SUPT5H

引言 基因的表达依赖于转录激活因子和转录辅助因子的参与，这一过程为通过调控基因表达来治疗疾病提供了有前途的策略。在这些辅助因子中，一种具有bromodomain（BD）与extraterminal （ET）结构域（BET）的家族蛋白质起到了关键作用，包括BRD2、BRD3、BRD4和BRDT。这类蛋白可以通过bromodomain（BD1和BD2）特异性地识别染色质的乙酰化标记从而调控基因表达，因此被视为基因表观调控的重要分子。 BET蛋白的BD结构域在基因调控中发挥着两个重要作用。首先，它们通过与乙酰化组蛋白的结合，帮助将BET蛋白招募到基因的增强子和启动子区域，促进基因的激活。其次，BD结构域还通过一种特殊的组蛋白伴侣活性，促进基因的转录延长过程。此外，BET蛋白的ET结构域，能与多种转录调控因子结合，进而进一步影响基因的表达。在这些蛋白中，BRD4因其支持癌基因MYC的转录表达而备受关注。大家长期认为它通过结合乙酰化组蛋白，招募正性转录延长因子（P-TEFb），促进癌基因的持续表达。 BET抑制剂（BETi）通过与BD结构域竞争性结合，阻断BET蛋白与染色质的相互作用，进而抑制癌基因的转录。这类抑制剂在多种癌症中显示出很强的抗肿瘤活性，尤其是抑制像MYC这样的致癌基因。然而，临床试验表明，对于雌激素受体阳性（ER+）乳腺癌患者，BET抑制剂并未显示出显著疗效。雌激素受体（ER）通过激活癌基因（如MYC和CCND1）的转录，促进肿瘤生长，因此对ER+乳腺癌患者而言，ER扮演了关键角色。尽管近年医学界在ER+乳腺癌治疗方面取得了一些进展，但治疗耐药性仍然是一个重大挑战，尤其是对于高风险患者，他们往往面临有限的治疗选择和较差的预后。而对于这些患者，BET抑制剂一度被寄予厚望。 2024年9月9日，洛克菲勒大学Robert G. Roeder博士 (Arnold and Mabel Beckman Professor) 和张思聪博士（research associate）在Nature Structural & Molecular Biology上发表了文章Resistance of estrogen receptor function to BET bromodomain inhibition is mediated by transcriptional coactivator cooperativity。该研究揭示了一个重要机制：在ER+乳腺癌中，尽管BET抑制剂能够抑制一些基因的表达，但MYC等关键癌基因的转录却依然活跃。这并非是由于乳腺癌细胞对BET抑制剂的演化适应，而是因为他们有一种与生俱来的转录机制——BRD4可以通过不依赖于BD结构域的机制，与其他转录辅助因子相互作用，继续激活这些癌基因。这一发现解释了BET抑制剂在ER+乳腺癌患者中疗效有限的原因。 研究人员首先仔细比较了使用BET抑制剂与去除BET蛋白后对转录的影响，并发现MYC等基因的转录依赖于BET蛋白而非BD结构域的功能。由于目前对哪一个BET蛋白参与ER介导的转录调控尚有争议，研究人员比较了所有BET蛋白的功能。研究人员发现，只有BRD4可以通过不依赖于BD结构域激活MYC与其他癌基因的转录。将BRD4从细胞中去除后，BET抑制剂的抗癌作用显著增强。通过生化手段，发现BRD4能够与Mediator蛋白复合物结合，这个复合物在基因转录过程中发挥着至关重要的调控作用并与ER直接结合。当利用细胞编辑的手段干扰Mediator的功能时，BRD4在基因增强子上的结合显著下降。由于BRD4与Mediator的结合不依赖于BD，研究人员发现了一种新的BRD4的招募途径。 此外，通过研究人员发明的一种专门分析RNA聚合酶II（Pol II）及其相关因子作用的方法——mass spectrometry with DTME crosslinking (MSDC)，发现了BET蛋白通过促进Pol II与转录延长因子（如SPT5、SPT6，PAF1复合物）相互作用，促进基因的转录延长过程。而且这些延长因子的与BET蛋白的作用也不依赖于BD结构域。不过，BET蛋白并并不参与招募P-TEFb。相反，BET蛋白被去除后，P-TEFb在一些基因区域的结合甚至有所增加。因此，我们阐明了BET蛋白如何在不依赖于BD的情况下促进转录延长。 该研究发现对理解ER驱动的基因调控机制具有重要意义。这不仅有助于揭示BET蛋白在癌症中的复杂作用机制，尤其是其在没有BD结构域参与下的关键功能，也为未来开发新型抗癌疗法提供了新的方向。通过更深入地研究这些机制，我们有望克服现有内分泌疗法的抗药性，改善包括乳腺癌在内的多种癌症的治疗效果。 参考文献 https://www.nature.com/articles/s41594-024-01384-6 责编|探索君 排版|探索君 文章来源|“BioArt” End 往期精选 围观 一文读透细胞死亡(Cell Death) | 24年Cell重磅综述（长文收藏版） 热文 Cell | 是什么决定了细胞的大小？ 热文 Nature | 2024年值得关注的七项技术 热文 Nature | 自身免疫性疾病能被治愈吗？科学家们终于看到了希望 热文 CRISPR技术进化史 | 24年Cell综述

引言高效终止是基因转录所必需的。真核生物使用保守的外核糖核酸酶介导的机制来终止RNA聚合酶II (Pol II)的mRNA转录。2024年3月27日，中国科学院分子植物科学卓越创新中心/中国科学院合成生物学重点实验室张余研究组（曾媛为论文的第一作者）在Nature 在线发表题为”Structural basis of exoribonuclease-mediated mRNA transcription termination“的研究论文，该研究报道了与5 '至3 '外核糖核酸酶Rat1及其伙伴Rai1结合的酿酒酵母 Pol II预终止转录复合物的两个冷冻电镜结构。结构表明，Rat1取代了延伸因子Spt5，停靠在Pol II茎结构域。Rat1屏蔽Pol II的RNA出口通道，引导新生RNA走向其活性中心，并在新生RNA的5 '端堆叠三个核苷酸。这些结构进一步表明，Rat1在缩短RNA时向Pol II旋转。总之，该研究揭示了酵母mRNA转录终止的分子机制。转录作为中心法则中必不可少的一步，在基因表达过程中承担重要作用。转录按照其反应过程可以分为转录起始，转录延伸，转录终止。近年来转录起始和转录延伸的机制逐渐被揭开。然而，由于转录终止的动态和不稳定特性，其机制仍不清楚。转录终止（transcription termination）是指RNA聚合酶（RNA polymerase，RNAP）停止RNA延伸、释放RNA并从DNA解离的过程。正确的转录终止对mRNA的稳定性和RNA聚合酶的高效循环利用至关重要。酵母细胞RNA聚合酶II（RNA polymerase II，Pol II）的转录终止机制根据其合成的RNA类型主要分为两类，第一类是mRNA的转录终止，第二类是non-coding RNA的转录终止。mRNA的转录终止机制在真核生物中普遍保守。当Pol II转录至基因末端的多聚腺苷化信号（Polyadenylation Site; PAS）时，pre-mRNA在PAS位点下游被切割分为两段。切割后的mRNA完成PolyA加尾后被转运至细胞质翻译，而Pol II仍然朝下游转录继续合成RNA，其转录终止依赖一个保守的5’-3’ RNA外切酶（酵母Rat1，哺乳动物XRN2，植物XRN3），针对真核细胞mRNA转录终止的机制已经被研究了多年，提出了包括“鱼雷”、“异构”等多个模型，但是其机制尚存争议。异构模型认为，核酸外切酶Rat1结合诱导Pol II构象变化，促进mRNA自发解离。鱼雷模型认为，核酸外切酶Rat1切割下游RNA并追赶Pol II，当Rat1赶上Pol II后触发mRNA解离。转录终止具体通过何种机制尚存争论。图1 外切酶Rat1与Pol II形成的复合物结构（Credit: Nature）鉴于mRNA转录终止过程快速连续，难以捕捉其中间状态。研究团队利用磷硫键修饰的不同长度RNA组装Pol II转录终止复合物，重构了外切酶缩短RNA的中间态过程，并解析了上述中间状态的复合物结构（图1）。研究团队发现外切酶稳定结合在Pol II的RNA通道外侧，RNA从Pol II的活性中心延伸到了外切酶的活性中心。外切酶的结合位置和Pol II的转录延伸因子互相重叠，外切酶结合促使了延伸因子的解离，解释了外切酶延缓Pol II转录延伸速率的原因。根据以上结构信息，研究团队提出了外切酶介导的mRNA转录终止机制：外切酶覆盖RNA通道出口，直接将Pol II转录的RNA引导至催化中心，利用其RNA外切酶和移位酶缩短RNA长度的同时，对mRNA产生牵引力从而将其从Pol II 催化中心拖拽出来，最终解离mRNA并终止Pol II 转录（图2）。研究团队的工作表明mRNA的转录终止符合修正版的“鱼雷”模型，Pol II类似航行的军舰、外切酶鱼雷追赶上Pol II之后，吸附在Pol II上，随后利用其水解RNA转化的机械力将RNA从Pol II中拖拽出来。图2 外切酶Rat1介导的转录终止模型（Credit: Nature）比较细菌、古菌和真核生物mRNA转录终止，研究团队提出三域生物利用相似的方式终止mRNA合成。这些终止因子均结合在聚合酶的RNA通道外侧，真核生物利用Rat1/XRN2/XRN3的外切酶活性解离mRNA，细菌利用Rho的RNA移位酶活性解离mRNA，古菌利用FttA的外切酶活性解离mRNA。原文链接https://www.nature.com/articles/s41586-024-07240-3责编|探索君排版|探索君文章来源|“iNature”End往期精选围观一文读透细胞死亡(Cell Death) | 24年Cell重磅综述（长文收藏版）热文Nature | 破除传统：为何我们需要重新思考肿瘤的命名方式热文Nature | 2024年值得关注的七项技术热文Nature | 自身免疫性疾病能被治愈吗？科学家们终于看到了希望热文CRISPR技术进化史 | 24年Cell综述

Protein arginine methyltransferases (PRMTs) are catalytic enzymes for arginine methylation, capable of methylating a variety of proteins and playing pivotal roles in biological processes such as gene expression, splicing, and DNA damage repair.  Currently, 9 members of the PRMTs family has been identified, divided into three major types. Type I, catalyzes the formation of asymmetric dimethylarginine and includes PRMT1, PRMT2, PRMT3, PRMT4, PRMT6, PRMT8. Type II catalyzes the formation of symmetric dimethylarginine and includes PRMT5 and PRMT9. Type III is responsible for the formation of monomethylarginine and includes only PRMT7.  PRMT5 is the most highly studied subtype of PRMT, forming a heterooctameric complex with Methylosome protein 50 (MEP50), enabling the symmetrical dimethylation of a variety of substrates, including histone and non-histone proteins. PRMT5 histone substrates primarily include H4 residue Arg3 (H4R3), H3 residue Arg2 (H3R2), Arg8 (H3R8), and H2A residue Arg3 (H2AR3), whose symmetric dimethylation is related to gene transcription regulation. Non-histone substrates of PRMT5 include Sm proteins, nucleolar proteins, RAF proteins, EGFR, androgen receptor (AR), tumor suppressor p53, IL-2, HOXA9, NFkB, KLF4, GATA4, PDCD4, E2F-1, SPT5, GM130, POLR2A, SREBP1a, Rad9, and more that are involved in regulating cellular signalling, differentiation, DNA repair, and mRNA splicing among other cellular processes.  Research has found that PRMT5 is overexpressed in various types of cancer, such as glioblastoma, melanoma, leukemia/lymphoma, multiple myeloma, prostate cancer, bladder urothelial carcinoma, ovarian cancer, lung cancer, gastric cancer, and colorectal cancer. Furthermore, PRMT5 expression is associated with a poor prognosis in tumor progression and metastasis. Further research has shown that PRMT5 directly affects the growth and differentiation of tumor cells by regulating multiple genes such as p53, IL-2, TRAIL receptor, MITF, transcription factor E2F-1, and tumor suppressor gene 7. Additionally, PRMT5 has been found to constitute "synthetic lethal" partnerships with gene mutations like methylthioadenosine phosphorylase (MTAP), metabolic enzyme methionine adenosyltransferase 2 alpha (MAT2A), and others.  Both in vitro and in vivo studies have shown that inhibition or knockdown of PRMT5 significantly reduces cell proliferation, migration, and colony formation, but promotes apoptosis and cell cycle arrest. Furthermore, oral PRMT5 inhibitors demonstrate significant antitumor activity in lung tumor xenograft models. Thus, PRMT5 has become a new target for the development of epigenetic anti-cancer drugs. PRMT5 Competitive LandscapeAccording to the data provided by Patsnap Synapse-Global Drug Intelligence Database: the following figure shows that as of 22 Sep 2023, there are a total of 41 PRMT5 drugs worldwide, from 39 organizations, covering 28 indications, and conducting 20 clinical trials. 👇Please click on the picture link below for free registration or login directly if you have freemium accounts, you can browse the latest research progress on drugs , indications, organizations, clinical trials, clinical results, and drug patents related to this target. Based on the analysis of target PRMT5, it can be concluded that several companies, including GSK Plc, Ipsen SA, Tango Therapeutics, Inc., Amgen, Inc., Mirati Therapeutics, Inc., and Prelude Therapeutics, Inc., are actively involved in the research and development of drugs targeting PRMT5. These companies have drugs in various stages of development, with some reaching advanced stages such as Phase 2. The indications for these drugs cover a wide range of cancer types, indicating the potential of PRMT5 as a therapeutic target. Small molecule drugs and chemical drugs are the most rapidly progressing drug types under the target PRMT5, with small molecule drugs being the primary focus. The United States, China, and Canada are the countries/locations with the highest development activity, with the United States leading in Preclinical stage and China showing progress in Phase 1. Overall, the current competitive landscape suggests a promising future for the development of PRMT5 inhibitors in the pharmaceutical industry. PRMT5 inhibitors entering Phase II clinical studies: GSK-3326595GSK-3326595 is a small molecule drug that is being developed by Epizyme, Inc. It is designed to target PRMT5, a protein involved in various cellular processes. The drug is being investigated for its potential therapeutic applications in neoplasms, immune system diseases, and hemic and lymphatic diseases. The active indications for GSK-3326595 include solid tumors, non-Hodgkin lymphoma, neoplasms, acute myeloid leukemia, and myelodysplastic syndromes. These are all serious and potentially life-threatening conditions that affect different parts of the body, including the blood, lymph nodes, and various organs. 👇Please click on the image below to directly access the latest data (R&D Status | Core Patent | Clinical Trial | Approval status in Global countries) of this drug. Currently, GSK-3326595 is in Phase 2 of clinical development, which means that it has already undergone initial testing in humans and has shown promising results in terms of safety and efficacy. Phase 2 trials typically involve a larger number of participants and aim to further evaluate the drug's effectiveness and side effects. As a small molecule drug, GSK-3326595 has the advantage of being able to penetrate cells and interact with specific targets, such as PRMT5. This targeted approach can potentially lead to more effective treatment options with fewer side effects compared to traditional chemotherapy or radiation therapy. Epizyme, Inc. is the originator organization behind GSK-3326595. They are a biopharmaceutical company focused on developing novel therapies for patients with genetically defined cancers and other diseases. With their expertise in epigenetics and targeted therapies, Epizyme is dedicated to advancing the field of biomedicine and improving patient outcomes. In summary, GSK-3326595 is a small molecule drug being developed by Epizyme, Inc. It targets PRMT5 and has potential applications in neoplasms, immune system diseases, and hemic and lymphatic diseases. The drug is currently in Phase 2 of clinical development and shows promise in treating solid tumors, non-Hodgkin lymphoma, acute myeloid leukemia, and myelodysplastic syndromes. Epizyme, Inc. is the organization behind the development of this innovative drug, aiming to provide new treatment options for patients in need. PRMT5 inhibitors entering PhaseⅠ/Ⅱclinical studies: AMG-193AMG-193 is a small molecule drug developed by Amgen, Inc. It falls under the therapeutic areas of neoplasms and respiratory diseases, specifically targeting PRMT5. The drug is primarily indicated for the treatment of non-small cell lung cancer and solid tumors. 👇Please click on the image below to directly access the latest data (R&D Status | Core Patent | Clinical Trial | Approval status in Global countries) of this drug. Amgen, Inc. is the originator organization behind AMG-193. Being a small molecule drug, AMG-193 is designed to interact with PRMT5, a target protein involved in various cellular processes. By specifically targeting PRMT5, the drug aims to inhibit its activity and potentially disrupt the growth and survival of cancer cells in non-small cell lung cancer and solid tumors. Neoplasms, or abnormal tissue growth, encompass a wide range of cancers. Non-small cell lung cancer is one of the most common types of lung cancer, accounting for approximately 85% of all cases. Solid tumors, on the other hand, refer to abnormal masses of tissue that can occur in various organs, such as the breast, colon, or prostate. The current development stage of AMG-193, Phase 1/2, suggests that the drug has undergone initial testing in a small group of patients to evaluate its safety, dosage, and potential efficacy. This phase typically involves assessing the drug's pharmacokinetics, side effects, and preliminary therapeutic effects. The fact that AMG-193 has obtained IND approval in China indicates that it has met the necessary regulatory requirements to proceed with clinical trials in the country. In summary, AMG-193 is a small molecule drug developed by Amgen, Inc. that targets PRMT5. It is indicated for the treatment of non-small cell lung cancer and solid tumors. The drug has reached Phase 1/2 on a global scale and has obtained IND approval in China, allowing for further clinical trials in the country.