Anaplasticlargecelllymphoma(ALCL)isadistinctentityofT-celllymphomathatcanbedivided into2subtypesbasedonthepresenceoftranslocationsinvolvingthe ALK gene(ALK 1 andALK 2 ALCL). The interferon regulatory factor 4 (IRF4) is known to be highly expressed in both ALK 1 and ALK 2 ALCLs. However, the role of IRF4 in the pathogenesis of these lymphomas remains unclear.HereweshowthatALCLsofbothsubtypesareaddictedtoIRF4signaling,asknockdown of IRF4 by RNA interference was toxic to ALCL cell lines in vitro and in ALCL xenograft mouse models in vivo. Gene expression profiling after IRF4 knockdown demonstrated a significant downregulation of a variety of known MYC target genes. Furthermore, our analyses revealed that MYC is a primary target of IRF4, identifying a novel regulatory mechanism of MYC expression and its target gene network in ALCL. MYC, itself, is essential for ALCL survival, as both knockdownofMYCandpharmacologicinhibitionofMYCsignalingweretoxictoALCLcelllines.Collectively,ourresultsdemonstratethatALCLs are dependent on IRF4 and MYC signaling and that MYC may represent a promising target for future therapies. ( Blood . 2015;125(1):124-132)

Disease-causing mutations in genes encoding transcription factors (TFs) can affect TF interactions with their cognate DNA-binding motifs. Whether and how TF mutations impact upon the binding to TF composite elements (CE) and the interaction with other TFs is unclear. Here, we report a distinct mechanism of TF alteration in human lymphomas with perturbed B cell identity, in particular classic Hodgkin lymphoma. It is caused by a recurrent somatic missense mutation c.295 T > C (p.Cys99Arg; p.C99R) targeting the center of the DNA-binding domain of Interferon Regulatory Factor 4 (IRF4), a key TF in immune cells. IRF4-C99R fundamentally alters IRF4 DNA-binding, with loss-of-binding to canonical IRF motifs and neomorphic gain-of-binding to canonical and non-canonical IRF CEs. IRF4-C99R thoroughly modifies IRF4 function by blocking IRF4-dependent plasma cell induction, and up-regulates disease-specific genes in a non-canonical Activator Protein-1 (AP-1)-IRF-CE (AICE)-dependent manner. Our data explain how a single mutation causes a complex switch of TF specificity and gene regulation and open the perspective to specifically block the neomorphic DNA-binding activities of a mutant TF.

Epigenetic control of gene expression occurs within discrete spatial chromosomal units called topologically associating domains (TADs), but the exact spatial requirements of most genes are unknown; this is of particular interest for genes involved in cancer. We therefore applied high-resolution chromosomal conformation capture sequencing to map the three-dimensional (3D) organization of the human locus encoding the key myeloid transcription factor PU.1 in healthy monocytes and acute myeloid leukemia (AML) cells. We identified a dynamic ∼75-kb unit (SubTAD) as the genomic region in which spatial interactions between PU.1 gene regulatory elements occur during myeloid differentiation and are interrupted in AML. Within this SubTAD, proper initiation of the spatial chromosomal interactions requires PU.1 autoregulation and recruitment of the chromatin-adaptor protein LDB1 (LIM domain-binding protein 1). However, once these spatial interactions have occurred, LDB1 stabilizes them independently of PU.1 autoregulation. Thus, our data support that PU.1 autoregulates its expression in a "hit-and-run" manner by initiating stable chromosomal loops that result in a transcriptionally active chromatin architecture.

Unfavorable patient survival coincides with lineage plasticity observed in human acute leukemias. These cases are assumed to arise from hematopoietic stem cells, which have stable multipotent differentiation potential. However, here we report that plasticity in leukemia can result from instable lineage identity states inherited from differentiating progenitor cells. Using mice with enhanced c-Myc expression, we show, at the single-cell level, that T-lymphoid progenitors retain broad malignant lineage potential with a high capacity to differentiate into myeloid leukemia. These T-cell-derived myeloid blasts retain expression of a defined set of T-cell transcription factors, creating a lymphoid epigenetic memory that confers growth and propagates myeloid/T-lymphoid plasticity. Based on these characteristics, we identified a correlating human leukemia cohort and revealed targeting of Jak2/Stat3 signaling as a therapeutic possibility. Collectively, our study suggests the thymus as a source for myeloid leukemia and proposes leukemic plasticity as a driving mechanism. Moreover, our results reveal a pathway-directed therapy option against thymus-derived myeloid leukemogenesis and propose a model in which dynamic progenitor differentiation states shape unique neoplastic identities and therapy responses.

The aggressive activated B cell-like subtype of diffuse large B-cell lymphoma is characterized by aberrant B-cell receptor (BCR) signaling and constitutive nuclear factor kappa-B (NF-kappa B) activation, which is required for tumor cell survival. BCR-induced NF-kappa B activation requires caspase recruitment domain-containing protein 11 (CARD11), and CARD11 gain-of-function mutations are recurrently detected in human diffuse large B-cell lymphoma (DLBCL). To investigate the consequences of dysregulated CARD11 signaling in vivo, we generated mice that conditionally express the human DLBCL-derived CARD11 (L225LI) mutant. Surprisingly, CARD11(L225LI) was sufficient to trigger aggressive B-cell lymphoproliferation, leading to early postnatal lethality. CARD11(L225LI) constitutively associated with B-cell CLL/lymphoma 10 (BCL10) and mucosa-associated lymphoid tissue lymphoma translocation gene 1 (MALT1) to simultaneously activate the NF-kappa B and c-Jun N-terminal kinase (JNK) signaling cascades. Genetic deficiencies of either BCL10 or MALT1 completely rescued the phenotype, and pharmacological inhibition of JNK was, similar to NF-kappa B blockage, toxic to autonomously proliferating CARD11(L225LI)expressing B cells. Moreover, constitutive JNK activity was observed in primary human activated B cell-like (ABC)-DLBCL specimens, and human ABC-DLBCL cells were also sensitive to JNK inhibitors. Thus, our results demonstrate that enforced activation of CARD11/BCL10/MALT1 signaling is sufficient to drive transformed B-cell expansion in vivo and identify the JNK pathway as a therapeutic target for ABC-DLBCL.

Tumor‐induced immunosuppression remains a major challenge for immunotherapy of cancer patients. To further elucidate why an allogeneic gene‐modified [interleukin‐7 (IL‐7)/CD80‐cotransfected] renal cell cancer (RCC) vaccine failed to induce clinically relevant TH‐1‐polarized immune responses, peripheral blood mononuclear cells from enrolled study patients were analyzed by gene expression profiling (GEP) both prior and after vaccination. At baseline before vaccination, a profound downregulation of gene signatures associated with antigen presentation, immune response/T cells, cytokines/chemokines and signaling/transcription factors was observed in RCC patients as compared to healthy controls. Vaccination led to a partial reversion of preexisting immunosuppression, however, GEP indicated that an appropriate TH‐1 polarization could not be achieved. Most interestingly, our results suggest that the nuclear factor‐kappa B signaling pathway might be involved in the impairment of immunological responsiveness and the observed TH‐2 deviation. In summary, our data suggest that GEP might be a powerful tool for the prediction of immunosuppression and the monitoring of immune responses within immunotherapy trials.

e15555 Background: Tumor-induced immunosuppression is a hallmark of cancer and a major obstacle for immunotherapies. It includes impaired T cell function, reduced antigen presentation, and diminished humoral and innate immune responses. Therefore, the gene expression profile (GEP) of peripheral blood mononuclear cells (PBMCs) is very likely to reflect the immunological state and might give insights into the underlying molecular mechanisms of immunosuppression. A previously performed phase-I trial with an allogeneic, human leucocyte antigen (HLA)-A2-matched RCC cell line transfected with interleukin-7 (IL-7) and CD80 as a vaccine in RCC patients was feasible and safe, however, clinical responses and/or significant TH1-polarized immune responses were not observed. Methods: To further elucidate why our allogeneic gene-modified RCC vaccine failed to induce TH-1-polarized immune responses, PBMCs of RCC patients (n=9) and healthy controls (n=9) were analyzed by GEP both prior and after vaccination in order to identify differences in the expression of immunologically relevant genes and cellular processes. Results: At baseline, a profound and statistically significant downregulation of gene signatures associated with antigen presentation, immune response/T cells, cytokines/chemokines, and signaling/transcription factors was observed in RCC patients as compared to healthy controls. Vaccination led to a partial reversion of preexisting immunosuppression, however, GEP did not show appropriate TH-1 polarization. Moreover, various central components and a variety of known target genes of the nuclear factor-kappa B (NF-κB) signaling pathway were significantly downregulated in RCC patients´ PBMCs, implying that suppression of this signaling cascade plays a key role for the impairment of immunological responsiveness. Conclusions: Our data implicate that the GEP of PBMCs may serve as a surrogate parameter for interactions between tumor and immune system. GEP of PBMCs appears to be a useful tool for the characterization of a) tumor-induced immunosuppression and b) systemic immunological effects of immunotherapeutics.

Anaplastic large cell lymphoma (ALCL) is a distinct entity of T-cell lymphoma that can be divided into 2 subtypes based on the presence of translocations involving the ALK gene (ALK(+) and ALK(-) ALCL). The interferon regulatory factor 4 (IRF4) is known to be highly expressed in both ALK(+) and ALK(-) ALCLs. However, the role of IRF4 in the pathogenesis of these lymphomas remains unclear. Here we show that ALCLs of both subtypes are addicted to IRF4 signaling, as knockdown of IRF4 by RNA interference was toxic to ALCL cell lines in vitro and in ALCL xenograft mouse models in vivo. Gene expression profiling after IRF4 knockdown demonstrated a significant downregulation of a variety of known MYC target genes. Furthermore, our analyses revealed that MYC is a primary target of IRF4, identifying a novel regulatory mechanism of MYC expression and its target gene network in ALCL. MYC, itself, is essential for ALCL survival, as both knockdown of MYC and pharmacologic inhibition of MYC signaling were toxic to ALCL cell lines. Collectively, our results demonstrate that ALCLs are dependent on IRF4 and MYC signaling and that MYC may represent a promising target for future therapies.