High terminal deoxynucleotidyl transferase activity in acute myelogenous leukemia

Although leukocytes from all 13 acute lymphoblastic leukemia patients examined had high terminal deoxynucleotidyl transferase (terminal transferase) activity (20 to 100 units/mg of cellular DNA, where 1 unit equals 1 nmole of nucleotide polymerized in 1 hr) and those from 21 acute myelocytic leukemia patients had low terminal transferase activity (0.2 to 2 units/mg of cellular DNA), the bone marrow and peripheral blood leukocytes from 2 patients with acute myelocytic leukemia, diagnosed on the basis of clinical features and the morphology, cytochemistry, and cytogenetics of the leukemic cells, had terminal transferase activity (39 to 52 units/mg of cellular DNA) equivalent to that found in leukemic lymphoblasts. These results bring under question the specificity of high terminal transferase activity outside of the thymus as a marker for leukemic lymphoblasts and, secondarily, the derivation of acute lymphoblastic leukemia cells in all cases from thymocytes. Perhaps malignant transformation in a pleuripotent stem cell with derepression of the genome for terminal transferase could account for high terminal transferase activity observed in certain leukemic cells.