An abnormal fetus with trisomy 20 and trisomy 9 mosaicism. J.K. Lundberg, M.G. Bialer, A. Yenamandra, L. Trinchitella, E.. Schneider, R. Perrone, X. Zhou, E. Kahn, P. Koduru. Dept. of Pediatrics, Laboratory and OB/GYN, North Shore Univ. Hospital/NYU School of Medicine, Manhasset, NY.

   A 39 year old Saudi Arabian/Asian Indian woman was seen for genetic counseling for advanced maternal age and consanguinity (1st and 2nd cousins). Family history was significant for a son with pulmonic stenosis. Screening revealed that she is an a-thalassemia carrier. At 15 3/7 wk a comprehensive ultrasound revealed bilateral cerebral ventriculomegaly, dilated cisterna magna, bilateral cystic lesions in the anterolateral neck, and pleural effusion. On amniocentesis, AF-AFP was normal, but chromosome analysis of 20 metaphases from in-situ cultures of the amniotic fluid showed 47,XY,+20. Because this is usually a mosaic finding, we counted additional cells. Of 56 total cells counted, 50 were 47,XY,+20 and 6 were 47,XY,+9. No normal cells or double trisomy cells were seen. Pregnancy was terminated by Prostin induction at 18 wk. Post-mortem examination revealed cystic hygroma, low-set ears, micrognathia, truncus arteriosus, VSD, left diaphragmatic hernia, and hypoplastic left lung. Microscopic examination of the skin revealed lymphangiectasia. Chromosome analysis of 20 metaphases each of cells cultured from placenta, umbilical cord, heart, blood, skin, lung, liver, right and left kidney (total of 160 metaphases) all showed a combination of trisomy 9 and trisomy 20 cells with no normal or double trisomy cells, except lung had only trisomy 20 cells. The clinical findings are consistent with trisomy 9. Trisomy 20 mosaicism is usually benign and not confirmed in blood. Trisomy 20 cells in blood may have phenotypic consequences, but this has not been proven. It is not clear how these chromosome anomalies arose. If there had been 2 separate non-disjunction events, there should be a normal cell line. If the pregnancy had started with a single trisomy and a second non-disjunction occurred, with subsequent loss of one of the original trisomic chromosomes, then a cell line with double trisomy should exist. If both trisomies were the result of an autosomal recessive condition predisposing to karyotypic instability, then additional abnormal cell lines would be expected.