Comparison of RNA Editing Activity of APOBEC1-A1CF and APOBEC1-RBM47 Complexes Reconstituted in HEK293T Cells

RNA editing enzymes: structure, biological functions and applications

With the advancement of sequencing technologies and bioinformatics, over than 170 different RNA modifications have been identified. However, only a few of these modifications can lead to base pair changes, which are called RNA editing. RNA editing is a ubiquitous modification in mammalian transcriptomes and is an important co/posttranscriptional modification that plays a crucial role in various cellular processes. There are two main types of RNA editing events: adenosine to inosine (A-to-I) editing, catalyzed by ADARs on double-stranded RNA or ADATs on tRNA, and cytosine to uridine (C-to-U) editing catalyzed by APOBECs. This article provides an overview of the structure, function, and applications of RNA editing enzymes. We discuss the structural characteristics of three RNA editing enzyme families and their catalytic mechanisms in RNA editing. We also explain the biological role of RNA editing, particularly in innate immunity, cancer biogenesis, and antiviral activity. Additionally, this article describes RNA editing tools for manipulating RNA to correct disease-causing mutations, as well as the potential applications of RNA editing enzymes in the field of biotechnology and therapy.

Identification of RBM46 as a novel APOBEC1 cofactor for C-to-U RNA-editing activity

Cytidine (C) to Uridine (U) RNA editing is a post-transcription modification that is involved in diverse biological processes. APOBEC1 (A1) catalyzes the conversion of C-to-U in RNA, which is important in regulating cholesterol metabolism through its editing activity on ApoB mRNA. However, A1 requires a cofactor to form an "editosome" for RNA editing activity. A1CF and RBM47, both RNA-binding proteins, have been identified as cofactors that pair with A1 to form editosomes and edit ApoB mRNA and other cellular RNAs. SYNCRIP is another RNA-binding protein that has been reported as a potential regulator of A1, although it is not directly involved in A1 RNA editing activity. Here, we describe the identification and characterization of a novel cofactor, RBM46 (RNA-Binding-Motif-protein-46), that can facilitate A1 to perform C-to-U editing on ApoB mRNA. Additionally, using the low-error circular RNA sequencing technique, we identified novel cellular RNA targets for the A1/RBM46 editosome. Our findings provide further insight into the complex regulatory network of RNA editing and the potential new function of A1 with its cofactors.

Unraveling the Enzyme-Substrate Properties for APOBEC3A-Mediated RNA Editing

The APOBEC3 family of human cytidine deaminases is involved in various cellular processes, including the innate and acquired immune system, mostly through inducing C-to-U in single-stranded DNA and/or RNA mutations. Although recent studies have examined RNA editing by APOBEC3A (A3A), its intracellular target specificity are not fully characterized. To address this gap, we performed in-depth analysis of cellular RNA editing using our recently developed sensitive cell-based fluorescence assay. Our findings demonstrate that A3A and an A3A-loop1-containing APOBEC3B (A3B) chimera are capable of RNA editing. We observed that A3A prefers to edit specific RNA substrates which are not efficiently deaminated by other APOBEC members. The editing efficiency of A3A is influenced by the RNA sequence contexts and distinct stem-loop secondary structures. Based on the identified RNA specificity features, we predicted potential A3A-editing targets in the encoding region of cellular mRNAs and discovered novel RNA transcripts that are extensively edited by A3A. Furthermore, we found a trend of increased synonymous mutations at the sites for more efficient A3A-editing, indicating evolutionary adaptation to the higher editing rate by A3A. Our results shed light on the intracellular RNA editing properties of A3A and provide insights into new RNA targets and potential impact of A3A-mediated RNA editing.

The roles of APOBEC-mediated RNA editing in SARS-CoV-2 mutations, replication and fitness

During COVID-19 pandemic, mutations of SARS-CoV-2 produce new strains that can be more infectious or evade vaccines. Viral RNA mutations can arise from misincorporation by RNA-polymerases and modification by host factors. Analysis of SARS-CoV-2 sequence from patients showed a strong bias toward C-to-U mutation, suggesting a potential mutational role by host APOBEC cytosine deaminases that possess broad anti-viral activity. We report the first experimental evidence demonstrating that APOBEC3A, APOBEC1, and APOBEC3G can edit on specific sites of SARS-CoV-2 RNA to produce C-to-U mutations. However, SARS-CoV-2 replication and viral progeny production in Caco-2 cells are not inhibited by the expression of these APOBECs. Instead, expression of wild-type APOBEC3 greatly promotes viral replication/propagation, suggesting that SARS-CoV-2 utilizes the APOBEC-mediated mutations for fitness and evolution. Unlike the random mutations, this study suggests the predictability of all possible viral genome mutations by these APOBECs based on the UC/AC motifs and the viral genomic RNA structure.

The Roles of APOBEC-mediated RNA Editing in SARS-CoV-2 Mutations, Replication and Fitness.. [PMID: 34981048 PMCID: PMC8722585 DOI: 10.1101/2021.12.18.473309]

During COVID-19 pandemic, mutations of SARS-CoV-2 produce new strains that can be more infectious or evade vaccines. Viral RNA mutations can arise from misincorporation by RNA-polymerases and modification by host factors. Analysis of SARS-CoV-2 sequence from patients showed a strong bias toward C-to-U mutation, suggesting a potential mutational role by host APOBEC cytosine deaminases that possess broad anti-viral activity. We report the first experimental evidence demonstrating that APOBEC3A, APOBEC1, and APOBEC3G can edit on specific sites of SARS-CoV-2 RNA to produce C-to-U mutations. However, SARS-CoV-2 replication and viral progeny production in Caco-2 cells are not inhibited by the expression of these APOBECs. Instead, expression of wild-type APOBEC3 greatly promotes viral replication/propagation, suggesting that SARS-CoV-2 utilizes the APOBEC-mediated mutations for fitness and evolution. Unlike the random mutations, this study suggests the predictability of all possible viral genome mutations by these APOBECs based on the UC/AC motifs and the viral genomic RNA structure.

Efficient Editing of SARS-CoV-2 genomic RNA by Host APOBEC deaminases and Its Potential Impacts on the Viral Replication and Emergence of New Strains in COVID-19 Pandemic

RNA binding motif 47 (RBM47): emerging roles in vertebrate development, RNA editing and cancer

RNA-binding proteins (RBPs) are critical players in the post-transcriptional regulation of gene expression and are associated with each event in RNA metabolism. The term 'RNA-binding motif' (RBM) is assigned to novel RBPs with one or more RNA recognition motif (RRM) domains that are mainly involved in the nuclear processing of RNAs. RBM47 is a novel RBP conserved in vertebrates with three RRM domains whose contributions to various aspects of cellular functions are as yet emerging. Loss of RBM47 function affects head morphogenesis in zebrafish embryos and leads to perinatal lethality in mouse embryos, thereby assigning it to be an essential gene in early development of vertebrates. Its function as an essential cofactor for APOBEC1 in C to U RNA editing of several targets through substitution for A1CF in the A1CF-APOBEC1 editosome, established a new paradigm in the field. Recent advances in the understanding of its involvement in cancer progression assigned RBM47 to be a tumor suppressor that acts by inhibiting EMT and Wnt/[Formula: see text]-catenin signaling through post-transcriptional regulation. RBM47 is also required to maintain immune homeostasis, which adds another facet to its regulatory role in cellular functions. Here, we review the emerging roles of RBM47 in various biological contexts and discuss the current gaps in our knowledge alongside future perspectives for the field.

RNA-binding motif protein RBM47 promotes tumorigenesis in nasopharyngeal carcinoma through multiple pathways

RNA binding motif proteins (RBMs) have been widely implicated in the tumorigenesis of multiple human cancers but scarcely studied in nasopharyngeal carcinoma (NPC). Here, we compare the mRNA levels of 29 RBMs between 87 NPC and 10 control samples. We find that RBM47 is frequently upregulated in NPC specimens, and its high expression is associated with the poor prognosis of patients with NPC. Biological experiments show that RBM47 plays an oncogenic role in NPC cells. Mechanically, RBM47 binds to the promoter and regulates the transcription of BCAT1, and its overexpression partially rescues the inhibitory effects of RBM47-knockdown on NPC cells. Moreover, transcriptome analysis reveals that RBM47 regulates alternative splicing of pre-mRNA, including those cancer-related, to a large extent in NPC cells. Furthermore, RBM47 binds to hnRNPM and cooperatively regulates multiple splicing events in NPC cells. In addition, we find that knockdown of hnRNPM inhibits proliferation and migration of NPC cells. Our study, taken together, shows that RBM47 promotes the progression of NPC through multiple pathways, acting as a transcriptional factor and a modulator of alternative splicing in cooperation with hnRNPM. Our study also highlights that RBM47 and hnRNPM could be prognostic factors and potential therapeutic targets for NPC.

APOBEC1 mediated C-to-U RNA editing: target sequence and trans-acting factor contribution to 177 RNA editing events in 119 murine transcripts in-vivo

Mammalian C-to-U RNA editing was described more than 30 years ago as a single nucleotide modification in small intestinal Apob RNA, later shown to be mediated by the RNA-specific cytidine deaminase APOBEC1. Reports of other examples of C-to-U RNA editing, coupled with the advent of genome-wide transcriptome sequencing, identified an expanded range of APOBEC1 targets. Here we analyze the cis-acting regulatory components of verified murine C-to-U RNA editing targets, including nearest neighbor as well as flanking sequence requirements and folding predictions. RNA secondary structure of the editing cassette was associated with editing frequency and exhibited minimal free energy values comparable to small nuclear RNAs. We summarize findings demonstrating the relative importance of trans-acting factors (A1CF, RBM47) acting in concert with APOBEC1. Co-factor dominance was associated with editing frequency, with RNAs targeted by both RBM47 and A1CF edited at a lower frequency than RBM47 dominant targets. Using this information, we developed a multivariable linear regression model to predict APOBEC1 dependent C-to-U RNA editing efficiency, incorporating factors independently associated with editing frequencies based on 103 Sanger-confirmed editing sites, which accounted for 84% of the observed variance. This model also predicted a composite score for available human C-to-U RNA targets, which again correlated with editing frequency.

Insights into the Structures and Multimeric Status of APOBEC Proteins Involved in Viral Restriction and Other Cellular Functions

Apolipoprotein B mRNA editing catalytic polypeptide-like (APOBEC) proteins belong to a family of deaminase proteins that can catalyze the deamination of cytosine to uracil on single-stranded DNA or/and RNA. APOBEC proteins are involved in diverse biological functions, including adaptive and innate immunity, which are critical for restricting viral infection and endogenous retroelements. Dysregulation of their functions can cause undesired genomic mutations and RNA modification, leading to various associated diseases, such as hyper-IgM syndrome and cancer. This review focuses on the structural and biochemical data on the multimerization status of individual APOBECs and the associated functional implications. Many APOBECs form various multimeric complexes, and multimerization is an important way to regulate functions for some of these proteins at several levels, such as deaminase activity, protein stability, subcellular localization, protein storage and activation, virion packaging, and antiviral activity. The multimerization of some APOBECs is more complicated than others, due to the associated complex RNA binding modes.

APOBEC1 cytosine deaminase activity on single-stranded DNA is suppressed by replication protein A

Many APOBEC cytidine deaminase members are known to induce ‘off-target’ cytidine deaminations in 5′TC motifs in genomic DNA that contribute to cancer evolution. In this report, we characterized APOBEC1, which is a possible cancer related APOBEC since APOBEC1 mRNA is highly expressed in certain types of tumors, such as lung adenocarcinoma. We found a low level of APOBEC1-induced DNA damage, as measured by γH2AX foci, in genomic DNA of a lung cancer cell line that correlated to its inability to compete in vitro with replication protein A (RPA) for ssDNA. This suggests that RPA can act as a defense against off-target deamination for some APOBEC enzymes. Overall, the data support the model that the ability of an APOBEC to compete with RPA can better predict genomic damage than combined analysis of mRNA expression levels in tumors and analysis of mutation signatures.

Live-Cell Quantification of APOBEC1-Mediated RNA Editing: A Comparison of RNA Editing Assays

APOBEC1 is a member of the AID/APOBECs, a group of deaminases responsible for the editing of C>U in both DNA and RNA. APOBEC1 is physiologically involved in C>U RNA editing: while hundreds of targets have been discovered in mice, in humans the only well-characterized target of APOBEC1 is the apolipoprotein B (ApoB) transcript. APOBEC1 edits a CAA codon into a stop codon, which causes the translation of a truncated form of ApoB. A number of assays have been developed to investigate this process. Early assays, poisoned primer extension and Sanger sequencing, have focused on accuracy and sensitivity but rely on extraction of the RNA from tissues and cells. More recently, the need to visualize the RNA editing process directly in live cells have led to the development of fluorescence-based tools. These assays detect RNA editing through reporters whose editing causes a change in cellular localization or a change in fluorescent properties. Here we review the available assays to quantify RNA editing, and we present the protocol for cytofluorimetric analysis using a double-fluorescent reporter.

Prognostic value of lipid metabolism-related genes in head and neck squamous cell carcinoma

BACKGROUND

Altered lipid metabolism is involved in the development of many tumors. However, the role of dissimilar lipid metabolism in head and neck squamous cell carcinoma (HNSCC) is not fully established.

AIMS

Here, we sought to determine the prognostic value of lipid metabolism-related genes in HNSCC.

METHODS

RNA-seq data and clinical features of 545 HNSCC cases were obtained from The Cancer Genome Atlas database. A regulatory network of transcription factors-lipid metabolism genes and a risk prognostic model of lipid metabolism-related genes was developed using bioinformatics and Cox regression modeling. We used tumor immune estimation resource to analyze immune cell infiltration in patients with HNSCC based on the prognostic index (PI) of lipid metabolism-related genes.

RESULTS

A total of 136 differentially expressed lipid metabolism genes were identified. Of these, 23 are related to prognosis. In addition to predicting HNSCC prognosis, 11 lipid metabolism-related genes (ARSI, CYP27B1, CYP2D6, DGKG, DHCR7, LPIN1, PHYH, PIP5K1B, PLA2G2D, RDH16, and TRIB3) also affect HNSCC clinical features (stage, gender, and pathological stage). The PI of lipid metabolism-related genes embodied the state of HNSCC tumor immune microenvironment.

The structure of APOBEC1 and insights into its RNA and DNA substrate selectivity

APOBEC1 (APO1), a member of AID/APOBEC nucleic acid cytosine deaminase family, can edit apolipoprotein B mRNA to regulate cholesterol metabolism. This APO1 RNA editing activity requires a cellular cofactor to achieve tight regulation. However, no cofactors are required for deamination on DNA by APO1 and other AID/APOBEC members, and aberrant deamination on genomic DNA by AID/APOBEC deaminases has been linked to cancer. Here, we present the crystal structure of APO1, which reveals a typical APOBEC deaminase core structure, plus a unique well-folded C-terminal domain that is highly hydrophobic. This APO1 C-terminal hydrophobic domain (A1HD) interacts to form a stable dimer mainly through hydrophobic interactions within the dimer interface to create a four-stranded β-sheet positively charged surface. Structure-guided mutagenesis within this and other regions of APO1 clarified the importance of the A1HD in directing RNA and cofactor interactions, providing insights into the structural basis of selectivity on DNA or RNA substrates.

Zebrafish RBM47 Promotes Lysosome-Dependent Degradation of MAVS to Inhibit IFN Induction

IFN is essential for hosts to defend against viral invasion, whereas it must be tightly regulated to prevent hyperimmune responses. Fish mitochondrial antiviral signaling protein (MAVS) is a vital factor for IFN production, but until now, there have been few studies on the regulation mechanisms of fish MAVS enabling IFN to be properly controlled. In this study, we show that zebrafish RNA-binding motif protein 47 (RBM47) promotes MAVS degradation in a lysosome-dependent manner to suppress IFN production. First, the transcription of IFN activated by polyinosinic/polycytidylic acid (poly I:C), spring viremia of carp virus, or retinoic acid-inducible gene I (RIG-I)-like receptor pathway components were significantly suppressed by RBM47. Second, RBM47 interacted with MAVS and promoted lysosome-dependent degradation of MAVS, changing the cellular location of MAVS from the cytoplasm to the lysosome region. Finally, RBM47 inhibited downstream MITA and IRF3/7 activation, impairing the host antiviral response. Collectively, these data suggest that zebrafish RBM47 negatively regulates IFN production by promoting lysosome-dependent degradation of MAVS, providing insights into the role of RBM47 in the innate antiviral immune response in fish.

Transcriptomic Analysis Identifies RNA Binding Proteins as Putative Regulators of Myelopoiesis and Leukemia

Acute myeloid leukemia (AML) is a common and aggressive hematological malignancy. Acquisition of heterogeneous genetic aberrations and epigenetic dysregulation lead to the transformation of hematopoietic stem cells (HSC) into leukemic stem cells (LSC), which subsequently gives rise to immature blast cells and a leukemic phenotype. LSCs are responsible for disease relapse as current chemotherapeutic regimens are not able to completely eradicate these cellular sub-populations. Therefore, it is critical to improve upon the existing knowledge of LSC specific markers, which would allow for specific targeting of these cells more effectively allowing for their sustained eradication from the cellular milieu. Although significant milestones in decoding the aberrant transcriptional network of various cancers, including leukemia, have been achieved, studies on the involvement of post-transcriptional gene regulation (PTGR) in disease progression are beginning to unfold. RNA binding proteins (RBPs) are key players in mediating PTGR and they regulate the intracellular fate of individual transcripts, from their biogenesis to RNA metabolism, via interactions with RNA binding domains (RBDs). In this study, we have used an integrative approach to systematically profile RBP expression and identify key regulatory RBPs involved in normal myeloid development and AML. We have analyzed RNA-seq datasets (GSE74246) of HSCs, common myeloid progenitors (CMPs), granulocyte-macrophage progenitors (GMPs), monocytes, LSCs, and blasts. We observed that normal and leukemic cells can be distinguished on the basis of RBP expression, which is indicative of their ability to define cellular identity, similar to transcription factors. We identified that distinctly co-expressing modules of RBPs and their subclasses were enriched in hematopoietic stem/progenitor (HSPCs) and differentiated monocytes. We detected expression of DZIP3, an E3 ubiquitin ligase, in HSPCs, knockdown of which promotes monocytic differentiation in cell line model. We identified co-expression modules of RBP genes in LSCs and among these, distinct modules of RBP genes with high and low expression. The expression of several AML-specific RBPs were also validated by quantitative polymerase chain reaction. Network analysis identified densely connected hubs of ribosomal RBP genes (rRBPs) with low expression in LSCs, suggesting the dependency of LSCs on altered ribosome dynamics. In conclusion, our systematic analysis elucidates the RBP transcriptomic landscape in normal and malignant myelopoiesis, and highlights the functional consequences that may result from perturbation of RBP gene expression in these cellular landscapes.