MLL Is Essential for NUP98-HOXA9-Induced Leukemia

NUP98, a component of the nuclear pore complex (NPC), plays an important role in molecular trafficking between the cytoplasm and the nucleus. NUP98 has two Phe-Gly (FG) repeat domains, which are characteristic of NPC proteins. The NUP98 gene is rearranged and fused to several partner genes, such as HOXA9 and DDX10, in acute myeloid leukemia and myelodysplastic syndromes. Leukemia with NUP98 rearrangement is associated with poor prognosis. However, the molecular mechanism of leukemogenesis induced by NUP98-fusions remains unclear.

To investigate the role of NUP98 of NUP98 fusion proteins in leukemogenesis, we generated a set of deletion mutants of NUP98-HOXA9 lacking each FG repeat domain. These mutants were transduced into normal murine progenitor cells and their activity in the immortalization of murine cells was examined by colony-formation assays. Deletion of the second FG domain resulted in a dramatic delay in leukemia induction when cells were transplanted into mice, suggesting that the second FG repeat domain is crucial for immortalization in vitro and leukemogenesis in vivo. qPCR and ChIP analyses showed that the FG repeat domain is necessary for Hoxa gene activation and recruitment to the Hoxa gene locus. To identify interacting proteins of the FG repeat domain of NUP98-HOXA9, protein complexes containing full-length and mutant NUP98-HOXA9 were purified. Mass spectrometry analysis showed that full-length NUP98-HOXA9, but not the mutant lacking the second FG repeat, interacted with MLL. ChIP analysis also showed co-localization of NUP98-HOXA9 and MLL at the Hoxa gene locus in NUP98-HOXA9-expressing cells. Furthermore, analysis of Mll null mice showed that MLL is essential for the recruitment of NUP98-HOXA9 to the Hoxa gene locus and for NUP98-HOXA9-induced immortalization and leukemogenesis. These results suggest that NUP98 fusion proteins interact with MLL to activate HOXA genes during leukemogenesis.