NUP98-fusions although rare, have been associated with de novo acute myeloid leukemia (AML), chronic myeloid leukemia, myelodysplastic syndrome, T-cell acute lymphoblastic leukemia (T-ALL) and therapy-related myeloid malignancies. The NUP98-PHF23 (NP23) gene fusion was cloned from an acute myeloid leukemia (AML) patient with a t(11;17)(p15;p13) chromosome translocation. The nucleoporin 98 protein (NUP98), normally a component of the nuclear pore complex, is known to be fused to at least 28 different fusion partners as a result of structural chromosomal rearrangements associated with hematological malignancies. PHF23 encodes the Plant homeodomain (PHD) finger 23 protein. PHF23 is largely uncharacterized, but the PHD finger motif has been shown to act as a reader of di- and tri-methylated histone 3 lysine 4 (H3K4me2/3) marks. This suggests that PHF23 may function in chromatin regulation and that the NP23 fusion protein may play a role in aberrant chromatin modification at domains of active gene transcription. To determine the oncogenic potential of NP23, we generated a transgenic mouse model that expressed the human fusion gene in hematopoietic tissues.
We have characterized two founder lines (C10 and B10) expressing the NP23 fusion in hematopoietic tissue. Most of the offspring from the C10 line developed an AML that closely resembled the human disease, with increased blasts in the blood or bone marrow, widespread organ infiltration, and myeloid immunophenotype. Onset of disease was as early as 4.5 months, and 70 percent of the NP23 mice succumbed to leukemia by 12 months of age. Of note, an independent line (B10) developed a wider spectrum of leukemias but with similar age of onset and penetrance. The B10 mice predominantly developed T-ALL and AML, and four cases of B-ALL and one erythroleukemia, indicating that the NP23 protein was oncogenic in several different hematopoietic cell types. AMLs typically demonstrated an aberrant Mac-1+/B220dim phenotype, which has previously been recognized in leukemias caused by overexpression of the Hoxa cluster genes Hoxa5,7,9,10,11. Microarray gene expression analysis identified the Hoxa cluster genes to be markedly overexpressed, and validation by RQ-PCR demonstrated that Hoxa5, a7, a9 and a10 overexpression ranged from 10- to greater than 1000-fold increased in both the AML and T-ALL samples compared to wild type hematopoietic tissues; these Hoxa cluster genes were also overexpressed in hematopoietic tissues from clinically healthy NP23 transgenic mice. We also identified a novel transcript, Gm525 (homologue of H. sapiens C17orf67), that is markedly (100x) elevated specifically in the T-ALL samples. Most T-ALL samples have HD or PEST domain Notch1 mutations, and Notch1 mRNA levels, as well as its downstream target Hes1, are elevated in the T-ALLs compared to WT thymus. Immortal cell lines were established from two of the NP23 T-ALLs, and ChIP-seq was used to assay the genome wide pattern of H3K4me3 and H3K27me3 histone marks. Results show abundant levels of H3K4me3 at the Hoxa locus which tightly correlates to the increased Hoxa cluster gene expression seen in the cell lines. The NP23 model will be useful for identifying oncoproteins involved in leukemic transformation, particularly those oncoproteins which play a role in chromatin modification or are downstream targets of the HOXA genes.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.