Germinal Center Lymphomas Research Articles

NF-κB Mutations in Germinal Center B-Cell Lymphomas: Relation to NF-κB Function in Normal B Cells.

Most B cell lymphomas arise from the oncogenic transformation of B cells that have undergone the germinal center (GC) reaction of the T cell-dependent immune response, where high-affinity memory B cells and plasma cells are generated. The high proliferation of GC B cells coupled with occasional errors in the DNA-modifying processes of somatic hypermutation and class switch recombination put the cell at a risk to obtain transforming genetic aberrations, which may activate proto-oncogenes or inactivate tumour suppressor genes. Several subtypes of GC lymphomas harbor genetic mutations leading to constitutive, aberrant activation of the nuclear factor-κB (NF-κB) signaling pathway. In normal B cells, NF-κB has crucial biological roles in development and physiology. GC lymphomas highjack these activities to promote tumour-cell growth and survival. It has become increasingly clear that the separate canonical and non-canonical routes of the NF-κB pathway and the five downstream NF-κB transcription factors have distinct functions in the successive stages of GC B-cell development. These findings may have direct implications for understanding how aberrant NF-κB activation promotes the genesis of various GC lymphomas corresponding to the developmentally distinct GC B-cell subsets. The knowledge arising from these studies may be explored for the development of precision medicine approaches aimed at more effective treatments of the corresponding tumours with specific NF-κB inhibitors, thus reducing systemic toxicity. We here provide an overview on the patterns of genetic NF-κB mutations encountered in the various GC lymphomas and discuss the consequences of aberrant NF-κB activation in those malignancies as related to the biology of NF-κB in their putative normal cellular counterparts.

The Role of Coinfections in the EBV–Host Broken Equilibrium

The Epstein–Barr virus (EBV) is a well-adapted human virus, and its infection is exclusive to our species, generally beginning in the childhood and then persisting throughout the life of most of the affected adults. Although this infection generally remains asymptomatic, EBV can trigger life-threatening conditions under unclear circumstances. The EBV lifecycle is characterized by interactions with other viruses or bacteria, which increases the probability of awakening its pathobiont capacity. For instance, EBV infects B cells with the potential to alter the germinal center reaction (GCR)—an adaptive immune structure wherein mutagenic-driven processes take place. HIV- and Plasmodium falciparum-induced B cell hyperactivation also feeds the GCR. These agents, along with the B cell tropic KSHV, converge in the ontogeny of germinal center (GC) or post-GC lymphomas. EBV oral transmission facilitates interactions with local bacteria and HPV, thereby increasing the risk of periodontal diseases and head and neck carcinomas. It is less clear as to how EBV is localized in the stomach, but together with Helicobacter pylori, they are known to be responsible for gastric cancer. Perhaps this mechanism is reminiscent of the local inflammation that attracts different herpesviruses and enhances graft damage and chances of rejection in transplanted patients. In this review, we discussed the existing evidence suggestive of EBV possessing the potential to synergize or cooperate with these agents to trigger or worsen the disease.

KDM5 inhibition offers a novel therapeutic strategy for the treatment of KMT2D mutant lymphomas

AbstractLoss-of-function mutations in KMT2D are a striking feature of germinal center (GC) lymphomas, resulting in decreased histone 3 lysine 4 (H3K4) methylation and altered gene expression. We hypothesized that inhibition of the KDM5 family, which demethylates H3K4me3/me2, would reestablish H3K4 methylation and restore the expression of genes repressed on loss of KMT2D. KDM5 inhibition increased H3K4me3 levels and caused an antiproliferative response in vitro, which was markedly greater in both endogenous and gene-edited KMT2D mutant diffuse large B-cell lymphoma cell lines, whereas tumor growth was inhibited in KMT2D mutant xenografts in vivo. KDM5 inhibition reactivated both KMT2D-dependent and -independent genes, resulting in diminished B-cell signaling and altered expression of B-cell lymphoma 2 (BCL2) family members, including BCL2 itself. KDM5 inhibition may offer an effective therapeutic strategy for ameliorating KMT2D loss-of-function mutations in GC lymphomas.

USP21 promotes cell proliferation by maintaining the EZH2 level in diffuse large B-cell lymphoma.

BackgroundDiffuse large B‐cell lymphoma (DLBCL) is the most common category of non‐Hodgkin lymphoma (NHL). However, the underlying molecular mechanism of DLBCL remains unclear.MethodsReal‐time PCR and Western blot analysis were performed to assess the expression of ubiquitin‐specific peptidase 21 (USP21) or enhancer of zeste 2 polycomb repressive complex 2 subunit (EZH2). CCK8 assay and cell death staining were carried out to examine the role of USP21 in cell proliferation and cell death, respectively.ResultsWe found that the deubiquitinase USP21 was highly expressed in the DLBCL lymphoid tissue. The expression of USP21 promoted DLBCL cell proliferation, while it had no obvious effect on cell death. In addition, we found that USP21 regulated cell proliferation via cysteine 221, the catalytic site of USP21. Furthermore, we identified that USP21 could stabilize EZH2, a protein required for germinal center formation and lymphoma formation.ConclusionThe deubiquitinase USP21 promotes cell proliferation by maintaining the EZH2 protein level in DLBCL.

Compromised counterselection by FAS creates an aggressive subtype of germinal center lymphoma.

Fas is highly expressed on germinal center (GC) B cells, and mutations of FAS have been reported in diffuse large B cell lymphoma (DLBCL). Although GC-derived DLBCL has better overall outcomes than other DLBCL types, some cases are refractory, and the molecular basis for this is often unknown. We show that Fas is a strong cell-intrinsic regulator of GC B cells that promotes cell death in the light zone, likely via T follicular helper (Tfh) cell-derived Fas ligand. In the absence of Fas, GCs were more clonally diverse due to an accumulation of cells that did not demonstrably bind antigen. FAS alterations occurred most commonly in GC-derived DLBCL, were associated with inferior outcomes and an enrichment of Tfh cells, and co-occurred with deficiency in HVEM and PD-L1 that regulate the Tfh-B cell interaction. This work shows that Fas is critically required for GC homeostasis and suggests that loss of Tfh-mediated counterselection in the GC contributes to lethality in GC-derived lymphoma.

Abstract IA26: What do the results from the trials tell us?

Abstract The relatively high prevalence of somatic mutations in several epigenetic modifying genes in germinal center lymphomas suggests a role in pathogenesis, with a variety of potential mechanisms for this. The first of these somatic changes to be successfully targeted by small-molecule inhibitors was the activating mutation of the enhancer of Zeste Homolog-2 (EZH2), which is found in up to a quarter of follicular lymphomas and germinal center-type diffuse large B-cell lymphomas, where it drives the methylation of histone H3 on lysine 27, inhibiting exit from the germinal center and accelerating lymphoma development in preclinical models. Two inhibitors of EZH2 have entered clinical trials, GSK2816126 and tazemetostat. GSK2816126 is a highly selective and potent inhibitor of wild-type (WT) and mutant (Y641N, A677G, and A687V) EZH2, with IC50 of 28–861 nM for proliferation in EZH2-mutant lymphoma cell lines. It is not orally bioavailable, but pharmacokinetic/pharmacodynamic studies suggested that therapeutic levels could be achieved with intravenous doses of between 900mg and 2,700mg twice a week. Tazemetostat is a similarly selective and orally bioavailable small-molecule inhibitor of EZH2 with an IC50 of 0.49-5800 nM for proliferation in EZH2-mutant lymphoma cell lines. Both of these molecules have been tested in phase I dose escalation studies. GSK2816126 was given by twice-weekly intravenous infusion at doses of up to 3,000mg, with dose-limiting liver enzyme rises at the highest dose, achieving trough levels of 410 ng/mL. No changes in H3K27me3 could be seen in peripheral blood mononuclear cells, and only one short-lived partial response was seen among 20 patients with B-cell lymphoma. Tazemetostat was given at doses of up to 1,600mg twice daily without reaching dose-limiting toxicity, although significant thrombocytopenia was seen at this dose level. The mean pre-dose trough level at the recommended phase II dose of 800mg bd was 75 ng/mL, and skin biopsies showed a reduction of H3K27me3 by approximately 40% from baseline. Tazemetostat has shown moderate efficacy as a single agent in relapsed/refractory follicular lymphoma (FL), with overall response rate (ORR) 33/43 (77%) and median duration of response (mDOR) 8.3 months in EZH2-mutant cases and ORR 18/53 (34%), mDOR 13 months in those without mutations. In relapsed/refractory diffuse large B-cell lymphoma (DLBL) the response rate was lower, ORR 6/36 (17%), mDOR 11+ months in mutant cases and 20/121 (17%), mDOR 7+ months in wild-type. In both studies the treatment was well tolerated, with the most frequently reported adverse events being thrombocytopenia, anemia, asthenia, vomiting, and fatigue, all in less than 5% of patients with FL and less than 20% in DLBL. Combination therapy of tazemetostat with atezolizumab did not increase the response rate in DLBL, but giving it with R-CHOP chemotherapy was well tolerated in a phase Ib study, and a dose of 800mg BD could be given without apparently increased toxicity over R-CHOP in patients between 60 and 80 years old, and without adverse effects on the pharmacology of the chemotherapy or tazemetostat. In keeping with many agents targeting a single molecular abnormality, inhibitors of EZH2 have demonstrated clear but transient efficacy, with the most promising results in follicular lymphoma. The higher response rate among cases with an activating mutation is notable, but wild-type lymphomas also show responsiveness in a minority of cases, indicating other potential mechanisms of action. The combination of tazemetostat with chemoimmunotherapy appears promising, although the relatively good prognosis for DLBL bearing an EZH2 mutation may make it difficult to demonstrate clear clinical benefit with this approach. Citation Format: Peter W. M. Johnson. What do the results from the trials tell us? [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr IA26.

Abstract PO-22: Compromised counterselection by FAS creates a lethal subtype of germinal center lymphoma

Abstract Fas is highly expressed on germinal center (GC) B cells, and mutations of FAS have been reported in diffuse large B-cell lymphoma (DLBCL); however, the function of Fas in the GC remains controversial. Although GC-derived DLBCL has better overall outcomes to therapy than other DLBCL types, some cases are refractory and the molecular basis for this is often unknown. We show that Fas is a strong cell-intrinsic regulator of GC B cells that promotes B-cell death in the light zone likely via T follicular helper (Tfh) cell-derived Fas ligand. In the absence of Fas, GCs were more clonally diverse due to an accumulation of cells that did not demonstrably bind antigen. We found that FAS alterations occurred most commonly in the GC-derived genetic subtype of DLBCL, EZB. FAS alterations in EZB were associated with inferior outcomes and an enrichment of Tfh cells. Alterations in FAS co-occurred with deficiency in HVEM and PD-1 ligands that regulate the Tfh-B cell interaction and were associated with increased diversity of B-cell receptor variable genes across samples. This work shows that Fas is critically required for GC homeostasis and suggests that loss of Tfh-mediated counterselection in the GC contributes to lethality in a distinct subtype of GC-derived lymphoma. Citation Format: Raud Razzaghi, Shreya Agarwal, Nikita Kotlov, Olga Plotnikova, Krystle Nomie, Da Wei Huang, George W. Wright, Moyi Li, Katsuyoshi Takata, Chen Yao, John J. O'Shea, James D. Phelan, Stefania Pittaluga, David W. Scott, Jagan R. Muppidi. Compromised counterselection by FAS creates a lethal subtype of germinal center lymphoma [abstract]. In: Proceedings of the AACR Virtual Meeting: Advances in Malignant Lymphoma; 2020 Aug 17-19. Philadelphia (PA): AACR; Blood Cancer Discov 2020;1(3_Suppl):Abstract nr PO-22.

PF513 MUTATIONS AFFECTING THE CREBBP HAT DOMAIN PREDICT RESPONSE TO MTOR INHIBITORS EVEROLIMUS AND TEMSIROLIMUS IN RELAPSED/REFRACTORY FOLLICULAR LYMPHOMA

Background:With an expanding array of novel treatment options in follicular lymphoma (FL), we largely lack effective biomarkers to predict response in this molecularly heterogeneous disease. Activating mutations focused on components of the nutrient‐sensing arm of the mTORC1 pathway (RRAGC, ATP6V1B2, ATP6AP1), are particularly enriched in FL, occurring in ∼30% of cases. Meanwhile, clinical mTOR inhibitors (mTORi) have shown some promise, with response rates of 50–60% in relapsed/refractory FL.Aims:We therefore sought to determine whether gene mutation status ‐ particularly in genes regulating the mTOR pathway – may represent biomarkers of mTORi‐response in FL.Methods:We retrospectively analyzed pre‐treatment tumor samples from relapsed/refractory FL patients treated on two phase II clinical trials with single agent mTOR inhibitors, everolimus (NCT00436618) or temsirolimus (NCT00290472), with well characterized clinical outcomes. In total, 21 patients had available biopsies, consisting of 15 everolimus‐ and 6 temsirolimus‐treated cases.Tumor DNA extracted from formalin fixed paraffin embedded (FFPE) tissue was subjected to targeted sequencing using a 22 gene panel comprising genes recurrently mutated in FL, and PI3K/AKT/mTOR pathway genes mutated in germinal center lymphomas. Libraries were prepared using capture‐based target enrichment (Nonacus), and sequenced on the NextSeq550 (Illumina) with a mean coverage depth of x700. A stringent bioinformatic pipeline was adopted with variants called with VarScan2, and a 10% variant allele frequency (VAF) threshold was employed to filter out sequencing artefacts due to the FFPE material. 100% (15/15) of a selection of variants were validated by Sanger sequencing. Staining for pS6 was undertaken on tissue slides to examine downstream mTOR activation by immunohistochemistry.Results:Overall response rate (CR and PR) was 48% (10/21). Mutational analysis showed no significant association between clinical response and mTORC1‐pathway mutations (mut) taken individually or collectively; although, absolute numbers of mutations in these genes were low, including RRAGC (responder: 0/10 mut vs non‐responder: 1/11 mut), ATP6AP1 (responder: 1/10 vs non‐responder: 1/11), and ATP6V1B2 (responder: 1/10 vs non‐responder 3/11). Notably, no difference in pS6 staining was seen between responders and non‐responders.However, we found mutations in the epigenetic regulator CREBBP to be enriched in the responder group (100% (10/10) responders vs 54.5% (6/11) non‐responders, p = 0.04). This effect was more pronounced when considering only mutations within the catalytic histone acetyltransferase (HAT) domain of CREBBP, with HAT mutations seen in 100% (10/10) responders, and 27.3% (3/11) non‐responders (p = 0.001).Summary/Conclusion:Mutations in the mTOR pathway did not predict for response to mTOR inhibition in our study. The enrichment of CREBBP mutations in responders, and particularly those affecting the catalytic HAT domain, points to CREBBP as a potential biomarker of response which warrants validation in trials of drugs modulating the PI3K/AKT/mTOR axis. More broadly our data may lend support to a distinct clinical and biological behavior of CREBBP HAT domain‐mutant disease that also warrants further evaluation. Taken together, our findings highlight the potential opportunities afforded by undertaking correlative studies, and underlines the need to incorporate prospective biomarker discovery into clinical trials in order to help focus future therapeutic strategies more rationally.

MYC in Germinal Center-derived lymphomas: Mechanisms and therapeutic opportunities.

The rearrangement of immunoglobulin loci during the germinal center reaction is associated with an increased risk of chromosomal translocations that activate oncogenes such as MYC, BCL2 or BCL6, thus contributing to the development of B-cell lymphomas. MYC and BCL2 activation are initiating events in Burkitt's (BL) and Follicular Lymphoma (FL), respectively, but can occur at later stages in other subtypes such as Diffuse Large-B Cell Lymphoma (DLBCL). MYC can also be activated during the progression of FL to the transformed stage. Thus, either DLBCL or FL can give rise to aggressive double-hit lymphomas (DHL) with concurrent activation of MYC and BCL2. Research over the last three decades has improved our understanding of the functions of these oncogenes and the basis for their cooperative action in lymphomagenesis. MYC, in particular, is a transcription factor that contributes to cell activation, growth and proliferation, while concomitantly sensitizing cells to apoptosis, the latter being blocked by BCL2. Here, we review our current knowledge about the role of MYC in germinal center B-cells and lymphomas, discuss MYC-induced dependencies that can sensitize cancer cells to select pharmacological inhibitors, and illustrate their therapeutic potential in aggressive lymphomas-and in particular in DHL, in combination with BCL2 inhibitors.

MRD Detection in B-Cell Non-Hodgkin Lymphomas Using Ig Gene Rearrangements and Chromosomal Translocations as Targets for Real-Time Quantitative PCR.

Minimal residual disease (MRD) diagnostics is of high clinical relevance in patients with indolent B-cell non-Hodgkin lymphomas (B-NHL) and serves as a surrogate parameter to evaluate treatment effectiveness and long-term prognosis. MRD diagnostics performed by real-time quantitative PCR (RQ-PCR) is still the gold standard and currently the most sensitive and the most broadly applied method in follicular lymphoma (FL) and mantle cell lymphoma (MCL). Alternatively, droplet digital PCR (ddPCR) can be used for MRD monitoring in multiple myeloma, mantle cell lymphoma, and follicular lymphoma with comparable sensitivity, accuracy, and reproducibility.The most broadly applicable MRD target in B-NHL is the junctional regions of the rearranged immunoglobulin heavy chain gene (IGHV). Chromosomal translocations like the t(14;18) translocation in FL and t(11;14) translocation in MCL can be used as MRD target in selected lymphoma subtypes. In patients with B-cell chronic lymphocytic leukemia, both flow-cytometry and RQ-PCR are equally suited for MRD assessment as long as a sensitivity of 10-4 shall be achieved.MRD diagnostics targeting the IGHV gene is complex and requires extensive knowledge and experience because the junctional regions of each lymphoma have to be identified before the patient-specific RQ-PCR assays can be designed for MRD monitoring. In addition, the presence and load of somatic hypermutation (SHM) within the rearranged IG heavy variable (IGHV) gene occurring as during B-cell development of germinal center and post-germinal center lymphomas may hamper appropriate primer binding leading to false-negative results. The translocations mentioned above have the advantage that consensus forward primers and probes, both placed in the breakpoint regions of chromosome 18 in FL and chromosome 11 in MCL, can be used in combination with a reverse primer placed in the IGH joining region of chromosome 14. RQ-PCR-based methods can reach a good sensitivity (≤10-4). This chapter provides all relevant background information and technical aspects for the complete laboratory process from detection of the clonal IGHV gene rearrangement and the chromosomal translocations at diagnosis to the actual MRD measurements in clinical follow-up samples of B-NHL. However, it should be noted that MRD diagnostics for clinical treatment protocols has to be accompanied by regular international quality control rounds to ensure the reproducibility and reliability of the MRD results.

PRMT5 interacts with the BCL6 oncoprotein and is required for germinal center formation and lymphoma cell survival

AbstractThe germinal center (GC) reaction plays an important role in generating humoral immunity and is believed to give rise to most B-cell lymphomas. GC entry and exit are tightly regulated processes, controlled by the actions of transcription factors such as BCL6. Herein, we demonstrate that protein arginine methyltransferase 5 (PRMT5), a symmetric dimethyl arginine methyltransferase, is also necessary for GC formation and affinity maturation. PRMT5 contributes to GC formation and affinity maturation at least in part through its direct interaction with and methylation of BCL6 at arginine 305 (R305), a modification necessary for the full transcriptional repressive effects of BCL6. Inhibition of PRMT5 in B-cell lymphoma lines led to significant upregulation of BCL6 target genes, and the concomitant inhibition of both BCL6 and PRMT5 exhibited synergistic killing of BCL6-expressing lymphoma cells. Our studies identify PRMT5 as a novel regulator of the GC reaction and highlight the mechanistic rationale of cotargeting PRMT5 and BCL6 in lymphoma.

Precision medicine and lymphoma.

The treatment of the germinal center lymphomas, diffuse large B cell (DLBCL) and follicular lymphoma, has changed little beyond the introduction of immunochemotherapies. However, there exists a substantial group of patients within both diseases for which improvements in care will involve appropriate tailoring of treatment. DLBCL consists of two major subtypes with striking differences in their clinical outcomes paralleling their underlying genetic heterogeneity. Recent studies have seen advances in the stratification of germinal center lymphomas, through comprehensive profiling of 1001 DLBCLs alongside refinements in the identification of high-risk follicular lymphoma patients using m7-FLIPI and 23G models. A new wave of novel therapeutic agents is now undergoing clinical trials for germinal center lymphomas, with BCR and EZH2 inhibitors demonstrating preferential benefit in subgroups of patients. The emergence of cell-free DNA has raised the possibility of dynamic disease monitoring to potentially mitigate the complexity of spatial and temporal heterogeneity, whilst predicting tumor evolution in real time. Altogether knowledge of the genomic landscape of germinal center lymphomas is offering welcome opportunities in patient risk stratification and therapeutics. The challenge ahead is to establish how best to combine upfront or dynamic prognostication with precision therapies, while retaining practicality in clinical trials and the real-world setting.

Kappa and Lambda Determination by Single Colored Immunohistochemistry and Chromogenic in Situ Hybridization Techniques in B-Cell Malignancies

Background: kappa and lambda light chains detection in bone marrow trephine sections help in the determination of B-cell clonality through evaluation of light chain restriction. Aim of the Work: was to compare the efficacy of single color detection-based immunohistochemistry (IHC) and chromogenic in situ hybridization (CISH) in evaluating kappa/lambda expression in tissues harboring B-lymphoid lesions. Patients and Methods: Forty patients were enrolled in this study. They were divided into three groups chronic lymphocytic leukemia (CLL/SLL) group I (n=13), non-Hodgkin lymphoma (NHL) group II (n=24) and hairy cell leukemia (HCL) group III (n=3). The 24 NHL cases comprised of (11 diffuse large B-cell lymphoma, 6 mantle cell lymphomas, 3 marginal zone lymphoma, 2 lymphoplasmacytic lymphoma, 1 follicular lymphomas and 1 Burkitt’s lymphoma). Kappa and lambda light chains were detected in their bone marrow trephine sections using single colored immunohistochemistry, chromogenic in situ hybridization and the results were compared to the flowcytometry as reference method. Results: Light chain restriction (LCR) was detected by FCM in 100% of the cases followed by CISH (52.1%; 12/23) of the cases and finally IHC (43%; 18/40). Background: kappa and lambda light chains detection in bone marrow trephine sections help in the determination of B-cell clonality through evaluation of light chain restriction. Aim of the Work: was to compare the efficacy of single color detection-based immunohistochemistry (IHC) and chromogenic in situ hybridization (CISH) in evaluating kappa/lambda expression in tissues harboring B-lymphoid lesions. Patients and Methods: Forty patients were enrolled in this study. They were divided into three groups chronic lymphocytic leukemia (CLL/SLL) group I (n=13), non-Hodgkin lymphoma (NHL) group II (n=24) and hairy cell leukemia (HCL) group III (n=3). The 24 NHL cases comprised of (11 diffuse large B-cell lymphoma, 6 mantle cell lymphomas, 3 marginal zone lymphoma, 2 lymphoplasmacytic lymphoma, 1 follicular lymphomas and 1 Burkitt’s lymphoma). Kappa and lambda light chains were detected in their bone marrow trephine sections using single colored immunohistochemistry, chromogenic in situ hybridization and the results were compared to the flowcytometry as reference method. Results: Light chain restriction (LCR) was detected by FCM in 100% of the cases followed by CISH (52.1%; 12/23) of the cases and finally IHC (43%; 18/40). Conclusion: Both conventional CISH and IHC are effective in determining monoclonality in cases of mature B- cell neoplasm that has plasmacytic differentiation and with high amount of cytoplasmic Ig light chains such as MZL and LP. However, they are not effective in determining monoclonality in cases with low amount of Ig light chain such as cases of pregerminal and germinal center lymphoma. Yet, CISH is more informative than IHC due to the lack of background staining which allowed for greater discrimination between absence and presence of monoclonality.

Abstract IA31: Targeting immune receptor mutations in lymphoma

Abstract The HVEM (TNFRSF14) receptor gene is among the most frequently mutated genes in germinal center lymphomas. We report that loss of HVEM leads to cell-autonomous activation of B cell proliferation and drives the development of GC lymphomas in vivo. HVEM-deficient lymphoma B cells also induce a tumor-supportive microenvironment marked by exacerbated lymphoid stroma activation and increased recruitment of T follicular helper (TFH) cells. These changes result from the disruption of inhibitory cell-cell interactions between the HVEM and BTLA (B and T lymphocyte attenuator) receptors. Accordingly, administration of the HVEM ectodomain protein (solHVEM(P37-V202)) binds BTLA and restores tumor suppression. To deliver solHVEM to lymphomas in vivo, we engineered CD19-targeted chimeric antigen receptor (CAR) T cells that produce solHVEM locally and continuously. These modified CAR-T cells show enhanced therapeutic activity against xenografted lymphomas. Hence, the HVEM-BTLA axis opposes lymphoma development, and our study illustrates the use of CAR-T cells as “micropharmacies” able to deliver an anticancer protein. Citation Format: Hans-Guido Wendel. Targeting immune receptor mutations in lymphoma [abstract]. In: Proceedings of the Second AACR Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; May 6-9, 2017; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(24_Suppl):Abstract nr IA31.

Molecular genetics of aggressive B-cell lymphoma.

Molecular genetics of aggressive B-cell lymphoma.

CREBBP Inactivation Promotes the Development of HDAC3-Dependent Lymphomas.

Our findings establish the tumor suppressor function of CREBBP in GC lymphomas in which CREBBP mutations disable acetylation and result in unopposed deacetylation by BCL6/SMRT/HDAC3 complexes at enhancers of B-cell signaling and immune response genes. Hence, inhibition of HDAC3 can restore the enhancer histone acetylation and may serve as a targeted therapy for CREBBP-mutant lymphomas. Cancer Discov; 7(1); 38-53. ©2016 AACR.See related commentary by Höpken, p. 14This article is highlighted in the In This Issue feature, p. 1.

Development of Mouse Model Systems of Germinal Center Lymphomas.

Germinal centers (GC) are the predominant origin of human B cell lymphomagenesis. Transgenic mice in which gene expression is altered specifically in GC B cells have broadened our knowledge about the mechanisms of malignant transformation. However, extensive resources are needed due to the genetic complexity of these mouse models. Thus, bone marrow (BM)-derived chimerism is an attractive approach to study GC B cell derived lymphomagenesis, as it allows for an efficient allocation of resources and reduces the number of animals used.

Loss of the HVEM Tumor Suppressor in Lymphoma and Restoration by Modified CAR-T Cells

The HVEM (TNFRSF14) receptor gene is among themost frequently mutated genes in germinal center lymphomas. We report that loss of HVEM leads tocell-autonomous activation of B cell proliferationand drives the development of GC lymphomasinvivo. HVEM-deficient lymphoma B cells also induce a tumor-supportive microenvironment marked by exacerbated lymphoid stroma activation and increased recruitment of T follicular helper (TFH) cells. These changes result from the disruption of inhibitory cell-cell interactions between the HVEM and BTLA (B and T lymphocyte attenuator) receptors. Accordingly, administration of the HVEM ectodomain protein (solHVEM(P37-V202)) binds BTLA and restores tumor suppression. To deliver solHVEM to lymphomas invivo, we engineered CD19-targeted chimeric antigen receptor (CAR) Tcells that produce solHVEM locally and continuously. These modified CAR-T cells show enhanced therapeutic activity against xenografted lymphomas. Hence, the HVEM-BTLA axis opposes lymphoma development, and our study illustrates the use of CAR-T cells as "micro-pharmacies" able to deliver an anti-cancer protein.

27. Aberrant Expression of the Stem Cell microRNA-126 Induces B Cell Malignancy

MicroRNAs are essential regulators of normal and malignant hematopoiesis. miRNAs are relevant for gene therapy, since they can be exploited to fine-tune the expression profile of vector constructs or to alter viral tropism (GentnerN Chiriaco et al, 2014; Escobar et al, 2014) and described the function of miR-126 in HSC where it regulates the balance between quiescence and self-renewal (Lechman et al, 2012). We here report a novel role for miR-126 in the induction and maintenance of high-grade B cell malignancies. By ectopically expressing miR-126 in transplanted BM cells, we observed that up to 60% of mice (n=71) developed B cell malignancies. LV insertion site (IS) analysis revealed that all tumors were monoclonal. We then tracked back leukemic clone to different hematopoietic lineages prospectively purified from the mice 2-6 months before disease onset. IS sharing between normal lineages and leukemic clone suggests stem or multipotent progenitor cell as origin for most tumors. Importantly, we show that miR-126 is the direct cause of genesis and maintenance of leukemia, since leukemogenesis is abolished when miRNA expression is inhibited by doxycycline (doxy) using a tetracycline-repressible miR-126 cassette, and established symptomatic leukemia completely regresses when miR-126 is switched off by doxy through induction of apoptosis. Transcriptional profiling indicated that miR-126 regulates multiple genes in p53 pathway both in murine blasts and in normal human CD34+ cells. Previous work suggested expression of miR-126 in acute lymphoblastic leukemia (ALL) and germinal center lymphoma. To further establish the relevance of miR-126 in human disease, we measured miR-126 expression in blasts from 16 adult patients with ALL. miR-126 was highly expressed in most studied ALL cases (Phil+: n=11, Phil-: n=5), at similar levels as CD34+ cells. We then down-regulated miR-126 in primary blasts from human B-ALL patients (n=5), and we observed increased apoptosis and impaired engraftment in xenograft models after primary and secondary transplantation (miR-126/KD: n=32 mice; Ctrl: n=37 mice), demonstrating the relevance of miR-126 in human B-ALL. In conclusion, we present a novel spontaneous mouse model for high grade B cell malignancies which are addicted to miR-126 expression, provide insight into the dynamic process of leukemogenesis by clonal IS tracking and unveil key tumor signaling pathways controlled by miR-126. Down-regulation of miR-126 could be exploited as therapeutic strategy in ALL, since it would deplete leukemic cells while expanding normal HSC, two ways to restore normal hematopoieis.

HDAC9 Expression Is Deregulated in Malignant B-Cell Lymphomas in Particular in Diffuse Large B Cell Lymphoma and Mantle Cell Lymphoma

Aberrant epigenetics leading to changes in chromatin structure and patterns of gene expression is an important factor in cancer pathogenesis. Histone Deacetylase 9 (HDAC9) is a class IIa chromatin-modifying enzyme that, within the haematopoietic system, is preferentially expressed in the B-cell lineage. Mice that constitutively express human HDAC9 from early stages of B-cell development, under the control of the immunoglobulin heavy chain (IgH) enhancer, develop lymphoproliferative disorders, including germinal center (GC) and post-GC lymphomas, demonstrating an oncogenic role for HDAC9 in B-cells and highlighting its importance as a therapeutic target. In order to examine the relationship between disease observed in the mouse model and human primary lymphoma, we have examined, using immunohistochemistry (IHC) the expression of full length HDAC9 isoform in a panel of various B-cell malignancies.The study group included 59 non-Hodgkin lymphomas (NHL), and 3 classical HL. Non-HL consisted of 34 diffuse large B cell lymphoma (DLBCL), 9 follicular lymphoma (FL), 5 marginal zone lymphoma (MZL), 6 mantle cell lymphoma (MCL), and 2 small lymphocytic lymphomas (SLL). HDAC9 expression was assessed by IHC using tissue microarray and/or routine tissue sections. Protein expression was scored as negative (0), low (1), or high (2) depending on the staining signal intensity. Expression of HDAC9 in the nuclei of the tumor cells was compared with that seen in adenocarcinoma cells; if equal or higher, then expression of HDAC9 was considered high and if lower, then expression of HDAC9 was considered low. Five reactive lymph nodes were studied to assess the baseline expression of HDAC9. Rectal adenocarcinomas were used as positive controls. In reactive lymph nodes, HDAC9 was weakly expressed in a subset of germinal center cells, a subset of lymphoid cells in the paracortex as well as in endothelial cells. HDAC9 expression was detected in all subsets of B-cell lymphomas analyzed and in most cases with a level of expression higer than those seen in reactive lymph nodes. DLBCL and MCL tumors have the highest frequency of high HDAC9 expression among the B-cell lymphomas analyzed, 77 and 83% (Fisher’s exact test P=1,0), respectively. No differences in HDAC9 expression were detected in DLBCL of GC and non-GC type. In contrast, most (69%) of the low-grade B cell lymphomas show no or lower expression of HDAC9 (Fisher’s exact test P=0.004; as compared to DLBCL). Classical HL showed frequently low-expression of HDAC9 in the tumor cells. In summary, HDAC9 is frequently expressed in B-cell lymphomas with the highest level of expression found in the most aggressive lymphomas such as DLBCL and MCL. These findings support the biological role of HDAC9 in the pathobiology of aggressive B cell neoplasms. DisclosuresNo relevant conflicts of interest to declare.