A placental trisomy 2 detected by NIPT evolved in a fetal small Supernumerary Marker Chromosome (sSMC)

BACKGROUND

Non-invasive prenatal testing (NIPT) is a rapidly developing and widely used method in the prenatal screening. Recently, the widespread use of the NIPT caused a neglecting of the limitations of this technology.

CASE PRESENTATION

The 38-year-old woman underwent amniocentesis because of a high risk of trisomy 2 revealed by the genome-wide Non-Invasive Prenatal Test (NIPT). The invasive prenatal diagnosis revealed the mosaicism for a small supernumerary marker chromosome sSMC derived from chromosome 2. Interphase fluorescence in situ hybridization (FISH) on uncultured amniocytes revealed three signals of centromere 2 in 30% of the cells. GTG-banded metaphases revealed abnormal karyotype (47,XX,+mar[21]/46,XX[19]) and was confirmed by array comparative genomic hybridization (aCGH). Cytogenetic analyses (FISH, aCGH, karyotype) on fetal skin biopsies were performed and confirmed the genomic gain of the centromeric region of chromosome 2. In the placenta, three cell lines were detected: a normal cell line, a cell line with trisomy 2 and a third one with only the sSMC.

CONCLUSION

Whole-genome Non-Invasive Prenatal Testing allows not only the identification of common fetal trisomies but also diagnosis of rare chromosomal abnormalities. Especially in such cases, it is extremely important to perform not only NIPT verification on a sample of material other than trophoblast, but also to apply appropriate research methods. Such conduct allows detailed analysis of the detected aberration, thus appropriate clinical validity.

[Usefulness of MLPA technique for rapid prenatal detection of aneuploidy. Results of 409 diagnostic studies]

OBJECTIVES

The study was aimed to determine diagnostic application of MLPA for rapid prenatal identification of chromosome 13, 18, 21 and X and Y aneuploidies.

MATERIAL AND METHODS

409 amniotic fluid samples from amniocentesis for fetal karyotyping were studied. DNA was isolated using the QIAmp DNA Blood Midi Kit (348 samples) or through proteinase K treatment (61 samples). SALSA MLPA P095 probes (mrc-Holland) were used to detect aneuploidy

RESULTS

In 324 studies (79.2%) diagnostic results were obtained. Chromosomal aberrations were found in 16 cases (4.9%). These results were concordant with standard karyotype. In 3 cases (0.92%) false negative results were found but all abnormalities were undetectable with MLPA.

CONCLUSIONS

MLPA is a reliable method of rapid prenatal detection of aneuploidy

[Results of 1043 prenatal cytogenetic studies: retrospective study in the context of applicability of interphase FISH in prenatal diagnosis]]

OBJECTIVES

The risk of aneuploidy in a fetus is the main reason for referral in approximately 80% of prenatal studies. Recently, a new method for rapid detection of the most frequent aneuploidies affecting chromosomes 13, 18, 21, X and Y has been developed. Fluorescence in situ hybridisation (FISH) on uncultured fetal cells with probes specific for these chromosomes has been described which enables diagnosing aneuploidies within 24 to 48 hours. The purpose of the study was evaluation of clinical utility of this new method in prenatal diagnostics.

MATERIALS AND METHODS

Retrospective analysis of the results of 1043 prenatal cytogenetic studies performed with conventional banding methods was done. Number and type of chromosomal abnormalities found in different categories of indications with special emphasis on aberrations undetectable by FISH were analysed.

RESULTS

Chromosomal aberrations were found in 4.7% studies. The frequency of aneuploidies was 1.8% accounting for 35.8% of all diagnosed chromosomal abnormalities. In the group of 854 studies performed for elevated risk of aneuploidy it accounted for 60.7% (17/28) abnormalities. All other aberrations could not be detected by FISH with probes for most frequent aneuploidies. Among them, there were 6 unbalanced: del (8) and pseudic (15) with known abnormal phenotype and 4 marker chromosomes with unknown clinical consequences. Three other abnormalities were balanced but familial origin of them was documented.

CONCLUSIONS

A karyotype using classical banding methods should be performed whatever the indication of prenatal study is. It is the only fully informative method able to detect all chromosomal abnormalities. Interphase FISH assay must be considered as a complementary procedure to fetal karyotype analysis as it is designed only for aneuploidy identification. However it may be a very useful method for rapid diagnosis in specific clinical conditions especially in the cases of high risk of aneuploidy.

Cytogenetic and molecular characterization of two isodicentric Y chromosomes

We report the results of detailed molecular-cytogenetic studies of two isodicentric Y [idic(Y)] chromosomes identified in patients with complex mosaic karyotypes. We used fluorescence in situ hybridization (FISH) and polymerase chain reaction (PCR) to determine the structure and genetic content of the abnormal chromosomes. In the first patient, classical cytogenetics and FISH analysis with Y chromosome-specific probes showed in peripheral blood lymphocytes a karyotype with 4 cell lines: 45,X[128]/46,X,+idic(Y)(p11.32)[65]/47,XY,+idic(Y)(p11.32)[2]/47,X,+2idic(Y)(p11.32)[1]. No Y chromosome material was found in the removed gonads. For precise characterization of the Yp breakpoint, FISH and fiberFISH analysis, using a telomeric probe and a panel of cosmid probes from the pseudoautosomal region PAR1, was performed. The results showed that the breakpoint maps approximately 1,000 Kb from Ypter. The second idic(Y) chromosome was found in a boy with mild mental retardation, craniofacial anomalies, and the karyotype in lymphocytes 47,X,+idic(Y)(q11.23),+i(Y)(p10)[77]/46,X,+i(Y)(p10)[23]. To our knowledge, such an association has not been previously described. FISH and PCR analysis indicated the presence of at least two copies of the SRY gene in all analyzed cells. Using 17 PCR primers, the Yq breakpoint was shown to map between sY123 (DYS214) and sY121 (DYS212) loci in interval 5O in AZFb region. Possible mechanisms of formation of abnormal Y chromosomes and karyotype-phenotype correlations are discussed.

Identification of supernumerary marker chromosomes derived from chromosomes 5, 6, 19, and 20 using FISH

A large number of cases with supernumerary marker chromosomes (SMCs) should be compared to achieve a better delineation of karyotype-phenotype correlations. Here we present four phenotypically abnormal patients with autosomal marker chromosomes analysed by fluorescence in situ hybridisation using centromeric, telomeric, and unique sequence probes, as well as forward and reverse painting. We also report the first case, to the best of our knowledge, of an SMC derived from chromosome 5. Furthermore, a marker chromosome 20 in a patient with sex differentiation abnormalities, a double mar(6) in a boy with psychomotor retardation, and the association of r(19) with dup(21q21.2q22.12) are described. Although the mar(6) was very small, the presence of euchromatin was shown, suggesting that the partial trisomy of pericentric region derived sequences is implicated in the aetiology of the abnormal phenotypes.