CRISPR/Cas9-mediated gene disruption determines the roles of MITF and CITED2 in human mast cell differentiation.

The differentiation of hematopoietic stem and progenitor cells into the various cell lineages is regulated by cell-intrinsic factors and the progenitors' microenvironment. Experiments in mice have allowed the identification of transcriptional modulators that control mast cell differentiation. However, a comprehensive approach that allows efficient disruption of individual transcriptional modulators in primary human hematopoietic progenitors coupled with a mast cell formation readout has not been described. Here, we report a simple electroporation- and ribonucleoprotein-based knockout system that allows the identification of genes that are required and dispensable for human mast cell differentiation. We show that the transcription factor MITF is upregulated in human mast cell progenitors and reveal that the loss of MITF results in the suppressed formation of mast cells. By contrast, CITED2, another transcriptional modulator that is upregulated along the mast cell differentiation trajectory, was dispensable for human mast cell differentiation. Taken together, we report a CRISPR/Cas9-based framework that serves to identify genes involved in regulating the formation of human mast cells, and the results uncover the role of two transcriptional modulators in controlling human mast cell differentiation.

MDM2/MDMX inhibition by Sulanemadlin synergizes with anti-Programmed Death 1 immunotherapy in wild-type p53 tumors.

Immunotherapy has revolutionized cancer treatment but its efficacy depends on a robust immune response in the tumor. Silencing of the tumor suppressor p53 is common in tumors and can affect the recruitment and activation of different immune cells, leading to immune evasion and poor therapy response. We found that the p53 activating stapled peptide MDM2/MDMX inhibitor Sulanemadlin (ALRN-6924) inhibited p53 wild-type cancer cell growth in vitro and in vivo. In mice carrying p53 wild-type CT26.WT tumors, monotherapy with the PD-1 inhibitor DX400 or Sulanemadlin delayed tumor doubling time by 50% and 37%, respectively, while combination therapy decreased tumor doubling time by 93% leading to an increased median survival time. Sulanemadlin treatment led to increased immunogenicity and combination treatment with PD-1 inhibition resulted in an increased tumor infiltration of lymphocytes. This combination treatment strategy could potentially turn partial responders into responders of immunotherapy, expanding the patient target group for PD-1-targeting immunotherapy.

High-content screening of drug combinations of an mPGES-1 inhibitor in multicellular tumor spheroids leads to mechanistic insights into neuroblastoma chemoresistance.

High-throughput drug screening enables the discovery of new anticancer drugs. Although monolayer cell cultures are commonly used for screening, their limited complexity and translational efficiency require alternative models. Three-dimensional cell cultures, such as multicellular tumor spheroids (MCTS), mimic tumor architecture and offer promising opportunities for drug discovery. In this study, we developed a neuroblastoma MCTS model for high-content drug screening. We also aimed to decipher the mechanisms underlying synergistic drug combinations in this disease model. Several agents from different therapeutic categories and with different mechanisms of action were tested alone or in combination with selective inhibition of prostaglandin E2 by pharmacological inhibition of microsomal prostaglandin E synthase-1 (mPGES-1). After a systematic investigation of the sensitivity of individual agents and the effects of pairwise combinations, GFP-transfected MCTS were used in a confirmatory screen to validate the hits. Finally, inhibitory effects on multidrug resistance proteins were examined. In summary, we demonstrate how MCTS-based high-throughput drug screening has the potential to uncover effective drug combinations and provide insights into the mechanism of synergy between an mPGES-1 inhibitor and chemotherapeutic agents.

Noninvasive PET Detection of CD69-Positive Immune Cells Before Signs of Clinical Disease in Inflammatory Arthritis.

Rheumatoid arthritis (RA) is the most common inflammatory joint disease, and early diagnosis is key for effective disease management. CD69 is one of the earliest cell surface markers seen at the surface of activated immune cells, and CD69 is upregulated in synovial tissue in patients with active RA. In this study, we evaluated the performance of a CD69-targeting PET agent, [68Ga]Ga-DOTA-ZCAM241, for early disease detection in a model of inflammatory arthritis. Methods: A model of inflammatory arthritis was induced by transferring splenocytes from KRN T-cell receptor transgenic B6 mice into T-cell-deficient I-Ag7 major histocompatibility complex class II-expressing recipient mice. The mice were examined longitudinally by [68Ga]Ga-DOTA-ZCAM241 PET/CT before and 3, 7, and 12 d after induction of arthritis. Disease progression was monitored by clinical parameters, including measuring body weight and scoring the swelling of the paws. The uptake of [68Ga]Ga-DOTA-ZCAM241 in the paws was analyzed and expressed as SUVmean Tissue biopsy samples were analyzed for CD69 expression by flow cytometry or immunostaining for a histologic correlate. A second group of mice was examined by a nonbinding, size-matched Affibody molecule as the control. Results: Clinical symptoms appeared 5-7 d after induction of arthritis. The uptake of [68Ga]Ga-DOTA-ZCAM241 in the joints was negligible at baseline but increased gradually after disease induction. An elevated PET signal was found on day 3, before the appearance of clinical symptoms. The uptake of [68Ga]Ga-DOTA-ZCAM241 correlated with the clinical score and disease severity. The presence of CD69-positive cells in the joints and lymph nodes was confirmed by flow cytometry and immunostaining. The uptake of the nonbinding tracer that was the negative control also increased gradually with disease progression, although to a lesser extent than with [68Ga]Ga-DOTA-ZCAM241 Conclusion: The uptake of [68Ga]Ga-DOTA-ZCAM241 in the inflamed joints preceded the clinical symptoms in the KRN T-cell transfer model of inflammatory arthritis, in accordance with immunostaining for CD69. [68Ga]Ga-DOTA-ZCAM241 is thus a promising PET imaging marker of activated immune cells in tissue during RA onset.

MDM2 inhibitors, nutlin-3a and navtemadelin, retain efficacy in human and mouse cancer cells cultured in hypoxia.

Activation of p53 by small molecule MDM2 inhibitors can induce cell cycle arrest or death in p53 wildtype cancer cells. However, cancer cells exposed to hypoxia can develop resistance to other small molecules, such as chemotherapies, that activate p53. Here, we evaluated whether hypoxia could render cancer cells insensitive to two MDM2 inhibitors with different potencies, nutlin-3a and navtemadlin. Inhibitor efficacy and potency were evaluated under short-term hypoxic conditions in human and mouse cancer cells expressing different p53 genotypes (wild-type, mutant, or null). Treatment of wild-type p53 cancer cells with MDM2 inhibitors reduced cell growth by > 75% in hypoxia through activation of the p53-p21 signaling pathway; no inhibitor-induced growth reduction was observed in hypoxic mutant or null p53 cells except at very high concentrations. The concentration of inhibitors needed to induce the maximal p53 response was not significantly different in hypoxia compared to normoxia. However, inhibitor efficacy varied by species and by cell line, with stronger effects at lower concentrations observed in human cell lines than in mouse cell lines grown as 2D and 3D cultures. Together, these results indicate that MDM2 inhibitors retain efficacy in hypoxia, suggesting they could be useful for targeting acutely hypoxic cancer cells.

Biosynthesis of prostaglandin 15dPGJ2 -glutathione and 15dPGJ2-cysteine conjugates in macrophages and mast cells via MGST3.

Inhibition of microsomal prostaglandin E synthase-1 (mPGES-1) results in decreased production of proinflammatory PGE2 and can lead to shunting of PGH2 into the prostaglandin D2 (PGD2)/15-deoxy-Δ12,14-prostaglandin J2 (15dPGJ2) pathway. 15dPGJ2 forms Michael adducts with thiol-containing biomolecules such as GSH or cysteine residues on target proteins and is thought to promote resolution of inflammation. We aimed to elucidate the biosynthesis and metabolism of 15dPGJ2 via conjugation with GSH, to form 15dPGJ2-glutathione (15dPGJ2-GS) and 15dPGJ2-cysteine (15dPGJ2-Cys) conjugates and to characterize the effects of mPGES-1 inhibition on the PGD2/15dPGJ2 pathway in mouse and human immune cells. Our results demonstrate the formation of PGD2, 15dPGJ2, 15dPGJ2-GS, and 15dPGJ2-Cys in RAW264.7 cells after lipopolysaccharide stimulation. Moreover, 15dPGJ2-Cys was found in lipopolysaccharide-activated primary murine macrophages as well as in human mast cells following stimulation of the IgE-receptor. Our results also suggest that the microsomal glutathione S-transferase 3 is essential for the formation of 15dPGJ2 conjugates. In contrast to inhibition of cyclooxygenase, which leads to blockage of the PGD2/15dPGJ2 pathway, we found that inhibition of mPGES-1 preserves PGD2 and its metabolites. Collectively, this study highlights the formation of 15dPGJ2-GS and 15dPGJ2-Cys in mouse and human immune cells, the involvement of microsomal glutathione S-transferase 3 in their biosynthesis, and their unchanged formation following inhibition of mPGES-1. The results encourage further research on their roles as bioactive lipid mediators.

Autoantibodies targeting malondialdehyde-modifications in rheumatoid arthritis regulate osteoclasts via inducing glycolysis and lipid biosynthesis.

Proteins subjected to post-translational modifications, such as citrullination, carbamylation, acetylation or malondialdehyde (MDA)-modification are targeted by autoantibodies in seropositive rheumatoid arthritis (RA). Epidemiological and experimental studies have both suggested the pathogenicity of such humoral autoimmunity, however, molecular mechanisms triggered by anti-modified protein antibodies have remained to be identified. Here we describe in detail the pathways induced by anti-MDA modified protein antibodies that were obtained from synovial B cells of RA patients and that possessed robust osteoclast stimulatory potential and induced bone erosion in vivo. Anti-MDA antibodies boosted glycolysis in developing osteoclasts via an FcγRI, HIF-1α and MYC-dependent mechanism and subsequently increased oxidative phosphorylation. Osteoclast development required robust phosphoglyceride and triacylglyceride biosynthesis, which was also enhanced by anti-MDA by modulating citrate production and expression of the glycerol-3-phosphate dehydrogenase 1 (GPD1) and glycerol-3-phosphate acyltransferase 2 (GPAT2) genes. In summary, we described novel metabolic pathways instrumental for osteoclast differentiation, which were targeted by anti-MDA antibodies, accelerating bone erosion, a central component of RA pathogenesis.

Modulating T-cell activation with antisense oligonucleotides targeting lymphocyte cytosolic protein 2.

Dysregulated T-cell activation is a hallmark of several autoimmune diseases such as rheumatoid arthritis (RA) and multiple sclerosis (MS). The lymphocyte cytosolic protein 2 (LCP2), also known as SLP-76, is essential for the development and activation of T cells. Despite the critical role of LCP2 in T-cell activation and the need for developing drugs that modify T-cell activation, no LCP2 inhibitors have been developed. This can be explained by the "undruggable" nature of LCP2, lacking a structure permissive to standard small molecule inhibitor modalities. Here, we explored an alternative drug modality, developing antisense oligonucleotides (ASOs) targeting LCP2 mRNAs, and evaluated its activity in modulating T-cell activation. We identified a set of 3' UTR targeting LCP2 ASOs, which knocked down LCP2 in a human T-cell line and primary human T cells and found that these suppressed T-cell receptor mediated activation. We also found that the ASOs suppressed FcεR1-mediated mast cell activation, in line with the role of LCP2 in mast cells. Taken together, our data provide examples of how immunomodulatory ASOs that interfere with undruggable targets can be developed and propose that such drug modalities can be used to treat autoimmune diseases.

A CRISPR-p53 interactome with potential implications for clinical CRISPR/Cas9 use.

CRISPR/Cas9-based tools are anticipated to transform the gene therapy field by facilitating the correction of disease-causing mutations. However, CRISPR/Cas9 generates DNA damage, which triggers a DNA damage response centered around the tumor-suppressor p53. In this research perspective, we discuss implications of this and describe a CRISPR-p53 interactome with cancer-related genes that, if mutated, can give cells a selective advantage following exposure to CRISPR/Cas9. We propose that the genes in the CRISPR-p53 interactome should be monitored in the clinical setting and describe that transient p53 inhibition could be used to limit the enrichment of cells with such mutations.

The MDM2 Inhibitor Navtemadlin Arrests Mouse Melanoma Growth In Vivo and Potentiates Radiotherapy.

The tumor suppressor protein p53 is mutated in close to 50% of human tumors and is dysregulated in many others, for instance by silencing or loss of p14ARF. Under steady-state conditions, the two E3 ligases MDM2/MDM4 interact with and inhibit the transcriptional activity of p53. Inhibition of p53-MDM2/4 interaction to reactivate p53 in tumors with wild-type (WT) p53 has therefore been considered a therapeutic strategy. Moreover, studies indicate that p53 reactivation may synergize with radiation and increase tumor immunogenicity. In vivo studies of most MDM2 inhibitors have utilized immunodeficient xenograft mouse models, preventing detailed studies of action of these molecules on the immune response. The mouse melanoma cell line B16-F10 carries functional, WT p53 but does not express the MDM2 regulator p19ARF. In this study, we tested a p53-MDM2 protein-protein interaction inhibitor, the small molecule Navtemadlin, which is currently being tested in phase II clinical trials. Using mass spectrometry-based proteomics and imaging flow cytometry, we identified specific protein expression patterns following Navtemadlin treatment of B16-F10 melanoma cells compared with their p53 CRISPR-inactivated control cells. In vitro, Navtemadlin induced a significant, p53-dependent, growth arrest but little apoptosis in B16-F10 cells. When combined with radiotherapy, Navtemadlin showed synergistic effects and increased apoptosis. In vivo, Navtemadlin treatment significantly reduced the growth of B16-F10 melanoma cells implanted in C57Bl/6 mice. Our data highlight the utility of a syngeneic B16-F10 p53+/+ mouse melanoma model for assessing existing and novel p53-MDM2/MDM4 inhibitors and in identifying new combination therapies that can efficiently eliminate tumors in vivo. Significance: The MDM2 inhibitor Navtemadlin arrests mouse tumor growth and potentiates radiotherapy. Our results support a threshold model for apoptosis induction that requires a high, prolonged p53 signaling for cancer cells to become apoptotic.

CRISPR/Cas9-Induced DNA Damage Enriches for Mutations in a p53-Linked Interactome: Implications for CRISPR-Based Therapies.

Inactivating p53 mutations are the most abundant genetic alterations found in cancer. Here we show that CRISPR/Cas9-induced double-stranded DNA breaks enrich for cells deficient in p53 and in genes of a core CRISPR-p53 tumor suppressor interactome. Such enrichment could predispose to cancer development and thus pose a challenge for clinical CRISPR use. Transient p53 inhibition could suppress the enrichment of cells with these mutations. The level of DNA damage response induced by an sgRNA influenced the enrichment of p53-deficient cells and could be a relevant parameter in sgRNA design to limit cellular enrichment. Furthermore, a dataset of >800 human cancer cell lines identified additional factors influencing the enrichment of p53-mutated cells, including strong baseline CDKN1A expression as a predictor for an active CRISPR-p53 axis. Taken together, these data provide details about p53 biology in the context of CRISPR-induced DNA damage and identify strategies to enable safer CRISPR use. SIGNIFICANCE: CRISPR-mediated DNA damage enriches for cells with escape mutations in a core CRISPR-p53 interactome, which can be suppressed by transient inhibition of p53.

A rapid CRISPR competitive assay for in vitro and in vivo discovery of potential drug targets affecting the hematopoietic system.

CRISPR/Cas9 can be used as an experimental tool to inactivate genes in cells. However, a CRISPR-targeted cell population will not show a uniform genotype of the targeted gene. Instead, a mix of genotypes is generated - from wild type to different forms of insertions and deletions. Such mixed genotypes complicate analysis of the role of the targeted gene in the studied cell population. Here, we present a rapid and universal experimental approach to functionally analyze a CRISPR-targeted cell population that does not involve generating clonal lines. As a simple readout, we leverage the CRISPR-induced genetic heterogeneity and use sequencing to identify how different genotypes are enriched or depleted in relation to the studied cellular behavior or phenotype. The approach uses standard PCR, Sanger sequencing, and a simple sequence deconvoluting software, enabling laboratories without specific in-depth experience to perform these experiments. As proof of principle, we present examples studying various aspects related to hematopoietic cells (T cell development in vivo and activation in vitro, differentiation of macrophages and dendritic cells, as well as a leukemia-like phenotype induced by overexpressing a proto-oncogene). In conclusion, we present a rapid experimental approach to identify potential drug targets related to mature immune cells, as well as normal and malignant hematopoiesis.

In Vivo Lentiviral Gene Delivery of HLA-DR and Vaccination of Humanized Mice for Improving the Human T and B Cell Immune Reconstitution.

Humanized mouse models generated with human hematopoietic stem cells (HSCs) and reconstituting the human immune system (HIS-mice) are invigorating preclinical testing of vaccines and immunotherapies. We have recently shown that human engineered dendritic cells boosted bonafide human T and B cell maturation and antigen-specific responses in HIS-mice. Here, we evaluated a cell-free system based on in vivo co-delivery of lentiviral vectors (LVs) for expression of a human leukocyte antigen (HLA-DRA*01/ HLA-DRB1*0401 functional complex, "DR4"), and a LV vaccine expressing human cytokines (GM-CSF and IFN-α) and a human cytomegalovirus gB antigen (HCMV-gB). Humanized NOD/Rag1null/IL2Rγnull (NRG) mice injected by i.v. with LV-DR4/fLuc showed long-lasting (up to 20 weeks) vector distribution and expression in the spleen and liver. In vivo administration of the LV vaccine after LV-DR4/fLuc delivery boosted the cellularity of lymph nodes, promoted maturation of terminal effector CD4+ T cells, and promoted significantly higher development of IgG+ and IgA+ B cells. This modular lentigenic system opens several perspectives for basic human immunology research and preclinical utilization of LVs to deliver HLAs into HIS-mice.

In vitro and ex vivo functional characterization of human HLA-DRB1∗04 restricted T cell receptors.

Recent advances in single-cell sequencing technologies enable the generation of large-scale data sets of paired TCR sequences from patients with autoimmune disease. Methods to validate and characterize patient-derived TCR data are needed, as well as relevant model systems that can support the development of antigen-specific tolerance inducing drugs. We have generated a pipeline to allow streamlined generation of 'artificial' T cells in a robust and reasonably high throughput manner for in vitro and in vivo studies of antigen-specific and patient-derived immune responses. Hereby chimeric (mouse-human) TCR alpha and beta constructs are re-expressed in three different formats for further studies: (i) transiently in HEK cells for peptide-HLA tetramer validation experiments, (ii) stably in the TCR-negative 58 â€‹T cell line for functional readouts such as IL-2 production and NFAT-signaling, and lastly (iii) in human HLA-transgenic mice for studies of autoimmune disease and therapeutic interventions. As a proof of concept, we have used human HLA-DRB1∗04:01 restricted TCR sequences specific for a type I diabetes-associated GAD peptide, and an influenza-derived HA peptide. We show that the same chimeric TCR constructs can be used in each of the described assays facilitating sequential validation and prioritization steps leading to humanized animal models.

Biased TCR gene usage in citrullinated Tenascin C specific T-cells in rheumatoid arthritis.

We aimed to search for common features in the autoreactive T cell receptor (TCR) repertoire in patients with rheumatoid arthritis (RA), focusing on the newly identified candidate antigen citrullinated Tenascin C (cit-TNC). Mononuclear cells from peripheral blood or synovial fluid of eight RA-patients positive for the RA-associated HLA-DRB1*04:01 allele were in-vitro cultured with recently identified citrullinated peptides from Tenascin C. Antigen-specific T cells were isolated using peptide-HLA tetramer staining and subsequently single-cell sequenced for paired alpha/beta TCR analyses by bioinformatic tools. TCRs were re-expressed for further studies of antigen-specificity and T cell responses. Autoreactive T cell lines could be grown out from both peripheral blood and synovial fluid. We demonstrate the feasibility of retrieving true autoreactive TCR sequences by validating antigen-specificity in T cell lines with re-expressed TCRs. One of the Tenascin C peptides, cit-TNC22, gave the most robust T cell responses including biased TCR gene usage patterns. The shared TCR-beta chain signature among the cit-TNC22-specific TCRs was evident in blood and synovial fluid of different patients. The identification of common elements in the autoreactive TCR repertoire gives promise to the possibility of both immune monitoring of the autoimmune components in RA and of future antigen- or TCR-targeted specific intervention in subsets of patients.

Identifying novel B-cell targets for chronic inflammatory autoimmune disease by screening of chemical probes in a patient-derived cell assay.

B-cell secretion of autoantibodies drives autoimmune diseases, including systemic lupus erythematosus and idiopathic inflammatory myositis. Few therapies are presently available for treatment of these patients, often resulting in unsatisfactory effects and helping only some of the patients. We developed a screening assay for evaluation of novel targets suspending B-cell maturation into antibody secreting cells, which could contribute to future drug development. The assay was employed for testing 43 high quality chemical probes and compounds inhibiting under-explored protein targets, using primary cells from patients with autoimmune disease. Probes inhibiting bromodomain family proteins and histone methyl transferases demonstrated abrogation of B-cell functions to a degree comparable to a positive control, the JAK inhibitor tofacitinib. Inhibition of each target rendered a specific functional cell and potential disease modifying effect, indicating specific epigenetic protein targets as potential new intervention points for future drug discovery and development efforts.

Designing custom CRISPR libraries for hypothesis-driven drug target discovery.

Over the last decade Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) has been developed into a potent molecular biology tool used to rapidly modify genes or their expression in a multitude of ways. In parallel, CRISPR-based screening approaches have been developed as powerful discovery platforms for dissecting the genetic basis of cellular behavior, as well as for drug target discovery. CRISPR screens can be designed in numerous ways. Here, we give a brief background to CRISPR screens and discuss the pros and cons of different design approaches, including unbiased genome-wide screens that target all known genes, as well as hypothesis-driven custom screens in which selected subsets of genes are targeted (Fig. 1). We provide several suggestions for how a custom screen can be designed, which could broadly serve as inspiration for any experiment that includes candidate gene selection. Finally, we discuss how results from CRISPR screens could be translated into drug development, as well as future trends we foresee in the rapidly evolving CRISPR screen field.

IL-4 controls activated neutrophil FcγR2b expression and migration into inflamed joints.

Neutrophils are the most abundant immune cells found in actively inflamed joints of patients with rheumatoid arthritis (RA), and most animal models for RA depend on neutrophils for the induction of joint inflammation. Exogenous IL-4 and IL-13 protect mice from antibody-mediated joint inflammation, although the mechanism is not understood. Neutrophils display a very strong basal expression of STAT6, which is responsible for signaling following exposure to IL-4 and IL-13. Still, the role of IL-4 and IL-13 in neutrophil biology has not been well studied. This can be explained by the low neutrophil surface expression of the IL-4 receptor α-chain (IL-4Rα), essential for IL-4- and IL-13-induced STAT6 signaling. Here we identify that colony stimulating factor 3 (CSF3), released during acute inflammation, mediates potent STAT3-dependent neutrophil IL-4Rα up-regulation during sterile inflammatory conditions. We further demonstrate that IL-4 limits neutrophil migration to inflamed joints, and that CSF3 combined with IL-4 or IL-13 results in a prominent neutrophil up-regulation of the inhibitory Fcγ receptor (FcγR2b). Taking these data together, we demonstrate that the IL-4 and CSF3 pathways are linked and play important roles in regulating proinflammatory neutrophil behavior.

An intronic deletion in megakaryoblastic leukemia 1 is associated with hyperproliferation of B cells in triplets with Hodgkin lymphoma.

Megakaryoblastic leukemia 1 (MKL1) is a coactivator of serum response factor and together they regulate transcription of actin cytoskeleton genes. MKL1 is associated with hematologic malignancies and immunodeficiency, but its role in B cells is unexplored. Here we examined B cells from monozygotic triplets with an intronic deletion in MKL1, two of whom had been previously treated for Hodgkin lymphoma (HL). To investigate MKL1 and B-cell responses in the pathogenesis of HL, we generated Epstein-Barr virus-transformed lymphoblastoid cell lines from the triplets and two controls. While cells from the patients with treated HL had a phenotype close to that of the healthy controls, cells from the undiagnosed triplet had increased MKL1 mRNA, increased MKL1 protein, and elevated expression of MKL1-dependent genes. This profile was associated with elevated actin content, increased cell spreading, decreased expression of CD11a integrin molecules, and delayed aggregation. Moreover, cells from the undiagnosed triplet proliferated faster, displayed a higher proportion of cells with hyperploidy, and formed large tumors in vivo This phenotype was reversible by inhibiting MKL1 activity. Interestingly, cells from the triplet treated for HL in 1985 contained two subpopulations: one with high expression of CD11a that behaved like control cells and the other with low expression of CD11a that formed large tumors in vivo similar to cells from the undiagnosed triplet. This implies that pre-malignant cells had re-emerged a long time after treatment. Together, these data suggest that dysregulated MKL1 activity participates in B-cell transformation and the pathogenesis of HL.