Comparative quantitative expression of bcl-2 by normal and leukaemic myeloid cells

Expression of the bcl-2 oncoprotein by AML blasts has previously been demonstrated to be heterogenous with high levels of bcl-2 expression being associated with a low complete remission rate and poor survival. We have quantified bcl-2 expression in AML blasts in relation to expression of the CD34 antigen and in comparison to CD34-positive cells from normal bone marrow. When expressed as molecules of equivalent soluble fluorochrome (MESF) per cell. AML blast cell bcl-2 expression varied from 11.1 to 99.9 x 10(3) (median 39.4 x 10(3), n = 56) with 28.5% of patients expressing high MESF values (> 50 x 10(3)) and 16% of patients expressing low MESF values (< 20 x 10(3), the remainder expressing intermediate values. There was no significant difference between intensity of bcl-2 expression and FAB classification in the de novo AML cases; and there was no significant differences between de novo and secondary AML cases. Blasts from CD34+ AML patients expressed significantly higher levels of bcl-2 (mean MESF 43.6 x 10(3), n = 36) than CD34- AML patients (mean MESF 31.7 x 10(3), n = 19). In five cases of CD34+ AML, bcl-2 expression was determined on purified CD34+ and CD34- blast cell populations. In all cases CD34+ blasts were found to express significantly higher bcl-2 MESF values compared to the CD34- fraction. Purified CD34+ cells from normal bone marrow consistently expressed high levels of bcl-2 (MESF > 75 x 10(3), n = 4), which was comparable to that found on CD34+ AML cells. Our results suggest that the poor prognosis previously associated with AML blasts expressing the CD34 antigen may in part be related to high expression of bcl-2. Also the ability to measure bcl-2 in AML blasts quantitatively by flow cytometry and to categorize patients into discrete groups may be of value as a prognostic indicator in AML.