Rapid prenatal diagnosis of common trisomies: discordant results between QF-PCR analysis and karyotype analysis on long-term culture for a case of trisomy 18 detected in CVS

Abstract

We report here an apparently normal CVS QF-PCR result that was completely discrepant with the trisomy 18 positive karyotype result on long-term culture. This has important implications regarding our current testing protocol.

The Replacement of Cytogenetic Analysis by Direct Chorionic Villi Sampling Preparation with Quantitative Fluorescence PCR

“…However, as has already been discussed, according to Grati et al [6] there is a risk of missing some rare cases of type IV discordance that are TFM. As shown from our results and those of most investigators [4,26] if QF-PCR is always followed by LTC the probability of false-negative or false-positive results is negligible.…”

“…A complete discrepancy in the prediction of fetal trisomies 18 and 21 in CVS by QF-PCR analysis of uncultured villi and karyotyping after LTC is rare, but it has been reported in certain cases [26][27][28][29] . It has also been shown that QF-PCR cannot detect mosaicism at a level lower than 15%, which is rarely the case when full cytogenetic analysis is performed in LTC [4,27] .…”

“…In our study the discrepancies between the QF-PCR and LTC results in the cases of mos +21, ( table 3 ; cases 1 and 2) and mos 45,X/46,XY ( table 3 ; case 3) could be attributed to sampling from different sites of a mosaic placenta and testing or culturing different cell populations. According to Allen et al [26] , QF-PCR should be performed by analyzing the DNA sample taken from the CVS cell digest with the cytotrophoblast removed. This should be more representative of the mesodermal cell population of the whole CVS and minimize the risk of a discrepant result between QF-PCR and the fetal karyotype.…”

Detection of Chromosome Aneuploidies in Chorionic Villus Samples by Multiplex Ligation-Dependent Probe Amplification

“…This notion is in line with previous studies demonstrating the cellular complexity of CVS tissues and the relevance of CVS preparation methods for molecular testing to minimize the risk of false-positive or -negative results. 15,16 As such, MLPA analysis of CVS on DNA isolation of whole villi digested with proteinase K as presented here, is only appropriate as a replacement for TK of STC villi. In case 7, in which the abnormal cells were confined to the LTC, the MLPA result for chromosome 18 was indicative for a mosaic trisomy 18, with a 95% CI of 1.08Ϫ1.28.…”

Détection de la trisomie 21 par l’étude de l’ADN

“…Dans ce cas, on observe sur les différents marqueurs, des allèles supplémentaires ou des rapports d'allèles biaisés d'un marqueur à l'autre. Pour détecter un mosaïcisme par la méthode QF-PCR, il est nécessaire que la population cellulaire la moins représentée soit au moins de 10-15 % de la population cellulaire étudiée [3]. L'interprétation est déli-cate car la signification d'une mosaïque est difficile.…”