PPM1D modulates hematopoietic cell fitness and response to DNA damage and is a therapeutic target in myeloid malignancy.

PPM1D encodes a phosphatase that is recurrently activated across cancer, most notably in therapy-related myeloid neoplasms. However, the function of PPM1D in hematopoiesis and its contribution to tumor cell growth remain incompletely understood. Using conditional mouse models, we uncover a central role for Ppm1d in hematopoiesis and validate its potential as a therapeutic target. We find that Ppm1d regulates the competitive fitness and self-renewal of hematopoietic stem cells (HSCs) with and without exogenous genotoxic stresses. We also show that although Ppm1d activation confers cellular resistance to cytotoxic therapy, it does so to a lesser degree than p53 loss, informing the clonal competition phenotypes often observed in human studies. Notably, loss of Ppm1d sensitizes leukemias to cytotoxic therapies in vitro and in vivo, even in the absence of a Ppm1d mutation. Vulnerability to PPM1D inhibition is observed across many cancer types and dependent on p53 activity. Importantly, organism-wide loss of Ppm1d in adult mice is well tolerated, supporting the tolerability of pharmacologically targeting PPM1D. Our data link PPM1D gain-of-function mutations to the clonal expansion of HSCs, inform human genetic observations, and support the therapeutic targeting of PPM1D in cancer.

Antimalarial Delivery with a Ferritin-Based Protein Cage: A Step toward Developing Smart Therapeutics against Malaria.

Over the past two decades, the utilization of protein cages has witnessed exponential growth driven by their extensive applications in biotechnology and therapeutics. In the context of the recent Covid-19 pandemic, protein-cage-based scaffolds played a pivotal role in vaccine development. Beyond vaccines, these protein cages have proven valuable in diverse drug delivery applications thanks to their distinctive architecture and structural stability. Among the various types of protein cages, ferritin-based cages have taken the lead in drug delivery applications. This is primarily attributed to their ease of production, exceptional thermal stability, and nontoxic nature. While ferritin-based cages are commonly employed in anticancer drug delivery and contrast agent delivery, their efficacy in malarial drug delivery had not been explored until this study. In this investigation, several antimalarial drugs were encapsulated within horse spleen ferritin, and the binding and loading processes were validated through both experimental and computational techniques. The data unequivocally demonstrate the facile incorporation of antimalarial drugs into ferritin without disrupting its three-dimensional structure. Computational docking and molecular dynamics simulations were employed to pinpoint the precise location of the drug binding site within ferritin. Subsequent efficacy testing on Plasmodium revealed that the developed nanoconjugate, comprising the drug-ferritin conjugate, exhibited significant effectiveness in eradicating the parasite. In conclusion, the findings strongly indicate that ferritin-based carrier systems hold tremendous promise for the future of antimalarial drug delivery, offering high selectivity and limited side effects.

Patterns of substrate affinity, competition, and degradation kinetics underlie biological activity of thalidomide analogs.

Pharmacologic agents that modulate ubiquitin ligase activity to induce protein degradation are a major new class of therapeutic agents, active in a number of hematologic malignancies. However, we currently have a limited understanding of the determinants of activity of these agents and how resistance develops. We developed and used a novel quantitative, targeted mass spectrometry (MS) assay to determine the relative activities, kinetics, and cell-type specificity of thalidomide and 4 analogs, all but 1 of which are in clinical use or clinical trials for hematologic malignancies. Thalidomide analogs bind the CRL4CRBN ubiquitin ligase and induce degradation of particular proteins, but each of the molecules studied has distinct patterns of substrate specificity that likely underlie the clinical activity and toxicities of each drug. Our results demonstrate that the activity of molecules that induce protein degradation depends on the strength of ligase-substrate interaction in the presence of drug, the levels of the ubiquitin ligase, and the expression level of competing substrates. These findings highlight a novel mechanism of resistance to this class of drugs mediated by competition between substrates for access to a limiting pool of the ubiquitin ligase. We demonstrate that increased expression of a nonessential substrate can lead to decreased degradation of other substrates that are critical for antineoplastic activity of the drug, resulting in drug resistance. These studies provide general rules that govern drug-dependent substrate degradation and key differences between thalidomide analog activity in vitro and in vivo.

PPM1D-truncating mutations confer resistance to chemotherapy and sensitivity to PPM1D inhibition in hematopoietic cells.

Truncating mutations in the terminal exon of protein phosphatase Mg2+/Mn2+ 1D (PPM1D) have been identified in clonal hematopoiesis and myeloid neoplasms, with a striking enrichment in patients previously exposed to chemotherapy. In this study, we demonstrate that truncating PPM1D mutations confer a chemoresistance phenotype, resulting in the selective expansion of PPM1D-mutant hematopoietic cells in the presence of chemotherapy in vitro and in vivo. Clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein-9 nuclease mutational profiling of PPM1D in the presence of chemotherapy selected for the same exon 6 mutations identified in patient samples. These exon 6 mutations encode for a truncated protein that displays elevated expression and activity due to loss of a C-terminal degradation domain. Global phosphoproteomic profiling revealed altered phosphorylation of target proteins in the presence of the mutation, highlighting multiple pathways including the DNA damage response (DDR). In the presence of chemotherapy, PPM1D-mutant cells have an abrogated DDR resulting in altered cell cycle progression, decreased apoptosis, and reduced mitochondrial priming. We demonstrate that treatment with an allosteric, small molecule inhibitor of PPM1D reverts the phosphoproteomic, DDR, apoptotic, and mitochondrial priming changes observed in PPM1D-mutant cells. Finally, we show that the inhibitor preferentially kills PPM1D-mutant cells, sensitizes the cells to chemotherapy, and reverses the chemoresistance phenotype. These results provide an explanation for the enrichment of truncating PPM1D mutations in the blood of patients exposed to chemotherapy and in therapy-related myeloid neoplasms, and demonstrate that PPM1D can be a targeted in the prevention of clonal expansion of PPM1D-mutant cells and the treatment of PPM1D-mutant disease.

Increased mitochondrial apoptotic priming with targeted therapy predicts clinical response to re-induction chemotherapy.

Most patients with relapsed or refractory (R/R) acute myeloid leukemia (AML) do not benefit from current re-induction or approved targeted therapies. In the absence of targetable genetic mutations, there is minimal guidance on optimal treatment selection particularly in the R/R setting highlighting an unmet need for clinically useful functional biomarkers. Blood and bone marrow samples from patients treated on two clinical trials were used to test the combination of lenalidomide (LEN) and MEC (mitoxantrone, etoposide, and cytarabine) chemotherapy in R/R AML patients. The bone marrow samples were available to test the clinical utility of the mitochondrial apoptotic BH3 and dynamic BH3 profiling (DBP) assays in predicting response, as there was no clear genetic biomarker identifying responders. To test whether LEN-induced mitochondrial priming predicted clinical response to LEN-MEC therapy, we performed DBP on patient myeloblasts. We found that short-term ex vivo treatment with lenalidomide discriminated clinical responders from non-responders based on drug-induced change in priming (delta priming). Using paired patient samples collected before and after clinical LEN treatment (prior to MEC dosing), we confirmed LEN-induced increased apoptotic priming in vivo, suggesting LEN enhanced vulnerability of myeloblasts to cytotoxic MEC chemotherapy. This is the first study demonstrating the potential role of DBP in predicting clinical response to a combination regimen. Our findings demonstrate that functional properties of relapsed AML can identify active therapies.

Anti-CD20-interleukin-21 fusokine targets malignant B cells via direct apoptosis and NK-cell-dependent cytotoxicity.

In spite of newly emerging therapies and the improved survival of patients with non-Hodgkin lymphoma (NHL), relapses or primary refractory disease are commonly observed and associated with dismal prognosis. Although discovery of the anti-CD20 antibody rituximab has markedly improved outcomes in B-cell NHL, rituximab resistance remains an important obstacle to successful treatment of these tumors. To improve the efficacy of CD20-targeted therapy, we fused interleukin 21 (IL-21), which induces direct lymphoma cytotoxicity and activates immune effector cells, to the anti-CD20 antibody (αCD20-IL-21 fusokine). We observed substantially enhanced IL-21R-mediated signaling by the fusokine compared with native IL-21 at equimolar concentrations. Fusokine treatment led to direct apoptosis of lymphoma cell lines and primary tumors that otherwise were resistant to native IL-21 treatment. In addition to direct cytotoxicity, the fusokine enhanced NK cell activation, effector functions, and interferon γ production, resulting in greater antibody-dependent cell-mediated cytotoxicity compared with IL-21 and/or anti-CD20 antibody treatments. Further, the αCD20-IL-21 fusokine stabilizes IL-21 and prolongs its half-life. In vivo αCD20-IL-21 therapy resulted in a significant tumor control in the rituximab-resistant A20-huCD20 tumors. Collectively, the dual functional ability of the αCD20-IL-21 fusokine to induce direct apoptosis and activate immune effector cells may provide benefit over existing treatments for NHL.

Inhibition of USP10 induces degradation of oncogenic FLT3.

Oncogenic forms of the kinase FLT3 are important therapeutic targets in acute myeloid leukemia (AML); however, clinical responses to small-molecule kinase inhibitors are short-lived as a result of the rapid emergence of resistance due to point mutations or compensatory increases in FLT3 expression. We sought to develop a complementary pharmacological approach whereby proteasome-mediated FLT3 degradation could be promoted by inhibitors of the deubiquitinating enzymes (DUBs) responsible for cleaving ubiquitin from FLT3. Because the relevant DUBs for FLT3 are not known, we assembled a focused library of most reported small-molecule DUB inhibitors and carried out a cellular phenotypic screen to identify compounds that could induce the degradation of oncogenic FLT3. Subsequent target deconvolution efforts allowed us to identify USP10 as the critical DUB required to stabilize FLT3. Targeting of USP10 showed efficacy in preclinical models of mutant-FLT3 AML, including cell lines, primary patient specimens and mouse models of oncogenic-FLT3-driven leukemia.

miR-181a negatively regulates NF-κB signaling and affects activated B-cell-like diffuse large B-cell lymphoma pathogenesis.

Distinct subgroups of diffuse large B-cell lymphoma (DLBCL) genetically resemble specific mature B-cell populations that are blocked at different stages of the immune response in germinal centers (GCs). The activated B-cell (ABC)-like subgroup resembles post-GC plasmablasts undergoing constitutive survival signaling, yet knowledge of the mechanisms that negatively regulate this oncogenic signaling remains incomplete. In this study, we report that microRNA (miR)-181a is a negative regulator of nuclear factor κ-light-chain enhancer of activated B-cells (NF-κB) signaling. miR-181a overexpression significantly decreases the expression and activity of key NF-κB signaling components. Moreover, miR-181a decreases DLBCL tumor cell proliferation and survival, and anti-miR-181a abrogates these effects. Remarkably, these effects are augmented in the NF-κB dependent ABC-like subgroup compared with the GC B-cell (GCB)-like DLBCL subgroup. Concordantly, in vivo analyses of miR-181a induction in xenografts results in slower tumor growth rate and prolonged survival in the ABC-like DLBCL xenografts compared with the GCB-like DLBCL. We link these outcomes to relatively lower endogenous miR-181a expression and to NF-κB signaling dependency in the ABC-like DLBCL subgroup. Our findings indicate that miR-181a inhibits NF-κB activity, and that manipulation of miR-181a expression in the ABC-like DLBCL genetic background may result in a significant change in the proliferation and survival phenotype of this malignancy.

Pathophysiological significance and therapeutic targeting of germinal center kinase in diffuse large B-cell lymphoma.

Diffuse large B-cell lymphoma (DLBCL) is the most common subtype of non-Hodgkin lymphoma, yet 40% to 50% of patients will eventually succumb to their disease, demonstrating a pressing need for novel therapeutic options. Gene expression profiling has identified messenger RNAs that lead to transformation, but critical events transforming cells are normally executed by kinases. Therefore, we hypothesized that previously unrecognized kinases may contribute to DLBCL pathogenesis. We performed the first comprehensive analysis of global kinase activity in DLBCL, to identify novel therapeutic targets, and discovered that germinal center kinase (GCK) was extensively activated. GCK RNA interference and small molecule inhibition induced cell-cycle arrest and apoptosis in DLBCL cell lines and primary tumors in vitro and decreased the tumor growth rate in vivo, resulting in a significantly extended lifespan of mice bearing DLBCL xenografts. GCK expression was also linked to adverse clinical outcome in a cohort of 151 primary DLBCL patients. These studies demonstrate, for the first time, that GCK is a molecular therapeutic target in DLBCL tumors and that inhibiting GCK may significantly extend DLBCL patient survival. Because the majority of DLBCL tumors (∼80%) exhibit activation of GCK, this therapy may be applicable to most patients.

Direct and immune-mediated cytotoxicity of interleukin-21 contributes to antitumor effects in mantle cell lymphoma.

Mantle cell lymphoma (MCL) is a distinct subtype of non-Hodgkin lymphoma characterized by overexpression of cyclin D1 in 95% of patients. MCL patients experience frequent relapses resulting in median survival of 3 to 5 years, requiring more efficient therapeutic regimens. Interleukin (IL)-21, a member of the IL-2 cytokine family, possesses potent antitumor activity against a variety of cancers not expressing the IL-21 receptor (IL-21R) through immune activation. Previously, we established that IL-21 exerts direct cytotoxicity on IL-21R-expressing diffuse large B-cell lymphoma cells. Herein, we demonstrate that IL-21 possesses potent cytotoxicity against MCL cell lines and primary tumors. We identify that IL-21-induced direct cytotoxicity is mediated through signal transducer and activator of transcription 3-dependent cMyc upregulation, resulting in activation of Bax and inhibition of Bcl-2 and Bcl-XL. IL-21-mediated cMyc upregulation is only observed in IL-21-sensitive cells. Further, we demonstrate that IL-21 leads to natural killer (NK)-cell-dependent lysis of MCL cell lines that were resistant to direct cytotoxicity. In vivo treatment with IL-21 results in complete FC-muMCL1 tumor regression in syngeneic mice via NK- and T-cell-dependent mechanisms. Together, these data indicate that IL-21 has potent antitumor activity against MCL cells via direct cytotoxic and indirect, immune-mediated effects.

CD30 targeting with brentuximab vedotin: a novel therapeutic approach to primary effusion lymphoma.

Primary effusion lymphoma (PEL) is an aggressive subtype of non-Hodgkin lymphoma characterized by short survival with current therapies, emphasizing the urgent need to develop new therapeutic approaches. Brentuximab vedotin (SGN-35) is an anti-CD30 monoclonal antibody (cAC10) conjugated by a protease-cleavable linker to a microtubule-disrupting agent, monomethyl auristatin E. Brentuximab vedotin is an effective treatment of relapsed CD30-expressing Classical Hodgkin and systemic anaplastic large cell lymphomas. Herein, we demonstrated that PEL cell lines and primary tumors express CD30 and thus may serve as potential targets for brentuximab vedotin therapy. In vitro treatment with brentuximab vedotin decreased cell proliferation, induced cell cycle arrest, and triggered apoptosis of PEL cell lines. Furthermore, in vivo brentuximab vedotin promoted tumor regression and prolonged survival of mice bearing previously reported UM-PEL-1 tumors as well as UM-PEL-3 tumors derived from a newly established and characterized Kaposi's sarcoma-associated herpesvirus- and Epstein-Barr virus-positive PEL cell line. Overall, our results demonstrate for the first time that brentuximab vedotin may serve as an effective therapy for PEL and provide strong preclinical indications for evaluation of brentuximab vedotin in clinical studies of PEL patients.

miR-155 regulates HGAL expression and increases lymphoma cell motility.

HGAL, a prognostic biomarker in patients with diffuse large B-cell lymphoma and classic Hodgkin lymphoma, inhibits lymphocyte and lymphoma cell motility by activating the RhoA signaling cascade and interacting with actin and myosin proteins. Although HGAL expression is limited to germinal center (GC) lymphocytes and GC-derived lymphomas, little is known about its regulation. miR-155 is implicated in control of GC reaction and lymphomagenesis. We demonstrate that miR-155 directly down-regulates HGAL expression by binding to its 3'-untranslated region, leading to decreased RhoA activation and increased spontaneous and chemoattractant-induced lymphoma cell motility. The effects of miR-155 on RhoA activation and cell motility can be rescued by transfection of HGAL lacking the miR-155 binding site. This inhibitory effect of miR-155 suggests that it may have a key role in the loss of HGAL expression on differentiation of human GC B cells to plasma cell. Furthermore, this effect may contribute to lymphoma cell dissemination and aggressiveness, characteristic of activated B cell-like diffuse large B-cell lymphoma typically expressing high levels of miR-155 and lacking HGAL expression.

Identification of LMO2 transcriptome and interactome in diffuse large B-cell lymphoma.

LMO2 regulates gene expression by facilitating the formation of multipartite DNA-binding complexes. In B cells, LMO2 is specifically up-regulated in the germinal center (GC) and is expressed in GC-derived non-Hodgkin lymphomas. LMO2 is one of the most powerful prognostic indicators in diffuse large B-cell (DLBCL) patients. However, its function in GC B cells and DLBCL is currently unknown. In this study, we characterized the LMO2 transcriptome and transcriptional complex in DLBCL cells. LMO2 regulates genes implicated in kinetochore function, chromosome assembly, and mitosis. Overexpression of LMO2 in DLBCL cell lines results in centrosome amplification. In DLBCL, the LMO2 complex contains some of the traditional partners, such as LDB1, E2A, HEB, Lyl1, ETO2, and SP1, but not TAL1 or GATA proteins. Furthermore, we identified novel LMO2 interacting partners: ELK1, nuclear factor of activated T-cells (NFATc1), and lymphoid enhancer-binding factor1 (LEF1) proteins. Reporter assays revealed that LMO2 increases transcriptional activity of NFATc1 and decreases transcriptional activity of LEF1 proteins. Overall, our studies identified a novel LMO2 transcriptome and interactome in DLBCL and provides a platform for future elucidation of LMO2 function in GC B cells and DLBCL pathogenesis.

Red Blood Cells as Therapeutic Target to Treat Sickle Cell Disease.

Significance: Sickle cell disease (SCD) is the most common inherited diathesis affecting mostly underserved populations globally. SCD is characterized by chronic pain and fatigue, severe acute painful crises requiring hospitalization and opioids, strokes, multiorgan damage, and a shortened life span. Symptoms may appear shortly after birth, and, in less developed countries, most children with SCD die before attaining age 5. Hematopoietic stem cell transplant and gene therapy offer a curative therapeutic approach, but, due to many challenges, are limited in their availability and effectiveness for a majority of persons with SCD. A critical unmet need is to develop safe and effective novel targeted therapies. A wide array of drugs currently undergoing clinical investigation hold promise for an expanded pharmacological armamentarium against SCD. Recent Advances: Hydroxyurea, the most widely used intervention for SCD management, has improved the survival in the Western world and more recently, voxelotor (R-state-stabilizer), l-glutamine, and crizanlizumab (anti-P-selectin antibody) have been approved by the Food and Drug Administration (FDA) for use in SCD. The recent FDA approval emphasizes the need to revisit the advances in understanding the core pathophysiology of SCD to accelerate novel evidence-based strategies to treat SCD. The biomechanical breakdown of erythrocytesis, the core pathophysiology of SCD, is associated with intrinsic factors, including the composition of hemoglobin, membrane integrity, cellular volume, hydration, andoxidative stress. Critical Issues and Future Directions: In this context, this review focuses on advances in emerging nongenetic interventions directed toward the therapeutic targets intrinsic to sickle red blood cells (RBCs), which can prevent impaired rheology of RBCs to impede disease progression and reduce the sequelae of comorbidities, including pain, vasculopathy, and organ damage. In addition, given the intricate pathophysiology of the disease, it is unlikely that a single pharmacotherapeutic intervention will comprehensively ameliorate the multifaceted complications associated with SCD. However, the availability of multiple drug options affords the opportunity for individualized therapeutic regimens tailored to specific SCD-related complications. Furthermore, it opens avenues for combination drug therapy, capitalizing on distinct mechanisms of action and profiles of adverse effects.

Therapeutic biomarkers in acute myeloid leukemia: functional and genomic approaches.

Acute myeloid leukemia (AML) is clinically and genetically a heterogeneous disease characterized by clonal expansion of abnormal hematopoietic progenitors. Genomic approaches to precision medicine have been implemented to direct targeted therapy for subgroups of AML patients, for instance, IDH inhibitors for IDH1/2 mutated patients, and FLT3 inhibitors with FLT3 mutated patients. While next generation sequencing for genetic mutations has improved treatment outcomes, only a fraction of AML patients benefit due to the low prevalence of actionable targets. In recent years, the adoption of newer functional technologies for quantitative phenotypic analysis and patient-derived avatar models has strengthened the potential for generalized functional precision medicine approach. However, functional approach requires robust standardization for multiple variables such as functional parameters, time of drug exposure and drug concentration for making in vitro predictions. In this review, we first summarize genomic and functional therapeutic biomarkers adopted for AML therapy, followed by challenges associated with these approaches, and finally, the future strategies to enhance the implementation of precision medicine.

Does deteriorating antioxidant defense and impaired γ-glutamyl cycle induce oxidative stress and hemolysis in individuals with sickle cell disease?

Sickle cell disease (SCD) affects two-thirds of African and Indian children. Understanding the molecular mechanisms contributing to oxidative stress may be useful for therapeutic development in SCD. We evaluated plasma elemental levels of Indian SCD patients, trait and healthy controls (n=10/per group) via ICP-MS. Additionally, erythrocyte metabolomics of Indian SCD and healthy (n=5/per group) was carried out using LC-MS mass-spectrometry. Followed by assessment of antioxidant defence enzymes namely glutathione reductase (GR), superoxide dismutase (SOD), and catalase (CAT) in erythrocytes and plasma of Indian SCD patients (n=31) compared to trait (n=8) and healthy (n=9). In SCD plasma an elevated plasma 24Mg, 44Ca, 66Zn, 208Pb, 39K and reduced 57Fe, 77Se, 85Rb levels indicating higher hemolysis and anemia. Erythrocyte metabolome of SCD patients clustered separately from heathy revealing 135 significantly deregulated metabolic features including trimethyllysine, pyroglutamate, glutathione, aminolevulinate, and D-glutamine indicating oxidative stress and membrane fragility. Repressed GR, SOD, and CAT activities were observed in SCD patients of which GR and CAT activities did not change under hypoxia. These findings lead to the hypothesis that SCD-associated metabolic deregulations and a shift to ATP-consuming aberrant γ-glutamyl cycle leads to anemia, dehydration, oxidative stress and hemolysis driving the biomechanical pathophysiology of erythrocyte of SCD patients.

Understanding the complex formation of falstatin; an endogenous macromolecular inhibitor of falcipains.

Proteolytic activity constitutes a fundamental process essential for the survival of the malaria parasite and is thus highly regulated. Falstatin, a protease inhibitor of Plasmodium falciparum, tightly regulates the activity of cysteine hemoglobinases, falcipain-2 and 3 (FP2, FP3), by inhibiting FP2 through a single surface exposed loop. However, the multimeric nature of falstatin and its interaction with FP2 remained unexplored. Here we report that the N-terminal falstatin region is highly disordered, and needs chaperone activity (heat-shock protein 70, HSP70) for its folding. Protein-protein interaction assays showed a significant interaction between falstatin and HSP70. Further, characterization of the falstatin multimer through a series of biophysical techniques identified the formation of a falstatin decamer, which was extremely thermostable. Computational analysis of the falstatin decamer showed the presence of five falstatin dimers, with each dimer aligned in a head-to-tail orientation. Further, the falstatin C-terminal region was revealed to be primarily involved in the oligomerization process. Stoichiometric analysis of the FP2-falstatin multimer showed the formation of a heterooligomeric complex in a 1:1 ratio, with the participation of ten subunits of each protein. Taken together, our results report a novel protease-inhibitor complex and strengthens our understanding of the regulatory mechanisms of major plasmodium hemoglobinases.

Acquired Multidrug Resistance in AML Is Caused by Low Apoptotic Priming in Relapsed Myeloblasts.

In many cancers, mortality is associated with the emergence of relapse with multidrug resistance (MDR). Thus far, the investigation of cancer relapse mechanisms has largely focused on acquired genetic mutations. Using acute myeloid leukemia (AML) patient-derived xenografts (PDX), we systematically elucidated a basis of MDR and identified drug sensitivity in relapsed AML. We derived pharmacologic sensitivity for 22 AML PDX models using dynamic BH3 profiling (DBP), together with genomics and transcriptomics. Using in vivo acquired resistant PDXs, we found that resistance to unrelated, narrowly targeted agents in distinct PDXs was accompanied by broad resistance to drugs with disparate mechanisms. Moreover, baseline mitochondrial apoptotic priming was consistently reduced regardless of the class of drug-inducing selection. By applying DBP, we identified drugs showing effective in vivo activity in resistant models. This study implies evasion of apoptosis drives drug resistance and demonstrates the feasibility of the DBP approach to identify active drugs for patients with relapsed AML. SIGNIFICANCE: Acquired resistance to targeted therapy remains challenging in AML. We found that reduction in mitochondrial priming and common transcriptomic signatures was a conserved mechanism of acquired resistance across different drug classes in vivo. Drugs active in vivo can be identified even in the multidrug resistant state by DBP.

Altered RNA export by SF3B1 mutants confers sensitivity to nuclear export inhibition.

SF3B1 mutations frequently occur in cancer yet lack targeted therapies. Clinical trials of XPO1 inhibitors, selinexor and eltanexor, in high-risk myelodysplastic neoplasms (MDS) revealed responders were enriched with SF3B1 mutations. Given that XPO1 (Exportin-1) is a nuclear exporter responsible for the export of proteins and multiple RNA species, this led to the hypothesis that SF3B1-mutant cells are sensitive to XPO1 inhibition, potentially due to altered splicing. Subsequent RNA sequencing after XPO1 inhibition in SF3B1 wildtype and mutant cells showed increased nuclear retention of RNA transcripts and increased alternative splicing in the SF3B1 mutant cells particularly of genes that impact apoptotic pathways. To identify novel drug combinations that synergize with XPO1 inhibition, a forward genetic screen was performed with eltanexor treatment implicating anti-apoptotic targets BCL2 and BCLXL, which were validated by functional testing in vitro and in vivo. These targets were tested in vivo using Sf3b1K700E conditional knock-in mice, which showed that the combination of eltanexor and venetoclax (BCL2 inhibitor) had a preferential sensitivity for SF3B1 mutant cells without excessive toxicity. In this study, we unveil the mechanisms underlying sensitization to XPO1 inhibition in SF3B1-mutant MDS and preclinically rationalize the combination of eltanexor and venetoclax for high-risk MDS.

Efficacy of bortezomib in a direct xenograft model of primary effusion lymphoma.

Primary effusion lymphoma (PEL) is an aggressive B-cell lymphoma most commonly diagnosed in HIV-positive patients and universally associated with Kaposi's sarcoma-associated herpesvirus (KSHV). Chemotherapy treatment of PEL yields only short-term remissions in the vast majority of patients, but efforts to develop superior therapeutic approaches have been impeded by lack of animal models that accurately mimic human disease. To address this issue, we developed a direct xenograft model, UM-PEL-1, by transferring freshly isolated human PEL cells into the peritoneal cavities of NOD/SCID mice without in vitro cell growth to avoid the changes in KSHV gene expression evident in cultured cells. We used this model to show that bortezomib induces PEL remission and extends overall survival of mice bearing lymphomatous effusions. The proapoptotic effects of bortezomib are not mediated by inhibition of the prosurvival NF-kappaB pathway or by induction of a terminal unfolded protein response. Transcriptome analysis by genomic arrays revealed that bortezomib down-regulated cell-cycle progression, DNA replication, and Myc-target genes. Furthermore, we demonstrate that in vivo treatment with either bortezomib or doxorubicin induces KSHV lytic reactivation. These reactivations were temporally distinct, and this difference may help elucidate the therapeutic window for use of antivirals concurrently with chemotherapy. Our findings show that this direct xenograft model can be used for testing novel PEL therapeutic strategies and also can provide a rational basis for evaluation of bortezomib in clinical trials.