Discordant prenatal diagnosis of trisomy 21 due to mosaic structural rearrangements of chromosome 21

A Case Report of a Feto-Placental Mosaicism Involving a Segmental Aneuploidy: A Challenge for Genome Wide Screening by Non-Invasive Prenatal Testing of Cell-Free DNA in Maternal Plasma

Non-invasive prenatal testing (NIPT) using cell-free DNA can detect fetal chromosomal anomalies with high clinical sensitivity and specificity. In approximately 0.1% of clinical cases, the NIPT result and a subsequent diagnostic karyotype are discordant. Here we report a case of a 32-year-old pregnant patient with a 44.1 Mb duplication on the short arm of chromosome 4 detected by NIPT at 12 weeks' gestation. Amniocentesis was carried out at 18 weeks' gestation, followed by conventional and molecular cytogenetic analysis on cells from the amniotic fluid. SNP array analysis found a de novo deletion of 1.2 Mb at chromosome 4, and this deletion was found to be near the critical region of the Wolf-Hirschhorn syndrome. A normal 46,XY karyotype was identified by G-banding analysis. The patient underwent an elective termination and molecular investigations on tissues from the fetus, and the placenta confirmed the presence of type VI true fetal mosaicism. It is important that a patient receives counselling following a high-risk call on NIPT, with appropriate diagnostic analysis advised before any decisions regarding the pregnancy are taken. This case highlights the importance of genetic counselling following a high-risk call on NIPT, especially in light of the increasing capabilities of NIPT detection of sub-chromosomal deletions and duplications.

Case Report: Two cases of apparent discordance between non-invasive prenatal testing (NIPT) and amniocentesis resulting in feto-placental mosaicism of trisomy 21. Issues in diagnosis, investigation and counselling

The sequencing of cell-free fetal DNA in the maternal plasma through non-invasive prenatal testing (NIPT) is an accurate genetic screening test to detect the most common fetal aneuploidies during pregnancy. The extensive use of NIPT, as a screening method, has highlighted the limits of the technique, including false positive and negative results. Feto-placental mosaicism is a challenging biological issue and is the most frequent cause of false positive and negative results in NIPT screening, and of discrepancy between NIPT and invasive test results. We are reporting on two cases of feto-placental mosaicism of trisomy 21, both with a low-risk NIPT result, identified by ultrasound signs and a subsequent amniocentesis consistent with a trisomy 21. In both cases, after the pregnancy termination, cytogenetic and/or cytogenomic analyses were performed on the placenta and fetal tissues, showing in the first case a mosaicism of trisomy 21 in both the placenta and the fetus, but a mosaicism in the placenta and a complete trisomy 21 in the fetus in the second case. These cases emphasize the need for accurate and complete pre-test NIPT counselling, as well as to identify situations at risk for a possible false negative NIPT result, which may underestimate a potential pathological condition, such as feto-placental mosaicism or fetal trisomy. Post-mortem molecular autopsy may discriminate between placental, fetal and feto-placental mosaicism, and between complete or mosaic fetal chromosomal anomalies. A multidisciplinary approach in counselling, as well as in the interpretation of biological events, is essential for the clarification of complex cases, such as feto-placental mosaicisms.

Study of Human T21 Placenta Suggests a Potential Role of Mesenchymal Spondin-2 in Placental Vascular Development

Placental development is particularly altered in trisomy of chromosome 21 (T21)-affected pregnancies. We previously described in T21-affected placentae an abnormal paracrine crosstalk between the villus mesenchymal core and villus trophoblasts. T21-affected placentae are known to be characterized by their hypovascularity. However, the causes of this anomaly remain not fully elucidated. Therefore, the hypothesis of an abnormal paracrine crosstalk between fetal mesenchymal core and placental endothelial cells (PLECs) was evocated. Villus mesenchymal cells from control (CMCs) and T21 placentae (T21MCs) were isolated and grown in culture to allow their characterization and collection of conditioned media for functional analyses (CMC-CM and T21MC-CM, respectively). Interestingly, PLEC proliferation and branching ability were less stimulated by T21MC-CM than by CMC-CM. Protein array analysis identified secreted proangiogenic growth factors in CMC-CM, which were reduced in T21MC-CM. Combined mass spectrometry and biochemical analysis identified spondin-2 as a factor decreased in T21MC-CM compared with CMC-CM. We found that exogenous spondin-2 stimulated PLEC proliferation and established that T21MC-CM supplemented with spondin-2 recovered conditioned media ability to induce PLEC proliferation and angiogenesis. Hence, this study demonstrates a crosstalk between villus mesenchymal and fetal endothelial cells, in which spondin-2 secreted from mesenchymal cells plays a central role in placental vascular functions. Furthermore, our results also suggest that a reduction in spondin-2 secretion may contribute to the pathogenesis of T21 placental hypovascularity.

Isochromosome 21q is overrepresented among false-negative cell-free DNA prenatal screening results involving Down syndrome

False-negative cell-free DNA (cfDNA) screening results involving Down syndrome are rare, but have high clinical impact on patients and their healthcare providers. Understanding the biology behind these results may allow for improved diagnostic follow-up and counseling. In 5 different centers offering cfDNA prenatal screening, 9 false-negative results were documented in 646 confirmed cases of trisomy 21; a false-negative rate of 1.4% (95% CI, 0.7-2.6). False-negative results included 4 cases of classical trisomy 21 and 5 cases with a de novo 21q;21q rearrangement. Two out of five rearrangements had molecular studies and were confirmed as isochromosomes. When combined with reports from the cfDNA screening literature, 8 out of 29 (28%) Down syndrome cases with a false-negative "non-invasive prenatal test" (NIPT) were associated with a 21q;21q rearrangement, compared with 2% reported in live born children with Down syndrome. In our laboratory series, evidence for placental or fetal mosaicism was present in 3 out of 3 true-positive cases involving a 21q;21q rearrangement and was confirmed in one false-negative case where placental material was available for study. Isochromosome 21q rearrangements are thus overrepresented among false-negative cfDNA screening results involving Down syndrome. Postzygotic isochromosome formation leading to placental mosaicism provides a biological cause for the increased prevalence of these rearrangements among false-negative cases. For clinical practice, a low trisomic fraction (z-score or equivalent measure) relative to the fetal fraction suggests placental mosaicism. Care should be taken as these cases may not reflect confined placental mosaicism, but rather full trisomy in the presence of a placenta containing normal cells.

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).

Mesenchymal activin-A overcomes defective human trisomy 21 trophoblast fusion

Placental development is markedly abnormal in trisomy 21 (T21) pregnancies. We hypothesized that abnormal paracrine cross talk between the fetal mesenchymal core and the trophoblast might be involved in the defect of syncytiotrophoblast formation and function. In a large series of primary cultured human cytotrophoblasts isolated from second-trimester control (n = 44) and T21 placentae (n = 71), abnormal trophoblast fusion and differentiation was observed in more than 90% of T21 cases. We then isolated and cultured villous mesenchymal cells from control (n = 10) and T21 placentae (n = 8) and confirmed their fetal origin. Conditioned medium of control mesenchymal cells overcame the abnormal trophoblast fusion of T21 cytotrophoblasts by activating the TGFβ signaling pathway, as shown by the phosphospecific protein microarray analysis and the use of TGFβ signaling pathway antagonists. Using protein arrays, we further analyzed the cytokines present in the conditioned medium from control and T21 mesenchymal cells. Activin-A was identified as strongly secreted by cells from both sources, but at a significantly (P < 0.01) lower level in the case of T21 mesenchymal cells. Recombinant activin-A stimulated T21 trophoblast fusion. Blocking activin-A antibody inhibited the fusion induced by conditioned medium and exogenous activin-A. Furthermore, follistatin, an activin-A binding protein largely secreted by T21 mesenchymal cells, inhibited the conditioned medium fusogenic activity. These results show that the defective trophoblast fusion and differentiation associated with T21 can be overcome in vitro and reveal the key role of the fetal mesenchymal core in human trophoblast differentiation.

[Trisomy 21 by isochromosome: a case report of true false negative of chorionic villi sampling]

We report a rare case of true false negative of chorionic villi sampling for a child with Down syndrome. A chorionic villi sampling was performed for a nuchal translucency at the first trimester. The karyotype was 46,XX for the short and the long-term culture. Because of facial dysmorphy and cardiopathy to the child, a karytoype was proposed. This postnatal karyotype showed a trisomy 21, by isochromosome 46,XX,i(21)(q10). We expose the mechanism of true false negative of chorionic villi sampling, and particularly the role of isochromosome in this case.

Comparison of modes of ascertainment for mosaic vs complete trisomy 21

OBJECTIVE

We sought to compare the indications for amniocentesis leading to the detection of either mosaicism of trisomy 21 (mosaic-T21) or complete trisomy 21 (T21).

STUDY DESIGN

A retrospective review of a large amniocentesis database (n = 494,163) was conducted. All specimens with mosaic-T21 (n = 124) were compared with a maternal age-matched group of T21 fetuses (n = 496). Samples with normal karyotypes were matched for maternal age and served as normal controls (n = 496). The chi(2) testing was used for statistical analysis.

RESULTS

The presence of an abnormal first-trimester screen, abnormal sonographic findings, and specifically the single sonographic abnormalities of either a cystic hygroma or a cardiac anomaly were significantly less common in the mosaic-T21 as compared with the T21 group. There were no such differences between the mosaic-T21 and the normal control group.

CONCLUSION

Fetuses with mosaic-T21, similar to those with normal karyotype, do not present with the same abnormal screening tests as fetuses with T21.

Complete discrepancy between QF-PCR analysis of uncultured villi and karyotyping of cultured cells in the prenatal diagnosis of trisomy 21 in three CVS

OBJECTIVE

To investigate complete discrepancies in the prenatal diagnosis of trisomy 21 between QF-PCR analysis of uncultured villi and karyotyping of cultured cells in three chorion villus samples.

METHODS

Clinical details were obtained from all three patients. Follow-up studies were undertaken where possible by evaluation of chromosome 21 copy number with QF-PCR, interphase FISH, MLPA and karyotyping, and by post-mortem examination.

RESULTS

Case 1: severe oligohydramnios and microcephaly on scan. QF-PCR: trisomy 21; MLPA: trisomy 21; cultured karyotype: 46,XY[48]. Placental and fetal tissue results and post-mortem examination indicated a euploid fetus with trisomy 21 mosaicism confined to the placenta. Case 2: Down screen risk 1:16; NT = 4.4 mm; absent nasal bone (Caucasian mother). QF-PCR: disomy 21; cultured karyotype: 47,XY,+ 21[23]. Neck thickening noted at delivery-post-mortem refused, no fetal tissue available. Placental tissue indicated mosaicism for trisomy 21. Case 3: Down screen risk 1:91; NT = 6.7 mm. QF-PCR: disomy 21; cultured karyotype: 46,XX,der(21;21)(q10;q10)[60]. No follow-up possible. PCR genotyping of cultured cells confirmed sample identity in all three cases. Chromosome 21 markers observed by PCR were biallelic in all three cases, indicating that a mitotic error could account for the presence of the abnormal cell lines in each case.

CONCLUSION

QF-PCR analysis of uncultured villi and cultured karyotyping may rarely show complete discrepancy in the prediction of fetal trisomy 21 in CVS. Within-biopsy sample mosaicism, together with the testing of different cell populations, provide an explanation for these results. Practical ways to minimise the risk of such discrepancy are proposed.

Very early prenatal diagnosis of genetic diseases based on coelomic fluid analysis: a feasibility study

BACKGROUND

Coelocentesis may represent the ideal technique for very early prenatal diagnosis. Although cell density in coelomic fluid (CF) is very low, the results of analyses on the cellular compartment have been proposed for prenatal diagnosis.

METHODS AND RESULTS

We aimed to evaluate the amount of total DNA (i.e. cellular and cell-free) in 14 samples (0.4-0.8 ml) of CF, taken from women at 8- to 9-week gestation, who are about to undergo termination of pregnancy, and to assess the feasibility of multiple single-gene analyses using multiplex real-time PCR. We found that the amount of total DNA in the CF was very low and varied widely. Genetic testing using multiplex real-time PCR was successfully achieved in 10 of 14 samples (71%). However, when considering samples that could provide a reliable prenatal diagnosis (i.e. successful PCR analysis and no marked maternal contamination), reliable CF-DNA-based prenatal diagnoses were obtained in only 8 of the 14 (58%) samples.

CONCLUSION

The development of highly reliable procedures adapted to pauci-cellular CF is crucially needed before coelocentesis could be proposed for early prenatal diagnosis of genetic diseases before 10 weeks.

Postzygotic isochromosome formation as a cause for false-negative results from chorionic villus chromosome examinations

OBJECTIVE

To investigate the origin and mechanisms of formation of isochromosomes 13q and 21q in instances where prenatal chromosome examination revealed a normal karyotype while postnatal chromosome examination from blood showed translocation trisomy 13 and 21.

METHODS

G and/or Q-banded chromosome examinations from CVS cultures and lymphocyte chromosome examinations from two newborns. Microsatellite marker analysis of DNA from the probands and their parents. Prenatal ultrasonic examinations of the fetuses and postnatal clinical examinations of the probands.

RESULTS

Short and long-term CVS examinations from two fetuses revealed normal karyotypes. Lymphocyte karyotypes of the newborns showed the karyotype 46,XY,i(21)(q10) in the first case and 46,XY,i(13)(q10) in the second. The isochromosomes 21q and 13q were shown, by microsatellite marker analysis of the patients and their parents, to be of maternal and paternal origin, respectively.

CONCLUSION

Postzygotic isochromosome formation is one of the possible mechanisms that may lead to false-negative results of chorionic villus chromosome examinations, even if both short-term and long-term cultures are performed and give normal results.

Sex-specific chromosome instability in early human development

The predominance of females segregating chromosome aberrations to their offspring has been explained mostly by selection disadvantage of unbalanced products of spermatogenesis. However, analysis of data from the literature supports the idea that somatic cells of early female embryos are similar to female germ cells in that they are prone to malsegregation. The goal of this study was to compare the sex ratio (male to female ratio) of carriers of presumably mitotic-occurring chromosome abnormalities to identify any sex biases. In examining the literature, we found a female prevalence in cases of mosaicism associated with uniparental disomy (UPD) (26 male individuals/conceptions and 45 female individuals/conceptions, sex ratio is 0.58, significantly different from 1.06 in newborn population, P = 0.0292). This predominance was highest at gestational age <16 week (8 male and 22 female conceptuses, sex ratio is 0.36, significantly different from expected figure of 1.28, P = 0.0025), which diminished at later stages of fetal development indicating potential correction of trisomies predominantly in females. There is a threefold prevalence of 46,XX/45,X mosaics over 46,XY/45,X mosaics in prenatally diagnosed cases, which also suggests a gender-specific postzygotic chromosome loss. The male prevalence in Prader-Willi syndrome with maternal UPD of chromosome 15 also can be explained by sex-specific trisomy correction, with predominant loss of a maternal chromosome causing biparental inheritance and therefore, complete correction of trisomy in females (without UPD). Finally, there is a female predominance in carriers of chromosome rearrangement with pericentromere break (mosaicism for Robertsonian translocation/isochromosome, centric fission, nonacrocentric isochromosome, and whole arm rearrangement), in both prenatal (21 males and 36 females, sex ratio is 0.58, P < 0.0184) and postnatal ill-defined cases (14 males and 35 females, sex ratio is 0.40, P = 0.001). Thus, the findings presented in this paper suggest that, in addition to reduction in male fertility, and to probable selection against abnormal cell line(s), there are two mechanisms that contribute to female preponderance among carriers of mosaicism: sex-specific chromosome loss and sex-specific centromere instability. The data obtained suggest that females may have gonadal mosaicism for aneuploidies and structural rearrangements more often than males. This may lead to the maternal origin bias in offspring with trisomies or structural rearrangements.