Prenatal diagnosis of a 16p11.2p11.1 mosaic small supernumerary marker chromosome (sSMC)

Background. Small supernumerary marker chromosome (sSMC) is a challenge in prenatal diagnosis. Its’ presence is associated with advanced maternal age and distinct ultrasound findings, prediction of postnatal clinical consequences and prenatal counselling is difficult. Exact characterization of an sSMC is augmented by the application of novel molecular methods such as fluorescent in situ hybridization (FISH) and array comparative genome hybridization (aCGH). Case presentation. Chorion villous sampling of a fetus of a 42-year old secundipara was carried out due to advanced maternal age and the conventional banding cytogenetic examination revealed a mosaic sSMC. Detailed prenatal ultrasound scan showed no fetal malformations. The karyotype from confirmatory amniocentesis was 47,XY,+mar[45]/46,XY. The level of mosaicism was 70% from chorionic villus and amnion cells as well. By using FISH and SNP based aCGH the exact origin of the marker was described and the final prenatal karyotype was determined as 47,XY,+mar[70].arr[GRCh38]16p11.2p11.1(31699804_35989873)x3dn/46,XY[30]. The increase in DNA dosage was 4,29 Mb affecting 20 genes with two OMIM ones (ZNF267 and TP53TG3), the SNP analysis excluded the possibility of uniparental disomy (UPD) of the chromosome. The application of the molecular cytogenetic methods allowed us the differentiate the mosaic marker chromosome from the known 16p11.2 duplication syndrome in close neighboring that is connected to developmental delay and autism spectrum disorder. The present case was identified as a harmless de novo euchromatic variant (EV) of the short arm of chromosome 16. Postnatal karyotyping from neonatal peripheral blood confirmed the presence of the marker. FISH analysis with probe D16Z2 on cultured lymphocyte and buccal smear samples revealed a 64% and 45% mosaic state of the sSMC, respectively. The precise karyotype was finally defined as 47,XY,+min(16)(:16p11.2->p11.1:)dn[64]/46,XY. Conclusions. Novel microarray methods combined with molecular cytogenetic analysis is particularly effective in the rapid and accurate diagnosis of sSMCs, especially in a prenatal situation when the exact characterization of a genomic imbalance is of utmost importance. Precise diagnosis facilitates proper genetic counseling, allows informed decision making and helps avoiding unnecessary pregnancy termination.