CYTOGENETIC AND MOLECULAR-GENETIC CHARACTERIZATION OF TRISOMY-20 MOSAICISM IN FETAL BLOOD AND TISSUES

We report a case of mosaic trisomy 20, the most common autosomal mosai cism identified in amniocytes, ascertained in a woman referred for amn iocentesis because of abnormal ultrasound at 18.1 weeks' gestation whi ch revealed short femurs and nuchal thickening. Metaphase analysis of 98 clones revealed 47,XY, +20 in 96 cells (98 per cent). Trisomy 20 wa s demonstrated in 6 cells (12 per cent) in a total of 50 cells from tw o fetal blood cultures obtained after pregnancy termination. Fluoresce nce in situ hybridization (FISH) analysis of interphase nuclei utilizi ng a chromosome 20 alpha-satellite centromeric DNA probe revealed thre e signals in 57/546 nuclei (10 per cent) in fetal blood. Metaphase ana lysis of 167 cells from seven different fetal tissue sources revealed trisomy 20 in 32 cells (19.2 per cent). The percentage of trisomy 20 c ells varied with tissue type, with the highest percentage (13/25 cells , 52 per cent) identified in the small intestine and lymph nodes and t he lowest percentage (1/34 cells, 2.9 per cent) identified in a specim en of chorionic villi. Molecular genetic analyses utilizing polymerase chain reaction (PCR)-formated dinucleotide repeat polymorphisms demon strated that the non-disjunctional event most likely occurred post-zyg otically and that the origin of the extra chromosome 20 was maternal. This study is the first to demonstrate trisomy 20 cells in fetal blood , suggesting that mosaic trisomy 20 can be embryonic in origin. In cas es of prenatally detected mosaic trisomy 20, examination of fetal bloo d should be considered, as well as study of placental membranes, skin, and urine sediment to confirm the karyotype and determine its signifi cance.