An I for an A: Dynamic Regulation of Adenosine Deamination-Mediated RNA Editing

A Targeted Deep Sequencing Method to Quantify Endogenous Retrovirus Gag Sequence Variants and Open Reading Frames Expressed in Nonobese Diabetic Mice

Endogenous retroviruses (ERVs) are involved in autoimmune diseases such as type 1 diabetes (T1D). ERV gene products homologous to murine leukemia retroviruses are expressed in the pancreatic islets of NOD mice, a model of T1D. One ERV gene, Gag, with partial or complete open reading frames (ORFs), is detected in the islets, and it contains many sequence variants. An amplicon deep sequencing analysis was established by targeting a conserved region within the Gag gene to compare NOD with T1D-resistant mice or different ages of prediabetic NOD mice. We observed that the numbers of different Gag variants and ORFs are linked to T1D susceptibility. More importantly, these numbers change during the course of diabetes development and can be quantified to calculate the levels of disease progression. Sequence alignment analysis led to identification of additional markers, including nucleotide mismatching and amino acid consensus at specific positions that can distinguish the early and late stages, before diabetes onset. Therefore, the expression of sequence variants and ORFs of ERV genes, particularly Gag, can be quantified as biomarkers to estimate T1D susceptibility and disease progression.

Adenosine deaminases that act on RNA, then and now

In this article, I recount my memories of key experiments that led to my entry into the RNA editing/modification field. I highlight initial observations made by the pioneers in the ADAR field, and how they fit into our current understanding of this family of enzymes. I discuss early mysteries that have now been solved, as well as those that still linger. Finally, I discuss important, outstanding questions and acknowledge my hope for the future of the RNA editing/modification field.

ADARs regulate cuticle collagen expression and promote survival to pathogen infection

BACKGROUND

In all organisms, the innate immune system defends against pathogens through basal expression of molecules that provide critical barriers to invasion and inducible expression of effectors that combat infection. The adenosine deaminase that act on RNA (ADAR) family of RNA-binding proteins has been reported to influence innate immunity in metazoans. However, studies on the susceptibility of ADAR mutant animals to infection are largely lacking.

RESULTS

Here, by analyzing adr-1 and adr-2 null mutants in well-established slow-killing assays, we find that both Caenorhabditis elegans ADARs are important for organismal survival to gram-negative and gram-positive bacteria, all of which are pathogenic to humans. Furthermore, our high-throughput sequencing and genetic analysis reveal that ADR-1 and ADR-2 function in the same pathway to regulate collagen expression. Consistent with this finding, our scanning electron microscopy studies indicate adr-1;adr-2 mutant animals also have altered cuticle morphology prior to pathogen exposure.

CONCLUSIONS

Our data uncover a critical role of the C. elegans ADAR family of RNA-binding proteins in promoting cuticular collagen expression, which represents a new post-transcriptional regulatory node that influences the extracellular matrix. In addition, we provide the first evidence that ADAR mutant animals have altered susceptibility to infection with several opportunistic human pathogens, suggesting a broader role of ADARs in altering physical barriers to infection to influence innate immunity.

Precision RNA base editing with engineered and endogenous effectors

RNA base editing refers to the rewriting of genetic information within an intact RNA molecule and serves various functions, such as evasion of the endogenous immune system and regulation of protein function. To achieve this, certain enzymes have been discovered in human cells that catalyze the conversion of one nucleobase into another. This natural process could be exploited to manipulate and recode any base in a target transcript. In contrast to DNA base editing, analogous changes introduced in RNA are not permanent or inheritable but rather allow reversible and doseable effects that appeal to various therapeutic applications. The current practice of RNA base editing involves the deamination of adenosines and cytidines, which are converted to inosines and uridines, respectively. In this Review, we summarize current site-directed RNA base-editing strategies and highlight recent achievements to improve editing efficiency, precision, codon-targeting scope and in vivo delivery into disease-relevant tissues. Besides engineered editing effectors, we focus on strategies to harness endogenous adenosine deaminases acting on RNA (ADAR) enzymes and discuss limitations and future perspectives to apply the tools in basic research and as a therapeutic modality. We expect the field to realize the first RNA base-editing drug soon, likely on a well-defined genetic disease. However, the long-term challenge will be to carve out the sweet spot of the technology where its unique ability is exploited to modulate signaling cues, metabolism or other clinically relevant processes in a safe and doseable manner.

Identification and analysis of microRNA editing events in recurrent bladder cancer based on RNA sequencing: MicroRNA editing level is a potential novel biomarker

Background: With the continued advancement of RNA-seq (RNA-sequencing), microRNA (miRNA) editing events have been demonstrated to play an important role in different malignancies. However, there is yet no description of the miRNA editing events in recurrent bladder cancer.

Objective: To identify and compare miRNA editing events in primary and recurrent bladder cancer, as well as to investigate the potential molecular mechanism and its impact on patient prognosis.

Methods: We examined the mRNA and miRNA transcriptomes of 12 recurrent bladder cancer cases and 13 primary bladder cancer cases. The differentially expressed mRNA sequences were analyzed. Furthermore, we identified the differentially expressed genes (DEGs) in recurrent bladder cancer. The Gene Ontology (GO) functional enrichment analyses on DEGs and gene set enrichment analysis were performed. The consensus molecular subtype (CMS) classification of bladder cancer was identified using the Consensus MIBC package in R (4.1.0); miRNA sequences were then further subjected to differentially expressed analysis and pathway enrichment analysis. MiRNA editing events were identified using miRge3.0. miRDB and TargetScanHuman were used to predict the downstream targets of specific differentially edited or expressed miRNAs. The expression levels of miR-154-5p and ADAR were validated by RT-qPCR. Finally, survival and co-expression studies were performed on the TCGA-BLCA cohort.

Results: First, the mRNA expression levels in recurrent bladder cancer changed significantly, supporting progression via related molecular signal pathways. Second, significantly altered miRNAs in recurrent bladder cancer were identified, with miR-154-5p showing the highest level of editing in recurrent bladder cancer and may up-regulate the expression levels of downstream targets HS3ST3A1, AQP9, MYLK, and RAB23. The survival analysis results of TCGA data revealed that highly expressed HS3ST3A1 and RAB23 exhibited poor prognosis. In addition, miR-154 editing events were found to be significant to CMS classification.

Conclusion: MiRNA editing in recurrent bladder cancer was detected and linked with poor patient prognosis, providing a reference for further uncovering the intricate molecular mechanism in recurrent bladder cancer. Therefore, inhibiting A-to-I editing of miRNA may be a viable target for bladder cancer treatment, allowing current treatment choices to be expanded and individualized.

RNA and neuronal function: the importance of post-transcriptional regulation

The brain represents an organ with a particularly high diversity of genes that undergo post-transcriptional gene regulation through multiple mechanisms that affect RNA metabolism and, consequently, brain function. This vast regulatory process in the brain allows for a tight spatiotemporal control over protein expression, a necessary factor due to the unique morphologies of neurons. The numerous mechanisms of post-transcriptional regulation or translational control of gene expression in the brain include alternative splicing, RNA editing, mRNA stability and transport. A large number of trans-elements such as RNA-binding proteins and micro RNAs bind to specific cis-elements on transcripts to dictate the fate of mRNAs including its stability, localization, activation and degradation. Several trans-elements are exemplary regulators of translation, employing multiple cofactors and regulatory machinery so as to influence mRNA fate. Networks of regulatory trans-elements exert control over key neuronal processes such as neurogenesis, synaptic transmission and plasticity. Perturbations in these networks may directly or indirectly cause neuropsychiatric and neurodegenerative disorders. We will be reviewing multiple mechanisms of gene regulation by trans-elements occurring specifically in neurons.

The E3 ubiquitin ligase SMURF2 stabilizes RNA editase ADAR1p110 and promotes its adenosine-to-inosine (A-to-I) editing function

Epitranscriptomic changes in RNA catalyzed by the RNA-editing enzyme ADAR1 play an essential role in the regulation of diverse molecular and cellular processes, both under physiological conditions and in disease states, including cancer. Yet, despite a growing body of evidence pointing to ADAR1 as a potential therapeutic target, the mechanisms regulating its cellular abundance and activity, particularly of its constitutively expressed and ubiquitous form, ADAR1p110, are poorly understood. Here, we report the HECT-type E3 ubiquitin ligase SMURF2 as a pivotal regulator of ADAR1p110. We show that SMURF2, which is primarily known to promote the ubiquitin-mediated degradation of its protein substrates, protects ADAR1p110 from proteolysis and promotes its A-to-I editase activity in human and mouse cells and tissues. ADAR1p110's interactome analysis performed in human cells also showed a positive influence of SMURF2 on the stability and function of ADAR1p110. Mechanistically, we found that SMURF2 directly binds, ubiquitinates and stabilizes ADAR1p110 in an E3 ubiquitin ligase-dependent manner, through ADAR1p110 ubiquitination at lysine-744 (K744). Mutation of this residue to arginine (K744R), which is also associated with several human disorders, including dyschromatosis symmetrica hereditaria (DSH) and some types of cancer, abolished SMURF2-mediated protection of ADAR1p110 from both proteasomal and lysosomal degradation and inactivated ADAR1p110-mediated RNA editing. Our findings reveal a novel mechanism underlying the regulation of ADAR1 in mammalian cells and suggest SMURF2 as a key cellular factor influencing the protein abundance, interactions and functions of ADAR1p110.

ADAR-Editing during Ostreid Herpesvirus 1 Infection in Crassostrea gigas: Facts and Limitations

Ostreid herpesvirus-1 (OsHV-1) RNAs are enzymatically modified by A-to-I conversions during the infection of Crassostrea gigas. The increase of ADAR1 expression and hyper-editing activity parallel to OsHV-1 RNAs suggests a functional connection between dsRNA editing and antiviral responses. We analyzed 87 RNA-seq data sets from immuno-primed, resistant, and susceptible oysters exposed to OsHV-1 to compare the ADAR hyper-editing levels on host and viral transcripts and trace hyper-editing on the oyster genes. Host RNAs were more hyper-edited than viral RNAs, despite the increased editing of viral RNAs in late infection phases. A set of genes, representing ∼0.5% of the oyster transcriptome and including several tripartite motif-containing sequences, were constantly hyper-edited. Conversely, we identified genes involved in antiviral response, miRNA maturation, and epigenetic regulation that were hyper-edited in specific conditions only. Despite technical and biological bottlenecks that hamper the understanding of the bivalve “RNA editome,” available tools and technologies can be adapted to bivalve mollusks.

IMPORTANCE Ostreid herpesvirus-1 (OsHV-1) is a harmful pathogen of bivalve species, such as oysters. However, knowledge is lacking about host–virus interactions at the molecular level, hampering the possibility of a correct management of viral outbreaks and related massive mortalities. Notably, OsHV-1 transcripts are massively modified by host RNA editing enzyme during infection, resulting in multiple A-to-I variations along RNAs assuming double-strand conformations. The impact of these modifications on host transcripts is, however, not completely clear. Analyzing RNA-seq data of oysters infected with OsHV-1, we revealed that ∼0.5% of the oyster transcriptome is always enzymatically modified by ADAR, whereas genes involved in antiviral response, miRNA maturation, and epigenetic regulation were hyper-edited in specific conditions only. Despite our results, relevant technical bottlenecks impair an accurate quantification of RNA editing events, making necessary an approach specifically dedicated to the progressive understanding of oyster “RNA editome.”

A Novel Model of Tumor-Infiltrating B Lymphocyte Specific RNA-Binding Protein-Related Genes With Potential Prognostic Value and Therapeutic Targets in Multiple Myeloma

Background: RNA-binding proteins (RBPs) act as important regulators in the progression of tumors. However, their role in the tumorigenesis and prognostic assessment in multiple myeloma (MM), a B-cell hematological cancer, remains elusive. Thus, the current study was designed to explore a novel prognostic B-cell-specific RBP signature and the underlying molecular mechanisms.

Methods: Data used in the current study were obtained from the Gene Expression Omnibus (GEO) database. Significantly upregulated RBPs in B cells were defined as B cell-specific RBPs. The biological functions of B-cell-specific RBPs were analyzed by the cluster Profiler package. Univariate and multivariate regressions were performed to identify robust prognostic B-cell specific RBP signatures, followed by the construction of the risk classification model. Gene set enrichment analysis (GSEA)-identified pathways were enriched in stratified groups. The microenvironment of the low- and high-risk groups was analyzed by single-sample GSEA (ssGSEA). Moreover, the correlations among the risk score and differentially expressed immune checkpoints or differentially distributed immune cells were calculated. The drug sensitivity of the low- and high-risk groups was assessed via Genomics of Drug Sensitivity in Cancer by the pRRophetic algorithm. In addition, we utilized a GEO dataset involving patients with MM receiving bortezomib therapy to estimate the treatment response between different groups.

Results: A total of 56 B-cell-specific RBPs were identified, which were mainly enriched in ribonucleoprotein complex biogenesis and the ribosome pathway. ADAR, FASTKD1 and SNRPD3 were identified as prognostic B-cell specific RBP signatures in MM. The risk model was constructed based on ADAR, FASTKD1 and SNRPD3. Receiver operating characteristic (ROC) curves revealed the good predictive capacity of the risk model. A nomogram based on the risk score and other independent prognostic factors exhibited excellent performance in predicting the overall survival of MM patients. GSEA showed enrichment of the Notch signaling pathway and mRNA cis-splicing via spliceosomes in the high-risk group. Moreover, we found that the infiltration of diverse immune cell subtypes and the expression of CD274, CD276, CTLA4 and VTCN1 were significantly different between the two groups. In addition, the IC50 values of 11 drugs were higher in the low-risk group. Patients in the low-risk group exhibited a higher complete response rate to bortezomib therapy.

Conclusion: Our study identified novel prognostic B-cell-specific RBP biomarkers in MM and constructed a unique risk model for predicting MM outcomes. Moreover, we explored the immune-related mechanisms of B cell-specific RBPs in regulating MM. Our findings could pave the way for developing novel therapeutic strategies to improve the prognosis of MM patients.

Identification of SRSF3 target mRNAs using inducible TRIBE

Serine and arginine-rich splicing factor 3 (SRSF3), the smallest member of the Ser/Arg-rich (SR) RNA-binding protein family, regulates multiple aspects of post-transcriptional gene expression program. Although SRSF3 is essential for early embryo development, reprogramming, and pluripotency maintenance, the RNA targets and specificity of RNA recognition of SRSF3 are not well understood in human pluripotent stem cells. In this study, we used inducible TRIBE (targets of RNA binding sites by editing) to identify RNA targets and binding motifs of SRSF3 in human embryonic stem cells (hESCs). We identified 3888 confident binding sites of SRSF3, corresponding to 1222 gene targets. Our results showed that nearly half of the binding sites were distributed in exons, reflecting the alternative splicing function of SRSF3. Motif analysis demonstrated that two of the SRSF3 recognition sequences were the same as the motifs identified in mouse embryonic stem cells, suggesting the recognition sequences of SRSF3 may be conserved in mammals. Overall, our analyses revealed the RNA targets of SRSF3 and uncovered its RNA recognition specificity, providing a valuable resource for understanding the function of SRSF3 in human embryonic stem cells.