Hematological changes in Down's syndrome.

The many hematological abnormalities observed in patients with Down's syndrome have intrigued hematologists for many years. This review summarizes recent studies concerning the hematological findings in newborn infants with Down's syndrome, the transient leukemoid/leukemia-like proliferative disorders in these infants, the increased incidence of leukemia and types of leukemia in patients with Down's syndrome, and immunological studies of these patients. In addition, studies concerning the significance of the extra genetic material in chromosome 21 found in patients with Down's syndrome are discussed. It appears that the extra genetic material in chromosome 21 confers a proliferative advantage to hematopoietic stem cells, and may make them more prone to further karyotypic changes leading to leukemia. Megakaryocytic proliferative disorders are more common in patients with Down's syndrome. The spectrum of myeloproliferative disorders including myelofibrosis, myeloid metaplasia, and megakaryoblastic leukemia is seen in these patients. Studies of the genetic loci on chromosome 21 have demonstrated loci for enzymes involved in purine biosyntheses. Further studies of specific loci on chromosome 21 may help explain the hematopoietic growth advantages found in the stem cells with an extra chromosome 21.