Increased Apoptosis as a Mechanism of Ineffective Erythropoiesis in Myelodysplastic Syndromes

Ineffective hematopoiesis, with premature death of marrow myeloid precursors, is a hallmark of myelodysplastic syndromes (MDS), with the apparent paradox of peripheral cytopenia associated with hypercellular bone marrow (BM). Excessive apoptosis appears relevant especially in low-risk MDS. Apoptosis, triggered by the BM microenvironment and/or intrinsic cellular defects, is regulated at different levels by numerous factors, such as oncogenes and their protein products, hematopoietic growth factors, immunologic factors, cell-cell or cellstromal interactions, critical adhesion receptors, and various cytokines. Deregulation of both the intrinsic and the extrinsic pathways have been reported in MDS cells. Many studies provide evidence that the activation of the Fas/Fas-ligand system might represent an important pathogenetic mechanism. Recently, it has been demonstrated that the erythroid apoptosis of low-risk MDS is initiated at a very early stage of stem cells and associated with mitochondrial dysfunction. There is a constitutive triggering to apoptosis via cytochrome C release from the mitochondrial intermembrane space, with subsequent activation of effector caspases and increased sensitivity to death ligands triggering the extrinsic apoptotic pathway. The role of the mitochondrial pathway might be relevant especially in refractory anemia with ring sideroblasts, where abnormalities in mitochondrial ferritin expression might directly influence iron homeostasis and contribute to alter the balance between cell growth and death. The pathogenesis of refractory anemia without ring sideroblasts seems to be more heterogeneous, with the involvement of alternative mechanisms, including T-cell–mediated BM failure. Elucidation of these pathogenetic mechanisms might lead to the development of novel therapeutic strategies.