NUP98 Is Fused to PMX1 Homeobox Gene in Human Acute Myelogenous Leukemia With Chromosome Translocation t(1;11)(q23;p15)

Trans-Repressive Effect of NUP98-PMX1 on PMX1-Regulated c-FOSGene through Recruitment of Histone Deacetylase 1 by FG Repeats

The formation of fusion genes between NUP98 and members of the HOX family represents a critical factor for the genesis of acute leukemia or acute transformation of chronic myeloid leukemia (CML). To gain insights into the molecular mechanisms underlying the leukemogenesis of NUP98-HOX fusion products, we cloned NUP98-PMX1 from a CML-blast crisis patient with t(1;11) as a secondary chromosomal translocation, and functionally studied the fusion products in detail through various molecular and protein biochemical assays. In addition to many interesting features, we have found that the NUP98-PMX1 fusion protein exerts a repressive effect on PMX1 or serum response factor-mediated c-FOS activation, probably through the recruitment of a common corepressor histone deacetylase 1 by FG domains of the NUP98-PMX1 fusion protein. Moreover, we have provided evidence that the FG domains of NUP98-PMX1 and two other NUP98-containing fusion proteins, i.e., NUP98-HOXA9 and NUP98-HOXC11, all exhibit dual binding ability to both CREB binding protein, a coactivator, and histone deacetylase 1, a corepressor. Accordingly, we have hypothesized that this dual binding activity is shared by most, if not all, NUP98-HOX-involved fusion proteins, enabling these fusion proteins to act as both trans-activators and trans-repressors, and contributing to the genesis of acute leukemia or acute transformation of CML.

NUP98 Is Fused to Topoisomerase (DNA) IIβ 180 kDa (TOP2B) in a Patient with Acute Myeloid Leukemia with a New t(3;11)(p24;p15)

PURPOSE

The nucleoporin 98 kDa (NUP98) gene has been reported to be fused to 17 different partner genes in various hematologic malignancies with 11p15 aberrations. Cytogenetic analysis of an adult de novo acute myelogenous leukemia (M5a) revealed a t(3;11)(p24;p15), suggesting rearrangement of NUP98 with a novel partner gene.

EXPERIMENTAL DESIGN

Fluorescence in situ hybridization (FISH) was used to confirm the involvement of NUP98 in the t(3;11)(p24;p15). Selection of possible NUP98 partner genes was done by computer-aided analysis of the 3p24 region using the University of California Santa Cruz genome browser. Fusion gene-specific FISH and reverse transcription-PCR analyses were done to verify the presence of the new NUP98 fusion.

RESULTS

FISH analysis using a NUP98-specific clone showed a split signal, indicating that the NUP98 gene was affected by the translocation. Of the genes localized at 3p24, TOP2B was selected as a possible fusion partner candidate gene. Dual-color fusion gene-specific FISH and reverse transcription-PCR analysis verified that NUP98 was indeed fused to TOP2B. In addition to reciprocal NUP98-TOP2B and TOP2B-NUP98 in-frame fusion transcripts, an alternatively spliced out-of-frame TOP2B-NUP98 transcript that resulted in a premature stop codon was detected. Analysis of the genomic breakpoints revealed typical signs of nonhomologous end joining resulting from error-prone DNA repair.

CONCLUSIONS

TOP2B encodes a type II topoisomerase, which is involved in DNA transcription, replication, recombination, and mitosis, and besides TOP1, represents the second NUP98 fusion partner gene that belongs to the topoisomerase gene family. This finding emphasizes the important role of topoisomerases in malignant transformation processes.

Nup98-HoxA9 immortalizes myeloid progenitors, enforces expression of Hoxa9, Hoxa7 and Meis1, and alters cytokine-specific responses in a manner similar to that induced by retroviral co-expression of Hoxa9 and Meis1

The association between acute myeloid leukaemia (AML) and the aberrant expression of Hoxa9 is evidenced by (1) proviral activation of Hoxa9 and Meis1 in BXH-2 murine AML, (2) formation of the chimeric Nup98-HoxA9 transactivator protein as a consequence of the t(7;11) translocation in human AML, and (3) the strong expression of HoxA9 and Meis1 in human AML. In mouse models, enforced retroviral expression of Hoxa9 alone in marrow is not sufficient to cause rapid AML, while co-expression of Meis1 and Hoxa9 induces rapid AML. In contrast, retroviral expression of Nup98-HoxA9 is sufficient to cause rapid AML in the absence of enforced Meis1 expression. Previously, we demonstrated that Hoxa9 could block the differentiation of murine marrow progenitors cultured in granulocyte-macrophage colony-simulating factor (GM-CSF). These progenitors lacked Meis1 expression, could not proliferate in stem cell factor (SCF), but could differentiate into neutrophils when switched into granulocyte colony-simulating factor (G-CSF). Ectopic expression of Meis1 in these Hoxa9 cells suppressed their G-CSF-induced differentiation, permitted proliferation in SCF, and therein offered a potential explanation of cooperative function. Because Meis1 binds N-terminal Hoxa9 sequences that are replaced by Nup98, we hypothesized that Nup98-HoxA9 might consolidate the biochemical functions of both Hoxa9 and Meis1 on target gene promoters and might evoke their same lymphokine-responsive profile in immortalized progenitors. Here we report that Nup98-HoxA9, indeed mimicks Hoxa9 plus Meis1 coexpression - it immortalizes myeloid progenitors, prevents differentiation in response to GM-CSF, IL-3, G-CSF, and permits proliferation in SCF. Unexpectedly, however, Nup98-Hoxa9 also enforced strong transcription of the cellular Hoxa9, Hoxa7 and Meis1 genes at levels similar to those found in mouse AML's generated by proviral activation of Hoxa9 and Meis1. Using Hoxa9(-/-) marrow, we demonstrate that expression of Hoxa9 is not required for myeloid immortalization by Nup98-HoxA9. Rapid leukaemogenesis by Nup98-HoxA9 may therefore result from both the intrinsic functions of Nup98-HoxA9, as well as of those of coexpressed HOX and MEIS1 genes.

Transcription factor fusions in acute leukemia: variations on a theme

The leukemia-associated fusion proteins share several structural or functional similarities, suggesting that they may impart a leukemic phenotype through common modes of transcriptional dysregulation. The fusion proteins generated by these translocations usually contain a DNA-binding domain, domains responsible for homo- or hetero-dimerization, and domains that interact with proteins involved in chromatin remodeling (e.g., co-repressor molecules or co-activator molecules). It is these shared features that constitute the 'variations on the theme' that underling the aberrant growth and differentiation that is the hallmark of acute leukemia cells.