Prenatal Diagnosis of Chromosomal Mosaicism in 18,369 Cases of Amniocentesis

OBJECTIVE

 The prenatal diagnosis of chromosomal mosaicism is fraught with uncertainty. Karyotyping, chromosomal microarray analysis (CMA), and fluorescence in situ hybridization (FISH) are three commonly used techniques. In this study, we evaluated these techniques for the prenatal diagnosis of chromosomal mosaicism and its clinical outcome.

STUDY DESIGN

 A retrospective review of mosaicism was conducted in 18,369 pregnant women from January 2016 to November 2021. The subjects underwent amniocentesis to obtain amniotic fluid for G-band karyotyping with or without CMA/FISH. Cases diagnosed with chromosomal mosaicism were selected for further analysis.

RESULTS

 In total, 101 cases of chromosomal mosaicism were detected in 100 pregnant women (0.54%, 100/18,369). Four were lost during follow-up, 61 opted to terminate their pregnancy, and 35 gave birth to a healthy singleton or twins. Among these 35 cases, postnatal cytogenetic testing was performed on eight and two exhibited mosaicism; however, nothing abnormal was observed in the postnatal phenotype follow-up. Karyotyping identified 96 incidents of chromosomal mosaicism including 13 with level II mosaicism and 83 with level III mosaicism, FISH identified 37 cases of mosaicism, and CMA identified 17. The most common form of chromosomal mosaicism involved monosomy X, of which the mosaic fraction in cultured karyotyping appeared higher or comparable to uncultured FISH/CMA (p < 0.05). Discordant mosaic results were observed in 34 of 101 cases (33.7%), most of which resulted from the detection limit of different techniques and/or the dominant growth of a certain cell line.

CONCLUSION

 Based on the postnatal follow-up results from the babies born, we obtained a more hopeful result for the prognosis of chromosomal mosaicism. Although karyotyping was the most sensitive method for detecting chromosomal mosaicism, artifacts and bias resulting from culture should be considered, particularly for sex chromosomal abnormalities involving X monosomy, in which the combination with uncultured FISH was necessary.

KEY POINTS

· Karyotyping combined with uncultured FISH or CMA is beneficial for prenatal diagnosis of chromosomal mosaicism.. · Fetuses without ultrasound structural anomalies with chromosomal mosaicism often have optimistic prognosis..

Clinical, Cytogenetic and Molecular Cytogenetic Outcomes of Cell-Free DNA Testing for Rare Chromosomal Anomalies

The scope of cell-free DNA (cfDNA) testing was expanded to the genome, which allowed screening for rare chromosome anomalies (RCAs). Since the efficiency of the test for RCAs remains below the common aneuploidies, there is a debate on the usage of expanded tests. This study focuses on the confirmatory and follow-up data of cases with positive cfDNA testing for RCAs and cases with screen-negative results in a series of 912 consecutive cases that underwent invasive testing following cfDNA testing. Chorion villus sampling (CVS), amniocentesis (AS), fetal blood sampling, and term placenta samples were investigated using classical cytogenetic and molecular cytogenetic techniques. Out of 593 screen-positive results, 504 (85%) were for common aneuploidies, 40 (6.7%) for rare autosomal trisomies (RATs), and 49 (8.3%) for structural chromosome anomalies (SAs). Of the screen-positives for RATs, 20 cases were evaluated only in fetal tissue, and confined placental mosaicism (CPM) could not be excluded. Among cases with definitive results (n = 20), the rates of true positives, placental mosaics, and false positives were 35%, 45%, and 10%, respectively. Among screen-positives for SAs, 32.7% were true positives. The confirmation rate was higher for duplications than deletions (58.3% vs. 29.4%). The rate of chromosomal abnormality was 10.9% in the group of 256 screen-negatives with pathological ultrasound findings. This study provides further data to assess the efficiency of expanded cfDNA testing for RATs and SAs. The test efficiency for cfDNA seems to be higher for duplications than for deletions, which is evidence of the role of expert ultrasound in identifying pregnancies at increased risk for chromosome anomalies, even in pregnancies with screen-negatives. Furthermore, we discussed the efficiency of CVS vs. AC in screen-positives for RATs.

Avoiding a second amniocentesis to corroborate prenatal diagnosis by using refrigerated samples

OBJECTIVE

This study is aimed to probe the usefulness of refrigerated aliquots of amniotic fluid to be used for fluorescence in situ hybridization (FISH) in order to perform an accurate prenatal diagnosis avoiding the risk related to an additional amniocentesis procedure and the psychological stress to the pregnant woman and her family.

METHODS

Non-cultured amniotic fluid (AF) samples were analyzed by FISH. Such samples were divided into two groups. The first one included fresh collected AF (FCAF, N = 30). The second one included refrigerated samples of AF (RSAF, N = 12) to corroborate uncertain chromosome aberration identification obtained by conventional methods. Sample refrigeration did not exceed 18 days.

RESULTS

No differences were found between groups. In the RSAF group, three cases of chromosomal mosaicismo and seven cases of pseudomosaicism were corroborated. No alteration adjudicated to aberrant chromosomal line presence was found in born children according to genetic specialists' criteria. In the two remaining cases, applied procedure allowed elucidating fetal sex in one case and the origin of a marker chromosome in the other.

CONCLUSIONS

Amniocytes obtained from RAFS are a useful biological material to be assayed by FISH, achieving an accurate prenatal diagnosis and avoiding an additional amniocentesis.

Comparable low-level mosaicism in affected and non affected tissue of a complex CDH patient

In this paper we present the detailed clinical and cytogenetic analysis of a prenatally detected complex Congenital Diaphragmatic Hernia (CDH) patient with a mosaic unbalanced translocation (5;12). High-resolution whole genome SNP array confirmed a low-level mosaicism (20%) in uncultured cells, underlining the value of array technology for identification studies. Subsequently, targeted Fluorescence In-Situ Hybridization in postmortem collected tissues demonstrated a similar low-level mosaicism, independently of the affected status of the tissue. Thus, a higher incidence of the genetic aberration in affected organs as lung and diaphragm cannot explain the severe phenotype of this complex CDH patient. Comparison with other described chromosome 5p and 12p anomalies indicated that half of the features presented in our patient (including the diaphragm defect) could be attributed to both chromosomal areas. In contrast, a few features such as the palpebral downslant, the broad nasal bridge, the micrognathia, microcephaly, abnormal dermatoglyphics and IUGR better fitted the 5p associated syndromes only. This study underlines the fact that low-level mosaicism can be associated with severe birth defects including CDH. The contribution of mosaicism to human diseases and specifically to congenital anomalies and spontaneous abortions becomes more and more accepted, although its phenotypic consequences are poorly described phenomena leading to counseling issues. Therefore, thorough follow-up of mosaic aberrations such as presented here is indicated in order to provide genetic counselors a more evidence based prediction of fetal prognosis in the future.

Rapid aneuploidy detection with multiplex ligation-dependent probe amplification: a prospective study of 4000 amniotic fluid samples

The introduction of prenatal screening requires rapid high-throughput diagnosis of common aneuploidies. Multiplex ligation-dependent probe amplification (MLPA) allows for quick, easily automated multiplex testing of these aneuploidies in one polymerase chain reaction. We performed a large prospective study using MLPA on 4000 amniotic fluid (AF) samples including all indications and compared its value to karyotyping and fluorescence in situ hybridization (FISH). MLPA can reliably determine common aneuploidies with 100% sensitivity and 100% specificity. Moreover, some mosaic cases and structural chromosome aberrations were detected as well. In cases of a male fetus, triploidies can be detected by an aberrant pattern of probe signals, which mimics maternal cell contamination (MCC). Macroscopic blood contamination was encountered in 3.2% of the AF samples. In 20% of these samples, an MLPA pattern was found consistent with MCC, although there were no false negatives of the most common aneuploidies. As the vast majority of inconclusive results (1.7%) is due to potential MCC, we designed a protocol in which we determine whether MLPA can be performed on blood-contaminated AF samples by testing if blood is of fetal origin. Then, the number of inconclusive results could be theoretically reduced to 0.05%. We propose an alternative interpretation of relative probe signals for rapid aneuploidy diagnosis (RAD). We discuss the value of MLPA for the detection of (submicroscopic) structural chromosome anomalies. MLPA is a reliable method that can replace FISH and could be used as a stand-alone test for RAD instead of karyotyping.

New approaches to fluorescence in situ hybridization

Fluorescence in situ hybridization (FISH) is a nonisotopic labeling and detection method that provides a direct way to determine the relative location or copy number of specific DNA sequences in nuclei or chromosomes. With recent advancements, this technique has found increased application in a number of research areas, including cytogenetics, prenatal diagnosis, cancer research and diagnosis, nuclear organization, gene loss and/or amplification, and gene mapping. The availability of different types of probe and the increasing number of FISH techniques has made it a widespread and diversely applied technology. Multicolor karyotyping by multicolor FISH and spectral karyotyping interphase FISH and comparative genomic hybridization allow genetic analysis of previously intractable targets. We present a brief overview of FISH technology and describe in detail methods of probe labeling and detection for different types of tissue sample, including microdissected nuclei from formalin-fixed paraffin-embedded tissue sections.

Reevaluating confined placental mosaicism

Chromosomal mosaicism was found in 38 of 4,000 chorionic villus samples examined from 1998 to 2003. A small fraction of these (5/38) were confirmed as true mosaics by analysis of amniotic fluid. Twenty-nine cases that fit the definition of confined placental mosaicism were followed with clinical and cytogenetic analysis throughout the pregnancy, at birth and in a few cases into infancy. This was done to determine the prognostic interpretation of prenatal cytogenetic results from multiple specimens in a single pregnancy and thus allow for reevaluation of the genetic counseling. In 2 of these 29 cases, low-level mosaicism was found in the neonate, and in 1 of these the chromosome abnormality is probably the cause of the resulting minor phenotypic abnormalities. Families face unique difficulties when confined placental mosaicism is the prenatal diagnosis, and it is extremely important that the counseling they receive takes into consideration the unlikely possibility of the placental abnormality appearing in fetal tissues.

Evidence for imprinting on chromosome 16: The effect of uniparental disomy on the outcome of mosaic trisomy 16 pregnancies

Although a number of infants with maternal uniparental disomy of chromosome 16 (upd(16)mat) have been reported, the evidence for imprinting on chromosome 16 is not yet conclusive. To test the hypothesis that upd(16)mat has a distinct phenotype, which would support the existence of imprinted gene(s) on chromosome 16, statistical analysis was performed on a large series (n = 83) of mosaic trisomy 16 cases with molecular determination of uniparental disomy status. The incidence of upd(16)mat was 40%, which is consistent with the expected one third from random chromosome loss during trisomy rescue (P = 0.262). In pairwise comparisons, upd(16)mat was found to be associated with fetal growth restriction (P = 0.029) and with increased risk of major malformation (RR = 1.43; P = 0.053). Regression modeling showed that the effect of upd(16)mat on fetal/neonatal weight and malformation is independent of the degree of trisomy detected in the fetus. Regression modeling to control for the degree of trisomy detected in the placenta was not possible due to limited sample size. We conclude that upd(16)mat is associated with more severe growth restriction, and possibly, with higher risk of malformation. Our hypothesis is that imprinted gene(s) exist on chromosome 16 and that abnormal expression of these gene(s) in upd(16)mat cells during development results in decreased cell proliferation. Although we do not advocate prenatal testing for upd(16), studies on the long-term outcome of upd(16)mat neonates is necessary for counseling purposes.

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.