Among the hematopoietic malignancies, MDS is perhaps the least studied, in part because of the difficulty of obtaining a suitable model. We report on a murine model of EVI1-positive MDS that we have generated by BM infection and transplantation. After a relatively long latency, the reconstituted animals develop a fatal pancytopenia, accompanied by hypercellular BM and severe dyserythropoiesis. These features resemble those reported in human EVI1-positive MDS (14
). We have also determined that among the genes responsible for terminal differentiation of erythroid lineage and platelet formation, EpoR
) are repressed in the BM of the diseased mice. The repression of these receptors does not require cooperating mutations but is observed as early as 4 months after BMT, at a time when the mice behave normally and the morphology and profile of their PB are normal. It was earlier reported that inappropriately expressed EVI1 blocks erythropoiesis by binding to the DNA consensus sequence of the transcription factor GATA1 and by repressing genes regulated by GATA1 (9
). Surprisingly, however, it was also reported that the expression of the GATA1-regulated EpoR
was not altered (9
). In contrast, by RQ-PCR we found that the expression of this gene as well as that of c-Mpl
is strongly downregulated in the BM of EVI1-positive mice. As previously suggested, it is quite probable that disruption of GATA1 functions is responsible for the downregulation of these two genes. However, the mechanism involved is probably more complex than straight DNA binding by EVI1 because, in contrast to probes that contain 9 GATA repeats (10
), electrophoretic mobility shift assays show that a canonic DNA probe that specifically binds to GATA1 fails to interact with EVI1 (data not shown). We find that, in vitro, EVI1 affects normal hematopoiesis in many contradictory ways. Whereas it downregulates erythroid colony formation, it very strongly upregulates the rate of cell proliferation in response to GM-CSF, while simultaneously doubling the differentiation time. These defects appear immediately in the isolated BM cells after expression of EVI1, and they occur early in the transplanted animals as well. On the other hand, because the mice survive and behave normally for about 9 months, it is improbable that these defects by themselves cause their death. On the basis of the BM analysis 3 months after transplantation, we propose that, as shown in vitro, EVI1 expression in murine hematopoietic progenitors induces erythroid differentiation impairment. Because the differentiation impairment is only partial and EVI1 cells proliferate in response to cytokines, in vivo homeostatic mechanisms are probably able to prevent the development of anemia in the mice for 8–10 months, presumably through the increased hematopoietic proliferation induced by EVI1, which was evident upon morphological examination of the BM and spleen biopsy specimens. Eventually, however, progressive cytopenia occurred in the mice. Hematopoietic cells isolated from the mice at this point demonstrated impaired colony formation potential in response to cytokines in vitro, in contrast to freshly infected EVI1 hematopoietic cells and cells 3 months after BMT, which had an impaired response only to Epo. The reason for this late-onset loss of hematopoietic stem cell functional capacity may be accumulation of genetic damage during the 10-month period of replicative stress or as a consequence of EVI1-induced genomic instability. In summary, disease progression in EVI1-positive mice demonstrates that hypercellular marrow with peripheral cytopenia, a feature of MDS, can result from specific molecular events that impair (without completely preventing) terminal differentiation. The EVI1-positive mice that we generated are a suitable platform in which to test the efficacy of and optimal timing for therapeutic strategies.