Coordination of transcription and processing of tRNA

tRNA-derived fragments: Unveiling new roles and molecular mechanisms in cancer progression

tRNA-derived fragments (tRFs) are novel small noncoding RNAs (sncRNAs) that range from approximately 14 to 50 nt. They are generated by the cleavage of mature tRNAs or precursor tRNAs (pre-tRNAs) at specific sites. Based on their origin and length, tRFs can be classified into three categories: (1) tRF-1 s; (2) tRF-3 s, tRF-5 s, and internal tRFs (i-tRFs); and (3) tRNA halves. They play important roles in stress response, signal transduction, and gene expression processes. Recent studies have identified differential expression of tRFs in various tumors. Aberrantly expressed tRFs have critical clinical value and show promise as new biomarkers for tumor diagnosis and prognosis and as therapeutic targets. tRFs regulate the malignant progression of tumors via various mechanisms, primarily including modulation of noncoding RNA biogenesis, global chromatin organization, gene expression regulation, modulation of protein translation, regulation of epigenetic modification, and alternative splicing regulation. In conclusion, tRF-mediated regulatory pathways could present new avenues for tumor treatment, and tRFs could serve as promising therapeutic targets for cancer therapy.

RNase P: Beyond Precursor tRNA Processing

Ribonuclease P (RNase P) was first described in the 1970's as an endoribonuclease acting in the maturation of precursor transfer RNAs (tRNAs). More recent studies, however, have uncovered non-canonical roles for RNase P and its components. Here, we review the recent progress of its involvement in chromatin assembly, DNA damage response, and maintenance of genome stability with implications in tumorigenesis. The possibility of RNase P as a therapeutic target in cancer is also discussed.

Transcription complexes recruit a chaperone to perform cotranscriptional processing of tRNA

Leone et al.1 reveal that Pol III transcription complexes recruit a chaperone, HSP70, to execute cotranscriptional cleavage of precursor tRNA. HSP70 binds to the polymerase and translocates to nascent precursor tRNA and then tRNA. The last complex facilitates Pol III to engage in a new, efficient transcription cycle with another HSP70.

An efficient CRISPR/Cas9 genome editing system based on a multiple sgRNA processing platform in Trichoderma reesei for strain improvement and enzyme production

BACKGROUND

The CRISPR/Cas9 technology is being employed as a convenient tool for genetic engineering of the industrially important filamentous fungus Trichoderma reesei. However, multiplex gene editing is still constrained by the sgRNA processing capability, hindering strain improvement of T. reesei for the production of lignocellulose-degrading enzymes and recombinant proteins.

RESULTS

Here, a CRISPR/Cas9 system based on a multiple sgRNA processing platform was established for genome editing in T. reesei. The platform contains the arrayed tRNA-sgRNA architecture directed by a 5S rRNA promoter to generate multiple sgRNAs from a single transcript by the endogenous tRNA processing system. With this system, two sgRNAs targeting cre1 (encoding the carbon catabolite repressor 1) were designed and the precise deletion of cre1 was obtained, demonstrating the efficiency of sgRNAs processing in the tRNA-sgRNA architecture. Moreover, overexpression of xyr1-A824V (encoding a key activator for cellulase/xylanase expression) at the ace1 (encoding a repressor for cellulase/xylanase expression) locus was achieved by designing two sgRNAs targeting ace1 in the system, resulting in the significantly enhanced production of cellulase (up to 1- and 18-fold on the Avicel and glucose, respectively) and xylanase (up to 11- and 41-fold on the Avicel and glucose, respectively). Furthermore, heterologous expression of the glucose oxidase gene from Aspergillus niger ATCC 9029 at the cbh1 locus with the simultaneous deletion of cbh1 and cbh2 (two cellobiohydrolase coding genes) by designing four sgRNAs targeting cbh1 and cbh2 in the system was acquired, and the glucose oxidase produced by T. reesei reached 43.77 U/mL. Besides, it was found the ER-associated protein degradation (ERAD) level was decreased in the glucose oxidase-producing strain, which was likely due to the reduction of secretion pressure by deletion of the major endogenous cellulase-encoding genes.

CONCLUSIONS

The tRNA-gRNA array-based CRISPR-Cas9 editing system was successfully developed in T. reesei. This system would accelerate engineering of T. reesei for high-level production of enzymes including lignocellulose-degrading enzymes and other recombinant enzymes. Furthermore, it would expand the CRISPR toolbox for fungal genome editing and synthetic biology.

Different modification pathways for m1A58 incorporation in yeast elongator and initiator tRNAs

As essential components of the protein synthesis machinery, tRNAs undergo a tightly controlled biogenesis process, which include the incorporation of numerous posttranscriptional modifications. Defects in these tRNA maturation steps may lead to the degradation of hypomodified tRNAs by the rapid tRNA decay (RTD) and nuclear surveillance pathways. We previously identified m1A58 as a late modification introduced after modifications Ψ55 and T54 in yeast elongator tRNAPhe. However, previous reports suggested that m1A58 is introduced early during the tRNA modification process, in particular on primary transcripts of initiator tRNAiMet, which prevents its degradation by RNA decay pathways. Here, aiming to reconcile this apparent inconsistency on the temporality of m1A58 incorporation, we examined its introduction into yeast elongator and initiator tRNAs. We used specifically modified tRNAs to report on the molecular aspects controlling the Ψ55 → T54 → m1A58 modification circuit in elongator tRNAs. We also show that m1A58 is efficiently introduced on unmodified tRNAiMet, and does not depend on prior modifications. Finally, we show that m1A58 has major effects on the structural properties of initiator tRNAiMet, so that the tRNA elbow structure is only properly assembled when this modification is present. This observation provides a structural explanation for the degradation of hypomodified tRNAiMet lacking m1A58 by the nuclear surveillance and RTD pathways.

DNA-dependent RNA polymerases in plants

DNA-dependent RNA polymerases (Pols) transfer the genetic information stored in genomic DNA to RNA in all organisms. In eukaryotes, the typical products of nuclear Pol I, Pol II, and Pol III are ribosomal RNAs, mRNAs, and transfer RNAs, respectively. Intriguingly, plants possess two additional Pols, Pol IV and Pol V, which produce small RNAs and long noncoding RNAs, respectively, mainly for silencing transposable elements. The five plant Pols share some subunits, but their distinct functions stem from unique subunits that interact with specific regulatory factors in their transcription cycles. Here, we summarize recent advances in our understanding of plant nucleus-localized Pols, including their evolution, function, structures, and transcription cycles.

The Pol III transcriptome: Basic features, recurrent patterns, and emerging roles in cancer

The RNA polymerase III (Pol III) transcriptome is universally comprised of short, highly structured noncoding RNA (ncRNA). Through RNA-protein interactions, the Pol III transcriptome actuates functional activities ranging from nuclear gene regulation (7SK), splicing (U6, U6atac), and RNA maturation and stability (RMRP, RPPH1, Y RNA), to cytoplasmic protein targeting (7SL) and translation (tRNA, 5S rRNA). In higher eukaryotes, the Pol III transcriptome has expanded to include additional, recently evolved ncRNA species that effectively broaden the footprint of Pol III transcription to additional cellular activities. Newly evolved ncRNAs function as riboregulators of autophagy (vault), immune signaling cascades (nc886), and translation (Alu, BC200, snaR). Notably, upregulation of Pol III transcription is frequently observed in cancer, and multiple ncRNA species are linked to both cancer progression and poor survival outcomes among cancer patients. In this review, we outline the basic features and functions of the Pol III transcriptome, and the evidence for dysregulation and dysfunction for each ncRNA in cancer. When taken together, recurrent patterns emerge, ranging from shared functional motifs that include molecular scaffolding and protein sequestration, overlapping protein interactions, and immunostimulatory activities, to the biogenesis of analogous small RNA fragments and noncanonical miRNAs, augmenting the function of the Pol III transcriptome and further broadening its role in cancer. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Processing of Small RNAs RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Transcriptome alterations in peripheral blood B cells of patients with multiple sclerosis receiving immune reconstitution therapy

BACKGROUND

Multiple sclerosis (MS) is a chronic, inflammatory and neurodegenerative disease that leads to irreversible damage to the brain and spinal cord. The goal of so-called "immune reconstitution therapies" (IRTs) is to achieve long-term disease remission by eliminating a pathogenic immune repertoire through intense short-term immune cell depletion. B cells are major targets for effective immunotherapy in MS.

OBJECTIVES

The aim of this study was to analyze the gene expression pattern of B cells before and during IRT (i.e., before B-cell depletion and after B-cell repopulation) to better understand the therapeutic effects and to identify biomarker candidates of the clinical response to therapy.

METHODS

B cells were obtained from blood samples of patients with relapsing-remitting MS (n = 50), patients with primary progressive MS (n = 13) as well as healthy controls (n = 28). The patients with relapsing MS received either monthly infusions of natalizumab (n = 29) or a pulsed IRT with alemtuzumab (n = 15) or cladribine (n = 6). B-cell subpopulation frequencies were determined by flow cytometry, and transcriptome profiling was performed using Clariom D arrays. Differentially expressed genes (DEGs) between the patient groups and controls were examined with regard to their functions and interactions. We also tested for differences in gene expression between patients with and without relapse following alemtuzumab administration.

RESULTS

Patients treated with alemtuzumab or cladribine showed on average a > 20% lower proportion of memory B cells as compared to before IRT. This was paralleled by profound transcriptome shifts, with > 6000 significant DEGs after adjustment for multiple comparisons. The top DEGs were found to regulate apoptosis, cell adhesion and RNA processing, and the most highly connected nodes in the network of encoded proteins were ESR2, PHB and RC3H1. Higher mRNA levels of BCL2, IL13RA1 and SLC38A11 were seen in patients with relapse despite IRT, though these differences did not pass the false discovery rate correction.

CONCLUSIONS

We show that B cells circulating in the blood of patients with MS undergoing IRT present a distinct gene expression signature, and we delineated the associated biological processes and gene interactions. Moreover, we identified genes whose expression may be an indicator of relapse risk, but further studies are needed to verify their potential value as biomarkers.

Human RNase P: overview of a ribonuclease of interrelated molecular networks and gene-targeting systems

The seminal discovery of ribonuclease P (RNase P) and its catalytic RNA by Sidney Altman has not only revolutionized our understanding of life, but also opened new fields for scientific exploration and investigation. This review focuses on human RNase P and its use as a gene-targeting tool, two topics initiated in Altman's laboratory. We outline early works on human RNase P as a tRNA processing enzyme and comment on its expanding nonconventional functions in molecular networks of transcription, chromatin remodeling, homology-directed repair, and innate immunity. The important implications and insights from these discoveries on the potential use of RNase P as a gene-targeting tool are presented. This multifunctionality calls to a modified structure-function partitioning of domains in human RNase P, as well as its relative ribonucleoprotein, RNase MRP. The role of these two catalysts in innate immunity is of particular interest in molecular evolution, as this dynamic molecular network could have originated and evolved from primordial enzymes and sensors of RNA, including predecessors of these two ribonucleoproteins.

Differentially expressed tRNA-derived fragments in bovine fetuses with assisted reproduction induced congenital overgrowth syndrome

Background: As couples struggle with infertility and livestock producers wish to rapidly improve genetic merit in their herd, assisted reproductive technologies (ART) have become increasingly popular in human medicine as well as the livestock industry. Utilizing ART can cause an increased risk of congenital overgrowth syndromes, such as Large Offspring Syndrome (LOS) in ruminants. A dysregulation of transcripts has been observed in bovine fetuses with LOS, which is suggested to be a cause of the phenotype. Our recent study identified variations in tRNA expression in LOS individuals, leading us to hypothesize that variations in tRNA expression can influence the availability of their processed regulatory products, tRNA-derived fragments (tRFs). Due to their resemblance in size to microRNAs, studies suggest that tRFs target mRNA transcripts and regulate gene expression. Thus, we have sequenced small RNA isolated from skeletal muscle and liver of day 105 bovine fetuses to elucidate the mechanisms contributing to LOS. Moreover, we have utilized our previously generated tRNA sequencing data to analyze the contribution of tRNA availability to tRF abundance. Results: 22,289 and 7,737 unique tRFs were predicted in the liver and muscle tissue respectively. The greatest number of reads originated from 5' tRFs in muscle and 5' halves in liver. In addition, mitochondrial (MT) and nuclear derived tRF expression was tissue-specific with most MT-tRFs and nuclear tRFs derived from LysUUU and iMetCAU in muscle, and AsnGUU and GlyGCC in liver. Despite variation in tRF abundance within treatment groups, we identified differentially expressed (DE) tRFs across Control-AI, ART-Normal, and ART-LOS groups with the most DE tRFs between ART-Normal and ART-LOS groups. Many DE tRFs target transcripts enriched in pathways related to growth and development in the muscle and tumor development in the liver. Finally, we found positive correlation coefficients between tRNA availability and tRF expression in muscle (R = 0.47) and liver (0.6). Conclusion: Our results highlight the dysregulation of tRF expression and its regulatory roles in LOS. These tRFs were found to target both imprinted and non-imprinted genes in muscle as well as genes linked to tumor development in the liver. Furthermore, we found that tRNA transcription is a highly modulated event that plays a part in the biogenesis of tRFs. This study is the first to investigate the relationship between tRNA and tRF expression in combination with ART-induced LOS.

Recent insights into the structure, function, and regulation of the eukaryotic transfer RNA splicing endonuclease complex

The splicing of transfer RNA (tRNA) introns is a critical step of tRNA maturation, for intron-containing tRNAs. In eukaryotes, tRNA splicing is a multi-step process that relies on several RNA processing enzymes to facilitate intron removal and exon ligation. Splicing is initiated by the tRNA splicing endonuclease (TSEN) complex which catalyzes the excision of the intron through its two nuclease subunits. Mutations in all four subunits of the TSEN complex are linked to a family of neurodegenerative and neurodevelopmental diseases known as pontocerebellar hypoplasia (PCH). Recent studies provide molecular insights into the structure, function, and regulation of the eukaryotic TSEN complex and are beginning to illuminate how mutations in the TSEN complex lead to neurodegenerative disease. Using new advancements in the prediction of protein structure, we created a three-dimensional model of the human TSEN complex. We review functions of the TSEN complex beyond tRNA splicing by highlighting recently identified substrates of the eukaryotic TSEN complex and discuss mechanisms for the regulation of tRNA splicing, by enzymes that modify cleaved tRNA exons and introns. Finally, we review recent biochemical and animal models that have worked to address the mechanisms that drive PCH and synthesize these studies with previous studies to try to better understand PCH pathogenesis. This article is categorized under: RNA Processing > tRNA Processing RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition.

Structural insights into nuclear transcription by eukaryotic DNA-dependent RNA polymerases

The eukaryotic transcription apparatus synthesizes a staggering diversity of RNA molecules. The labour of nuclear gene transcription is, therefore, divided among multiple DNA-dependent RNA polymerases. RNA polymerase I (Pol I) transcribes ribosomal RNA, Pol II synthesizes messenger RNAs and various non-coding RNAs (including long non-coding RNAs, microRNAs and small nuclear RNAs) and Pol III produces transfer RNAs and other short RNA molecules. Pol I, Pol II and Pol III are large, multisubunit protein complexes that associate with a multitude of additional factors to synthesize transcripts that largely differ in size, structure and abundance. The three transcription machineries share common characteristics, but differ widely in various aspects, such as numbers of RNA polymerase subunits, regulatory elements and accessory factors, which allows them to specialize in transcribing their specific RNAs. Common to the three RNA polymerases is that the transcription process consists of three major steps: transcription initiation, transcript elongation and transcription termination. In this Review, we outline the common principles and differences between the Pol I, Pol II and Pol III transcription machineries and discuss key structural and functional insights obtained into the three stages of their transcription processes.

Characteristics of Transfer RNA-Derived Fragments Expressed during Human Renal Cell Development: The Role of Dicer in tRF Biogenesis

tRNA-derived fragments participate in the regulation of many processes, such as gene silencing, splicing and translation in many organisms, ranging from bacteria to humans. We were interested to know how tRF abundance changes during the different stages of renal cell development. The research model used here consisted of the following human renal cells: hESCs, HEK-293T, HK-2 and A-489 kidney tumor cells, which, together, mimic the different stages of kidney development. The characteristics of the most abundant tRFs, tRFGly(CCC), tRFVal(AAC) and tRFArg(CCU), were presented. It was found that these parental tRNAs present in cells are the source of many tRFs, thus increasing the pool of potential regulatory RNAs. Indeed, a bioinformatic analysis showed the possibility that tRFGly(CCC) and tRRFVal(AAC) could regulate the activity of a range of kidney proteins. Moreover, the distribution of tRFs and the efficiency of their expression is similar in adult and embryonic stem cells. During the formation of tRFs, HK-2 cells resemble A-498 cancer cells more than other cells. Additionally, we postulate the involvement of Dicer nuclease in the formation of tRF-5b in all the analyzed tRNAs. To confirm this, 293T NoDice cells, which in the absence of Dicer activity do not generate tRF-5b, were used.

Structural basis for impaired 5' processing of a mutant tRNA associated with defects in neuronal homeostasis

SignificanceUnderstanding and treating neurological disorders are global priorities. Some of these diseases are engendered by mutations that cause defects in the cellular synthesis of transfer RNAs (tRNAs), which function as adapter molecules that translate messenger RNAs into proteins. During tRNA biogenesis, ribonuclease P catalyzes removal of the transcribed sequence upstream of the mature tRNA. Here, we focus on a cytoplasmic tRNAArgUCU that is expressed specifically in neurons and, when harboring a particular point mutation, contributes to neurodegeneration in mice. Our results suggest that this mutation favors stable alternative structures that are not cleaved by mouse ribonuclease P and motivate a paradigm that may help to understand the molecular basis for disease-associated mutations in other tRNAs.

A novel homozygous synonymous variant further expands the phenotypic spectrum of POLR3A-related pathologies

Pathogenic biallelic variants in POL3RA have been associated with different disorders characterized by progressive neurological deterioration. These include the 4H leukodystrophy syndrome (hypomyelination, hypogonadotropic hypogonadism, and hypodontia) and adolescent-onset progressive spastic ataxia, as well as Wiedemann-Rautenstrauch syndrome (WRS), a recognizable neonatal progeroid syndrome. The phenotypic differences between these disorders are thought to occur mainly due to different functional effects of underlying POLR3A variants. Here we present the detailed clinical course of a 37-year-old woman in whom we identified a homozygous synonymous POLR3A variant c.3336G>A resulting in leaky splicing r.[3336ins192, =, 3243_3336del94]. She presented at birth with intrauterine growth retardation, lipodystrophy, muscular hypotonia, and several WRS-like facial features, albeit without sparse hair and prominent scalp veins. She had no signs of developmental delay or intellectual disability. Over the years, above characteristic facial features, she showed severe postnatal growth retardation, global lipodystrophy, joint contractures, thoracic hypoplasia, scoliosis, anodontia, spastic quadriplegia, bilateral hearing loss, aphonia, hypogonadotropic hypogonadism, and cerebellar peduncles hyperintensities in brain imaging. These manifestations partially overlap the clinical features of the previously reported POLR3A-associated disorders, mostly mimicking the WRS. Thus, our study expands the POLR3A-mediated phenotypic spectrum and suggests existence of a phenotypic continuum underlying biallelic POLR3A variants.