Prenatal detection of trisomy 21 in uncultured amniocytes by fluorescence in situ hybridization: a prospective study

Prenatal Detection of Chromosome Aneuploidy by Quantitative Fluorescence PCR

Autosomal chromosome aneuploid pregnancies that survive to term, namely, trisomies 13, 18, and 21, account for 89% of chromosome abnormalities with a severe phenotype identified in prenatal samples. They are traditionally detected by full karyotype analysis of cultured cells. The average reporting time for a prenatal karyotype analysis is approximately 14 days, and in recent years, there has been increasing demand for more rapid prenatal results with respect to the common chromosome aneuploidies, to relieve maternal anxiety and facilitate options in pregnancy. The rapid tests that have been developed negate the requirement for cultured cells, instead directly testing cells from the amniotic fluid or chorionic villus sample, with the aim of generating results within 48 h of sample receipt. Interphase fluorescence in situ hybridization is the method of choice in some genetic laboratories, usually because the expertise and equipment are readily available. However, a quantitative fluorescence (QF)-PCR-based approach is now widely used and reported as a clinical diagnostic service in many studies. It may be used as a stand-alone test or as an adjunct test to full karyotype or array CGH analysis, which scan for other chromosome abnormalities not detected by the QF-PCR assay.

Rapid Prenatal Diagnosis of Aneuploidy Using Quantitative Fluorescence-PCR (QF-PCR)

Molecular cytogenetic aneuploidy testing for pregnant women at increased risk of chromosome abnormality leads to rapid reassurance for those with normal results and earlier decisions on pregnancy management in the case of abnormality. We tested 9080 prenatal samples using a one-tube QF-PCR test for trisomies 13, 18, and 21; the abnormality rate was 5.9%. There were no misdiagnoses for non-mosaic trisomy. A sex chromosome multiplex was developed that detects structural sex chromosome abnormalities as well as aneuploidies. The sex chromosome test was targeted at pregnancies (272) with specific abnormalities suggestive of Turner syndrome; 13.2% showed 45,X, confirmed by follow-up analysis.

Context and Applications of Targeted Genetic Testing, with Emphasis on Copy Number Variants

There has been a huge acceleration in our technical ability to detect variation in the human genome in recent years, and there has been a corresponding effort in clinical diagnostic laboratories to take advantage of this progress for the benefit of patients. There has, however, not been an equivalent increase in our understanding of human genetics and disease, not for lack of effort but due to the far greater complexity of understanding variation than the difficulties of detecting it. This chapter describes how software tools can be used to target clinical genetic diagnostic testing in order to exploit technical and scientific advances both efficiently and cost-effectively, while maximizing clinical utility.

Performance of QF-PCR in targeted prenatal aneuploidy diagnosis: Indian scenario

OBJECTIVE

Among the rapid aneuploidy detection methods, QF-PCR has now become an alternative tool for prenatal aneuploidy diagnosis concomitant with karyotyping. This method has been validated in many of the western clinics but in India no study was conducted to assess its utility as standalone procedure. The study was designed to answer the question whether QF-PCR can be implemented as a standalone diagnostic method for rapid aneuploidy diagnosis in our present clinical setup?

MATERIALS AND METHODS

Study was conducted during March 2012 to August 2014 consisting of 270 prenatal samples that underwent for aneuploidy diagnosis. In addition to karyotyping, QF-PCR was also performed on these samples and the results were compared.

RESULTS

Of 270 samples screened, 262 samples showed euploid genome (125 normal male and 137 normal female). Eight samples were consistent with aneuploidy--four trisomy 21 male sample, 2 trisomy 21 female sample, 1 trisomy 18 samples and 1 Klinefelter sample. The specificity, sensitivity, positive prediction value and negative prediction values were 100% while false positive rate and false negative rate were 0%.

CONCLUSION

Outcome of this study strongly suggests that QF-PCR can be used as standalone procedure for targeted rapid aneuploidy diagnosis.

Chromosome aberrations identified by cytogenetic analysis of the first two clones of cultured amniotic fluid cells compared with QF-PCR results

We report on our experience of studying amniotic fluid cells by cytogenetic analysis (CA) of the first 2 clones. We investigated the incidence and types of chromosome aberrations detected by CA of 196 amniocenteses performed on pregnant women at high risk. Of these cases, 178 were analysed by QF-PCR (risk group A). The results were compared with the data obtained by CA of 1,263 amniocenteses carried out in patients with other indications. QF-PCR was used to investigate 1,030 of these cases (risk group B). The combined average turnaround time for a CA result of the first 2 clones in both risk groups was within 9 ± 2 days. The final CA results (≥6 clones) were obtained within 12 ± 4 days. In risk group A, CA was not possible in 2 cases due to cell culture failure. The foetal karyotype was abnormal in 13.8% of the cases by CA of ≥6 clones and in 13.5% of the cases by QF-PCR. Together, CA of ≥6 clones and QF-PCR detected chromosome aberrations in 14.8% of the cases. With the exception of 2 cases of in vitro culture failure and 1 case with low gonosomal mosaicism, CA of the first 2 clones detected all cases with chromosome aberrations. Five cases with clinically significant chromosome aberrations were not detected by QF-PCR. In risk group B, the foetal karyotype was found to be abnormal in 2.2% of the cases by CA of ≥6 clones and in 1.0% of the cases by QF-PCR. Together, CA of ≥6 clones and QF-PCR revealed chromosome aberrations in 2.2% of the cases. With the exception of 1 case with low gonosomal mosaicism, CA of the first 2 clones detected all other cases with chromosome aberrations. The majority of these cases were inherited chromosome aberrations. Eighteen cases with chromosome aberrations were not detected by QF-PCR. Based on our results, CA of ≥6 clones, together with QF-PCR as a first test, should be performed in all prenatal cases with abnormal ultrasound findings. In pregnancies with other indications, CA of the first 2 clones alone is sufficient to identify all clinically significant (and inherited) chromosome aberrations.

Rapid detection of fetal aneuploidies by quantitative fluorescent-polymerase chain reaction for prenatal diagnosis in the Turkish population

Prenatal diagnosis is testing for diseases or conditions in a fetus or embryo before it is born. It employs a variety of techniques to determine the health and condition of an unborn fetus. The main goal of this process is to perform prenatal diagnosis at the earliest possible stage of gestation. In this regard, quantitative fluorescent-polymerase chain reaction (QF-PCR), a novel technique that is fast and reliable, was employed to detect aneuploidies (13, 18, 21, X and Y) without the need of the time-consuming culturing process. The QF-PCR method can detect five different chromosome aneuploidies with 98.6% accuracy. In this study, 1874 amniotic fluid samples of pregnant subjects, who were referred to the Department of Medical Biology and Genetics, Adana, Turkey (molecular biology section), were analyzed with the QF-PCR technique by employing 27 short tandem repeat (STR) markers to detect chromosomes 13, 18, 21, X and Y aneuploidies. We detected 31 subjects (1.7%) with aneuploidies or euploidies out of the 1874 subjects. The average age of the pregnant subjects was 32 (range: 14-49). Abnormal karyotypes detected were as follows: 47,XX,+21 (19.4%, 6/31), 47,XY,+21 (48.4%, 15/31), 48,XXX,+21 (3.2%, 1/31), 69,XXX (3.2%, 1/31), 47,XY,+13 (3.2%, 1/31), 47,XXY (9.6%, 3/31), 47,XXX (9.6%, 3/31) and 45,X (3.2%, 1/31). Moreover, some STR markers were found to be more specific to the Turkish population. In conclusion, QF-PCR can be regarded as an alternative method of conventional cytogenetic analysis as it is a rapid and reliable method; however, in most cases it is required to be supported or validated with conventional cytogenetic karyotyping and some STR markers employed for QF-PCR can be more informative for a given population.

Prenatal detection of chromosome aneuploidy by quantitative-fluorescence PCR

QF-PCR refers to the amplification of chromosome-specific polymorphic microsatellite markers using fluorescence-labelled primers, followed by semi-quantitative analysis of the products on a genetic analyser to determine copy number and/or imbalances of specific chromosomal material. This approach is now widely used for rapid prenatal diagnosis of the common trisomies. In addition, it can successfully detect maternal cell contamination and mosaicism in prenatal material.

Assessment of QF-PCR as the first approach in prenatal diagnosis

Quantitative fluorescent PCR (QF-PCR) has been used by many laboratories for prenatal diagnosis of the most common aneuploidies. QF-PCR is rapid, cost-effective, and suitable for automation and can detect most abnormalities diagnosed by conventional karyotyping. Whether QF-PCR should be used alone in most of the samples and in which karyotyping should also be offered is currently a topic of debate. We evaluated and compared the results obtained from 7679 prenatal samples in which conventional karyotype and QF-PCR had been performed, including 1243 chorionic villi and 6436 amniotic fluid samples. Concordant QF-PCR and karyotype results were obtained in 98.75% of the samples. An abnormal karyotype associated with adverse clinical outcome undetected by QF-PCR was found in 0.05% of samples. Therefore, QF-PCR can be used alone in a large number of samples studied in a prenatal laboratory, thereby reducing both the workload in cytogenetic laboratories and parental anxiety when awaiting results.

Prenatal detection of chromosome aneuploidy by quantitative fluorescence PCR

Autosomal chromosome aneuploid pregnancies that survive to term, namely, trisomies 13, 18, and 21, account for 89% of chromosome abnormalities with a severe phenotype. They are normally detected by full karyotype analysis of cultured cells. The average reporting time for a prenatal karyotype analysis is approximately 14 days, and in recent years, there has been increasing demand for more rapid prenatal results with respect to the common chromosome aneuploidies, to relieve maternal anxiety and facilitate options in pregnancy. The rapid tests that have been developed negate the requirement for cultured cells, instead directly testing cells from the amniotic fluid or chorionic villus sample, with the aim of generating results within 48 h of sample receipt. Interphase fluorescence in situ hybridization is the method of choice in some genetic laboratories, usually because the expertise and equipment are readily available. However, a quantitative fluorescence (QF)-PCR-based approach is more suited to a high-throughput diagnostic service. This approach has been investigated in a small number of pilot studies and reported as a clinical diagnostic service in many studies. It may be used as a stand-alone test or as an adjunct test to full karyotype analysis, which subsequently confirms the rapid result and scans for other chromosome abnormalities not detected by the QF-PCR assay.

External quality assessment of rapid prenatal detection of numerical chromosomal aberrations using molecular genetic techniques: 3 years experience

OBJECTIVES

Prenatal diagnosis using rapid molecular genetic techniques is now a widely used method for detecting the most prevalent chromosomal aneuploidies. The object of this work was to develop a methodology for delivering external quality assessment (EQA) appropriate to the needs of routine diagnostic testing laboratories.

METHODS

We have provided three rounds of EQA using 15 different samples over 3 years. The scheme has developed to assess both the genotyping accuracy of the results and the appropriateness of the clinical reports issued to the referring clinician.

RESULTS

Participation in the EQA scheme has increased from 9 to 27 laboratories from across Europe over the three sample distributions. All laboratories have used quantitative fluorescence-PCR (QF-PCR) to analyse these samples except for a sole participant in 2006 who used multiplex ligation-dependent probe amplification (MLPA). In total 265 samples have been distributed, of which four (1.5%) were not reported due to technical failures and one (0.4%) was reported incorrectly and must be regarded as a genotyping error.

CONCLUSIONS

We have demonstrated a significant and increasing demand for EQA in the rapid detection of aneuploidies in UK and other European laboratories. Using the methodologies described, we have had a very low rate of technical failures and demonstrated a high level of genotyping accuracy. However, the quality of the clinical reports was variable and suggestions are made for improvement.

Prenatal Diagnosis by Array-CGH

Microscopic karyotype analysis of cultured cells has been regarded as the gold standard for prenatal diagnosis for over 30 years. Since the first application of this technique to prenatal testing in the early 1970's, this procedure has proved to be highly reliable for identifying chromosome copy number abnormalities (aneuploidy) and large structural rearrangements in foetal cells obtained invasively by either amniocentesis or chorionic villus sampling (CVS). Recognising the need for more rapid testing methods which do not require cell culture, fluorescence in situ hybridisation (FISH) and quantitative fluorescence PCR (QF-PCR) have been introduced to this field in order to answer specific diagnostic questions. However, both FISH and QF-PCR suffer the disadvantage in that they are difficult to scale to a comprehensive, genome-wide screen. Array-comparative genomic hybridisation (array-CGH) in contrast is a comprehensive, genome-wide screening strategy for detecting DNA copy number imbalances which can be rapid, less labour-intensive than karyotype banding analysis and is highly amenable to automation. Array-CGH has the potential to be used for prenatal diagnosis and may address many of the limitations of both conventional microscopic cytogenetic analyses and the more recently employed rapid-screening strategies.

Prenatal diagnosis for chromosome abnormalities: past, present and future

Prenatal diagnosis for chromosome abnormalities has been available for over 30 years. The most common referral indication is a raised risk of Down's syndrome, and diagnosis has, until recently, been carried out by culture of cells from invasive prenatal sampling, followed by full karyotype analysis, with a waiting time of around 2 weeks for results. More recent developments in fluorescence in situ hybridisation (FISH) and quantitative fluorescence-PCR techniques have led to rapid 1-2 d reporting for Down's syndrome, opening the way to the possibility of targeted testing based on referral indication, thus reducing the incidence of difficult counselling issues and potentially unnecessary pregnancy terminations following the unexpected discovery of anomalies such as balanced chromosome rearrangements. The future of prenatal diagnosis must lie in the non-invasive diagnosis of Down's syndrome using fetal cells from maternal circulation.

False-positive diagnosis of trisomy 21 using fluorescence in situ hybridisation (FISH) on uncultured amniotic fluid cells

An amniocentesis was performed on a 22-week pregnancy following the detection of foetal abnormalities on ultrasound. A rapid aneuploid screen using fluorescence in situ hybridisation on uncultured amniotic fluid cells revealed 3 signals for chromosome 21, consistent with trisomy 21. In situ metaphase cultures revealed a 46,XY normal male karyotype. These discordant results may be explained by a sub-standard batch of the commercially available probe or alternatively, a very specific variation within the sample interacting with the probe. Alternative strategies are proposed in order to safeguard against inappropriate clinical action as a consequence of discordant results.

Prenatal detection of aneuploidies using fluorescence in situ hybridization: a preliminary experience in an Indian set up

Fluorescence in situ hybridization (FISH) is a powerful molecular cytogenetic technique which allows rapid detection of aneuploidies on interphase cells and metaphase spreads. The aim of the present study was to evaluate FISH as a tool in prenatal diagnosis of aneuploidies in high risk pregnancies in an Indian set up. Prenatal diagnosis was carried out in 88 high-risk pregnancies using FISH and cytogenetic analysis. Multicolour commercially available FISH probes specific for chromosomes 13, 18, 21, X and Y were used. Interphase FISH was done on uncultured cells from chorionic villus and amniotic fluid samples. FISH on metaphase spreads was done from cord blood samples. The results of FISH were in conformity with the results of cytogenetic analysis in all the normal and aneuploid cases except in one case of structural chromosomal abnormality. The hybridization efficiency of the 5 probes used for the detection of aneuploidies was 100%. Using these probes FISH assay yielded discrete differences in the signal profiles between cytogenetically normal and abnormal samples. The overall mean interphase disomic signal patterns of chromosomes 13, 18, 21, X and Y were 94.45%; for interphase trisomic signal pattern of chromosome 21 was 97.3%. Interphase FISH is very useful in urgent high risk cases. The use of FISH overcomes the difficulties of conventional banding on metaphase spreads and reduces the time of reporting. However, with the limited number of probes used, the conventional cytogenetic analysis serves as a gold standard at present. It should be employed as an adjunctive tool to conventional cytogenetics.

Rapid prenatal diagnosis of trisomy 21 in 5049 consecutive uncultured amniotic fluid samples by fluorescence in situ hybridisation (FISH)

OBJECTIVES

This was a retrospective study on the results of interphase fluorescence in situ hybridization (FISH), performed routinely for chromosome 21 and on ultrasonographic indications for chromosomes 13, 18, X and Y in a series of 5049 amniotic fluid samples.

METHODS

Interphase FISH for chromosome 21 was performed in 5049 consecutive amniotic fluid samples for the rapid prenatal diagnosis of Down syndrome. Aneuploidy for four other chromosomes (13, 18, X and Y) was tested following ultrasonographic indications. Karyotypes from standard cytogenetic analysis were compared to the FISH results.

RESULTS

Using conventional cytogenetics 3.6% (183/5049) chromosomal anomalies were detected. After exclusion of familial chromosome rearrangements, i.e. balanced autosomal reciprocal or Robertsonian translocations (30/5049) and inversions (19/5049), 2.65% chromosomal anomalies (134/5049) were diagnosed. Of this group 0.18% (9/5049) were chromosomal rearrangements not detectable by FISH and 2.47% (125/5049) were numerical chromosomal anomalies detectable by interphase FISH for chromosomes 13, 18, 21, X and Y. With routine interphase FISH for chromosome 21 and FISH on echographic indication for the other four chromosomes we detected 107/125 of these numerical chromosomal anomalies, i.e. 85.6%. All 70 cases of trisomy 21 were detected by FISH and confirmed with conventional cytogenetics (sensitivity=100%) and there were no false-positive results (specificity=100%). Maternal cell contamination of amniotic fluid samples occurred in 1.27% (64/5049) of samples; 0.26% (13/5049) of these samples were uninformative by FISH due to maternal cell contamination (12/5049) or absence of nuclei in one sample (1/5049).

CONCLUSION

In this group of 5049 samples we found that FISH is a reliable technique for the rapid prenatal diagnosis of trisomy 21. The number of uninformative cases due to maternal cell contamination was low. The strategy to perform FISH for chromosome 21 in all samples and only on ultrasonographic indication for the four other chromosomes (13, 18, X and Y) followed by standard cytogenetics is effective.

Development and implementation of a new rapid aneuploidy diagnostic service within the UK National Health Service and implications for the future of prenatal diagnosis

BACKGROUND

Prenatal diagnosis for chromosome abnormality is routinely undertaken by full karyotype analysis of chromosomes from cultured cells; pregnant women must wait on average 13-14 days for their results. Autosomal trisomies, which account for around 80% of significant abnormalities, can be detected by quantitative fluorescence (QF) PCR. We report on the development and implementation of this technique as the first such routine service within a diagnostic department of the UK National Health Service (NHS).

METHODS

We designed a "one-tube test" comprising four primer pairs for polymorphic tetranucleotide repeat sequences on chromosome 21, four primer pairs for sequences on chromosome 18, three primer pairs for sequences on chromosome 13, and one primer pair to identify the sex chromosomes. All prenatal samples received by our NHS diagnostic department between April, 2000, and April, 2001, were tested. After DNA extraction, PCR amplification was done and the products separated on a capillary-based genetic analyser; the results were interpreted with dedicated software. Follow-up karyotype analysis was done on all samples.

FINDINGS

1148 amniotic fluid samples, 188 chorionic villus samples, and 37 fetal tissue samples were tested; the amplification failure rate was zero with our current protocol. QF-PCR results were obtained and reported on 1314 (98%) of the prenatal samples; the remaining 22 (2%) were uninformative because of maternal-cell contamination. One case of mosaicism in a chorionic villus sample, and two cases indicating somatic expansion of a tetranucleotide repeat were found. No false positive or false negative results were obtained. The mean reporting time for the last 4 months of data collection was 1.25 working days.

INTERPRETATION

QF-PCR aneuploidy testing is an efficient and accurate technique for the detection of autosomal trisomies in prenatal samples. Implementation of this service has led to the rapid diagnosis of abnormalities and early reassurance for women with normal results.

Prenatal diagnosis using interphase fluorescence in situ hybridization (FISH): 2-year multi-center retrospective study and review of the literature

Since 1993, the position of the American College of Medical Genetics (ACMG) has been that prenatal interphase fluorescence in situ hybridization (FISH) is investigational. In 1997, the FDA cleared the AneuVysion assay (Vysis, Inc.) to enumerate chromosomes 13, 18, 21, X and Y for prenatal diagnosis. Data is presented from the clinical trial that led to regulatory clearance (1379 pregnancies) and from retrospective case review on 5197 new pregnancies. These studies demonstrated an extremely high concordance rate between FISH and standard cytogenetics (99.8%) for specific abnormalities that the AneuVysion assay is designed to detect. In 29 039 informative testing events (6576 new and 22 463 cases in the literature) only one false positive (false positive rate = 0.003%) and seven false negative results (false negative rate = 0.024%) occurred. A historical review of all known accounts of specimens tested is presented (29 039 using AneuVysion and 18 275 specimens tested with other probes). These performance characteristics support a prenatal management strategy that includes utilization of FISH for prenatal testing when a diagnosis of aneuploidy of chromosome 13, 18, 21, X or Y is highly suspected by virtue of maternal age, positive maternal serum biochemical screening or abnormal ultrasound findings.

Fluorescence in situ hybridization (FISH) for rapid detection of aneuploidy: experience in 911 prenatal cases

Fluorescence in situ hybridization (FISH) was performed with probes specific for chromosomes 13, 18, 21, X and Y on 911 of 11123 (8.2%) amniotic fluid samples submitted to the present authors' laboratory for cytogenetic analysis over an 8-year period. Altogether 3516 hybridizations were performed with an interpretable FISH result on all chromosomes requested in 884/911 (97%) of cases. An uninformative FISH result occurred in 44 hybridizations among 27 cases (3%). Of a total of 89 karyotypically proven cases with aneuploidy that might have been detected by FISH, the overall detection rate was 84%. An inconclusive or incomplete FISH result occurred in 9/89 (10%) of these proven aneuploid cases. In the remaining 80 informative proven aneuploid cases, correct detection of aneuploidy was accomplished in 75/80 (94%) of samples. A false-negative result occurred in the remaining 5/80 (6%) of such informative cases. Eighteen cases had karyotypically proven abnormalities that could not have been detected by the targeted FISH. Aside from these 18 cases, FISH allowed correct detection of normal disomy in 785/804 (98%) of such cases. An incomplete FISH result occurred in 18 normal disomic cases. There was a single possible 'false-positive' FISH result for chromosome 21. Interphase FISH analysis of uncultured amniotic fluid cells has been shown to be a useful laboratory tool for rapid fetal aneuploidy screening during pregnancy. As with all clinical laboratory diagnostic tests, incomplete or inconclusive results (or even interpretive errors) occur in a small percentage of cases. Nevertheless, FISH results accompanied by other data and by appropriate counseling provide clinicians and patients with valuable information for clinical decision-making surrounding family planning and pregnancy management.

Fluorescence in situ hybridization of chorionic interphase cells for prenatal screening of Down syndrome

OBJECTIVE

Our purpose was to determine the usefulness and reliability of fluorescence in situ hybridization on interphase chorionic villi cells in the prenatal diagnosis of Down syndrome.

METHODS

A total of 336 samples of chorionic villi were analysed by direct chromosome preparation and FISH with a DNA probe specific to chromosome 21. The samples were obtained as part of the routine obstetric investigation and management.

RESULTS

The sampling and direct karyotyping was successful in all cases. At least 50 cells were valuable by FISH in 331 of 336 samples. Both methods showed Down syndrome in 12 cases. The follow-up investigations showed that there was no false-negative or false-positive result following these procedures.

CONCLUSION

Based on these results and the fact that it is possible to analyse by interphase FISH at least ten times more cells than by conventional cytogenetic methods, and these cells originate from different tissues of chorionic villi, it is concluded that FISH increases the reliability of the diagnosis. Nevertheless, more data are needed for correct statistical analysis. Since this method is cheaper and gives diagnosis earlier than cell culture, the combination of direct chromosome preparation and FISH on chorionic villi is offered for prenatal Down syndrome screening.

Assessing the chromosome copy number in metaphase II oocytes by sequential fluorescence in situ hybridization

PURPOSE

Aneuploidy in oocytes is the main cause of failed embryo implantation and of miscarriage. At present, only limited data on the prevalence of aneuploidy in freshly collected human oocytes are available and all studies have been performed with conventional methods for karyotyping. In this feasibility study, multiple-hybridization fluorescence in situ hybridization (FISH) was evaluated as an alternative method to determine the number of chromosomes in oocytes.

METHODS

Fifty-two spare oocytes were collected from 23 patients treated with gonadotropins for intrauterine insemination or intracytoplasmic sperm injection. A conventional dual color FISH approach using mixtures of chromosome-specific standard alpha-satellite probes was applied consecutively to the chromosomes of the same metaphase II oocyte. Mixtures of three to six probes were designed in order to allow chromosome identification based on signal color and centromeric index.

RESULTS

One hybridization cycle was possible in 52 uninseminated metaphase II oocytes, two hybridizations in 43 oocytes (82.7%), three hybridizations in 30 oocytes (57.6%), four hybridizations in 27 oocytes (51.9%), and five hybridizations in 15 oocytes (28.8%). Altogether, 591 chromosomes could be marked (47.4% of the entire chromosome complement, 11.4 chromosomes per oocyte). The most important single factor contributing to technical failure was loss of the oocyte from the slide.

CONCLUSION

This feasibility study demonstrates that multiple-hybridization FISH can be used for the assessment of a larger proportion of the chromosome complement in oocyte as compared to previous studies based on FISH.

An assessment of the use of interphase FISH with chromosome specific probes as an alternative to cytogenetics in prenatal diagnosis

We have assessed the effects that would have been observed if we had changed from standard prenatal diagnosis to interphase fluorescence in situ hybridization (FISH) on our amniocentesis samples. We aimed to estimate the number of cases with aberrations other than chromosomes 13, 18, 21, X and Y, which would not have been detectable by FISH and to assess the potential clinical implications for these cases. In 1687 prenatal diagnoses, 111 cases had abnormal cytogenetic reports (6.5% aneuploidy rate). Out of those 111 cases, 14 had chromosomal abnormalities not detectable by FISH but four of these had major structural abnormalities diagnosed on ultrasound, which would have lead to counselling of a very poor prognosis anyway. In 10 cases without abnormal ultrasound findings, if FISH had been used rather than cytogenetics, it appears that there may have had no detrimental effects on the clinical outcomes of the cases studied. Out of those 10 cases, two pregnancies were terminated because of abnormal cytogenetic results (one was due to maternal age and the second one was due to abnormal biochemical screening) (mosaic 46,XY, /47,XY,+mar and 46,X,del(8)(p21) respectively) and their post-mortem results also did not show any abnormalities. One pregnancy was continued in spite of a de novo chromosomal rearrangement and resulted in an apparently normal live birth. Five cases (including a set of twins) with inherited balanced translocations resulted in four normal live births and one unexplained intrauterine death at 32 weeks' gestation and post-mortem was declined. One case with a paternally derived abnormal chromosome 21, decided to continue the pregnancy and resulted in a normal live birth. The last case in this group resulted in a rhesus related intrauterine death in the second trimester, and although an abnormal chromosome 13 insertion (paternally derived known aberration) there was no abnormality found at post-mortem. Therefore, we suggest that it is reasonable to use FISH as an alternative prenatal diagnosis for indications such as advanced maternal age and abnormal maternal serum biochemical screening when high quality ultrasound scanning is performed, but FISH should only be used as an additional test to conventional cytogenetics for the other indications, especially when abnormalities are found on ultrasound scan.

Use of interphase fluorescence in situ hybridization in third trimester fetuses with anomalies and growth retardation

In the last few years, attention has been focused on the use of interphase fluorescence in situ hybridization (FISH) for prenatal diagnosis with chromosome-specific DNA probes in the second trimester. This technique is accurate, rapid, and detects the most common aneuploidies. We present a preliminary study using FISH technique on uncultured amniotic cells derived from 30 fetuses with ultrasonographic evidence of intrauterine growth retardation (IUGR) in the third trimester. Fifteen fetuses were males and 15 were females. Seven fetuses (23.3%) had abnormal chromosomal constitution: five (18.6%) had trisomy 21, one (2.35%) had trisomy 18, and one (2.35%) showed a mosaic trisomy 18. No abnormalities were detected in the other 23 fetuses. Amniocentesis combined with FISH appears to be a safe, rapid, and accurate alternative to blood sampling in the third trimester, reducing the clinical and emotional stress of the time required to complete chromosome analysis by routine cytogenetics.

Rapid and simple prenatal DNA diagnosis of Down's syndrome

BACKGROUND

Prenatal diagnosis of chromosomal abnormality requires cytogenetic analysis of amniotic fetal cells. The necessary culture time delays diagnosis, is expensive, and requires substantial scientific expertise. In a masked prospective study, we investigated the feasibility of PCR amplification of chromosome 21 markers for the prenatal diagnosis of Down's syndrome.

METHODS

The study population consisted of 2167 pregnant women, undergoing amniocentesis for prenatal diagnosis. In this cohort at least 1.5 mL amniotic fluid was available surplus to the requirements for traditional diagnostic methods. DNA was extracted from the surplus amniotic fluid and amplified in fluorescence-based PCR reactions, with three small-tandem-repeat markers located on chromosome 21. The products of the reactions were analysed on a DNA sequencer to identify the presence of two or three copies of chromosome 21.

FINDINGS

In 2083 (97.4%) of 2139 samples of amniotic fluid that were not macroscopically blood-stained, two DNA markers gave an informative and correct result, identifying 2053 fetuses as normal and 30 as having trisomy 21 Down's syndrome (as confirmed by cytogenetic analysis). An extra marker was informative in 32 of 41 other clear samples. Thus a total of 99.6% informative results was achieved with these three markers. Macroscopically blood-stained samples (28 [1.3%]) were unsuitable for DNA testing. They gave a typical but non-informative result. There were no false-positive or false-negative results.

INTERPRETATION

The PCR-based DNA diagnostic test has great potential for improved prenatal diagnosis of Down's syndrome, with the advantage that results may be available within a day.

Quantitative evaluation of fluorescence in situ hybridization (FISH) signals in uncultured coelomic cells

Uncultured coelomic cells were hybridized with alpha satellite DNA probes representing chromosomes X, Y, 18, and 13/21 in order to evaluate the distribution of hybridization signals obtained by fluorescence in situ-hybridization (FISH) analysis of this cell type. Cells from 26 samples were hybridized with the X probe and the Y probe was hybridized with cells from 25 of the samples. Cells from 16 and 11 samples were hybridized with an 18 alpha satellite DNA probe and 13/21 alpha satellite probe, respectively. The evaluation demonstrated that FISH with X, Y, 18, and 13/21 alpha satellite DNA probes on uncultured coelomic cells is a reliable technique since the distribution of hybridization signals is comparable to that seen in uncultured amniotic fluid cells.

Prenatal detection of chromosome aneuploidies by fluorescence in situ hybridization: experience with 2000 uncultured amniotic fluid samples in a prospective preclinical trial

Successful rapid prenatal detection of selected numerical chromosome abnormalities by using fluorescence in situ hybridization (FISH) on uncultured amniotic fluid samples has been described by Klinger et al. (1992) and Ward et al. (1993, 1997). Using essentially the same FISH protocol and identical probes specific for chromosomes 21, 18, 13, X, and Y, we prospectively compared the results of FISH and conventional cytogenetics on 2000 amniotic fluid cell samples. The 1-day FISH assay yielded discrete differences in the signal profiles between cytogenetically disomic, i.e., normal, and trisomic samples. Due to intermittent absent Y-signals, the assay differentiated less well between samples with cytogenetically normal and abnormal sex chromosome complements. The assay efficiency, and thus the clinical utility, was affected by (1) unsuccessful hybridizations (7 per cent of all hybridizations), (2) hybridizations with less than 50 scorable nuclei (19 per cent of all hybridizations), and (3) visibly contaminated samples with possible maternal cell contamination (14 per cent of all samples). As a result, we were not able to reproduce the results of Klinger et al. (1992) and Ward et al. (1993, 1997).

Maternal cell contamination in uncultured amniotic fluid

The presence of maternal cells in uncultured amniotic fluid may result in error in the interpretation of prenatal tests such as direct DNA analysis and rapid aneuploidy detection by fluorescence in situ hybridization (FISH). Using simultaneous dual colour X and Y FISH, we assessed maternal cell contamination in uncultured amniotic fluids from 500 women carrying male fetuses. The presence of maternal cells was correlated with the amount of blood present in the amniotic fluid as defined by visual examination of the cell pellet after centrifugation. The overall rate of maternal cell contamination in uncultured amniotic fluid as identified using X and Y-specific probes was 21.4 per cent, compared with 0.2 per cent in cultured fluid. Sixteen per cent of slightly bloody and 55 per cent of moderately bloody uncultured fluids had at least 20 per cent maternal cells and were classified as uninformative according to our protocol for rapid aneuploidy detection. Maternal and fetal cells could not be distinguished based on morphological characteristics alone.

Cross-hybridization of the chromosome 13/21 alpha satellite DNA probe to chromosome 22 in the prenatal screening of common chromosomal aneuploidies by FISH

In a routine application of commercially available centromeric DNA probes for the prenatal screening of common trisomies involving the autosomes 13, 18, and 21, and sex chromosomes, four cases of discrepancy between fluorescence in situ hybridization (FISH) results and follow-up cytogenetic analysis were observed from a total of 516 cases of amniocentesis. In three of these cases, the results were false negative, and in one false positive. In this case, amniocentesis was performed because of a positive triple test in a 34-year-old woman with previous infertility treatment. The alpha satellite DNA probe for chromosomes 13/21 revealed five signals in 50 per cent of uncultured amniocytes, while standard cytogenetic analysis showed a normal karyotype. FISH analysis on metaphase chromosomes demonstrated the location of the additional signal in the centromeric region of chromosome 22. This additional signal was also present in the centromeric region of chromosome 22 of the mother, providing evidence for a possible inherited polymorphism in chromosome 22 responsible for unspecific hybridization with the alpha satellite probe for chromosomes 13/21 in this case. The observed polymorphism in centromeric regions may contribute to unreliability of the use of the 13/21 alpha satellite probe for prenatal screening by FISH.

A chromosome 21-specific cosmid cocktail for the detection of chromosome 21 aberrations in interphase nuclei

Fluorescent in situ hybridization (FISH) with a 21q11-specific probe (CB21c1) consisting of three non-overlapping cosmids has been applied to interphase amniocytes of pregnancies at increased risk for fetal aneuploidy (N = 78) and to interphase lymphocytes, cultured and uncultured, of patients referred for Down syndrome (N = 19 and 28, respectively). In the uncultured amniocytes, six chromosome aberrations were detected: three cases of trisomy 21, a triploidy, a de novo 46,XX,t(21q21q), and a mosaic 46,XY/47,XY,+dic(21)(q11)/48,XY,+dic(21)(q11),+del(21)(q11). In 15 cultured and 20 uncultured blood samples, FISH correctly diagnosed trisomy 21 (full or mosaic) at the interphase level, which was confirmed in all cases by subsequent karyotyping. Because of specific and strong signals in interphase nuclei, CB21c1 appears to be a useful tool for the rapid detection of chromosome 21 abnormalities.

Prenatal aneuploidy detection in interphase cells by fluorescence in situ hybridization (FISH)

FISH is a quick, inexpensive, accurate, sensitive and relatively specific method for aneuploidy detection in samples of uncultured chorionic villus cells and amniotic fluid cells. FISH allows detection of the autosomal trisomies 13, 18 and 21 and X and Y abnormalities and any other chromosome abnormality for which a specific probe is available. The detection rate of these abnormalities is high in informative samples which have a concordance of > 99.5% with cytogenetic results. A relatively high number of abnormal cases are found in uninformative samples. However, such samples should be regarded as samples to be investigated further. Clinical experience with the use of FISH for prenatal diagnosis is now beyond 10,000 cases; a number of clinical protocols and smaller trials have also been carried out, resulting in 90% of attempted analyses giving informative results with a high detection rate and extraordinarily low false-positive and false-negative rates. Unsolved problems remain, such as occasional technical failures, admixtures of maternal blood and up to 20% uninformative scoring results, especially for abnormal specimens. FISH is at present used as an adjunct to classical cytogenetic analysis. However, this should not be interpreted as meaning that FISH could not be used as a methodology in its own right. If FISH were to be considered a diagnostic test then this might be the case, due to the risk of false-negative and false-positive results and the fact that FISH does not allow a diagnosis of certain structural abnormalities. If, on the other hand, FISH is considered a screening test, which means that in all abnormal (or indeterminate) cases, classical cytogenetic analysis would follow the abnormal screening test, the accuracy which is potentially higher than for other screening methods, for example in cases of trisomy 21, justifies FISH as a prenatal screening test in its own right.