High-resolution specificity profiling and off-target prediction for site-specific DNA recombinases

Engineering spacer specificity of the Cre/loxP system

Translational research on the Cre/loxP recombination system focuses on enhancing its specificity by modifying Cre/DNA interactions. Despite extensive efforts, the exact mechanisms governing Cre discrimination between substrates remains elusive. Cre recognizes 13 bp inverted repeats, initiating recombination in the 8 bp spacer region. While literature suggests that efficient recombination proceeds between lox sites with non-loxP spacer sequences when both lox sites have matching spacers, experimental validation for this assumption is lacking. To fill this gap, we investigated target site variations of identical pairs of the loxP 8 bp spacer region, screening 6000 unique loxP-like sequences. Approximately 84% of these sites exhibited efficient recombination, affirming the plasticity of spacer sequences for catalysis. However, certain spacers negatively impacted recombination, emphasizing sequence dependence. Directed evolution of Cre on inefficiently recombined spacers not only yielded recombinases with enhanced activity but also mutants with reprogrammed selective activity. Mutations altering spacer specificity were identified, and molecular modelling and dynamics simulations were used to investigate the possible mechanisms behind the specificity switch. Our findings highlight the potential to fine-tune site-specific recombinases for spacer sequence specificity, offering a novel concept to enhance the applied properties of designer-recombinases for genome engineering applications.

Efficient site-specific integration of large genes in mammalian cells via continuously evolved recombinases and prime editing

Methods for the targeted integration of genes in mammalian genomes suffer from low programmability, low efficiencies or low specificities. Here we show that phage-assisted continuous evolution enhances prime-editing-assisted site-specific integrase gene editing (PASSIGE), which couples the programmability of prime editing with the ability of recombinases to precisely integrate large DNA cargoes exceeding 10 kilobases. Evolved and engineered Bxb1 recombinase variants (evoBxb1 and eeBxb1) mediated up to 60% donor integration (3.2-fold that of wild-type Bxb1) in human cell lines with pre-installed recombinase landing sites. In single-transfection experiments at safe-harbour and therapeutically relevant sites, PASSIGE with eeBxb1 led to an average targeted-gene-integration efficiencies of 23% (4.2-fold that of wild-type Bxb1). Notably, integration efficiencies exceeded 30% at multiple sites in primary human fibroblasts. PASSIGE with evoBxb1 or eeBxb1 outperformed PASTE (for 'programmable addition via site-specific targeting elements', a method that uses prime editors fused to recombinases) on average by 9.1-fold and 16-fold, respectively. PASSIGE with continuously evolved recombinases is an unusually efficient method for the targeted integration of genes in mammalian cells.

Protocol for the establishment of a serine integrase-based platform for functional validation of genetic switch controllers in eukaryotic cells

Serine integrases (Ints) are a family of site-specific recombinases (SSRs) encoded by some bacteriophages to integrate their genetic material into the genome of a host. Their ability to rearrange DNA sequences in different ways including inversion, excision, or insertion with no help from endogenous molecular machinery, confers important biotechnological value as genetic editing tools with high host plasticity. Despite advances in their use in prokaryotic cells, only a few Ints are currently used as gene editors in eukaryotes, partly due to the functional loss and cytotoxicity presented by some candidates in more complex organisms. To help expand the number of Ints available for the assembly of more complex multifunctional circuits in eukaryotic cells, this protocol describes a platform for the assembly and functional screening of serine-integrase-based genetic switches designed to control gene expression by directional inversions of DNA sequence orientation. The system consists of two sets of plasmids, an effector module and a reporter module, both sets assembled with regulatory components (as promoter and terminator regions) appropriate for expression in mammals, including humans, and plants. The complete method involves plasmid design, DNA delivery, testing and both molecular and phenotypical assessment of results. This platform presents a suitable workflow for the identification and functional validation of new tools for the genetic regulation and reprogramming of organisms with importance in different fields, from medical applications to crop enhancement, as shown by the initial results obtained. This protocol can be completed in 4 weeks for mammalian cells or up to 8 weeks for plant cells, considering cell culture or plant growth time.

Bacteriophage lambda site-specific recombination

The site-specific recombination pathway of bacteriophage λ encompasses isoenergetic but highly directional and tightly regulated integrative and excisive reactions that integrate and excise the vial chromosome into and out of the bacterial chromosome. The reactions require 240 bp of phage DNA and 21 bp of bacterial DNA comprising 16 protein binding sites that are differentially used in each pathway by the phage-encoded Int and Xis proteins and the host-encoded integration host factor and factor for inversion stimulation proteins. Structures of higher-order protein-DNA complexes of the four-way Holliday junction recombination intermediates provided clarifying insights into the mechanisms, directionality, and regulation of these two pathways, which are tightly linked to the physiology of the bacterial host cell. Here we review our current understanding of the mechanisms responsible for regulating and executing λ site-specific recombination, with an emphasis on key studies completed over the last decade.

Preclinical toxicity analyses of lentiviral vectors expressing the HIV-1 LTR-specific designer-recombinase Brec1

Drug-based antiretroviral therapies (ART) efficiently suppress HIV replication in humans, but the virus persists as integrated proviral reservoirs in small numbers of cells. Importantly, ART cannot eliminate HIV from an infected individual, since it does not target the integrated provirus. Therefore, genome editing-based strategies that can inactivate or excise HIV genomes would provide the technology for novel curative therapies. In fact, the HIV-1 LTR-specific designer-recombinase Brec1 has been shown to remove integrated proviruses from infected cells and is highly efficacious on clinical HIV-1 isolates in vitro and in vivo, suggesting that Brec1 has the potential for clinical development of advanced HIV-1 eradication strategies in people living with HIV. In line with the preparation of a first-in-human advanced therapy medicinal product gene therapy trial, we here present an extensive preclinical evaluation of Brec1 and lentiviral vectors expressing the Brec1 transgene. This included detailed functional analysis of potential genomic off-target sites, assessing vector safety by investigating vector copy number (VCN) and the risk for potential vector-related insertional mutagenesis, as well as analyzing the potential of Brec1 to trigger an undesired strong T cell immune response. In conclusion, the antiviral designer-recombinase Brec1 is shown to lack any detectable cytopathic, genotoxic or T cell-related immunogenic effects, thereby meeting an important precondition for clinical application of the therapeutic lentiviral vector LV-Brec1 in novel HIV-1 curative strategies.

Next generation synthetic memory via intercepting recombinase function

Here we present a technology to facilitate synthetic memory in a living system via repurposing Transcriptional Programming (i.e., our decision-making technology) parts, to regulate (intercept) recombinase function post-translation. We show that interception synthetic memory can facilitate programmable loss-of-function via site-specific deletion, programmable gain-of-function by way of site-specific inversion, and synthetic memory operations with nested Boolean logical operations. We can expand interception synthetic memory capacity more than 5-fold for a single recombinase, with reconfiguration specificity for multiple sites in parallel. Interception synthetic memory is ~10-times faster than previous generations of recombinase-based memory. We posit that the faster recombination speed of our next-generation memory technology is due to the post-translational regulation of recombinase function. This iteration of synthetic memory is complementary to decision-making via Transcriptional Programming - thus can be used to develop intelligent synthetic biological systems for myriad applications.

CRISPR technology: A decade of genome editing is only the beginning

The advent of clustered regularly interspaced short palindromic repeat (CRISPR) genome editing, coupled with advances in computing and imaging capabilities, has initiated a new era in which genetic diseases and individual disease susceptibilities are both predictable and actionable. Likewise, genes responsible for plant traits can be identified and altered quickly, transforming the pace of agricultural research and plant breeding. In this Review, we discuss the current state of CRISPR-mediated genetic manipulation in human cells, animals, and plants along with relevant successes and challenges and present a roadmap for the future of this technology.

VCre/VloxP and SCre/SloxP as Reliable Site-Specific Recombination Systems for Genome Engineering

The Cre/loxP system is a versatile and powerful tool that has been used to develop many kinds of genetically modified mice, such as conditional knockout mice and mutant protein-expressing mice through the excision of a STOP cassette. However, while numerous in vivo and in vitro applications of the Cre/loxP system have been reported, it remains difficult to target at one time more than one set of recognition sites in an identical single cell in mice using the Cre/loxP system. To overcome this barrier, we developed two novel site-specific recombination systems called VCre/VloxP and SCre/SloxP. These systems allow multiple independent site-specific recombination, for example, multiple targeted deletions in the same cell at different times. In this chapter, I describe the features of VCre/VloxP and SCre/SloxP, practical protocols and tips on how to use them in genomic engineering applications, potential problems in their use, and how problems can be identified and solved.

Prediction of designer-recombinases for DNA editing with generative deep learning

Site-specific tyrosine-type recombinases are effective tools for genome engineering, with the first engineered variants having demonstrated therapeutic potential. So far, adaptation to new DNA target site selectivity of designer-recombinases has been achieved mostly through iterative cycles of directed molecular evolution. While effective, directed molecular evolution methods are laborious and time consuming. Here we present RecGen (Recombinase Generator), an algorithm for the intelligent generation of designer-recombinases. We gather the sequence information of over one million Cre-like recombinase sequences evolved for 89 different target sites with which we train Conditional Variational Autoencoders for recombinase generation. Experimental validation demonstrates that the algorithm can predict recombinase sequences with activity on novel target-sites, indicating that RecGen is useful to accelerate the development of future designer-recombinases.

Model-guided engineering of DNA sequences with predictable site-specific recombination rates

Site-specific recombination (SSR) is an important tool in synthetic biology, but its applications are limited by the inability to predictably tune SSR reaction rates. Facile rate manipulation could be achieved by modifying the DNA substrate sequence; however, this approach lacks rational design principles. Here, we develop an integrated experimental and computational method to engineer the DNA attachment sequence attP for predictably modulating the inversion reaction mediated by the recombinase Bxb1. After developing a qPCR method to measure SSR reaction rate, we design, select, and sequence attP libraries to inform a machine-learning model that computes Bxb1 inversion rate as a function of attP sequence. We use this model to predict reaction rates of attP variants in vitro and demonstrate their utility in gene circuit design in Escherichia coli. Our high-throughput, model-guided approach for rationally tuning SSR reaction rates enhances our understanding of recombinase function and expands the synthetic biology toolbox.

A Novel Cre/lox71-Based System for Inducible Expression of Recombinant Proteins and Genome Editing

In this study, we developed a novel Cre/lox71-based system for the controlled transient expression of target genes. We used the bacteriophage P1 Cre recombinase, which harbors a short, highly specific DNA-binding site and does not have endogenous binding sites within mouse or human genomes. Fusing the catalytically inactive form of Cre recombinase and the VP64 transactivation domain (VP16 tetramer), we constructed the artificial transcription factor Cre-VP64. This transcription factor binds to the lox71 sites within the promoter region of the target gene and, therefore, upregulates its expression. We tested the Cre-VP64/lox71 system for the controlled expression of several genes, including growth factors and the genome editor CRISPR/Cas9, and obtained superior efficiency in the regulation of transgene expression, achieving a high expression level upon induction together with low basal activity. This system or its modified forms can be suggested as a novel effective tool for the transitory controlled expression of target genes for functional genomic studies, as well as for gene therapy approaches.

HIV-1 Reservoir Persistence and Decay: Implications for Cure Strategies

PURPOSE OF REVIEW

Despite suppressive antiretroviral therapy (ART), a viral reservoir persists in individuals living with HIV that can reignite systemic replication should treatment be interrupted. Understanding how HIV-1 persists through effective ART is essential to develop cure strategies to induce ART-free virus remission.

RECENT FINDINGS

The HIV-1 reservoir resides in a pool of CD4-expressing cells as a range of viral species, a subset of which is genetically intact. Recent studies suggest that the reservoir on ART is highly dynamic, with expansion and contraction of virus-infected cells over time. Overall, the intact proviral reservoir declines faster than defective viruses, suggesting enhanced immune clearance or cellular turnover. Upon treatment interruption, rebound viruses demonstrate escape from adaptive and innate immune responses, implicating these selective pressures in restriction of virus reactivation. Cure strategies employing immunotherapy are poised to test whether host immune pressure can be augmented to enhance reservoir suppression or clearance. Alternatively, genomic engineering approaches are being applied to directly eliminate intact viruses and shrink the replication-competent virus pool. New evidence suggests host immunity exerts selective pressure on reservoir viruses and clears HIV-1 infected cells over years on ART. Efforts to build on the detectable, but insufficient, reservoir clearance via empiric testing in clinical trials will inform our understanding of mechanisms of viral persistence and the direction of future cure strategies.

Programmable deletion, replacement, integration and inversion of large DNA sequences with twin prime editing

The targeted deletion, replacement, integration or inversion of genomic sequences could be used to study or treat human genetic diseases, but existing methods typically require double-strand DNA breaks (DSBs) that lead to undesired consequences, including uncontrolled indel mixtures and chromosomal abnormalities. Here we describe twin prime editing (twinPE), a DSB-independent method that uses a prime editor protein and two prime editing guide RNAs (pegRNAs) for the programmable replacement or excision of DNA sequences at endogenous human genomic sites. The two pegRNAs template the synthesis of complementary DNA flaps on opposing strands of genomic DNA, which replace the endogenous DNA sequence between the prime-editor-induced nick sites. When combined with a site-specific serine recombinase, twinPE enabled targeted integration of gene-sized DNA plasmids (>5,000 bp) and targeted sequence inversions of 40 kb in human cells. TwinPE expands the capabilities of precision gene editing and might synergize with other tools for the correction or complementation of large or complex human pathogenic alleles.

Mechanisms of Cre recombinase synaptic complex assembly and activation illuminated by Cryo-EM

Cre recombinase selectively recognizes DNA and prevents non-specific DNA cleavage through an orchestrated series of assembly intermediates. Cre recombines two loxP DNA sequences featuring a pair of palindromic recombinase binding elements and an asymmetric spacer region, by assembly of a tetrameric synaptic complex, cleavage of an opposing pair of strands, and formation of a Holliday junction intermediate. We used Cre and loxP variants to isolate the monomeric Cre-loxP (54 kDa), dimeric Cre₂-loxP (110 kDa), and tetrameric Cre₄-loxP₂ assembly intermediates, and determined their structures using cryo-EM to resolutions of 3.9, 4.5 and 3.2 Å, respectively. Progressive and asymmetric bending of the spacer region along the assembly pathway enables formation of increasingly intimate interfaces between Cre protomers and illuminates the structural bases of biased loxP strand cleavage order and half-the-sites activity. Application of 3D variability analysis to the tetramer data reveals constrained conformational sampling along the pathway between protomer activation and Holliday junction isomerization. These findings underscore the importance of protein and DNA flexibility in Cre-mediated site selection, controlled activation of alternating protomers, the basis for biased strand cleavage order, and recombination efficiency. Such considerations may advance development of site-specific recombinases for use in gene editing applications.

Pairing of single mutations yields obligate Cre-type site-specific recombinases

Tyrosine site-specific recombinases (SSRs) represent a versatile genome editing tool with considerable therapeutic potential. Recent developments to engineer and evolve SSRs into heterotetramers to improve target site flexibility signified a critical step towards their broad utility in genome editing. However, SSR monomers can form combinations of different homo- and heterotetramers in cells, increasing their off-target potential. Here, we discover that two paired mutations targeting residues implicated in catalysis lead to simple obligate tyrosine SSR systems, where the presence of all distinct subunits to bind as a heterotetramer is obligatory for catalysis. Therefore, only when the paired mutations are applied as single mutations on each recombinase subunit, the engineered SSRs can efficiently recombine the intended target sequence, while the subunits carrying the point mutations expressed in isolation are inactive. We demonstrate the utility of the obligate SSR system to improve recombination specificity of a designer-recombinase for a therapeutic target in human cells. Furthermore, we show that the mutations render the naturally occurring SSRs, Cre and Vika, obligately heteromeric for catalytic proficiency, providing a straight-forward approach to improve their applied properties. These results facilitate the development of safe and effective therapeutic designer-recombinases and advance our mechanistic understanding of SSR catalysis.

Nearest-Neighbor Effects Modulate loxP Spacer DNA Chemical Shifts and Guide Oligonucleotide Design for Nuclear Magnetic Resonance Studies

The Cre-loxP gene editing tool enables site-specific editing of DNA without leaving lesions that must be repaired by error-prone cellular processes. Cre recombines two 34-bp loxP DNA sites that feature a pair of palindromic recombinase-binding elements flanking an asymmetric 8-bp spacer region, via assembly of a tetrameric intasome complex and formation of a Holliday junction intermediate. Recombination proceeds by coordinated nucleophilic attack by pairs of catalytic tyrosine residues on specific phosphodiester bonds in the spacer regions of opposing strands. Despite not making base-specific contacts with the asymmetric spacer region of the DNA, Cre exhibits a preference for initial cleavage on one of the strands, suggesting that intrinsic properties of the uncontacted 8-bp spacer region give rise to this preference. Furthermore, little is known about the structural and dynamic features of the loxP spacer that make it a suitable target for Cre. To enable NMR spectroscopic studies of the spacer, we have aimed to identify a fragment of the 34-bp loxP site that retains the structural features of the spacer while minimizing the spectral crowding and line-broadening seen in longer oligonucleotides. Sequence-specific chemical shift differences between spacer oligos of different lengths, and of a mutant that inverts strand cleavage order, reveal how both nearest-neighbor and next-nearest-neighbor effects dominate the chemical environment experienced by the spacer. We have identified a 16-bp oligonucleotide that preserves the structural environment of the spacer, setting the stage for NMR-based structure determination and dynamics investigations.

DNA methylation occurring in Cre-expressing cells inhibits loxP recombination and silences loxP-sandwiched genes

The low DNA recombination efficiency of site-specific recombinase systems in plants limits their application; however, the underlying mechanism is unknown. We evaluate the gene deletion performance of four recombinase systems (Cre/loxP, Flp/FRT, KD/KDRT and B3/B3RT) in tobacco where the recombinases are under the control of germline-specific promoters. We find that the expression of these recombinases results mostly in gene silencing rather than gene deletion. Using the Cre/loxP system as a model, we reveal that the region flanked by loxP sites (floxed) is hypermethylated, which prevents floxed genes from deletion while silencing the expression of the genes. We further show CG methylation alone in the recombinase binding element of the loxP site is unable to impede gene deletion; instead, CHH methylation in the crossover region is required to inhibit loxP recombination. Our study illustrates the important role of recombinase-induced DNA methylation in the inhibition of site-specific DNA recombination and uncovers the mechanism underlying recombinase-associated gene silence in plants.

Sequence modification on demand: search and replace tools for precise gene editing in plants

Until recently, our ability to generate allelic diversity in plants was limited to introduction of variants from domesticated and wild species by breeding via uncontrolled recombination or the use of chemical and physical mutagens-processes that are lengthy and costly or lack specificity, respectively. Gene editing provides a faster and more precise way to create new variation, although its application in plants has been dominated by the creation of short insertion and deletion mutations leading to loss of gene function, mostly due to the dependence of editing outcomes on DNA repair pathway choices intrinsic to higher eukaryotes. Other types of edits such as point mutations and precise and pre-designed targeted sequence insertions have rarely been implemented, despite providing means to modulate the expression of target genes or to engineer the function and stability of their protein products. Several advancements have been developed in recent years to facilitate custom editing by regulation of repair pathway choices or by taking advantage of alternative types of DNA repair. We have seen the advent of novel gene editing tools that are independent of DNA double-strand break repair, and methods completely independent of host DNA repair processes are being increasingly explored. With the aim to provide a comprehensive review of the state-of-the-art methodology for allele replacement in plants, I discuss the adoption of these improvements for plant genome engineering.

Expanding the Potential of Mammalian Genome Engineering via Targeted DNA Integration

Inserting custom designed DNA sequences into the mammalian genome plays an essential role in synthetic biology. In particular, the ability to introduce foreign DNA in a site-specific manner offers numerous advantages over random DNA integration. In this review, we focus on two mechanistically distinct systems that have been widely adopted for targeted DNA insertion in mammalian cells, the CRISPR/Cas9 system and site-specific recombinases. The CRISPR/Cas9 system has revolutionized the genome engineering field thanks to its high programmability and ease of use. However, due to its dependence on linearized DNA donor and endogenous cellular pathways to repair the induced double-strand break, CRISPR/Cas9-mediated DNA insertion still faces limitations such as small insert size, and undesired editing outcomes via error-prone repair pathways. In contrast, site-specific recombinases, in particular the Serine integrases, demonstrate large-cargo capability and no dependence on cellular repair pathways for DNA integration. Here we first describe recent advances in improving the overall efficacy of CRISPR/Cas9-based methods for DNA insertion. Moreover, we highlight the advantages of site-specific recombinases over CRISPR/Cas9 in the context of targeted DNA integration, with a special focus on the recent development of programmable recombinases. We conclude by discussing the importance of protein engineering to further expand the current toolkit for targeted DNA insertion in mammalian cells.

Genome editing with CRISPR-Cas nucleases, base editors, transposases and prime editors

The development of new CRISPR-Cas genome editing tools continues to drive major advances in the life sciences. Four classes of CRISPR-Cas-derived genome editing agents-nucleases, base editors, transposases/recombinases and prime editors-are currently available for modifying genomes in experimental systems. Some of these agents have also moved rapidly into the clinic. Each tool comes with its own capabilities and limitations, and major efforts have broadened their editing capabilities, expanded their targeting scope and improved editing specificity. We analyze key considerations when choosing genome editing agents and identify opportunities for future improvements and applications in basic research and therapeutics.

Nucleofection of phiC31 Integrase Protein Mediates Sequence-Specific Genomic Integration in Human Cells

The phage-derived phiC31 integrase is a useful tool for mediating sequence-specific genomic integration in mammalian cells, recombining donor plasmids bearing the attB recognition site with introduced genomic attP sites or endogeneous pseudo-attP sites having partial identity to attP. In most prior studies, phiC31 integrase has been introduced as plasmid DNA or mRNA. The current report examines whether phiC31 integrase functions efficiently in mammalian cells when co-nucleofected as a purified protein, along with attB-containing donor plasmids or PCR fragments. We describe preparation of phiC31 integrase protein and evidence that it can mediate genomic integration in human 293 cells, including PCR evidence for integration at an endogenous pseudo-attP site. This work demonstrates for the first time the ability of 605- and 613-amino-acid versions of phiC31 integrase protein to mediate efficient, site-specific integration into the genome of human cells when co-nucleofected with full-sizedattB-containing donor plasmids or linear 2.5-kb PCR fragments. This protein-mediated approach may be especially useful for integration of exogenous sequences into valuable therapeutic target cells, such as hematopoietic stem cells or T cells, that are sensitive to introduced DNA.

Sources of off-target expression from recombinase-dependent AAV vectors and mitigation with cross-over insensitive ATG-out vectors

In combination with transgenic mouse lines expressing Cre or Flp recombinases in defined cell types, recombinase-dependent adeno-associated viruses (AAVs) have become the tool of choice for localized cell-type-targeted gene expression. Unfortunately, applications of this technique when expressing highly sensitive transgenes are impeded by off-target, or "leak" expression, from recombinase-dependent AAVs. We investigated this phenomenon and find that leak expression is mediated by both infrequent transcription from the inverted transgene in recombinant-dependent AAV designs and recombination events during bacterial AAV plasmid production. Recombination in bacteria is mediated by homology across the antiparallel recombinase-specific recognition sites present in recombinase-dependent designs. To address both of these issues we designed an AAV vector that uses mutant "cross-over insensitive" recognition sites combined with an "ATG-out" design. We show that these CIAO (cross-over insensitive ATG-out) vectors virtually eliminate leak expression. CIAO vectors provide reliable and targeted transgene expression and are extremely useful for recombinase-dependent expression of highly sensitive transgenes.