NUP98 is a
nucleoporin that plays complex roles in the nucleocytoplasmic trafficking of macromolecules. Rearrangements of the
NUP98 gene in human
leukemia result in the expression of numerous fusion
oncoproteins whose effect on nucleocytoplasmic trafficking is poorly understood. The present study was undertaken to determine the effects of leukemogenic
NUP98 fusion
proteins on CRM1-mediated nuclear export. NUP98-HOXA9, a prototypic
NUP98 fusion, inhibited the nuclear export of two known CRM1 substrates: mutated cytoplasmic
nucleophosmin and HIV-1 Rev. In vitro binding assays revealed that NUP98-HOXA9 binds CRM1 through the FG repeat motif in a Ran-
GTP-dependent manner similar to but stronger than the interaction between CRM1 and its export substrates. Two
NUP98 fusions, NUP98-HOXA9 and NUP98-DDX10, whose fusion partners are structurally and functionally unrelated, interacted with endogenous CRM1 in myeloid cells as shown by co-immunoprecipitation. These leukemogenic
NUP98 fusion
proteins interacted with CRM1, Ran, and the
nucleoporin NUP214 in a manner fundamentally different from that of wild-type
NUP98. NUP98-HOXA9 and NUP98-DDX10 formed characteristic aggregates within the nuclei of a myeloid cell line and primary human CD34+ cells and caused aberrant localization of CRM1 to these aggregates. These
NUP98 fusions caused nuclear accumulation of two
transcription factors, NFAT and NFkappaB, that are regulated by CRM1-mediated export. The nuclear entrapment of NFAT and NFkappaB correlated with enhanced transcription from promoters responsive to these
transcription factors. Taken together, the results suggest a new mechanism by which
NUP98 fusions dysregulate transcription and cause
leukemia, namely, inhibition of CRM1-mediated nuclear export with aberrant nuclear retention of transcriptional regulators.