Introduction: Transcription factor GATA-1 is essential for erythroid and megakaryocytic maturation. A role of GATA-1 in cell cycle control is suggested by the fact that GATA-1 mutations are associated with hematopoietic precursor proliferation and leukemogenesis and that defective GATA-1 expression is observed in in vitro cultures of erythroid myelodysplastic precursors. In order to study more in detail a potential role of GATA-1 dysregulation in myelodysplastic syndromes (MDS), we constructed lentiviral vectors with the aim to overexpress GATA-1 protein or to inhibit its production in erythroid progenitors.
Methods and Results: Using RNA interference technology we tested how GATA-1 inhibition interfered with erythroid differentiation. We selected one GATA-1 specific siRNA, which abolished expression of GATA-1 protein in K562 and HEL erythroleukemic cell lines, as verified by Western blot. Interestingly, we observed in parallel to the disappearance of GATA-1 protein, decreased proliferation rates (170x for K562 and 30x for HEL after 17 days of culture) and increased apoptosis. Normal CD34+ cells cultured in our culture system and transduced with the siRNA vector were practically blocked in their erythroid differentiation: 14 % glyco+/CD36- mature erythroid cells versus 81 % in untransduced and 80 % in cultures transduced with control lentivector (obtained after 14 days of culture). Differentiation into myeloid cells was not affected.
To overexpress GATA-1 we cloned the wild-type as well as a mutated, caspase-resistant, form of GATA-1 in a pWPIR-ires-GFP bicistronic lentivector. Functionality of both lentivectors was validated in HeLa cells. For the study of GATA-1 in primary human hematopoietic cells we used an in vitro culture system in which CD34+ progenitors differentiate into mature red blood cells in the presence of erythropoietin, IL-3, and SCF. Transduction of CD34+ cells with lentivectors led to increase of GATA-1 mRNA (400-fold) measured by Realtime RT-PCR and to detection of protein. No difference was observed in cell numbers, expression of erythroid differentiation markers and survival between cells transduced with control vector and with pWPIR-GATA-1-ires-GFP. CD34+ cells from 3 patients with low-risk MDS in this culture system proliferated less (15x ± 13 amplification after 14 days of culture versus 72x ± 35 for normal precursors) differentiated less, and became apoptotic earlier than normal cells. However, overexpression of GATA-1 did not restore proliferation rate, nor did it lead to increased erythroid differentiation, or increase in survival.
Conclusion: GATA-1 overexpression was not able to overcome defective erythroid differentiation of myelodysplastic progenitors, nor did it increase differentiation of normal erythroid progenitors. On the other hand, GATA-1 inhibition in normal erythroid precursors led to blockage of erythroid differentiation. We therefore assume that either factors upstream of GATA-1 or additional, GATA-1 independent factors, are responsible for the myelodysplastic phenotype.