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Pseudoleukemia in Down's syndrome. Analysis of immunophenotype and gene rearrangement.

This report demonstrates a case of transient abnormal myelopoiesis (TAM) evolving in a patient with Down's syndrome. A diagnosis was established after the patient's blast cell count decreased considerably three weeks after the initial leukemic phase. The blast population in the authors' case expressed Leu-9 (CD7), 6D1, and TdT+. Cytochemistries showed some of the blast population to be peroxidase positive and Sudan black positive. Platelet peroxidase by electron microscopic examination showed some positive blasts. Therefore, surface markers and cytochemical studies in this case suggested an abnormal proliferation involving a pluripotential stem cell capable of expressing myeloid and lymphoid characteristics. Cytogenetics was performed at birth and showed 47,XY,+21/48,XY,+21,+mar, confirming the diagnosis of Down's syndrome. The origin of the chromosomal fragment was uncertain. It was of interest that during the remission phase of his pseudoleukemia there was a concomitant decrease in the extra chromosomal fragment. Immunoglobulin and T-cell antigen receptor gene rearrangement studies showed only germline patterns, indicating that the lymphoid cells in the blast population were not clonally expanded. Therefore, immunoglobulin and T-cell antigen receptor rearrangement analysis and immunophenotyping are extremely valuable techniques in distinguishing between TAM and acute lymphoblastic leukemia in patients with Down's syndrome.