Trisomy 9 confined to the placenta: prenatal diagnosis and neonatal follow-up

Confined placental mosaicism and the association with pregnancy outcome and fetal growth: a review of the literature

BACKGROUND

Chromosomal mosaicism can be detected in different stages of early life: in cleavage stage embryos, in blastocysts and biopsied cells from blastocysts during preimplantation genetic testing for aneuploidies (PGT-A) and later during prenatal testing, as well as after birth in cord blood. Mosaicism at all different stages can be associated with adverse pregnancy outcomes. There is an onward discussion about whether blastocysts diagnosed as chromosomally mosaic by PGT-A should be considered safe for transfer. An accurate diagnosis of mosaicism remains technically challenging and the fate of abnormal cells within an embryo remains largely unknown. However, if aneuploid cells persist in the extraembryonic tissues, they can give rise to confined placental mosaicism (CPM). Non-invasive prenatal testing (NIPT) uses cell-free (cf) DNA released from the placenta in maternal blood, facilitating the detection of CPM. In literature, conflicting evidence is found about whether CPM is associated with fetal growth restriction (FGR) and/or other pregnancy outcomes. This makes counselling for patients by clinicians challenging and more knowledge is needed for clinical decision and policy making.

OBJECTIVE AND RATIONALE

The objective of this review is to evaluate the association between CPM and prenatal growth and adverse pregnancy outcomes. All relevant literature has been reviewed in order to achieve an overview on merged results exploring the relation between CPM and FGR and other adverse pregnancy outcomes.

SEARCH METHODS

The following Medical Subject Headings (MESH) terms and all their synonyms were used: placental, trophoblast, cytotrophoblast, mosaicism, trisomy, fetal growth, birth weight, small for gestational age and fetal development. A search in Embase, PubMed, Medline Ovid, Web of Science, Cochrane Central Register of Controlled Trials (CENTRAL) and Google Scholar databases was conducted. Relevant articles published until 16 July 2020 were critically analyzed and discussed.

OUTCOMES

There were 823 articles found and screened based on their title/abstract. From these, 213 articles were selected and full text versions were obtained for a second selection, after which 70 publications were included and 328 cases (fetuses) were analyzed. For CPM in eight different chromosomes (of the total 14 analyzed), there was sufficient evidence that birth weight was often below the 5th percentile of fetal growth standards. FGR was reported in 71.7% of CPM cases and preterm birth (<37 weeks of delivery) was reported in 31.0% of cases. A high rate of structural fetal anomalies, 24.2%, in cases with CPM was also identified. High levels of mosaicism in CVS and presence of uniparental disomy (UPD) were significantly associated with adverse pregnancy outcomes.

WIDER IMPLICATIONS

Based on the literature, the advice to clinicians is to monitor fetal growth intensively from first trimester onwards in case of CPM, especially when chromosome 2, 3, 7, 13, 15, 16 and 22 are involved. In addition to this, it is advised to examine the fetuses thoroughly for structural fetal anomalies and raise awareness of a higher chance of (possibly extreme) premature birth. Despite prematurity in nearly a fifth of cases, the long-term follow-up of CPM life borns seems to be positive. More understanding of the biological mechanisms behind CPM will help in prioritizing embryos for transfer after the detection of mosaicism in embryos through PGT-A.

Antenatal detection of mosaic trisomy 9 by ultrasound: a case report and literature review

This paper presents a fetus with mosaic trisomy 9 diagnosed by chorionic villus sampling and confirmed by cordocentesis, and compares this case with published cases in order better to define the ultrasound markers confined to trisomy 9 syndrome. Detailed fetal ultrasound examination was carried out, revealing shortened femur, placental cysts and oligohydramnios. All published trisomy 9 cases with abnormal ultrasound findings were extracted from the MEDLINE database in the period from 1973 to 2002. We found 12 non-mosaic and 13 mosaic cases, including our case. The most frequent ultrasound abnormalities included characteristic cardiac, skeletal, craniofacial and central nervous system malformations. Intrauterine growth restriction and single umbilical artery were prevalent non-specific findings in both non-mosaic and mosaic groups. Parental chromosomal variations, as in our case, were not uncommon findings. When a fetus shows structural anomalies suggesting the presence of trisomy 9, karyotyping should be performed on both chorionic villi or amniocytes and fetal blood lymphocytes to enable a correct diagnosis to be made.

Chorionic villus sampling

Chorionic villus sampling (CVS) has been used a successful and safe first-trimester prenatal diagnostic technique for over 12 years. Developed to avoid the medical and psychological complications of later prenatal diagnosis by amniocentesis, CVS rapidly has become a primary tool for the diagnosis of fetal cytogenetic, molecular, and biochemical disorders. In addition, its development has led to an improved understanding of several biological processes, including confined placental mosaicism and uniparental disomy.

Prenatal diagnosis of trisomy 9. Six cases and a review of the literature

Six prenatally diagnosed cases of trisomy 9 are reported and 22 previously reported cases are reviewed; the difficulty of genetic counselling for such cases and the variation in the percentage of trisomic cells in different tissues, thus making accurate diagnosis of trisomy 9 difficult, are emphasized. In addition to karyotyping results, ultrasound findings are important in achieving diagnoses. Finally, a course of action when prenatal trisomy 9 is detected is proposed.

Confined placental chimerism: prenatal and postnatal cytogenetic and molecular analysis, and pregnancy outcome

The presence of two cell lines in chorionic villi sampling (CVS) represents a significant complication in CVS analysis, interpretation, and counseling. We report on the cytogenetic and molecular analysis of a pregnancy that was conceived on clomiphene citrate. Two cell lines (46,XX and 47,XY,+9) were discovered in CVS analysis done for maternal age; 94% of the cells in the culture were 46,XX and 6% were 47,XY, +9 (the direct preparation was 46,XX). As neither line could have derived from the other, chimerism and not mosaicism was suspected, with the 47,XY,+9 cells deriving from a co-twin whose demise was the result of the autosomal trisomy. At a subsequent amniocentesis, only normal female cells were observed and a normal female infant was delivered at term. Cytogenetic analysis done on the infant's peripheral blood and on a sample of an umbilical vessel showed only 46,XX cells, while amnion and a fibrotic area of the placenta contained 2 cell lines, 46,XX and 47,XY,+9. Molecular analysis of 3 different tissues was done by the polymerase chain reaction (PCR) and Southern blotting, using Y specific primers and probes, respectively. The presence of Y specific DNA was detected in the placenta and amnion, but not in the umbilical blood vessel. These data excluded true chimerism in the fetal tissues at the level of about 1 in 10(5) cells and have defined for the first time probable confined placental chimerism (CPC), the result most likely of a "vanishing twin." Whenever two cell lines are found in CVS, especially in the setting of pharmacologically stimulated ovulation, the possibility of CPC should be considered. The effects of CPC on placental function and fetal outcome merit further study.

Cytogenetic diagnoses after chorionic villus sampling are less reliable in very-high-or very-low-risk pregnancies

An increasing number of cytogenetic prenatal diagnoses are performed on chorionic villus samplings. The accuracy of this method is influenced by chromosomal mosaicism, mostly confined to direct preparation methods. Especially those investigators who have experienced false-negative and false-positive findings propagate the combined use of direct and culture methods. Yet large collaborative studies have shown that in approximately two-thirds of diagnostic cases only one procedure is applied. Moreover, the accuracy of a cytogenetic investigation depends not only on the ontogenetic origin of the tissues investigated, but also on interacting factors such as the 'a priori risk' and the 'predictive value of a cytogenetic finding'. On this basis a differentiated prenatal diagnostic procedure is discussed, including either sole short-term culture (STC), combined STC and long-term culture (LTC), primary amniocentesis (AC), or primary percutaneous umbilical blood sampling (PUBS). The predictive value of the cytogenetic diagnosis from CVS varies significantly dependent on the a priori risk of a chromosome aberration and, in the case of an abnormal karyotype, on the specific chromosome involved. A non-mosaic and 'non-lethal' trisomy detected in STC is highly representative of the embryo/fetus, but there are exceptions of limited predictive value, e.g., trisomy 18. Guided by the strategy of an optional follow-up by LTC, AC, or PUBS in 1317 successive CV samplings, we are not aware of a false-negative diagnosis, but probably had one false-positive diagnosis: 47,XXY after STC; 46,XY after LTC. When referring to the rate of fetuses with an unbalanced karyotype expected in the different indication groups, a relative increase of false-positive findings in the very-low-risk group (maternal age < or = 35 years of age) and of false-negative findings in the very-high-risk group (abnormal ultrasonographic findings) of pregnant women when only performing CVS becomes obvious. Because of this dilemma, AC or--especially in the latter group--PUBS might be primarily offered to these indication groups instead of CVS.