A New Mononuclear Cell (MNC) RNase H Activity-Based Parameter (ψ) With Possible Prognostic Value in Assessing Progression in Acute Myeloid Leukaemia.

A new biological parameter (ψ) has been obtained and proposed here to serve in the assessment of acute myeloid leukemia (AML) progression. It is a function of the activity of RNase H (EC 3.1.4.34), the latter determined in mononuclear cells from the peripheral blood of AML patients. Using a series of patients at the time of diagnosis and after 1-2 cycles of chemotherapy, the enzyme was assayed before the several times during chemotherapy. The derivation of 4 was based on evidence suggesting that the enzyme level correlates with the proliferating leukaemic blasts and their progenitors. Values ψ>1 signify the presence of clonogenic leukaemic progenitor cells in the peripheral circulation. When these high (> 1) ψ values were found during chemotherapy, in these cases it was possible to predict an increase of the peripheral blast pool, with 82% success, occurring 5-35 days before cytologic relapse. In the patients in whom at some stage during treatment, ψ acquired values above unity or in whom ψ increased progressively, survival time was in linear correlation with the time period from the initiation of treatment to the documentation of this high ψ estimate. These results suggest that a patient's relapse risk can be defined by ψ with some degree of precision.