PRENATAL-DIAGNOSIS OF TRISOMY-21 USING INTERPHASE FLUORESCENCE IN-SITU HYBRIDIZATION OF POST-REPLICATED CELLS WITH SITE-SPECIFIC COSMID ANDCOSMID CONTIG PROBES

Interphase fluorescence in situ hybridization (FISH) with chromosome 2 1-specific cosmid clones was used to identify trisomy 21 in cultured a nd uncultured amniotic cells. Two novel site-specific cosmid clones (r egions 21q22 and 21qtel) were compared with a cosmid contig (Zheng et al., 1992). Correct identification of chromosome 21 copy number was ma de 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 dire ctly fluorophorated telomeric probe using Cy3-dCTP and FluorX-dCTP all ows accurate detection of chromosome 21 in a fast 'one-step' FISH proc edure on uncultured interphase nuclei. In addition, we compared the ef ficacy of FISH analysis for the total population of interphase cells a nd cells in the post-replication (late S, G2) periods of the cell cycl e. Selective scoring of cells in the post-replicative period (showing a pair of hybridization signals on each chromatid of the replicated in terphase chromosome) increased the number of informative nuclei by up to 95-97 per cent. This approach allows cells with overlapping chromos omes, artificial double hybridization signals on separate chromatids i n interphase chromosomes, background hybridization, and polyploid cell s to be analysed. Application of directly labelled telomeric cosmid pr obes and integral analysis of hybridized nuclei in the pre- and post-r eplication periods of the cell cycle may help to further improve the p renatal detection of trisomy 21.