ALBERT

Description: PU.1 is a member of the ETS family of transcription factors and is required for the development of multiple hematopoietic lineages. PU.1-/- mice die from hematopoietic failure at about embryonic day 18.5 (e18.5) and show a complete absence of B cells, mature T cells, and macrophages. This phenotype suggests that PU.1 may function at the level of the hematopoietic stem cell (HSC) or a multilineage progenitor. To investigate the role of PU.1 in the regulation of HSCs, PU.1-/- embryos were analyzed at various stages of embryonic development. The absolute number and frequency of HSCs were determined by flow cytometric analysis of c-Kit+Thy-1.1loLin-Sca-1+ (KTLS) cells. We found that KTLS cells were absent or severely reduced in PU.1-/- fetal liver from e12.5 to e15.5. Progenitor cells with a c-Kit+Lin-AA4.1+ and c-Kit+Lin-CD34+ phenotype were also severely reduced. In addition, PU.1-/- fetal liver at e14.5 lacked common myeloid progenitors (CMPs) and granulocyte-macrophage progenitors (GMPs) but retained megakaryocyteerythroid progenitors (MEPs). Consistent with the loss of HSC activity, a 10-fold reduction in erythroid progenitors (mature erythroid burst-forming units [BFUEs]) was observed between e14.5 and e16.5. These data suggest that PU.1 plays an important role in the maintenance or expansion of HSC number in murine fetal liver. (Blood. 2004;104:3894-3900)

Description: Acute myeloid leukemia (AML) is self-renewal by immature myeloid precursors that fail to differentiate. An influential 'leukemia stem cell' model thus proposes that leukemogenic proteins augment or introduce a stem cell-like self-renewal program into cells, e.g., by upregulating signaling or transcription factors (TF) emblematic of stem cells (e.g., HOX). We investigated how the most recurrently mutated protein in AML, mutant nucleophosmin (mNPM1), causes leukemic cell expansion. The results challenge this model, but most importantly, open the door to rational targeted therapy for mNPM1 AML. One way of examining for stem cell programs in AML cells is to look at expression patterns of master TF that regulate expression of hundreds of genes and dictate cell fates. Of these select TF, the master TF that create hematopoietic stem cells (HLF etc.) are minimally or not expressed. Instead, there are very high levels of the master TF that drive monocyte and granulocyte lineage fates, PU.1 (SPI1) and CEBPA. Clearly, however, the lineage-programs intended by PU.1/CEBPA are inefficiently executed if at all - mNPM1 AML patient bone marrows had 85-97% cells with a granulocyte-monocyte progenitor phenotype, accumulated at the expense of downstream mature cells (Quek et al, JEM 2016). This aggregation at a lineage-committed, intermediate, naturally proliferative level of the hematopoietic hierarchy suggests an alternative model - instead of introducing a poorly-defined stem cell self-renewal program, mutant proteins disable differentiation programs which usually quench MYC-driven proliferation intrinsic to lineage-progenitors. To better understand how mNPM1 interacts with cellular machinery, we used mass-spectrometry to comprehensively document the protein interactions of endogenous NPM1 in AML cell nuclear and cytoplasmic fractions, the first analysis of this kind. Notably, the NPM1 protein interactome was enriched for PU.1. Critically, wild-type (wt) NPM1/PU.1 was in the nucleus of wtNPM1 AML cells, but mNPM1/PU.1 was in the cytoplasm of mNPM1 AML cells. This was evident clearly also by Western blot of cell fractions and by IF microscopy of primary AML cells and cell lines. Is cytoplasmic dis-location of PU.1 sufficient to explain persistent hematopoietic precursor proliferation? We used murine Pu.1 knock-out hematopoietic precursors transduced to express Pu.1 fused with the estrogen receptor (Pu.1-ER) to answer this question - Pu.1 relocation from the cytoplasm to the nucleus by tamoxifen triggered monocytic differentiation that terminated proliferation. Moreover, Pu.1-ER cells, like mNPM1 AML cells, highly express Hox genes, rapidly suppressed upon Pu.1 relocation to the nucleus. Thus, Pu.1 dominantly controls Hox and proliferation, as befitting of a master TF, and persistent HOX expression, like persistent progenitor proliferation, can be caused by Pu.1 loss-of-function. Protein macromolecules like NPM1 require transport factors to exit (exportins) the nucleus. A specific exportin, XPO1, was the major exportin found in the NPM1 interactome. XPO1 interactions with transported cargo can be inhibited by the small molecule drug KPT330. KPT330 10-20 nM rapidly re-located mNPM1 and PU.1 to the nucleus, downregulated MYC, upregulated p27/CDKN1B, upregulated monocyte but not granulocyte differentiation markers, induced morphologic changes of monocyte differentiation, and terminated proliferation of mNPM1 AML cells. The same low nanomolar treatment did not induce differentiation of wtNPM1 AML cells (THP1). Moreover, these KPT330 levels are not toxic to normal hematopoiesis (also shown by others). Thus, rather than gain-of-function of elusive stem cell-like self-renewal, the most frequently mutated protein in AML creates self-renewal by disabling a differentiation program that quenches intrinsic MYC-driven proliferation of lineage-progenitors. These observations are a mechanistic rationale to select refractory/relapsed mNPM1 AML patients for treatment with low well-tolerated doses of KPT330, with a defined molecular pharmacodynamic objective of returning PU.1 to the nucleus, to produce cell cycle exits by differentiation rather than p53-mediated apoptosis (to address chemotherapy resistance), to spare precious normal HSC (good therapeutic index), and directly reverse the basis for leukemic self-renewal (proliferation without differentiation). Figure. Figure. Disclosures Landesman: Karyopharm Therapeutics Inc: Employment, Other: stockholder. Saunthararajah:EpiDestiny: Consultancy, Other: patents around decitabine and tetrahydrouridine.

Description: The inversion of chromosome 16 in the inv(16)(p13q22) is one of the most frequent cytogenetic abnormalities observed in acute myeloid leukemia (AML). The inv(16) fuses the core binding factor (CBF) beta subunit with the coiled-coil rod domain of smooth muscle myosin heavy chain (SMMHC). Expression of CBFβ-SMMHC in mice does not promote AML in the absence of secondary mutations. Patient samples with the inv(16) also possess mutually exclusive activating mutations in either N-RAS, K-RAS, or the receptor tyrosine kinases, c-KIT and FLT3, in almost 70% of cases. To test whether an activating mutation of FLT3 (FLT3-ITD) would cooperate with CBFβ-SMMHC to promote AML, we coexpressed both mutations in hematopoietic progenitor cells used to reconstitute lethally irradiated mice. Analysis of transplanted animals showed strong selection for CBFβ-SMMHC/FLT3-ITD–expressing cells in bone marrow and peripheral blood. Compared with animals transplanted with only CBFβ-SMMHC–expressing cells, FLT3-ITD further restricted early myeloid differentiation and promoted peripheralization of primitive myeloblasts as early as 2.5 weeks after transplantation. FLT3-ITD also accelerated disease progression in all CBFβ-SMMHC/FLT3-ITD–reconstituted animals, which died of a highly aggressive and transplantable AML within 3 to 5 months. These results indicate that FLT3-activating mutations can cooperate with CBFβ-SMMHC in an animal model of inv(16)-associated AML.

Description: The c-kit (CD117) receptor is expressed on 〉 10% blasts in 64% of de novo AMLs and mediates proliferation and anti-apoptotic effects. High c-kit levels [defined as mean fluorescent intensity (MFI) 〉 20] correlate with a shorter time to relapse and decreased overall survival (OS). Imatinib mesylate (IM), a c-kit inhibitor, has activity against relapsed/ refractory AML. The primary objective of this study was to determine whether adding maintenance IM for 1 yr after completion of standard induction (IT) and post-remission therapy (PRT) in pts with newly diagnosed c-kit + AML improves progression-free survival (PFS) compared to historical controls. We previously presented our toxicity and correlative data at ASH 2012 (Abstract 3597). Here, we present our long term follow-up results. Methods: Pts were treated at Cleveland Clinic, Duke, Roswell Park, and University Hospitals of Cleveland from 2008 to 2012. IM was supplied by Novartis. Eligibility criteria: pts age ≥ 18 yrs, AML in first complete remission (CR1), ≥ 20% c-kit+ blasts at diagnosis (dx), ECOG performance status 0-2. Cytogenetics (CG) were classified per CALGB 8461. Pts must have received IT (7+3 [continuous infusion cytarabine and an anthracycline] or ADE [cytarabine, daunorubicin, etoposide]) and PRT (≥ 1 course for pts ≥ 60 yrs; ≥ 2 courses for pts 〈 60 yrs). CR was confirmed by bone marrow analysis prior to study enrollment. MDR expression was analyzed by IHC on diagnostic samples (n=19); AF1q gene expression was analyzed by RT-PCR on RNA from available diagnostic pt samples (n=9). C-kit MFI was calculated as the mean channel number (MCN) of the blasts/ MCN auto fluorescence using a CD45/orthogonal light scatter gate to isolate blasts and lymphocytes. All pts received IM 600 mg/day for 12 months (mos) unless they experienced toxicity or disease progression. Dose modifications were made for Grade 2-4 non-hematologic toxicity and Grades 3-4 neutropenia and thrombocytopenia. PFS was measured from the CR date to the time of relapse or death. Primary endpoints: Based on historical data from the Cleveland Clinic and SWOG, the median PFS for all AML pts undergoing IT 〈 60 yrs of age is 13 mos and for pts ≥ 60 yrs of age is 8 mos. The goal of this study was to see a 30% improvement in PFS at these time points in the respective age groups (i.e. 65% PFS at 13 mos for pts 〈 60 yrs; 65% PFS at 8 mos for pts ≥ 60 yrs). Results: Of 32 pts enrolled, the median age was 54 yrs (range 19-81), median WBC at dx 22.13 K/ uL (1.55-98.44), median peripheral blood blasts at dx 23.6% (range 0-85), and 44% were male. CG risk included: 16% (5) good, 66% (21) intermediate, 16% (5) poor, 3% (1) miscellaneous. Of the pts with normal CG, 10 were NPM1+, FLT3 ITD negative; and 1 pt was FLT3 ITD+. The median c-kit+ blast % was 79.9, and median c-kit MFI 39.8 (range 6.5-120.1). Median AF1q expression was 9.59 (range 1.83-161.85) (〉 9 is considered high and is associated with a poor prognosis; high AF1q is also associated with high c-kit expression). Eight-four percent of pts had moderate or high levels of drug resistance factors (GST1, MDR1, LRP1, and/or MRP1); almost half (47%) had high expression. There was no correlation between MDR and c-kit MFI. Pts received IM for a median of 4.0 mos (range 0.1-12.2) and the median daily dose was 600 mg. Twelve pts (38%) were dose reduced to 400 mg. Forty-five percent of pts experienced Grade 3 reactions possibly related to treatment, with the majority (31%) being myelosuppression. With a median follow-up time of 56.3 mos, the estimated median OS was 51.3 mos and estimated median relapse-free survival (RFS) 18.9 mos. The estimated PFS at 13 mos for pts 〈 60 yrs of age was 71 ± 10% (p=0.017, compared to the null hypothesis); and the estimated PFS at 8 mos for pts ≥ 60 yrs of age was 64 ± 15% (p=0.166, compared to the null hypothesis). Predictors of worse RFS included: age, WBC at dx, % peripheral blasts at dx, CG risk, and MDR expression. C-kit MFI and Af1q were not associated with RFS or OS. Conclusions: Use of IM maintenance therapy appeared to be associated with improved PFS compared to historical controls in pts 〈 60 yrs of age. In addition to a high c-kit MFI, these pts had other adverse characteristics (moderate to high levels of MDR. high AF1q). Though previous studies have demonstrated that c-kit MFI 〉 20.3 was an independent adverse prognostic factor for RFS and OS (median RFS 10.7 months) in AML, use of IM maintenance therapy in this study appeared to mitigate this, supporting further investigation. Disclosures Off Label Use: imatinib in the treatment of AML. Rao:Boehringer-Ingelheim: Other: Advisory Board; amgen: Other: ad board; novartis: Other: ad board. Rizzieri:Teva: Other: ad board, Speakers Bureau; Celgene: Other: ad board, Speakers Bureau. Wang:Immunogen: Research Funding. Griffiths:Alexion Pharmaceuticals: Honoraria; Astex: Research Funding; Celgene: Honoraria. Sekeres:TetraLogic: Membership on an entity's Board of Directors or advisory committees; Amgen: Membership on an entity's Board of Directors or advisory committees; Celgene Corporation: Membership on an entity's Board of Directors or advisory committees.

Description: Helios is a zinc-finger protein belonging to the Ikaros family of transcriptional regulators. It is expressed, along with Ikaros, throughout early stages of thymocyte development where it quantitatively associates with Ikaros through C-terminal zinc-finger domains that mediate heterodimerization between Ikaros family members. To understand the role of Helios in T-cell development, we used a retroviral vector to express full-length Helios or a Helios isoform that lacked the N-terminal DNA-binding domain in hematopoietic progenitor cells of reconstituted mice. Constitutive expression of full-length Helios resulted in an inhibition of T-cell development at the double-negative stage within the thymus. Although expression of the DNA-binding mutant of Helios did not contribute to developmental abnormalities at early times after transplantation, 60% of animals that expressed the Helios DNA-binding mutant developed an aggressive and transplantable T-cell lymphoma 4 to 10 months after transplantation. These results demonstrate a vital function for Helios in maintaining normal homeostasis of developing T cells and formally show that non–DNA-binding isoforms of Helios are lymphomagenic if aberrantly expressed within the T-cell lineage.

Description: Progenitor B cells deficient in Pax5 are developmentally multipotent, suggesting that Pax5 is necessary to maintain commitment to the B-cell lineage. Commitment may be mediated, in part, by Pax5 repression of myeloid-specific genes. To determine whether Pax5 expression in multipotential cells is sufficient to restrict development to the B-cell lineage in vivo, we enforced expression of Pax5 in hematopoietic stem cells using a retroviral vector. Peripheral blood analysis of all animals reconstituted with Pax5-expressing cells indicated that more than 90% of Pax5-expressing cells were B220+ mature B cells that were not malignant. Further analysis showed that Pax5 completely blocked T-lineage development in the thymus but did not inhibit myelopoiesis or natural killer (NK) cell development in bone marrow. These results implicate Pax5 as a critical regulator of B- versus T-cell developmental fate and suggest that Pax5 may promote commitment to the B-cell lineage by mechanisms that are independent of myeloid gene repression.

Description: NPM1 is the most frequently mutated gene in acute myeloid leukemia (AML). Unfortunately, there are no 'precision' or rational treatments for this subtype of AML.To elucidate molecular mechanisms of pathogenesis, we performed the first comprehensive, unbiased analysis of the endogenous NPM1 protein-interactome using mass-spectrometry (LC-MS/MS). This approach identified abundant amounts of the master transcription factor driver of monocyte lineage-differentiation PU.1 (SPI1). The NPM1/PU.1 interaction causes PU.1 functional deficiency when NPM1 is mutated, because mutant-NPM1 dislocates PU.1 into the cytoplasm with it.This was confirmed using six different methods: (i) Immunoprecipitation (IP)-LC-MS/MS from nuclear and cytoplasmic fractions of wildtype (wt) and NPM1 -mutated AML cell lines (n=2); (ii) IP-Western blot (WB) from nuclear/cytoplasmic fractions of NPM1 -mutated/wt AML cell lines (n=2); (iii) WB of nuclear/cytoplasmic fractions of NPM1 -mutated/wt AML cell lines (n=5); (iv) immunofluorescence microscopy (IF) of NPM1 -mutated/wt AML cell lines (n=5); (v) IF of NPM1 -mutated/wt primary AML cells (n=6); and (vi) cotransfection of HEK293 cells to express PU.1 + mutant vs wt-NPM1 followed by IF. Re-introduction of Pu.1 into the nucleus of Pu.1-null myeloid precursors which are differentiation-arrested and exponentially proliferating repressed key myeloid precursor genes (e.g., Hoxa9) and triggered terminal monocytic differentiation. Even though primary AML cells (n=900) express the PU.1/RUNX1/CEBPA monocyte differentiation-driving master transcription factor circuit at levels comparable to or exceeding that in normal monocytes, the expression of ~300 monocyte terminal-differentiation genes was suppressed. Importantly, the genes affected are strongly positively correlated (avg. rho 0.7) with PU.1 expression in normal hematopoiesis, consistent with functional disruption of PU.1 in AML. To translate these observations into a treatment option for NPM1 -mutated AMLs, we were guided by additional observations. First, the NPM1/PU.1 protein complex is exported by the nuclear export protein XPO1. XPO1-mediated nuclear export is inhibited by the small molecule selinexor. We found that sub-cytotoxic /low nanomolar concentrations of selinexor locked mutant-NPM1/PU.1 in the nucleus, releasing terminal monocytic differentiation of NPM1 -mutated AML cells both in vitro and in vivo . Briefly, NSG mice were xenotransplanted with NPM1 / FLT3 -mutated primary AML cells and observed until AML engraftment (≥20%) was confirmed. Selinexor was then administered by oral gavage at 2 mg/kg 4X/week (Fig1A), which is 10-fold lower than the usual in vivo cytotoxic dose (15-20 mg/kg) - low doses are well-tolerated and sufficient to promote non-cytotoxic differentiation of AML cells. After 50 days of treatment, bone marrow (Fig1B) and spleen (Fig1C) AML burden was significantly lower in selinexor vs vehicle treated mice. In addition, selinexor treated mice preserved murine hematopoiesis (Fig1D) and IF confirmed the partial nuclear restoration of PU.1 (Fig1E). Terminal monocytic differentiation of AML cells was evident by Giemsa-stained morphology (Fig1F) and flow cytometry (Fig1G). RUNX1 and CEBPA remain in NPM1 -mutated AML cell nuclei at high levels - PU.1 usually cooperates with these master transcription factor partners to exchange corepressors for coactivators and activate differentiation genes. Accordingly, IP-LC-MS/MS of endogenous nuclear CEBPA demonstrated enrichment for corepressors. Depletion of one of these corepressors, DNMT1, using non-cytotoxic concentrations of decitabine or 5-azacytidine (clinical DNMT1-depletors), also induced terminal-differentiation. Moreover, the granulocytic direction of differentiation naturally downregulated NPM1, an event inherent to CEBPA-driven granulocytic (but not monocytic) differentiation. This approach readily induced terminal-differentiation in NPM1 -mutated AML cells selected over 52 weeks of culture for resistance to selinexor, shown by exponential growth in selinexor 20 nM. The mechanisms by which mutant-NPM1 creates leukemic self-replication (proliferation uncoupled from differentiation) are thus reversed by non-cytotoxic molecular targeted clinical drugs. We are evaluating the combination of selinexor with non-cytotoxic DNMT1-depletion in vivo and clinical trials are planned. Disclosures Landesman: Karyopharm Therapeutics: Employment.