Fourteen patients with acute nonlymphocytic leukemia (ANLL) or dysmyelopoietic syndromes were found to have abnormalities involving the long arm of chromosome 3. In eight patients, the structural rearrangements involved both bands 3q21 and 3q26 and included t(3;3) (four patients), inv(3) (three patients), and ins(5;3) (one patient). Before treatment, seven of these eight patients had platelet counts above 100,000 per microliter, five had normal or elevated platelet counts, and four had significantly elevated platelet counts (600,000 to 1,731,000 per microliter). In each of the eight cases, normal or elevated platelet counts were associated with marked abnormalities of megakaryocytopoiesis, including increased numbers of megakaryocytes and numerous micromegakaryocytes. Classification within the French-American- British system was difficult in most of these cases; however, the leukemia in five of the eight patients with abnormalities of chromosome 3 that involved both bands 3q21 and 3q26 was classified as M4. The remaining six of the 14 patients had translocations between chromosome 3 and another chromosome. None involved both bands 3q21 and 3q26, and a break in either q21 or q26 was noted in only two patients. One of the six, who had ANLL (M4) with a normal platelet count, had a 3;5 translocation which involved band 3q25. These data suggest that in patients with ANLL, abnormalities of chromosome 3 which simultaneously involve bands 3q21 and 3q26 are associated with unusually high platelet counts.

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