Transient abnormal myelopoiesis followed by acute megakaryoblastic leukemia with extramedullary tumors. An autopsied case of Down's syndrome.

An autopsied case of acute megakaryoblastic leukemia is described in a 20 months old, Japanese female infant with Down's syndrome. She presented spontaneous remission of transient abnormal myelopoiesis in her neonatal period, which was followed by acute megakaryoblastic leukemia 1 year later. The clinical picture of acute megakaryoblastic leukemia was initially characterized by peripheral pancytopenia with a few blasts, the absence of hepatosplenomegaly, but ended in overt leukemia characterized by increase of blasts, marked hepatosplenomegaly, and elevated LDH. She died 6 months after the onset at 20 months of age. Autopsy findings revealed widespread leukemic infiltration comprised of megakaryoblasts and megakaryocytes, and extramedullary tumors in the left tibia, the liver, both kidneys, and the endocardium of the heart. Identification of the megakaryocytic cell line was performed in immunohistochemistry and electron microscopy. Chromosomal analyses of peripheral blood disclosed 47, XX, +21, in her neonatal period but disclosed 48, XX, +G, +G, in acute megakaryoblastic leukemia.