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      Chemically induced proximity in biology and medicine

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      Science

      American Association for the Advancement of Science (AAAS)

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          Abstract

          <p class="first" id="P2">Proximity, or the physical closeness of molecules, is a pervasive regulatory mechanism in biology. For example, most posttranslational modifications such as phosphorylation, methylation, and acetylation promote proximity of molecules to play deterministic roles in cellular processes. To understand the role of proximity in biologic mechanisms, chemical inducers of proximity (CIPs) were developed to synthetically model biologically regulated recruitment. Chemically induced proximity allows for precise temporal control of transcription, signaling cascades, chromatin regulation, protein folding, localization, and degradation, as well as a host of other biologic processes. A systematic analysis of CIPs in basic research, coupled with recent technological advances utilizing CRISPR, distinguishes roles of causality from coincidence and allows for mathematical modeling in synthetic biology. Recently, induced proximity has provided new avenues of gene therapy and emerging advances in cancer treatment. </p><p id="P3"> <div class="figure-container so-text-align-c"> <img alt="" class="figure" src="/document_file/5ca85e76-535c-4135-b9dc-54b351037838/PubMedCentral/image/nihms-1010308-f0001.jpg"/> </div> </p><p id="P4"> <b>Chemically induced proximity.</b> (Top) Left: Small molecules (hexagons) bind proteins of interest (crescents), dimerizing them to increase the effective molarity of reactions. [A] monomeric protein and [AB*] dimer concentrations; arrows, position coordinates. Middle: Synthetic dimerizers tag proteins (blue circles) for proteasomal degradation (red rods). Right: Homodimerizing molecules form kill switches for apoptosis. (Bottom) CIPs mimic cellular processes. Left: Protein transport mechanisms—nuclear import and export, membrane fusion, and protein folding. Middle: Regulation of gene activation by binding to DNA or chromatin (spheres with white strands), through recruitment of transcriptional activators or repressors (blue and red arrows). Right: Signal transduction pathways. </p><div class="section"> <a class="named-anchor" id="S1"> <!-- named anchor --> </a> <h5 class="section-title" id="d8637795e165">BACKGROUND:</h5> <p id="P5">Nature has evolved elegant mechanisms to regulate the physical distance between molecules, or proximity, for a wide variety of purposes. Whether it is activation of cell-membrane receptors, neuronal transmission across the synapse, or quorum sensing in bacterial biofilms, proximity is a ubiquitous regulatory mechanism in biology. Over the past two decades, chemically induced proximity has revealed that many essential features and processes, including protein structure, chromosomal architecture, chromatin accessibility, transcription, and cellular signaling, are governed by the proximity of molecules. We review the critical advances in chemical inducers of proximity (CIPs), which have informed active areas of research in biology ranging from basic advances to the development of cellular and molecular therapeutics. </p> </div><div class="section"> <a class="named-anchor" id="S2"> <!-- named anchor --> </a> <h5 class="section-title" id="d8637795e170">ADVANCES:</h5> <p id="P6">Until the 1990s, it was unclear whether proximity was sufficient to initiate signaling events or drive their effect on transcription. Synthetic small molecule–induced dimerization of the T cell receptor provided the first evidence that proximity could be used to understand signal transduction. A distinguishing feature of small-molecule induced-proximity systems (compared to canonical knockdown or knockout methods) is the ability to initiate a process midway and discern the ensuing order of events with precise temporal control. The rapid reversibility of induced proximity has enabled precise analysis of cellular and epigenetic memory and enabled the construction of synthetic regulatory circuits. Integration of CRISPR-Cas technologies into CIP strategies has broadened the scope of these techniques to study gene regulation on time scales of minutes, at any locus, in any genetic context. Furthermore, CIPs have been used to dissect the mechanisms governing seemingly well-understood processes, ranging from transport of proteins between the Golgi and endoplasmic reticulum to synaptic vesicle transmission. Recent advances in proximity-induced apoptosis, inhibition of aggregation, and selective degradation of endogenous proteins will likely yield new classes of drugs in the near future. </p> </div><div class="section"> <a class="named-anchor" id="S3"> <!-- named anchor --> </a> <h5 class="section-title" id="d8637795e175">OUTLOOK:</h5> <p id="P7">We review fundamental conceptual advances enabled by synthetic proximity as well as emerging CIP-based therapeutic approaches. Gene therapy with precise regulation and fully humanized systems are now possible. Integration of proximity-based apoptosis through caspase activation with chimeric antigen receptor (CAR) T cell therapies provides a safety switch, enabling mitigation of complications from engineered immune cells, such as graft-versus-host disease and B cell aplasia. Furthermore, this integration facilitates the potential for repopulation of a patient’s cells after successful transplantation. With the recent approval of CTL019, a CAR T cell therapeutic from Novartis, integrated strategies involving the use of CIP-based safety switches are emerging. Innovative exemplars include BPX-601 (NCT02744287) and BPX-701 (NCT02743611), which are now in phase 1 clinical trials. By using a similar proximity-based approach, conditional small-molecule protein degraders are also expected to have broad clinical utility. This approach uses bifunctional small molecules to degrade pathogenic proteins by dimerizing with E3 ubiquitin ligases. Degradation-by-dimerization strategies are particularly groundbreaking, because they afford the ability to repurpose any chemical probe that binds tightly with its pathogenic protein but which may not have previously provided a direct therapeutic effect. We anticipate that the translation of CIP methodology through both humanized gene therapies and degradation-by-dimerization approaches will have far-reaching clinical impact. </p> </div>

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          Most cited references77

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          Cellular therapies could play a role in cancer treatment and regenerative medicine if it were possible to quickly eliminate the infused cells in case of adverse events. We devised an inducible T-cell safety switch that is based on the fusion of human caspase 9 to a modified human FK-binding protein, allowing conditional dimerization. When exposed to a synthetic dimerizing drug, the inducible caspase 9 (iCasp9) becomes activated and leads to the rapid death of cells expressing this construct. We tested the activity of our safety switch by introducing the gene into donor T cells given to enhance immune reconstitution in recipients of haploidentical stem-cell transplants. Patients received AP1903, an otherwise bioinert small-molecule dimerizing drug, if graft-versus-host disease (GVHD) developed. We measured the effects of AP1903 on GVHD and on the function and persistence of the cells containing the iCasp9 safety switch. Five patients between the ages of 3 and 17 years who had undergone stem-cell transplantation for relapsed acute leukemia were treated with the genetically modified T cells. The cells were detected in peripheral blood from all five patients and increased in number over time, despite their constitutive transgene expression. A single dose of dimerizing drug, given to four patients in whom GVHD developed, eliminated more than 90% of the modified T cells within 30 minutes after administration and ended the GVHD without recurrence. The iCasp9 cell-suicide system may increase the safety of cellular therapies and expand their clinical applications. (Funded by the National Heart, Lung, and Blood Institute and the National Cancer Institute; ClinicalTrials.gov number, NCT00710892.).
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            The current predominant therapeutic paradigm is based on maximizing drug-receptor occupancy to achieve clinical benefit. This strategy, however, generally requires excessive drug concentrations to ensure sufficient occupancy, often leading to adverse side effects. Here, we describe major improvements to the proteolysis targeting chimeras (PROTACs) method, a chemical knockdown strategy in which a heterobifunctional molecule recruits a specific protein target to an E3 ubiquitin ligase, resulting in the target's ubiquitination and degradation. These compounds behave catalytically in their ability to induce the ubiquitination of super-stoichiometric quantities of proteins, providing efficacy that is not limited by equilibrium occupancy. We present two PROTACs that are capable of specifically reducing protein levels by >90% at nanomolar concentrations. In addition, mouse studies indicate that they provide broad tissue distribution and knockdown of the targeted protein in tumor xenografts. Together, these data demonstrate a protein knockdown system combining many of the favorable properties of small-molecule agents with the potent protein knockdown of RNAi and CRISPR.
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              Hijacking the E3 Ubiquitin Ligase Cereblon to Efficiently Target BRD4.

              BRD4, a bromodomain and extraterminal domain (BET) family member, is an attractive target in multiple pathological settings, particularly cancer. While BRD4 inhibitors have shown some promise in MYC-driven malignancies such as Burkitt's lymphoma (BL), we show that BRD4 inhibitors lead to robust BRD4 protein accumulation, which may account for their limited suppression of MYC expression, modest antiproliferative activity, and lack of apoptotic induction. To address these limitations we designed ARV-825, a hetero-bifunctional PROTAC (Proteolysis Targeting Chimera) that recruits BRD4 to the E3 ubiquitin ligase cereblon, leading to fast, efficient, and prolonged degradation of BRD4 in all BL cell lines tested. Consequently, ARV-825 more effectively suppresses c-MYC levels and downstream signaling than small-molecule BRD4 inhibitors, resulting in more effective cell proliferation inhibition and apoptosis induction in BL. Our findings provide strong evidence that cereblon-based PROTACs provide a better and more efficient strategy in targeting BRD4 than traditional small-molecule inhibitors.
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                Author and article information

                Journal
                Science
                Science
                American Association for the Advancement of Science (AAAS)
                0036-8075
                1095-9203
                March 08 2018
                March 08 2018
                : 359
                : 6380
                : eaao5902
                Article
                10.1126/science.aao5902
                6417506
                29590011
                e7f0e920-ab04-4838-bb45-885c91b3e3c5
                © 2018

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