Confirmation of CVS mosaicism in term placentae and high frequency of intrauterine growth retardation association with confined placental mosaicism

About 2 per cent of specimens from chorionic villus sampling (CVS) analysed either on direct preparation of cytotrophoblast cells or afterculture of mesenchymal stroma reveal confined placental mosaicism (CPM), most commonly involving chromosomal trisomy. A significantly higher rate of prenatal loss (22 per cent) as well as the presence of intrauterine growth retardation (IUGR) has been reported among pregnancies with CPM. To evaluate more precisely the effect of these aneuploid cell lines confined to the placenta on intrauterine fetal growth and fetal survival, we have studied 34 term placentae from pregnancies with CPM diagnosed on CVS and confirmed identical mosaicism in 17 of these placentae. There was a direct correlation between a high number of aneuploid cells present at CVS and a high likelihood of their detection in term placenta. Also, the proportion of aneuploid cells in the mosaic term placentae correlated with that observed in CVS specimens. Among 17 gestations with confirmed CPM at delivery, there were six cases of IUGR identified, five in liveborns and one associated with intrauterine death.     

Molecular cytogenetic analysis of term placentae suspected of mosaicism using fluorescence in situ hybridization

In first-trimester chorionic villus sampling (CVS) for prenatal diagnosis, abnormal chromosomal findings, such as mosaicism, trisomies, or suspect abnormal karyotypes, are found more frequently than at amniocentesis. The fact that these chromosomal abnormalities do not always reflect the fetal karyotype but may be restricted to the placenta is a major problem in diagnosis and counselling. In this paper we present the results of fluorescence in situ hybridization (FISH) studies on interphase nuclei of three term placentae investigated because of false-positive findings at first-trimester CVS. The chorionic villi of the first case showed a mosaic chromosome pattern involving a trisomy 10 cell line and a normal cell line, those of the second case a total trisomy 8 cell line, while in the third case a complete monosomy X was found. Follow-up amniocentesis in each of these three cases revealed a normal karyotype. By using FISH, we were able to confirm the presence of the aberrant cell lines, which were all confined to one part of the placenta. FISH on interphase nuclei allows the investigation of large numbers of cells for the existence of numerical chromosome aberrations in a quick and reliable way.     

Mosaicism and accuracy of prenatal cytogenetic diagnoses after chorionic villus sampling and placental biopsies

Discrepant chromosome findings in placenta and fetus (false negative and false positive) after chorionic villus sampling (CVS) are mainly due to confined mosaicism. Non-mosaic normal or abnormal chromosome counts after direct preparation and culture nearly always correctly reflect the fetal chromosome constitution. False-negative results have almost exclusively been restricted to cytotrophoblast cells not representing a fetal chromosome abnormality. Diagnosis of placental mosaicism definitely requires an adequate follow-up by amniocentesis, fetal blood sampling, or sonography before a pregnancy is terminated. When direct preparations and cultured cells are used for cytogenetic diagnoses and placental mosaicism is not taken as proof for a chromosomal abnormality in the fetus, CVS is an accurate diagnostic tool.     

The significance of trisomy 7 mosaicism in chorionic villus cultures

Two cases of mosaic trisomy 7 confined to the cultured cells and not found in direct preparation were detected from 200 consecutive first-trimester chorionic villus samples (CVS) analysed. The mosaicism was similar in the two cases, but the pregnancy outcome was different. In both cases, the direct metaphases from the CVS were 46, XY. Culture metaphases were mos46,XY/47,XY, + 7; the trisomy 7 was seen in 34 per cent of cells from case 1 and 53 per cent from case 2. A sonogram at 15¹/₂ weeks revealed fetal death in utero in case 1, and the patient declined amniocentesis. The fetal tissue failed to grow in culture, but the placental cultured cells were 47,XY, + 7 in 28 (100 per cent) cells analysed. In the second case, all the amniotic fluid cells were 46,XY and the pregnancy resulted in a normal male with a 46,XY karyotype in the cord blood and foreskin fibroblast cultures. The term placenta was mosaic with 13/163 (8 per cent) trisomy 7 cells. Extensive cytogenetic studies on the placenta for the first time confirmed trisomy 7 mosaicism confined to the villus cultures.     

A second case of intrauterine growth retardation and primary hypospadias associated with a trisomy 22 placenta but with biparental inheritance of chromosome 22 in the fetus

We report a case of severe intrauterine growth retardation (IUGR) and hypospadias in association with trisomy 22 diagnosed following chorionic villus sampling (CVS). Subsequent analysis of amniotic fluid cultures showed a normal male karyotype, 46,XY. As a previous case had been reported with similar abnormalities, in association with maternal uniparental disomy (UPD) 22, molecular studies were also performed. Microsatellite marker studies showed biparental inheritance. Follow-up studies after delivery showed a normal cell line in lymphocytes with the trisomy appearing to be confined to the placenta. The present case concurs with other earlier reports that maternal UPD for chromosome 22 has no impact on the phenotype. The features seen in the fetus are most likely the result of placental dysfunction due to trisomy, tissue-specific mosaicism and/or the effects of local growth restriction. Copyright © 2002 John Wiley & Sons, Ltd.     

Trisomy 8 mosaicism in chorionic villus sampling: Case report and counselling issues

We report an unusual case involving chorionic villus sampling (CVS) and trisomy 8 mosaicism. CVS showed a normal direct preparation while the culture showed mosaicism for trisomy 8. Subsequent amniocentesis revealed only normal chromosomes. A peripheral blood culture after birth revealed low-level trisomy 8 mosaicism. The patient appeared phenotypically and developmentally normal at 30 months of age. We conclude that prenatal counselling for similar cases needs to include the rare but real possibility that chromosome mosaicism detected prenatally may be found postnatally with largely unknown consequences. Secondly, low-level chromosomal mosaicism may be more common than previously recognized. Thirdly, very low-level trisomy 8 mosaicism may be compatible with a normal phenotype but long-term follow-up is required. And lastly, the use of fetal blood sampling is questionable in these cases because the phenotype may not be accurately predicted. Further studies of such cases are needed to address these important and unanswered issues, including the potential implication of mosaicism on academic performance and cognitive functioning.     

Trisomy 12 mosaicism detected by mid–trimester amniocentesis

Trisomy 12 mosaicism was found in about 15 per cent of cultured amniocytes obtained from a 32-year-old white female at 17·6 weeks of gestation. Termination of pregnancy was elected and multiple tissues were obtained for chromosome analysis. Of 158 cells examined, only 1 cell in placenta was found with an extra number 12 chromosome. Pathological examination of the fetus did not reveal significant physical abnormalities. This report illustrates the difficulty of confirming trisomy 12 mosaicism which has been detected on prenatal diagnosis. The presence of trisomy 12 in one placental cell obtained from the curettage specimen suggests the possibility of confined placental mosaicism in this case.     

Management of prenatally detected trisomy 8 mosaicism

We report on ten pregnancies with trisomy 8 mosaicism. Nine cases were prenatally detected in chorionic villi (n=6), amniotic fluid (AF) cells (n=2) or fetal blood (FB) lymphocytes (n=1). Follow-up laboratory investigations showed confined placental mosaicism (CPM) or pseudomosaicism in eight cases. In one case with ultrasound abnormalities, trisomy 8 mosaicism was detected in FB cells although cultured AF cells showed normal cells only. Another case of mosaic trisomy 8 was prenatally missed; cytogenetic analysis of short-term cultured villi revealed a normal male karyotype, while postnatally, trisomy 8 mosaicism was detected in peripheral blood lymphocytes and skin fibroblasts of the affected child. These findings indicate the difficulties in the prenatal diagnosis of trisomy 8 mosaicism. When found in chorionic villi, it mostly represented CPM, while in a case of true fetal trisomy 8 mosaicism, the cytotrophoblast cells showed a normal karyotype. So, the cytotrophoblast compartment of chorionic villi is a poor indicator of the presence or absence of fetal trisomy 8 mosaicism. Follow-up investigations including amniocentesis and especially fetal blood sampling are required to come to a definite prenatal diagnosis of trisomy 8 mosaicism. Copyright © 2001 John Wiley & Sons, Ltd.     

An audit of trisomy 16 in man

Sufficient information is now available from the literature to produce an audit of trisomy 16, in a theoretical cohort of 100 000 recognized pregnancies, from gametogenesis to term and onwards. Recent reports of premature separation of chromosome 16 bivalents during maternal meiosis I provide a novel mechanism for generation of this aneuploidy. Most, if not all, errors resulting in recognized mosaic and non-mosaic trisomy 16 pregnancies investigated using polymorphic DNA markers appear to originate at that stage. The incidence of this maternally derived trisomy 16 in the late first trimester is equivalent to 1500 cases in 100 000 recognized pregnancies, a figure which now corresponds very closely to the reinterpreted oogenesis data. Most trisomy 16 pregnancies are lost around 12 weeks' gestation, but of the order of 10 per cent (120–150 in this audit) undergo reduction to disomy, with 30 of these excluding aneuploidy from the fetal cell lineage (trisomic zygote rescue) and continuing into the second trimester. Maternal uniparental disomy (UPD) in one-third of this latter group is associated with loss later in pregnancy or severe intrauterine growth retardation, but can be compatible with a viable pregnancy. Adverse pregnancy outcomes are not restricted to those with UPD. Analysis of reports of confined placental mosaicism for chromosome 16 without associated UPD indicates that the presence of high levels of trisomic cells in the placenta alone consistently produces a more variable inhibition of fetal growth, which may also, in cases, be associated with late pregnancy loss.     

Increased incidence of cytogenetic abnormalities in chorionic villus samples from pregnancies established by in vitro fertilization and embryo transfer (IVF–ET)

We studied 201 pregnancies that were established by in vitro fertilization and embryo transfer (IVF–ET) and compared the frequency of cytogenetic abnormalities with that found in a large control population matched for indication group (advanced maternal age) and time of sampling. A total of 252 IVF–ET fetuses were cytogenetically analysed by either chorionic villus sampling (CVS; n = 80) or amniocentesis (n = 172). Eleven chromosome abnormalities were found in the CVS group (13·8 per cent); among them, a 45, X/46, X, dic(q11)/46, X, del(Y)(q11) mosaic that was found in an IVF pregnancy established by intracytoplasmic sperm injection (ICSI), four cases of trisomy 21, and three cases of trisomy 7 confined to the placenta. The results indicate a statistically significant three-to five-fold increase in both confined placental abnormalities (P<0·008) and true fetal chromosome anomalies (P<0·04). In the amniocentesis group, identical rates (1·7 per cent) of chromosome abnormalities were found in the IVF–ET and control groups. It is concluded that late first trimester, but not early second trimester, IVF–ET pregnancies are characterized by an increased frequency of cytogenetic abnormalities found at prenatal diagnosis.     

Postnatal placental confirmation of trisomy 2 and trisomy 16 detected at chorionic villus sampling: A possible association with intrauterine growth retardation and elevated maternal serum alpha-fetoprotein

Detection of trisomy 2 and trisomy 16 mosaicism through chorionic villus sampling (CVS) is not an infrequent finding. We describe here two cases, one of non-mosaic trisomy 2 and the other of high level mosaicism for trisorny 16. Amniocentesis in both cases demonstrated non-mosaic 46,XY karyotypes. Each pregnancy continued to delivery of liveborn, normal-appearing boys; both pregnancies were complicated by severe intrauterine growth retardation (IUGR). Postnatal studies of placental biopsies in both cases confirmed the original CVS findings, whereas cord blood karyotypes were normal in both boys. Both children have demonstrated adequate catch-up growth.     

Identification of a case of maternal uniparental disomy of chromosome 10 associated with confined placental mosaicism

We report a case of maternal uniparental disomy of chromosome 10 discovered after chorionic villus sampling (CVS). Direct preparations revealed mosaic trisomy 10, while cultured CVS cells, as well as amniotic fluid cells, showed only a normal 46,XY complement. DNA analysis using microsatellite markers showed both chromosomes 10 to have been inherited from the mother. The pregnancy was complicated by polyhydramnios. A phenotypically normal male infant of appropriate size was delivered by Caesarean section at 41 weeks' gestation. Since only the direct preparations showed trisomy 10, this case illustrates the importance of CVS direct preparations in the detection of pregnancies at risk of uniparental disomy (UPD). Although the increased frequency of confined placental mosaicism (CPM) diagnosed when direct preparations are performed has been viewed negatively, identification of both CPM and UPD may have biological and clinical significance for a pregnancy. Even though only a single case of maternal disomy 10 is reported here, the apparently normal phenotype provides evidence that there are no major imprinted loci on chromosome 10 that affect in utero growth and development. However, other potential effects such as mental retardation will require long-term follow-up of this as well as additional cases.     

An embryogenic model to explain cytogenetic inconsistencies observed in chorionic villus versus fetal tissue

While the fetus and placenta have a common ancestry, chorionic villus tissue does not always reflect fetal genotype. Data are presented from 15 CVS subjects in whom cytogenetic inconsistencies were observed when comparing (1) cultured chorionic villi, (2) direct chromosome preparations of intact villi, and (3) cultured fetal tissue. Embryogenic models are presented to explain these discrepancies. Mosaicism confined to direct chromosome preparations was the most commonly observed inconsistency. This can be explained by postzygotic non-disjunction limited to cytotrophoblast. In all but one instance, the abnormal cell line was limited to the placenta, with the normal cell line reflecting fetal genotype. Analysis of direct chromosome preparations from multiple individually processed villus fragments may be helpful in recognizing mosaicism confined to the placenta. While both direct chromosome preparations and villus cultures can be misleading, the latter are more likely to reflect fetal genetic status since they are derived from the extraembryonic mesoderm.     

Isochromosome 5p mosaicism at prenatal diagnosis: observations and outcomes in six cases at chorionic villus sampling and one at amniocentesis

We present six cases of 47,+i(5p)/46 mosaicism diagnosed at chorionic villus sampling (CVS), this being the first prospective series to be reported. The clinical indication in each was advanced maternal age. Further prenatal studies in four (amniocentesis, plus fetal blood sampling in one) did not show the isochromosome. In one case, subsequent amniocentesis showed 1/48 in situ colonies with the isochromosome, but fetal blood was karyotypically normal. These five pregnancies resulted in phenotypically normal livebirths; further normal follow-up reports (from age 4 months through 4 years) are noted in four of these. Analysis of placental tissue in one case confirmed the presence of the i(5p) mosaicism. In the remaining case, in which 100% of CVS cultured cells had the i(5p), the pregnancy was terminated. Fetal skin fibroblasts did not show the i(5p). Thus, in none of these six cases was true fetal mosaicism detected, nor an abnormal phenotype noted. We suggest that a 47,+i(5p)/46 karyotype, detected at CVS, may frequently reflect confined placental mosaicism. In addition, we report a case of the primary diagnosis of 47,+i(5p)/46 mosaicism at amniocentesis. The infant appeared normal at birth, but a brain malformation was subsequently identified. Copyright © 2002 John Wiley & Sons, Ltd.     

‘False-negative’ and ‘false-positive’ prenatal cytogenetic results due to ‘true’ mosaicism

A 37-year-old gravida was referred for CVS because of advanced maternal age. A trisomy 21 was present in all cells after short-term incubation (direct processing (DP)) and long-term culture. According to our policy, a retap was offered for confirmation of the result during the legally required 3-day waiting period between communication of the result and termination of pregnancy. Unexpectedly all cells after DP showed a normal male chromosome complement. Further investigations revealed mosaicism in trophoblast tissue and a normal karyotype in amniotic fluid cells and fetal blood (50 mitoses each). The parents elected to continue the pregnancy after extensive ultrasound examinations did not show suspicious findings. After the birth of a healthy child, cell cultures from ten different placental sites confirmed mosaicism. Four out of 100 mitoses from a lymphocyte culture showed an additional chromosome 21. The child had no dysmorphic features and the development was normal at the age of 10 weeks. This case demonstrates the restricted validity of prenatal cytogenetic analysis in the presence of true fetal mosaicism. It also stresses the benefit of our policy to offer a retap in cases with abnormal cytogenetic results prior to termination of pregnancy which is considered unnecessary by many cytogeneticists.     

Trisomy 16 confined to the placenta

Two cases with trisomy 16 confined to the placenta are presented. Prenatal diagnosis was indicated because of fetal growth retardation. In case 1, a phenotypically normal but small-for-date boy was born. In case 2, the fetus turned out to be triploid on cordocentesis. In both instances the trisomy 16 was recovered from the placenta. Recovery indicates that the abnormality was present in the placenta during the time of fetal growth retardation, which supports an aetiological relationship. Strict appliance of the current models cannot readily explain the observed discrepancies. In case 2, a chimeric placenta as a result of a vanishing twin is assumed. Cases of placental trisomy 16 published after 1988 are reviewed. It is concluded that confined placental trisomy 16 can cause intrauterine growth retardation if present in both the direct preparation and the villus culture. The chances of finding a chromosomally abnormal fetus (mosaic trisomy 16, triploidy) after diagnosis of trisomy 16 in chorionic villi are low but warrant further investigations.     

Chromosome mosaicism of the placenta—a cause of developmental failure of the fetus?

Fourteen (2.5 per cent) of 568 chromosome preparations after CVS showed discrepancies between the placental and fetal karyotype, mainly due to placental mosaicism. The presence of a second cell line within the placenta was confirmed in all but one case, in which cytogenetic reinvestigations were carried out. Our clinical data indicate that severe developmental retardation in the newborn is not to be expected if only the placenta carries the chromosomally abnormal cell line.     

Confined placental mosaicism,IUGR, and adverse pregnancy outcome: A controlled retrospective U.K. collaborative survey

In a retrospective collaborative study involving 21 U.K. laboratories and 11775 CVS prenatal diagnostic procedures, a total of 73 cases of confined placental mosaicism (CPM) were identified among the 8004 first-trimester referrals because of advanced maternal age, a previous child with a numerical chromosome abnormality, or a family history of the same. Data were collected on subsequent cytogenetic follow-up and pregnancy outcome for each case and a referral matched control. Comparison with the control population failed to demonstrate a marked increase in adverse pregnancy outcome in the CPM group, but a significant increase in both low and high birth weight infants was recorded. In a parallel study, 7 out of 108 cases, referred for prenatal diagnosis because of ultrasound detection of isolated intrauterine growth retardation (IUGR) in the second or third trimester, were shown to have a chromosome abnormality restricted to the extraembryonic tissues. These included cases of CPM involving trisomy 9 and del(13)(q13), neither of which has previously been reported in association with IUGR.     

Confined placental mosaicism in term placentae: Analysis of 125 cases

In order to determine the incidence of confined placental mosaicism (CPM) in term placentae and to show the presence of specific sites and the effect on fetal development, 125 placentae from uneventful pregnancies were analysed by cytogenetic methods. The incidence was at least 4.8 per cent and there were no specific sites on the placenta. Although the number of cases is still too small, we found CPM to be associated with intrauterine growth retardation in six cases.     

Retrospective study of trisomy 18 in chorionic villi with fluorescent in situ hybridization on archival direct preparations

Trisomy 18 in direct chorionic villus preparations needs further investigation since the chromosome abnormality may be confined to the placenta and may not represent the actual fetal karyotype. We performed, retrospectively, fluorescent in situ hybridization (FISH) with the chromosome 18 centromere probe (L1.84) on interphase nuclei of destained slides of all cases of full trisomy 18 (n=22) and mosaic trisomy 18 (n=8) detected among 7600 first-trimester chorionic villus samples during an 8-year period (1985–1992). More nuclei displaying three signals were encountered in cases of full and mosaic trisomy 18 confirmed in fetal tissue than in non-confirmed cases. FISH can be useful for the verification of trisomy 18 in direct chorionic villus preparations.