Increased apoptosis in mononucleated cells but not in CD34+ cells in blastic forms of myelodysplastic syndromes.

INTRODUCTION Myelodysplastic syndromes are characterized by peripheral refractory cytopenias together with normo or hyper cellular marrow. Increased apoptosis has been shown to be involved in the process leading to this paradox. MATERIALS AND METHODS Early apoptosis detection, based on the modification of mitochondrial transmembrane potential (deltapsim), was performed on bone marrow cells from 42 MDS patients (21 RA, four RARS, 10 RAEB, two RAEB-t, three sAML, and two CMML) and seven normal healthy donors. Phosphatidylserine (PS) expression, a late/intermediate marker of the apoptotic cascade, was also quantified. Apoptosis was analysed by flow cytometry both on unsorted mononuclear cells and on progenitor cells after CD34+ magnetic cell sorting. RESULTS A significant increase of apoptosis of MNC was observed in RA, RARS, RAEB and to a lower extent in RAEB-t and AML samples. In the progenitor compartment, RA and RARS samples presented a high level of apoptosis, whereas a switch to a low level of apoptosis was detected in the blastic forms RAEB, RAEB-t and sAML. Fas (CD95/APO-1), a member of the death domain receptor family, has been reported to be overexpressed on MDS CD34+ marrow cells. A functional assay of Fas cross-linking using the CH11 antibody on CD34+ marrow cells was performed on samples of 17 MDS patients; 8/17 were found to be sensitive to Fas-induced apoptosis. However, no correlation was observed with the level of in vivo spontaneous apoptosis. CONCLUSION This study demonstrates increased apoptosis of MNC in all MDS subgroups as measured by deltapsim collapse. Moreover, while important apoptosis is still observed at the progenitor level in early MDS, blastic forms show a clear reduction of apoptosis. Study of Fas functionality modulates the implication of this receptor in the pathophysiology of the disease.