Mol. Hum. Reprod. Advance Access originally published online on September 10, 2004
Molecular Human Reproduction 2004 10(11):839-846; doi:10.1093/molehr/gah108
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Rapid prenatal diagnosis of common chromosome aneuploidies by QF-PCR. Assessment on 18 000 consecutive clinical samples
1Departament de Genètica Molecular, 2Departament de Citogenètica General Lab, 08021 Barcelona, 3Unitat de Biologia, Departament de Biologia Cel·lular, Fisiologia i Immunologia, Universitat Autònoma de Barcelona E-08193, Bellaterra, Barcelona, Spain, 4Molecular Pathology Lab, Sant' Anna Hospital, 1026 Turin, 5Molecular Genetics and Cytogenetics Lab, Promea–Day Surgery, 1026 Turin, 6Department of Obstetrics and Gynaecology, University of Turin, Sant' Anna Hospital, 1026 Turin, Italy and 7The Galton Laboratory, University College London, London NWI 2HE, UK
8 To whom correspondence should be addressed at: General Lab, Genética Molecular, c/Amigo 12, 08021 Barcelona, Spain. Email: vc{at}general-lab.com
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Abstract |
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The quantitative fluorescent PCR (QF-PCR) assay, introduced during the last few years, allows prenatal diagnoses of common chromosome aneuploidies in a few hours after sampling. We report the first assessment of QF-PCR performed on a large cohort of 18 000 consecutive clinical specimens analysed in two different Centres. All samples were analysed by QF-PCR using several selected STR markers together with amelogenin and, occasionally, SRY for fetal sexing. Results were compared with those obtained by conventional cytogenetic analysis. In 17 129 tests, normal fetuses were detected by QF-PCR. No false positives were observed. All 732 cases of trisomy 21, 18, 13, triploidies, double trisomies as well as all but one fetuses with X and Y aneuploidies were correctly diagnosed. Chromosome mosaicism could also be suspected in several samples. In some cases of in vitro culture failures, QF-PCR was the only evidence of fetal X, Y, 21, 18 and 13 chromosome complement. QF-PCR proved to be efficient and reliable in detecting major numerical chromosome disorders. The main advantages of the molecular assay are its very low cost, speed and automation enabling a single operator to perform up to 40 assays per day. QF-PCR relieves anxiety of most parents within 24 h from sampling and accelerates therapeutic interventions in the case of an abnormal result. In countries where large scale conventional cytogenetics is hampered by its high cost and lack of technical expertise, QF-PCR may be used as the only prenatal diagnostic test.
Key words: aneuploidy/QF-PCR/rapid prenatal diagnosis/STR
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Introduction |
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Prenatal diagnoses of chromosome abnormalities are performed by conventional cytogenetic analysis using in vitro culture of fetal nucleated cells retrieved by amniocentesis, chorionic biopsy or fetal blood sampling. Improvements in culture media have only partially reduced the interval between the collection of the samples and the reporting of the results. This is a very anxious time for the parents, particularly if non-invasive screening tests (biochemical and/or ultrasound), performed during the first trimester of pregnancy, have suggested an increased risk of chromosome disorders (Sjogren and Uddenberg, 1990
; Marteau et al., 1992). The need for rapid prenatal diagnoses prompted the use of the fluorescence in situ hybridization assay (FISH) for the detection of major chromosome aneuploidies using uncultured cells (Ward et al., 1993; Adinolfi and Crolla, 1994; Eiben et al., 1998). However, the cost of this labour-intensive procedure has limited its application mainly to high-risk pregnancies (Evans et al., 1999; Pergament et al., 2000; Tepperberg et al., 2001). During the past 10 years, quantitative fluorescent PCR (QF-PCR) has been introduced to perform rapid prenatal diagnoses of common chromosome aneuploidies (Mansfield 1993; Pertl et al., 1994, 1996; Adinolfi et al., 1997, 2000). The clinical utility of this assay has repeatedly been confirmed together with its high sensitivity and specificity in detecting major chromosome abnormalities (Pertl et al., 1999a,b; Schmidt et al., 2000; Adinolfi and Sherlock, 2001; Cirigliano et al., 2001a,b; Levett et al., 2001; Mann et al., 2001). One of the advantages of QF-PCR is the automation of part of the procedure that allows high throughput of samples at a very low cost (Adinolfi et al., 2000; Adinolfi and Sherlock, 2001).
QF-PCR tests are now performed in several prenatal Centres in Europe for the detection of major numerical abnormalities affecting chromosomes X, Y, 21, 18 and 13, with results provided in 24 h (Shmidt et al., 2000; Cirigliano et al., 2001b, 2002, 2003; Mann et al., 2001; Voglino et al., 2002; Bili et al., 2002; Andonova et al., 2004).
Here we report the results of screening 18 000 consecutive fetal samples, using both QF-PCR and conventional cytogenetic analysis.
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Materials and methods |
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Samples were collected between December 1999 and October 2003 in two different genetic Centres (General Lab, Barcelona, Spain and Sant' Anna Hospital, Turin, Italy). Both institutions were also offering rapid QF-PCR service to other cytogenetic laboratories, private clinics and public hospitals, so that several high-risk pregnancy samples were received nationwide on a daily basis.
The majority of prenatal samples (93%) were amniotic fluids (16 746) collected between 12 and 31 weeks of gestation; 264 samples were heavily bloodstained and suspected of being contaminated with maternal cells; these samples were analysed together with a maternal buccal wash or blood sample in order to distinguish between maternal and fetal STR profiles.
A total of 625 chorionic villous samples (CVS, 3.5%), collected between 11 and 13 weeks of gestation, 123 fetal blood samples (0.7%) and 506 fetal tissues from aborted fetuses (2.8%) were also investigated.
In 146 prenatal cases, QF-PCR was not requested at the time of collecting the samples, but it was only performed afterwards on cytogenetic cultures. These included slow in vitro growing samples and failed cultures contaminated by bacterial or fungal cells; QF-PCR was also performed in some cases of long-term CVS cultures and cultured amniotic fluids to assess the fetal origin of the predominant cell population.
The most frequent indication for the request of rapid QF-PCR diagnosis of common chromosome aneuploidies was advanced maternal age, followed by abnormal serum screenings, parental anxiety and abnormal ultrasound findings.
All women received genetic counselling, including detailed information on the advantages and limitations of the rapid QF-PCR assay; routine informed consent was obtained in all cases.
Genomic DNA was extracted from 0.5–1 ml amniotic fluids or cell cultures, 5 µl of fetal bloods, 200 µl buccal washes, a small villous fragment or 1–2 mm tissue sections using a resin-based procedure (Instagene Matrix; Bio-Rad Laboratories, USA), as previously described (Cirigliano et al., 2001b). QF-PCR was performed in both Centres using the large set of STR markers for chromosomes X, Y, 21, 18 and 13 listed in Table I. Primer sequences, retrieved from The Genome Data Base (http://www.gdb.org) and Cooperative Human Linkage Center (http://www.chlc.org), were used for multiplex QF-PCR without modifications. A total of six STR for the X and Y chromosomes, six for chromosome 21, five for chromosome 18 and four for chromosome 13 were selected because of their high level of heterozygosity, thus reducing the number of uninformative samples (Cirigliano et al., 1999, 2001a,b). Furthermore, non-polymorphic sequence of the amelogenin (AMXY) gene and, occasionally, SRY were also included to detect fetal sex.
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All STR were stable tri-, tetra- and penta-nucleotides, suitable for multiplex PCR because of their minimal production of artefacts during amplification. The STR locations along the examined chromosomes were also taken into account in order to increase the possibility of detecting partial trisomies (Figure 1). All forward primers (Roche-TIB Molbiol, Germany) were labelled with fluorescent molecules allowing accurate sizing and quantification of QF-PCR products. Primers producing amplicons of similar sizes were labelled with different fluorochromes in order to be amplified and analysed in the same multiplex QF-PCR reactions (Sherlock et al., 1998
).
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In the course of this study the two Centres developed different combinations of primers in multiplex QF-PCR reactions in order to simplify sample handling and data analysis (Cirigliano et al., 2001a
,b, 2002; Voglino et al., 2002).
Fetal sex and chromosomes X and Y copy number were assessed for all cases by amplification of the homologous gene AMXY together with the pseudoautosomal X22 and chromosome X-specific markers (Table I) (Cirigliano et al., 1999, 2001a,b). In the course of this study the D21S1411 STR was used as internal control to quantify the X-linked HPRT marker in order to assess X chromosome copy number in all samples (Cirigliano et al., 2002).
Aneuploidy screenings were initially performed (almost the first 6000 samples) using two multiplex QF-PCR assays that included two STR on chromosomes X, 21, 18, 13, and the AMXY. Samples showing homozygosity for two chromosome-specific markers, as well as all aneuploid cases, were re-tested with chromosome-specific multiplex PCR using up to four STR on chromosome 21, four STR on the X, and three markers on each chromosome 18 and 13 (Cirigliano et al., 2001b). The following group of 
12 000 samples was tested adding two more STR specific for chromosomes X and 18 and one more marker for chromosome 13 (Table I).
The fluorescent QF-PCR products and size standards were analysed by capillary electrophoresis on ABI 310 automated DNA sequencers and GeneScan 3.7 Software (Applied Biosystems, USA) as previously described (Pertl et al., 1996; Adinolfi et al., 1997; Cirigliano et al., 2001a, 2002). In both Centres, after the first 18 months of activity of the QF-PCR service, this model of automated DNA sequencer was replaced with a four-capillary instrument (ABI 3100-Avant; Applied Biosystems) in order to increase sample throughput. Final data were generated using GeneScan and Genotyper Softwares (Applied Biosystems) as previously described (Adinolfi et al., 1997; Pertl et al., 1994, 1996; Cirigliano et al., 1999, 2001a,b, 2002, 2003).
Normal chromosome complement was assessed by detecting, for the correspondent STR, two different alleles with a ratio between fluorescent peak areas of 1:1 (ranging between 0.9 and 1.3:1). Trisomies were detected, with the correspondent chromosome specific STR, either as three peaks of fluorescent activity with area ratio close to 1:1:1 or as two unbalanced peaks with area ratio of 2:1 (ranging from 1.7 to 2.4:1) (Adinolfi et al., 1997, 2000; Cirigliano et al., 1999, 2001a,b). The presence of trisomic triallelic or diallelic patterns for at least three different STR on the same chromosome was considered as evidence of trisomy. The same trisomic patterns observed for all chromosome-specific STR were indicative of triploidy. All prenatal samples were processed and reported within 24–48 h; in 98% of cases results were made available within the next working day.
Conventional cytogenetic analyses were performed on all prenatal samples, cultured and harvested according to standard procedures; depending on the specimen, the results were issued between 5 and 28 days (mean reporting time of 2 weeks for amniotic fluids).
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Results |
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DNA extraction and QF-PCR amplification were successful in 17 986 cases (99.9%). Only 14 samples failed to amplify; all these cases were failed cytogenetic cultures heavily contaminated by bacterial or fungal cells. These samples had not been analysed by QF-PCR soon after collection since the molecular tests were requested by cytogeneticists only after cell culture failures. Conventional cytogenetic analysis was not achieved in a total of 26 prenatal cases because of cell culture failures (24 samples) or maternal cell overgrowth (two cases). In 12 cases QF-PCR tests were successful and gave normal results; thus, a second invasive procedure could be avoided. In 111 bloodstained amniotic fluids, QF-PCR showed the presence of a high level of maternal cell contamination and no result could be achieved other than fetal sex.
Detection of normal samples
The results of rapid aneuploidy screenings and conventional cytogenetic analyses are reported in Table II. In 17 052 samples the QF-PCR tests were informative for the five chromosomes examined and provided evidence that fetuses had normal chromosome complement. The detection of heterozygous patterns with fluorescent peak ratios close to 1:1 for at least two chromosome-specific STR was sufficient to perform the diagnosis. No false positive results were observed.
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Seventy-seven cases (0.4%) were uninformative for copy number of either chromosome 18 or 13, due to the homozygosity of the employed polymorphic markers. These samples were later diagnosed as normal for these chromosomes by conventional cytogenetic analysis. However, 64 of these uninformative samples (88%) were tested using only three markers on chromosomes 18 and 13 during the first 2 years (
6000 samples). To reduce the number of uninformative results, two more STRs specific for chromosome 18 and one for chromosome 13 were selected. The inclusion of these markers markedly reduced the frequency of uninformative samples to nine in the next 12 000 samples.
Detection of fetal sex
Fetal sex and chromosome X and Y copy number were determined in all cases by amplification of the non-polymorphic sequences of the amelogenin gene (AMXY). QF-PCR of this marker generates chromosome X- and Y-specific products differing 6 bp in length that can be identified and quantified (Figure 1). The results were in agreement with cytogenetic analysis in 17 972 cases (99.9%). In two prenatal samples, amplification of the AMXY marker resulted in apparently normal female fetuses that were later diagnosed as males by cytogenetic analysis. Further investigations showed that the discordant QF-PCR results were due to a Y chromosome microdeletion that included the AMXY, but not the SRY, DNA sequences. In one case this rare abnormality could be demonstrated by QF-PCR assay as inherited from the father.
The Y-specific product of the AMXY marker was found duplicated in two normal males that produced fluorescent peaks in a ratio of 2:1 similar to that observed in 47,XYY males. In both cases, the correct diagnosis was achieved by the simultaneous amplification of the X22 marker that confirmed the presence of a single Y chromosome.
The X chromosome-specific product of the amelogenin was also found duplicated in three normal males; in these cases, the QF-PCR of this sequence generated fluorescent peaks with ratios of 2:1, similar to those observed in patients with Klinefelter syndrome (47,XXY). However, in all these cases the correct diagnosis could be performed by the simultaneous amplification of five X chromosome STRs and the pseudoautosomal X22 marker that allowed the correct assessment of normal XY chromosome complements.
Detection of aneuploidies
In the present series, 800 abnormal karyotypes were diagnosed by conventional cytogenetic analysis. A total of 746 aneuploidies was also detected by the rapid QF-PCR assay (93.3% sensitivity) within 24 h from collection of the samples (Table II). The rapid test demonstrated 100% specificity for chromosomes X, Y, 21, 18 and 13 aneuploidies with positive predictive value (PPV) of 100% and negative predictive value (NPV) of 99.7%.
Detection of autosomal trisomies
All 618 samples with trisomies of chromosomes 21, 18 or 13 and triploidies were readily diagnosed by the multiplex QF-PCR assay without false negative results (100% sensitivity and specificity). A total of 344 fetuses with trisomy 21, 162 with trisomy 18, 61 with trisomy 13 and 51 triploidies were identified in this series (Table II).
Two cases of partial trisomies involving chromosome 18 [46,XY.dup(18)(q12.2q21.2) and 46,XYdup(18)(q11.2q22)] were also diagnosed by detecting trisomic patterns for two out of the five markers used for this chromosome (Figure 1).
As shown in Table II, only 19 cases of numerical abnormalities were not detected by QF-PCR; they involved chromosomes for which specific STRs were not included, such as trisomies of chromosomes 22, 16, 15 and small extra chromosome markers.
STR duplications
Submicroscopic polymorphic duplications of microsatellites were observed in 19 cases as clear trisomic triallelic or diallelic patterns for one chromosome-specific STRs (Table III, Figure 2). All these samples were diagnosed as normal by cytogenetic analysis. Duplications were observed in three samples for one STR on chromosome 21, in nine cases for one of the markers selected on chromosome 13; in six samples in the pseudoautosomal X22 and in the remaining case with the D18S386 marker.
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The maternal origin of the duplication could be demonstrated in nine cases by QF-PCR analysis of the same marker in both parents; in six samples the polymorphism was found as inherited from the father (Table III, Figure 2). In two out of 19 samples the duplication was found to be de novo; in both cases paternity was confirmed by amplification of all other available STR markers. In the remaining two cases, parental DNA was not available for further investigations. Three more normal samples (one CVS, one amniotic fluid and one maternal blood) produced unbalanced triallelic patterns for a single marker consistent with somatic microsatellite instability (Mann et al., 2003
). In two cases the D18S386 was involved and in the remaining one the HPRT marker. However, these three samples were analysed using other informative chromosome-specific STR that allowed correct prenatal diagnoses.
Detection of sex chromosome aneuploidies
The diagnosis of Turner syndrome was based on the detection of a single QF-PCR product for all the X chromosome polymorphic markers, in the absence of the Y-specific amelogenin product (Cirigliano et al., 1999, 2000, 2001a,b, 2002). The QF-PCR detection of Turner's syndrome was successful in 55 out of 56 cases (Table II). The only false negative QF-PCR result in our series was a 45,X fetus that was diagnosed as normal female. In this case, three out of four employed STRs produced single PCR peaks, while the X22 marker suggested that the sample was derived from a normal heterozygous female; this X22 pattern probably resulted from a polymorphic submicroscopic duplication of this STR sequence (Figure 2). In the course of this study, the introduction of D21S1411 used as internal control to quantify the X-linked HPRT marker (Cirigliano et al., 2002) and the inclusion of up to six STR on the X chromosome, further improved the detection of X monosomy. This approach also allowed the prenatal detection of rare normal females homozygous for up to four STR markers. As shown in Table II, two out of six samples diagnosed as 47,XXX by QF-PCR showed discordant results when analysed by conventional cytogenetic tests. The first amniotic fluid resulted in a 45,X karyotype. This discordant result suggested the possibility of a mosaic 45,X/46,XX with the normal cell line not grown during in vitro cultures (Figure 3). Amniocentesis was repeated together with fetal blood sampling (FBS) and the suspected X chromosome mosaicism was confirmed by cytogenetics analysis of the amniotic fluid; however, only normal cells were detected in the fetal blood sample. In the second case of amniotic fluid, diagnosed as trisomy X by QF-PCR, cytogenetic analysis documented the presence of a mosaic fetus 45,X[75]/46,XX[25].
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All 53 non-mosaic aneuploidies involving both the X and Y chromosomes, including 47,XXY, 47,XYY and rare karyotypes such as X,Y pentasomies or double trisomies were correctly diagnosed by QF-PCR (Table II and Figure 1).
Detection of chromosome mosaicisms
QF-PCR was effective in detecting 12 out of 25 fetuses with chromosome mosaicism (Table II). The presence of two different cell lines in the prenatal samples was detected in all cases of 45,X/46,XY mosaicism (Figure 3). In two of these cases, the presence of a cell line with the Y chromosome could only be demonstrated by the QF-PCR detection, in the uncultured samples, of a small Y-specific fluorescent PCR product of the AMXY; in these two cases cytogenetic analysis resulted in pure 45,X. In three fetal samples, a mosaicism 45,X/46,XX could be suspected by QF-PCR as unbalanced allele ratios for the employed X chromosome markers (Figure 3). Chromosomes 21 and 18 mosaic could be diagnosed in two and one cases respectively as small extra alleles for at least two chromosome-specific STRs (Figure 3). In agreement with previous studies (Cirigliano et al., 1999, 2001a,b; Schmidt et al., 2000), low level chromosome mosaicisms could not be detected by QF-PCR in the remaining cases.
Analysis of samples with maternal cell contamination
Rapid diagnosis of aneuploidies by QF-PCR may be hampered by the presence of contaminating maternal cells in the samples, but in the present series 264 heavily bloodstained amniotic fluids were also analysed (Table IV). Normal fetuses were diagnosed in 151 cases. The prenatal diagnosis of normal female fetuses was made possible by testing the corresponding maternal DNA with the same STR markers. In samples retrieved from male fetuses, quantification of maternal cell contamination was possible by comparing fluorescence of the X-specific AMXY sequence with the purely fetal Y product.
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Two aneuploidies were also identified in this subset of samples (Table IV). In the remaining 111 bloodstained amniotic fluids only the diagnosis of fetal sex was possible by detecting Y-specific chromosome sequences.
Cytogenetic analysis was successfully performed in 262 out of 264 bloodstained samples; it could also diagnose three aneuploidies that were undetectable by QF-PCR because of a very high level of maternal cell contamination (Table IV). In two samples, only maternal cells were present in the in vitro culture. However, in both cases the QF-PCR detection of Y sequences in the uncultured samples revealed the culture artefact and allowed correct sexing of the fetus.
According to previous studies (Cirigliano et al., 2003), the STR markers selected for multiplex QF-PCR assays were informative for the assessment of zygosity in all 184 multiple pregnancies analysed in the course of this study.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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The analysis of this large series of prenatal diagnoses (18 000 consecutive clinical specimens), performed by both the QF-PCR assay and conventional cytogenetics, should help to assess the advantages of applying the molecular technique on fetal samples collected by amniocentesis, chorionic biopsy and cordocentesis. The multiplex QF-PCR tests were based on the use of selected STR markers that could detect the most frequent chromosome disorders.
The overall results show that QF-PCR is a rapid, simple and accurate diagnostic test. In 17 129 normal pregnancies, which were correctly diagnosed by QF-PCR, without false positive results, parents could be informed about the outcome of the test within 24–48 h from the collection of the samples. Thus, the rapid assay reached the purpose of greatly reducing the anxiety in 95% of all parents waiting for full karyotype analysis. QF-PCR performed on clear amniotic fluids correctly diagnosed 100% of trisomies for chromosomes 21, 18, 13, tripolidies and non-mosaic aneuploidies involving both chromosomes X and Y without false negative results. A total of 746 aneuploidies, detected by QF-PCR, accounts for 93.2% of fetuses with abnormal karyotypes diagnosed by conventional cytogenetic tests. The sensitivity of the molecular assay increased up to 95.4% if only clinically significant cytogenetic abnormalities were taken into account. No false positives were observed and, in several cases, the great efficiency and reliability of QF-PCR allowed early termination of affected fetuses without further waiting for completion of cytogenetic analysis (Adinolfi et al., 2001; Cirigliano et al., 2001b, 2003). This policy is now followed in other selected genetic units, especially if ultrasound examination confirms the presence of affected fetuses.
The STRs selected on chromosomes X, Y and 21 were informative in 100% of samples; the markers on chromosomes 13 and 18 allowed correct assessment of chromosome copy number in 99.6% of cases; only 77 normal samples were found homozygous for all the selected markers, but with the inclusion of more STRs for these chromosomes, this number declined to 0.075%.
Molecular detection of fetal sex by amplification of the AMXY sequence was in agreement with cytogenetic analysis in all but two cases. Only in two male fetuses was the Y sequence of the gene found to be deleted, thus generating a pattern that was indistinguishable from a normal female. In both cases the X22 pseudoautosomal marker was heterozygous and thus compatible with the diagnosis of normal female fetuses. It has to be stressed that X22 was the only marker available to detect two sex chromosomes in these samples; thus, homozygosity of this STR with concomitant AMY deletion could have resulted in misdiagnosis of Turner's syndrome. Accordingly, soon after the detection of the first AMY deletion case, in both laboratories QF-PCR diagnoses of 45,X in the absence of suggestive ultrasound evidence, were further investigated using the SRY marker. Our results confirm a previous finding (Santos et al., 1998) that deletion of the amelogenin gene on the Y chromosome is a very rare event in our population. However, a higher frequency of this polymorphism (up to 3.6% of males) has recently been reported in Indian and Malay ethnic groups (Chang et al., 2003). To screen these populations it is strongly advisable to include different Y chromosome sequences, such as SRY, in multiplex assays in order to increase the reliability of sex detection.
Either the X- or the Y-specific products of the amelogenin gene were found duplicated in five male fetuses with normal karyotype. Also in these cases multiplex QF-PCR of the pseudoautosomal X22 marker together with several X-linked STRs allowed the correct diagnosis. The homologous region of the sex chromosomes containing the AMXY gene allows generation of chromosome-specific PCR products and is widely used for sex determination of DNA samples. We show that, beside deletions of the Y sequence, duplications of both the X and the Y specific products are also possible and, although very rare, these can only be detected by QF-PCR analysis in male subjects.
The comparison of fluorescent PCR products of the X-linked HPRT and D21S1411 markers allowed the assessment of X chromosome copy number in female fetuses homozygous for up to four X chromosome STRs (Cirigliano et al., 2002). It was also possible to confirm that multiplex of these two sequences was of diagnostic value for the prenatal detection of fetuses with Turner syndrome independently from allele frequencies and heterozygosity of the employed markers (Cirigliano et al., 2002).
As shown in previous investigations (Cirigliano et al., 1999, 2001a,b), prenatal diagnoses of Turner mosaicisms by QF-PCR were hampered by the difficulty of assessing the type and ratios of the various subpopulations of cells. However, in the majority of cases, the QF-PCR tests allowed at least to deduce the diagnosis of mosaicisms by the detection of abnormal fluorescent peak ratios for the employed X chromosome STRs. In some mosaic fetuses, the ratios between 46,XX/45,X or 45,X/47,XXX or even 46,XX/ 45,X /47,XXX cells may be such as to produce fluorescent peak ratios similar to those of normal 46,XX or 47,XXX fetuses, as previously observed (Cirigliano et al., 1999, 2001a,b). In some cases of X chromosome mosaicism, discrepancies between the QF-PCR and cytogenetics results regarding the percentages of abnormal cell subpopulations are probably due to the different in vitro cell growth of normal (46,XX or 46,XY) and 45,X cells (Cirigliano et al., 2001a) with the aneuploid cell line generally growing faster than the normal. For the same reason, in this series, only the comparison between QF-PCR result on direct sample and karyotype of cultured cells allowed correct detection of three mosaic cases.
The difficulty of detecting some sex chromosome abnormalities must be evaluated in the context of the present view about the advantages of performing targeted prenatal tests for selected chromosome disorders (Adinolfi and Sherlock, 2001; Mann et al., 2001; Donaghue et al., 2003; Brun et al., 2004). In some genetic Centres, sex chromosome-specific QF-PCR assays are not routinely performed for all cases (Mann et al., 2001); only samples retrieved from fetuses suspected by ultrasound of having such chromosome disorders are tested (Donaghue et al., 2003). However, sex chromosome aneuploidies such as 47,XXY, 47,XXX, and 47,XYY are generally not referred for abnormal ultrasound, as well as, in the present study, one 45,X fetus. Routine inclusion of primers for QF-PCR diagnosis of sex chromosome aneuploidies allows detection of all such cases before completion of fetal karyotype, being of great help for pregnancy management and leaving more time for parents to make a decision.
QF-PCR amplification of microsatellites has proven to be a very safe procedure for rapid and simple prenatal diagnosis. In clinical settings it has the added value to allow testing samples suspected of being contaminated by maternal cells. In this series, 155 normal male and female fetuses together with two aneuploidies were identified with great confidence, even if in the presence of heavily bloodstained amniotic fluid samples. Maternal cell contamination, if present, was always detected as characteristic QF-PCR patterns with extra peaks or skewed ratios for all chromosome markers that could not be confused with triploidy or mosaicism.
Using at least two informative sequences on each chromosome, rare polymorphic STR duplications were detected in 19 samples. In these cases the trisomic pattern observed for a single STR was in contrast with the normal heterozygous profile of all the other markers on the same chromosome. While the same result could also be due to partial trisomies (i.e. unbalanced translocation), the analysis of both parents with the same marker allowed us to distinguish the rare inherited polymorphism in 15 out of 19 cases.
The finding of submicroscopic duplications in microsatellites should also be a warning about the risk in reporting an autosomal trisomy detected with a single informative STR. Several chromosome-specific markers (at least three different sequences) should always be included in multiplex assays to confirm the diagnosis of aneuploidy.
Samples showing normal heterozygous pattern for a single STR should also be considered carefully; mutation in primer binding site causing null alleles, even if very rare, could hamper the detection of trisomies. In our series the only false negative result was observed in the diagnosis of a Turner syndrome fetus with a normal heterozygous pattern in only one of the four employed X-linked markers. This result is highly likely to be due to a submicroscopic duplication of this sequence that caused the artefact. This diagnosis was performed during the first year of activity when only three STRs on the X and the pseudoautosomal X22 marker were analysed. The inclusion of up to six markers and the D21S1411 used as internal control to quantify the X-linked HPRT greatly improved the efficiency of the QF-PCR assay in detecting X monosomy.
In our study, somatic microsatellite instability was only observed in three cases. This is in contrast with a recent report of high frequencies of the phenomenon in prenatal samples (Mann et al., 2003). In their study, 4.2% CVS samples exhibited unbalanced triallelic patterns for a single STR marker only in a proportion of cells; this was interpreted as evidence of somatic generation of a de novo allele. In our series this was an extremely rare pattern only observed in one out of 625 CVS (0.2%). However, it should be stressed that these mutations do not affect the QF-PCR diagnosis when several STR markers on each chromosome are employed in multiplex assays.
Compared with FISH analysis, the clinical application of QF-PCR has the same limitation of detecting selected major numerical chromosome disorders, but, as shown in the present study, with several advantages that increase its clinical utility. The assay is very sensitive so that only a very small volume of sample is needed for the analysis. The use of highly polymorphic STR markers allows the testing of samples suspected of being contaminated with maternal cells as well as the assessment of zygosity in multiple pregnancies. The use of several highly polymorphic and chromosome-specific STRs also makes it possible to detect partial trisomies. The efficiency of the assay is not influenced by the gestational age and the high rate of uninformative samples, described using FISH (Bryndorf et al., 1997), has not been observed in the course of this study.
In conclusion, the present results confirm that QF-PCR is a rapid, simple and accurate prenatal diagnostic test; in the great majority of cases it has been straightforward. The inclusion of primers for chromosomes X, Y, 21, 18 and 13 allows the detection of the great majority of clinically significant chromosome abnormalities. Because of a residual risk, in developed countries, QF-PCR can be used as a preliminary tool to reduce parental anxiety and, eventually, to accelerate therapeutic interventions, but conventional cytogenetic analysis should still be performed to confirm the prenatal diagnoses of normal fetuses. The automation of the procedure easily allowed a single operator to handle up to 40 samples at the same time and the results made available on the next working day. The availability of multicapillary DNA sequencers enables high throughput of samples at very low cost, thus allowing prenatal diagnosis of common chromosome aneuploidies in countries with large populations and limited economic resources for setting up conventional cytogenetic tests (Adinolfi et al., 2000).
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Acknowledgements |
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The authors are grateful to Rebeca Font (Institut Català de Oncologia, Barcelona) for her valuable help with statistical analysis.
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References |
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Submitted on June 9, 2004; resubmitted on August 15, 2004; accepted on August 19, 2004.
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