A case of maternal uniparental disomy of chromosome 9 diagnosed prenatally and 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 advanced maternal age. Follow-up amniocentesis revealed level 2 mosaicism for trisomy 9. Trisomy 9 was not detected at fetal blood sampling. Molecular analysis 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 pregnancy continued and neonatal blood testing showed a mosaic trisomy 9 karyotype, i.e. 4/50 cells analysed. Clinical postnatal follow-up for a period of 1 year has documented only minor facial dysmorphism and skeletal abnormalities. Development appears unremarkable. This case is the second report of maternal uniparental disomy for chromosome 9 detected prenatally and is the first case followed up post-term. This report highlights the difficulty of making informed prognostic assessments in such cases despite extensive laboratory investigation.