BCL-2 EXPRESSION IN ACUTE MYELOBLASTIC-LEUKEMIA - RELATIONSHIP WITH AUTONOMOUS GROWTH AND CD34 ANTIGEN EXPRESSION

The bcl-2 gene encodes a mitochondrial protein that inhibits the onset of apoptosis induced by growth factor withdrawal or cytotoxic agents. Using quantitative flow cytometry and expressing bcl-2 levels as the number of molecules of equivalent soluble fluorochrome (MESF) per cell , we have shown that bcl-2 protein expression in the blast cells from patients with acute myeloblastic leukaemia (AML) is heterogeneous, but not related to FAB type. The blast cells from AML patients with the c apacity to grow and survive autonomously in vitro were found to have h igher bcl-2 MESF values than those that were dependent upon exogenous growth factors. We have previously reported that the blast cells from 70% of AML patients exhibit autonomous growth and autocrine growth fac tor production in vitro and that this has been shown to be an importan t indicator of poor prognosis in AML. High bcl-2 expression has also b een associated with a low complete remission rate and poor survival in AML. In the patients whose blast cells exhibited autonomous growth, n eutralisation of endogenous GM-CSF resulted in down-regulation of bcl- 2 protein, whereas in blast cells from patients whose cells proliferat ed only in the presence of added growth factors, incorporation of reco mbinant human (rh) GM-CSF in the culture media resulted in up-regulati on of bcl-2. Because CD34 positivity has been reported as another indi cator of poor prognosis in AML, we compared bcl-2 expression in cases of CD34 positive AML, CD34 negative AML and CD34 positive normal bone marrow cells. Bcl-2 was found to be strongly expressed on the CD34+ no rmal bone marrow cells. The blast cells from CD34+ AML patients expres sed significantly higher bcl-2 levels than CD34- AML patients. In five cases of CD34+ AML, the bcl-2 levels were determined on purified CD34 + and CD34- blast cell populations. The CD34+ blast cells were found t o express significantly higher bcl-2 levels compared with the CD34- bl ast cells. Our data would suggest that quantification of bcl-2 in AML blast cells may be useful as a prognostic indicator in AML.