A Knock-In Npm1 Mutation in Mice Results in Myeloproliferation and Implies a Perturbation in Hematopoietic Microenvironment

Abstract

Somatic Nucleophosmin (NPM1) mutation frequently occurs in acute myeloid leukemia (AML), but its role in leukemogenesis remains unclear. This study reports the first “conventional” knock-in mouse model of Npm1 mutation, which was achieved by inserting TCTG after nucleotide c.857 (c.854_857dupTCTG) to mimic human mutation without any “humanized” sequence. The resultant mutant peptide differed slightly different from that in humans but exhibited cytoplasmic pulling force. Homozygous (Npm1c+/c+) mice showed embryonic lethality before day E8.5, wheras heterozygous (Npm1wt/c+) mice appeared healthy at birth and were fertile. Approximately 36% of Npm1wt/c+ mice developed myeloproliferative disease (MPD) with extramedullary hematopoiesis. Those Npm1wt/c+ mice that did not develop MPD nevertheless gradually developed monocytosis and showed increased numbers of marrow myeloid precursors. This second group of Npm1wt/c+ mice also showed compromised cobblestone area formation, suggesting pathology in the hematopoietic niche. Microarray experiments and bioinformatic analysis on mice myeloid precursor cells and 227 human samples revealed the expression of CXCR4/CXCL12-related genes was significantly suppressed in mutant cells from both mice and humans. Thus, our mouse model demonstrated that Npm1 mutation can result in MPD, but is insufficient for leukemogenesis. Perturbation of hematopoietic niche in mutant hematopoietic stem cells (implied by underrepresentation of CXCR4/CXCL12-related genes) may be important in the pathogenesis of NPM1 mutations.

CXC Chemokine Receptor 4 Expression, CXC Chemokine Receptor 4 Activation, and Wild-Type Nucleophosmin Are Independently Associated With Unfavorable Prognosis in Patients With Acute Myeloid Leukemia

“…In fact, recent studies by Chou et al utilizing both murine knock-in model of AML and human AML specimens demonstrated remarkable suppression of CXCR4/CXCL12-related gene signatures, attesting to the role of bone marrow niche defect in NPM1-mutant leukemias. 42 Interestingly, CXCR4 is not listed as one of the differentially expressed genes in their study, suggesting that differences in CXCL12 ligand expression could possibly account for the observed functional differences. Importantly, two recent papers in Nature confirmed the critical importance of CXCL12 produced by specialized stromal and endothelial cells within the bone marrow microenvironment for the support of normal HSC.…”

“…42 and demonstrated that the expression of CXCR4/CXCL12-related genes was significantly suppressed in mutant myeloid precursors compared to myeloid precursors with wild type NPM1 ; similarly, suppression of CXCL12 and CXCR4 pathway signatures was detected by genome-wide expression microarray analysis on BM mononuclear cells in NPM1 -mutated AML patients compared to NPM1 -wild AML. 42 …”

Retinoic acid and arsenic trioxide trigger degradation of mutated NPM1, resulting in apoptosis of AML cells

“…In these 3 patients, blast counts reincreased with discontinuation of treatment. Two additional patients 7,8 were treated; patient 7 died of invasive Aspergillosis at day 21 from RA/arsenic treatment with no evidence of response and patient 8 rapidly died of bilateral interstitial pneumonia at day 10 from RA/arsenic treatment (data not shown). Thus, RA and arsenic exerted a transient in vivo antileukemic effect in this subset of AML patients.…”

“…NPM1 mutations drive leukemogenesis, as hematopoietic disorders were observed in transgenics or knock-in mice. [6][7][8][9][10][11] Some studies suggested that addition of retinoic acid (RA) to conventional chemotherapy improves survival, selectively in AML patients harboring the NPM1 mutation in the absence of FLT3-ITD. 12 Other clinical studies reported negative results.…”

Localization of AML-related nucleophosmin mutant depends on its subtype and is highly affected by its interaction with wild-type NPM

“…the patients were heterozygous for the mutation and retained a wild-type allele [5,9,10]. Homozygous Npm1 mutant knock-in mice were reported to show embryonic lethality [14]. …”