Terminal deoxynucleotidyl transferase-positive acute leukemias evolving from a myelodysplastic syndrome

We have analyzed the distribution of terminal deoxynucleotidyl transferase (TdT) in 30 patients with myelodysplastic syndromes (MDS), in connection with morphology, cytochemistry, and cytogenetics. All cases in preleukemic phase were strictly TdT-negative. At variance, 4 (19%) of 21 cases with subsequent leukemia were TdT-positive and showed a pure lymphoid (one case) or a mixed lymphoid-myelomonocytic (three cases) morphology. The TdT enzymatic activity ranged from 2 to 21 U/10⁸ cells and the percent of positive cells in the immunofluorescence test ranged from 10 to 80% of total. All cases were Philadelphia chromosome-negative. The clonal origin of MDS from the pluripotent stem cell, capable of originating both a lymphoid and myeloid progeny, is further corroborated.