Establishing the Role of EPS15, DVL2 and Cortactin in CALM-AF10 Leukemogenesis

Abstract Background: Leukemia is the most common type of childhood cancer. Although the prognosis for many pediatric leukemias has improved, leukemias associated with the t(10;11) CALM-AF10 translocation remain difficult to treat. CALM-AF10 leukemias account for ~5-10% of childhood T-cell acute lymphoblastic leukemia (T-ALL)as well as a subset of acute myeloid leukemia (AML). CALM-AF10 leukemias exhibit increased expression of proleukemic HOXA genes, but relatively little is known about the cellular mechanisms that drive CALM-AF10 leukemogenesis. Our laboratory has demonstrated that the CALM protein contains a nuclear export signal (NES) that is critical for CALM-AF10-dependent leukemogenesis. The NES interacts with the CRM1/XPO1 nuclear export receptor, which shuttles proteins from the nucleus to the cytoplasm through the nuclear pore complex. We have shown that transcriptional activation of HOXA genes by CALM-AF10 is dependent on its interaction with CRM1. Importantly, CRM1 does not contain a recognized DNA binding domain, and it is not currently understood how the CALM-AF10/CRM1 complex interacts with regulatory regions of HOXA genes. To identify proteins that mediate the interaction between the CALM-AF10/CRM1 complex and DNA, we took advantage of a proximity-based labeling approach using BioID2, a second-generation biotin ligase. When fused to a protein of interest and in the presence of biotin, BioID2 biotinylates proteins in close proximity to the ligase. These biotinylated proteins can then be identified by mass spectrometry (MS). Methods: We prepared an expression plasmid in which BioID2 was cloned in-frame with CALM-AF10. Human Embryonic Kidney 293 (HEK293) cells were transiently transfected with BioID2-CALM-AF10 and grown in the presence or absence of biotin. MS was performed to identify candidate interacting proteins. We validated direct interactions of candidate proteins with CALM-AF10 using co-immunoprecipitation experiments in HEK293 cells transfected with a CALM-AF10 plasmid. We confirmed that candidate proteins are present in murine CALM-AF10 leukemia cells via Western blotting. In order to efficiently knockout (KO) candidate proteins, we have generated a human U937 cell line (which harbors a t(10;11) CALM-AF10 translocation) with a stable incorporated Cas9. To assess whether KO of EPS15, DVL2 or CTTN affects HOXA5 expression, we performed RT-qPCR in U937-Cas9 cells lines with confirmed KO. Results: We carried out three independent transfections/MS experiments, which identified 71, 95 and 61 proteins, respectively. Of the proteins identified, 12 candidates were common to all three experiments . Importantly, we identified Disruptor Of Telomeric silencing 1-Like (DOT1L), a protein known to interact with AF10, and Nuclear pore complex protein 214 (NUP214), a protein that interacts with CRM1 and that is involved in leukemogenic translocations. We chose EPS15, DVL2 and CTTN for further study, as each of these proteins plays a role in leukemogenesis. We performed initial validation of direct interactions via co-immunoprecipitation and found that all three proteins co-precipitate with CALM-AF10. Western blotting showed that all three proteins are expressed in a murine CALM-AF10 leukemia cell line. We effectively knocked out EPS15 protein expression in U937 cells, and showed that HOXA5 expression is reduced in the setting of EPS15 knockout. Conclusion: We used biotin ligase-dependent proximity-based labeling to identify candidate proteins that potentially interact with the CALM-AF10 fusion protein. Our identification of DOT1L validates the approach, since DOT1L is known to interact with CALM-AF10. We have started to investigate three candidate proteins - EPS15, DVL2 and CTTN - all of which are involved in leukemogenic transformation. We have shown that EPS15, DVL2 and CTTN are expressed in murine CALM-AF10 leukemia cells and directly interact with the CALM-AF10 fusion protein. Knockout of EPS15 in U937 cells results in decreased HOXA5 expression, suggesting the importance of EPS15 in CALM-AF10 leukemogenesis. Evaluation of the roles of these proteins in leukemogenesis may lead to identification of novel pathways involved in CALM-AF10 leukemogenesis. Disclosures No relevant conflicts of interest to declare.

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Characterizing Proteins That Mediate CALM-AF10 Leukemogenesis

Blood ◽

10.1182/blood-2020-138816 ◽

2020 ◽

Vol 136 (Supplement 1) ◽

pp. 14-14

Author(s):

Rafi Kazi ◽

Waitman K. Aumann ◽

Pritha Bagchi ◽

Daniel S. Wechsler

Keyword(s):

Mass Spectrometry ◽

Growth Factor ◽

Epidermal Growth Factor ◽

Nuclear Pore Complex ◽

Transcriptional Activation ◽

Nuclear Export ◽

Nuclear Pore ◽

Biotin Ligase ◽

Epidermal Growth ◽

Hoxa Genes

Background: Leukemia is the most common type of childhood cancer. Although the prognosis for many pediatric leukemias has improved, leukemias associated with the t(10;11) CALM-AF10 translocation remain difficult to treat. CALM-AF10 leukemias account for ~5-10% of childhood T-cell acute lymphoid leukemia (T-ALL) as well as a subset of acute myeloid leukemia (AML). CALM-AF10 leukemias exhibit increased expression of proleukemic HOXA genes, but relatively little is known about the cellular mechanisms that drive CALM-AF10 leukemogenesis. Our laboratory has demonstrated that the CALM protein contains a nuclear export signal (NES) that is critical for CALM-AF10-dependent leukemogenesis. The NES interacts with the CRM1/XPO1 nuclear export receptor, which shuttles proteins from the nucleus to the cytoplasm through the nuclear pore complex. We have shown that transcriptional activation of HOXA genes by CALM-AF10 is critically dependent on its interaction with CRM1. Importantly, CRM1 does not contain a recognized DNA binding domain, and it is not currently understood how the CALM-AF10/CRM1 complex interacts with regulatory regions of HOXAgenes. In order to identify proteins that mediate the interaction between the CALM-AF10/CRM1 complex and DNA, we took advantage of a proximity-based labeling approach using BioID2, a second-generation biotin ligase. When fused to a protein of interest and in the presence of biotin, BioID2 biotinylates proteins in close proximity to the ligase. These biotinylated proteins can then be identified by mass spectrometry (MS). Methods: We prepared an expression plasmid in which BioID2 was cloned in-frame with CALM-AF10. We then transiently transfected Human Embryonic Kidney 293 (HEK293) cells with the BioID2-CALM-AF10 plasmid, grew them in the presence or absence of biotin, and performed streptavidin-pulldown followed by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) to identify candidate interacting proteins. Proteins were considered candidates if they had a peptide spectrum match (PSM) score > 10 and at least a two-fold greater PSM score versus negative control. We validated direct interactions of candidate proteins with CALM-AF10 by performing co-immunoprecipitation experiments. Results: We first confirmed that the addition of BioID2 to CALM-AF10 does not affect the transcriptional activation of HOXA genes or CALM-AF10 mediated immortalization of hematopoietic stem cells. We carried out three independent transfections/LC-MS/MS experiments, which identified 71, 95 and 61 proteins, respectively. Of the proteins identified, 11 candidates were common to all three experiments.Of particular interest, we identified Disruptor Of Telomeric silencing 1-Like (DOT1L), a protein known to interact with AF10, and Nuclear pore complex protein 214 (NUP214), a protein that has been identified in leukemogenic translocations. The nine additional candidate proteins included: EPS15, DVL2, DVL3, and DDX3X -all known to play a role in leukemogenesis. We performed initial validation of direct interactions via co-immunoprecipitation and found that Epidermal Growth Factor Receptor Substrate 15(EPS15) co-precipitates with CALM-AF10. Conclusion: We used biotin ligase-dependent proximity-based labeling to identify candidate proteins that potentially interact with the CALM-AF10 fusion protein. Our identification of DOT1L validates the approach, since DOT1L is known to interact with CALM-AF10. We have started to investigate other candidate proteins, focusing on known translocation partners in various leukemias. Our screen identified EPS15, a protein involved in receptor-mediated endocytosis of epidermal growth factor and a known translocation partner for MLL/KMT2A. KMT2A-EPS15 translocations (t(1;11)(p32;q23)) have been identified in both AML and ALL, and KMT2A-EPS15 is among the eight most common KMT2A rearrangements. We have shown that EPS15 co-immunoprecipitates with CALM-AF10, suggesting that EPS15 may also play a role in CALM-AF10 leukemogenesis. Further evaluation of this interaction is underway, and may lead to identification of novel pathways involved in CALM-AF10 leukemogenesis. Disclosures No relevant conflicts of interest to declare.

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The CRM1 Nuclear Export Receptor Activates HOXA Gene Expression in Leukemogenesis

Blood ◽

10.1182/blood.v124.21.2190.2190 ◽

2014 ◽

Vol 124 (21) ◽

pp. 2190-2190

Author(s):

Catherine P Lavau ◽

Jessica L Heath ◽

William H Lee ◽

Amanda E Conway ◽

Daniel S Wechsler

Keyword(s):

Gene Expression ◽

Bone Marrow ◽

Transcriptional Activation ◽

Nuclear Export ◽

Leukemia Cells ◽

Nuclear Pore ◽

Myeloid Neoplasms ◽

Hoxa Genes ◽

Hoxa Gene Expression ◽

Hoxa Gene

Abstract HOXA genes are effectors of oncogenic transformation that are frequently upregulated in myeloid and T-cell acute leukemias. Chromosomal translocation-derived oncoproteins, including MLL fusions, NUP (NUP98 or NUP214) fusions or CALM-AF10, bind to HOXA genes and result in their overexpression. We have previously demonstrated that a CRM1-dependent Nuclear Export Signal (NES) within CALM is essential for CALM-AF10’s ability to upregulate HOXA genes and cause leukemia in mice. Interfering with the CRM1/CALM-AF10 interaction by either genetic or pharmacologic inhibition abolishes CALM-AF10’s ability to bind to and activate HOXA gene expression. Furthermore, we showed that CRM1 binds to HOXA loci, suggesting that CRM1 recruits CALM-AF10 to its target genes. To explore whether CRM1 is also involved in the upregulation of Hoxa genes associated with MLL- and NUP98-fusion genes, we measured Hoxa transcript levels in murine leukemia cells treated with the CRM1 inhibitor Leptomycin B (LMB). LMB is a small molecule that covalently binds to the NES binding domain of CRM1 and blocks its ability to interact with NES partner proteins. We found that treatment of MLL-AF10, MLL-ENL, NUP98-HOXA9 or NUP98-AF10 leukemia cells with LMB (1 nM, 2 hours) causes a 50% reduction of Hoxa7, Hoxa9, Hoxa10 and Hoxa11 levels, similar to what is observed in CALM-AF10 leukemia cells. This suggests that in addition to its ability to interact with CALM-AF10, CRM1 may also participate in the transcriptional activation of Hoxa genes caused by MLL- or NUP98-fusion proteins. To demonstrate the importance of the CRM1/CALM interaction in CALM-AF10-dependent oncogenesis, we studied the biological activity of an artificial CRM1-AF10 fusion protein. Using a murine bone marrow clonogenic progenitor replating assay, we found that while native CRM1 overexpression did not result in transformation, the CRM1-AF10 fusion significantly increased the self-renewal of clonogenic progenitors. This effect was even more pronounced when CRM1 was fused to the MLL partner ENL: transduction with a CRM1-ENL fusion gene caused the immortalization of clonogenic bone marrow progenitors. Both CRM1-AF10- and CRM1-ENL-transduced progenitors displayed overexpression of Hoxa genes. To investigate the leukemogenic potential of CRM1-AF10in vivo, we transplanted mice with retrovirally transduced bone marrow progenitors and found that CRM1-AF10 induces myeloid neoplasms with a low penetrance and long latency (after more than a year of observation, 5 of 15 mice developed myeloid neoplasms between 160 and 220 days). These primary CRM1-AF10 leukemias could be transplanted to secondary recipients and cause myeloid leukemias with a shorter latency. Experiments to determine the leukemogenic potential of CRM1-ENLin vivo are ongoing. In contrast to CRM1-AF10, CRM1-ENL-transduced progenitors displayed a marked proliferative advantage in all transplanted mice (assessed by the elevation in the percentage of GFP-expressing CRM1-ENL-transduced cells in the peripheral blood over time); mice transplanted 74 days ago will be followed to determine survival curves. In summary, our results demonstrate that CRM1 regulates the expression of Hoxa genes in mouse leukemia cells, and alteration of CRM1’s activity can drive murine leukemogenesis. This has implications for understanding the mechanisms of HOXA deregulation in human leukemias induced by various fusion oncoproteins. It is noteworthy that in addition to interacting directly with CALM-AF10 through the NES, CRM1 physiologically interacts with NUP98 and NUP214 to facilitate transport through the nuclear pore. Our data also suggest that the anti-tumor effects of CRM1 inhibitors (Selective Inhibitors of Nuclear Export, SINEs) currently undergoing clinical trials, could be mediated, at least in part, by their ability to block the transcriptional activation of tumor-promoting genes by CRM1. Disclosures No relevant conflicts of interest to declare.

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Early Different Downstream Target of Glucocorticoid Receptor Contributing to Glucocorticoids Sensitivity in Kasumi-1 Cells

Blood ◽

10.1182/blood.v128.22.5132.5132 ◽

2016 ◽

Vol 128 (22) ◽

pp. 5132-5132

Author(s):

Wenbin Gu ◽

Meng Li ◽

Liang Liang ◽

Jian Zhang ◽

Chongye Guo ◽

...

Keyword(s):

Glucocorticoid Receptor ◽

Fusion Protein ◽

Cell Line ◽

Myeloid Leukemia ◽

Target Genes ◽

Conflicts Of Interest ◽

Critical Role ◽

Leukemia Cell ◽

Leukemia Cells ◽

Non Coding Rnas

Abstract The t(8;21) chromosome translocation frequently occurs in acute myeloid leukemia (AML), resulting in an in-frame fusion between the DNA-binding domain of AML1 and almost the entire of ETO gene. The fusion AML1-ETO protein is thought to play a critical role in the abnormal proliferation and differentiation of myeloid leukemia cells, such as Kasumi-1 and SKNO-1 cells. Glucocorticoids (GC) can induce apoptosis in these cells at low concentrations, whereas most other myeloid leukemia cell lines are resistant to glucocorticoid-induced apoptosis. To experimentally address possible sensitive mechanisms in leukemia cells with AML1-ETO translocation, we generated aGC-resistant Kasumi-1 cell line by induction of 10-6 M dexamethasone (Dex) for three weeks. The IC50 of Dex to cells is increased from 2.5×10-8 M for original GC-sensitive Kasumi-1 cell line ( K-S cell line) to more than 1×10-5 M for induced GC-resistant Kasumi-1 cell line (K-R cell line). Since GC resistance often results from mutations in the glucocorticoid receptor (GR), all the exons of GR gene were sequenced and no mutation was found in K-R cells. Comparing to those in K-S cells, the GR protein level didn't decrease in K-R cells after 2h, 4h, 8h, 12h and 24h exposure to dexamethasone. Given that the difference of direct GR downstream genes between K-S and K-R cells may play a key role in the GC sensitivity, we systematically analyzed the changes of gene expression induced by Dex versus ethanol vehicle for 8h in K-S and K-R cells by high throughput RNA sequencing. The time point of 8h was selected according to the expression peaks of several foregone GR target genes after Dex induction. There were found 32 genes conversely regulated in K-S and K-R cells, including 14 mRNAs and 18 long non-coding RNAs. Pathway analysis indicated that the upregulated genes in K-S cells might promote the AML1-ETO fusion protein degradation by proteasomes, while the component genes of this pathway were downregulated in K-R cells. Further validation and function studies of these mRNAs and long non-coding RNAs are ongoing. Our data suggested that the downstream targets of GR among GC-sensitive and -resistant Kasumi-1 cells were significant different and they may contribute to the GC sensitivity and resistance by degradation or reservation of AML-ETO fusion protein and the regulation of apoptosis in t(8;21) leukemia cell subtype. Disclosures No relevant conflicts of interest to declare.

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The importin-beta family member Crm1p bridges the interaction between Rev and the nuclear pore complex during nuclear export

Current Biology ◽

10.1016/s0960-9822(06)00335-6 ◽

1997 ◽

Vol 7 (10) ◽

pp. 767-775 ◽

Cited By ~ 289

Author(s):

Megan Neville ◽

Francoise Stutz ◽

Linda Lee ◽

Laura I Davis ◽

Michael Rosbash

Keyword(s):

Family Member ◽

Nuclear Pore Complex ◽

Nuclear Export ◽

Nuclear Pore ◽

Importin Beta

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The nuclear basket proteins Mlp1p and Mlp2p are part of a dynamic interactome including Esc1p and the proteasome

Molecular Biology of the Cell ◽

10.1091/mbc.e13-07-0412 ◽

2013 ◽

Vol 24 (24) ◽

pp. 3920-3938 ◽

Cited By ~ 63

Author(s):

Mario Niepel ◽

Kelly R. Molloy ◽

Rosemary Williams ◽

Julia C. Farr ◽

Anne C. Meinema ◽

...

Keyword(s):

Gene Expression ◽

Nuclear Pore Complex ◽

Nuclear Export ◽

Regulation Of Gene Expression ◽

Nuclear Pore ◽

Docking Site ◽

Discrete Structure ◽

Transport Channel ◽

Macromolecular Complexes ◽

Nuclear Stability

The basket of the nuclear pore complex (NPC) is generally depicted as a discrete structure of eight protein filaments that protrude into the nucleoplasm and converge in a ring distal to the NPC. We show that the yeast proteins Mlp1p and Mlp2p are necessary components of the nuclear basket and that they also embed the NPC within a dynamic protein network, whose extended interactome includes the spindle organizer, silencing factors, the proteasome, and key components of messenger ribonucleoproteins (mRNPs). Ultrastructural observations indicate that the basket reduces chromatin crowding around the central transporter of the NPC and might function as a docking site for mRNP during nuclear export. In addition, we show that the Mlps contribute to NPC positioning, nuclear stability, and nuclear envelope morphology. Our results suggest that the Mlps are multifunctional proteins linking the nuclear transport channel to multiple macromolecular complexes involved in the regulation of gene expression and chromatin maintenance.

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A Role for RanBP1 in the Release of CRM1 from the Nuclear Pore Complex in a Terminal Step of Nuclear Export

The Journal of Cell Biology ◽

10.1083/jcb.145.4.645 ◽

1999 ◽

Vol 145 (4) ◽

pp. 645-657 ◽

Cited By ~ 158

Author(s):

Ralph H. Kehlenbach ◽

Achim Dickmanns ◽

Angelika Kehlenbach ◽

Tinglu Guan ◽

Larry Gerace

Keyword(s):

Nuclear Pore Complex ◽

Nuclear Export ◽

Nuclear Pore ◽

Activated T Cells ◽

Permeabilized Cells ◽

Binding Domains ◽

Biochemical Fractionation ◽

Nuclear Export Receptor ◽

Gtpase Ran

We recently developed an assay in which nuclear export of the shuttling transcription factor NFAT (nuclear factor of activated T cells) can be reconstituted in permeabilized cells with the GTPase Ran and the nuclear export receptor CRM1. We have now used this assay to identify another export factor. After preincubation of permeabilized cells with a Ran mutant that cannot hydrolyze GTP (RanQ69L), cytosol supports NFAT export, but CRM1 and Ran alone do not. The RanQ69L preincubation leads to accumulation of CRM1 at the cytoplasmic periphery of the nuclear pore complex (NPC) in association with the p62 complex and Can/Nup214. RanGTP-dependent association of CRM1 with these nucleoporins was reconstituted in vitro. By biochemical fractionation and reconstitution, we showed that RanBP1 restores nuclear export after the RanQ69L preincubation. It also stimulates nuclear export in cells that have not been preincubated with RanQ69L. RanBP1 as well as Ran-binding domains of the cytoplasmic nucleoporin RanBP2 promote the release of CRM1 from the NPC. Taken together, our results indicate that RanGTP is important for the targeting of export complexes to the cytoplasmic side of the NPC and that RanBP1 and probably RanBP2 are involved in the dissociation of nuclear export complexes from the NPC in a terminal step of transport.

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GLE2, a Saccharomyces cerevisiae homologue of the Schizosaccharomyces pombe export factor RAE1, is required for nuclear pore complex structure and function.

Molecular Biology of the Cell ◽

10.1091/mbc.7.12.1921 ◽

1996 ◽

Vol 7 (12) ◽

pp. 1921-1937 ◽

Cited By ~ 119

Author(s):

R Murphy ◽

J L Watkins ◽

S R Wente

Keyword(s):

Saccharomyces Cerevisiae ◽

Schizosaccharomyces Pombe ◽

Nuclear Pore Complex ◽

Nuclear Export ◽

Protein Import ◽

Nuclear Transport ◽

Complex Structure ◽

Striking Similarity ◽

Nuclear Pore ◽

Pore Complexes

To identify and characterize novel factors required for nuclear transport, a genetic screen was conducted in the yeast Saccharomyces cerevisiae. Mutations that were lethal in combination with a null allele of the gene encoding the nucleoporin Nup100p were isolated using a colony-sectoring assay. Three complementation groups of gle (for GLFG lethal) mutants were identified. In this report, the characterization of GLE2 is detailed. GLE2 encodes a 40.5-kDa polypeptide with striking similarity to that of Schizosaccharomyces pombe RAE1. In indirect immunofluorescence and nuclear pore complex fractionation experiments, Gle2p was associated with nuclear pore complexes. Mutated alleles of GLE2 displayed blockage of polyadenylated RNA export; however, nuclear protein import was not apparently diminished. Immunofluorescence and thin-section electron microscopic analysis revealed that the nuclear pore complex and nuclear envelope structure was grossly perturbed in gle2 mutants. Because the clusters of herniated pore complexes appeared subsequent to the export block, the structural perturbations were likely indirect consequences of the export phenotype. Interestingly, a two-hybrid interaction was detected between Gle2p and Srp1p, the nuclear localization signal receptor, as well as Rip1p, a nuclear export signal-interacting protein. We propose that Gle2p has a novel role in mediating nuclear transport.

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Deguelin regulates nuclear pore complex proteins Nup98 and Nup88 in U937 cells in vitro1

Acta Pharmacologica Sinica ◽

10.1111/j.1745-7254.2005.00192.x ◽

2005 ◽

Vol 26 (10) ◽

pp. 1265-1273 ◽

Cited By ~ 8

Author(s):

Hong-li LIU ◽

Yan CHEN ◽

Guo-hui CUI ◽

Qiu-ling WU ◽

Jing HE ◽

...

Keyword(s):

Nuclear Pore Complex ◽

Nuclear Pore ◽

U937 Cells

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The Great Escape: mRNA Export through the Nuclear Pore Complex

International Journal of Molecular Sciences ◽

10.3390/ijms222111767 ◽

2021 ◽

Vol 22 (21) ◽

pp. 11767

Author(s):

Paola De Magistris

Keyword(s):

Conformational Changes ◽

Nuclear Pore Complex ◽

Nuclear Export ◽

Messenger Rna ◽

Mrna Export ◽

Protein Translation ◽

Nuclear Pore ◽

Body Of Knowledge ◽

Transport Receptors ◽

Basic Characteristics

Nuclear export of messenger RNA (mRNA) through the nuclear pore complex (NPC) is an indispensable step to ensure protein translation in the cytoplasm of eukaryotic cells. mRNA is not translocated on its own, but it forms ribonuclear particles (mRNPs) in association with proteins that are crucial for its metabolism, some of which; like Mex67/MTR2-NXF1/NXT1; are key players for its translocation to the cytoplasm. In this review, I will summarize our current body of knowledge on the basic characteristics of mRNA export through the NPC. To be granted passage, the mRNP cargo needs to bind transport receptors, which facilitate the nuclear export. During NPC transport, mRNPs undergo compositional and conformational changes. The interactions between mRNP and the central channel of NPC are described; together with the multiple quality control steps that mRNPs undergo at the different rings of the NPC to ensure only proper export of mature transcripts to the cytoplasm. I conclude by mentioning new opportunities that arise from bottom up approaches for a mechanistic understanding of nuclear export.

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SIX1 Is a Novel Effector Gene in CALM-AF10 Leukemia

Blood ◽

10.1182/blood-2019-130178 ◽

2019 ◽

Vol 134 (Supplement_1) ◽

pp. 5035-5035

Author(s):

Waitman K. Aumann ◽

Catherine P. Lavau ◽

Amanda Harrington ◽

Amanda E. Conway ◽

Daniel S. Wechsler

Keyword(s):

Nuclear Export ◽

Conflicts Of Interest ◽

Fetal Liver ◽

Critical Role ◽

Leukemia Cell ◽

The Self ◽

Hek293 Cells ◽

Hematopoietic Progenitors ◽

Self Renewal ◽

Hoxa Genes

Background: The CALM-AF10 translocation is detected in ~10% of T-cell acute lymphoblastic leukemias (T-ALLs), and in some acute myeloid leukemias (AMLs). CALM-AF10 leukemias are characterized by high expression of proleukemic HOXA genes, which serve a critical role in hematopoiesis. We hypothesized that identification of novel CALM-AF10 effector genes may yield new therapeutic targets in this difficult to treat leukemia. We took advantage of our prior observation that the nuclear export factor CRM1/XPO1 tethers CALM-AF10 to HOXA genes by interacting with a nuclear export signal (NES) in CALM. Using next generation sequencing, we determined that, SIX1, similar to HOXA genes, is increased in CALM-AF10 leukemias and decreased in response to CRM1 inhibition with Leptomycin B (LMB). Design/Methods: RT-qPCR and Chromatin Immunoprecipitation were performed using both bone marrow progenitors and murine embryonic fibroblasts (MEFs) transduced with CALM-AF10 or an empty vector, with and without LMB. The ability of SIX1 to enhance self-renewal of hematopoietic progenitors was examined by measuring the colony-forming ability of transduced fetal liver hematopoietic progenitor cells. CRISPR-Cas9 was used to silence SIX1 in Human Embryonic Kidney 293 (HEK293) cells. Results: RT-qPCR confirmed overexpression of SIX1 in both CALM-AF10 transduced MEFs and CALM-AF10 leukemias, with decreased SIX1 expression observed in the presence of LMB. ChIP analysis showed that CALM-AF10 binds to the SIX1 gene locus. Overexpression of SIX1 in fetal liver cells was sufficient to increase the self-renewal potential of colony-forming progenitors. SIX1 was successfully knocked out in HEK293 cells without a significant effect on HEK293 proliferation. Conclusions: The SIX1 homeobox gene is highly expressed during development and its expression is silenced post-embryogenesis. Increased SIX1 expression has been reported in numerous solid tumors. We have determined that SIX1 is upregulated in CALM-AF10 leukemias, and increases the self-renewal potential of hematopoietic progenitors. Using CRISPR-Cas9 to silence SIX1, we have demonstrated that SIX1 is not essential for cell survival, and that its inhibition may impair CALM-AF10 leukemia cell proliferation. Thus, SIX1 may play a pathogenic role in leukemogenesis and is a potential therapeutic target in CALM-AF10 leukemias. Disclosures No relevant conflicts of interest to declare.

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