Trisomy 7 CVS mosaicism: pregnancy outcome, placental and DNA analysis in 14 cases

Clinical Significance of Non-Invasive Prenatal Screening for Trisomy 7: Cohort Study and Literature Review

Trisomy 7 is the most frequently observed type of rare autosomal trisomies in genome-wide non-invasive prenatal screening (NIPS). Currently, the clinical significance of trisomy 7 NIPS-positive results is still unknown. We reviewed two independent cohorts from two laboratories where similar NIPS metrics were applied. A total of 70,441 singleton cases who underwent genome-wide NIPS were analyzed, among which 39 pregnancies were positive for trisomy 7, yielding a screen-positive rate of 0.055% (39/70,441). There were 28 cases with invasive testing results available; the positive predictive value (PPV) was 3.6% (1/28). We then searched the published NIPS studies to generate a large cohort of 437,873 pregnancies and identified 247 cases (0.056%) that were screened positive for trisomy 7. The overall PPV was 3.4% (4/118) in the combined data. The presence of uniparental disomy 7 was not detected in the NIPS trisomy 7-positive pregnancies with normal fetal karyotype. Among the 85 cases with pregnancy outcome available in combined data, 88.2% were normal live births, 14.1% had intrauterine growth restriction, preterm birth or low birth weight, 3.5% presented with ultrasound abnormality, and no fetal loss was observed. Our data provide valuable information for counseling and management of trisomy 7-positive NIPS pregnancies.

The significance of trisomy 7 mosaicism in noninvasive prenatal screening

Background

This study was an evaluation of the role of noninvasive prenatal testing (NIPT) in the detection of trisomy 7 in prenatal diagnosis.

Method

A total of 35 consecutive cases underwent screening for trisomies by cell-free DNA testing between April 2015 and November 2017 due to suspicious NIPT results; these cases represented 0.11% of patients (35/31,250) with similar frequencies of abnormal results among the laboratories performing the tests. NIPT was offered to further screen for common fetal chromosomal abnormalities. Karyotype analysis, chromosomal microarray analysis (CMA), and next-generation sequencing (NGS) were used to detect 20, 14, and 25 patients, respectively, who accepted confirmatory diagnostic testing.

Results

High-risk results by NIPT were recorded for trisomy 7 alone in 29 women: dual aneuploidy in 4 patients and multiple aneuploidy in 2 patients. Karyotype analysis of amniotic fluid cells was normal in all 20 pregnancies, suggesting a probability of confined placental mosaicism. Further CMA data were obtained in 14 of the cases mentioned above, and 2 fetuses were detected with positive results with copy number variation. The NGS results suggested that all these samples were placental chimerisms of chromosome 7, except for one sample that was found to be an additional chimerism of chromosome 2, which was also consistent with the NIPT result.

Conclusion

Our results may be useful for the counseling of pregnant women in the detection of trisomy 7 by NIPT.

Unexpected finding of uniparental disomy mosaicism in term placentas: Is it a common feature in trisomic placentas?

Objective

Non‐invasive prenatal testing (NIPT) detects placental chromosome aberrations. When amniocentesis reveals a normal karyotype, confined placental mosaicism (CPM) may be assumed. In order to confirm this, placental cytogenetic studies were performed.

Method

NIPT was conducted in the course of the Dutch TRIDENT study. Placentas of 10 cases with NIPT results indicating an autosomal trisomy and showing a normal (N = 9) or low mosaic karyotype (N = 1) in amniotic fluid (AF) were investigated. The cytotrophoblast as well as the mesenchymal core of two to four placental chorionic villi biopsies were studied with single nucleotide polymorphism (SNP) array. Clinical outcome data were collected.

Results

In 10/10 cases, CPM was proven. In 3/10 cases trisomy/uniparental disomy (UPD)/biparental disomy (BPD) mosaicism was discovered. In 2/3 cases, all three cell lines were present in the placenta, whereas BPD was found in AF. In 1/3 cases trisomy 22/UPD22 was present in AF while trisomy 22/BPD22 mosaicism was found in the placenta. Five of 10 pregnancies were affected with pre‐eclampsia, low birth weight, preterm delivery, and/or congenital malformations.

Conclusion

The presence of trisomy/UPD/BPD mosaicism in 3/10 cases that we investigated proves that trisomic zygote rescue may involve multiple rescue events during early embryogenesis. UPD mosaicism, when present in crucial fetal tissues, may explain the abnormal phenotype in undiagnosed cases.

What's already known about this topic?

Trisomic zygote rescue is the main mechanism for uniparental disomy (UPD) formation.

Confined placental mosaicism (CPM) is the major source of discordant NIPT results.

CPM is associated with a risk for adverse pregnancy outcome.

What does this study add?

Trisomic zygote rescue may involve multiple rescue events based on the co‐occurrence of a trisomy‐, UPD‐, and BPD‐cell line in half of the rescued cases as revealed by placental studies.

Formation of upd(7)mat by trisomic rescue: SNP array typing provides new insights in chromosomal nondisjunction

BACKGROUND

Maternal uniparental disomy (UPD) of chromosome 7 (upd(7)mat) accounts for approximately 10% of patients with Silver-Russell syndrome (SRS). For upd(7)mat and trisomy 7, a significant number of mechanisms have been proposed to explain the postzygotic formation of these chromosomal compositions, but all have been based on as small number of cases. To obtain the ratio of isodisomy and heterodisomy in UPDs (hUPD, iUPD) and to determine the underlying formation mechanisms, we analysed a large cohort of upd(7)mat patients (n = 73) by SNP array typing. Based on these data, we discuss the UPDs and their underlying trisomy 7 formation mechanisms.

RESULTS

A whole chromosome 7 maternal iUPD was confirmed in 28.8%, a mixture or complete maternal hUPD in 71.2% of patients.

CONCLUSIONS

We could demonstrate that nondisjunction mechanism affecting chromosome 7 are similar to that of the chromosomes more frequently involved in trisomy (and/or UPD), and that mechanisms other than trisomic rescue have a lower significance than previously suspected. Furthermore, we suggest SNP array typing for future parent- and cell-stage-of origin studies in human aneuploidies as they allow the definite classification of trisomies and UPDs, and provide information on recombinational events and their suggested association with aneuploidy formation.

New developments in Silver-Russell syndrome and implications for clinical practice

Silver-Russell syndrome is a clinically and genetically heterogeneous disorder, characterized by prenatal and postnatal growth restriction, relative macrocephaly, body asymmetry and characteristic facial features. It is one of the imprinting disorders, which results as a consequence of aberrant imprinted gene expressions. Currently, maternal uniparental disomy of chromosome 7 accounts for approximately 10% of Silver-Russell syndrome cases, while ~50% of patients have hypomethylation at imprinting control region 1 at chromosome 11p15.5 locus, leaving ~40% of cases with unknown etiologies. This review aims to provide a comprehensive list of molecular defects in Silver-Russell syndrome reported to date and to highlight the importance of multiple-loci/tissue testing and trio (both parents and proband) screening. The epigenetic and phenotypic overlaps with other imprinting disorders will also be discussed.

Neonatal Silver-Russell syndrome with maternal uniparental heterodisomy, trisomy 7 mosaicism, and dysplasia of the cerebellum

OBJECTIVES

We report here the unusual association of Silver-Russell syndrome (SRS) and cerebellar dysplasia with trisomy 7 mosaicism and maternal uniparental disomy of chromosome 7 [UPD(7)m].

METHODS

Low-level trisomy 7 mosaicism was diagnosed prenatally on amniocytes, and UPD(7)m was confirmed after birth.

RESULTS

Medical examination at birth showed dysmorphic facial features of SRS. Cytogenetic analysis on several tissues and cells confirmed mosaic trisomy 7. Unusual severe psychomotor retardation, hypotonia, and choreoathetoid movement were noted at 6 months. Brain magnetic resonance imaging showed both cerebellar hypoplasia and dysplasia.

CONCLUSIONS

This unusual association of SRS and dysplasia of the cerebellum might be related to the presence of the trisomy 7 mosaicism on the cerebellum. Our observation strengthens the hypothesis that the phenotype observed in patients with SRS with UPD(7)m might also result from an undetected low level of trisomy 7 mosaicism that could best be revealed by performing cytogenetic investigations.

Chromosomal Mosaicism in Human Feto-Placental Development: Implications for Prenatal Diagnosis

Chromosomal mosaicism is one of the primary interpretative issues in prenatal diagnosis. In this review, the mechanisms underlying feto-placental chromosomal mosaicism are presented. Based on the substantial retrospective diagnostic experience with chorionic villi samples (CVS) of a prenatal diagnosis laboratory the following items are discussed: (i) The frequency of the different types of mosaicism (confined placental, CPM, and true fetal mosaicisms, TFM); (ii) The risk of fetal confirmation after the detection of a mosaic in CVS stratified by chromosome abnormality and placental tissue involvement; (iii) The frequency of uniparental disomy for imprinted chromosomes associated with CPM; (iv) The incidence of false-positive and false-negative results in CVS samples analyzed by only (semi-)direct preparation or long term culture; and (v) The implications of the presence of a feto-placental mosaicism for microarray analysis of CVS and non-invasive prenatal screening (NIPS).

The origin, mechanisms, incidence and clinical consequences of chromosomal mosaicism in humans

BACKGROUND

Chromosomal mosaicism, the presence of two or more distinct cell lines, is prevalent throughout human pre- and post-implantation development and can lead to genetic abnormalities, miscarriages, stillbirths or live births. Due to the prevalence and significance of mosaicism in the human species, it is important to understand the origins, mechanisms and incidence of mosaicism throughout development.

METHODS

Literature searches were conducted utilizing Pubmed, with emphasis on human pre- and post-implantation mosaicism.

RESULTS

Mosaicism persists in two separate forms: general and confined. General mosaicism is routine during human embryonic growth as detected by preimplantation genetic screening at either the cleavage or blastocyst stage, leading to mosaicism within both the placenta and fetus proper. Confined mosaicism has been reported in the brain, gonads and placenta, amongst other places. Mosaicism is derived from a variety of mechanisms including chromosome non-disjunction, anaphase lagging or endoreplication. Anaphase lagging has been implicated as the main process by which mosaicism arises in the preimplantation embryo. Furthermore, mosaicism can be caused by any one of numerous factors from paternal, maternal or exogenous factors such as culture media or possibly controlled ovarian hyperstimulation during in vitro fertilization (IVF). Mosaicism has been reported in as high as 70 and 90% of cleavage- and blastocyst-stage embryos derived from IVF, respectively.

CONCLUSIONS

The clinical consequences of mosaicism depend on which chromosome is involved, and when and where an error occurs. Mitotic rescue of a meiotic error or a very early mitotic error will typically lead to general mosaicism while a mitotic error at a specific cell lineage point typically leads to confined mosaicism. The clinical consequences of mosaicism are dependent on numerous aspects, with the consequences being unique for each event.

Placental weight in pregnancies with trisomy confined to the placenta

OBJECTIVE

Mosaicism with trisomy confined to the placenta is present in ~1% of ongoing pregnancies at the time of chorionic villus sampling. Some studies have found reduced fetal growth in confined placental trisomy. The objective of this study was to assess placental weight and feto-placental weight ratio in pregnancies with trisomy confined to the placenta, and to correlate them with the level of trisomy in the three major placental lineages.

METHODS

We conducted a retrospective study of 69 pregnancies with prenatally diagnosed mosaic trisomy in which the trisomic cells were confined to the placenta. Placental weight and feto-placental weight ratio were compared to those of matched controls, and placental weight was also analyzed for associations with the type and level of trisomy. Placental pathology was also reviewed.

RESULTS

The pregnancies with mosaic trisomy were found to have lower placental weights than matched controls, but normal feto-placental weight ratios. Placental weight was not associated with the type or level of trisomic cells in the three placental lineages at term (chorionic plate, chorionic villus mesenchyme, and trophoblast). There were no pathognomonic findings on routine placental pathology of the trisomic placentas.

CONCLUSION

Although placental weight was reduced (with normal feto-placental weight ratio) in pregnancies with trisomy confined to the placenta, the level of placental trisomy was not correlated with placental weight. Thus, trisomy may alter placental function rather than have a direct hypoplastic effect on placental growth. More in-depth studies beyond routine pathology are required to identify how trisomy affects placental function.

Epigenetics in Silver-Russell syndrome

Silver-Russell syndrome (SRS) is a clinically heterogeneous syndrome characterized by intra-uterine and postnatal growth retardation with spared cranial growth, dysmorphic features and frequent body asymmetry. Various cytogenetic abnormalities have been described in a small number of SRS or SRS-like cases involving chromosomes 7, 8, 11, 15, 17 and 18. However, until recent data became available involving imprinted genes on chromosome 7 and chromosome 11p15, the molecular cause of the syndrome was unknown in most cases. Genomic imprinting is the best example of transcriptional control of genes by epigenetic modifications. Many imprinted genes play key roles in fetal and placental growth and behaviour. This is illustrated in SRS, which can now be considered as a new imprinting disease model. These new findings in the pathophysiology of SRS allow long-term follow-up studies to be performed based on molecular diagnosis. This could help to define appropriate clinical guidelines regarding growth and feeding difficulties.

Trisomy 7 mosaicism, maternal uniparental heterodisomy 7 and Hirschsprung's disease in a child with Silver–Russell syndrome

Prenatal trisomy 7 is usually a cell culture artifact in amniocytes with normal diploid karyotype at birth and normal fetal outcome. In the same way, true prenatal trisomy 7 mosaicism usually results in a normal child except when trisomic cells persist after birth or when trisomy rescue leads to maternal uniparental disomy, which is responsible for 5.5-7% of patients with Silver-Russell syndrome (SRS). We report here on the unusual association of SRS and Hirschsprung's disease (HSCR) in a patient with maternal uniparental heterodisomy 7 and trisomy 7 mosaicism in intestine and skin fibroblasts. HSCR may be fortuitous given its frequency, multifactorial inheritance and genetic heterogeneity. However, the presence of the trisomy 7 mosaicism in intestine as well as in skin fibroblasts suggests that SRS and HSCR might possibly be related. Such an association might result from either an increased dosage of a nonimprinted gene due to trisomy 7 mosaicism in skin fibroblasts (leading to SRS) and in intestine (leading to HSCR), or from an overexpression, through genomic imprinting, of maternally expressed imprinted allele(s) in skin fibroblasts and intestine or from a combination of trisomy 7 mosaicism and genomic imprinting. This report suggests that the SRS phenotype observed in maternal uniparental disomy 7 (mUPD(7)) patients might also result from an undetected low level of trisomy 7 mosaicism. In order to validate this hypothesis, we propose to perform a conventional and molecular cytogenetic analysis in different tissues every time mUPD7 is displayed.

Uniparental disomy (UPD) other than 15: Phenotypes and bibliography updated

Uniparental disomy (UPD) describes the inheritance of a pair of chromosomes from only one parent. The concept was introduced in Medical Genetics by Engel (1980); Am J Med Genet 6:137-143. Aside UPD 15, which is the most frequent one, up to now (February 2005) 197 cases with whole chromosome maternal UPD other than 15 (124 X heterodisomy, 59 X isodisomy, and 14 cases without information of the mode of UPD) and 68 cases with whole chromosome paternal UPD other than 15 (13 X heterdisomy, 53 X isodisomy, and 2 cases without information of the mode of UPD) have been reported. In this review we discuss briefly the problems associated with UPD and provide a comprehensive clinical summary with a bibliography for each UPD other than 15 as a guide for genetic counseling.

Intrauterine growth restriction--genetic causes and consequences

Intrauterine growth restriction is known to be associated with many medical problems for the baby, both before and after delivery. The mechanisms involved in fetal growth are not well understood, with an increasing range of metabolic diseases being implicated. Several key genes involved in normal embryonic and fetal growth and development are now known to be imprinted. Disruption of this parent-specific mono-allelic expression causes phenotypic changes, many of which are important for growth and development. Two growth disorders, Beckwith-Wiedemann syndrome and Silver-Russell syndrome, are discussed in detail as they represent well-characterized phenotypes that arise as a consequence of disrupted imprinting. These human models will allow us to elucidate key genes and mechanisms important in normal fetal growth.

X chromosome inactivation patterns in Russell-Silver syndrome patients and their mothers

Russell-Silver syndrome (RSS) is a disorder characterized by pre- and post-natal growth deficiency, triangular facies, relative macrocephaly, and body asymmetry. Maternal uniparental disomy for chromosome 7 has been found in approximately 10% of RSS cases, while the cause in the remaining cases is unknown. Although most cases of RSS are sporadic, at least 25 families have been reported with familial RSS and inheritance patterns of RSS consistent with an X-linked dominant mutation. Thus, we hypothesized that skewed X-chromosome inactivation (XCI) could be increased in some females with RSS as a consequence of a tendency to have the mutant allele on the predominantly active chromosome. Alternatively, unaffected mothers of children with RSS may tend to be skewed due to preferential inactivation of the mutant allele. In support of this last hypothesis, a significant increase in extremely skewed XCI (>95%) was found in mothers of children with RSS (4 of 21, 19%) compared to controls (2 of 101, 2%) (P = 0.008). However, an increase in skewed XCI was also observed in female patients who had unexplained short stature but did not fulfill the criteria for RSS (3 of 17, 18%) (P = 0.02), but not in those diagnosed as RSS (0 of 7, n.s.). Different mechanisms may underlie the increase in skewing in each group, possibly being due to different X-linked mutations or growth restriction during very early in utero development.

A multiplex methylation PCR assay for identification of uniparental disomy of chromosome 7

Uniparental disomy of chromosome 7 (UPD7) is associated with abnormal phenotypic effects because of inappropriate expression of imprinted genes on chromosome 7. Based on the differential methylation of the promoter region of the imprinted PEG1/MEST locus at 7q32, we designed a multiplex methylation PCR (mPCR) assay to rapidly distinguish UPD7 from biparental inheritance of chromosome 7. Primers were designed to produce different sized PCR amplicons based on the parent of origin-specific methylation at this locus; electrophoresis of PCR amplicons showed a 189-bp product from the methylated maternal allele and a 109-bp product from the unmethylated paternal allele. This mPCR assay correctly predicted the chromosome 7 imprinting status in normal control and UPD7 samples. Previous assays for UPD7 required genotyping of the proband and parents, or separate maternal- and paternal-specific mPCR reactions. The advantage of this assay is that parental samples are not required and that amplification of both alleles in the same reaction is simpler and provides an internal control. This multiplex mPCR assay will be useful in screening for UPD7 in patients with Silver-Russell syndrome (SRS; also Russell-Sliver syndrome, RSS), primordial growth retardation, and in patients with supernumerary marker chromosomes or chromosome rearrangements of chromosome 7 origin.

Prenatal diagnosis of a trisomy 7/maternal uniparental heterodisomy 7 mosaic fetus

Chromosomal segregation anomalies often result in trisomy of a single chromosome, which can lead to a disastrous phenotype in the fetus. However, this trisomy is often "rescued" via loss of one of the triploid chromosomes. Depending on which chromosome is eliminated in the rescue, a condition known as uniparental disomy may arise where both copies of a particular chromosome pair originate from the same parent. Here we describe the case of a 32-year-old woman who was prenatally diagnosed with trisomy 7 mosaicism via amniocentesis. Forty-one percent of the colonies analyzed displayed trisomy 7 (two maternal chromosomes and one paternal chromosome); whereas, 59% of the colonies demonstrated a normal, disomic karyotype. Further analysis revealed that the "normal" cell lineage displayed maternal uniparental heterodisomy. Chorionic villus tissue was homogeneously trisomic, and cord blood cells were uniformly disomic. This is the first case of trisomy 7/uniparental disomy 7 mosaic fetus reported in the literature. It is important to note that not all such cases will result in lethality or a fetus with major anatomic, developmental, or cognitive abnormalities. In addition, the benefit of pre- and post-screening test counseling and support is discussed in reference to fetuses with chromosomal abnormalities in general and our case in particular.

Origin of amnion and implications for evaluation of the fetal genotype in cases of mosaicism

OBJECTIVE

To investigate presence of trisomy in amniotic epithelium (uncultured amnion) and mesenchyme (cultured amnion) from mosaic cases to understand the origins of these tissues and their relationship to pregnancy outcome.

METHODS

Polymerase chain reaction (PCR) of microsatellite loci was used to determine the presence of trisomy (of meiotic origin only) in amnion samples from 33 placentas previously ascertained because of a prenatal diagnosis of trisomy mosaicism that was predominantly confined to the placental tissues.

RESULTS

In 16 (48%) of 33 cases, trisomy was confirmed to be present by molecular analysis of uncultured amnion. In contrast, cytogenetic analysis of cultured amnion showed trisomy in only 2 of 20 informative cases. The molecular detection of trisomy in amnion was strongly associated with poor pregnancy outcome (intrauterine growth restriction, fetal anomalies and/or intrauterine/neonatal death) even when analysis was limited to cases negative for the trisomy on amniotic fluid (N = 22, p = 0.0005).

CONCLUSIONS

We infer that amniotic mesenchyme (usually diploid) derives from early embryonic mesoderm of the primitive streak and not from the hypoblast as is commonly cited. Trisomy in amniotic epithelium suggests that high numbers of abnormal cells were present in the epiblast, and this correlates with poor outcome even when the subsequently derived fetus and amniotic mesenchyme appear to carry only diploid cells.

Review and meta-analysis of systematic searches for uniparental disomy (UPD) other than UPD 15

All systematic searches for uniparental disomy (UPD) so far published and comprising clinically defined populations (Silver-Russell syndrome/primordial growth retardation (SRS/PGR) (n = 14), multiple malformations (n = 2), or rare syndromes (n = 12)) or situations at risk (confined placental mosaicism (CPM) (n = 13), spontaneous abortions (n = 6), additional marker chromosomes (n = 15), balanced non-Robertsonian translocations (n = 3), or balanced Robertsonian translocations (n = 15)) were reviewed. In many studies clinical and/or cytogenetic information on fluorescent in situ hybridization (FISH) results was very scarce. Meta-analysis concerning an adequate number of cases was possible for SRS/PGR, CPM, additional marker chromosomes, and balanced Robertsonian translocations only. As expected, the highest risk for UPD was found in cases with translocations between homologous acrocentric chromosomes (11 cases with UPD of 15 investigated) and in CPM due to a meiotic error (25 of 51 cases). In prenatal investigations or in cases with a normal phenotype, translocations between nonhomologous acrocentric chromosomes implied a risk for UPD of less than 0.5%. The risks for maternal UPD 7 in cases with SRS/PGR, for UPD 15 in cases with an additional inv dup(15) marker chromosome, and for UPD of any chromosome in cases with multiple malformation/mental retardation were approximately 5.5%, and approximately 1.3%, respectively. Searches for UPD in well-defined syndromes (Brachmann-De Lange syndrome, Sotos syndrome, Rett syndrome, Weaver syndrome, or XX true hermaphroditism) were disappointing. Not a single case was found.

Genetics and metabolic causes of stillbirth

Approximately 25% of stillbirths have been attributed to cytogenetic, mendelian, or biochemical causes with 75% still unknown. The most common autosomal trisomies are 21, 18, and 13, and the most common karyotypic abnormality is 45x. The remaining are sporadic multiple malformation syndromes and single organ malformations. Little is known about the genetic and metabolic causes of stillbirth; however, with new cytogenetic techniques such as fluorescent in situ hybridization, comparative genomic hybridization, and telomeric probes, cytogenetic errors will be identified more accurately. Advances in diagnosis will provide additional information for appropriate genetic counseling.

Silver-Russell syndrome: a dissection of the genetic aetiology and candidate chromosomal regions

The main features of Silver-Russell syndrome (SRS) are pre- and postnatal growth restriction and a characteristic small, triangular face. SRS is also accompanied by other dysmorphic features including fifth finger clinodactyly and skeletal asymmetry. The disorder is clinically and genetically heterogeneous, and various modes of inheritance and abnormalities involving chromosomes 7, 8, 15, 17, and 18 have been associated with SRS and SRS-like cases. However, only chromosomes 7 and 17 have been consistently implicated in patients with a strict clinical diagnosis of SRS. Two cases of balanced translocations with breakpoints in 17q23.3-q25 and two cases with a hemizygous deletion of the chorionic somatomammatropin gene (CSH1) on 17q24.1 have been associated with SRS, strongly implicating this region. Maternal uniparental disomy for chromosome 7 (mUPD(7)) occurs in up to 10% of SRS patients, with disruption of genomic imprinting underlying the disease status in these cases. Recently, two SRS patients with a maternal duplication of 7p11.2-p13, and a single proband with segmental mUPD for the region 7q31-qter, were described. These key patients define two separate candidate regions for SRS on both the p and q arms of chromosome 7. Both the 7p11.2-p13 and 7q31-qter regions are subject to genomic imprinting and the homologous regions in the mouse are associated with imprinted growth phenotypes. This review provides an overview of the genetics of SRS, and focuses on the newly defined candidate regions on chromosome 7. The analyses of imprinted candidate genes within 7p11.2-p13 and 7q31-qter, and gene candidates on distal 17q, are discussed.

Post-zygotic origin of complete maternal chromosome 7 isodisomy and consequent loss of placental PEG1/MEST expression

Maternal UPD of chromosome 7 is associated with pre- and postnatal growth retardation (IUGR, PNGR) and Silver-Russell syndrome (SRS [MIM 180860]). We report a case of IUGR in a newborn with SRS stigmata. Using combined haplotyping and cytogenetic-FISH studies we characterized the lymphocytes, umbilical cord and four placental cotyledons. The results are consistent with complete maternal isodisomy 7 and trisomy 7 mosaicism of post-zygotic origin. The trisomic cell line was prevalent in trophoblast cells from two placental cotyledons. Trisomy 7 of post-zygotic origin is a frequent finding, but maternal isodisomy 7, due to trisomic rescue has never been reported. PEG1/MEST expression was evaluated on placenta cDNA and a specific transcript was revealed only in the cotyledons with a high percentage of trisomic cells and the presence of the paternal chromosome 7 contribution, but not in the placental biopsies with maternal isodisomy 7. The histological features of the four placental fragments revealed that isodisomy 7 correlates with a pattern of cotyledonary hyper-ramification due to an increase of the branching angiogenesis, which could be the result of a defect of angiogenesis caused by the absence of PEG1 product. The severe hypo-ramification of the two cotyledons, showing trisomy 7 mosaicism, may be due to the triplicate dosage of genes on chromosome 7. The delayed fetal growth could be the phenotypic effect of the imbalance between imprinted and non-imprinted genes on chromosome 7 in the fetus or the result of abnormal placental function during pregnancy.

Cytogenetic analyses of culture failures by comparative genomic hybridisation (CGH)-Re-evaluation of chromosome aberration rates in early spontaneous abortions

Comparative genomic hybridisation (CGH) represents an alternative molecular-cytogenetic technique capable of detecting chromosomal imbalances by reverse fluorescence in situ hybridisation. As the technique uses genomic DNA for assessment it does not rely on metaphase chromosomes in the test material and thus circumvents technical problems associated with tissue culturing. In the present study, we applied CGH to identify chromosome anomalies in 60 spontaneous abortions of the first trimester, that had failed to grow in culture. In 57 out of 60 cases CGH analyses were successful. The overall aneuploidy rate detected was 72%. Trisomy was the predominant chromosome anomaly accounting for 68.0% of abnormal abortions, followed by triploidy (17.1%) and monosomy X (9.8%). An unbalanced structural rearrangement was found in one (2.4%) abortion. Most frequently involved in trisomies were chromosomes 16 (32.1%), 7 and 22 (10.7% each), 4, 13, 15, and 21 (7.2 % each). Three triploid cases and one complete mole were detected by microsatellite analysis as supplementary method. CGH data on culture failures were compared with data derived from 4693 successfully karyotyped first trimester spontaneous abortions, resulting in a chromosome aberration rate of 64.8%. The distribution of the different chromosome anomalies was similar with the exception of a higher rate of trisomies 7 and of XYY-triploidies in the culture failures. Based on our data we suggest that the genetic contribution to pregnancy loss is still underestimated. Investigating abortion tissues hitherto unassessed by conventional methods, we suggest that the contribution of chromosome aberrations to first trimester pregnancy loss is nearly 70%.

Correlation of confined placental mosaicism with fetal intrauterine growth retardation. A case control study of placentas at delivery

OBJECTIVE

Our purpose was to determine if the frequency of confined placental mosaicism in newborns with unexplained intrauterine growth retardation (IUGR) was higher compared with infants with appropriate growth in utero and the outcome of these pregnancies.

STUDY DESIGN

A total of 20 cases with unexplained IUGR and 20 cases with appropriate growth for gestational age has been studied. Amnion, chorion and villi biopsy specimens were obtained from growth-retarded cases and controls at delivery. Cord blood specimens for 48-hour lymphocyte cultures were obtained from all infants with IUGR.

RESULTS

Karyotype analysis revealed confined placental mosaicism in two of 20 (10%) cases with IUGR. In one growth retarded case and one appropriate growth for gestational age case, mosaicism was also confirmed in the amnion. Cytogenetic analysis from peripheral blood of newborns showed normal karyotype in all cases. Three pregnancies in the group of fetuses with IUGR (15%) ended with fetal death compared with normal fetal surveillance of all cases from the control group.

CONCLUSION

Confined placental mosaicism was detected two times more frequently from placentas of growth- retarded infants compared with those of newborns with appropriate growth. The fetal loss was significantly higher in the group of cases with IUGR compared with the control group.

An association between skewed X-chromosome inactivation and abnormal outcome in mosaic trisomy 16 confined predominantly to the placenta

Skewed X-chromosome inactivation (XCI) is frequently found in the diploid fetal tissues of individuals with mosaic trisomy that originated from a 'trisomic zygote rescue' event. This may result from a high number of trisomic cells in the embryonic cell pool at the time of XCI, which are subsequently eliminated by selection. We hypothesize that extremely skewed XCI in these mosaic cases will be associated with a poor fetal outcome due to failure to completely eliminate the trisomy from all fetal tissues. To test this hypothesis, XCI status was evaluated in 17 cases of prenatally detected trisomy 16 mosaicism. Ten of the 15 informative cases showed extreme XCI skewing ( > or = 90% inactivation of one allele) in blood or other diploid fetal tissues compared to six of the 111 controls (p < 0.001). Among these 10 'skewed' cases, 6 showed an abnormal outcome, defined as developmental abnormalities and/or intrauterine or neonatal death. In contrast, of the 5 cases without extreme skewing, none showed abnormal outcome, although outcome information was incomplete in 1 case. An additional 6 cases analyzed, involving trisomy mosaicism for other chromosomes, showed similar results. Further studies are warranted to determine if XCI status adds useful information to the prediction of pregnancy outcome in prenatally detected mosaic trisomy.

Pathogenesis of chromosomal mosaicism and its effect on early human development

Constitutional chromosomal mosaicism is the result of postfertilization mitotic error, the mechanism of which is not fully understood. The distribution of mosaicism in the conceptus depends on the timing, cell lineage(s) involved, cell viability, and chromosome involved. The developmental consequences of mosaicism also are related to its meiotic or somatic type. Meiotic mosaicism often is associated with a more severely adverse effect on the conceptus (see trisomy zygote rescue) due to the presence of uniparental disomy in the embryo/fetus and/or to dysfunction of a trisomic placenta. As mosaicism can be tissue specific, the result of a normal karyotype in cultured lymphocytes does not exclude the presence of mosaicism elsewhere in the conceptus. Mosaicism can best be detected by a combination of traditional cytogenetic analysis with molecular cytogenetic techniques such as comparative genomic hybridization and fluorescence in situ hybridization.

Maternal uniparental disomy of chromosome 2 and confined placental mosaicism for trisomy 2 in a fetus with intrauterine growth restriction, hypospadias, and oligohydramnios

We present a case of maternal uniparental heterodisomy for chromosome 2 (UPD 2) detected after trisomy 2 mosaicism was found on placental biopsy. This case presented prenatally with severe intrauterine growth restriction (IUGR) and oligohydramnios. The diploid newborn had hypospadias and features consistent with oligohydramnios sequence. He died shortly after birth of severe pulmonary hypoplasia. The term placenta had high levels of trisomy 2 in both the trophoblast and the stroma. A comparison of this case with others reported in the literature suggests that the IUGR and oligohydramnios are likely related to placental insufficiency due to the high levels of trisomy 2 present in the trophoblast of the term placenta and the presence of UPD 2 in the diploid placental line.