The double-domain cytidine deaminase APOBEC3G is a cellular site-specific RNA editing enzyme

Cytosine-to-uracil RNA editing is upregulated by pro-inflammatory stimulation of myeloid cells

Myeloid cells undergo large changes to their gene expression profile in response to inflammatory stimulation. This includes an increase in post-transcriptional modifications carried out by adenosine-to-inosine (A-to-I) and cytosine-to-uracil (C-to-U) RNA editing enzymes. However, the precise RNA editing targets altered by stimulation and the consequences of RNA editing on gene expression and the proteome have been understudied. We present a comprehensive RNA editing analysis of stimulated myeloid cells across three independent cohorts totalling 297 samples, including monocytes and IPS-derived microglia. We observed that C-to-U editing, while less abundant, has a higher effect size in response to stimulation than A-to-I, and has a greater potential to recode the proteome. We investigated the consequences of RNA editing on RNA stability and gene expression using in silico and in vitro reporter methods, and identified a recoding C-to-U site in ARSB that mimics a reported lysosomal storage disorder mutation.

APOBEC3-Related Editing and Non-Editing Determinants of HIV-1 and HTLV-1 Restriction

The apolipoprotein B mRNA editing enzyme catalytic polypeptide-like 3 (APOBEC3/A3) family of cytosine deaminases serves as a key innate immune barrier against invading retroviruses and endogenous retroelements. The A3 family's restriction activity against these parasites primarily arises from their ability to catalyze cytosine-to-uracil conversions, resulting in genome editing and the accumulation of lethal mutations in viral genomes. Additionally, non-editing mechanisms, including deaminase-independent pathways, such as blocking viral reverse transcription, have been proposed as antiviral strategies employed by A3 family proteins. Although viral factors can influence infection progression, the determinants that govern A3-mediated restriction are critical in shaping retroviral infection outcomes. This review examines the interactions between retroviruses, specifically human immunodeficiency virus type 1 and human T-cell leukemia virus type 1, and A3 proteins to better understand how editing and non-editing activities contribute to the trajectory of these retroviral infections.

Restoration of cytidine to uridine genetic code using an MS2-APOBEC1 artificial enzymatic approach

By employing site-directed RNA editing (SDRE) to restore point-mutated RNA molecules, it is possible to change gene-encoded information and synthesize proteins with different functionality from a single gene. Thymine (T) to cytosine (C) point mutations cause various genetic disorders, and when they occur in protein-coding regions, C-to-uridine (U) RNA changes can lead to non-synonymous alterations. By joining the deaminase domain of apolipoprotein B messenger RNA (mRNA) editing catalytic polypeptide 1 (APOBEC1) with a guide RNA (gRNA) complementary to a target mRNA, we created an artificial RNA editase. We used an mRNA encoding blue fluorescent protein (BFP), obtained from the green fluorescent protein (GFP) gene through the introduction of a T > C mutation, as our target RNA. In a proof of principle experiment, we reverted the T > C mutation at the RNA level using our APOBEC1 site-directed RNA editing system, recovering GFP signal. Sanger sequencing of cDNA from transfected cells and polymerase chain reaction-restriction length polymorphism analysis validated this result, indicating an editing of approximately 21 %. Our successful development of an artificial RNA editing system using the deaminase APOBEC1, in conjunction with the MS2 system, may lead to the development of treatments for genetic diseases based on the restoration of specific types of wild type sequences at the mRNA level.

Fluorescent shift assay for APOBEC-mediated RNA editing

Cytidine (C) to Uridine (U) RNA editing is a post-transcriptional modification that is involved in diverse biological processes. The APOBEC deaminase family acts in various cellular processes mostly through inducing C-to-U mutation in single-stranded RNA (or DNA). However, comparing the activity of different RNA editing enzymes to one another is difficult due to the limited number of systems that can provide direct and efficient readout. In this report, a system in which RNA editing directly prompts a change in the subcellular localization of a modified eGFP structure is described in detail. This approach allows us to compare relative fluorescence intensity based on the RNA editing level. When observed through a fluorescence detection system, like a scanning confocal microscope, the cellular nucleus can be readily identified using a DNA-binding stain, such as DAPI or Hoechst, so that the accurate calculation of the ratio of nuclear to cytosolic eGFP intensity can be applied for an individual cell. This method provides a useful and flexible tool to examine and quantify RNA editing activity within cells, and it is not only limited to APOBEC proteins, but can also be applied more generally to other RNA editing enzymatic assays.

Validation of the APOBEC3A-Mediated RNA Single Base Substitution Signature and Proposal of Novel APOBEC1, APOBEC3B, and APOBEC3G RNA Signatures

Mutational signature analysis gained significant attention for providing critical insights into the underlying mutational processes for various DNA single base substitution (SBS) signatures and their associations with different cancer types. Recently, RNA single base substitution (RNA-SBS) signatures were defined and described by decomposing RNA variants found in non-small cell lung cancer. Through statistical association, they attributed Apolipoprotein B mRNA Editing Enzyme, Catalytic Polypeptide 3A (APOBEC3A) mutagenesis to the RNA-SBS2 signature. Here, we provide the first validation of an RNA-SBS mutational signature by decomposing novel exogenous and endogenous APOBEC3A RNA editing signatures into COSMICv3.4 RNA-SBS reference signatures. Additionally, we have identified novel RNA-SBS signatures for APOBEC1, APOBEC3B, and APOBEC3G.

In vitro deamination assay to measure the activity and processivity of AID/APOBEC enzymes

The AID/APOBEC family of enzymes are cytidine/deoxycytidine deaminases that primarily catalyze the deamination of deoxycytidines (dCs) into deoxyuridines (dUs) on single-stranded DNA (ssDNA). In humans, there are 11 members within the family. AID and APOBEC3 (A3) enzymes have been extensively characterized for their ability to introduce promutagenic dUs during antibody gene diversification and intrinsic immune defenses against viruses and retrotransposons, respectively. In order to search for a local dC deamination target to effectively catalyze the deamination reaction, AID/APOBEC enzymes adopt facilitated diffusion as a mechanism to search for the target deamination sites on ssDNA substrates, which includes one-dimensional (1D) movements termed sliding, and three-dimensional (3D) movements termed jumping and intersegment transfer. This type of diffusional mechanism enables AID/APOBEC enzymes to processively scan ssDNA substrates and serves as a key determinant to the mutagenic potential of AID/APOBEC enzymes in vivo. The catalysis and processive ssDNA scanning behaviors of AID/APOBEC enzymes can be assessed using purified proteins and synthetic ssDNA through an in vitro deamination assay. In this Chapter, we describe how to perform deamination assays where DNA scanning mechanisms and processivity can be measured under single-hit conditions using a fluorescently labeled ssDNA substrate. The in vitro deamination assay can also be applied to determine AID/APOBEC activity in cell lysates or in kinetic reactions to determine the specific activity of purified enzymes.

Purification of enzymatically active APOBEC proteins from an insect cell expression system

The APOBEC cytidine/deoxycytidine deaminase family of enzymes has 11 members in humans. These enzymes carry out essential developmental, metabolic, and immunological functions through the deamination of cytosine to form uracil in RNA or single-stranded DNA. The known physiological functions relate to lipid absorption (APOBEC1), immunoglobulin gene diversification (AID), virus restriction (APOBEC3A-H, excluding E), and muscle differentiation (APOBEC2). The ability to characterize in vitro how APOBEC enzymes interact with and catalyze cytidine/deoxycytidine deamination of their substrate has provided key insights and understanding of their physiological functions. Having the most highly active and soluble enzyme to carry out in vitro experiments is essential. For APOBEC enzymes this requires purification from a mammalian or insect cell system. Since mammalian cell expression is lower than robustly engineered recombinant systems such as the Spodoptera frugiperda 9 (Sf9) and baculovirus systems, we have developed recombinant baculovirus expression and purification methods for APOBEC enzymes from Sf9 cells. The yield for all family members is suitable for biochemical assays, with some enzymes yielding milligram amounts (suitable for structural studies). Here we describe the expression and purification of APOBEC3A, APOBEC3B, APOBEC3C, APOBEC3F, APOBEC3G, APOBEC3H (Haplotypes II, V, VII), and APOBEC1 using existing molecular biology reagents. We also describe how to clone a novel gene into the system for expression and purification. Due to different expression levels and solubility, three purification methods are detailed that enable high, medium, and low expressing APOBECs to be purified.

Viral N protein hijacks deaminase-containing RNA granules to enhance SARS-CoV-2 mutagenesis

Host cell-encoded deaminases act as antiviral restriction factors to impair viral replication and production through introducing mutations in the viral genome. We sought to understand whether deaminases are involved in SARS-CoV-2 mutation and replication, and how the viral factors interact with deaminases to trigger these processes. Here, we show that APOBEC and ADAR deaminases act as the driving forces for SARS-CoV-2 mutagenesis, thereby blocking viral infection and production. Mechanistically, SARS-CoV-2 nucleocapsid (N) protein, which is responsible for packaging viral genomic RNA, interacts with host deaminases and co-localizes with them at stress granules to facilitate viral RNA mutagenesis. N proteins from several coronaviruses interact with host deaminases at RNA granules in a manner dependent on its F17 residue, suggesting a conserved role in modulation of viral mutagenesis in other coronaviruses. Furthermore, mutant N protein bearing a F17A substitution cannot localize to deaminase-containing RNA granules and leads to reduced mutagenesis of viral RNA, providing support for its function in enhancing deaminase-dependent viral RNA editing. Our study thus provides further insight into virus-host cell interactions mediating SARS-CoV-2 evolution.

Molecular mechanism for regulating APOBEC3G DNA editing function by the non-catalytic domain

APOBEC3G, part of the AID/APOBEC cytidine deaminase family, is crucial for antiviral immunity. It has two zinc-coordinated cytidine-deaminase domains. The non-catalytic N-terminal domain strongly binds to nucleic acids, whereas the C-terminal domain catalyzes C-to-U editing in single-stranded DNA. The interplay between the two domains is not fully understood. Here, we show that DNA editing function of rhesus macaque APOBEC3G on linear and hairpin loop DNA is enhanced by AA or GA dinucleotide motifs present downstream in the 3'-direction of the target-C editing sites. The effective distance between AA/GA and the target-C sites is contingent on the local DNA secondary structure. We present two co-crystal structures of rhesus macaque APOBEC3G bound to ssDNA containing AA and GA, revealing the contribution of the non-catalytic domain in capturing AA/GA DNA. Our findings elucidate the molecular mechanism of APOBEC3G's cooperative function, which is critical for its antiviral role and its contribution to mutations in cancer genomes.

Potential Role of APOBEC3 Family Proteins in SARS-CoV-2 Replication

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has acquired multiple mutations since its emergence. Analyses of the SARS-CoV-2 genomes from infected patients exhibit a bias toward C-to-U mutations, which are suggested to be caused by the apolipoprotein B mRNA editing enzyme polypeptide-like 3 (APOBEC3, A3) cytosine deaminase proteins. However, the role of A3 enzymes in SARS-CoV-2 replication remains unclear. To address this question, we investigated the effect of A3 family proteins on SARS-CoV-2 replication in the myeloid leukemia cell line THP-1 lacking A3A to A3G genes. The Wuhan, BA.1, and BA.5 variants had comparable viral replication in parent and A3A-to-A3G-null THP-1 cells stably expressing angiotensin-converting enzyme 2 (ACE2) protein. On the other hand, the replication and infectivity of these variants were abolished in A3A-to-A3G-null THP-1-ACE2 cells in a series of passage experiments over 20 days. In contrast to previous reports, we observed no evidence of A3-induced SARS-CoV-2 mutagenesis in the passage experiments. Furthermore, our analysis of a large number of publicly available SARS-CoV-2 genomes did not reveal conclusive evidence for A3-induced mutagenesis. Our studies suggest that A3 family proteins can positively contribute to SARS-CoV-2 replication; however, this effect is deaminase-independent.

The cytidine deaminase APOBEC3A regulates nucleolar function to promote cell growth and ribosome biogenesis

Cancer initiates as a consequence of genomic mutations and its subsequent progression relies in part on increased production of ribosomes to maintain high levels of protein synthesis for unchecked cell growth. Recently, cytidine deaminases have been uncovered as sources of mutagenesis in cancer. In an attempt to form a connection between these 2 cancer driving processes, we interrogated the cytidine deaminase family of proteins for potential roles in human ribosome biogenesis. We identified and validated APOBEC3A and APOBEC4 as novel ribosome biogenesis factors through our laboratory's established screening platform for the discovery of regulators of nucleolar function in MCF10A cells. Through siRNA depletion experiments, we highlight APOBEC3A's requirement in making ribosomes and specific role within the processing and maturation steps that form the large subunit 5.8S and 28S ribosomal (r)RNAs. We demonstrate that a subset of APOBEC3A resides within the nucleolus and associates with critical ribosome biogenesis factors. Mechanistic insight was revealed by transient overexpression of both wild-type and a catalytically dead mutated APOBEC3A, which both increase cell growth and protein synthesis. Through an innovative nuclear RNA sequencing methodology, we identify only modest predicted APOBEC3A C-to-U target sites on the pre-rRNA and pre-mRNAs. Our work reveals a potential direct role for APOBEC3A in ribosome biogenesis likely independent of its editing function. More broadly, we found an additional function of APOBEC3A in cancer pathology through its function in ribosome biogenesis, expanding its relevance as a target for cancer therapeutics.

Navigating the landscape of epitranscriptomics and host immunity

RNA modifications, also termed epitranscriptomic marks, encompass chemical alterations to individual nucleotides, including processes such as methylation and editing. These marks contribute to a wide range of biological processes, many of which are related to host immune system defense. The functions of immune-related RNA modifications can be categorized into three main groups: regulation of immunogenic RNAs, control of genes involved in innate immune response, and facilitation of adaptive immunity. Here, we provide an overview of recent research findings that elucidate the contributions of RNA modifications to each of these processes. We also discuss relevant methods for genome-wide identification of RNA modifications and their immunogenic substrates. Finally, we highlight recent advances in cancer immunotherapies that aim to reduce cancer cell viability by targeting the enzymes responsible for RNA modifications. Our presentation of these dynamic research avenues sets the stage for future investigations in this field.

Synthesis of 1,4-azaphosphinine nucleosides and evaluation as inhibitors of human cytidine deaminase and APOBEC3A

Nucleoside and polynucleotide cytidine deaminases (CDAs), such as CDA and APOBEC3, share a similar mechanism of cytosine to uracil conversion. In 1984, phosphapyrimidine riboside was characterised as the most potent inhibitor of human CDA, but the quick degradation in water limited the applicability as a potential therapeutic. To improve stability in water, we synthesised derivatives of phosphapyrimidine nucleoside having a CH2 group instead of the N3 atom in the nucleobase. A charge-neutral phosphinamide and a negatively charged phosphinic acid derivative had excellent stability in water at pH 7.4, but only the charge-neutral compound inhibited human CDA, similar to previously described 2'-deoxyzebularine (Ki = 8.0 ± 1.9 and 10.7 ± 0.5 µM, respectively). However, under basic conditions, the charge-neutral phosphinamide was unstable, which prevented the incorporation into DNA using conventional DNA chemistry. In contrast, the negatively charged phosphinic acid derivative was incorporated into DNA instead of the target 2'-deoxycytidine using an automated DNA synthesiser, but no inhibition of APOBEC3A was observed for modified DNAs. Although this shows that the negative charge is poorly accommodated in the active site of CDA and APOBEC3, the synthetic route reported here provides opportunities for the synthesis of other derivatives of phosphapyrimidine riboside for potential development of more potent CDA and APOBEC3 inhibitors.

Molecular mechanism for regulating APOBEC3G DNA editing function by the non-catalytic domain

APOBEC3G (A3G) belongs to the AID/APOBEC cytidine deaminase family and is essential for antiviral immunity. It contains two zinc-coordinated cytidine-deaminase (CD) domains. The N-terminal CD1 domain is non-catalytic but has a strong affinity for nucleic acids, whereas the C-terminal CD2 domain catalyzes C-to-U editing in single-stranded DNA. The interplay between the two domains in DNA binding and editing is not fully understood. Here, our studies on rhesus macaque A3G (rA3G) show that the DNA editing function in linear and hairpin loop DNA is greatly enhanced by AA or GA dinucleotide motifs present downstream (in the 3'-direction) but not upstream (in the 5'-direction) of the target-C editing sites. The effective distance between AA/GA and the target-C sites depends on the local DNA secondary structure. We present two co-crystal structures of rA3G bound to ssDNA containing AA and GA, revealing the contribution of the non-catalytic CD1 domain in capturing AA/GA DNA and explaining our biochemical observations. Our structural and biochemical findings elucidate the molecular mechanism underlying the cooperative function between the non-catalytic and the catalytic domains of A3G, which is critical for its antiviral role and its contribution to genome mutations in cancer.

Improvement of C-to-U RNA editing using an artificial MS2-APOBEC system

RNA cytidine deamination (C-to-U editing) has been achieved using the MS2-apolipoprotein B-editing catalytic polypeptide-like (APOBEC)1 editing system. Here, we fused the cytidine deaminase (CDA) enzymes APOBEC3A and APOBEC3G with the MS2 system and examined their RNA editing efficiencies in transfected HEK 293T cells. Given the single-stranded RNA preferences of APOBEC3A and APOBEC3G, we designed unconventional guide RNAs that induced a loop at the target sequence, allowing the target to form a single-stranded structure. Because APOBEC3A and APOBEC3G have different base preferences (5'-TC and 5'-CC, respectively), we introduced the D317W mutation into APOBEC3G to convert its base preference to that of APOBEC3A. Upon co-transfection with a guide RNA that induced the formation of a 14 nt loop on the target sequence, MS2-fused APOBEC3A and APOBEC3G showed high editing efficiency. While the D317W mutation of APOBEC3G led to a slight improvement in editing efficiency, the difference was not statistically significant. These findings indicate that APOBEC3A and APOBEC3G can induce C-to-U RNA editing when transfected with a loop guide RNA. Moreover, the editing efficiency of APOBEC3G can be enhanced by site-specific mutation to alter the base preference. Overall, our results demonstrate that the MS2 system can fuse and catalyze reactions with different enzymes, suggesting that it holds an even greater potential for RNA editing than is utilized currently.

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.

Alternative Splicing, RNA Editing, and the Current Limits of Next Generation Sequencing

The advent of next generation sequencing (NGS) has fostered a shift in basic analytic strategies of a gene expression analysis in diverse pathologies for the purposes of research, pharmacology, and personalized medicine. What was once highly focused research on individual signaling pathways or pathway members has, from the time of gene expression arrays, become a global analysis of gene expression that has aided in identifying novel pathway interactions, the discovery of new therapeutic targets, and the establishment of disease-associated profiles for assessing progression, stratification, or a therapeutic response. But there are significant caveats to this analysis that do not allow for the construction of the full picture. The lack of timely updates to publicly available databases and the "hit and miss" deposition of scientific data to these databases relegate a large amount of potentially important data to "garbage", begging the question, "how much are we really missing?" This brief perspective aims to highlight some of the limitations that RNA binding/modifying proteins and RNA processing impose on our current usage of NGS technologies as relating to cancer and how not fully appreciating the limitations of current NGS technology may negatively affect therapeutic strategies in the long run.

Genomic-transcriptomic evolution in lung cancer and metastasis

Intratumour heterogeneity (ITH) fuels lung cancer evolution, which leads to immune evasion and resistance to therapy1. Here, using paired whole-exome and RNA sequencing data, we investigate intratumour transcriptomic diversity in 354 non-small cell lung cancer tumours from 347 out of the first 421 patients prospectively recruited into the TRACERx study2,3. Analyses of 947 tumour regions, representing both primary and metastatic disease, alongside 96 tumour-adjacent normal tissue samples implicate the transcriptome as a major source of phenotypic variation. Gene expression levels and ITH relate to patterns of positive and negative selection during tumour evolution. We observe frequent copy number-independent allele-specific expression that is linked to epigenomic dysfunction. Allele-specific expression can also result in genomic-transcriptomic parallel evolution, which converges on cancer gene disruption. We extract signatures of RNA single-base substitutions and link their aetiology to the activity of the RNA-editing enzymes ADAR and APOBEC3A, thereby revealing otherwise undetected ongoing APOBEC activity in tumours. Characterizing the transcriptomes of primary-metastatic tumour pairs, we combine multiple machine-learning approaches that leverage genomic and transcriptomic variables to link metastasis-seeding potential to the evolutionary context of mutations and increased proliferation within primary tumour regions. These results highlight the interplay between the genome and transcriptome in influencing ITH, lung cancer evolution and metastasis.

Lactobacillus for ribosome peptide editing cancer

This study reviews newly discovered insect peptide point mutations as new possible cancer research targets. To interpret newly discovered peptide point mutations in insects as new possible cancer research targets, we focused on the numerous peptide changes found in the 'CSP' family on the sex pheromone gland of the female silkworm moth Bombyx mori. We predict that the Bombyx peptide modifications will have a significant effect on cancer CUP (cancers of unknown primary) therapy and that bacterial peptide editing techniques, specifically Lactobacillus combined to CRISPR, will be used to regulate ribosomes and treat cancer in humans.

Cellular APOBEC3A deaminase drives mutations in the SARS-CoV-2 genome

The number of genetic variations in the SARS-CoV-2 genome has been increasing primarily due to continuous viral mutations. Here, we report that the human APOBEC3A (A3A) cytidine deaminase plays a critical role in the induction of C-to-U substitutions in the SARS-CoV-2 genome. Bioinformatic analysis of the chronological genetic changes in a sequence database indicated that the largest UC-to-UU mutation signature, consistent with APOBEC-recognized nucleotide motifs, was predominant in single-stranded RNA regions of the viral genome. In SARS-CoV-2-infected cells, exogenous expression of A3A but not expression of other APOBEC proteins induced UC-to-UU mutations in viral RNA (vRNA). Additionally, the mutated C bases were often located at the tips in bulge or loop regions in the vRNA secondary structure. Interestingly, A3A mRNA expression was drastically increased by interferons (IFNs) and tumour necrosis factor-α (TNF-α) in epithelial cells derived from the respiratory system, a site of efficient SARS-CoV-2 replication. Moreover, the UC-to-UU mutation rate was increased in SARS-CoV-2 produced from lung epithelial cells treated with IFN-ß and TNF-α, but not from CRISPR/Cas9-based A3A knockout cells. Collectively, these findings demonstrate that A3A is a primary host factor that drives mutations in the SARS-CoV-2 RNA genome via RNA editing.

Viral genome sequence datasets display pervasive evidence of strand-specific substitution biases that are best described using non-reversible nucleotide substitution models

Background

The vast majority of phylogenetic trees are inferred from molecular sequence data (nucleotides or amino acids) using time-reversible evolutionary models which assume that, for any pair of nucleotide or amino acid characters, the relative rate of X to Y substitution is the same as the relative rate of Y to X substitution. However, this reversibility assumption is unlikely to accurately reflect the actual underlying biochemical and/or evolutionary processes that lead to the fixation of substitutions. Here, we use empirical viral genome sequence data to reveal that evolutionary non-reversibility is pervasive among most groups of viruses. Specifically, we consider two non-reversible nucleotide substitution models: (1) a 6-rate non-reversible model (NREV6) in which Watson-Crick complementary substitutions occur at identical relative rates and which might therefor be most applicable to analyzing the evolution of genomes where both complementary strands are subject to the same mutational processes (such as might be expected for double-stranded (ds) RNA or dsDNA genomes); and (2) a 12-rate non-reversible model (NREV12) in which all relative substitution types are free to occur at different rates and which might therefore be applicable to analyzing the evolution of genomes where the complementary genome strands are subject to different mutational processes (such as might be expected for viruses with single-stranded (ss) RNA or ssDNA genomes).

Results

Using likelihood ratio and Akaike Information Criterion-based model tests, we show that, surprisingly, NREV12 provided a significantly better fit to 21/31 dsRNA and 20/30 dsDNA datasets than did the general time reversible (GTR) and NREV6 models with NREV6 providing a better fit than NREV12 and GTR in only 5/30 dsDNA and 2/31 dsRNA datasets. As expected, NREV12 provided a significantly better fit to 24/33 ssDNA and 40/47 ssRNA datasets. Next, we used simulations to show that increasing degrees of strand-specific substitution bias decrease the accuracy of phylogenetic inference irrespective of whether GTR or NREV12 is used to describe mutational processes. However, in cases where strand-specific substitution biases are extreme (such as in SARS-CoV-2 and Torque teno sus virus datasets) NREV12 tends to yield more accurate phylogenetic trees than those obtained using GTR.

Conclusion

We show that NREV12 should, be seriously considered during the model selection phase of phylogenetic analyses involving viral genomic sequences.

Structural basis of sequence-specific RNA recognition by the antiviral factor APOBEC3G

An essential step in restricting HIV infectivity by the antiviral factor APOBEC3G is its incorporation into progeny virions via binding to HIV RNA. However, the mechanism of APOBEC3G capturing viral RNA is unknown. Here, we report crystal structures of a primate APOBEC3G bound to different types of RNAs, revealing that APOBEC3G specifically recognizes unpaired 5'-AA-3' dinucleotides, and to a lesser extent, 5'-GA-3' dinucleotides. APOBEC3G binds to the common 3'A in the AA/GA motifs using an aromatic/hydrophobic pocket in the non-catalytic domain. It binds to the 5'A or 5'G in the AA/GA motifs using an aromatic/hydrophobic groove conformed between the non-catalytic and catalytic domains. APOBEC3G RNA binding property is distinct from that of the HIV nucleocapsid protein recognizing unpaired guanosines. Our findings suggest that the sequence-specific RNA recognition is critical for APOBEC3G virion packaging and restricting HIV infectivity.

Competition for DNA binding between the genome protector replication protein A and the genome modifying APOBEC3 single-stranded DNA deaminases

The human APOBEC family of eleven cytosine deaminases use RNA and single-stranded DNA (ssDNA) as substrates to deaminate cytosine to uracil. This deamination event has roles in lipid metabolism by altering mRNA coding, adaptive immunity by causing evolution of antibody genes, and innate immunity through inactivation of viral genomes. These benefits come at a cost where some family members, primarily from the APOBEC3 subfamily (APOBEC3A-H, excluding E), can cause off-target deaminations of cytosine to form uracil on transiently single-stranded genomic DNA, which induces mutations that are associated with cancer evolution. Since uracil is only promutagenic, the mutations observed in cancer genomes originate only when uracil is not removed by uracil DNA glycosylase (UNG) or when the UNG-induced abasic site is erroneously repaired. However, when ssDNA is present, replication protein A (RPA) binds and protects the DNA from nucleases or recruits DNA repair proteins, such as UNG. Thus, APOBEC enzymes must compete with RPA to access their substrate. Certain APOBEC enzymes can displace RPA, bind and scan ssDNA efficiently to search for cytosines, and can become highly overexpressed in tumor cells. Depending on the DNA replication conditions and DNA structure, RPA can either be in excess or deficient. Here we discuss the interplay between these factors and how despite RPA, multiple cancer genomes have a mutation bias at cytosines indicative of APOBEC activity.

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.

C-to-U RNA Editing: A Site Directed RNA Editing Tool for Restoration of Genetic Code

The restoration of genetic code by editing mutated genes is a potential method for the treatment of genetic diseases/disorders. Genetic disorders are caused by the point mutations of thymine (T) to cytidine (C) or guanosine (G) to adenine (A), for which gene editing (editing of mutated genes) is a promising therapeutic technique. In C-to-Uridine (U) RNA editing, it converts the base C-to-U in RNA molecules and leads to nonsynonymous changes when occurring in coding regions; however, for G-to-A mutations, A-to-I editing occurs. Editing of C-to-U is not as physiologically common as that of A-to-I editing. Although hundreds to thousands of coding sites have been found to be C-to-U edited or editable in humans, the biological significance of this phenomenon remains elusive. In this review, we have tried to provide detailed information on physiological and artificial approaches for C-to-U RNA editing.

Compositional biases in RNA viruses: Causes, consequences and applications

If each of the four nucleotides were represented equally in the genomes of viruses and the hosts they infect, each base would occur at a frequency of 25%. However, this is not observed in nature. Similarly, the order of nucleotides is not random (e.g., in the human genome, guanine follows cytosine at a frequency of ~0.0125, or a quarter the number of times predicted by random representation). Codon usage and codon order are also nonrandom. Furthermore, nucleotide and codon biases vary between species. Such biases have various drivers, including cellular proteins that recognize specific patterns in nucleic acids, that once triggered, induce mutations or invoke intrinsic or innate immune responses. In this review we examine the types of compositional biases identified in viral genomes and current understanding of the evolutionary mechanisms underpinning these trends. Finally, we consider the potential for large scale synonymous recoding strategies to engineer RNA virus vaccines, including those with pandemic potential, such as influenza A virus and Severe Acute Respiratory Syndrome Coronavirus Virus 2. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Evolution and Genomics > Computational Analyses of RNA RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition.

A SARS-CoV-2 Delta vVariant cContaining mMutation in the pProbe bBinding rRegion uUsed for RT-qPCR tTest in Japan eExhibited aAtypical PCR aAmplification and mMight iInduce fFalse nNegative rResult

Introduction

A recent pandemic of SARS-CoV-2 infection has caused severe health problems and substantially restricted social and economic activities. RT-qPCR plays a vital role in the diagnosis of SARS-CoV-2 infection. The N protein-coding region is widely analyzed in RT-qPCR to diagnose SARS-CoV-2 infection in Japan. We recently encountered two cases of SARS-CoV-2-positive specimens showing atypical amplification curves in the RT-qPCR.

Methods

We performed whole-genome sequencing of 63 samples (2 showing aberrant RT-qPCR curve and 61 samples infected with SARS-CoV-2 simultaneously in the same area) followed by Phylogenetic tree analysis.

Results

We found that the viruses showing abnormal RT-qPCR curves were Delta-type variants of SARS-CoV-2 with a single nucleotide mutation in the probe-binding site. There were no other cases with the same mutation, indicating that the variant had not spread in the area. After searching the database, hundreds of variants were reported globally, and one in Japan contained the same mutation. Phylogenetic analysis showed that the variant was very close to other Delta variants endemic in Japan but quite far from the variants containing the same mutation reported from outside Japan, suggesting sporadic generation of mutant in some domestic areas.

Conclusions

These findings propose two key points: i) mutations in the region used for SARS-CoV-2 RT-qPCR can cause abnormal amplification curves, and ii) various mutations can be generated sporadically and unpredictably; therefore, efficient and robust screening systems are needed to promptly monitor the emergence of de novo variants.

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

The optimal pH of AID is skewed from that of its catalytic pocket by DNA-binding residues and surface charge

Activation-induced cytidine deaminase (AID) is a member of the apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like (APOBEC) family of cytidine deaminases. AID mutates immunoglobulin loci to initiate secondary antibody diversification. The APOBEC3 (A3) sub-branch mutates viral pathogens in the cytosol and acidic endosomal compartments. Accordingly, AID functions optimally near neutral pH, while most A3s are acid-adapted (optimal pH 5.5-6.5). To gain a structural understanding for this pH disparity, we constructed high-resolution maps of AID catalytic activity vs pH. We found AID's optimal pH was 7.3 but it retained most (>70%) of the activity at pH 8. Probing of ssDNA-binding residues near the catalytic pocket, key for bending ssDNA into the pocket (e.g R25) yielded mutants with altered pH preference, corroborating previous findings that the equivalent residue in APOBEC3G (H216) underlies its acidic pH preference. AID from bony fish exhibited more basic optimal pH (pH 7.5-8.1) and several R25-equivalent mutants altered pH preference. Comparison of pH optima across the AID/APOBEC3 family revealed an inverse correlation between positive surface charge and overall catalysis.  The paralogue with the most robust catalytic activity (APOBEC3A) has the lowest surface charge, most acidic pH preference, while the paralogue with the most lethargic catalytic rate (AID) has the most positive surface charge and highest optimal pH. We suggest one possible mechanism is through surface charge dictating an overall optimal pH that is different from the optimal pH of the catalytic pocket microenvironment. These findings illuminate an additional structural mechanism that regulates AID/APOBEC3 mutagenesis.

APOBEC3C, a nucleolar protein induced by genotoxins, is excluded from DNA damage sites

The human genome contains 11 APOBEC (apolipoprotein B mRNA editing catalytic polypeptide‐like) cytidine deaminases classified into four families. These proteins function mainly in innate antiviral immunity and can also restrict endogenous retrotransposable element multiplication. The present study focuses on APOBEC3C (A3C), a member of the APOBEC3 subfamily. Some APOBEC3 proteins use their enzymatic activity on genomic DNA, inducing mutations and DNA damage, while other members facilitate DNA repair. Our results show that A3C is highly expressed in cells treated with DNA‐damaging agents. Its expression is regulated by p53. Depletion of A3C slightly decreases proliferation and does not affect DNA repair via homologous recombination or nonhomologous end joining. The A3C interactomes obtained from control cells and cells exposed to the genotoxin etoposide indicated that A3C is a nucleolar protein. This was confirmed by the detection of either endogenous or ectopic A3C in nucleoli. Interestingly, we show that A3C is excluded from areas of DNA breaks in live cells. Our data also indicate that the C‐terminal part of A3C is responsible for its nucleolar localization and exclusion from DNA damage sites.

Evolutionary Signatures Governing the Codon Usage Bias in Coronaviruses and Their Implications for Viruses Infecting Various Bat Species

Many viruses that cause serious diseases in humans and animals, including the betacoronaviruses (beta-CoVs), such as SARS-CoV, MERS-CoV, and the recently identified SARS-CoV-2, have natural reservoirs in bats. Because these viruses rely entirely on the host cellular machinery for survival, their evolution is likely to be guided by the link between the codon usage of the virus and that of its host. As a result, specific cellular microenvironments of the diverse hosts and/or host tissues imprint peculiar molecular signatures in virus genomes. Our study is aimed at deciphering some of these signatures. Using a variety of genetic methods we demonstrated that trends in codon usage across chiroptera-hosted CoVs are collaboratively driven by geographically different host-species and temporal-spatial distribution. We not only found that chiroptera-hosted CoVs are the ancestors of SARS-CoV-2, but we also revealed that SARS-CoV-2 has the codon usage characteristics similar to those seen in CoVs infecting the Rhinolophus sp. Surprisingly, the envelope gene of beta-CoVs infecting Rhinolophus sp., including SARS-CoV-2, had extremely high CpG levels, which appears to be an evolutionarily conserved trait. The dissection of the furin cleavage site of various CoVs infecting hosts revealed host-specific preferences for arginine codons; however, arginine is encoded by a wider variety of synonymous codons in the murine CoV (MHV-A59) furin cleavage site. Our findings also highlight the latent diversity of CoVs in mammals that has yet to be fully explored.

APOBECs orchestrate genomic and epigenomic editing across health and disease

APOBEC proteins can deaminate cytosine residues in DNA and RNA. This can lead to somatic mutations, DNA breaks, RNA modifications, or DNA demethylation in a selective manner. APOBECs function in various cellular compartments and recognize different nucleic acid motifs and structures. They orchestrate a wide array of genomic and epigenomic modifications, thereby affecting various cellular functions positively or negatively, including immune editing, viral and retroelement restriction, DNA damage responses, DNA demethylation, gene expression, and tissue homeostasis. Furthermore, the cumulative increase in genomic and epigenomic editing with aging could also, at least in part, be attributed to APOBEC function. We synthesize our cumulative understanding of APOBEC activity in a unifying overview and discuss their genomic and epigenomic impact in physiological, pathological, and technological contexts.

Replication catastrophe induced by cyclic hypoxia leads to increased APOBEC3B activity

Tumor heterogeneity includes variable and fluctuating oxygen concentrations, which result in the accumulation of hypoxic regions in most solid tumors. Tumor hypoxia leads to increased therapy resistance and has been linked to genomic instability. Here, we tested the hypothesis that exposure to levels of hypoxia that cause replication stress could increase APOBEC activity and the accumulation of APOBEC-mediated mutations. APOBEC-dependent mutational signatures have been well-characterized, although the physiological conditions which underpin them have not been described. We demonstrate that fluctuating/cyclic hypoxic conditions which lead to replication catastrophe induce the expression and activity of APOBEC3B. In contrast, stable/chronic hypoxic conditions which induce replication stress in the absence of DNA damage are not sufficient to induce APOBEC3B. Most importantly, the number of APOBEC-mediated mutations in patient tumors correlated with a hypoxia signature. Together, our data support the conclusion that hypoxia-induced replication catastrophe drives genomic instability in tumors, specifically through increasing the activity of APOBEC3B.

A mark of disease: how mRNA modifications shape genetic and acquired pathologies

RNA modifications have recently emerged as a widespread and complex facet of gene expression regulation. Counting more than 170 distinct chemical modifications with far-reaching implications for RNA fate, they are collectively referred to as the epitranscriptome. These modifications can occur in all RNA species, including messenger RNAs (mRNAs) and noncoding RNAs (ncRNAs). In mRNAs the deposition, removal, and recognition of chemical marks by writers, erasers and readers influence their structure, localization, stability, and translation. In turn, this modulates key molecular and cellular processes such as RNA metabolism, cell cycle, apoptosis, and others. Unsurprisingly, given their relevance for cellular and organismal functions, alterations of epitranscriptomic marks have been observed in a broad range of human diseases, including cancer, neurological and metabolic disorders. Here, we will review the major types of mRNA modifications and editing processes in conjunction with the enzymes involved in their metabolism and describe their impact on human diseases. We present the current knowledge in an updated catalog. We will also discuss the emerging evidence on the crosstalk of epitranscriptomic marks and what this interplay could imply for the dynamics of mRNA modifications. Understanding how this complex regulatory layer can affect the course of human pathologies will ultimately lead to its exploitation toward novel epitranscriptomic therapeutic strategies.

The effects of RNA editing in cancer tissue at different stages in carcinogenesis

RNA editing is one of the most prevalent and abundant forms of post-transcriptional RNA modification observed in normal physiological processes and often aberrant in diseases including cancer. RNA editing changes the sequences of mRNAs, making them different from the source DNA sequence. Edited mRNAs can produce editing-recoded protein isoforms that are functionally different from the corresponding genome-encoded protein isoforms. The major type of RNA editing in mammals occurs by enzymatic deamination of adenosine to inosine (A-to-I) within double-stranded RNAs (dsRNAs) or hairpins in pre-mRNA transcripts. Enzymes that catalyse these processes belong to the adenosine deaminase acting on RNA (ADAR) family. The vast majority of knowledge on the RNA editing landscape relevant to human disease has been acquired using in vitro cancer cell culture models. The limitation of such in vitro models, however, is that the physiological or disease relevance of results obtained is not necessarily obvious. In this review we focus on discussing in vivo occurring RNA editing events that have been identified in human cancer tissue using samples surgically resected or clinically retrieved from patients. We discuss how RNA editing events occurring in tumours in vivo can identify pathological signalling mechanisms relevant to human cancer physiology which is linked to the different stages of cancer progression including initiation, promotion, survival, proliferation, immune escape and metastasis.

RNA editing enzyme APOBEC3A promotes pro-inflammatory M1 macrophage polarization

Pro-inflammatory M1 macrophage polarization is associated with microbicidal and antitumor responses. We recently described APOBEC3A-mediated cytosine-to-uracil (C > U) RNA editing during M1 polarization. However, the functional significance of this editing is unknown. Here we find that APOBEC3A-mediated cellular RNA editing can also be induced by influenza or Maraba virus infections in normal human macrophages, and by interferons in tumor-associated macrophages. Gene knockdown and RNA_Seq analyses show that APOBEC3A mediates C>U RNA editing of 209 exonic/UTR sites in 203 genes during M1 polarization. The highest level of nonsynonymous RNA editing alters a highly-conserved amino acid in THOC5, which encodes a nuclear mRNA export protein implicated in M-CSF-driven macrophage differentiation. Knockdown of APOBEC3A reduces IL6, IL23A and IL12B gene expression, CD86 surface protein expression, and TNF-α, IL-1β and IL-6 cytokine secretion, and increases glycolysis. These results show a key role of APOBEC3A cytidine deaminase in transcriptomic and functional polarization of M1 macrophages.

Potential APOBEC-mediated RNA editing of the genomes of SARS-CoV-2 and other coronaviruses and its impact on their longer term evolution

Members of the APOBEC family of cytidine deaminases show antiviral activities in mammalian cells through lethal editing in the genomes of small DNA viruses, herpesviruses and retroviruses, and potentially those of RNA viruses such as coronaviruses. Consistent with the latter, APOBEC-like directional C→U transitions of genomic plus-strand RNA are greatly overrepresented in SARS-CoV-2 genome sequences of variants emerging during the COVID-19 pandemic. A C→U mutational process may leave evolutionary imprints on coronavirus genomes, including extensive homoplasy from editing and reversion at targeted sites and the occurrence of driven amino acid sequence changes in viral proteins. If sustained over longer periods, this process may account for the previously reported marked global depletion of C and excess of U bases in human seasonal coronavirus genomes. This review synthesizes the current knowledge on APOBEC evolution and function and the evidence of their role in APOBEC-mediated genome editing of SARS-CoV-2 and other coronaviruses.

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SARS-CoV-2 sequence variants contain an overabundance of C- > U transitions

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C- > U transitions are the hallmark of the activity of APOBEC cytosine deaminases

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Further work is needed to determine APOBEC's role in coronavirus evolution

Coronavirus genomes carry the signatures of their habitats

Coronaviruses such as SARS-CoV-2 regularly infect host tissues that express antiviral proteins (AVPs) in abundance. Understanding how they evolve to adapt or evade host immune responses is important in the effort to control the spread of infection. Two AVPs that may shape viral genomes are the zinc finger antiviral protein (ZAP) and the apolipoprotein B mRNA editing enzyme-catalytic polypeptide-like 3 (APOBEC3). The former binds to CpG dinucleotides to facilitate the degradation of viral transcripts while the latter frequently deaminates C into U residues which could generate notable viral sequence variations. We tested the hypothesis that both APOBEC3 and ZAP impose selective pressures that shape the genome of an infecting coronavirus. Our investigation considered a comprehensive number of publicly available genomes for seven coronaviruses (SARS-CoV-2, SARS-CoV, and MERS infecting Homo sapiens, Bovine CoV infecting Bos taurus, MHV infecting Mus musculus, HEV infecting Sus scrofa, and CRCoV infecting Canis lupus familiaris). We show that coronaviruses that regularly infect tissues with abundant AVPs have CpG-deficient and U-rich genomes; whereas those that do not infect tissues with abundant AVPs do not share these sequence hallmarks. Among the coronaviruses surveyed herein, CpG is most deficient in SARS-CoV-2 and a temporal analysis showed a marked increase in C to U mutations over four months of SARS-CoV-2 genome evolution. Furthermore, the preferred motifs in which these C to U mutations occur are the same as those subjected to APOBEC3 editing in HIV-1. These results suggest that both ZAP and APOBEC3 shape the SARS-CoV-2 genome: ZAP imposes a strong CpG avoidance, and APOBEC3 constantly edits C to U. Evolutionary pressures exerted by host immune systems onto viral genomes may motivate novel strategies for SARS-CoV-2 vaccine development.

The interesting relationship between APOBEC3 deoxycytidine deaminases and cancer: a long road ahead

Cancer is considered a group of diseases characterized by uncontrolled growth and spread of abnormal cells and is propelled by somatic mutations. Apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3) family of enzymes are endogenous sources of somatic mutations found in multiple human cancers. While these enzymes normally act as an intrinsic immune defence against viruses, they can also catalyse 'off-target' cytidine deamination in genomic single-stranded DNA intermediates. The deamination of cytosine forms uracil, which is promutagenic in DNA. Key factors to trigger the APOBEC 'off-target' activity are overexpression in a non-normal cell type, nuclear localization and replication stress. The resulting uracil-induced mutations contribute to genomic variation, which may result in neutral, beneficial or harmful consequences for the cancer. This review summarizes the functional and biochemical basis of the APOBEC3 enzyme activity and highlights their relationship with the most well-studied cancers in this particular context such as breast, lung, bladder, and human papillomavirus-associated cancers. We focus on APOBEC3A, APOBEC3B and APOBEC3H haplotype I because they are the leading candidates as sources of somatic mutations in these and other cancers. Also, we discuss the prognostic value of the APOBEC3 expression in drug resistance and response to therapies.

Programmable C-to-U RNA editing using the human APOBEC3A deaminase

Programmable RNA cytidine deamination has recently been achieved using a bifunctional editor (RESCUE-S) capable of deaminating both adenine and cysteine. Here, we report the development of "CURE", the first cytidine-specific C-to-U RNA Editor. CURE comprises the cytidine deaminase enzyme APOBEC3A fused to dCas13 and acts in conjunction with unconventional guide RNAs (gRNAs) designed to induce loops at the target sites. Importantly, CURE does not deaminate adenosine, enabling the high-specificity versions of CURE to create fewer missense mutations than RESCUE-S at the off-targets transcriptome-wide. The two editing approaches exhibit overlapping editing motif preferences, with CURE and RESCUE-S being uniquely able to edit UCC and AC motifs, respectively, while they outperform each other at different subsets of the UC targets. Finally, a nuclear-localized version of CURE, but not that of RESCUE-S, can efficiently edit nuclear RNAs. Thus, CURE and RESCUE are distinct in design and complementary in utility.

Point mutation bias in SARS-CoV-2 variants results in increased ability to stimulate inflammatory responses

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection induces severe pneumonia and is the cause of a worldwide pandemic. Coronaviruses, including SARS-CoV-2, have RNA proofreading enzymes in their genomes, resulting in fewer gene mutations than other RNA viruses. Nevertheless, variants of SARS-CoV-2 exist and may induce different symptoms; however, the factors and the impacts of these mutations are not well understood. We found that there is a bias to the mutations occurring in SARS-CoV-2 variants, with disproportionate mutation to uracil (U). These point mutations to U are mainly derived from cytosine (C), which is consistent with the substrate specificity of host RNA editing enzymes, APOBECs. We also found the point mutations which are consistent with other RNA editing enzymes, ADARs. For the C-to-U mutations, the context of the upstream uracil and downstream guanine from mutated position was found to be most prevalent. Further, the degree of increase of U in SARS-CoV-2 variants correlates with enhanced production of cytokines, such as TNF-α and IL-6, in cell lines when compared with stimulation by the ssRNA sequence of the isolated virus in Wuhan. Therefore, RNA editing is a factor for mutation bias in SARS-CoV-2 variants, which affects host inflammatory cytokines production.

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.

Viral CpG Deficiency Provides No Evidence That Dogs Were Intermediate Hosts for SARS-CoV-2

Due to the scope and impact of the COVID-19 pandemic there exists a strong desire to understand where the SARS-CoV-2 virus came from and how it jumped species boundaries to humans. Molecular evolutionary analyses can trace viral origins by establishing relatedness and divergence times of viruses and identifying past selective pressures. However, we must uphold rigorous standards of inference and interpretation on this topic because of the ramifications of being wrong. Here, we dispute the conclusions of Xia (2020. Extreme genomic CpG deficiency in SARS-CoV-2 and evasion of host antiviral defense. Mol Biol Evol. doi:10.1093/molbev/masa095) that dogs are a likely intermediate host of a SARS-CoV-2 ancestor. We highlight major flaws in Xia's inference process and his analysis of CpG deficiencies, and conclude that there is no direct evidence for the role of dogs as intermediate hosts. Bats and pangolins currently have the greatest support as ancestral hosts of SARS-CoV-2, with the strong caveat that sampling of wildlife species for coronaviruses has been limited.

Extreme Genomic CpG Deficiency in SARS-CoV-2 and Evasion of Host Antiviral Defense

Wild mammalian species, including bats, constitute the natural reservoir of betacoronavirus (including SARS, MERS, and the deadly SARS-CoV-2). Different hosts or host tissues provide different cellular environments, especially different antiviral and RNA modification activities that can alter RNA modification signatures observed in the viral RNA genome. The zinc finger antiviral protein (ZAP) binds specifically to CpG dinucleotides and recruits other proteins to degrade a variety of viral RNA genomes. Many mammalian RNA viruses have evolved CpG deficiency. Increasing CpG dinucleotides in these low-CpG viral genomes in the presence of ZAP consistently leads to decreased viral replication and virulence. Because ZAP exhibits tissue-specific expression, viruses infecting different tissues are expected to have different CpG signatures, suggesting a means to identify viral tissue-switching events. The author shows that SARS-CoV-2 has the most extreme CpG deficiency in all known betacoronavirus genomes. This suggests that SARS-CoV-2 may have evolved in a new host (or new host tissue) with high ZAP expression. A survey of CpG deficiency in viral genomes identified a virulent canine coronavirus (alphacoronavirus) as possessing the most extreme CpG deficiency, comparable with that observed in SARS-CoV-2. This suggests that the canine tissue infected by the canine coronavirus may provide a cellular environment strongly selecting against CpG. Thus, viral surveys focused on decreasing CpG in viral RNA genomes may provide important clues about the selective environments and viral defenses in the original hosts.

Single C-to-T substitution using engineered APOBEC3G-nCas9 base editors with minimum genome- and transcriptome-wide off-target effects

Cytosine base editors (CBEs) enable efficient cytidine-to-thymidine (C-to-T) substitutions at targeted loci without double-stranded breaks. However, current CBEs edit all Cs within their activity windows, generating undesired bystander mutations. In the most challenging circumstance, when a bystander C is adjacent to the targeted C, existing base editors fail to discriminate them and edit both Cs. To improve the precision of CBE, we identified and engineered the human APOBEC3G (A3G) deaminase; when fused to the Cas9 nickase, the resulting A3G-BEs exhibit selective editing of the second C in the 5'-CC-3' motif in human cells. Our A3G-BEs could install a single disease-associated C-to-T substitution with high precision. The percentage of perfectly modified alleles is more than 6000-fold for disease correction and more than 600-fold for disease modeling compared with BE4max. On the basis of the two-cell embryo injection method and RNA sequencing analysis, our A3G-BEs showed minimum genome- and transcriptome-wide off-target effects, achieving high targeting fidelity.

Rampant C→U Hypermutation in the Genomes of SARS-CoV-2 and Other Coronaviruses: Causes and Consequences for Their Short- and Long-Term Evolutionary Trajectories

The pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has motivated an intensive analysis of its molecular epidemiology following its worldwide spread. To understand the early evolutionary events following its emergence, a data set of 985 complete SARS-CoV-2 sequences was assembled. Variants showed a mean of 5.5 to 9.5 nucleotide differences from each other, consistent with a midrange coronavirus substitution rate of 3 × 10-4 substitutions/site/year. Almost one-half of sequence changes were C→U transitions, with an 8-fold base frequency normalized directional asymmetry between C→U and U→C substitutions. Elevated ratios were observed in other recently emerged coronaviruses (SARS-CoV, Middle East respiratory syndrome [MERS]-CoV), and decreasing ratios were observed in other human coronaviruses (HCoV-NL63, -OC43, -229E, and -HKU1) proportionate to their increasing divergence. C→U transitions underpinned almost one-half of the amino acid differences between SARS-CoV-2 variants and occurred preferentially in both 5' U/A and 3' U/A flanking sequence contexts comparable to favored motifs of human APOBEC3 proteins. Marked base asymmetries observed in nonpandemic human coronaviruses (U ≫ A > G ≫ C) and low G+C contents may represent long-term effects of prolonged C→U hypermutation in their hosts. The evidence that much of sequence change in SARS-CoV-2 and other coronaviruses may be driven by a host APOBEC-like editing process has profound implications for understanding their short- and long-term evolution. Repeated cycles of mutation and reversion in favored mutational hot spots and the widespread occurrence of amino acid changes with no adaptive value for the virus represent a quite different paradigm of virus sequence change from neutral and Darwinian evolutionary frameworks and are not incorporated by standard models used in molecular epidemiology investigations.IMPORTANCE The wealth of accurately curated sequence data for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), its long genome, and its low substitution rate provides a relatively blank canvas with which to investigate effects of mutational and editing processes imposed by the host cell. The finding that a large proportion of sequence change in SARS-CoV-2 in the initial months of the pandemic comprised C→U mutations in a host APOBEC-like context provides evidence for a potent host-driven antiviral editing mechanism against coronaviruses more often associated with antiretroviral defense. In evolutionary terms, the contribution of biased, convergent, and context-dependent mutations to sequence change in SARS-CoV-2 is substantial, and these processes are not incorporated by standard models used in molecular epidemiology investigations.

Integrative analyses of the RNA modification machinery reveal tissue- and cancer-specific signatures

BACKGROUND

RNA modifications play central roles in cellular fate and differentiation. However, the machinery responsible for placing, removing, and recognizing more than 170 RNA modifications remains largely uncharacterized and poorly annotated, and we currently lack integrative studies that identify which RNA modification-related proteins (RMPs) may be dysregulated in each cancer type.

RESULTS

Here, we perform a comprehensive annotation and evolutionary analysis of human RMPs, as well as an integrative analysis of their expression patterns across 32 tissues, 10 species, and 13,358 paired tumor-normal human samples. Our analysis reveals an unanticipated heterogeneity of RMP expression patterns across mammalian tissues, with a vast proportion of duplicated enzymes displaying testis-specific expression, suggesting a key role for RNA modifications in sperm formation and possibly intergenerational inheritance. We uncover many RMPs that are dysregulated in various types of cancer, and whose expression levels are predictive of cancer progression. Surprisingly, we find that several commonly studied RNA modification enzymes such as METTL3 or FTO are not significantly upregulated in most cancer types, whereas several less-characterized RMPs, such as LAGE3 and HENMT1, are dysregulated in many cancers.

CONCLUSIONS

Our analyses reveal an unanticipated heterogeneity in the expression patterns of RMPs across mammalian tissues and uncover a large proportion of dysregulated RMPs in multiple cancer types. We provide novel targets for future cancer research studies targeting the human epitranscriptome, as well as foundations to understand cell type-specific behaviors that are orchestrated by RNA modifications.

RNA Editing Alters miRNA Function in Chronic Lymphocytic Leukemia

Chronic lymphocytic leukemia (CLL) is a high incidence B cell leukemia with a highly variable clinical course, leading to survival times ranging from months to several decades. MicroRNAs (miRNAs) are small non-coding RNAs that regulate the expression levels of genes by binding to the untranslated regions of transcripts. Although miRNAs have been previously shown to play a crucial role in CLL development, progression and treatment resistance, their further processing and diversification by RNA editing (specifically adenosine to inosine or cytosine to uracil deamination) has not been addressed so far. In this study, we analyzed next generation sequencing data to provide a detailed map of adenosine to inosine and cytosine to uracil changes in miRNAs from CLL and normal B cells. Our results reveal that in addition to a CLL-specific expression pattern, there is also specific RNA editing of many miRNAs, particularly miR-3157 and miR-6503, in CLL. Our data draw further light on how miRNAs and miRNA editing might be implicated in the pathogenesis of the disease.

Structural perspectives on HIV-1 Vif and APOBEC3 restriction factor interactions

Human immunodeficiency virus (HIV) is a retroviral pathogen that targets human immune cells such as CD4+ T cells, macrophages, and dendritic cells. The human apolipoprotein B mRNA- editing catalytic polypeptide 3 (APOBEC3 or A3) cytidine deaminases are a key class of intrinsic restriction factors that inhibit replication of HIV. When HIV-1 enters the cell, the immune system responds by inducing the activation of the A3 family proteins, which convert cytosines to uracils in single-stranded DNA replication intermediates, neutralizing the virus. HIV counteracts this intrinsic immune response by encoding a protein termed viral infectivity factor (Vif). Vif targets A3 to an E3 ubiquitin ligase complex for poly-ubiquitination and proteasomal degradation. Vif is unique in that it can recognize and counteract multiple A3 restriction factor substrates. Structural biology studies have provided significant insights into the overall architectures and functions of Vif and A3 proteins; however, a structure of the Vif-A3 complex has remained elusive. In this review, we summarize and reanalyze experimental data from recent structural, biochemical, and functional studies to provide key perspectives on the residues involved in Vif-A3 protein-protein interactions.