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While Ras/mitogen-activated protein kinase (MAPK) signaling is activated in most human cancers, attempts to target this pathway using kinase active site inhibitors have not typically led to durable, clinical benefit. To address this shortcoming, we sought to test the feasibility of an alternative targeting strategy, focused on the ERK2 substrate binding domains, D and DBP. We found that disabling the ERK2-DBP domain in mice caused baseline erythrocytosis. Consequently, we investigated the role of the ERK2-D and -DBP domains in disease, using a JAK2-dependent model of polycythemia vera (PV). Importantly, inactivation of the ERK2-DBP domain promoted the progression of disease from PV to myelofibrosis (MF), suggesting that the ERK2-DBP domain normally opposes progression. ERK2-DBP inactivation also prevented oncogenic JAK2 kinase (JAK2V617F) from promoting oncogene-induced senescence (OIS) in vitro. The ERK2-DBP mutation attenuated JAK2-mediated OIS by preventing the physical interaction of ERK2 with the transcription factor, Egr1. Because inactivation of the ERK2-DBP created a functional ERK2 kinase limited to binding substrates through its D-domain, these data suggested that the D-domain substrates were responsible for promoting oncogene-induced progenitor growth and tumor progression, and that pharmacologic targeting of the ERK2-D domain might attenuate cancer cell growth. Indeed, pharmacologic agents targeting the ERK2-D domain were effective in attenuating the growth of JAK2-dependent myeloproliferative neoplasm cell lines. Taken together, these data indicate that the ERK-D and -DBP domains can play distinct roles in the progression of neoplasms and that the D-domain has the potential to be potent therapeutic target in Ras/MAPK dependent cancers.

The interaction between ERK2 and its substrates is critically mediated by two domains, the common docking (CD) D-domain and DEF-binding pocket (DBP) domain. Previous studies have suggested that ERK2 is not only necessary to drive hematopoietic and myelo-erythroid development, but it is also important for the pathogenesis of hematological cancers, as revealed by recurrent ERK2 somatic mutations in many types of lymphoma and leukemia. Here we show that the activation of ERK2 in JAK2V617F-driven myeloproliferative neoplasm (MPN) enhance polycythemia vera (PV) progression from erythrocytosis to myelofibrosis when ERK2-DBP binding capacity is disabled. Conversely, targeting the ERK2 D-domain preserves ERK2 catalytic function while reducing outgrowth and proliferation of human and murine MPN cell lines. To determine whether ERK2 binding domains play a role in the pathogenesis of JAK2V617F-driven MPN, we generated an ERK2-Y261A (Erk2 Y261A) mutant knockin mouse model in which the ERK2-DBP domain was inactivated, but ERK2 kinase activity and D-domain function was preserved. We observed splenomegaly in Erk2 Y261A/Y261A, characterized by a 10-fold expansion of CD44+/Ter119+ primitive erythroblasts and Ter119+CD44+/ FSC hi immature erythroid progenitors. We then transplanted Jak2 V617F-expressing ERK2 wildtype (WT), knockout (KO) or DBP hematopoietic progenitors (HSPCs) into irradiated, immunodeficient mice (Rag2 −/−Il2rg −/−) and observed disease resembling human PV, with elevated RBC counts, hematocrit, and hemoglobin levels. ERK2-DBP mutant recipients exhibited mild but sustained erythrocytosis with a prevalence of circulating GFP+ monocytes and neutrophils. At 12-weeks post-transplantation, ERK2-DBP mutant recipients had significant splenic burden and decreased bone marrow cellularity relative to WT and KO recipients. Reticulin staining confirmed progression towards myelofibrosis with interstitial infiltration in both the BM and the spleen of ERK2 DBP mutant recipients. To understand the molecular basis of ERK-DBP domain in JAK2V617F-driven MPN, we performed in vitro colony forming unit assay to assess the clonogenic potential of JAK2V617F-expressing HSPCs in cytokine supplemented-methylcellulose supporting myeloid or erythroid progenitor development. Ectopic JAK2V617F expression markedly reduced colony formation by both ERK2 wildtype and knockout HSPCs. On the other hand, JAK2V617F expression did not reduce colony formation by ERK2-DBP HSPCs. These differences were not evident in methylcellulose cultures that supported erythroid development, suggesting that disabling ERK-DBP binding capacity primarily affects the myeloid lineage. Furthermore, we found that JAK2V617F increased senescence associated β-galactosidase (SA-βGa) activity in ERK2-WT HSPCs. SA-βGal activity was modestly affected by ERK deficiency but was strongly attenuated by the ERK2-DBP mutation, supporting the notion that ERK2-DBP domain promotes oncogene-induced senescence (OIS). Previously, we identified transcription factor, Early growth response 1 (Egr1) which interacts with ERK2-DBP domain and plays a critical role as a tumor suppressor in myeloid neoplasms. We observed that JAK2V617F expression failed to suppress colony formation and senescence by Egr1-deficient HSPCs. The ability of EGR1 to restore these effects is abrogated by inactivating the DEF motif (EGR1-Y252A). Together, this data demonstrates that the interaction between ERK2-DBP and Egr1 is required for JAK2V617F-mediated colony suppression and senescence induction. Finally, to determine whether ERK-D domain acts to promote progression by targeting substrates distinct from those that bind to the DBP domain, we developed an inhibitor (#76) of the ERK-D domain. Indeed, 76 spared ERK phosphorylation and EGR1 induction but impaired RSK phosphorylation, a critical D-domain substrate involved in cancer progression. Moreover, 76 suppressed colony formation by JAK2V61F-expressing HSPCs akin to the active site MEK/ERK inhibitor, U0126. By altering the 4-ethoxybenylidene moiety of 76, we were able to enhance anti-proliferative effects of the parent compound against human MPN cell lines, SET-2, UKE-1, and Ku812. Our findings identify ERK2-domain specific roles in the pathogenesis of JAK2V617F driven MPN and supports a novel therapeutic approach to targeting JAK2 and MAPK dependent MPN. Disclosures Levine: Celgene: Research Funding; QIAGEN: Membership on an entity's Board of Directors or advisory committees; Auron: Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria; Mission Bio: Membership on an entity's Board of Directors or advisory committees; Zentalis: Membership on an entity's Board of Directors or advisory committees; Ajax: Membership on an entity's Board of Directors or advisory committees; C4 Therapeutics: Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy; Lilly: Honoraria; Janssen: Consultancy; Astellas: Consultancy; Gilead: Honoraria; Prelude: Membership on an entity's Board of Directors or advisory committees; Isoplexis: Membership on an entity's Board of Directors or advisory committees; Morphosys: Consultancy; Roche: Honoraria, Research Funding; Imago: Membership on an entity's Board of Directors or advisory committees. Wiest: Janssen Pharmaceuticals: Research Funding.

Introduction: While most patients (pts) with cutaneous T-cell lymphoma (CTCL) have an indolent course, survival for stages ≥IIB is usually less than 5 years (Kim YH et al, Arch Dermatol 2003). When an aggressive approach of combination cytotoxic therapies and radiation in CTCL was compared to conservative sequential therapies in newly diagnosed pts, the increase in response rate with combination therapy was offset by toxicities, and no benefit in disease-free or overall survival was seen (Kaye FS et al, NEJM, 1989). Since then several novel agents have been approved in CTCL, including the CD30-directed antibody-drug conjugate brentuximab vedotin (BV) [in relapsed primary cutaneous ALCL and CD30-expressing mycosis fungoides (MF)], and HDAC inhibitors (HDACi). However, response rates of single agents are modest. Tolerable and more efficacious therapies are needed, including rational combinations of active biological agents. Evidence suggests that HDAC inhibition may upregulate CD30 expression (Hasanali ZS et al, Sci Transl Med 2015), supporting the combination of the HDACi romidepsin (R) with Brentuximab vedotin (BV) in pts with CTCL. Methods: In this multicenter phase I clinical trial, pts age ≥18 with stage ≥IB CTCL, good organ function, ECOG PS≤2, Results: At the time of abstract submission, 7 pts have been enrolled (DL0: n=3; DL1: n=3; expansion cohort: n=1). No pt experienced a cycle 1 DLT and DL1 was deemed the MTD. No pts were enrolled in the de-escalation cohorts. Enrollment in the expansion cohort is ongoing. Median age of pts was 64 years (range 51-79); 72% (n=5) were male; median ECOG PS 1 (0-2); median prior lines of systemic therapy were 4 (0-4), including 1 pt with prior HDACi, and 1 pt with prior BV and R exposure. All pts had MF. Stage at enrollment was stage IIB in 5 (72%), IB and IVA2 in 1 each (14% each). No pts have experienced G4 or 5 adverse events (AE). The only G3 AEs observed during treatment were transaminitis and fever (n=1 each), both resolved spontaneously. The most common AEs were nausea (71%), vomiting (43%), gastro-esophageal reflux, constipation, peripheral sensory neuropathy, anorexia, fatigue, and thrombophlebitis (29% each; see Table 1). Response assessment is available for 5 of 7 pts. The overall response rate was 80% (4/5), all of which partial responses, including 1 pt who had received both prior R and BV. The median change in mSWAT was a decrease of 59% from baseline (range -19.5 to -81.8%). After a median follow up of 6.1 months, median estimated progression-free survival was 12 months (PFS probability 0.42; 95%CI 0.1-1.0). Four pts came off treatment: 2 due to progression, 1 due to non-adherence related to COVID-19 concerns, 1 because of recurrent thrombophlebitis; 3 pts remain on treatment. Conclusion: Preliminary findings from this phase I study exploring the combination of R+BV indicate that R+BV is well tolerated at a dose of R 14mg/m2 and BV 1.2mg/kg given every 2 weeks and appears efficacious in CTCL. Updated results will be presented at the time of the meeting. Enrollment in the expansion cohort and correlative studies, including analysis of changes in CD30 expression after 1 dose of R, and association of response with CD30 expression, are ongoing. Disclosures Barta: Atara: Honoraria; Monsanto: Consultancy; Seattle Genetics: Honoraria, Research Funding; Janssen: Honoraria; Pfizer: Honoraria. Feldman:AstraZeneca: Consultancy; Janssen: Speakers Bureau; Portola: Research Funding; Pfizer: Research Funding; Kyowa Kirin: Consultancy, Research Funding; Eisai: Research Funding; Cell Medica: Research Funding; Amgen: Research Funding; Pharmacyclics: Honoraria, Other, Speakers Bureau; Abbvie: Honoraria; Bayer: Consultancy, Honoraria; Trillium: Research Funding; Viracta: Research Funding; Rhizen: Research Funding; Corvus: Research Funding; BMS: Consultancy, Honoraria, Research Funding; Kite: Honoraria, Other: Travel expenses, Speakers Bureau; Celgene: Honoraria, Research Funding; Takeda: Honoraria, Other: Travel expenses; Seattle Genetics, Inc.: Consultancy, Honoraria, Other: Travel expenses, Research Funding, Speakers Bureau. DeSimone:Sanofi/Genzyme: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Soligenix: Other: Investigator; Helsinn: Speakers Bureau. Fung:Genentech: Honoraria, Other: speakers' bureau, travel support; Sanotif: Honoraria, Other: speakers' bureau, travel support; AstraZeneca: Honoraria, Other: speakers' bureau, travel support; Kite, a Gilead Company: Honoraria, Other: speakers' bureau, travel support; Takeda: Honoraria, Other: speakers' bureau, travel support; Janssen Oncology: Honoraria, Other: speakers' bureau, travel support; AbbVie: Honoraria, Other: speakers' bureau, travel support. Khan:Celgene: Research Funding; Seattle Genetics: Research Funding; Janssen: Honoraria; Pharmacyclics: Honoraria; Bristol Myers Squibb: Research Funding. OffLabel Disclosure: Combination of romidepsin and brentuximab vedotin.

Although it has long been reported that mutations in ribosome proteins (RP) are associated with increased cancer risk in humans, the molecular basis why which RP mutations do so remains unclear. Nevertheless, the prevailing view is that RP mutations, such as Rps19, are thought to alter transformation potential through general impairment of ribosome biogenesis or function. Importantly, recent observations are beginning to challenge this notion as too simplistic. We have determined that the RP, Rpl22, is not essential for ribosome biogenesis or global protein synthesis; however, its inactivation impairs the development of normal T lymphocytes and increases their transformation potential. Indeed, RPL22 is inactivated in human T acute lymphoblastic leukemia (T-ALL) and this is associated with reduced survival. Moreover, Rpl22-deficiency accelerates development of leukemia in a myristylated Akt2 transgenic (MyrAkt2 Tg) mouse model of T-ALL. To gain insight into how Rpl22 inactivation facilitates development of leukemia, we are performing unbiased transcriptomic and proteomic analysis on Rpl22+/+ and Rpl22-/- thymic lymphomas arising in the MyrAkt2 Tg model, and in an Rpl22-/- lymphoma reconstituted with Rpl22. Interestingly, relatively few changes in mRNA transcript read depth were observed; however, substantial differences in the proteome were observed. Pathway analysis revealed that the loss of Rpl22 altered the expression of proteins regulating RNA-processing, in particular RNA-splicing. Interestingly, interrogation of the transrciptome data for alternative splicing revealed that alterations in exon usage. The ability of Rpl22 to influence splicing appears to be conserved across species as alternative splicing was also observed in zebrafish embryos in which Rpl22 was knocked down using morpholino oligonucleotides. Consequently, we hypothesize that Rpl22 regulates biological events through its ability of binding to RNA targets, and controlling the expression of their protein products at least in part through altering mRNA splicing. How Rpl22 changes mRNA splicing pattern is currently under investigation. Disclosures No relevant conflicts of interest to declare.

While mutations in ribosomal proteins (RP) have long been associated with increased cancer risk, the molecular basis for this association remains unclear. The prevailing view is that mutations in RP increase cancer risk either by impairing the biogenesis or function of the ribosome. Here, we report that loss of the RP, Rpl22, is frequently observed in human myelodysplastic syndrome (MDS) and is associated with reduced survival in acute myelogenous leukemia (AML). The link between Rpl22 loss and aggressiveness of myeloid disease is causal, since it is replicated in mice lacking Rpl22. Importantly, the development of an MDS-like syndrome in Rpl22-deficient mice and the accelerated development of myeloid leukemia are not caused by reduced protein synthesis in hematopoietic stem cells (HSC). Instead, whole transcriptome analysis of HSC reveals that Rpl22 loss alters the expression of genes controlling cell metabolism and results in an increase in fatty acid oxidation. Moreover, the increased fatty acid oxidation is required for the increased self-renewal and reduced differentiation displayed by Rpl22-deficient HSC, as well as the survival of Rpl22-deficient myeloid leukemia cells. Together, these findings indicate that RP are capable of suppressing transformation potential in ways that do not involve effects on global protein synthesis. Indeed, Rpl22 suppresses transformation by suppressing lipid oxidation, suggesting that MDS and AML patients with reduced Rpl22 expression might benefit from pharmacologic interventions targeting lipid oxidation. Disclosures No relevant conflicts of interest to declare.

Ribosomal protein (RP) mutations in diseases such as 5q− syndrome both disrupt hematopoiesis and increase the risk of developing hematologic malignancy. However, the mechanism by which RP mutations increase cancer risk has remained an important unanswered question. We show here that monoallelic, germline inactivation of the ribosomal protein L22 (Rpl22) predisposes T-lineage progenitors to transformation. Indeed, RPL22 was found to be inactivated in ∼ 10% of human T-acute lymphoblastic leukemias. Moreover, monoallelic loss of Rpl22 accelerates development of thymic lymphoma in both a mouse model of T-cell malignancy and in acute transformation assays in vitro. We show that Rpl22 inactivation enhances transformation potential through induction of the stemness factor, Lin28B. Our finding that Rpl22 inactivation promotes transformation by inducing expression of Lin28B provides the first insight into the mechanistic basis by which mutations in Rpl22, and perhaps some other RP genes, increases cancer risk.

The first-line treatment for acute myeloid leukemia (AML) patients is a combination of cytarabine (Ara-C) and an antracycline, such as daunorubicin (Dnr). However, one third of AML patients are refractory to first-line therapy and 35-60% of those who do initially respond to chemotherapy relapse with resistant disease. Therefore, identifying and deciphering the molecular abnormalities that contribute to chemotherapy resistance in AML is the critical first step for developing new therapies that overcome chemotherapy resistance. In our study, we found that expression of the proto-oncogene c-JUN is elevated in patient-derived AML samples compared with normal hematopoietic progenitor cells. Additionally, expression and phosphorylation of c-JUN further increased in human AML cells exposed to Ara-C or Dnr. Confirming functional functional importance of c-JUN in human AML cells, we found that short hairpin RNAs (shRNAs) that specifically target c-JUN suppress cell growth and induce apoptosis in several genetically distinct human AML cell lines (Kasumi-1, MOLM-14, MV4-11, NOMO-1, OCI-AML3, SKM-1, THP-1 and U937). Furthermore, we have observed that shRNA-mediated inhibition of c-JUN also significantly potentiated the cytotoxic effects of Ara-C (p To elucidate the molecular mechanism by which c-JUN promotes AML cell survival and chemotherapy resistance, we performed global gene expression analyses (RNA-seq) of THP-1 cells expressing either non-targeting control or c-JUN shRNAs. From these analyses, we found that c-JUN inhibition results in decreased expression of numerous genes that are activated in response to endoplasmic reticulum (ER) stress, including a significant reduction in the expression of genes regulated by the ER stress-activated transcription factors ATF4 and DDIT3 (p=2.44x10-20). We therefore hypothesized that c-JUN may control ER stress signaling pathways to promote cell survival and chemotherapy resistance in AML. In support of this hypothesis, we have found that c-JUN inhibition blunts the activation of ER-stress related pathways stimulated by chemical inducers of ER stress, thapsigargin and tunicamycin. Additionally, we found that Ara-C treatment activates ER stress-related signaling pathways and that this activation can be reversed by c-JUN inhibition, which correlates with the increased death of c-JUN shRNA-expressing AML cells exposed to Ara-C. To further elucidate the role of ER-stress related pathways in AML, we focused on the serine/threonine kinase, PERK (encoded by EIF2AK3), which activates signaling pathways that aim to ameliorate ER stress and promote cell survival. Similar to c-JUN inhibition, shRNA-mediated inhibition of PERK not only increased cell death but also enhanced the cytotoxicity of Ara-C in AML cells, indicating that this ER stress pathway is a potential therapeutic target to increase the effectiveness of standard chemotherapy. To investigate the therapeutic potential of targeting the PERK pathway, we treated human AML cells with a chemical inhibitor of PERK, GSK2656157, in combination with Ara-C and found that certain concentrations of the two drugs synergistically increased AML cell killing versus each drug alone. Using leukemia cells derived from a mouse model of AML driven by the leukemogenic fusion allele MLL-AF9 alone or in combination with a FLT3 internal tandem duplication, we observed that the combination of GSK2656157 and Ara-C significantly reduced colony growth in methylcellulose by 80% compared with each drug alone (p These findings suggest that c-JUN plays a vital role in mitigating ER stress in AML cells and that targeting c-JUN and/or ER stress signaling pathways is a novel potential therapeutic approach for improving the effectiveness of current first-line AML therapies. Disclosures No relevant conflicts of interest to declare.

Mutations and deletions in ribosomal proteins are associated with a group of diseases termed ribosomopathies. Collectively, these diseases are characterized by ineffective hematopoiesis, bone marrow failure, and an increased risk of developing myelodysplastic syndrome (MDS) and subsequently acute myeloid leukemia (AML). This observation highlights the role of dysregulation of this class of proteins in the development and progression of myeloid neoplasms. Analysis of gene expression in CD34+ hematopoietic stem cells (HSC) from 183 MDS patients demonstrated that ribosomal protein L22 (Rpl22) was the most significantly reduced ribosomal protein gene in MDS. Interestingly, we observed that AML patients with lower expression of Rpl22 had a significant reduction in their survival (TCGA cohort, N=200, Log Rank P value Disclosures No relevant conflicts of interest to declare.

CD3zeta is a subunit of the T-cell antigen receptor (TCR) complex required for its assembly and surface expression that also plays an important role in TCR-mediated signal transduction. We report here a patient with T(-)B(+)NK(+) severe combined immunodeficiency (SCID) who was homozygous for a single C insertion following nucleotide 411 in exon 7 of the CD3zeta gene. The few T cells present contained no detectable CD3zeta protein, expressed low levels of cell surface CD3epsilon, and were nonfunctional. CD4(+)CD8(-)CD3epsilon(low), CD4(-)CD8(+)CD3epsilon(low), and CD4(-)CD8(-)CD3epsilon(low) cells were detected in the periphery, and the patient also exhibited an unusual population of CD56(-)CD16(+) NK cells with diminished cytolytic activity. Additional studies demonstrated that retrovirally transduced patient mutant CD3zeta cDNA failed to rescue assembly of nascent complete TCR complexes or surface TCR expression in CD3zeta-deficient MA5.8 murine T-cell hybridoma cells. Nascent transduced mutant CD3zeta protein was also not detected in metabolically labeled MA5.8 cells, suggesting that it was unstable and rapidly degraded. Taken together, these findings provide the first demonstration that complete CD3zeta deficiency in humans can cause SCID by preventing normal TCR assembly and surface expression.