A case of maternal uniparental disomy of chromosome 9 diagnosed prenatallyand the related problem of residual trisomy

Non-mosaic trisomy 9 was found in a chorionic villus (CV) sample taken from a 43-year-old woman referred for prenatal chromosome analysis due to advan ced maternal age. Follow-up amniocentesis revealed level 2 mosaicism for tr isomy 9. Trisomy 9 was not detected at fetal blood sampling. Molecular anal ysis of fetal (amniocyte) DNA showed maternal uniparental heterodisomy (UPD ) for chromosome 9, Two crossovers resulted in a region of isodisomy in the distal long arm. Trisomy rescue of a meiosis 1 segregation error seems to have been responsible for the uniparental disomy of chromosome 9, The pregn ancy continued and neonatal blood testing showed a mosaic trisomy 9 karyoty pe, i.e. 4/50 cells analysed. Clinical postnatal follow-up for a period of 1 year has documented only minor facial dysmorphism and skeletal abnormalit ies. Development appears unremarkable. This case is the second report of ma ternal uniparental disomy for chromosome 9 detected prenatally and is the f irst case followed up post-term. This report highlights the difficulty of m aking informed prognostic assessments in such cases despite extensive labor atory investigation. Copyright (C) 2000 John Wiley & Sons, Ltd.