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Background: Trabectedin is an FDA-approved DNA minor groove binder (MGB) that has activity against translocation-associated sarcomas. Lurbinectedin is a next-generation MGB with pre-clinical activity against myeloid leukemia cells. A dose-finding phase 1 clinical trial was performed in patients with acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS) with further assessment of safety and tolerability. Methods: Forty-two patients with relapsed/refractory AML/MDS received lurbinectedin administered as a 1-hour intravenous infusion in a 3+3 study design. Two dosing schedules were used: 3.5 mg, 5 mg, 7 mg, or 6 mg on days 1 and 8 or 2 mg, 3 mg, 1 mg, or 1.5 mg for 3 consecutive days on days 1 to 3. Patients 18 years or older with a diagnosis of advanced, relapsed/refractory AML (non-acute promyelocytic leukemia) and MDS were eligible and treated on study. Eligible patients had adequate hepatic, renal, and cardiac function and an Eastern Cooperative Oncology Group (ECOG) performance status of 0 to 2. Patients with uncontrolled infection, human immunodeficiency virus, cardiac and neurological disorders, or those who were pregnant were ineligible. Clinical trial information: NCT01314599. Results: Three patients experienced dose-limiting toxicities of rhabdomyolysis (grade 4), hyperbilirubinemia (grade 3), and oral herpes (grade 3) with the days 1 and 8 schedule. Otherwise, adverse events mainly consisted of gastrointestinal manifestations (n=11), febrile neutropenia/infections (n=4), pulmonary toxicity (n=2), and renal failure (n=2). The most common laboratory abnormalities observed were an increase in creatinine (93%) and anemia, neutropenia, and thrombocytopenia (100%). Overall, 33 of 42 patients (79%) had reduction in blasts in peripheral blood or bone marrow. One patient achieved a partial response and two patients a morphologic leukemia-free state. Most (n=30, 71%) were discontinued due to progressive disease. Early deaths occurred from disease-related causes that were not attributable to lurbinectedin. Four patients with a chromosome 11q21-23 abnormality had significantly greater bone marrow blast reduction than those without such abnormality, with decrease of 31±14% (n=4) vs. 8±8% (n=16), respectively (P=0.04). Conclusions: Overall, lurbinectedin was safe and tolerated using the schedules and dose levels tested. While no sustained remissions were observed, single-agent lurbinectedin was transiently leukemia suppressive for some patients. Disclosures Rodríguez: PharmaMar: Employment. Ravandi:Bristol-Myers Squibb: Research Funding; Jazz: Honoraria; Abbvie: Research Funding; Seattle Genetics: Research Funding; Abbvie: Research Funding; Amgen: Honoraria, Research Funding, Speakers Bureau; Orsenix: Honoraria; Seattle Genetics: Research Funding; Astellas Pharmaceuticals: Consultancy, Honoraria; Sunesis: Honoraria; Orsenix: Honoraria; Macrogenix: Honoraria, Research Funding; Astellas Pharmaceuticals: Consultancy, Honoraria; Jazz: Honoraria; Xencor: Research Funding; Bristol-Myers Squibb: Research Funding; Amgen: Honoraria, Research Funding, Speakers Bureau; Macrogenix: Honoraria, Research Funding; Xencor: Research Funding; Sunesis: Honoraria. Daver:Sunesis: Consultancy; Alexion: Consultancy; Daiichi-Sankyo: Research Funding; BMS: Research Funding; ImmunoGen: Consultancy; Kiromic: Research Funding; Incyte: Research Funding; Pfizer: Consultancy; Incyte: Consultancy; ARIAD: Research Funding; Karyopharm: Consultancy; Pfizer: Research Funding; Karyopharm: Research Funding; Novartis: Consultancy; Sunesis: Research Funding; Novartis: Research Funding; Otsuka: Consultancy. Jain:Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; ADC Therapeutics: Research Funding; Infinity: Research Funding; Pfizer: Research Funding; Novimmune: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Research Funding; Servier: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Research Funding; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; Genentech: Research Funding; ADC Therapeutics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologioes: Research Funding; Pharmacyclics: Research Funding; Incyte: Research Funding; Verastem: Research Funding; Seattle Genetics: Research Funding; Pharmacyclics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Astra Zeneca: Research Funding; Astra Zeneca: Research Funding; Servier: Research Funding; Celgene: Research Funding; Seattle Genetics: Research Funding; Verastem: Honoraria, Membership on an entity's Board of Directors or advisory committees; Astra Zeneca: Honoraria, Membership on an entity's Board of Directors or advisory committees; Infinity: Research Funding; Adaptive Biotechnologies: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Research Funding; Cellectis: Research Funding; ADC Therapeutics: Research Funding; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologioes: Research Funding; Pharmacyclics: Research Funding; Cellectis: Research Funding; Verastem: Research Funding; Servier: Research Funding; Abbvie: Research Funding; Incyte: Research Funding; Genentech: Research Funding; Abbvie: Research Funding; BMS: Research Funding; Astra Zeneca: Honoraria, Membership on an entity's Board of Directors or advisory committees; Verastem: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; ADC Therapeutics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees; Servier: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novimmune: Honoraria, Membership on an entity's Board of Directors or advisory committees; Adaptive Biotechnologies: Honoraria, Membership on an entity's Board of Directors or advisory committees; Janssen: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pharmacyclics: Honoraria, Membership on an entity's Board of Directors or advisory committees; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees. Maiti:Celgene Corporation: Other: Research funding to the institution. Martinez:PharmaMar: Employment. Siguero:PharmaMar: Employment. Al-Kali:Novartis: Research Funding.

Background Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and aggressive hematologic malignancy characterized by neoplastic cells that are positive for CD123, CD4, BDCA2, and TCL1 and aberrant expression of CD56. Historically, patients with BPDCN have an unfavorable prognosis and the optimal treatment is not established due to lack of prospective data. Case report In this report we describe a patient with Felty’s syndrome and myelodysplastic syndrome (MDS) in whom a population of aberrant plasmacytoid dendritic cells emerged while on treatment with decitabine. Approximately 4 months later he transformed to leukemic BPDCN with skin and eye manifestations. Cytogenetic analysis showed diploid karyotype and molecular analysis showed mutations in KRAS, NOTCH1, and RUNX1 genes. He was treated with CD123-targeted therapy and had significant response in his marrow, skin, eyes, and functional status after one cycle. Conclusion The case demonstrates that minimal transformative disease of BPDCN may be detectable in patients with MDS well before fulminant progression. Early detection of emerging leukemic clones may allow for alternative monitoring and treatment considerations.

Tropomyosin-related kinase A (TRKA) translocations have oncogenic potential and have been found in rare cases of solid tumors. Accumulating evidence indicates that TRKA and its ligand, nerve growth factor (NGF), may play a role in normal hematopoiesis and may be deregulated in leukemogenesis. Here, we report a comprehensive evaluation of TRKA signaling in normal and leukemic cells. TRKA expression is highest in common myeloid progenitors and is overexpressed in core binding factor and megakaryocytic leukemias, especially Down syndrome-related AML. Importantly, NGF can rescue GM-CSF dependent TF-1 AML cells, but does not drive proliferation in other TRKA-expressing lines. Although TRKA expression is heterogeneous between and within AML samples, NGF stimulation broadly induces ERK signaling, demonstrating the functional ability of AML cells to respond to NGF/TRKA signaling. However, neither shRNA knockdown nor pharmacologic inhibition have significant anti-proliferative effects on human AML cells in vitro and in vivo. Thus, despite functional NGF/TRKA signaling, the importance of TRKA in AML remains unclear.

As new drugs targeting MYC show clinical activity in acute myeloid leukemia (AML), understanding MYC expression in AML is of critical importance. We assessed MYC protein expression by immunohistochemistry in bone marrow of patients with untreated AML (n = 265). Overall, 90% of patients demonstrated MYC overexpression and MYC immunopositivity ≤6% was associated with superior complete remission (CR) duration of 23 months versus 12 months for MYC immunopositivity 6% (p = .028). Among 241 patients at higher risk for relapse, including those ≥55 years of age and patients with intermediate- and high-risk AML, MYC immunopositivity ≤6% conferred significantly superior median overall survival (OS) (24 versus 13 months; p = .042), event-free survival (EFS) (14 versus 6 months; p = .048), and relapse-free survival (RFS) (25 versus 12 months; p = .024). The prognostic impact of MYC-immunopositivity was retained on multivariate analysis of OS, EFS, and RFS. We conclude that MYC immunopositivity is an important prognostic factor in patients with untreated AML, particularly those at higher risk for relapse.

The classification of a gene as an oncogene or a tumor suppressor has been a staple of cancer biology for decades. However, as we delve deeper into the biology of these genes, this simple classification has become increasingly difficult for some. In the case of heterogeneous nuclear ribonuclear protein K (hnRNP K), its role as a tumor suppressor has recently been described in acute myeloid leukemia and demonstrated in a haploinsufficient mouse model. In contrast, data from other clinical correlation studies suggest that hnRNP K may be more fittingly described as an oncogene, due to its increased levels in a variety of malignancies. hnRNP K is a multifunctional protein that can regulate both oncogenic and tumor suppressive pathways through a bevy of chromatin-, DNA-, RNA-, and protein-mediated activates, suggesting its aberrant expression may have broad-reaching cellular impacts. In this review, we highlight our current understanding of hnRNP K, with particular emphasis on its apparently dichotomous roles in tumorigenesis.

Purpose Acute promyelocytic leukemia (APL) is the most prognostically favorable subtype of Acute myeloid leukemia (AML). Defining the features that allow identification of APL patients likely to relapse after therapy remains challenging. Experimental Design Proteomic profiling is performed on 20 newly diagnosed APL, 205 non-APL AML, and 10 normal CD34+ samples using Reverse Phase Protein Arrays probed with 230 antibodies. Results Comparison between APL and non-APL AML samples identifies 8.3% of the proteins to be differentially expressed. Proteins higher expressed in APL are involved in the pro-apoptotic pathways or are linked to higher proliferation. The "MetaGalaxy" approach that considers proteins in relation to other assayed proteins stratifies the APL patients into two protein signatures. All of the relapse patients (n = 4/4) are in protein signature 2 (S2). Comparison of proteins between the signatures shows significant differences in relative expression for 38 proteins. Protein expression summary plots suggest less translational activity in combination with a less proliferative character for S2 compared to signature 1. Conclusions and Clinical Relevance This study provides a potential proteomic-based classification of APL patients that may be useful for risk stratification and therapeutic guidance. Validation in a larger independent cohort is required.

Acute myeloid leukemia (AML) is a conglomerate of hematologic malignancies characterized by recurrent genetic and/or chromosomal aberrations. Recent development of targeted agents has bolstered our armamentarium of therapeutic options and improved outcomes for many patients. Acquiring deeper mechanistic understanding of myeloid leukemogenesis will provide a basis for development of even more therapeutic strategies and further improve patient outcomes. Our clinical data have revealed that overexpression of HNRNPK is a recurrent abnormality that occurs in upwards of 20% of AML cases at both the RNA and protein levels. Using bone marrow samples from a similar proportion of patients, we have recently discovered a supernumerary marker chromosome containing an extra copy of the HNRNPK locus that is not detectable with routine cytogenetic testing. We have further associated high hnRNP K protein levels with decreased overall survival in de novo AML, emphasizing the need to understand the role of hnRNP K in myeloid malignancy. To directly evaluate the oncogenic capacity of hnRNP K, we have overexpressed hnRNP K in murine fetal liver cells (FLCs). Using CyTOF and colony formation assays, we demonstrated that hnRNP K-overexpressing FLCs have altered differentiation potential and self-renewal capacity compared to empty vector controls in vitro. These findings are recapitulated in vivo, as murine recipients of hnRNP K-overexpressing FLCs develop myeloid lineage disease, often manifesting as fatal megakaryocytic leukemia. To elucidate a mechanism by which hnRNP K causes myeloid disease, we performed hnRNP K immunoprecipitation followed by mass spectrometry in an AML cell line and identified that hnRNP K preferentially interacts with translational machinery, ribosomal subunits, and proteins involved in RNA processing. In conjunction with data from our hnRNP K overexpression models that indicate overexpression of hnRNP K occurs primarily in the cytoplasm, we then performed hnRNP K-RNA immunoprecipitation followed by sequencing (RIP-Seq). We determined that hnRNP K interacts with the transcript of RUNX1-a master regulator of hematopoiesis and a critical player in a myriad of leukemias-including megakaryocytic leukemias like those observed in our mouse models. Using biochemical assays, we have demonstrated that hnRNP K directly binds to consensus sequences in the RUNX1 transcript, and ultimately alters RUNX1 translation. Indeed, mice exhibiting hnRNP K overexpression have increased protein levels of Runx1 in hematopoietic tissues. Our data demonstrate that hnRNP K overexpression drives myeloid malignancy. Currently, we are screening compounds that will disrupt the interaction between hnRNP K and RUNX1 in our efforts to further understand myeloid biology and ultimately improve outcomes for patients with these diseases. Disclosures No relevant conflicts of interest to declare.

INTRODUCTION The TET2 (Ten‐Eleven Translocation 2) gene, located at chromosome 4q24.1, belongs to TET family of proteins that possesses the capacity of catalyzing the conversion of 5‐methylcytosine into 5‐hydroxymethylcytosine. It is considered to be a putative tumor suppressor gene during cancer initiation and development. TET2 mutations are extensively studied and reported in a variety of human hematological malignancies including acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS), suggesting a crucial role of TET2 in the pathogenesis of blood cancers. The landscape of rare TET2 variants which may be of potential clinical relevance is not fully described. The landscape of TET2 variants from a large cohort of MDS and AML patients was explored. METHODS We analyzed data from bone marrow samples of 2634 patients with AML or MDS who presented at MD Anderson between the years 2012-2016. Seven hundred seventy seven patients were found to have TET2 variants that were not classifiable as known mutations. Two hundred sixty-three patients were diagnosed with MDS, 144 with secondary AML transformed from MDS, and 370 with AML without prior history of MDS. The coding regions of TET2 gene was analyzed by next-generation sequencing in the molecular diagnostics laboratory. Non-mutation variants were classified into 2 groups: 1) those reported as germline polymorphisms (GP) in population studies, literature or matched tumor-normal analysis on patients at MDACC; and 2) variants for which a germline versus somatic origin could not be determined unequivocally (variants of uncertain origin, VUO). Very common GP variants (defined as those with a population frequency of over 20% in laboratory sample cohort) were not evaluated. Variants' predicted frequency in the population according to gnomAD Exomes 2.0.2, and classification according to American College of Medical Genetics and Genomics (ACMG) guidelines for interpretation of sequence variants, were assessed using the VarSome platform by Saphetor. RESULTS Two-hundred and twenty-eight unique variants were identified. The amino acid substitutions from missense variants were widely distributed throughout TET2. Fifty-four unique variants were classified as GP and several of these recurrent variants were found to be present in our population at a significantly higher frequency (p CONCLUSIONS The landscape of rare TET2 variants in a large group of MDS and AML patients was frequent, diverse, and included both GP and VUO. GP variants were more likely benign by ACMG classification and found in both MDS and AML patients. A large number of singleton VUO were found among MDS patients, and many were predicted to be pathogenic or had uncertain significance. VUO's also contained many fs and nonsense variants. These result support the idea that rare pathogenic variants in TET2 are currently underrecognized as contributory, although they are likely to help drive leukemogenesis. Greater understanding of the importance of individual TET2 variants and additional classification metrics are needed. Disclosures DiNardo: Karyopharm: Honoraria; Abbvie: Honoraria; Agios: Consultancy; Medimmune: Honoraria; Celgene: Honoraria; Bayer: Honoraria. Daver:Karyopharm: Research Funding; Incyte: Consultancy; Novartis: Consultancy; ImmunoGen: Consultancy; Sunesis: Consultancy; Pfizer: Consultancy; Daiichi-Sankyo: Research Funding; Kiromic: Research Funding; Alexion: Consultancy; Otsuka: Consultancy; Sunesis: Research Funding; Incyte: Research Funding; ARIAD: Research Funding; BMS: Research Funding; Karyopharm: Consultancy; Novartis: Research Funding; Pfizer: Research Funding. Kadia:Celgene: Research Funding; Takeda: Consultancy; BMS: Research Funding; Abbvie: Consultancy; Novartis: Consultancy; Novartis: Consultancy; Amgen: Consultancy, Research Funding; Jazz: Consultancy, Research Funding; Abbvie: Consultancy; Pfizer: Consultancy, Research Funding; Takeda: Consultancy; Amgen: Consultancy, Research Funding; BMS: Research Funding; Pfizer: Consultancy, Research Funding; Jazz: Consultancy, Research Funding; Celgene: Research Funding. Konopleva:Stemline Therapeutics: Research Funding; abbvie: Research Funding; cellectis: Research Funding; Immunogen: Research Funding. Andreeff:Amgen: Consultancy, Research Funding; Astra Zeneca: Research Funding; Jazz Pharma: Consultancy; Oncolyze: Equity Ownership; Aptose: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Oncoceutics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; SentiBio: Equity Ownership; United Therapeutics: Patents & Royalties: GD2 inhibition in breast cancer ; Reata: Equity Ownership; Celgene: Consultancy; Eutropics: Equity Ownership, Membership on an entity's Board of Directors or advisory committees; Daiichi-Sankyo: Consultancy, Patents & Royalties: MDM2 inhibitor activity patent, Research Funding.

hnRNP K is an RNA binding protein that controls a multitude of cellular processes and is aberrantly expressed in cancers. We have previously shown that hnRNP K functions as a haploinsufficient tumor suppressor in AML patients with 9q deletions. However, overexpression of hnRNP K is the more commonly observed clinical phenomenon. We have recently discovered that hnRNP K overexpression in patients with diffuse large B-cell lymphoma correlates with dismal outcomes and directly resulted in the development of lymphomas in transgenic mice. To understand the mechanistic basis for the oncogenicity of hnRNP K overexpression and to identify therapeutic vulnerabilities, we performed RNA-sequencing, RNA immunoprecipitation following by sequencing (RIP-Seq), mass spectrometry, and polysome assays. We observed that hnRNP K regulates both global transcription and translational processes within the cell via modulation of 7SK and translation initiation proteins (such as the eIFs and PABP), respectively. Consequently, we hypothesized that aberrant hnRNP K expression primarily perturbs oncogenes with short half-lives. Mechanistically, we identified that hnRNP K binds to the RNA and regulates the expression of a plethora of critical oncogenes and tumor suppressors involved in hematologic malignancies such as c-Myc, RUNX1, and Cyclin D1. As proof of concept for clinical applications, we have demonstrated that hnRNP K-driven c-Myc overexpression renders tumors susceptible to bromodomain inhibition. Given that hnRNP K directs global transcription and translation, it is likely that hnRNP K overexpressing tumors will also be sensitive to transcriptional and translational inhibitors such as CDK9 inhibitors and omacetaxine mepesuccinate, respectively. However, since hnRNP K also regulates a plethora of additional cellular processes that extend far beyond mRNA and protein synthesis, there is a need to develop hnRNP K-specific inhibitors that will only target these activities. Thus, we have recently begun to identify small molecule compounds that can directly inhibit hnRNP K-RNA binding function on specific targets using an in vitro fluorescent binding assay. Using this assay, we are currently screening a library of 70,000 small molecule compounds to identify agents that can prevent hnRNP K-RNA interactions. In summary, we have established that hnRNP K is a bona fide oncogene that drives lymphomagenesis. Global analyses have revealed therapeutic vulnerabilities of hnRNP K overexpressing tumors. Furthermore, using our in vitro RNA binding assays, we anticipate identification of novel hnRNP K-specific inhibitors. Disclosures No relevant conflicts of interest to declare.

OBJECTIVES/SPECIFIC AIMS: Acute myeloid leukemia (AML) is a devastating hematologic malignancy wherein 20% of circulating white blood cells harboring markers of immature stem cells in conjunction with positive myeloperoxidase staining and blast-appearing morphology. RPPA revealed expression of c-Myc positively correlated with increased hnRNP K levels. In HnrnpkTg mice, c-Myc protein was increased, yet MYC RNA was invariably decreased compared to wildtype. To decipher a mechanism by which this may occur, we demonstrated a post-transcriptional interaction between hnRNP K and c-Myc in vivo. JQ1, a BRD4 inhibitor, that epigenetically decreases c-Myc expression showed preferential activity against myeloid cells expressing high levels of hnRNP K both in vitro and in vivo. DISCUSSION/SIGNIFICANCE OF IMPACT: These preliminary studies demonstrate that hnRNP K overexpression causes myeloid malignancies in both mouse and man. We have determined that c-Myc contributes in part to hnRNP K-mediated leukemogenesis, and that targeting c-Myc may be an effective strategy for hnRNP K-overexpressing AML. We are currently validating other potential targets for interaction with hnRNP K by performing RNA-Seq and hnRNP K immunoprecipitation followed by mass spectrometry. Fortunately, several of our putative targets are druggable—allowing for viable translational outputs to these mechanistic studies.

Acute myeloid leukemia (AML) is a disease largely defined by recurrent genetic and chromosomal abnormalities. As such, The Cancer Genome Atlas' recent detailed examination of the most common genetic abnormalities that drive AML uncovered relatively few novel alterations. However, within this list of recurrently mutated genes was heterogeneous nuclear ribonucleoprotein K (HNRNPK). While previously unknown to impact AML, we recently identified HNRNPK as a haploinsufficient tumor suppressor at the 9q21.32 locus. In contrast to its tumor suppressive roles, hnRNP K is most customarily overexpressed, rather than underexpressed, in several solid tumors. In this study, we show through FISH analyses, RNA expression profiling, and reverse phase protein array analyses that the HNRNPK gene is amplified with corresponding mRNA and protein overexpression in a sizeable cohort of patients with AML that do not carry a 9q.21.32 deletion. Together, this indicates that hnRNP K may be an uncharacterized oncogene in this context. Increased hnRNP K correlated with decreased event-free and overall survival, increased WBC, and increased bone marrow and peripheral blood blast percentage. High hnRNP K levels also correlated with elevated expression of histone modifiers (e.g., ASH2L and TRIM24), regulators of ribogenesis (e.g., nucleolin and EBP1), and potent oncogenes, such as c-Myc. Together, these observations indicate that increased hnRNP K expression significantly alters the landscape of myeloid biology and may thus contribute to the etiology of AML. To directly interrogate this possibility in vivo, we generated a Vav-Cre;HnrnpkTg mouse model. In addition to significant differences in hnRNP K expression compared to Vav-Cre mice, Vav-Cre;HnrnpkTg mice exhibited reduced survival, development of transplantable myeloid malignancies, and enhanced HSPC proliferation and differentiation potential. Reflecting data from our patients, Vav-Cre;HnrnpkTg mice also exhibited elevated expression of critical oncogenic pathways such as c-Myc. Currently, we are examining pharmacologic methods to abrogate the effects of aberrant hnRNP K expression on various activated pathways. Transplant models using diseased bone marrow from Vav-Cre;HnrnpkTg mice are being utilized to evaluate the efficacy of specific pathway inhibitors (i.e., bromodomain inhibitors or bortezomib) in prolonging survival and decreasing leukemic burden. These studies will provide the rationale for potential clinical trails that employ these agents for patients with hnRNP K-overexpressing AML. Disclosures No relevant conflicts of interest to declare.

Deletion of the short-arm of chromosome 17 (17p-) is one of the most critical genetic alterations used in chronic lymphocytic leukemia (CLL) risk stratification. The tumor suppressor TP53 maps to this region, and its loss or mutation accelerates CLL progression, hampers response to chemotherapy and shortens survival. Although florescent in situ hybridization analyses for 17p deletions are routinely performed during clinical diagnoses, p53 mutational status is often unexamined. Given the limited clinical data that exists for frontline treatment of patients with CLL harboring TP53 mutations, there is a need to understand the biology of CLL with TP53 mutations and identify treatment strategies for this subset of patients. Herein, we used a CLL mouse model (Eμ-TCL1) harboring one of the most common TP53 hot-spot mutations observed in CLL (p53R172H, corresponding to p53R175H in humans) to evaluate its impact on disease progression, survival, response to therapy and loss of the remaining wild-type Trp53 allele following ibrutinib treatment. We show that ibrutinib was effective in increasing survival, activating cellular programs outside the p53 pathway and did not place selective pressure on the remaining wild-type Trp53 allele. These data provide evidence that ibrutinib acts as an effective treatment for aggressive forms of CLL with TP53 mutations.

Approximately 30% of patients with acute myeloid leukemia (AML) harbor activating mutations in the fms-like tyrosine kinase receptor 3 (FLT3). Such mutations are associated with increased propensity to relapse and dramatically decreased survival. FLT3 tyrosine kinase inhibitors (TKI), such as quizartinib, have shown modest clinical effects as single agents, and resistance remains a significant clinical problem. In addition to constitutive activity of the FLT3 receptor, AML cells with an internal tandem duplication of FLT3 (FLT3-ITD) exhibit elevated levels of reactive oxygen species (ROS). Antioxidants such as heme-oxygenase 1 (HO-1) are often upregulated in conjunction with increased ROS levels. In addition to its role as an antioxidant, HO-1 also has known proliferative and anti-apoptotic functions in some cell types. A study in non-small cell lung carcinoma recently suggested that HO-1 inhibits the Notch pathway through binding the Notch receptor. As Notch signaling exerts pro-apoptotic effects in AML, targeting HO-1 may induce Notch signaling, representing a potential therapeutic approach. Our data show that HO-1 is particularly elevated in FLT3-ITD expressing AML cells. Interestingly, HO-1 protein was found to be further elevated in AML cells with acquired resistance to FLT3-directed TKI compared to TKI-sensitive cells. Importantly, knockdown of HO-1, or inhibition of HO-1 catalytic activity with zinc protoporphyrin (ZnPP), leads to decreased survival and proliferation, demonstrating a critical pro-survival role of HO-1. We have determined that expression of HO-1 is under the control of NRF2, a transcriptional regulator of the antioxidant response, in FLT3-ITD positive AML cells. Inhibition of NRF2 with brusatol resulted in increased sensitivity to quizartinib in TKI-sensitive cell lines. Further, brusatol restored sensitivity to low nanomolar concentrations of quizartinib in TKI-resistant cells. Together, our work shows that combined inhibition of FLT3 and NRF2 using quizartinib and brusatol, respectively, exhibits synergy in the context of FLT3-ITD AML. Given the reported interaction between HO-1 and Notch, we sought to determine whether this interaction occurs in AML. Using the TCGA AML database, we found significant co-occurrence of HMOX1 (HO-1) and NOTCH2 mRNA expression in 200 patient samples. In vitro immunoprecipitation of the Notch2 receptor revealed that HO-1 directly interacts with Notch2 in FLT3-ITD positive AML cells. Importantly, brusatol treatment resulted in activation of the Notch pathway as demonstrated by cleavage of the Notch receptor and increased transcript levels of HES1, a canonical downstream target of Notch signaling. Given that HO-1 interacts with Notch2 and is under NRF2 control, we propose that decreasing HO-1 levels with brusatol treatment will modulate the Notch pathway such that its anti-leukemic effects can be exerted. In conclusion, inhibition of HO-1, directly or through NRF2 inhibition, demonstrates synergistic effects with FLT3 inhibition in both TKI-sensitive and -resistant AML. Furthermore, NRF2 inhibition induces Notch activation, providing a potential mechanism for this synergistic combination. Our data suggest that antioxidant modulation is a potential therapeutic approach for FLT3-ITD positive AML. Disclosures No relevant conflicts of interest to declare.