Y homologous amelogenin gene in which the Y counterpart has a 189 bp deletion within one of the introns. This deletion has made it possible to identify individual X and Y counterparts based on the difference in size between them. None of the 18 pregnant women studied showed a positive Y-signal although eight of them bear male fetuses excluding the presence of fetal cells at one in 25,000 maternal cells. The results presented here show that a sensitivity of greater than one in 25,000 is required for detection of fetal genetic disease using maternal peripheral blood.

Reliability of gender determination using the polymerase chain reaction (PCR) for single cells

Contamination with extraneous DNA sequences is a frequent problem when performing PCR analysis of single cells. This report describes our experience with eliminating contaminating DNA sequences from PCR reagents for the purposes of gender identification. We have used amplification of Y-specific sequences to identify the gender of single human amniocytes. Female cells consistently showed no Y-specific bands but only 80% of male cells showed the expected intense Y-specific band. This phenomenon could lead to incorrect gender identification of single cells. We developed a technique of simultaneous amplification of X- and Y-specific sequences to prevent misdiagnosis because of failed PCR, which allows accurate preimplantation gender determination for women at risk for conceiving children with X-linked genetic diseases. We analyzed the gender of 141 consecutive single cells in a blinded manner without a single incorrect gender assignment.

Preimplantation diagnosis: a patient perspective

A new dimension in the prevention of birth defects will be achieved when genetic diseases can be routinely diagnosed in embryos prior to implantation. The impressions and attitudes towards preimplantation diagnosis were studied in prospective patients, women at high reproductive risk for a genetic disease. Their perspective highlighted not only the advantages and disadvantages of this new approach, but also those changes necessary in order for preimplantation diagnosis to become a useful and practical technique. The data presented are based on information obtained by a mailed questionnaire answered by 58 women. The main benefit of preimplantation diagnosis for these high-risk women would be the ability to undertake a pregnancy without having to be subjected to the physical and/or emotional trauma of elective termination. Their major concerns related to possible damage to the embryo following biopsy, the cost of the procedure, and the low success rate of completed pregnancies. Other issues to be addressed before preimplantation diagnosis could begin to compare favourably with existing forms of prenatal testing were that the methods of obtaining oocytes or embryos should be simple, well tolerated, highly efficient, and low in maternal risk, and that the genetic analysis of embryonic or extraembryonic cells should be unequivocally accurate.

Isolation of bovine Y-derived sequence: potential use in embryo sexing

To obtain bovine Y-derived probes, we have constructed a bovine plasmid library enriched for Y-specific DNA sequences by the deletion enrichment method. The resulting clones were analyzed by hybridization to Southern blots of male and female genomic DNA. From 200 clones tested, two (BC1.2 and BC1.34) were entirely male specific, six gave a male-female differential hybridization pattern, and the remaining reacted similarly with male and female DNA. Interspecies somatic cell hybrid studies and chromosomal in situ hybridization confirmed that the BC1.2 sequence was derived from the Y chromosome. This 54-bp fragment is present at about 2000-2500 copies in the bovine male genome. No polymorphism was revealed with any of the restriction enzymes used, suggesting enzyme site conservation within blocks of repeats. Evolutionary study has shown that the BC1.2 sequence is conserved within Bos and Bison genera and remains male specific. The male specificity and repeated nature of the BC1.2 sequence have enabled us to use it as a molecular probe for sex determination on small numbers of cells by in situ hybridization.

Sexing of forensic samples using PCR

A rapid protocol has been established for sexing forensic samples by the Polymerase Chain Reaction method. Three sets of primer were used, two specific for Y chromosome repetitive sequences and one specific for X chromosome repetitive sequences. Detailed procedures of experiments, the controls and the applications to testing bloodstains and a vaginal swab are presented. The sensitivity of the test and problems due to contamination are discussed.

Medical genetics

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Recent advances in the polymerase chain reaction

The polymerase chain reaction (PCR) has dramatically altered how molecular studies are conducted as well as what questions can be asked. In addition to simplifying molecular tasks typically carried out with the use of recombinant DNA technology, PCR has allowed a spectrum of advances ranging from the identification of novel genes and pathogens to the quantitation of characterized nucleotide sequences. PCR can provide insights into the intricacies of single cells as well as the evolution of species. Some recent developments in instrumentation, methodology, and applications of the PCR are presented in this review.

Preimplantation embryo biopsy: detection of trisomy in a single cell biopsied from a four-cell mouse embryo

This paper describes an efficient technique for the production of metaphase spreads from single blastomeres biopsied from four-cell preimplantation mouse embryos. The karyotype obtained by chromosomal analysis of single biopsied cells is shown to be fully predictive of subsequent fetal karyotype. The data in this study also demonstrate that the entire process of embryo biopsy and karyotypic analysis of biopsied blastomeres does not adversely affect the ability of biopsied embryos to form fetuses after transfer into pseudopregnant recipients. This study has potential clinical relevance in that it demonstrates that chromosomally defective embryos can be accurately identified before implantation. In addition, the techniques developed in this study may facilitate more efficient procedures for the genesis of animal models for human disorders such as Down syndrome and Alzheimers disease.

A simple method for quantifying specific mRNAs in small numbers of early mouse embryos

Amplification of sequences by the polymerase chain reaction (PCR) has become a powerful tool in the study of gene expression. The technique is, in fact, so powerful that it may detect 'leaky transcription'. Thus, it is now important to be able to quantify the transcripts that are amplified to determine whether or not they represent legitimate transcription of target genes. In this paper, we describe a one-step amplification reaction coupled to solution hybridization/RNase protection that is capable of quantitating specific transcripts in total RNA from one to ten preimplantation mouse embryos and is generally applicable to any cloned mRNA sequence.

Assisted conception. Health services and evaluation

Comparison of assisted conception in Australia, the United Kingdom, and the United States indicates that further rapid growth in services is likely in many countries. Better data on pregnancy rates and the outcome of pregnancy, as well as standardized reporting of national results, are needed to monitor the effectiveness of treatment.

Preembryo biopsy and analysis of blastomeres by in situ hybridization

We developed a method for the biopsy of preimplantation mouse embryos (preembryos) at the four- to eight-cell stage, which uses partial zona pellucida dissection. The preembryos were collected in calcium- and magnesium-free phosphate-buffered saline solution with 0.01% ethylenediaminetetraacetic acid, 0.1 mol/L sucrose, and 4 mg/ml of bovine serum albumin to facilitate removal of blastomeres. This allows entry of a fine micropipette into the perivitelline cavity with subsequent removal of a single blastomere by gentle suction. The majority of embryos (75%) from which biopsy specimens were obtained in this fashion developed to the blastocyst stage. The blastomeres obtained were mainly intact and they were fixed to glass slides. After permeabilization, in situ hybridization was performed with chromosome X- and chromosome 3-specific probes. Human unfertilized eggs and blastomeres from human polyspermic embryos also have been analyzed by in situ hybridization with chromosome specific probes. The combination of nondestructive embryo biopsy and in situ hybridization is a possible approach for preimplantation genetic diagnosis.

Antenatal diagnosis of beta-thalassemia in Sardinia

This paper reviews the characteristics and the results of 15 years of experience with a preventive program, based on carrier screening and prenatal diagnosis, designed to control thalassemia major in the Sardinian population. The education of the population about thalassemia and the modalities for its prevention was accomplished via the mass media. Carrier screening was carried out voluntarily on couples of child-bearing age. Prenatal diagnosis was initially carried out by fetal blood analysis; since 1983, it has been done by DNA analysis on non-amplified or amplified DNA. Different chorionic villous sampling procedures have been used. Nowadays, we have adopted the transabdominal approach because, in our experience, it seems to be associated with a low risk (2%) of fetal mortality. At the present time, the beta-thalassemia mutations are detected directly by dot-blot analysis of amplified DNA with 32P- or horseradish peroxidase-labeled allele-specific oligonucleotide probes. Two oligonucleotide probes, one complementary to the codon-39 nonsense mutation, which accounts for 95.7% of the beta-thalassemia chromosomes in the Sardinian population, and the other complementary to the frameshift at codon 6, which is the second most common mutation in our population (2.1%), allow us to make prenatal diagnosis in the large majority of cases. Notwithstanding a careful dissection of maternal decidua from chorionic villi, co-amplification of maternal sequence was detected in 4 out of 425 cases tested by this procedure. In order to avoid this pitfall, the simultaneous amplification of highly polymorphic VNTR (variable number of tandem repeats) segments could be used. On the whole we have so far carried out 2711 prenatal tests: 1130 by fetal blood analysis, 1156 by oligonucleotide hybridization on electrophoretically separated DNA fragments, and 425 by dot-blot analysis on amplified DNA with allele-specific oligonucleotide probes. Two errors occurred by fetal blood analysis and none by DNA analysis. The incidence of thalassemia major declined from 1:250 live births in the absence of prevention to 1:1000 after the establishment of this program, indicating that carrier screening and prenatal diagnosis are effective means for preventing thalassemia major at the population level.

The polymerase chain reaction: miracle or mirage? A critical review of its uses and limitations in diagnosis and research

Since publication of the polymerase chain reaction (PCR) technique in 1985 (Saiki et al. Science 1985; 230: 1350-1354), there has been an explosion of reports on its use in medicine and science. We critically review its use both as a diagnostic technique and as a research tool, and show the pathologist how to evaluate PCR data and how to avoid the pitfalls of overinterpretation. We discuss the value of PCR in the characterization of genetic defects, prenatal diagnosis, carrier testing, HLA typing, detecting micro-organisms, identifying activated oncogenes, and in the characterization of leukaemias and lymphomas, and summarize the main applications in biomedical research.

Chorionic villus sampling for prenatal diagnosis in Wales using DNA probes--5 years' experience

Chorionic villi were sampled from 125 women who requested prenatal diagnosis, either for genetic disorder or because of advanced maternal age. Of these, 105 samples were obtained by the transcervical route and 20 were obtained by the transabdominal approach. The sampling success rate was 97 per cent (122/125). The mean maternal age of the patients was 31 years (range 17-44) and the mean gestational age at which the chorionic villus sampling was performed was 10 weeks (range 7-13 weeks). Seventy-four of these diagnoses involved the use of DNA markers. The minimal size of the sample used for DNA diagnosis was 5 mg. Maternal contamination was detected in two samples. A diagnosis was provided on all but two samples. The fetal loss rate was high initially but fell to 1.9 per cent in 1988.

Direct electrophoretic detection of the allelic state of single DNA molecules in human sperm by using the polymerase chain reaction

We have developed a procedure that allows the detection of polymerase chain reaction (PCR) products derived from a single target DNA molecule in a human sperm without using radioactive probes. With this method, three genetic loci present in a single sperm can be amplified simultaneously. The amplification procedure is specific as well as efficient and permits detection of the PCR product by ethidium bromide staining after polyacrylamide gel electrophoresis. When allele-specific PCR primers that differ in length are used, the size of the PCR products of different alleles also vary in length, allowing the allelic state at each locus to be determined electrophoretically. Studies on individual sperm by using this procedure should facilitate the measurement of genetic recombination in humans over small physical distances. The ability to directly analyze the allelic state of PCR products from one cell rapidly and simply will also be useful for the prenatal diagnosis of genetic disease, especially in the analysis of single blastomeres taken from in vitro fertilized eggs prior to implantation.