Fetoplacental mosaicism: potential implications for false-positive and false-negative noninvasive prenatal screening results.

PURPOSE: Noninvasive prenatal screening for fetal aneuploidy analyzes cell-free fetal DNA circulating in the maternal plasma. Because cell-free fetal DNA is mainly of placental trophoblast origin, false-positive and false-negative findings may result from placental mosaicism. The aim of this study was to calculate the potential contribution of placental mosaicism in discordant results of noninvasive prenatal screening. METHODS: We performed a retrospective audit of 52,673 chorionic villus samples in which cytogenetic analysis of the cytotrophoblast (direct) and villus mesenchyme (culture) was performed, which was followed by confirmatory amniocentesis in chorionic villi mosaic cases. Using cases in which cytogenetic discordance between cytotrophoblast and amniotic fluid samples was identified, we calculated the potential contribution of cell line-specific mosaicism to false-positive and false-negative results of noninvasive prenatal screening. RESULTS: The false-positive rate, secondary to the presence of abnormal cell line with common trisomies in cytotrophoblast and normal amniotic fluid, ranged from 1/1,065 to 1/3,931 at 10% and 100% mosaicism, respectively; the false-negative rate was calculated from cases of true fetal mosaicism, in which a mosaic cell line was absent in cytotrophoblast and present in the fetus; this occurred in 1/107 cases. CONCLUSION: Despite exciting advances, underlying biologic mechanisms will never allow 100% sensitivity or specificity.

De novo small supernumerary marker chromosomes detected on 143,000 consecutive prenatal diagnoses: chromosomal distribution, frequencies, and characterization combining molecular cytogenetics approaches.

OBJECTIVE: The risk of clinical consequences in prenatal cases with de novo small supernumerary marker chromosomes (sSMC), often in mosaic conditions, is not easy to predict, which results in difficulties in genetic counseling. METHOD: In this study, we evaluated the frequency, the chromosomal origin, and the clinical indication of 104 de novo sSMC detected in a monocenter survey on the basis of 143,000 consecutive prenatal diagnoses, and we assessed the reliability of molecular cytogenetics technologies for sSMC characterization. RESULTS: We detected a de novo sSMC frequency of 0.072%. Its incidence in advanced maternal age group is statistically different from that found in maternal anxiety indication (<35 years old). A higher prevalence of mosaicism in chorionic villi sampling (CVS) than in amniotic fluids was also revealed related to confined placental mosaicisms. The risk of confirmation in amniotic fluids of mosaics previously revealed at CVS was 33.3%. No uniparental disomy conditions were found when imprinted chromosomes were involved in the occurrence of de novo sSMC. The majority of de novo sSMC were acrocentric derived-chromosomes, and a neocentromere formation was observed in one pregnancy. CONCLUSION: Our data support that array comparative genomic hybridization has improved sSMC characterization and demonstrate its utility in supporting genetic counseling. We propose a workflow for de novo sSMC characterization.

QF-PCR as a substitute for karyotyping of cytotrophoblast for the analysis of chorionic villi: advantages and limitations from a cytogenetic retrospective audit of 44,727 first-trimester prenatal diagnoses.

OBJECTIVES: Karyotyping on chorionic villous samples (CVS) includes the analysis of both cytotrophoblast (STC) and mesenchyme (LTC). This approach requires complex laboratory organization and trained technicians. The introduction of quantitative fluorescent polymerase chain reaction (QF-PCR) instead of conventional karyotyping in low-risk pregnancies opened its application in CVS analysis. Discordant QF-PCR and CVS cytogenetic results were reported, and strategies for CVS analysis were introduced to minimize this risk. The possibility to substitute the STC with QF-PCR was reported. The aim of this study is to evaluate benefits and limitations of the approach QF-PCR + LTC compared with the traditional method STC + LTC and to quantify the associated risks of false results. METHOD: This study is based on a retrospective cytogenetic audit of CVS results (n = 44 727) generated by the STC + LTC analytic approach. False-negative risks related to true fetal mosaicism type IV, imprinting syndromes and maternal contamination in LTC were calculated. RESULTS: Compared with STC + LTC, QF-PCR + LTC approach is associated with a cumulative false-negative risk of ~1/3100-1/4400. Costs and reporting time of STC in a high-throughput cytogenetic lab are similar to a CE-IVD marked QF-PCR analysis. CONCLUSIONS: These results should be clearly highlighted in the pre-test counseling and extensively discussed with the couple prior to testing for informed consent.

Chromosome abnormalities investigated by non-invasive prenatal testing account for approximately 50% of fetal unbalances associated with relevant clinical phenotypes.

During the past 20 years non-invasive screening tests have been increasingly utilized in prenatal diagnosis (PD) practice. Considerable effort has been exerted by multicenter consortia to evaluate the reliability of non-invasive screening tests in detecting those women with an increased risk of having a pregnancy affected by trisomies 21, 18, and 13, monosomy X, and triploidies. To what extent this group of abnormal karyotypes accounts for the total number of phenotypically relevant fetal chromosome abnormalities has, however, never been investigated. The present report is an attempt aimed to quantify this proportion. A retrospective analysis of a homogeneous survey of 115,128 consecutive invasive prenatal tests was undertaken. All cases were classified in accordance with the indication given for the invasive testing. Cytogenetic results regarding 96,416 karyotype analyses performed because of advanced maternal age (>or=35 years) or gestational anxiety (<35 years) were considered since these are the patients who usually undergo non-invasive screening tests. We calculated the number of cases (T21, T18, T13, 45,X, and triploidy) that would have been detected by prenatal screening on the basis of the published detection rate of the combined-2 test or the quadruple test. Our findings indicate that the chromosomal abnormalities investigated by screening tests represent <50% of the fetal chromosomal abnormalities associated with an abnormal outcome ranging from intermediate-to-severe in women <35 years (45.8% and 39.6% in the first and second trimesters, respectively), and sensitivity >50% in women >or=35 years (65.1% and 61.8%, respectively). To conclude, approximately 50% of the phenotypically relevant abnormal karyotypes cannot be detected by non-invasive prenatal screening tests.

Quantitative fluorescence-polymerase chain reaction assay for the detection of the duplication of the Charcot Marie Tooth disease type 1A critical region.

Charcot Marie Tooth (CMT) syndrome is the most common hereditary peripheral neuropathy, with an incidence of about 1 in 2500. The subtype 1A (CMT1A) is caused by a tandem duplication of a 1.5-Mb region encompassing the PMP22 gene. Conventional short tandem repeat (STR) analysis can reveal this imbalance if a triallelic pattern, defining with certainty the presence of duplication, is present. In case of duplication with a biallelic pattern, it can only indicate a semiquantitative dosage of the fluorescence intensity ratio of the two fragments. In this study we developed a quantitative fluorescence-PCR using seven highly informative STRs within the CMT1A critical region that successfully disclosed or excluded the presence of the pathogenic imbalance in a cohort of 60 samples including 40 DNAs from samples with the CMT1A duplication previously characterized with two different molecular approaches, and 20 diagnostic samples from 10 members of a five-generation pedigree segregating CMT1A, 8 unrelated cases and 2 prenatal samples. The application of the quantitative fluorescence-PCR using STRs located in the critical region could be a reliable method to evaluate the presence of the PMP22 duplication for the diagnosis and classification of hereditary neuropathies in asymptomatic subjects with a family history of inherited neuropathy, in prenatal samples in cases with one affected parent, and in unrelated patients with a sporadic demyelinating neuropathy with clinical features resembling CMT (i.e., pes cavus with hammer toes) or with conduction velocities in the range of CMT1A.

Pure monosomy and pure trisomy of 13q21.2-31.1 consequent to a familial insertional translocation: exclusion of PCDH9 as the responsible gene for autosomal dominant auditory neuropathy (AUNA1).

Insertional translocations (IT) are rare structural rearrangements. Offspring of IT balanced carriers are at high risk to have either pure partial trisomy or monosomy for the inserted segment as manifested by "pure" phenotypes. We describe an IT between chromosomes 3 and 13 segregating in a three-generation pedigree. Short tandem repeat (STR) segregation analysis and array-comparative genomic hybridization were used to define the IT as a 25.1 Mb segment spanning 13q21.2-q31.1. The phenotype of pure monosomy included deafness, duodenal stenosis, developmental and growth delay, vertebral anomalies, and facial dysmorphisms; the trisomy was manifested by only minor dysmorphisms. As the AUNA1 deafness locus on 13q14-21 overlaps the IT in the PCDH9 (protocadherin-9) gene region, PCDH9 was investigated as a candidate gene for deafness in both families. Genotyping of STRs and single nucleotide polymorphisms defined the AUNA1 breakpoint as 35 kb 5' to PCDH9, with a 2.4 Mb area of overlap with the IT. DNA sequencing of coding regions in the AUNA1 family and in the retained homologue chromosome in the monosomic patient revealed no mutations. We conclude that AUNA1 deafness does not share a common etiology with deafness associated with monosomy 13q21.2-q31.3; deafness may result from monosomy of PCHD9 or another gene in the IT, as has been demonstrated in contiguous gene deletion syndromes. Precise characterization of the breakpoints of the translocated region is useful to identify which genes may be contributing to the phenotype, either through haploinsufficiency or extra dosage effects, in order to define genotype-phenotype correlations.

Prenatal detection by subtelomeric FISH and MLPA of unbalanced meiotic recombinants resulting from parental pericentric inversions.

OBJECTIVE: Pericentric inversion carriers are predisposed to produce unbalanced gametes that result in conceptuses having either a partial trisomy for one distal segment and a partial monosomy for the other or vice versa. Larger inversions result in smaller unbalanced distal segments and a higher likelihood of a viable fetus. In these cases the structure of the recombinant chromosome is similar to the original balanced inverted or normal ones despite the (unbalanced) genetic content. Such cases may not be detected prenatally by conventional cytogenetic analysis. METHODS: In all prenatal samples from the pericentric inversion carriers we applied subtelomeric FISH probes specific for the chromosome involved in order to detect parental meiotic recombinants resulting from a single cross-over event. Confirmatory MLPA was also applied in unbalanced fetuses. RESULTS: The occurrence of a duplication deficiency unbalance from pericentric inversion carriers was successfully detected in all three fetuses by FISH. MLPA assays applied in two cases confirmed these results. CONCLUSIONS: The application of commercial FISH subtelomeric probes is a reliable method that could be routinely applied for the detection of single cross-over meiotic recombinants. MLPA is a sound alternative technique.

Confirmation of mosaicism and uniparental disomy in amniocytes, after detection of mosaic chromosome abnormalities in chorionic villi.

Chromosome mosaicism is detected in about 1-2% of chorionic villi samples (CVS), and may be due to a postzygotic nondisjunction event generating a trisomic cell line in an initially normal conceptus (mitotic origin) or the postzygotic loss of one chromosome in an initially trisomic conceptus (meiotic origin and trisomy rescue). Depending on the distribution of the abnormal cell line, the mosaic can be confined to the placenta (CPM) or generalised to the fetus (TFM, true fetal mosaicism). Trisomy rescue could theoretically be associated with a 33.3% probability of uniparental disomy (UPD) in the fetus. The aim of this study was to determine the risk of fetal involvement in a cohort of numerical and structural chromosome mosaics revealed in chorionic villi by means of combined direct and long-term culture analyses; we also determined the incidence of UPD associated with mosaic aneuploidies and supernumerary markers involving imprinted chromosomes. A total of 273 of a consecutive series of 15,109 CVS evaluated during a period of 5 years showed a mosaic condition in direct preparations and/or long-term cultures; confirmatory amniocentesis was performed in 203 cases. The abnormal cell line was extended to the fetus in 12.8% cases in terms of structural and numerical abnormalities involving autosomes and sex chromosomes; the risk of TFM varied and depended on the placental tissue distribution of the abnormal cell line. One of the 51 cases in which the mosaic involved an imprinted chromosome showed UPD, thus indicating a risk of 1.96%.

Three cases with de novo 6q imbalance and variable prenatal phenotype.

We describe two families in which three fetuses had a de novo 6q imbalance and abnormal phenotypes. We determined the boundaries and the parental origin of the chromosomal alterations by segregation analysis using a panel of short tandem repeats (STRs) located on 6q. Cases 1 and 2 (family A) were two sibs with 6q imbalance involving different regions. Case 1 was a female fetus with arthrogryposis, who had a complex rearrangement resulting in two deleted regions (6q22 and 6q25.1-q25.2) and a duplication of 6q23-q25.1. This latter imbalance was reported previously and is associated with joint contractures and short neck, also present in this fetus. The sib (case 2) had intrauterine growth restriction (IUGR) and agenesis of the ductus venosus. This male died shortly after birth; postnatal karyotype and molecular investigations showed a 6q21 de novo deletion. Case 3 (family B) had a prenatally detected deletion of 6q14-q16. Autopsy of the fetus documented minor facial anomalies and contractures of the limbs. All rearrangements were de novo and of paternal origin. Our data and the consistent number of cases of de novo 6q alterations previously reported suggest that chromosome arm 6q could be prone to rearrangements resulting in heterogeneous phenotypes. In family A, chromosome 6q imbalances involving different chromosomal regions were present in two consecutive pregnancies. In such cases counseling should suggest the impossibility of excluding recurrence of a chromosomal imbalance, and should discuss the option of early prenatal diagnosis in subsequent pregnancies.

Prenatal search for UPD 14 and UPD 15 in 83 cases of familial and de novo heterologous Robertsonian translocations.

OBJECTIVES: The presence in the conceptus of a Robertsonian translocation predisposes to UPD formation, mainly by post-zygotic events of chromosome abnormality rescue. This is due to the increased risk of generating aneuploid zygotes because the rearranged chromosome and the respective homologues are prone to non-disjunction errors. Given this, carriers and karyotypically normal individuals conceived from a parent with a Robertsonian translocation are at risk for UPD. Abnormal phenotypes due to an imprinting effect have been found to be associated with UPD 14 and 15. The aim of the study was to refine, at the time of prenatal diagnosis, the risk for UPD 14 and 15 in a population with Robertsonian translocations involving these chromosomes. METHODS: Sixty-five cases of familial and de novo heterologous Robertsonian translocations involving chromosomes 14 and 15 and 18 fetuses with a normal karyotype, but conceived by a Robertsonian translocation carrier were prenatally studied to investigate the presence of UPD for chromosomes 14 and 15. RESULTS: Of the 65 Robertsonian translocation carriers, one fetus with a de novo der(14;21) showed maternal UPD 14. None of the 18 fetuses with a normal karyotype had UPD. CONCLUSION: Our data, combined with other previous prenatal investigations provide a general risk estimate for UPD 14 and 15 of 0.6%. Nevertheless, combining our data and those previously reported, all three fetuses with UPD had a de novo Robertsonian translocation, thus suggesting a risk of UPD formation of about 3% for this specific group of translocation carriers.