TERMINAL DEOXYNUCLEOTIDYL TRANSFERASE (TDT) EXPRESSION IN ACUTE MYELOID-LEUKEMIA

Terminal deoxynucleotidyl transferase (TdT) was initially considered a s a marker of immature lymphoid cells, but many studies have since pro vided conclusive evidence for the existence of TdT+ cases of acute mye loid leukemia (AML). The reported incidence of TdT + AML cases varies largely (from 0 % to 55 %, average of combined data of the literature 18%, children 19%, and adults 21 %) suggesting interlaboratory differe nces in the types of AML examined, the sensitivity of the method used, and the percentage of positive blasts taken as cut-off value. Signifi cantly higher frequencies of TdT+ AML were reported in studies employi ng immunocytochemical staining (alkaline phosphatase anti-alkaline pho sphatase or immunoperoxidase) than in series using immunofluorescence microscopy or biochemical assays. Statistical analysis of various cut- off levels demonstrates an inverse correlation between cut-off point a nd incidence. The combined data show that TdT-positivity is more commo n in the immature cell types (M0, M1), with no correlation with age or sex. Except for contested suggestions of an association with t(6;9) a nd t(8;21), no clear relationship between karyotype and TdT status has been documented. Although an association between T-cell receptor or i mmunoglobulin gene rearrangements and expression of TdT in AML was pos tulated, subsequent studies could not demonstrate this correlation. Th ere was no significant relationship with other immunophenotypic marker s except for CD34 positivity suggesting that the TdT + cells represent an immature population. The percentage of positive cells was usually lower in AML than in ALL; in most cases only a subpopulation of the AM L cells was TdT+. Thus, TdT could be viewed as a marker of hematopoiet ic immaturity. In about one-half of the studies on adults, TdT express ion was reported to indicate a poor prognosis; others did not find any prognostic difference between TdT + and TdT - AML cases. No correlati on between TdT-positivity and prognosis was found in childhood AML.