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Site-specific gene integration by recombinase-mediated cassette exchange in anhydrobiotic Pv11 cells.

Pv11 cells, derived from Polypedilum vanderplanki, uniquely tolerate complete desiccation (anhydrobiosis). Although a CRISPR/Cas9-based precise integration method (CRIS-PITCh) has been developed for Pv11 cells, a CRISPR-free strategy that fixes both the genomic locus and transgene copy number has not yet been established. Here, we implement recombinase-mediated cassette exchange (RMCE) in Pv11 cells and generate master cell lines that retain anhydrobiosis following genetic engineering. We first evaluated the activity of multiple site-specific recombinases in Pv11 cells using a transient two-plasmid reporter assay. Flp, Cre, and Bxb1 recombinases all excised a test cassette and activated a green fluorescent protein (GFP) reporter, whereas phiC31 integrase mediated recombination at the DNA sequence level but did not induce reporter expression under our construct configuration. To enable genomic RMCE, we inserted an FRT/FRT3-landing pad (LP) into a previously identified genomic safe-harbor locus using CRIS-PITCh and isolated clonal master cell lines by single-cell sorting. Using the established master line, Flp-based RMCE achieved site-specific cassette exchange at the LP, producing HaloTag fluorescence and drug resistance upon successful exchange. In addition, the expected post-exchange sequence was confirmed by sequencing. We further established an all-in-one RMCE vector combining the Flp recombinase and donor cassette on a single plasmid. Together, these results demonstrate locus-defined, single-copy transgene integration in anhydrobiotic Pv11 cells via RMCE and provide a standardized, CRISPR-free workflow for routine genetic manipulation in this unique cell line. This workflow facilitates both fundamental research and applied biotechnological applications using desiccation-tolerant cells.

Anhydrobiosis

Small serine recombinases are markers for antiphage defense system discovery.

Renewed interest in phage therapy has highlighted a need to understand how bacteria subvert phage infection through antiphage defense systems. Traditionally, strategies to identify antiphage defense systems lack throughput or have limitations for bacterial species where antiphage defense systems are understudied. Herein, we developed a bioinformatic pipeline that uses a small serine recombinase to identify known and unknown antiphage defense systems. Using this approach to query reference genomes and metagenomes, we show that small serine recombinase genes are genetically linked to antiphage defense systems and serve as bait for finding these systems across diverse bacterial phyla. Using co-transcription predictions and statistical analysis of protein domain abundances, we experimentally validated our bioinformatic approach by discovering that KAP P-loop NTPases are fused to putative antiphage domains and reinforce prokaryotic Schlafen proteins as a new class of antiphage defense. Our work shows that small serine recombinases are a reliable genetic marker for the discovery of antiphage defenses across diverse bacterial phyla.

Bacteriophages

Efficient site-specific integration of kilobase-length DNA fragments in plant cells via Kp03 recombinase.

Targeted insertion of large DNA sequences into plant genomes remains a major challenge in synthetic biology. Here, we evaluate the large serine recombinase Kp03 for site-specific integration of DNA fragments in rice and Arabidopsis. In transient protoplast assays, Kp03 mediates efficient insertion of donor DNA up to 27.3 kilobases (kb), with plasmid integration efficiencies reaching 99.1% for fragments up to 3.4 kb. Truncation experiments reveal that a minimal 15-bp attB sequence is necessary for integration. As a proof of concept, Kp03 successfully incorporates a 3.4-kb donor DNA into the rice genome at a locus containing this minimal attB sequence. Moreover, in rice callus, combining Kp03 with the NM-PE genome editing system to install a 26-bp attB site enables targeted integration of a 3.4-kb donor at the desired genomic locus. These findings establish Kp03 as a versatile tool for plant genome engineering, with broad applications for synthetic biology.

Oryza

A genetic manipulation tool based on the GP35 recombinase for targeted gene editing in mycoplasmas of ruminants.

Pathogenic ruminant mycoplasmas are major etiological agents in cattle and small ruminants and are responsible for substantial economic losses in the livestock industry. Progress in pathogenesis research and vaccine development has been hampered by a lack of effective genetic tools. The applicability of common genome editing platforms, such as CRISPR, is inherently restricted in these organisms owing to their minimal genomes, the absence of a cell wall, and low homologous recombination efficiency. Although transposon-mediated random mutagenesis and single-base editing are currently used in the editing of bovine mycoplasma, the stochastic nature of transposons, the risk of single-base random deamination, and limitations in editing window selection hinder the genetic manipulation of bovine mycoplasma. Here, we introduce a plasmid-based methodology that employs the GP35 recombinase from bacteriophage SPP1 to mediate long single-stranded DNA (ssDNA) recombineering, thereby enabling precise gene insertions and deletions in Mycoplasma bovis, with a positive-editing rate of 77.78% - 100%. This targeted system eliminates the risk of random deamination. Leveraging this tool, we generated a panel of M. bovis mutants affecting metabolic and virulence genes and obtained key insights into Mb0564, identified as a novel adhesin. The 192 to 287 aa region of GP35 is critical for interaction with SSB. Structural conservation analysis further suggested that this GP35-ssDNA editing system possesses a high potential for translation to other ruminant pathogens. Collectively, our approach expands the existing genetic toolkit for M. bovis, advances synthetic biology and M. bovis pathobiology, facilitates vaccine development, and strengthens the control of high-impact livestock diseases in line with the One Health framework.

Animals

Evaluating detection of Histophilus somni immunoglobulin-binding protein A DR2 Fic: A species-specific gene target for recombinase polymerase amplification relative to long-read sequencing of respiratory samples from feedlot calves.

Histophilosis is an important cause of morbidity and mortality as well as antimicrobial use in feedlot cattle across North America. Detection of Histophilus somni by culture is challenging, and there is no standardized tool for distinguishing isolates that carry virulence factors most likely to contribute to disease. The DR2 repeat of H. somni-associated virulence factor 'immunoglobulin-binding protein A' (ibpA DR2) harbors a Fic domain that mediates host cell cytotoxicity and is essential for histophilosis. For rapid detection of ibpA DR2 in extracted DNA, we developed a real-time recombinase polymerase amplification (RPA) assay with a runtime of 24&#xa0;min at 39&#xa0;&#xb0;C. DNA from H. somni-RPA-positive respiratory swabs (n&#xa0;=&#xa0;73) was screened for ibpA DR2 using the novel RPA assay and long-read metagenomic sequencing, as well as nanopore whole-genome sequencing (WGS) of H. somni isolated from the same samples. IbpA DR2 was identified in 71% and 70% of tested samples using RPA and WGS, respectively, and in &#x2264;41% of samples using metagenomic sequencing. The likelihood of detection by RPA did not differ (OR 1.1, 95% CI (0.42, 2.9), P&#xa0;>&#xa0;0.99) from WGS; however, agreement between these assays was only fair (&#x3ba;&#xa0;=&#xa0;0.31). Conversely, RPA (OR 3.4, 95% CI (1.6, 8.2)) and WGS (OR 8.0, 95% CI (2.4, 42)) were more likely (P&#xa0;<&#xa0;0.001) to detect ibpA DR2 than metagenomic sequencing, likely reflecting limited coverage of H. somni by metagenomics. This study demonstrated that RPA and long-read WGS detected ibpA DR2 with similar frequencies in extracted DNA and H. somni isolates, respectively. Further testing of non-target isolates confirmed the analytical specificity of ibpA DR2 to H. somni. Further investigation of the diagnostic validity for RPA-based ibpA DR2 detection is required in a larger cohort of field samples, as a rapid screening tool for H. somni most likely to contribute to disease.

Animals

Engineered genomic attachment sites for site-specific recombinases enable high-efficiency integration in plants and human cells.

Here we developed a DNA-centric strategy for optimizing site-specific recombination by rationally engineering chimeric attachment sites. The high-activity att variants enhance Bxb1-mediated integration efficiency in human cells and plants. Among these att variants, the engineered attB(V111) site achieved 51.9% integration efficiency in HEK293T cells (1.7-fold versus wild-type attB) and 35.6% in rice protoplasts (4.4-fold versus wild-type attB). When paired with an engineered single protein mutant in the Bxb1 catalytic domain, the optimized system achieved targeted integration efficiencies of 31% for a CD19 chimeric antigen receptor cassette and 25% for an ornithine transcarbamylase expression cassette in human cells. In rice, these engineered variants enabled integration of a 5.8&#x2009;kb herbicide-resistance cassette at a targeted genomic locus, with stable integration detected in 24% of regenerated plants. Oxford Nanopore-based long-read sequencing of edited plants reveals complete and precise insertion with high specificity. Propagation of edited seedlings to T1 plants confirms heritable editing to future generations. This approach provides a safe, broadly applicable approach for recombinase-based genome editing.

Journal Article

Cre-loaded integrase-defective lentiviral vectors for targeted cassette exchange in CHO cells.

Genome-modifying enzymes, such as recombinases and CRISPR-associated nucleases, enable targeted gene insertion when delivered transiently to minimize off-target effects. Precise genome engineering requires controlled enzyme activity, as well as efficient donor DNA transfer. Integrase-defective lentiviral vectors (IDLVs) provide a promising platform for transient episomal DNA transfer; however, their integration efficiency depends on complementary genome-targeting strategies. Here, we engineered Cre-loaded IDLVs (Cre-IDLVs) that co-package lentiviral vector genomes together with bioactive Cre recombinase. Cre was inserted into the Gag region of an integrase-defective gag-pol construct, allowing for efficient encapsidation and protease-mediated release during virion maturation without compromising the viral titer. The resulting particles carried donor cassettes flanked by heterospecific loxP sites. When applied to CHO founder cells harboring compatible genomic loxP landing pads, Cre-IDLVs efficiently mediated recombination-mediated cassette exchange, producing the highest number of G418-resistant colonies among the plasmid ratios tested. Genomic PCR and sequencing confirmed precise locus-specific insertion without detectable random integration in the analyzed clones. These findings establish Cre-IDLVs as a streamlined dual-delivery platform that couples transient recombinase activity with episomal donor DNA transfer. This hybrid lentiviral strategy provides a programmable approach for controlled and site-specific genome modification in mammalian cells.

Integrases

Advances in large-scale DNA engineering with the CRISPR system.

In recent years, DNA engineering technology has undergone significant advancements, with clustered regularly interspaced short palindromic repeats (CRISPR)-based target-specific DNA insertion emerging as one of the most rapidly expanding and widely studied approaches. Traditional DNA insertion technologies employing recombinases typically involve introducing foreign DNA into genes in vivo by either pre-engineering recognition sequences specific to the recombinase or through genetic crossing to incorporate the requisite recognition sequence into the target gene. However, CRISPR-based gene insertion technologies have advanced to streamline this engineering process by combining the CRISPR-Cas module with recombinase enzymes. This process enables accurate and efficient one-step insertion of foreign DNA into the target gene in vivo. Here we provide an overview of the latest developments in CRISPR-based gene insertion technologies and discusses their potential future applications.

CRISPR-Cas Systems

Distinct roles of the two BRCA2 DNA-binding domains in DNA damage repair and replication fork preservation.

Homologous recombination (HR) removes DNA double-strand breaks (DSBs) and preserves stressed DNA replication forks. Successful HR execution requires the tumor suppressor BRCA2, which harbors distinct DNA-binding domains (DBDs): one that possesses three oligonucleotide/oligosaccharide-binding (OB) folds (OB-DBD) and another residing in the C-terminal recombinase binding domain (CTRB-DBD). Here, we employ multi-faceted approaches to delineate the contributions of these domains toward HR and replication fork maintenance. We show that OB-DBD and CTRB-DBD confer single-strand DNA (ssDNA)- and dsDNA-binding capabilities, respectively, and that BRCA2 variants mutated in either domain are impaired in their ability to load the recombinase RAD51 onto ssDNA pre-occupied by RPA. While the CTRB-DBD mutant is modestly affected by DNA break repair, it exhibits a strong defect in the protection of stressed replication forks. In contrast, the OB-DBD is indispensable for both BRCA2 functions. Our study thus defines the unique contributions of the two BRCA2 DBDs in genome maintenance.

BRCA2 Protein

Programmable promoter editing for precise control of transgene expression.

Subtle changes in gene expression direct cells to distinct cellular states. Identifying and controlling dose-dependent transgenes require tools for precisely titrating expression. Here, we develop a highly modular, extensible framework called DIAL for building editable promoters that allow for fine-scale, heritable changes in transgene expression. Using DIAL, we increase expression by recombinase-mediated excision of spacers between the binding sites of a synthetic zinc finger transcription factor and the core promoter. By nesting varying numbers and lengths of spacers, DIAL generates a tunable range of unimodal setpoints from a single promoter. Through small-molecule control of transcription factors and recombinases, DIAL supports temporally defined, user-guided control of transgene expression that is extensible to additional transcription factors. Lentiviral delivery of DIAL generates multiple setpoints in primary cells and induced pluripotent stem cells. As promoter editing generates stable states, DIAL setpoints are heritable, facilitating mapping of transgene levels to phenotype and fate in direct conversion to induced motor neurons. The DIAL framework opens opportunities for tailoring transgene expression and improving the predictability and performance of gene circuits across diverse applications.

Promoter Regions, Genetic

An Update on Inborn Errors of V(D)J Recombination.

V(D)J recombination is the fundamental process by which developing T and B lymphocytes generate diverse antigen receptors, enabling adaptive immunity. This tightly regulated program operates exclusively in lymphoid precursors during G1 phase and depends on the lymphocyte-specific RAG1-RAG2 recombinase to introduce programmed DNA double-strand breaks at recombination signal sequences, followed by repair through the classical nonhomologous end joining (c-NHEJ) pathway. Disruption of any step in this molecular choreography compromises antigen receptor diversity and underlies a spectrum of inborn errors of immunity (IEIs), ranging from severe combined immunodeficiency (SCID) to immune dysregulation with autoimmunity and granulomatous disease. In this review, we place disorders of V(D)J recombination within the broader framework of T-cell development, detailing the temporal waves of recombinase activity, chromatin accessibility, and DNA damage responses that guide thymocyte differentiation. We discuss pathogenic variants affecting the cleavage phase [RAG1, RAG2, and the recently identified RAG cochaperone NudC domain-containing 3 (NUDCD3)], end processing (ARTEMIS), ligation and repair (LIG4, XLF, XRCC4, PRKDC), and genome surveillance pathways (ATM, MRN complex, RNF168), highlighting genotype-phenotype correlations and mechanisms driving immune deficiency and dysregulation. We briefly review recent diagnostic advances, including newborn screening using T-cell receptor excision circles, repertoire sequencing, and functional assays, alongside current therapeutic strategies. Finally, we outline key unanswered questions and argue that continued integration of clinical observation with molecular discovery is essential to improve outcomes and deepen understanding of adaptive immune development.

Humans

An RPA-assisted homogeneous electrochemical DNA sensor for on-site eDNA detection toward early warning of crown-of-thorns starfish outbreaks.

Crown-of-thorns starfish (COTS) outbreaks seriously threaten coral reef ecosystems, while conventional monitoring approaches are time-consuming and often lack sufficient sensitivity for early warning. Existing electrochemical DNA sensors usually require complex electrode-surface immobilization procedures, which can lead to uneven probe distribution, significant steric hindrance, and poor stability. Meanwhile, the low concentration of environmental DNA (eDNA) in marine environments further complicates detection. To overcome these challenges, this study developed a homogeneous electrochemical DNA sensor assisted by recombinase polymerase amplification (RPA) for COTS eDNA detection. Target DNA was first amplified by RPA, and the amplification products were then hybridized in solution with capture probe (CP)-modified magnetic beads (MB) and biotin-labeled signal probe (SP) to form sandwich-structured MB complexes. These complexes were subsequently magnetically enriched and immobilized on the electrode surface for electrochemical signal readout. Under optimized conditions, the sensor displayed a linear response to COTS genomic DNA from 3.77&#xa0;fg/&#x3bc;L to 1&#xa0;ng/&#x3bc;L, with an LOD of 2.02&#xa0;fg/&#x3bc;L and an LOQ of 3.77&#xa0;fg/&#x3bc;L. The sensor was applied to Xisha Islands samples, and the results agreed with droplet digital PCR (ddPCR) (P&#xa0;>&#xa0;0.05), demonstrating its potential for sensitive and reliable on-site COTS eDNA detection.

Animals

How to Study Gene Expression and Gain of Function of Hoxb1 in Mouse Heart Development.

Anterior Hox genes are required for genetic identity and anterior posterior patterning of the second heart field (SHF), which contributes to the formation of the embryonic heart in vertebrates. Defective contribution of SHF cells to the arterial or venous pole of the heart is often associated with severe congenital heart defects. The mouse Cre-lox system allows the activation of expression of any gene of interest in restricted tissues. We developed a gain of function approach that relies on the use of a CAG transgene to ectopically activate Hoxb1 expression in SHF cardiac progenitor cells through specific Cre activation. Therefore, we generated a floxed transgenic mouse line, CAG-Hoxb1-eGFP, which upon recombination by Cre recombinase conditionally induces robust Hoxb1 and eGFP expression. When induced within the anterior SHF lineage, we detected heart defects in mouse embryos such as right ventricular hypoplasia. Here, we describe the strategy for generating and genetically crossing this transgenic mouse line. We also provide detailed protocols for whole-mount embryo and paraffin section in situ RNAscope hybridization and X-gal staining allowing investigation of SHF contribution during heart development.

Animals

Small-Molecule Inhibitors Targeting DNA Repair and DNA Repair Deficiency in Research and Cancer Therapy.

To maintain stable genomes and to avoid cancer and aging, cells need to repair a multitude of deleterious DNA lesions, which arise constantly in every cell. Processes that support genome integrity in normal cells, however, allow cancer cells to develop resistance to radiation and DNA-damaging chemotherapeutics. Chemical inhibition of the key DNA repair proteins and pharmacologically induced synthetic lethality have become instrumental in both dissecting the complex DNA repair networks and as promising anticancer agents. The difficulty in capitalizing on synthetically lethal interactions in cancer cells is that many potential targets do not possess well-defined small-molecule binding determinates. In this review, we discuss several successful campaigns to identify and leverage small-molecule inhibitors of the DNA repair proteins, from PARP1, a paradigm case for clinically successful small-molecule inhibitors, to coveted new targets, such as RAD51 recombinase, RAD52 DNA repair protein, MRE11 nuclease, and WRN DNA helicase.

DNA Helicases

Molecular diagnostics and integrated management challenges of tobacco streak virus: Current status and future perspectives.

Tobacco streak virus (TSV) is an economically important viral pathogen causing severe yield and quality losses in several agricultural, horticultural and medicinal crops worldwide. Its complex epidemiology involving sap transmission, infected pollen and pollen-feeding thrips, together with symptom similarity to other necrosis-inducing pathogens, frequently results in misdiagnosis and delayed disease management. This review critically evaluates recent advances in TSV diagnostics and integrated disease management strategies. Particular emphasis is placed on the transition from conventional biological and serological assays to advanced molecular diagnostics including reverse transcription polymerase chain reaction (RT-PCR), quantitative real-time PCR, multiplex PCR and emerging isothermal amplification technologies such as recombinase polymerase amplification (RPA) and loop-mediated isothermal amplification (LAMP). The review also highlights emerging innovations including CRISPR/Cas-based diagnostics in addition, integrated management approaches involving phytosanitation, weed reservoir management, vector ecology-based, host resistance breeding, RNA interference (RNAi) and genome editing technologies are critically analysed. Major challenges including inadequate field validation, limited multiplex capability, poor assay standardization and scarcity of resistant cultivars are discussed. Future objectives to develop quick, field-adaptable and durable TSV detection and management methods are additionally discussed.

CRISPR/Cas diagnostics

DNA binding and RAD51 engagement by the BRCA2 C-terminus orchestrate DNA repair and replication fork preservation.

The tumor suppressor BRCA2 participates in DNA double-strand break repair by RAD51-dependent homologous recombination and protects stressed DNA replication forks from nucleolytic attack. We demonstrate that the C-terminal Recombinase Binding (CTRB) region of BRCA2, encoded by gene exon 27, harbors a DNA binding activity. CTRB alone stimulates the DNA strand exchange activity of RAD51 and permits the utilization of RPA-coated ssDNA by RAD51 for strand exchange. Moreover, CTRB functionally synergizes with the Oligonucleotide Binding fold containing DNA binding domain and BRC4 repeat of BRCA2 in RPA-RAD51 exchange on ssDNA. Importantly, we show that the DNA binding and RAD51 interaction attributes of the CTRB are crucial for homologous recombination and protection of replication forks against MRE11-mediated attrition. Our findings shed light on the role of the CTRB region in genome repair, reveal remarkable functional plasticity of BRCA2, and help explain why deletion of Brca2 exon 27 impacts upon embryonic lethality.

DNA Replication

BCDX2-CX3 and DX2-CX3 complexes assemble and stabilize RAD51 filaments.

The repair of DNA double-strand breaks by homologous recombination is essential for genomic integrity, and its dysregulation is a hallmark of cancer1. Central to homologous recombination is the RAD51 recombinase, whose assembly into a nucleoprotein filament is governed by five RAD51 paralogues (RAD51B, RAD51C, RAD51D, XRCC2 and XRCC3)2. Mutations in any of these proteins predispose individuals to multiple cancers or genetic disorders3-6. These paralogues are thought to form two functionally separate complexes RAD51B-RAD51C-RAD51D-XRCC2 (BCDX2) and RAD51C-XRCC3 (CX3), that act independently at different stages of homologous recombination7-11. Here we demonstrate that all five paralogues can assemble into a single, ATP-dependent BCDX2-CX3-RAD51 supercomplex. The architecture of this assembly bound to single-stranded DNA reveals a contiguous filament where the CX3 module stacks atop BCDX2, creating a protofilament template for RAD51 filament formation. We further identify a novel, RAD51B-independent DX2-CX3 complex (RAD51D-XRCC2-RAD51C-XRCC3) functioning as a stable RAD51 anchor on single-stranded DNA, and we capture it in multiple states, including capping RAD51 filament segment. These distinct assemblies are differentially regulated by ATPase activity, defining a dynamic BCDX2-CX3 'loader' and a stable DX2-CX3 'anchor' that provide functional modularity to the homologous recombination machinery. This work provides a unifying mechanism for human RAD51 paralogue function and delivers an atomic blueprint for interpreting disease-causing mutations.

Rad51 Recombinase

Moss BRCA2 lacking the canonical DNA-binding domain promotes homologous recombination and binds to DNA.

BRCA2 is crucial for mediating homology-directed DNA repair (HDR) through its binding to single-stranded DNA (ssDNA) and the recombinases RAD51 and DMC1. Most BRCA2 orthologs have a canonical DNA-binding domain (DBD) with the exception of Drosophila melanogaster. It remains unclear whether such a noncanonical BRCA2 variant without DBD possesses a DNA-binding activity. Here, we identify a new noncanonical BRCA2 in the model plant Physcomitrium patens (PpBRCA2). We establish that PpBRCA2 is essential for genome integrity maintenance, somatic DNA double-strand break (DSB) repair, HDR-mediated gene targeting, and RAD51 foci recruitment at DNA break sites. PpBRCA2 is also critical for DSB repair during meiosis. Interestingly, PpBRCA2 interacts strongly with RAD51 but weakly with DMC1, suggesting a distinct meiotic function compared to other BRCA2 homologs. Despite lacking the canonical DBD, PpBRCA2 binds ssDNA through its disordered N-terminal region and efficiently promotes HDR. Our work highlights that the ssDNA binding capacity of BRCA2 homologs is conserved regardless of the presence of a canonical DBD and provides a deeper understanding of BRCA2's functional diversity across species.

BRCA2 Protein