Herein, we report the prenatal diagnosis and molecular cytogenetic characterization of the mosaicism for sSMC derived from chromosome 2. The indication for the prenatal examination in this case was a high risk of trisomy 2 revealed by NIPT test. Our case underlines the screening nature of the NIPT test and also demonstrates that performing only the FISH analyses in uncultured amniocytes, as the most rapid way to verify the NIPT result, would be insufficient for the correct diagnosis. Therefore, bearing in mind that the fetal and placental karyotypes might be different, a good follow-up of abnormal NIPT results is necessary, but as the investigation showed, techniques as well as the tested tissue should be properly selected.
Chromosomal mosaicism can be associated with a wide spectrum of phenotypes extending from apparently normal to severe or lethal. Hsu and coauthors [17] summarized the outcome of 11 fetuses with whole chromosome trisomy 2 detected in mosaicism, which ranged from 4–33% and has been confirmed in other tissues. Within these cases, 1 with the lowest percentage of abnormal cells (4% in the amniocytes but without trisomy in blood and placenta) resulted in an apparently normal livebirth; 1 newborn presented IUGR, 1 had IUGR and multiple anomalies, 3 stillbirths or intrauterine deaths, and 4 in elective terminations (11–33% mosaic cells, all with abnormal findings).
Clinical consequences in cases of mosaicism of partial trisomy can be harder to predict. Marker chromosomes derivative from chromosome 2, described in the literature, have been associated with multiple clinical consequences. Children diagnosed after birth might present: developmental delay, brain malformations, seizures, heart defect, kidney malformations, hypotonia, mental retardation, growth retardation, microcephaly [18]. However, little is known about prenatal ultrasound abnormalities in such cases.
Microarray analysis defined the gain of 14,83 Mb in the 2q11.1q13 region. Our case is highly similar to a case reported by Riegel and Schinzel [19]. Authors described a 4-year-old boy with multiple clinical features and a duplicated segment of 2q11.1-q13.2 presented in all analyzed cells. Low birth weight (< 10th centile), a left cleft lip with cleft palate and natal left upper incisor were noted at birth. Consecutive examination revealed mental retardation, low set ears, irregular teeth, cryptorchidism and epilepsy. However, also in this case, three ultrasound routine examinations performed prenatally did not disclose abnormal findings. The difference in the clinical presentation between our case and the patient described by Riegel and Schinzel [19], might be due to the percentage of the level of normal cells, but also could be because of different impact of the simple region duplication in the genome versus marker chromosome.
Furthermore, cytogenetic discrepancy between results of uncultured fetal cells (skin 64%, amniocytes 30%) and cultured cells (skin 30%, amniocytes 52%) provides an additional challenge in genetic counselling. FISH analyses in uncultured skin cells presented higher percentage of mosaicism (64%) than was found later in cultured skin cells (30%). The discrepancy has also been observed between uncultured and cultured placenta cells. In uncultured placenta we observed three cell lines; with a normal number of signals from chromosome 2 (39%), trisomy of whole chromosome 2 (40%), and 21% of cells carrying the sSMC. Whereas cultured placenta did not reveal the whole trisomy 2. The variations in cell numbers carrying an abnormality before and after cultivation are compatible with results of other authors and presumably stem from a selection against the trisomic cells after long-term culture [20]. Different values of mosaicism indicated in uncultured (30%) and cultured (52%) amniocytes can be caused by the contamination of amniotic fluid with maternal cells. Moreover, the number of available for analysis metaphases was relatively low (n = 40) in comparison to uncultured nuclei analyzed with FISH (n = 150). The fact that the abnormal cells may divide slower and undergo apoptosis more easily causes an increase in the proportion of normal cells, should be taken into consideration during genetic consultation [21].
In the presented case, based on FISH and GTG-banding results, the sSMC was in fact a small ring chromosome. There is a high probability that this ring chromosome formation is caused by low repetitive elements present in the pericentric region of chromosome 2 [22, 23].
We have observed a discrepancy between the indication for invasive testing, and the final genetic diagnosis results. However, the presence of the trisomy of whole chromosome 2 was confirmed in the placenta. It is probable that the marker chromosome has arisen from trisomic embryo cells, which existed at an initial stage of development. The presence of the marker chromosome mosaicism in subsequent studies points to the existence of a functional trisomy rescue mechanism in this case.
A genetic counselling of fetuses with mosaicism is especially problematic because of the relatively poor phenotypic data and time-limitation. Considering the fate of the pregnancy in the case of detecting abnormalities, NIPT can be assessed as a screening test, and should be accompanied by an ultrasound examination. Hence, invasive diagnosis is necessary to confirm the non-invasive results. In the presented case, it should be emphasized that there is a discrepancy between the result of the NIPT study and the results of the genetic diagnostic tests performed in the fetal tissues. Therefore it is crucial to choose the most suitable investigation strategy in order to perform the most rapid genetic diagnosis.