CEITEC, Masaryk University, Kamenice 735/5, E35, Brno 62500, Czechia.
Acc Chem Res. 2023 Nov 21;56(22):3165-3174. doi: 10.1021/acs.accounts.3c00433. Epub 2023 Oct 31.
The adenosine deaminase acting on RNA (ADAR) enzymes that catalyze the conversion of adenosine to inosine in double-stranded (ds)RNA are evolutionarily conserved and are essential for many biological functions including nervous system function, hematopoiesis, and innate immunity. Initially it was assumed that the wide-ranging biological roles of ADARs are due to inosine in mRNA being read as guanosine by the translational machinery, allowing incomplete RNA editing in a target codon to generate two different proteins from the same primary transcript. In humans, there are approximately seventy-six positions that undergo site-specific editing in tissues at greater than 20% efficiency that result in recoding. Many of these transcripts are expressed in the central nervous system (CNS) and edited by ADAR2. Exploiting mouse genetic models revealed that transgenic mice lacking the gene encoding Adar2 die within 3 weeks of birth. Therefore, the role of ADAR2 in generating protein diversity in the nervous system is clear, but why is ADAR RNA editing activity essential in other biological processes, particularly editing mainly involving ADAR1? ADAR1 edits human transcripts having embedded Alu element inverted repeats (AluIRs), but the link from this activity to innate immunity activation was elusive. Mice lacking the gene encoding Adar1 are embryonically lethal, and a major breakthrough was the discovery that the role of Adar1 in innate immunity is due to its ability to edit such repetitive element inverted repeats which have the ability to form dsRNA in transcripts. The presence of inosine prevents activation of the dsRNA sensor melanoma differentiation-associated protein 5 (Mda5). Thus, inosine helps the cell discriminate self from non-self RNA, acting like a barcode on mRNA. As innate immunity is key to many different biological processes, the basis for this widespread biological role of the ADAR1 enzyme became evident.Our group has been studying ADARs from the outset of research on these enzymes. In this Account, we give a historical perspective, moving from the initial purification of ADAR1 and ADAR2 and cloning of their encoding genes up to the current research focus in the field and what questions still remain to be addressed. We discuss the characterizations of the proteins, their localizations, posttranslational modifications, and dimerization, and how all of these affect their biological activities. Another aspect we explore is the use of mouse and genetic models to study ADAR functions and how these were crucial in determining the biological functions of the ADAR proteins. Finally, we describe the severe consequences of rare mutations found in the human genes encoding ADAR1 and ADAR2.
腺苷脱氨酶作用于 RNA（ADAR）酶可催化双链 (ds)RNA 中的腺苷转化为肌苷，这些酶在许多生物学功能中是保守的，包括神经系统功能、造血和先天免疫。最初，人们认为 ADAR 的广泛生物学作用是由于 mRNA 中的肌苷被翻译机制读为鸟苷，允许靶密码子中的不完全 RNA 编辑，从而从同一初级转录本产生两种不同的蛋白质。在人类中，大约有 76 个位置在组织中以大于 20%的效率进行特异性编辑，从而导致重新编码。这些转录本中有许多在中枢神经系统 (CNS) 中表达，并由 ADAR2 编辑。利用小鼠遗传模型表明，缺乏编码 Adar2 基因的转基因小鼠在出生后 3 周内死亡。因此，ADAR2 在神经系统中产生蛋白质多样性的作用是明确的，但为什么 ADAR RNA 编辑活性在其他生物学过程中是必要的，特别是主要涉及 ADAR1 的编辑？ADAR1 编辑人类转录本中嵌入的 Alu 元件反向重复 (AluIR)，但从这种活性到先天免疫激活的联系还不清楚。缺乏编码 Adar1 基因的小鼠在胚胎期是致命的，一个重大突破是发现 Adar1 在先天免疫中的作用是由于其编辑这些重复元件反向重复的能力，这些重复元件在转录本中具有形成 dsRNA 的能力。肌苷的存在阻止了双链 RNA 传感器黑色素瘤分化相关蛋白 5 (Mda5)的激活。因此，肌苷有助于细胞区分自身和非自身 RNA，就像 mRNA 上的条形码一样。由于先天免疫是许多不同生物学过程的关键，ADAR1 酶的这种广泛生物学作用的基础变得显而易见。我们的研究小组从研究这些酶的一开始就一直在研究 ADAR。在本报告中，我们从 ADAR1 和 ADAR2 的最初纯化和其编码基因的克隆开始，介绍了该领域的当前研究重点和仍然存在的问题。我们讨论了蛋白质的特性、它们的定位、翻译后修饰和二聚化，以及所有这些如何影响它们的生物学活性。我们探索的另一个方面是使用小鼠和遗传模型来研究 ADAR 功能，以及这些模型在确定 ADAR 蛋白的生物学功能方面是多么关键。最后，我们描述了在编码 ADAR1 和 ADAR2 的人类基因中发现的罕见突变的严重后果。
