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.

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.