Prenatal detection of isochromosome 21 by QF-PCR. A comparison between FISH and traditional karyotyping.

Objective: To compare rapid aneuploidy diagnostic tests with traditional karyotyping in the prenatal detection of Down syndrome due to isochromosome 21.

Methods: Quantitative fluorescence PCR (QF-PCR) and fluorescent in situ hybridization (FISH) for chromosomes 13, 18, 21, X and Y were performed on uncultured amniotic fluid, followed by routine karyotyping. chromosomal and microsatellite analysis of peripheral blood from parents was also carried out.

Results: The QF-PCR screening showed a trisomic diallelic pattern for 5 of 6 markers spanning the long arm of chromosome 21. FISH showed 3 signals in the interphase cells for the region 21q22.13-q22 during LSI 21 probe mapping. Cultured amniotic fluid revealed an isochromosome 21 resulting in a 46,XX,i(21)(q10),+21 karyotype. Parental microsatellite analysis proved that the isochromosome was paternal in origin.

Conclusion: The most informative analytical tool in this case appears to be QF-PCR, although a combination of QF-PCR and karyotyping provided the most evidence.