Abstract • 156

We report the occurrence of monosomy 7 transformation in cytogenetically normal donor cells, in a girl with severe aplastic anemia, three years following matched-unrelated same sex donor allogeneic bone marrow transplantation (MUD-BMT). The patient was initially diagnosed with congenital amegakaryocytosis which later progressed to severe aplastic anemia. Her BMT preparative regimen included total body irradiation (200 cGy, BID ×3 days), Ara-C (25 mg/kg/d × 3 days) and Cytoxan (50 mg/kg/d × 2 days). Cytogenetic studies prior to bone marrow transplantation were normal on multiple occasions. However, three years after MUD-BMT, she developed transfusion-dependent thrombocytopenia. Conventional cytogenetic study revealed monosomy 7 while restriction fragment length polymorphism indicated the genetic constitution of her hematopoietic cells was 100% donor. Since cells with monosomy 7 may have growth advantage and it is unreliable to distinguish between donor and recipient by conventional cytogenetic methods, when they are of the same sex, we employed PCR to amplify microsatellite markers on chromosome 7 and found that the transformation of monosomy 7 occurred in genetically normal donor cells. The donor remains clinically well and genetically normal five years after transplantation. This suggests that the microenvironment of the bone marrow of our patient with severe aplastic anemia may have played a critical role in the development of monosomy 7 of normal donor cells and we conclude that chromosomal microsatellite marker analysis is a valuable tool for precise donor/recipient differentiation in engraftment monitoring.