Mutations in calreticulin (CALR) are phenotypic drivers in the pathogenesis of myeloproliferative neoplasms. Mechanistic studies have demonstrated that mutant CALR binds to the thrombopoietin receptor MPL, and that the positive electrostatic charge of the mutant CALR C terminus is required for mutant CALR-mediated activation of JAK-STAT signaling. Here we demonstrate that although binding between mutant CALR and MPL is required for mutant CALR to transform hematopoietic cells; binding alone is insufficient for cytokine independent growth. We further show that the threshold of positive charge in the mutant CALR C terminus influences both binding of mutant CALR to MPL and activation of MPL signaling. We find that mutant CALR binds to the extracellular domain of MPL and that 3 tyrosine residues within the intracellular domain of MPL are required to activate signaling. With respect to mutant CALR function, we show that its lectin-dependent function is required for binding to MPL and for cytokine independent growth, whereas its chaperone and polypeptide-binding functionalities are dispensable. Together, our findings provide additional insights into the mechanism of the pathogenic mutant CALR-MPL interaction in myeloproliferative neoplasms.
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UNLABELLED: Somatic mutations in calreticulin (CALR) are present in approximately 40% of patients with myeloproliferative neoplasms (MPN), but the mechanism by which mutant CALR is oncogenic remains unclear. Here, we demonstrate that expression of mutant CALR alone is sufficient to engender MPN in mice and recapitulates the disease phenotype of patients with CALR-mutant MPN. We further show that the thrombopoietin receptor MPL is required for mutant CALR-driven transformation through JAK-STAT pathway activation, thus rendering mutant CALR-transformed hematopoietic cells sensitive to JAK2 inhibition. Finally, we demonstrate that the oncogenicity of mutant CALR is dependent on the positive electrostatic charge of the C-terminus of the mutant protein, which is necessary for physical interaction between mutant CALR and MPL. Together, our findings elucidate a novel paradigm of cancer pathogenesis and reveal how CALR mutations induce MPN. SIGNIFICANCE: The mechanism by which CALR mutations induce MPN remains unknown. In this report, we show that the positive charge of the CALR mutant C-terminus is necessary to transform hematopoietic cells by enabling binding between mutant CALR and the thrombopoietin receptor MPL.
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Impaired erythropoiesis in the deletion 5q (del(5q)) subtype of myelodysplastic syndrome (MDS) has been linked to heterozygous deletion of RPS14, which encodes the ribosomal protein small subunit 14. We generated mice with conditional inactivation of Rps14 and demonstrated an erythroid differentiation defect that is dependent on the tumor suppressor protein p53 (encoded by Trp53 in mice) and is characterized by apoptosis at the transition from polychromatic to orthochromatic erythroblasts. This defect resulted in age-dependent progressive anemia, megakaryocyte dysplasia and loss of hematopoietic stem cell (HSC) quiescence. As assessed by quantitative proteomics, mutant erythroblasts expressed higher levels of proteins involved in innate immune signaling, notably the heterodimeric S100 calcium-binding proteins S100a8 and S100a9. S100a8--whose expression was increased in mutant erythroblasts, monocytes and macrophages--is functionally involved in the erythroid defect caused by the Rps14 deletion, as addition of recombinant S100a8 was sufficient to induce a differentiation defect in wild-type erythroid cells, and genetic inactivation of S100a8 expression rescued the erythroid differentiation defect of Rps14-haploinsufficient HSCs. Our data link Rps14 haploinsufficiency in del(5q) MDS to activation of the innate immune system and induction of S100A8-S100A9 expression, leading to a p53-dependent erythroid differentiation defect.
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© 2015 The Authors. JAK2V617F is the most common oncogenic lesion in patients with myeloproliferative neoplasms (MPNs). Despite the ability of JAK2V617F to instigate DNA damage in vitro, MPNs are nevertheless characterized by genomic stability. In this study, we address this paradox by identifyingthe DNA helicase RECQL5 as a suppressor of genomic instability in MPNs. We report increased RECQL5 expression in JAK2V617F-expressing cells and demonstrate that RECQL5 is required to counteract JAK2V617F-induced replication stress. Moreover, RECQL5 depletion sensitizes JAK2V617F mutant cells to hydroxyurea (HU), a pharmacological inducer of replication stress and the most common treatment for MPNs. Using single-fiber chromosome combing, we show that RECQL5 depletion in JAK2V617F mutant cells impairs replication dynamics following HU treatment, resulting in increased double-stranded breaks andapoptosis. Cumulatively, these findings identify RECQL5 as a critical regulator of genome stability in MPNs and demonstrate that replication stress-associated cytotoxicity can be amplified specifically in JAK2V617F mutant cells through RECQL5-targeted synthetic lethality.
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The JAK2V617F mutation is found in most patients with a myeloproliferative neoplasm (MPN). This gain-of-function mutation dysregulates cytokine signaling and is associated with increased accumulation of DNA damage, a process likely to drive disease evolution. JAK2V617F inhibits NHE-1 upregulation in response to DNA damage and consequently represses Bcl-xL deamidation and apoptosis, thus giving rise to inappropriate cell survival. However, the mechanism whereby NHE-1 expression is inhibited by JAK2V617F is unknown. In this study, we demonstrate that the accumulation of reactive oxygen species (ROS) in cells expressing JAK2V617F compromises the NHE-1/Bcl-xL deamidation pathway by repressing NHE-1 upregulation in response to DNA damage. In JAK2V617F-positive cells, increased ROS levels results from aberrant PI3K signaling, which decreases nuclear localization of FOXO3A and decreases catalase expression. Furthermore, when compared with autologous control erythroblasts, clonally derived JAK2V617F-positive erythroblasts from MPN patients displayed increased ROS levels and reduced nuclear FOXO3A. However, in hematopoietic stem cells (HSCs), FOXO3A is largely localized within the nuclei despite the presence of JAK2V617F mutation, suggesting that JAK2-FOXO signaling has a different effect on progenitors compared with stem cells. Inactivation of FOXO proteins and elevation of intracellular ROS are characteristics common to many cancers, and hence these findings are likely to be of relevance beyond the MPN field.
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Clonal proliferation in myeloproliferative neoplasms (MPN) is driven by somatic mutations in JAK2, CALR or MPL, but the contribution of inherited factors is poorly characterized. Using a three-stage genome-wide association study of 3,437 MPN cases and 10,083 controls, we identify two SNPs with genome-wide significance in JAK2(V617F)-negative MPN: rs12339666 (JAK2; meta-analysis P=1.27× 10(-10)) and rs2201862 (MECOM; meta-analysis P=1.96 × 10(-9)). Two additional SNPs, rs2736100 (TERT) and rs9376092 (HBS1L/MYB), achieve genome-wide significance when including JAK2(V617F)-positive cases. rs9376092 has a stronger effect in JAK2(V617F)-negative cases with CALR and/or MPL mutations(Breslow-Day P=4.5 × 10(-7)), whereas in JAK2(V617F)-positive cases rs9376092 associates with essential thrombocythemia (ET) rather than polycythemia vera (allelic χ(2) P=7.3 × 10(-7)). Reduced MYB expression, previously linked to development of an ET-like disease in model systems, associates with rs9376092 in normal myeloid cells. These findings demonstrate that multiple germline variants predispose to MPN and link constitutional differences in MYB expression to disease phenotype.
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Signaling mutations (eg, JAK2V617F) and mutations in genes involved in epigenetic regulation (eg, TET2) are the most common cooccurring classes of mutations in myeloproliferative neoplasms (MPNs). Clinical correlative studies have demonstrated that TET2 mutations are enriched in more advanced phases of MPNs such as myelofibrosis and leukemic transformation, suggesting that they may cooperate with JAK2V617F to promote disease progression. To dissect the effects of concomitant Jak2V617F expression and Tet2 loss within distinct hematopoietic compartments in vivo, we generated Jak2V617F/Tet2 compound mutant genetic mice. We found that the combination of Jak2V617F expression and Tet2 loss resulted in a more florid MPN phenotype than that seen with either allele alone. Concordant with this, we found that Tet2 deletion conferred a strong functional competitive advantage to Jak2V617F-mutant hematopoietic stem cells (HSCs). Transcriptional profiling revealed that both Jak2V617F expression and Tet2 loss were associated with distinct and nonoverlapping gene expression signatures within the HSC compartment. In aggregate, our findings indicate that Tet2 loss drives clonal dominance in HSCs, and Jak2V617F expression causes expansion of downstream precursor cell populations, resulting in disease progression through combinatorial effects. This work provides insight into the functional consequences of JAK2V617F-TET2 comutation in MPNs, particularly as it pertains to HSCs.
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A decade on from the discovery of the JAK2V617F mutation in the majority of patients with myeloproliferative neoplasms (MPNs), JAK2V617F is now firmly installed in the hematology curriculum of medical students and the diagnostic-testing algorithm of clinicians. Furthermore, the oral JAK1/JAK2 inhibitor ruxolitinib, rationally designed to target activated JAK2 signaling in MPN, has been approved by the Food and Drug Administration (FDA) of the United States for the past 3 years for the treatment of intermediate- and advanced-phase myelofibrosis. Notwithstanding this, JAK2V617F continues to stimulate the MPN research community and novel insights into understanding the mechanisms by which JAK2V617F contributes to the pathogenesis of MPN are continually emerging. In this chapter, we focus on recent advances in 4 main areas: (1) the molecular processes coopted by JAK2V617F to induce MPN, (2) the role that JAK2V617F plays in phenotypic diversity in MPN, (3) the functional impact of JAK2V617F on hematopoietic stem cells, and (4) therapeutic strategies to target JAK2V617F. Although great strides have been made, significant deficits still exist in our understanding of the precise mechanisms by which JAK2V617F-mutant hematopoietic stem cells emerge and persist to engender clonal hematopoiesis in MPN and in developing strategies to preferentially target the JAK2V617F-mutant clone therapeutically. Critically, although myelofibrosis remains arguably the greatest clinical challenge in JAK2V617F-mediated MPN, the current understanding of myelofibrosis-specific disease biology remains quite rudimentary. Therefore, many important biological questions pertaining to JAK2V617F will continue to engage and challenge the MPN research community in the coming decade.
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Cancers result from the accumulation of genetic lesions, but the cellular consequences of driver mutations remain unclear, especially during the earliest stages of malignancy. The V617F mutation in the JAK2 non-receptor tyrosine kinase (JAK2V617F) is present as an early somatic event in most patients with myeloproliferative neoplasms (MPNs), and the study of these chronic myeloid malignancies provides an experimentally tractable approach to understanding early tumorigenesis. Introduction of exogenous JAK2V617F impairs replication fork progression and is associated with activation of the intra-S checkpoint, with both effects mediated by phosphatidylinositide 3-kinase (PI3K) signaling. Analysis of clonally derived JAK2V617F-positive erythroblasts from MPN patients also demonstrated impaired replication fork progression accompanied by increased levels of replication protein A (RPA)-containing foci. However, the associated intra-S checkpoint response was impaired in erythroblasts from polycythemia vera (PV) patients, but not in those from essential thrombocythemia (ET) patients. Moreover, inhibition of p53 in PV erythroblasts resulted in more gamma-H2Ax (γ-H2Ax)-marked double-stranded breaks compared with in like-treated ET erythroblasts, suggesting the defective intra-S checkpoint function seen in PV increases DNA damage in the context of attenuated p53 signaling. These results demonstrate oncogene-induced impairment of replication fork progression in primary cells from MPN patients, reveal unexpected disease-restricted differences in activation of the intra-S checkpoint, and have potential implications for the clonal evolution of malignancies.
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Genomic regions of acquired uniparental disomy (UPD) are common in malignancy and frequently harbor mutated oncogenes. Homozygosity for such gain-of-function mutations is thought to modulate tumor phenotype, but direct evidence has been elusive. Polycythemia vera (PV) and essential thrombocythemia (ET), 2 subtypes of myeloproliferative neoplasms, are associated with an identical acquired JAK2V617F mutation but the mechanisms responsible for distinct clinical phenotypes remain unclear. We provide direct genetic evidence and demonstrate that homozygosity for human JAK2V617F in knock-in mice results in a striking phenotypic switch from an ET-like to PV-like phenotype. The resultant erythrocytosis is driven by increased numbers of early erythroid progenitors and enhanced erythroblast proliferation, whereas reduced platelet numbers are associated with impaired platelet survival. JAK2V617F-homozygous mice developed a severe hematopoietic stem cell defect, suggesting that additional lesions are needed to sustain clonal expansion. Together, our results indicate that UPD for 9p plays a causal role in the PV phenotype in patients as a consequence of JAK2V617F homozygosity. The generation of a JAK2V617F allelic series of mice with a dose-dependent effect on hematopoiesis provides a powerful model for studying the consequences of mutant JAK2 homozygosity.
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Subclones homozygous for JAK2V617F are more common and larger in patients with polycythemia vera compared to essential thrombocythemia, but their role in determining phenotype remains unclear. We genotyped 4564 erythroid colonies from 59 patients with polycythemia vera or essential thrombocythemia to investigate whether the proportion of JAK2V617F -homozygous precursors, compared to heterozygous precursors, is associated with clinical or demographic features. In polycythemia vera, a higher proportion of homozygous-mutant precursors was associated with more extreme blood counts at diagnosis, consistent with a causal role for homozygosity in polycythemia vera pathogenesis. Larger numbers of homozygous-mutant colonies were associated with older age, and with male gender in polycythemia vera but female gender in essential thrombocythemia. These results suggest that age promotes development or expansion of homozygous-mutant clones and that gender modulates the phenotypic consequences of JAK2V617F homozygosity, thus providing a potential explanation for the long-standing observations of a preponderance of men with polycythemia vera but of women with essential thrombocythemia.
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Subclones homozygous for JAK2V617F are more common in polycythemia vera (PV) than essential thrombocythemia (ET), but their prevalence and significance remain unclear. The JAK2 mutation status of 6495 BFU-E, grown in low erythropoietin conditions, was determined in 77 patients with PV or ET. Homozygous-mutant colonies were common in patients with JAK2V617F-positive PV and were surprisingly prevalent in JAK2V617F-positive ET and JAK2 exon 12-mutated PV. Using microsatellite PCR to map loss-of-heterozygosity breakpoints within individual colonies, we demonstrate that recurrent acquisition of JAK2V617F homozygosity occurs frequently in both PV and ET. PV was distinguished from ET by expansion of a dominant homozygous subclone, the selective advantage of which is likely to reflect additional genetic or epigenetic lesions. Our results suggest a model in which development of a dominant JAK2V617F-homzygous subclone drives erythrocytosis in many PV patients, with alternative mechanisms operating in those with small or undetectable homozygous-mutant clones.
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Here we present the Transcription Factor Encyclopedia (TFe), a new web-based compendium of mini review articles on transcription factors (TFs) that is founded on the principles of open access and collaboration. Our consortium of over 100 researchers has collectively contributed over 130 mini review articles on pertinent human, mouse and rat TFs. Notable features of the TFe website include a high-quality PDF generator and web API for programmatic data retrieval. TFe aims to rapidly educate scientists about the TFs they encounter through the delivery of succinct summaries written and vetted by experts in the field. TFe is available at http://www.cisreg.ca/tfe.
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Genetic alterations affecting members of the Janus kinase (JAK) family have been discovered in a wide array of cancers and are particularly prominent in hematological malignancies. In this review, we focus on the role of such lesions in both myeloid and lymphoid tumors. Oncogenic JAK molecules can activate a myriad of canonical downstream signaling pathways as well as directly interact with chromatin in noncanonical processes, the interplay of which results in a plethora of diverse biological consequences. Deciphering these complexities is shedding unexpected light on fundamental cellular mechanisms and will also be important for improved diagnosis, identification of new therapeutic targets, and the development of stratified approaches to therapy.
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In 2005, several groups identified a single gain-of-function point mutation in the JAK2 kinase that was present in the majority of patients with myeloproliferative neoplasms (MPNs). Since this discovery, much effort has been dedicated to understanding the molecular consequences of the JAK2V617F mutation in the haematopoietic system. Three waves of mouse models have been produced recently (bone marrow transplantation, transgenic and targeted knock-in), which have facilitated the understanding of the molecular pathogenesis of JAK2V617F-positive MPNs, providing potential platforms for designing and validating novel therapies in humans. This Commentary briefly summarises the first two types of mouse models and then focuses on the more recently generated knock-in models.
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The JAK2V617F mutation is associated with distinct myeloproliferative neoplasms, including polycythemia vera (PV) and essential thrombocythemia (ET), but it remains unclear how it generates disparate disorders. By comparing clonally-derived mutant and wild-type cells from individual patients, we demonstrate that the transcriptional consequences of JAK2V617F are subtle, and that JAK2V617F-heterozygous erythroid cells from ET and PV patients exhibit differential interferon signaling and STAT1 phosphorylation. Increased STAT1 activity in normal CD34-positive progenitors produces an ET-like phenotype, whereas downregulation of STAT1 activity in JAK2V617F-heterozygous ET progenitors produces a PV-like phenotype. Our results illustrate the power of clonal analysis, indicate that the consequences of JAK2V617F reflect a balance between STAT5 and STAT1 activation and are relevant for other neoplasms associated with signaling pathway mutations.
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BACKGROUND: The HOX11/TLX1 (hereafter referred to as HOX11) homeobox gene was originally identified at a t(10;14)(q24;q11) translocation breakpoint, a chromosomal abnormality observed in 5-7% of T cell acute lymphoblastic leukemias (T-ALLs). We previously reported a predisposition to aberrant spindle assembly checkpoint arrest and heightened incidences of chromosome missegregation in HOX11-overexpressing B lymphocytes following exposure to spindle poisons. The purpose of the current study was to evaluate cell cycle specific expression of HOX11. RESULTS: Cell cycle specific expression studies revealed a phosphorylated form of HOX11 detectable only in the mitotic fraction of cells after treatment with inhibitors to arrest cells at different stages of the cell cycle. Mutational analyses revealed phosphorylation on threonine-247 (Thr247), a conserved amino acid that defines the HOX11 gene family and is integral for the association with DNA binding elements. The effect of HOX11 phosphorylation on its ability to modulate expression of the downstream target, cyclin B1, was tested. A HOX11 mutant in which Thr247 was substituted with glutamic acid (HOX11 T247E), thereby mimicking a constitutively phosphorylated HOX11 isoform, was unable to bind the cyclin B1 promoter or enhance levels of the cyclin B1 protein. Expression of the wildtype HOX11 was associated with accelerated progression through the G2/M phase of the cell cycle, impaired synchronization in prometaphase and reduced apoptosis whereas expression of the HOX11 T247E mutant restored cell cycle kinetics, the spindle checkpoint and apoptosis. CONCLUSIONS: Our results demonstrate that the transcriptional activity of HOX11 is regulated by phosphorylation of Thr247 in a cell cycle-specific manner and that this phosphorylation modulates the expression of the target gene, cyclin B1. Since it is likely that Thr247 phosphorylation regulates DNA binding activity to multiple HOX11 target sequences, it is conceivable that phosphorylation functions to regulate the expression of HOX11 target genes involved in the control of the mitotic spindle checkpoint.
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The JAK2 V617F mutation is found in most patients with a myeloproliferative neoplasm and is sufficient to produce a myeloproliferative phenotype in murine retroviral transplantation or transgenic models. However, several lines of evidence suggest that disease phenotype is influenced by the level of mutant JAK2 signaling, and we have therefore generated a conditional knock-in mouse in which a human JAK2 V617F is expressed under the control of the mouse Jak2 locus. Human and murine Jak2 transcripts are expressed at similar levels, and mice develop modest increases in hemoglobin and platelet levels reminiscent of human JAK2 V617F-positive essential thrombocythemia. The phenotype is transplantable and accompanied by increased terminal erythroid and megakaryocyte differentiation together with increased numbers of clonogenic progenitors, including erythropoietin-independent erythroid colonies. Unexpectedly, JAK2(V617F) mice develop reduced numbers of lineage(-)Sca-1(+)c-Kit(+) cells, which exhibit increased DNA damage, reduced apoptosis, and reduced cell cycling. Moreover, competitive bone marrow transplantation studies demonstrated impaired hematopoietic stem cell function in JAK2(V617F) mice. These results suggest that the chronicity of human myeloproliferative neoplasms may reflect a balance between impaired hematopoietic stem cell function and the accumulation of additional mutations.
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Acute myeloid leukemia (AML) may follow a JAK2-positive myeloproliferative neoplasm (MPN), although the mechanisms of disease evolution, often involving loss of mutant JAK2, remain obscure. We studied 16 patients with JAK2-mutant (7 of 16) or JAK2 wild-type (9 of 16) AML after a JAK2-mutant MPN. Primary myelo-fibrosis or myelofibrotic transformation preceded all 7 JAK2-mutant but only 1 of 9 JAK2 wild-type AMLs (P = .001), implying that JAK2-mutant AML is preceded by mutation(s) that give rise to a "myelofibrosis" phenotype. Loss of the JAK2 mutation by mitotic recombination, gene conversion, or deletion was excluded in all wild-type AMLs. A search for additional mutations identified alterations of RUNX1, WT1, TP53, CBL, NRAS, and TET2, without significant differences between JAK2-mutant and wild-type leukemias. In 4 patients, mutations in TP53, CBL, or TET2 were present in JAK2 wild-type leukemic blasts but absent from the JAK2-mutant MPN. By contrast in a chronic-phase patient, clones harboring mutations in JAK2 or MPL represented the progeny of a shared TET2-mutant ancestral clone. These results indicate that different pathogenetic mechanisms underlie transformation to JAK2 wild-type and JAK2-mutant AML, show that TET2 mutations may be present in a clone distinct from that harboring a JAK2 mutation, and emphasize the clonal heterogeneity of the MPNs.© 2010 by The American Society of Hematology.
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The discovery of JAK2V617F as an acquired mutation in the majority of patients with myeloproliferative disorders (MPDs) and the key role of the JAK2-STAT5 signaling cascade in normal hematopoiesis has focused attention on the downstream transcriptional targets of STAT5. Despite evidence of its vital role in normal erythropoiesis and its ability to recapitulate many of the features of myeloid malignancies, including the MPDs, few functionally validated targets of STAT5 have been described. Here we used a combination of comparative genomics and chromatin immunoprecipitation assays to identify ID1 as a novel target of the JAK2-STAT5 signaling axis in erythroid cells. STAT5 binds and transactivates a downstream enhancer of ID1, and ID1 expression levels correlate with the JAK2V617F mutation in both retrovirally transfected fetal liver cells and polycythemia vera patients. Knockdown and overexpression studies in a well-characterized erythroid differentiation assay from primary murine fetal liver cells demonstrated a survival-promoting action of ID1. This hitherto unrecognized function implicates ID1 in the expansion of erythroblasts during terminal differentiation and suggests that ID1 plays an important role in the pathogenesis of polycythemia vera. Furthermore, our findings contribute to an increasing body of evidence implicating ID proteins in a wider range of cellular functions than initially appreciated.© 2009 by The American Society of Hematology.
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The TLX1/HOX11 homeobox gene was originally identified at the recurrent t(10;14)(q24;q11) translocation breakpoint, a chromosomal abnormality observed in 5-7% of T-cell acute lymphoblastic leukemias (T-ALLs). Proviral insertional mutagenesis studies performed on transgenic mice ectopically expressing TLX1/HOX11 in B lymphocytes (IgHmu-HOX11(Tg) mice) revealed the Ubr1 gene locus as a frequent site of proviral insertion, concomitant with accelerated development of diffuse large B-cell lymphoma. Insertion into this genomic region was confirmed by Southern blotting and by the ability to generate a polymerase chain reaction (PCR) amplicon across the viral-genome junction. Western immunoblot and semiquantitative reverse transcriptase-PCR analysis revealed downregulated expression of the Ubr1 gene product subsequent to viral integration. Loss or reduced levels of Ubr1 expression was associated with 5/14 spontaneous B-cell lymphomas in IgHmu-HOX11(Tg) mice and one of nine primary human T-ALLs. To gain mechanistic insight into the cooperativity between TLX1/HOX11 and Ubr1, IgHmu-HOX11(Tg)/Ubr1(-/-) mice were generated. IgHmu-HOX11(Tg)/Ubr1(-/-) mice exhibited a modest but statistically significant acceleration of disease onset relative to IgHmu-HOX11(Tg)/Ubr1(+/-) mice. Moreover, micronucleus assays to detect for chromosome missegregation were conducted and revealed increased presence of micronuclei in IgHmu-HOX11(Tg)/Ubr1(-/-) primary B lymphocyte cultures, and in both TLX1/HOX11-overexpressing T cell lines and fibroblast cultures following transfection with short interfering RNAs (siRNAs) targeting Ubr1. Karyotyping of primary B lymphocyte cultures revealed increased incidences of hypodiploid karyotypes. Finally, mitotic figures analysed from Ubr1 siRNA-transfected fibroblast cultures revealed no defects in chromosome congression to the metaphase plate, but increased incidences of atypical anaphase figures, including the development of anaphase bridges and lagging chromosomes. Based on these findings, we identify a synergistic role between TLX1/HOX11 overexpression and Ubr1 inactivation in promoting chromosome missegregation, permitting the accrual of additional chromosome losses and cytogenic abnormalities en route to malignancy.
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Dysregulated expression of the homeobox gene, HOX11 is a frequent etiologic event in T-cell acute lymphoblastic leukemias. HOX11-transgenic mice (IgHmu-HOX11Tg)-expressing HOX11 in the B-cell compartment develop B-cell lymphomas with extended latency. The latency suggests that additional genetic events are required prior to the onset of malignant lymphoma. We report the identification of 17 HOX11 collaborating genes, revealed through their propensity to be targeted in a proviral insertional mutagenesis screen. Seven integrations disrupted genes in mitotic spindle checkpoint control, suggesting that cells with elevated HOX11 expression are especially sensitive to dysregulation of chromosome segregation during mitosis. IgHmu-HOX11Tg primary B-lymphocyte cultures exposed to the aneugenic agents, colchicine and colcemid, exhibited increased incidences of chromosome missegregation as assessed by cytokinesis-block micronucleus assays. Additionally, IgHmu-HOX11Tg cultures were shown to exhibit aberrant bypass of spindle checkpoint arrest, as assessed by the increased presence of cycling cells determined by assessment of DNA content and by BrdU immunolabelling. Western immunoblotting revealed elevated expression of the mitotic effector molecules, cyclin A, cyclin B1 and cdc20 in IgHmu-HOX11Tg cultures. Moreover, spontaneously arising lymphoid neoplasms in IgHmu-HOX11Tg mice frequently exhibit aberrant expression of mitotic regulators, concomitant with increased development of micronuclei, abnormal mitotic checkpoint control and increased incidences of abnormal karyotypes when expanded in culture. Collectively, these findings indicate that abnormal regulation of spindle checkpoint control as a result of HOX11 overexpression leads to a heightened predisposition for development of aneuploidy, contributing to oncogenesis.
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