Erythropoiesis Is Highly Stimulated in CD34⁺ Cells in Low-Risk Myelodysplastic Syndromes (MDS) with an Improper Mitochondrial Function.

The apoptosis of early erythroblasts from patients with low-risk MDS, refractory anemia (RA) and RA with ringed sideroblasts (RARS), is mediated through a constitutive cytochrome c (cyt c) release from mitochondria (Tehranchi etal, 2003). Moreover, mature erythroblasts in RARS, but not in RA, show mitochondrial accumulation of aberrant ferritin (MtF) (Cazzola etal, 2003). This study aimed at further describing the pathophysiology of ineffective hematopoiesis in low-risk MDS, by studying cyt c release and MtF expression during erythroid differentiation and mitochondria ATP production in MDS bone marrow cells. We assessed freshly isolated CD34⁺ cells and day 4–14 erythroblasts from RARS, RA and normal bone marrow (NBM). CD34⁺ cells from all individuals were negative for MtF. NBM showed only few positive cells (0–4%, d4–14), and RA erythroblasts a median of 3% (0–8%) MtF⁺ cells. RARS erythroblasts, on the contrary, showed an early increase in MtF⁺ cells and a continuous increase during the culture period (d4=10%, d7=17%, d14=19%). There was a positive correlation between MtF expression and cyt c at day 14 ( r²=0.8). There was no significant difference in mitochondria ATP production between RARS, RA and NBM (all complexes or cyt -dependent complex IV). We found a significant over-expression (mRNA) of the pro-apoptotic genes for cyt c, Bid and Bax at day 0. Moreover, genes involved in erythroid differentiation were significantly up-regulated in MDS CD34⁺ cells: 6-fold for GATA-1 and 23-fold for β-globin; p<,0005 for both. GATA-1 and β-globin expression increased during normal erythroid maturation, but in MDS erythroblasts GATA-1 declined and β-globin showed only a weak increase. Comparing RARS with RA, the former showed both higher expression of the β-globin and GATA-1 genes, and a higher degree of cyt c release and MtF expression. This indicates that the cellular abnormalities leading to erythroid apoptosis as well as efforts to compensate for these defects are present at the stem cell level in RARS. G-CSF that reduces cyt c release in MDS erythroblasts (RARS>RA) showed no effect at all on ATP production or cyt c mRNA. Moreover, G-CSF tended to increase MtF expression in some RARS erythroblast cultures, indicating that it allows survival of pro-apoptotic MDS erythroblasts rather than addressing the cause of apoptosis. In conclusion, the aberrant MtF expression of RARS erythroblasts occurs at a very early stage of erythroid differentiation and is paralleled by an up-regulation of genes involved in erythroid differentiation. Alternative mechanisms may be involved in RA pathogenesis, since these cells show cyt c release but only moderate MtF expression, and less gene up-regulation.