Rapid detection of numerical aberrations of chromosomes 13, 18 and 21 in chorionic mesenchymal cells

Prenatal diagnosis of trisomy 21 using interphase fluorescence in situ hybridization of post-replicated cells with site-specific cosmid and cosmid contig probes

Interphase fluorescence in situ hybridization (FISH) with chromosome 21-specific cosmid clones was used to identify trisomy 21 in cultured and uncultured amniotic cells. Two novel site-specific cosmid clones (regions 21q22 and 21qtel) were compared with a cosmid contig (Zheng et al., 1992). Correct identification of chromosome 21 copy number was made in 65-75 per cent of trisomic cells and in 70-75 per cent of normal disomic cells by using all the tested probes. However, the chromosome 21-specific telomeric probe (cos 17F8) showed the best results due to more intense and clearly visible hybridization. Utilization of a directly fluorophorated telomeric probe using Cy3-dCTP and FluorX-dCTP allows accurate detection of chromosome 21 in a fast 'one-step' FISH procedure on uncultured interphase nuclei. In addition, we compared the efficacy of FISH analysis for the total population of interphase cells and cells in the post-replication (late S, G2) periods of the cell cycle. Selective scoring of cells in the post-replicative period (showing a pair of hybridization signals on each chromatid of the replicated interphase chromosome) increased the number of informative nuclei by up to 95-97 per cent. This approach allows cells with overlapping chromosomes, artificial double hybridization signals on separate chromatids in interphase chromosomes, background hybridization, and polyploid cells to be analysed. Application of directly labelled telomeric cosmid probes and integral analysis of hybridized nuclei in the pre- and post-replication periods of the cell cycle may help to further improve the prenatal detection of trisomy 21.

Screening for Down's syndrome

Down's syndrome (DS) is the commonest cause of severe mental retardation in children. It is the result of trisomy of chromosome 21 which is usually a random event though it is commoner in older mothers. DS can be diagnosed by chorionic villus sampling (CVS) and amniocentesis followed by karyotyping. Because of the risks associated with these invasive procedures, they can only be offered to a high-risk group. At one time the sole basis for identifying this increased risk was maternal age, but within the past ten years a series of biochemical and ultrasound abnormalities have been shown in DS pregnancies. The biochemical abnormalities include changes in the levels of most fetal and placental products in the maternal circulation. The best-known of these changes are the reduced levels of alphafetoprotein (AFP) and oestriol (E3) and increased levels of human chorionic gonadotrophin (hCG). The mechanism underlying these biochemical phenomena is unknown. Screening programmes involving the measurement of hCG and AFP, with or without additional parameters such as E3, at 15-18 weeks of pregnancy can typically identify 60% or more of cases of DS with a screen-positive rate of 5%. The combined risk derived from the various biochemical parameters, together with maternal age, is calculated by one of a number of computer programmes which have been developed for this purpose. There has been considerable discussion as to the exact biochemical tests which should be used for DS screening. This had led to controversy as to whether measurement of E3 has a place, and whether or not measurement of the free beta-subunit of hCG should replace measurement of the intact molecule. A notable recent development is the suggestion that measurement of the urinary beta-core of the hCG could be a highly discriminatory marker. A number of factors can affect the results of biochemical screening for DS. These include maternal weight, gestational age, ethnic origin, smoking, and diabetes. In addition, abnormal levels of the biochemical products may be found in other chromosome 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.

Fluorescence in situ hybridization with a chromosome 21-specific cosmid contig: 1-day detection of trisomy 21 in uncultured mesenchymal chorionic villus cells

We present a modified, fast trisomy 21 detection assay using fluorescence in situ hybridization (FISH) on uncultured mesenchymal chorionic villus cells. The whole test takes about 24 h. We used a cosmid contig as a probe and modified an in situ sample preparation method first described by Klinger et al. (1992). The assays saves time and cost of culture in comparison with a previously described trisomy 21 detection FISH assay (Bryndorf et al., 1993). A small blind clinical study comparing the modified and the previously described FISH assays using mesenchymal chorionic villus cells showed comparable results and concordance with conventional cytogenetic analysis. The frequency of nuclei with three hybridization signals from samples disomic for chromosome 21 ranged from 0 to 8 per cent with both assays, while trisomic samples had 60-80 and 54-90 per cent of the mesenchymal nuclei with three signals in the modified and previously described assays, respectively. Normal (disomic) and trisomic mesenchymal chorionic villus samples can be distinguished clearly and rapidly without culture in the modified assay.