PRENATAL-DIAGNOSIS AND DYSMORPHIC FINDINGS IN MOSAIC TRISOMY-16

We report two cases of mosaic trisomy 16 diagnosed by amniocentesis, w ith dysmorphic findings in both cases evident upon delivery. Following elective termination, case 1 demonstrated a trisomy 16 cell line in f etal skin (4 per cent) and placental tissue (64 per cent). Molecular s tudies on the disomic cell line indicated that both chromosome 16s wer e maternal in origin, suggesting loss of the paternal chromosome 16 fr om a trisomic zygote (uniparental heterodisomy). At birth, case 2 demo nstrated only disomic cells in skin and blood, with trisomy 16 present in 4 per cent of cells from the amnion. Molecular studies confirmed b oth maternal and paternal contributions of the chromosome 16s. We anal ysed DNA from one previously reported case of mosaic trisomy 16 (Willi ams et al., 1992) and failed to find signs of uniparental disomy in th is child with congenital heart defects. These cases had distinctive bu t different dysmorphic features. We suggest that trisomy 16 embryos ma y revert to disomy during the course of pregnancy, allowing for longer survival with various abnormalities in growth and morphogenesis. The clinical significance of prenatally detected mosaic trisomy 16 may not be completely defined by additional cytogenetic, molecular, and ultra sound studies.