Acute myeloid leukemia (AML) is the most common and lethal leukemia in adults. AML consists of many genetic subtypes, which limits broad applicability of targeted therapy. We discovered that the hematopoiesis-restricted tetraspanin CD37 is expressed on the majority of primary AML blasts and thus may represent a common therapeutic target for AML regardless of subtype. We demonstrate that the internalization properties of CD37 are distinct in AML blasts when compared with normal blood cells, and that CD37 rapidly accumulates inside AML blasts via dynamin-dependent endocytosis. Our work revealed that the clinically relevant anti-CD37 antibody-drug conjugate (ADC) Debio 1562 (alpha CD37DM1) is highly cytotoxic to AML blasts, but not normal hematopoietic stem cells. We found that alpha CD37-DM1 improved clinical outcomes and overall survival in multiple in vivo models of AML. Together, these data demonstrate that targeting CD37 with an ADC such as alpha CD37DM1 is a feasible and promising therapeutic option for the treatment of AML.

Supplemental Table 4: Abundance information of top 20 CDR3 sequences and top 10 heavy chains and top 10 light chains (see supplemental Excel file) Supplemental Table 5: DESeq2 normalized counts, log2-fold change, and p-values and adjusted p-values for selected oncogenes, Myc and TCL1A (see supplemental Excel file)

Supplemental Figure 1: Commonly used murine cell surface markers of B-cell development Supplemental Figure 2: Multicolor flow cytometry gating strategy and representative flow plots from a WT spleen sample Supplemental Figure 3: Histopathological assessment of bone marrow aspirates Supplemental Figure 4: Relative changes of mature B-cell subsets in marrow and blood Supplemental Figure 5: Myc and TCL1 expression in CD21-IgM-, CD21-IgM+ and CD19+CD5+ populations Supplemental Figure 6: Characterization of BCR and V gene usage Supplemental Figure 7: B220low/+CD19+CD5+ CLL-like population in recipients of flow-sorted dTG malignancy components Supplemental Figure 8: Immunophenotype of B220+ non-CLL cells in recipients of flow-sorted dTG malignancy components Supplemental Table 1: Fluorophores, concentrations and suppliers of antibodies used to stain murine cells Supplemental Table 2: Low-input RNA-seq post-alignment quality control metrics Supplemental Table 3: Alignment statistics of heavy- and light chain- (lambda and kappa) B-cell receptor (BCR) reads Supplemental Table 6: Characteristics of recipient mice and engrafted diseases after injection of flow-sorted CLL, CD21-IgM- and CD21-IgM+ cells obtained from pooled diseased dTG donor mice (n=3)

Richter’s Transformation (RT) is a poorly understood and fatal progression of chronic lymphocytic leukemia (CLL) manifesting histologically as diffuse large B-cell lymphoma. Protein arginine methyltransferase 5 (PRMT5) is implicated in lymphomagenesis, but its role in CLL or RT progression is unknown. We demonstrate herein that tumors uniformly overexpress PRMT5 in patients with progression to RT. Furthermore, mice with B-specific overexpression of hPRMT5 develop a B-lymphoid expansion with increased risk of death, and Eµ-PRMT5/TCL1 double transgenic mice develop a highly aggressive disease with transformation that histologically resembles RT; where large-scale transcriptional profiling identifies oncogenic pathways mediating PRMT5-driven disease progression. Lastly, we report the development of a SAM-competitive PRMT5 inhibitor, PRT382, with exclusive selectivity and optimal in vitro and in vivo activity compared to available PRMT5 inhibitors. Taken together, the discovery that PRMT5 drives oncogenic pathways promoting RT provides a compelling rationale for clinical investigation of PRMT5 inhibitors such as PRT382 in aggressive CLL/RT cases.

Background: Richter's transformation (RT) is the progression of chronic lymphocytic leukemia (CLL) to a high-grade lymphoma, most commonly resembling a diffuse large B-cell-like lymphoma, that has a bleak prognosis of 6-12 months. Many of the recurrent somatic alterations identified in RT are also associated with the CLL-phase of disease, suggesting additional unidentified genetic, epigenetic, or molecular events underlie transformation. Aberrant PRMT5 expression is known to promote and maintain oncogenic signaling in non-Hodgkin's lymphomas but its role in CLL and RT remains unclear. Methods: Tissue microarrays and western blots were used to evaluate PRMT5 expression in CLL/SLL and RT lymph nodes (LN). Differentially expressed genes between patient-matched RT LNs and CLL PBMCs were identified with scRNA-seq & scV(D)J-seq. ScRNA-seq was utilized to identify transcriptional changes between Eμ-PRMT5/TCL1 & Eμ-TCL1 mice. Tumor allografts were generated by administering 5 x 10^6 CD19+CD5+ spleen cells from Eμ-PRMT5/TCL1 mice intravenously to C57/BL6J mice. One week post-engraftment mice were enrolled in treatment groups (PRT382 10mg/kg, n=10; vehicle, n=7) with all treatments administered for 4 contiguous days per week by oral gavage. Results: Tissue microarray analysis on 70 CLL and 15 RT LN cases showed robust PRMT5 staining in RT LNs and minimal staining in CLL tissues. Immunoblot analysis revealed that CLL cells from patients that eventually undergo transformation displayed significant increase in PRMT5 expression months prior to transformation, whereas CLL patients maintained variable to minimal PRMT5 expression. ScRNA-seq & scV(D)J-seq on RT LN biopsies and CLL PBMCs from two patients revealed that PRMT5 expression was restricted to distinct nodal B cell subpopulations co-expressing MYC, BIRC5, CD83, and CD69. One PRMT5+ cluster displayed overlapping expression of proliferative markers (Mki67, TOP2A, PCNA, CALM2/3, and HMGB1/2) with enriched EIF4 signaling. Anti-apoptotic and immune modulating genes (BCL21A, IL4I1, TCL1A, and CCL3/4) were enriched in the second PRMT5+ nodal cluster. Interestingly, transcriptional diversity was enhanced in clonally related RT B cell populations with an increased proportion of cells expressing PRMT5 when compared to the CLL-phase of disease. To further evaluate the role of PRMT5 in CLL disease progression, we conducted histopathologic analysis on the spleen and lymph nodes of 6 month old animals from Eµ-PRMT5/TCL1 & Eµ-TCL1 mouse models, revealing a RT-like histology with loss of germinal centers and effacement of normal architecture exclusively in Eµ-PRMT5/TCL1 animals. Transcriptional changes underlying phenotypic differences between Eµ-PRMT5/TCL1 & Eµ-TCL1 mice were also assessed by scRNA-seq, where splenic B cells of the Eµ-PRMT5/TCL1 exhibited increased expression of oncogenic and immune regulating genes (Myc, Mki67, Egr1, Cxcr5, Ccr7, Il-10, Ctla4, and Pd-L1). Interestingly, the dominant B-cell clusters enriched in Eµ-PRMT5/TCL1 spleen and LNs showed significant transcriptional overlap with human RT PRMT5 expressing LN clusters (e.g., Myc, Cd83, and Cd69). With the observation that PRMT5 may play a significant role in CLL-to-RT evolution, we then aimed to evaluate the use of targeted PRMT5 inhibition against murine RT-like tumors in vivo. Using the selective SAM-competitive PRMT5 inhibitor, PRT382, in Eµ-PRMT5/TCL1 allografts, we observed PRT382 treatment delayed leukemic expansion, increased median survival (p<0.0001), and reduced spleen mass compared to vehicle treated mice. Conclusion: We show that PRMT5 is upregulated in the lymph nodes of CLL patients prior to and after transformation, suggesting a role for PRMT5 in CLL transformation to RT. Likewise, the Eμ-PRMT5/TCL1 mouse model develops a RT-like histology and shows significant overlap in the transcriptional profile to human RT tumors. And further, inhibition of PRMT5 with PRT382 provides significant improvement in median overall survival and slows disease progression in Eμ-PRMT5/TCL1 adoptive transfer models. Our preclinical data identifies PRMT5 as a high-risk marker in CLL and provides rationale for the clinical evaluation of PRMT5 inhibition in the treatment of high-risk CLL/RT.

Abstract Chronic lymphocytic leukemia (CLL) is the most prevalent adult leukemia in Western countries and is spelled by substantial genetic and clinical heterogeneity. During CLL transformation, loss or gain of genetic material appears to be a key determinant of disease phenotype and clinical outcome, with major chromosome aberrations observed in up to 80% of patients. Alternatively, balanced translocations, specifically those resulting in constitutive over-expression of various proto-oncogenes under the immunoglobulin heavy chain locus (IGH; 14q32), occur far less frequently. Despite their infrequence, molecular profiling of these rare rearrangements have revealed broad importance of un-recognized genes critical to the pathogenesis of CLL. Employing this strategy, we identified a young CLL patient with a previously undescribed t(X;14)(q28;q32) translocation, co-localization of the mature T cell proliferation 1 (MTCP1; Xq28) coding region with the IGH locus, triggering overexpression of MTCP1 in the CLL cells. Translocations involving MTCP1 are a driving factor in T-prolymphocytic leukemia; however, a role for MTCP1 in CLL has not been described. Inspired by this observation, we screened >1700 suspected CLL cases and evaluated gene expression data for further evidence of MCTP1 aberrations. This query identified seven additional Xq28 rearrangements, revealed MTCP1 mRNA was globally over-expressed in CLL cells compared to normal B-cells, and increased MTCP1 mRNA expression portends a poor response to chemoimmunotherapy. To establish a role for MTCP1 as an oncogene in B cell malignancies, we generated a mouse model with B cell-specific MTCP1 overexpression (Eµ-MTCP1). Longitudinal evaluation revealed a majority of Eµ-MTCP1 mice developed a lethal hematologic malignancy between 5-12 months of age, highlighted by the progressive emergence of clonally related CLL-like B lymphocytes (CD19+/CD5+ B cells) in the blood and accumulating in the spleen and lymph nodes. To support the use of the newly generated Eµ-MTCP1 mouse as a tool for pre-clinical evaluation of CLL therapeutics, we demonstrate that continuous ibrutinib administration in Eµ-MTCP1 mice was sufficient to delay the onset of the CLL-like disease and significantly prolonged survival. In summary, we report Xq28 translocations as rare genetic abnormalities in CLL, yet being one mechanism by which CLL cells amplify expression of MTCP1 compared to normal B cell subsets. Further, the Eµ-MTCP1 mouse model should be considered as an alternative tool for both biologic assessment of co-expressed genes and pre-clinical evaluation of novel CLL therapeutics. Lastly, relevant to all cancer types, successful application of a strategy pursuing the functional consequence of genes involved in rare translocations contributed to the understanding of this disease and identified a novel target for future therapeutic consideration. Citation Format: Janek S. Walker, Zachary A. Hing, Steven Sher, James Cronin, Katie Williams, Bonnie Harrington, Jordan N. Skinner, Casey B. Cempre, Charles T. Gregory, Max Yano, Larry P. Beaver, Brandi R. Walker, Jadwiga M. Labanowska, Nyla A. Heerema, Krzysztof Mrozek, Jennifer A. Woyach, Amy S. Ruppert, Amy Lehman, Hatice Gulcin Ozer, Vincenzo Coppola, John C. Byrd, James S. Blachly, Rosa Lapalombella. Evaluating a rare t(X;14)(q28;q32) translocation reveals MTCP1 as a driving factor in chronic lymphocytic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2260.

Rare, recurrent balanced translocations occur in a variety of cancers but are often not functionally interrogated. Balanced translocations with the immunoglobulin heavy chain locus ( IGH ; 14q32) in chronic lymphocytic leukemia (CLL) are infrequent but have led to the discovery of pathogenic genes including CCND1 , BCL2 , and BCL3 . Following identification of a t(X;14)(q28;q32) translocation that placed the mature T cell proliferation 1 gene ( MTCP1 ) adjacent to the immunoglobulin locus in a CLL patient, we hypothesized that this gene may have previously unrecognized importance. Indeed, here we report overexpression of human MTCP1 restricted to the B cell compartment in mice produces a clonal CD5 + /CD19 + leukemia recapitulating the major characteristics of human CLL and demonstrates favorable response to therapeutic intervention with ibrutinib. We reinforce the importance of genetic interrogation of rare, recurrent balanced translocations to identify cancer driving genes via the story of MTCP1 as a contributor to CLL pathogenesis.

Background Exportin 1 (XPO1/CRM1) is a key mediator of nuclear export with relevance to multiple cancers, including chronic lymphocytic leukemia (CLL). Whole exome sequencing has identified hot-spot somatic XPO1 point mutations which we found to disrupt highly conserved biophysical interactions in the NES-binding groove, conferring novel cargo-binding abilities and forcing cellular mis-localization of critical regulators. However, the pathogenic role played by change-in-function XPO1 mutations in CLL is not fully understood. Methods We performed a large, multi-center retrospective analysis of CLL cases ( N = 1286) to correlate nonsynonymous mutations in XPO1 (predominantly E571K or E571G; n = 72) with genetic and epigenetic features contributing to the overall outcomes in these patients. We then established a mouse model with over-expression of wildtype (wt) or mutant (E571K or E571G) XPO1 restricted to the B cell compartment (Eµ-XPO1). Eµ-XPO1 mice were then crossed with the Eµ-TCL1 CLL mouse model. Lastly, we determined crystal structures of XPO1 (wt or E571K) bound to several selective inhibitors of nuclear export (SINE) molecules (KPT-185, KPT-330/Selinexor, and KPT-8602/Eltanexor). Results We report that nonsynonymous mutations in XPO1 associate with high risk genetic and epigenetic features and accelerated CLL progression. Using the newly-generated Eµ-XPO1 mouse model, we found that constitutive B-cell over-expression of wt or mutant XPO1 could affect development of a CLL-like disease in aged mice. Furthermore, concurrent B-cell expression of XPO1 with E571K or E571G mutations and TCL1 accelerated the rate of leukemogenesis relative to that of Eµ-TCL1 mice. Lastly, crystal structures of E571 or E571K-XPO1 bound to SINEs, including Selinexor, are highly similar, suggesting that the activity of this class of compounds will not be affected by XPO1 mutations at E571 in patients with CLL. Conclusions These findings indicate that mutations in XPO1 at E571 can drive leukemogenesis by priming the pre-neoplastic lymphocytes for acquisition of additional genetic and epigenetic abnormalities that collectively result in neoplastic transformation.

AbstractPurpose:Aberrant Myc expression is a major factor in the pathogenesis of aggressive lymphoma, and these lymphomas, while clinically heterogeneous, often are resistant to currently available treatments and have poor survival. Myc expression can also be seen in aggressive lymphomas that are observed in the context of CLL, and we sought to develop a mouse model that could be used to study therapeutic strategies for aggressive lymphoma in the context of CLL.Experimental Design:We crossed the Eμ-TCL1 mouse model with the Eμ-Myc mouse model to investigate the clinical phenotype associated with B-cell–restricted expression of these oncogenes. The resulting malignancy was then extensively characterized, from both a clinical and biologic perspective.Results:Eμ-TCL1xMyc mice uniformly developed highly aggressive lymphoid disease with histologically, immunophenotypically, and molecularly distinct concurrent CLL and B-cell lymphoma, leading to a significantly reduced lifespan. Injection of cells from diseased Eμ-TCL1xMyc into WT mice established a disease similar to that in the double-transgenic mice. Both Eμ-TCL1xMyc mice and mice with disease after adoptive transfer failed to respond to ibrutinib. Effective and durable disease control was, however, observed by selective inhibition of nuclear export protein exportin-1 (XPO1) using a compound currently in clinical development for relapsed/refractory malignancies, including CLL and lymphoma.Conclusions:The Eμ-TCL1xMyc mouse is a new preclinical tool for testing experimental drugs for aggressive B-cell lymphoma, including in the context of CLL.

Abstract Bromodomain and extra-terminal (BET) family proteins are key regulators of gene expression in cancer. Herein, we utilize BRD4 profiling to identify critical pathways involved in pathogenesis of chronic lymphocytic leukemia (CLL). BRD4 is overexpressed in CLL and is enriched proximal to genes upregulated or de novo expressed in CLL with known functions in disease pathogenesis and progression. These genes, including key members of the B-cell receptor (BCR) signaling pathway, provide a rationale for this therapeutic approach to identify new targets in alternative types of cancer. Additionally, we describe PLX51107, a structurally distinct BET inhibitor with novel in vitro and in vivo pharmacologic properties that emulates or exceeds the efficacy of BCR signaling agents in preclinical models of CLL. Herein, the discovery of the involvement of BRD4 in the core CLL transcriptional program provides a compelling rationale for clinical investigation of PLX51107 as epigenetic therapy in CLL and application of BRD4 profiling in other cancers. Significance: To date, functional studies of BRD4 in CLL are lacking. Through integrated genomic, functional, and pharmacologic analyses, we uncover the existence of BRD4-regulated core CLL transcriptional programs and present preclinical proof-of-concept studies validating BET inhibition as an epigenetic approach to target BCR signaling in CLL. Cancer Discov; 8(4); 458–77. ©2018 AACR. This article is highlighted in the In This Issue feature, p. 371

Background: Epigenetic alterations that occur as a result of arginine methyltransferase activity play a critical role in normal biological functions, and increasingly their importance has been shown in cancer initiation and progression. In particular, protein arginine methyltransferase 5 (PRMT5) is an important epigenetic regulator that influences differentiation, cell cycle progression, and many other processes. Aberrant PRMT5 activity has been implicated in tumorigenesis, however its role in the biology of chronic lymphocytic leukemia (CLL) has not been established. Despite recent improvements in therapy, CLL remains an incurable disease. While newer therapies targeting the B cell receptor pathway have shown remarkable efficacy, complete responses are not frequent. In particular, transformation of CLL to aggressive lymphoma (Richter syndrome; RS) occurs in up to 15% of patients and confers a poor prognosis; however, the mechanisms by which RS occurs are poorly understood. A better understanding of CLL progression may facilitate the development of new targeted therapies. We hypothesized that PRMT5 dysregulation initiates an epigenetic program in CLL that contributes to disease progression and potentially transformation. Moreover, we provide evidence that inhibition of PRMT5 is a promising strategy in CLL.

Abstract Background Exportin 1 (XPO1/CRM1) is a critical component of the nuclear to cytoplasmic export machinery. As such, XPO1 shuttles tumor suppressor proteins (e.g. p53, IkB) out of the nucleus thereby preventing their anti-apoptotic function and contributing to enhanced proliferation. XPO1 also transports a subset of mRNAs (e.g. c-fos, IFN-alpha) by virtue of 3'-UTR AU-rich elements. XPO1 is upregulated in most hematologic malignancies, including CLL, where elevated XPO1 expression correlates with poor prognosis and resistance to therapy. To date, the role of nuclear export in CLL is poorly understood. Interestingly, recurrent mutations at a highly conserved residue (E571G; E571K) in the cargo binding pocket of XPO1 have been described in chronic lymphocytic leukemia (CLL) and are associated with worse overall survival. We hypothesized that the E571G/K mutation confers novel cargo-binding abilities to XPO1 and may contribute to the pathogenesis of CLL. Methods To interrogate this question in an unbiased, transcriptome-wide manner we developed CLL cell lines expressing mutant XPO1 and a novel RNA immunoprecipitation procedure using RNase I digestion followed by RNA-seq to capture direct interaction sites or 'footprints'. Reads were mapped to the transcriptome and peaks (regions where RNA footprints are enriched) were identified. We examined footprints for enriched sequence motifs using the MEME suite. Mass-array screening identified a cohort of CLL patients bearing the E571G/K mutation for validation purposes. The XPO1 transgenic mice were generated via pronuclear injection of linearized plasmid containing human cDNA corresponding to XPO1WT, XPO1E571G, or XPO1E571K under the heavy chain promoter/enhancer. Results We identified a transcriptome-wide pool of >100 XPO1-interacting RNAs in CLL cells and identified RNA footprints of wild-type XPO1 contributing to the first XPO1 RNA profile of a cell line model of CLL. The majority of bound RNA species associated with XPO1 are ribosomal, as expected; however, we identified multiple additional classes of RNAs such as miRs, mRNA, long-non coding RNAs, and many un-annotated RNAs. These data are consistent with additional preliminary data we have generated indicating that XPO1 may regulate mRNAs important in CLL development. We are currently searching for structural motifs. Validation experiments using patient-derived CLL cells with and without XPO1 mutations are ongoing. To further assess the independent transforming role of XPO1 dysfunction in B-cells, we generated transgenic mice overexpressing XPO1WT, XPO1E571G, or XPO1E571K under the heavy chain promoter/enhancer in the B cell compartment of C57BL/6 mice. Overexpression of XPO1 at the protein level in B cells was confirmed in multiple transgenic founder lines. Preliminary studies were performed to characterize the XPO1 transgenic animals for defects or alterations in major stages of B cell maturation. We analyzed the pro-B (IgM-CD43+), pre-B (IgM-CD43-), and immature (IgM+CD43-) B cell stages of development from the bone marrow of XPO1 transgenic animals and littermate controls. We also examined immature (IgM+IgD-) and mature (IgM+IgD+) B cells isolated from the spleen of these animals. In this preliminary analysis no significant differences in the B cell compartment were observed between transgenic and non-transgenic animals. Functional studies of these B-cells are ongoing. A cohort of animals from each founder line is being followed for evidence of B-cell expansion and overall survival analysis. In addition, the XPO1 transgenic mice were crossed with the Eµ-TCL1 mice to establish the XPO1WT/E571K/G/TCL1 double transgenic mouse model whose disease is currently being monitored. Conclusions XPO1 is a recurring, high variant allele mutation that occurs in a small subset of CLL patients. XPO1 transports select mRNA and also novel RNA species whose function in CLL have yet to be characterized, but points to a novel role of this oncoprotein in the pathogenesis of this disease. Disclosures No relevant conflicts of interest to declare.

Despite the therapeutic efficacy of ibrutinib in chronic lymphocytic leukemia (CLL), complete responses are infrequent, and acquired resistance to Bruton agammaglobulinemia tyrosine kinase (BTK) inhibition is being observed in an increasing number of patients. Combination regimens that increase frequency of complete remissions, accelerate time to remission, and overcome single agent resistance are of considerable interest. We previously showed that the XPO1 inhibitor selinexor is proapoptotic in CLL cells and disrupts B-cell receptor signaling via BTK depletion. Herein we show the combination of selinexor and ibrutinib elicits a synergistic cytotoxic effect in primary CLL cells and increases overall survival compared with ibrutinib alone in a mouse model of CLL. Selinexor is effective in cells isolated from patients with prolonged lymphocytosis following ibrutinib therapy. Finally, selinexor is effective in ibrutinib-refractory mice and in a cell line harboring the BTK C481S mutation. This is the first report describing the combined activity of ibrutinib and selinexor in CLL, which represents a new treatment paradigm and warrants further evaluation in clinical trials of CLL patients including those with acquired ibrutinib resistance.

Abstract Chronic Lymphocytic Leukemia (CLL) is a B-cell malignancy with aberrant activation of the B-cell receptor (BCR) pathway. Despite durable remissions with targeted therapies (e.g., ibrutinib) in CLL, it remains an incurable disease. Epigenetic modifications, including DNA methylation and dysregulation of chromatin regulators have been shown to contribute to the neoplastic phenotype and the differential biologic behavior of tumor cells, including leukemia. An additional layer of epigenetic complexity in cancer cells is the acquisition of super-enhancer regions enriched at genes with known oncogenic function including MYC and BCL2. Super-enhancers in multiple myeloma cells and other tumors have been found strongly enriched for binding of BRD4, a member of the human bromodomain and extraterminal (BET) domain family of proteins which includes BRD2, BRD3, BRD4, and the testis-specific member BRDT. BRD4 binds to acetylated lysines on histones and regulates the expression of important oncogenes (e.g., MYC and BCL2). We investigated the therapeutic benefit of BET inhibition in cell culture and in vivo disease models of leukemia/lymphoma using PLX51107, a novel BRD4 inhibitor with unique binding mode. Results: We report that BRD4 is significantly overexpressed in CLL patient-derived B-cells compared to B-cells from healthy donors on both transcript and protein level (p < .001). RNA-seq analysis of 55 CLL patients revealed expression of various BRD4 isoforms with marked abundance of BRD4-long and BRD4-short. Next we sought to investigate the anti-tumor activity of PLX51107 in multiple malignant B-cell lines and patient-derived CLL cells. PLX51107 inhibited cell growth in MEC1, OCI-Ly2 and OCI-Ly6 (p < .001) dose-dependently with IC50 of 1.0 ± 0.09, 1.2 ± 0.05, 1.8 ± 0.05 μM, respectively. Notably, PLX51107 antagonized CpG-induced increase in cell proliferation of primary CLL cells (p < .01) which was consistent with the downmodulation of MYC and MCL1 along with the accumulation of the cyclin-dependent kinase inhibitor p21 and IκBα (p < .005). Furthermore, the efficacy of PLX51107 to disrupt survival signaling from the microenvironment was investigated under co-culture conditions with two different bone marrow stroma cell lines, wherein PLX51107 treatment significantly induced cytotoxicity in B-CLL cells (p < .01) without affecting stromal cell viability. By employing microarray analysis we identified possible novel targets of BRD4 in CLL. Validation of those targets is currently ongoing. Particularly, Bruton's tyrosine kinase (BTK) and phospholipase C gamma 2 (PLCG2) were markedly decreased with PLX51107 treatment (p < .005), thereby signifying potential therapeutic effect(s) for dual targeting of BRD4 and BCR-associated kinases to achieve deeper and durable responses in relapsed/refractory B-cell malignancies. Lastly, anti-tumor effects of BRD4 inhibition were evaluated in vivo using Eμ-TCL1 and cMYC/TCL1 adoptive transfer models of leukemia and lymphoma, respectively. In the Eμ-TCL1 engraftment model of aggressive CLL, PLX51107 treatment resulted in prolonged survival (p < .001) accompanied with decreased disease burden, lymphocyte infiltration and proliferation when compared to vehicle-treated mice. Next, the cMYC/TCL1 adoptive transfer mouse model was used to evaluate BRD4 inhibition in a highly penetrant, malignant leukemia/lymphoma phenotype analogous to high grade lymphoma wherein PLX51107 prolonged survival (p < .0001), decreased peripheral lymphocyte counts and neoplastic cell infiltration and proliferation in both spleen and lymph nodes. Conclusion: Collectively our findings reveal BRD4 as a valid and novel target for epigenetic therapy directed against core transcriptional programs in malignant/proliferating B-cells and provide support for use of PLX51107 as an effective treatment in clinical trials for relapsed/refractory CLL patients and related aggressive forms of B-cell malignancies, with the ultimate goal of improving the outcome of these patients. Disclosures Byrd: Acerta Pharma BV: Research Funding.

Background: Aggressive B-cell lymphomas occurring in the setting of Chronic Lymphocytic Leukemia (CLL) as a large cell transformation are an important clinical problem, and improved mouse models to test novel and targeted therapeutics are needed. The Eµ-Myc mouse overexpresses c-Myc gene which is placed under control of the Myc promoter and lymphoid-specific IgH enhancer (Eµ), resulting in c-Myc overexpression and spontaneous B-cell lymphoma development. The Eµ-Myc mice have been used in drug development, however malignancy develops at variable ages and with differing genetics and response to therapeutic agents, making drug studies difficult. The Eµ-TCL1 transgenic mouse overexpresses the human TCL1 oncogene, under the control of the B-cell specific IgVH promoter and Eµ enhancer. Mice develop a spontaneous mature B-cell leukemia after a long latency period and represent a well-established model of human CLL. We crossed the Eµ-Myc and TCL1 mice to create a new model of aggressive B-cell lymphoma to test novel therapeutics that would be more homogeneous than the Eµ-Myc model.

Abstract Background Although autosomal recessive hematologic disorders are individually rare and difficult to ascertain, studies involving one or more homozygous affected children and their unaffected heterozygous parents have led to expanded understanding of known and discovery of previously unknown processes. The son and daughter of two Salvadoran parents were diagnosed with congenital thrombotic thrombocytopenic purpura (cTTP) at 6 and 2 years of age, respectively, after presenting with fever, respiratory symptoms, hemolytic anemia, and thrombocytopenia and being found to have ADAMTS13 activities <1% without neutralizing IgG antibodies. They remain without long-term neurologic or renal sequelae following prophylactic infusions of fresh plasma (10 mL/kg every 2.5-weeks). The purpose of this study was to characterize and correlate single-nucleotide variations (SNVs) in each parent's, non-mutant ADAMTS13 allele with its mRNA and protein expression, activity, and enzyme kinetics. Methods Prior to a plasma infusion, blood samples were collected from the children and parents. Genomic DNA was isolated for polymerase chain reaction (PCR), and direct Sanger sequencing of all ADAMTS13 exons and flanking intronic segments was performed; all variants identified were confirmed by bidirectional sequencing of a second, independently generated amplicon. Total RNA was isolated and the steady-state level of ADAMTS13 mRNA was measured using a quantitative real-time PCR (q-RT-PCR)-based assay. ADAMTS13 was characterized enzymatically using the fluorogenic FRETS-VWF73 substrate and antigenically by ELISA. Results Both children were found to be homozygous and parents to be heterozygous for the previously described, cTTP-causing ADAMTS13 single-base-substitution mutation 20506C>T, a missense mutation that encodes cDNA-nucleotide 2518 (c.2518C>T) and ADAMTS13 residue 692 (692Arg>Cys [692R>C]) (Fig. A). As expected, the children's ADAMTS13 antigen and activity levels were undetectable, although notably, steady-state levels of the ADAMTS13 mRNA were >2.5-fold higher in the daughter than in the son. The re-sequenced regions of the ADAMTS13 loci segregating within this family contained 26 additional SNVs, seven of which were nonsynonymous (ns) including two previously unreported ns-SNVs: 27852C>T (c.3362C>T; 972Arg>Trp) and 33325G>A (c.3733G>A; 1096Arg>His) (Fig. A, left panel). The parents' genotypes differed at nine positions, including three ns-SNVs, creating two distinct, non-mutant haplotypes (designated I and III) at the gene, mRNA and protein levels. The q-RT-PCR assay revealed >4-fold higher steady-state mRNA levels in the father compared to the mother (p<0.001; Fig. B). Plasma ADAMTS13 activity and antigen levels were ∼2-fold greater in the father than in the mother (p=0.00164 and p=0.0633, respectively), but the specific activities of these structurally distinct ADAMTS13 proteins were notably almost identical (253.5 vs. 256.2 U/μg). Moreover, initial velocity kinetic analysis using the Michelis-Menten equation demonstrated that the Vmax of the father's ADAMTS13 was twice that of the mother's (1.4 vs. 0.7; p < 0.0001) while its affinity for substrate was one-third that of her ADAMTS13 (Km = 0.3 vs. 0.1; p = 0.0585). Discussion We capitalized on the fortuitous finding of children with complete homozygosity across ancestrally-related ADAMTS13 alleles harboring a null-type, loss-of-function mutation, as this enabled the substantially different levels of gene expression and function observed in the parents to be attributed to their two previously unreported, SNV-based, ADAMTS13 haplotypes. Additional investigation at the molecular, biochemical, cellular, and organismal levels will be necessary to determine which of the myriad potential individual SNV- and/or haplotype-based mechanisms are responsible for the observed parental differences in circulating ADAMTS13 antigen and activity. Disclosures: Kim: Haplomics, Inc.: Membership on an entity’s Board of Directors or advisory committees; Baxter: Honoraria. Marder:Baxter: Research Funding. Howard:Haplomics, Inc.: Equity Ownership, Membership on an entity’s Board of Directors or advisory committees; Baxter: Research Funding.

Congenital thrombotic thrombocytopenic purpura (cTTP) is a rare, recessively inherited genetic disorder with varying clinical presentation that is caused by ADAMTS13 mutations. Several studies have found limited associations between ADAMTS13 mutations and cTTP phenotype. The use of in silico tools that examine multiple mutation characteristics may better predict phenotype. We analysed 118 ADAMTS13 mutations found in 144 cTTP patients reported in the literature and examined associations of several mutation characteristics, including N-terminal proximity, the evolutionary conservation of the affected amino acid position, as well as amino acid charge/phosphorylation and genetic codon usage to disease phenotype. Structure-altering mutations were examined for their impact on ADAMTS13 function based on existing ADAMTS13 crystallographic data (AA 77-685). Our in silico data indicate that: (i) The position of the mutation in the N- or C-terminus, (ii) evolutionary conservation and (iii) codon usage of the affected mutation position are associated with disease parameters, such as age of onset, organ damage and fresh frozen plasma prophylaxis. In conclusion, the usage of multiple in silico tools presents a promising strategy in refining predictions for the diverse presentation of cTTP. Enhancing our utilization of in silico tools to find genotype-phenotype associations will create better-tailored approaches for individual patient treatment.

Synonymous variations, which are defined as codon substitutions that do not change the encoded amino acid, were previously thought to have no effect on the properties of the synthesized protein(s). However, mounting evidence shows that these "silent" variations can have a significant impact on protein expression and function and should no longer be considered "silent". Here, the effects of six synonymous and six non-synonymous variations, previously found in the gene of ADAMTS13, the von Willebrand Factor (VWF) cleaving hemostatic protease, have been investigated using a variety of approaches. The ADAMTS13 mRNA and protein expression levels, as well as the conformation and activity of the variants have been compared to that of wild-type ADAMTS13. Interestingly, not only the non-synonymous variants but also the synonymous variants have been found to change the protein expression levels, conformation and function. Bioinformatic analysis of ADAMTS13 mRNA structure, amino acid conservation and codon usage allowed us to establish correlations between mRNA stability, RSCU, and intracellular protein expression. This study demonstrates that variants and more specifically, synonymous variants can have a substantial and definite effect on ADAMTS13 function and that bioinformatic analysis may allow development of predictive tools to identify variants that will have significant effects on the encoded protein.