RESUMO
Trisomy 21 is the most common chromosomal aberration in live births. In this study we employed human chromosome 21-specific short tandem repeat (STR) DNA markers to determine the numbers of chromosome 21 present in fetal cells. Forty amniotic fluid samples from pregnancies complicated with fetal Down syndrome and 98 samples from euploid pregnancies were analyzed for D21S11 and interferon-alpha receptor (IFNAR) gene intervening sequence. Fluorescent dye-labeled primers were used in PCR amplification of these 2 markers. The PCR amplicon was analyzed with an automatic DNA sequence analyzer. The results showed that 35 of 40 fetal Down syndrome samples analyzed for IFNAR showed 3 distinct peaks, while 24 of 30 cases analyzed for D21S11 showed 3 distinct peaks. Two Down syndrome samples showed two uneven peaks. By analyzing 98 euploid pregnancies as controls, the ratios of area under the peaks were determined to be 1.31 +/- 0.22 and 1.96 +/- 0.18 (mean +/- SD) for the euploid pregnancies and pregnancies complicated by fetal Down syndrome with 2 peaks, respectively. Our data showed that altogether 39 of 40 (97.5%) Down syndrome cases were correctly identified based on either the 3-peak pattern in one or more of the DNA markers or the relative peak area ratio calculation. In conclusion, polymorphic STR DNA markers are useful for determining the numbers of chromosome 21 in fetal cells. The high sensitivity and automation of the procedures suggest a good prospect for use of this method in prenatal detection of fetal Down syndrome. However, this is a preliminary investigation and a large-scale study is necessary to validate the clinical application of this protocol.
