This is a proposal for an Independent Scientist Award (K02 - NHLBI). The studies proposed are an extension of our recently funded project R01 HL082549-01A1. The nucleoporin Nup98 gene is frequently rearranged in acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS), producing chimeras where the N-terminal portion of Nup98 is fused to one of 16 different proteins. The best-characterized Nup98 chimera is Nup98-HOXA9, which contains the N-terminal portion of Nup98 and the DMA-binding domain of the homeobox transcription factor HOXA9. The studies in the R01 proposal were designed to test the hypothesis that Nup98-HOXA9 is an aberrant transcription factor, identify its mechanisms of action, and elucidate the mechanisms by which it causes AML. Our most recent data indicate that Nup98-HOXA9 drastically increases the numbers of human self-renewing primitive hematopoietic cells. In contrast, its effects on the rest of human CD34+ cells are predominantly inhibitory, with increased apoptosis, reduced myeloid proliferation and differentiation, and increased numbers of erythroid cells. These findings are reminiscent of the findings in human MDS. Indeed, Nup98 chimeras, including Nup98-HOXA9 are associated with some cases of human MDS. In addition, recent studies in a mouse model show that expression of a Nup98-HOX chimera induces a severe MDS. In contrast, overexpression of HOXA9 has been linked to AML in humans and in mouse models, but not with MDS. Our hypothesis is that Nup98- HOXA9 induces MDS by inducing proliferation of primitive hematopoietic cells while inducing growth arrest and apoptosis in more mature committed progenitors. To test this hypothesis, we will determine the effects of Nup98-HOXA9 on several subsets of primary human CD34+ cells, compare them to those of HOXA9, and extend our studies of patient material to include patients with MDS. The Principal;Investigator is a physician-scientist with a tenure-track appointment as Assistant Professor of Pathology at Northwestern University. With the help of a K22 Award that ended recently, he was able to devote 75% of his time to research with the remainder spent primarily in diagnostic hematopathology. A K22 Award would enable the Principal Investigator to continue to devote at least 75% of his time to research aimed at understanding an important group of blood disorders, and establish his career as an independent investigator. (End of Abstract)