Prenatal diagnosis and molecular cytogenetic characterization of low-level mosaicism for tetrasomy 18p at amniocentesis in a pregnancy with a favorable outcome

We present prenatal diagnosis of low-level mosaicism for tetrasomy 18p at amniocentesis in a pregnancy with a favorable outcome. A 40-year-old woman underwent amniocentesis at 17 weeks of gestation because of advanced maternal age. Amniocentesis revealed a de novo supernumerary isochromosome 18p in eight of 39 colonies of cultured amniocytes. The karyotype was 47,XX,+i(18)(p10)[8]/46,XX[31]. Array comparative genomic hybridization (aCGH) analysis using uncultured amniocytes revealed arr 18p11.32p11.21 [hg 19] (148,963–14,081,887) × 2–3. Repeat amniocentesis was performed at 20 weeks of gestation. Interphase fluorescence in situ hybridization (FISH) analysis showed four 18p11.22-specific probe (RP11-918F20) signals in 11.7% (12/103 cells) of uncultured amniocytes. aCGH analysis on uncultured amniocytes did not detect genomic imbalance in chromosome 18. The parental karyotypes were normal. Polymorphic DNA marker analysis excluded uniparental disomy 18. Cytogenetic analysis of cultured amniocytes at repeat amniocentesis revealed a karyotype of 47,XX,+i(18)(p10)[2]/46,XX[12]. Prenatal ultrasound was unremarkable. The pregnancy was carried to 38 weeks of gestation, and a 2742-g phenotypically normal female baby was delivered with a cord blood karyotype of 46,XX. When examined at 8 months of age, the infant was normal in growth and psychomotor development. Interphase FISH analysis on 21 uncultured urinary cells revealed normal signals in all cells and no mosaic tetrasomy 18p. Low-level mosaic tetrasomy 18p at amniocentesis without ultrasound abnormalities can be associated with a favorable outcome.