Trisomy 15 mosaicism owing to familial reciprocal translocation t(1;15): implication for prenatal diagnosis

Rare autosomal trisomies detected by non-invasive prenatal testing: an overview of current knowledge

Non-invasive prenatal testing has been introduced for the detection of Trisomy 13, 18, and 21. Using genome-wide screening also other "rare" autosomal trisomies (RATs) can be detected with a frequency about half the frequency of the common trisomies in the large population-based studies. Large prospective studies and clear clinical guidelines are lacking to provide adequate counseling and management to those who are confronted with a RAT as a healthcare professional or patient. In this review we reviewed the current knowledge of the most common RATs. We compiled clinical relevant parameters such as incidence, meiotic or mitotic origin, the risk of fetal (mosaic) aneuploidy, clinical manifestations of fetal mosaicism for a RAT, the effect of confined placental mosaicism on placental function and the risk of uniparental disomy (UPD). Finally, we identified gaps in the knowledge on RATs and highlight areas of future research. This overview may serve as a first guide for prenatal management for each of these RATs.

Cytogenetic discrepancy between uncultured amniocytes and cultured amniocytes in mosaic trisomy 15 at amniocentesis

OBJECTIVE

We present mosaic trisomy 15 at amniocentesis.

MATERIALS AND METHODS

A 41-year-old woman underwent amniocentesis at 16 weeks of gestation because of an abnormal non-invasive prenatal testing (NIPT) result suspicious of trisomy 15. Amniocentesis revealed a karyotype of 46,XY. Array comparative genomic hybridization (aCGH) on uncultured amniocytes revealed 26% mosaicism for trisomy 15. She was referred for repeat amniocentesis. aCGH, interphase fluorescence in situ hybridization (FISH), quantitative fluorescent polymerase chain reaction (QF-PCR) assays and/or conventional cytogenetic analysis were applied on various cells and tissues including uncultured amniocytes, cultured amniocytes, cord blood, placenta, parental bloods and/or buccal mucosal cells.

RESULTS

Repeat amniocentesis at 21 weeks of gestation revealed a karyotype of 46, XY in cultured amniocytes, and 30% mosaicism for trisomy 15 by aCGH and 32% mosaicism for trisomy 15 by FISH in uncultured amniocytes. Repeat amniocentesis at 29 weeks of gestation revealed a karyotype of 46, XY in cultured amniocytes, and 15% mosaicism for trisomy 15 by aCGH and 7.2% mosaicism for trisomy 15 by FISH in uncultured amniocytes. QF-PCR on cultured amniocytes excluded uniparental disomy (UPD) 15. A phenotypically normal baby was delivered subsequently with a karyotype of 46, XY in cord blood and 2% mosaicism for trisomy 15 by FISH in buccal mucosal cells. The aCGH analysis revealed trisomy 15 in placenta and no genomic imbalance in cord blood. QF-PCR assays determined a maternal origin of trisomy 15 in placenta.

CONCLUSION

Cytogenetic discrepancy may occur between uncultured and cultured amniocytes in mosaic trisomy 15 at amniocentesis. The cells of trisomy 15 cell line in prenatally detected mosaic trisomy 15 may decrease in number as the fetus grows. Whenever NIPT suspects trisomy 15, a confirmatory amniocentesis should include genetic analysis on both uncultured and cultured amniocytes to exclude mosaic trisomy 15 and maternal UPD 15, especially when the cultured amniocytes have a normal karyotype.

Delineating the Mosaic Trisomy 15 Phenotype Using a Serendipitous Mechanism as a Clue

Parental balanced translocation is one of the traditional indications for invasive prenatal diagnosis. Usually, the diagnostic process is straightforward. Sometimes, however, results are not entirely clear and may reveal unexpected biological processes. We performed chorionic villi sampling for a paternal 8;15 reciprocal translocation in the sixth pregnancy of a Caucasian woman. Cytogenetic analysis of chorionic villi, after both short- and long-term cultures, revealed the presence of the same rearrangement found in the father as well as a trisomy 15. Surprisingly, the trisomy, which was initially expected to derive from aberrant segregation during paternal meiosis, resulted instead from maternal nondisjunction. Although a sonogram of the fetus appeared to be normal, follow-up amniocentesis demonstrated a low-level mosaic trisomy 15 in cells extracted from the amniotic fluid, while 10% of cells from fetal tissues sampled after termination of the pregnancy were also found to be trisomic. Fetal autopsy showed dysmorphic features, confirming the diagnosis of mosaic trisomy 15 and enabled deeper insight into the prenatal phenotype of this rare condition.

Candidate genes for congenital diaphragmatic hernia from animal models: sequencing of FOG2 and PDGFRα reveals rare variants in diaphragmatic hernia patients

Congenital diaphragmatic hernia (CDH) is a common, life threatening birth defect. Although there is strong evidence implicating genetic factors in its pathogenesis, few causative genes have been identified, and in isolated CDH, only one de novo, nonsense mutation has been reported in FOG2 in a female with posterior diaphragmatic eventration. We report here that the homozygous null mouse for the Pdgfralpha gene has posterolateral diaphragmatic defects and thus is a model for human CDH. We hypothesized that mutations in this gene could cause human CDH. We sequenced PDGFRalpha and FOG2 in 96 patients with CDH, of which 53 had isolated CDH (55.2%), 36 had CDH and additional anomalies (37.5%), and 7 had CDH and known chromosome aberrations (7.3%). For FOG2, we identified novel sequence alterations predicting p.M703L and p.T843A in two patients with isolated CDH that were absent in 526 and 564 control chromosomes respectively. These altered amino acids were highly conserved. However, due to the lack of available parental DNA samples we were not able to determine if the sequence alterations were de novo. For PDGFRalpha, we found a single variant predicting p.L967V in a patient with CDH and multiple anomalies that was absent in 768 control chromosomes. This patient also had one cell with trisomy 15 on skin fibroblast culture, a finding of uncertain significance. Although our study identified sequence variants in FOG2 and PDGFRalpha, we have not definitively established the variants as mutations and we found no evidence that CDH commonly results from mutations in these genes.