Homeodomain-only protein homeobox (HOPX) is the smallest homeodomain protein. It was regarded as a stem cell marker in several non-hematopoietic systems. While the prototypic homeobox genes such as the HOX family have been well characterized in acute myeloid leukemia (AML), the clinical and biological implications of HOPX in the disease remain unknown. Thus we analyzed HOPX and global gene expression patterns in 347 newly diagnosed de novo AML patients in our institute. We found that higher HOPX expression was closely associated with older age, higher platelet counts, lower white blood cell counts, lower lactate dehydrogenase levels, and mutations in RUNX1, IDH2, ASXL1, and DNMT3A, but negatively associated with acute promyelocytic leukemia, favorable karyotypes, CEBPA double mutations and NPM1 mutation. Patients with higher HOPX expression had a lower complete remission rate and shorter survival. The finding was validated in two independent cohorts. Multivariate analysis revealed that higher HOPX expression was an independent unfavorable prognostic factor irrespective of other known prognostic parameters and gene signatures derived from multiple cohorts. Gene set enrichment analysis showed higher HOPX expression was associated with both hematopoietic and leukemia stem cell signatures. While HOPX and HOX family genes showed concordant expression patterns in normal hematopoietic stem/progenitor cells, their expression patterns and associated clinical and biological features were distinctive in AML settings, demonstrating HOPX to be a unique homeobox gene. Therefore, HOPX is a distinctive homeobox gene with characteristic clinical and biological implications and its expression is a powerful predictor of prognosis in AML patients.

Background Additional sex combs-like 1 (ASXL1) is frequently mutated in myeloid malignancies. Recent studies showed that hematopoietic-specific deletion of Asxl1 or overexpression of mutant ASXL1 resulted in myelodysplasia-like disease in mice. However, actual effects of a “physiological” dose of mutant ASXL1 remain unexplored.MethodsWe established a knock-in mouse model bearing the most frequent Asxl1 mutation and studied its pathophysiological effects on mouse hematopoietic system.ResultsHeterozygotes (Asxl1 tm/+) marrow cells had higher in vitro proliferation capacities as shown by more colonies in cobblestone-area forming assays and by serial re-plating assays. On the other hand, donor hematopoietic cells from Asxl1 tm/+ mice declined faster in recipients during transplantation assays, suggesting compromised long-term in vivo repopulation abilities. There were no obvious blood diseases in mutant mice throughout their life-span, indicating Asxl1 mutation alone was not sufficient for leukemogenesis. However, this mutation facilitated engraftment of bone marrow cell overexpressing MN1. Analyses of global gene expression profiles of ASXL1-mutated versus wild-type human leukemia cells as well as heterozygote versus wild-type mouse marrow precursor cells, with or without MN1 overexpression, highlighted the association of in vivo Asxl1 mutation to the expression of hypoxia, multipotent progenitors, hematopoietic stem cells, KRAS, and MEK gene sets. ChIP-Seq analysis revealed global patterns of Asxl1 mutation-modulated H3K27 tri-methylation in hematopoietic precursors.ConclusionsWe proposed the first Asxl1 mutation knock-in mouse model and showed mutated Asxl1 lowered the threshold of MN1-driven engraftment and exhibited distinct biological functions on physiological and malignant hematopoiesis, although it was insufficient to lead to blood malignancies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13045-017-0508-x) contains supplementary material, which is available to authorized users.

Plant homeodomain finger gene 6 (PHF6) encodes a 365-amino-acid protein containing 2 plant homology domain fingers. Germline mutations of human PHF6 cause Börjeson-Forssman-Lehmann syndrome, a congenital neurodevelopmental disorder. Loss-of-function mutations of PHF6 are detected in patients with acute leukemia, mainly of T-cell lineage and in a small proportion of myeloid lineage. The functions of PHF6 in physiological hematopoiesis and leukemogenesis remain incompletely defined. To address this question, we generated a conditional Phf6 knockout mouse model and investigated the impact of Phf6 loss on the hematopoietic system. We found that Phf6 knockout mice at 8 weeks of age had reduced numbers of CD4+ and CD8+ T cells in the peripheral blood compared with the wild-type littermates. There were decreased granulocyte-monocytic progenitors but increased Lin–c-Kit+Sca-1+ cells in the marrow of young Phf6 knockout mice. Functional studies, including competitive repopulation unit and serial transplantation assays, revealed an enhanced reconstitution and self-renewal capacity in Phf6 knockout hematopoietic stem cells (HSCs). Aged Phf6 knockout mice had myelodysplasia-like presentations, including decreased platelet counts, megakaryocyte dysplasia, and enlarged spleen related to extramedullary hematopoiesis. Moreover, we found that Phf6 loss lowered the threshold of NOTCH1-induced leukemic transformation at least partially through increased leukemia-initiating cells. Transcriptome analysis on the restrictive rare HSC subpopulations revealed upregulated cell cycling and oncogenic functions, with alteration of key gene expression in those pathways. In summary, our studies show the in vivo crucial roles of Phf6 in physiological and malignant hematopoiesis.

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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.

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762 Nucleophosmin (NPM1) is a ubiquitous multifunctional phosphoprotein, which has nucleocytoplasmic shuttling activity. Somatic mutations in NPM1 gene result in cytoplasmic dislocation of NPM1 (NPM1c) and are frequently associated with acute myeloid leukemia (AML). The pathogenetic effects of mutated NPM1 protein have been explored by animal models including transgenic or “humanized” knock-in mouse models. Here, we demonstrate the first “canonical” mouse Npm1 mutant knock-in model. Different from the previously report of humanized NPM1 mutant knock-in model, we inserted TCTG after nucleotide c.857 of murine Npm1 coding sequence (c.854–857dupTCTG), a pattern identical to human NPM1 mutation, without any “humanized” sequence. This mutation caused a shift of peptide sequence from WQWRKSL* (amino acid 286–292) to LCLAVEEISLRKGFKQFEIFCLHFCNS* (amino acid 285 to 311), a pattern mildly different from the change in human NPM1 mutation, but is still predicted to generate a nuclear export signal. NPM1c+ genotype and protein accumulation in cytoplasm were confirmed with PCR and immunocytochemistry, respectively. We found that the homozygous NPM mutant (NPMc+/c+) mice were embryonic lethal before E10.5 day, while hetrozygote (NPMwt/c+) mice survived and were fertile, and born with Mendelian ratio. Most NPMwt/c+ mice had normal hematologic parameters and remained disease-free, however, these mice developed a delayed-onset aberration on the distribution of granulocyte-monocye progenitors (GMP), monocytes, and B lymphocytes in blood, spleen, and bone marrow. Colony forming unit assay showed normal hematopoietic development of marrow hematopoietic stem cells (HSC), but poor cobblestone formation while HSCs contacted with stroma microenvironment in NPMwt/c+ mice, suggesting the NPMc mutant may affect the ability of HSCs on contact signal expression. Three (12.5%) of 24 NPMwt/c+ mice developed leukocytosis and splenomegaly mimicking myeloproliferative neoplasm of human. Microscopic examination showed panmyelosis of the bone marrow and existence of hematopoiesis in the spleen. In addition, the immune activities of NPMwt/c+ mice's splenocytes and thymocytes with mitogen stimulation were decreased. In summary, our “canonical” NPMwt/c+ mouse model demonstrated subtle but definitive phenotypes in hematopoietic cells and provided insight into the pathogenesis of NPM1 mutation in human acute myeloid leukemia. A second hit may be necessary for the development of AML in NPMwt/c+ mice since no AML was detected in these mice till 20 months of age. Disclosures: No relevant conflicts of interest to declare.