DICER ribonuclease removes harmful R-loops

R-loop homeostasis in genome dynamics, gene expression and development

R-loops are double-edge swords with functional roles that in many cases constitute a threat to genome integrity and gene expression with relevant consequences in cell physiology and development. A number of factors have evolved to control R-loop homeostasis by acting at the levels of R-loop prevention, resolution, or the repair of the R-loop-induced DNA lesion. Deciphering the role of R-loops generated under different stresses and the plethora of processes controlling their homeostasis has become crucial to evaluate their impact in cell physiology and the biological significance of their association with development and disease. Here, we review publications of the last 2 years that help understand their biological role.

DNA-RNA hybrids in inflammation: sources, immune response, and therapeutic implications

Cytoplasmic DNA-RNA hybrids are emerging as important immunogenic nucleic acids, that were previously underappreciated. DNA-RNA hybrids, formed during cellular processes like transcription and replication, or by exogenous pathogens, are recognized by pattern recognition receptors (PRRs), including cGAS, DDX41, and TLR9, which trigger immune responses. Post-translational modifications (PTMs) including ubiquitination, phosphorylation, acetylation, and palmitoylation regulate the activity of PRRs and downstream signaling molecules, fine-tuning the immune response. Targeting enzymes involved in DNA-RNA hybrid metabolism and PTMs regulation offers therapeutic potential for inflammatory diseases. Herein, we discuss the sources, immune response, and therapeutic implications of DNA-RNA hybrids in inflammation, highlighting the significance of DNA-RNA hybrids as potential targets for the treatment of inflammation.

DICER1 Mutational Spectrum in Intracranial CNS-Neoplasias-A Review and a Report from the CNS-InterREST GPOH Study Center

DICER1 tumor predisposition syndrome is a genetic condition that increases the risk of developing certain cancer types. While thyroid tumors are the main tumors caused by this condition in adult oncology, children and adolescents with DICER1 germline mutations may suffer from a broader spectrum of tumors, including Sertoli-Leydig cell tumors, pleuropulmonary blastomas, embryonal rhabdomyosarcomas, and pineoblastomas. Although these diseases-many of which are hallmark tumors of DICER1 syndrome and rarely occur sporadically-have been known for several years, the more recent identification of DICER1 mutations in embryonal tumors with multilayered rosettes (ETMR) and DICER1-associated intra- and extracranial sarcomas has expanded the spectrum of tumor types potentially linked to DICER1 syndrome. This review sought to investigate the presence and characteristics of DICER1 mutations in rare CNS tumors and to discuss their potential implications for early recognition of DICER1-related syndromes. To address this, we conducted a comprehensive systematic literature review and analyzed data from our nationwide German database (CNS-InterREST) regarding these entities. When present, DICER1 mutation status, mutation type (somatic vs. germline), and localization within the gene were recorded. Demographic and clinical data-including age at diagnosis and tumor localization-were also evaluated where available. We found that the prevalence of DICER1 mutations in the cohort of ETMR patients included in the CNS-InterREST study was exceedingly low (1/31). The distribution of DICER1 mutations in patients with ETMR or intracranial sarcomas is comparable to that in other previously identified DICER1-mutant tumors. Our literature review demonstrates that within the 248 cases, which include three intracranial DICER1-mutated neoplasias and one reference group, most somatic mutations accumulate in the RNase IIIb domain, while germline mutations are usually evenly distributed throughout the gene. Overall, further research is necessary to unravel the cell-of-origin of the respective tumor types and whether other, hitherto undescribed, genetic factors may contribute to the development of ETMR and DICER1-associated intracranial sarcomas.

Transcription-Coupled Repair and R-Loop Crosstalk in Genome Stability

Transcription-coupled repair (TCR) and R-loops are two interrelated processes critical to the maintenance of genome stability during transcription. TCR, a specialized sub-pathway of nucleotide excision repair, rapidly removes transcription-blocking lesions from the transcribed strand of active genes, thereby safeguarding transcription fidelity and cellular homeostasis. In contrast, R-loops, RNA-DNA hybrid structures formed co-transcriptionally, play not only regulatory roles in gene expression and replication but can also contribute to genome instability when persistently accumulated. Recent experimental evidence has revealed dynamic crosstalk between TCR and R-loop resolution pathways. This review highlights current molecular and cellular insights into TCR and R-loop biology, discusses the impact of their crosstalk, and explores emerging therapeutic strategies aimed at optimizing DNA repair and reducing disease risk in conditions such as cancer and neurodegenerative disorders.

Regulation of CRISPR trans-cleavage activity by an overhanging activator

The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas12a system exhibits extraordinary capability in the field of molecular diagnosis and biosensing, attributed to its trans-cleavage ability. The precise modulation of performance has emerged as a significant challenge in advancing CRISPR technology to the next stage of development. Herein, we reported a CRISPR/Cas12a regulation strategy based on an overhanging activator. The presence of overhanging domains in activators creates steric hindrances that have a substantial impact on the trans-cleavage activity and activation timing of Cas12a. The trans-cleavage activity of Cas12a can be finely tuned by adjusting the position, length, and complementarity of the overhanging domains. Moreover, specific structures exhibit characteristics of automatic delayed activation. The presence of overhanging domains enables precise and timely activation of Cas12a, facilitating multifunctional applications. This system effectively accomplishes dynamic regulation, programmable release of cargo, logical operations, and multi-enzyme detection. The flexibility and versatility of this simple and powerful CRISPR regulatory strategy will pave the way for expanded applications of CRISPR/Cas in biotechnology, bioengineering, and biomedicine.

Looping forward: exploring R-loop processing and therapeutic potential

Recently, there has been increasing interest in the complex relationship between transcription and genome stability, with specific attention directed toward the physiological significance of molecular structures known as R-loops. These structures arise when an RNA strand invades into the DNA duplex, and their formation is involved in a wide range of regulatory functions affecting gene expression, DNA repair processes or cell homeostasis. The persistent presence of R-loops, if not effectively removed, contributes to genome instability, underscoring the significance of the factors responsible for their resolution and modification. In this review, we provide a comprehensive overview of how R-loop processing can drive either a beneficial or a harmful outcome. Additionally, we explore the potential for manipulating such structures to devise rationalized therapeutic strategies targeting the aberrant accumulation of R-loops.

The molecular chaperone ALYREF promotes R-loop resolution and maintains genome stability

Unscheduled R-loops usually cause DNA damage and replication stress, and are therefore a major threat to genome stability. Several RNA processing factors, including the conserved THO complex and its associated RNA and DNA-RNA helicase UAP56, prevent R-loop accumulation in cells. Here, we investigate the function of ALYREF, an RNA export adapter associated with UAP56 and the THO complex, in R-loop regulation. We demonstrate that purified ALYREF promotes UAP56-mediated R-loop dissociation in vitro, and this stimulation is dependent on its interaction with UAP56 and R-loops. Importantly, we show that ALYREF binds DNA-RNA hybrids and R-loops. Consistently, ALYREF depletion causes R-loop accumulation and R-loop-mediated genome instability in cells. We propose that ALYREF, apart from its known role in RNA metabolism and export, is a key cellular R-loop coregulator, which binds R-loops and stimulates UAP56-driven resolution of unscheduled R-loops during transcription.

The Smc5/6 complex counteracts R-loop formation at highly transcribed genes in cooperation with RNase H2

The R-loop is a common transcriptional by-product that consists of an RNA-DNA duplex joined to a displaced strand of genomic DNA. While the effects of R-loops on health and disease are well established, there is still an incomplete understanding of the cellular processes responsible for their removal from eukaryotic genomes. Here, we show that a core regulator of chromosome architecture -the Smc5/6 complex- plays a crucial role in the removal of R-loop structures formed during gene transcription. Consistent with this, budding yeast mutants defective in the Smc5/6 complex and enzymes involved in R-loop resolution show strong synthetic interactions and accumulate high levels of RNA-DNA hybrid structures in their chromosomes. Importantly, we demonstrate that the Smc5/6 complex acts on specific types of RNA-DNA hybrid structures in vivo and promotes R-loop degradation by the RNase H2 enzyme in vitro. Collectively, our results reveal a crucial role for the Smc5/6 complex in the removal of toxic R-loops formed at highly transcribed genes and telomeres.

The coexistence of a BRCA2 germline and a DICER1 somatic variant in two first-degree cousins suggests their potential synergic effect

Cancer predisposition syndromes are recognized in about 10% of pediatric malignancies with several genes specifically involved in a subset of pediatric tumors such as DICER1, in pleuropulmonary blastoma, cystic nephroma, and brain sarcomas. By contrast, the role of BRCA1/2 in pediatric cancer predisposition is still under investigation. We present two cases of young first-degree cousins, both carrying a germline BRCA2 variant and developing tumors characterized by somatic DICER1 mutations. Patient 1 presented with a cystic nephroma harboring a somatic DICER1 variant (p.Asp1810Tyr), while patient 2 had a primary intracranial DICER1-mutated sarcoma showing a distinct somatic DICER1 variant (p.Asp1709Glu) as well as biallelic inactivation of TP53 (p.Val173Leu, VAF 91%) and APC (p.Ile1307Lys, VAF 95%) and a pathogenic variant in KRAS (p.Gln61His). Both patients carried the same germline BRCA2 variant (p.Arg2842Cys) of unknown significance. The same variant was found in the mother of patient 2 and in the father of patient 1, who are siblings. A homologous recombination deficiency signature was not identified in any of the two tumors, possibly suggesting a reduction of BRCA2 activity. The association of BRCA2 and DICER1 variants in our cases hints at a potential cooperative role in cancer pathogenesis. Further studies are warranted to elucidate the interplay between BRCA1/2 and DICER1 variants and their implications for cancer predisposition and treatment in pediatric patients.

RNA biogenesis and RNA metabolism factors as R-loop suppressors: a hidden role in genome integrity

Genome integrity relies on the accuracy of DNA metabolism, but as appreciated for more than four decades, transcription enhances mutation and recombination frequencies. More recent research provided evidence for a previously unforeseen link between RNA and DNA metabolism, which is often related to the accumulation of DNA-RNA hybrids and R-loops. In addition to physiological roles, R-loops interfere with DNA replication and repair, providing a molecular scenario for the origin of genome instability. Here, we review current knowledge on the multiple RNA factors that prevent or resolve R-loops and consequent transcription-replication conflicts and thus act as modulators of genome dynamics.

Chromosomal R-loops: who R they?

R-loops, composed of DNA-RNA hybrids and displaced single-stranded DNA, are known to pose a severe threat to genome integrity. Therefore, extensive research has focused on identifying regulatory proteins involved in controlling R-loop levels. These proteins play critical roles in preventing R-loop accumulation and associated genome instability. Herein I summarize recent knowledge on R-loop regulators affecting R-loop homeostasis, involving a wide array of R-loop screening methods that have enabled their characterization, from forward genetic and siRNA-based screens to proximity labeling and machine learning. These approaches not only deepen our understanding on R-loop formation processes, but also hold promise to find new targets in R-loop dysregulation associated with human pathologies.

Substrate promiscuity of Dicer toward precursors of the let-7 family and their 3'-end modifications

The human let-7 miRNA family consists of thirteen members that play critical roles in many biological processes, including development timing and tumor suppression, and their levels are disrupted in several diseases. Dicer is the endoribonuclease responsible for processing the precursor miRNA (pre-miRNA) to yield the mature miRNA, and thereby plays a crucial role in controlling the cellular levels of let-7 miRNAs. It is well established that the sequence and structural features of pre-miRNA hairpins such as the 5'-phosphate, the apical loop, and the 2-nt 3'-overhang are important for the processing activity of Dicer. Exceptionally, nine precursors of the let-7 family (pre-let-7) contain a 1-nt 3'-overhang and get mono-uridylated in vivo, presumably to allow efficient processing by Dicer. Pre-let-7 are also oligo-uridylated in vivo to promote their degradation and likely prevent their efficient processing by Dicer. In this study, we systematically investigated the impact of sequence and structural features of all human let-7 pre-miRNAs, including their 3'-end modifications, on Dicer binding and processing. Through the combination of SHAPE structural probing, in vitro binding and kinetic studies using purified human Dicer, we show that despite structural discrepancies among pre-let-7 RNAs, Dicer exhibits remarkable promiscuity in binding and cleaving these substrates. Moreover, the 1- or 2-nt 3'-overhang, 3'-mono-uridylation, and 3'-oligo-uridylation of pre-let-7 substrates appear to have little effect on Dicer binding and cleavage rates. Thus, this study extends current knowledge regarding the broad substrate specificity of Dicer and provides novel insight regarding the effect of 3'-modifications on binding and cleavage by Dicer.

DNA damage and transcription stress

Genome damage and transcription are intimately linked. Tens to hundreds of thousands of DNA lesions arise in each cell each day, many of which can directly or indirectly impede transcription. Conversely, the process of gene expression is itself a source of endogenous DNA lesions as a result of the susceptibility of single-stranded DNA to damage, conflicts with the DNA replication machinery, and engagement by cells of topoisomerases and base excision repair enzymes to regulate the initiation and progression of gene transcription. Although such processes are tightly regulated and normally accurate, on occasion, they can become abortive and leave behind DNA breaks that can drive genome rearrangements, instability, or cell death.

The Non-Coding RNA Journal Club: Highlights on Recent Papers-13

We are delighted to share with you our thirteenth Journal Club and highlight some of the most interesting papers published recently [...].