mRNA表观修饰方式及m6A功能研究进展

mRNA上能发生100多种化学修饰,其中N~6-腺嘌呤(m~6A)是mRNA修饰中最广泛的表观修饰方式之一。在细胞分化、胚胎发育和应激等生物学过程中,特定的mRNA会发生包括N~1-腺嘌呤甲基化、N~5-胞嘧啶甲基化、假尿嘧啶以及N`6-腺嘌呤甲基化等修饰,它们共同形成了mRNA转录后调控的表观修饰转录组,实现对mRNA翻译成蛋白质过程的精确时空调控,特别是m~6A修饰能通过调控mRNA的代谢和翻译等进而调控细胞的一系列生物学过程。文中主要综述mRNA的表观修饰类型和特点,特别是m~6A修饰参与调控mRNA和细胞生物学功能的最新研究进展,并展望了将来m~6A表观修饰的研究重点和方向。     

The detection and functions of RNA modification m6A based on m6A writers and erasers

N⁶-methyladenosine (m⁶A) is the most frequent chemical modification in eukaryotic mRNA and is known to participate in a variety of physiological processes, including cancer progression and viral infection. The reversible and dynamic m⁶A modification is installed by m⁶A methyltransferase (writer) enzymes and erased by m⁶A demethylase (eraser) enzymes. m⁶A modification recognized by m⁶A binding proteins (readers) regulates RNA processing and metabolism, leading to downstream biological effects such as promotion of stability and translation or increased degradation. The m⁶A writers and erasers determine the abundance of m⁶A modifications and play decisive roles in its distribution and function. In this review, we focused on m⁶A writers and erasers and present an overview on their known functions and enzymatic molecular mechanisms, showing how they recognize substrates and install or remove m⁶A modifications. We also summarize the current applications of m⁶A writers and erasers for m⁶A detection and highlight the merits and drawbacks of these available methods. Lastly, we describe the biological functions of m⁶A in cancers and viral infection based on research of m⁶A writers and erasers and introduce new assays for m⁶A functionality via programmable m⁶A editing tools.     

外泌体介导的miRNAs对恶性肿瘤调控作用研究的新进展

外泌体是一种直径为30 nm^100 nm的细胞外脂质囊泡,几乎可以被所有类型的细胞释放,包括癌细胞。作为细胞间通讯的重要介质,宿主细胞或癌细胞分泌的外泌体可以介导包括miRNA、mRNA、DNA片段及蛋白质在内的多种物质参与肿瘤的发生、生长、侵袭及转移过程。尤其是miRNA已经被证实是肿瘤衍生的外泌体用于实现自身功能机制的重要组成部分。因此,外泌体miRNA在调节肿瘤发生发展、侵袭转移、肿瘤免疫应答、肿瘤血管生成及肿瘤耐药方面具有显著功能。本文就外泌体介导的miRNA对肿瘤的相关调控作用作一综述。     

Role of m6A modification in female infertility and reproductive system diseases

Gamete abnormalities and reproductive system tumors have become a dominant cause of infertility, troubling people globally. In recent years, increasing evidence emerged and found that N6-methyladenosine (m6A) played a leading role in reproduction. The biological effects of m6A modification are dynamically and reversibly regulated by methyltransferases (writers), WTAP, METTL3, METTL14 and KIAA1429, demethylases (erasers), FTO and ALKBH5, and m6A binding proteins (readers), including YTH domain. In this review, we highlight the change of m6A modification in abnormal oogenesis, female reproductive system diseases including reproductive system tumors, adenomyosis, endometriosis, premature ovarian failure and polycystic ovary syndrome. Moreover, we review some of the mechanisms and the specific modified genes that have been identified. Especially, with the underlying mechanisms being uncovered, m6A and its protein machineries are expected to be the markers and targets for the diagnosis and treatment of female reproductive dysfunction.     

METTL3-mediated m6A modification stabilizes TERRA and maintains telomere stability

Telomeric repeat-containing RNA (TERRA) is a type of long non-coding RNA transcribed from telomeres, and it forms R-loops by invasion into telomeric DNA. Since either an excessive or inadequate number of R-loops leads to telomere instability, the TERRA levels need to be delicately modulated. In this study, we found that m⁶A modification presents on the subtelomeric regions of TERRA and stabilizes it, and the loss of METTL3 impacts telomere stability. Mechanically, the m⁶A modification on TERRA is catalyzed by METTL3, recognized and stabilized by the m⁶A reader YTHDC1. Knockdown of either METTL3 or YTHDC1 enhances TERRA degradation. The m⁶A-modified TERRA forms R-loops and promotes homologous recombination which is essential for the alternative lengthening of telomeres (ALT) pathway in cancer cells. METTL3 depletion leads to R-loop reduction, telomere shortening and instability. Altogether, these findings reveal that METTL3 protects telomeres by catalyzing m⁶A modification on TERRA, indicating that inhibition or deletion of METTL3 is potentially a new avenue for ALT cancer therapy.     

The functional roles of m6A modification in T lymphocyte responses and autoimmune diseases

RNA N6-methyladenosine (m⁶A) modification is abundant in eukaryotes, bacteria and archaea. It is an RNA modification mainly existing in messenger RNA (mRNAs) and has a significant effect on the metabolism and function of mRNAs. m⁶A modification is controlled by three types of proteins, namely methyltransferase as the “writers”, demethylase as the “erasers”, and specific m⁶A recognized protein (YTHDF1–3) as the “readers”. Recent studies have shown that m⁶A modification plays an important role in cancer, viral infection and autoimmune diseases. In this review, we will elaborate on the m⁶A modifications in the homeostasis and differentiation of T cells. Then we will further summarize the effects of m⁶A modification on the T cell responses and T cell-mediated autoimmune diseases. This will advance T cell epigenetics research and provide potential biomarkers and therapeutic targets for autoimmune diseases.     

RNA m6A modification in prostate cancer: A new weapon for its diagnosis and therapy

Prostate cancer (PCa) is the most common malignant tumor and the second leading cause of cancer-related mortality in men worldwide. Despite significant advances in PCa therapy, the underlying molecular mechanisms have yet to be fully elucidated. Recently, epigenetic modification has emerged as a key player in tumor progression, and RNA-based N6-methyladenosine (m⁶A) epigenetic modification was found to be crucial. This review summarizes comprehensive state-of-art mechanisms underlying m⁶A modification, its implication in the pathogenesis, and advancement of PCa in protein-coding and non-coding RNA contexts, its relevance to PCa immunotherapy, and the ongoing clinical trials for PCa treatment. This review presents potential m⁶A-based targets and paves a new avenue for diagnosing and treating PCa, providing new guidelines for future related research through a systematic review of previous results.     

m6A modification of HSATIII lncRNAs regulates temperature‐dependent splicing

Nuclear stress bodies (nSBs) are nuclear membraneless organelles formed around stress‐inducible HSATIII architectural long noncoding RNAs (lncRNAs). nSBs repress splicing of hundreds of introns during thermal stress recovery, which are partly regulated by CLK1 kinase phosphorylation of temperature‐dependent Ser/Arg‐rich splicing factors (SRSFs). Here, we report a distinct mechanism for this splicing repression through protein sequestration by nSBs. Comprehensive identification of RNA‐binding proteins revealed HSATIII association with proteins related to N⁶‐methyladenosine (m⁶A) RNA modification. 11% of the first adenosine in the repetitive HSATIII sequence were m⁶A‐modified. nSBs sequester the m⁶A writer complex to methylate HSATIII, leading to subsequent sequestration of the nuclear m⁶A reader, YTHDC1. Sequestration of these factors from the nucleoplasm represses m⁶A modification of pre‐mRNAs, leading to repression of m⁶A‐dependent splicing during stress recovery phase. Thus, nSBs serve as a common platform for regulation of temperature‐dependent splicing through dual mechanisms employing two distinct ribonucleoprotein modules with partially m⁶A‐modified architectural lncRNAs.     

A cancer-associated METTL14 mutation induces aberrant m6A modification,affecting tumor growth

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The interplay between m6A modification and non-coding RNA in cancer stemness modulation: mechanisms,signaling pathways,and clinical implications

Cancer stemness, mainly consisting of chemo-resistance, radio-resistance, tumorigenesis, metastasis, tumor self-renewal, cancer metabolism reprogramming, and tumor immuno-microenvironment remodeling, play crucial roles in the cancer progression process and has become the hotspot of cancer research field in recent years. Nowadays, the exact molecular mechanisms of cancer stemness have not been fully understood. Extensive studies have recently implicated that non-coding RNA (ncRNA) plays vital roles in modulating cancer stemness. Notably, N6-methyladenosine (m6A) modification is of crucial importance for RNAs to exert their biological functions, including RNA splicing, stability, translation, degradation, and export. Emerging evidence has revealed that m6A modification can govern the expressions and functions of ncRNAs, consequently controlling cancer stemness properties. However, the interaction mechanisms between ncRNAs and m6A modification in cancer stemness modulation are rarely investigated. In this review, we elucidate the recent findings on the relationships of m6A modification, ncRNAs, and cancer stemness. We also focus on some key signaling pathways such as Wnt/β-catenin signaling, MAPK signaling, Hippo signaling, and JAK/STAT3 signaling to illustrate the underlying interplay mechanisms between m6A modification and ncRNAs in cancer stemness. In particular, we briefly highlight the clinical potential of ncRNAs and m6A modifiers as promising biomarkers and therapeutic targets for indicating cancer stemness properties and improving the diagnostic precision for a wide variety of cancers.     

Fat mass and obesity-associated protein regulates lipogenesis via m6A modification in fatty acid synthase mRNA

As the first identified N⁶-methyladenosine (m⁶A) demethylase, fat mass and obesity-associated (FTO) protein is associated with fatty acid synthase (FASN) and lipid accumulation. However, little is known about the regulatory role of FTO in the expression of FASN and de novo lipogenesis through m⁶A modification. In this study, we used FTO small interfering RNA to explore the effects of FTO knockdown on hepatic lipogenesis and its underlying epigenetic mechanism in HepG2 cells. We found that knockdown of FTO increased m⁶A levels in total RNA and enhanced the expression of YTH domain family member 2 which serves as the m⁶A-binding protein. The de novo lipogenic enzymes and intracellular lipid content were significantly decreased under FTO knockdown. Mechanistically, knockdown of FTO dramatically enhanced m⁶A levels in FASN messenger RNA (mRNA), leading to the reduced expression of FASN mRNA through m⁶A-mediated mRNA decay. The protein expressions of FASN along with acetyl CoA carboxylase and ATP-citrate lyase were further decreased, which inhibited de novo lipogenesis, thereby resulting in the deficiency of lipid accumulation in HepG2 cells and the induction of cellular apoptosis. The results reveal that FTO regulates hepatic lipogenesis via FTO-dependent m⁶A demethylation in FASN mRNA and indicate the critical role of FTO-mediated lipid metabolism in the survival of HepG2 cells. This study provides novel insights into a unique RNA epigenetic mechanism by which FTO mediates hepatic lipid accumulation through m⁶A modification and indicates that FTO could be a potential target for obesity-related diseases and cancer.     

The functional roles,cross-talk and clinical implications of m6A modification and circRNA in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related deaths worldwide. HCC has high rates of death and recurrence, as well as very low survival rates. N6-methyladenosine (m6A) is the most abundant modification in eukaryotic RNAs, and circRNAs are a class of circular noncoding RNAs that are generated by back-splicing and they modulate multiple functions in a variety of cellular processes. Although the carcinogenesis of HCC is complex, emerging evidence has indicated that m6A modification and circRNA play vital roles in HCC development and progression. However, the underlying mechanisms governing HCC, their cross-talk, and clinical implications have not been fully elucidated. Therefore, in this paper, we elucidated the biological functions and molecular mechanisms of m6A modification in the carcinogenesis of HCC by illustrating three different regulatory factors ("writer", "eraser", and "reader") of the m6A modification process. Additionally, we dissected the functional roles of circRNAs in various malignant behaviors of HCC, thereby contributing to HCC initiation, progression and relapse. Furthermore, we demonstrated the cross-talk and interplay between m6A modification and circRNA by revealing the effects of the collaboration of circRNA and m6A modification on HCC progression. Finally, we proposed the clinical potential and implications of m6A modifiers and circRNAs as diagnostic biomarkers and therapeutic targets for HCC diagnosis, treatment and prognosis evaluation.