Acute megakaryoblastic leukaemia associated with intrinsic platelet dysfunction and constitution ring 21 chromosome in a young boy.

A 3-year-old boy with pancytopenia and a paucity of circulating blast cells was found to have acute megakaryoblastic leukaemia. Histopathologic investigation of the bone marrow biopsy disclosed replacement by megakaryoblasts and mild-to-moderate reticulin fibrosis, the megakaryocytic origin of these cells was confirmed by their staining properties and by cross-reactivity with rabbit anti-rat platelet serum. Treatment with adriamycin and cytosine arabinoside induced a complete remission of this otherwise rapidly fatal disease. Before chemotherapy, the patient's platelets showed decreased aggregation in response to thrombin and adenosine diphosphate, as well as a defective thrombin-induced serotonin release reaction. Neither functional defect resolved after remission induction, indicating that the platelets were intrinsically abnormal. Most striking was the finding of a constitution chromosomal defect, a ring No. 21 chromosome, in addition to an abnormal malignant stem line. This appears to be the first reported instance of a constitutional r(21) chromosome associated with acute leukaemia.