Biochemical DSB-repair model for mammalian cells in G1 and early S phases of the cell cycle

qTAG: an adaptable plasmid scaffold for CRISPR-based endogenous tagging

Endogenous tagging enables the study of proteins within their native regulatory context, typically using CRISPR to insert tag sequences directly into the gene sequence. Here, we introduce qTAG, a collection of repair cassettes that makes endogenous tagging more accessible. The cassettes support N- and C-terminal tagging with commonly used selectable markers and feature restriction sites for easy modification. Lox sites also enable the removal of the marker gene after successful integration. We demonstrate the utility of qTAG with a range of diverse tags for applications in fluorescence imaging, proximity labeling, epitope tagging, and targeted protein degradation. The system includes novel tags like mStayGold, offering enhanced brightness and photostability for live-cell imaging of native protein dynamics. Additionally, we explore alternative cassette designs for conditional expression tagging, selectable knockout tagging, and safe-harbor expression. The plasmid collection is available through Addgene, featuring ready-to-use constructs for common subcellular markers and tagging cassettes to target genes of interest. The qTAG system will serve as an open resource for researchers to adapt and tailor their own experiments.

Enhancing CRISPR-Cas-based gene targeting in tomato using a dominant-negative ku80

The CRISPR-Cas-based gene targeting (GT) method has enabled precise modifications of genomic DNA ranging from single base to several kilobase scales through homologous recombination (HR). In plant somatic cells, canonical non-homologous end-joining (cNHEJ) is the predominant mechanism for repairing double-stranded breaks (DSBs), thus limiting the HR-mediated GT. In this study, we implemented an approach to shift the repair pathway preference toward HR by using a dominant-negative ku80 mutant protein (KUDN) to disrupt the initiation of cNHEJ. The employment of KUDN conferred a 1.71- to 3.55-fold improvement in GT efficiency at the callus stage. When we screened transformants, there was a more remarkable increase in GT efficiency, ranging from 1.62- to 9.84-fold, at two specific tomato loci, SlHKT1;2 and SlEPSPS1. With practical levels of efficiency, this enhanced KUDN-based GT tool successfully facilitated a 9-bp addition at an additional locus, SlCAB13. These findings provide another promising method for more efficient and precise plant breeding.

Integrating chromosome conformation and DNA repair in a computational framework to assess cell radiosensitivity. Phys Med Biol 2024;69:245017. [PMID: 39569898 DOI: 10.1088/1361-6560/ad94c6] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Objective.The arrangement of chromosomes in the cell nucleus has implications for cell radiosensitivity. The development of new tools to utilize Hi-C chromosome conformation data in nanoscale radiation track structure simulations allows forin silicoinvestigation of this phenomenon. We have developed a framework employing Hi-C-based cell nucleus models in Monte Carlo radiation simulations, in conjunction with mechanistic models of DNA repair, to predict not only the initial radiation-induced DNA damage, but also the repair outcomes resulting from this damage, allowing us to investigate the role chromosome conformation plays in the biological outcome of radiation exposure.Approach.In this study, we used this framework to generate cell nucleus models based on Hi-C data from fibroblast and lymphoblastoid cells and explore the effects of cell type-specific chromosome structure on radiation response. The models were used to simulate external beam irradiation including DNA damage and subsequent DNA repair. The kinetics of the simulated DNA repair were compared with previous results.Main results.We found that the fibroblast models resulted in a higher rate of inter-chromosome misrepair than the lymphoblastoid model, despite having similar amounts of initial DNA damage and total misrepairs for each irradiation scenario.Significance.This framework represents a step forward in radiobiological modeling and simulation allowing for more realistic investigation of radiosensitivity in different types of cells.

Modelling radiobiology

Radiotherapy has played an essential role in cancer treatment for over a century, and remains one of the best-studied methods of cancer treatment. Because of its close links with the physical sciences, it has been the subject of extensive quantitative mathematical modelling, but a complete understanding of the mechanisms of radiotherapy has remained elusive. In part this is because of the complexity and range of scales involved in radiotherapy-from physical radiation interactions occurring over nanometres to evolution of patient responses over months and years. This review presents the current status and ongoing research in modelling radiotherapy responses across these scales, including basic physical mechanisms of DNA damage, the immediate biological responses this triggers, and genetic- and patient-level determinants of response. Finally, some of the major challenges in this field and potential avenues for future improvements are also discussed.

Cosmic Radiations and the Cardiovascular System: A Narrative Review

In recent times, space flights receive continued interest. Humankind's next two goals are to return to the Moon and, a few years later, to land on the surface of Mars. Although technology will improve enough to enable long voyages, there are still some unresolved questions about the effects of the space environment on human health, including the effects of such long voyages on organs. Specifically, there is no information on the effects of radiation in space on the human cardiovascular system. To better understand the adaptation of the cardiovascular system to radiation exposure, the physical properties of radiation and the cellular and molecular mechanisms underlying tissue changes are essential. To this end, this article aims to provide an overview of the effects of radiation on the cardiovascular system by analyzing the physical properties of radiation and their relationship to cellular and molecular mechanisms and potential changes. Each type of radiation triggers different responses in the cardiovascular system. Radiation plays a relevant role in altering endothelial function and arterial wall stiffness by inducing vascular changes that accelerate atherosclerosis and affect endothelial adhesiveness. Clinical studies have shown that vascular changes due to radiation depend on the delayed manifestations of early radiation damage. To reduce the effects of radiation in space, some pharmacological treatments that seem to be able to counteract oxidative stress during flight are being used. At the same time, new shielding systems that can reduce or eliminate radiation exposure must be developed. Future studies should aim to replicate flights in the deep space environment to study in more detail the harmful effects of radiation on the whole cardiovascular system.

Modelling DNA damage-repair and beyond

The paper presents a review of mechanistic modelling studies of DNA damage and DNA repair, and consequences to follow in mammalian cell nucleus. We hypothesize DNA deletions are consequences of repair of double strand breaks leading to the modifications of genome that play crucial role in long term development of genetic inheritance and diseases. The aim of the paper is to review formation mechanisms underlying naturally occurring DNA deletions in the human genome and their potential relevance for bridging the gap between induced DNA double strand breaks and deletions in damaged human genome from endogenous and exogenous events. The model of the cell nucleus presented enables simulation of DNA damage at molecular level identifying the spectrum of damage induced in all chromosomal territories and loops. Our mechanistic modelling of DNA repair for double stand breaks (DSB), single strand breaks (SSB) and base damage (BD), shows the complexity of DNA damage is responsible for the longer repair times and the reason for the biphasic feature of mammalian cells repair curves. In the absence of experimentally determined data, the mechanistic model of repair predicts the in vivo rate constants for the proteins involved in the repair of DSB, SSB, and of BD.

DNA break clustering as a predictor of cell death across various radiation qualities: influence of cell size, cell asymmetry, and beam orientation

Cosmic radiation, composed of high charge and energy (HZE) particles, causes cellular DNA damage that can result in cell death or mutation that can evolve into cancer. In this work, a cell death model is applied to several cell lines exposed to HZE ions spanning a broad range of linear energy transfer (LET) values. We hypothesize that chromatin movement leads to the clustering of multiple double strand breaks (DSB) within one radiation-induced foci (RIF). The survival probability of a cell population is determined by averaging the survival probabilities of individual cells, which is function of the number of pairwise DSB interactions within RIF. The simulation code RITCARD was used to compute DSB. Two clustering approaches were applied to determine the number of RIF per cell. RITCARD outputs were combined with experimental data from four normal human cell lines to derive the model parameters and expand its predictions in response to ions with LET ranging from ~0.2 keV/μm to ~3000 keV/μm. Spherical and ellipsoidal nuclear shapes and two ion beam orientations were modeled to assess the impact of geometrical properties on cell death. The calculated average number of RIF per cell reproduces the saturation trend for high doses and high-LET values that is usually experimentally observed. The cell survival model generates the recognizable bell shape of LET dependence for the relative biological effectiveness (RBE). At low LET, smaller nuclei have lower survival due to increased DNA density and DSB clustering. At high LET, nuclei with a smaller irradiation area-either because of a smaller size or a change in beam orientation-have a higher survival rate due to a change in the distribution of DSB/RIF per cell. If confirmed experimentally, the geometric characteristics of cells would become a significant factor in predicting radiation-induced biological effects. Insight Box: High-charge and energy (HZE) ions are characterized by dense linear energy transfer (LET) that induce unique spatial distributions of DNA damage in cell nuclei that result in a greater biological effect than sparsely ionizing radiation like X-rays. HZE ions are a prominent component of galactic cosmic ray exposure during human spaceflight and specific ions are being used for radiotherapy. Here, we model DNA damage clustering at sub-micrometer scale to predict cell survival. The model is in good agreement with experimental data for a broad range of LET. Notably, the model indicates that nuclear geometry and ion beam orientation affect DNA damage clustering, which reveals their possible role in mediating cell radiosensitivity.

A fast Monte Carlo cell-by-cell simulation for radiobiological effects in targeted radionuclide therapy using pre-calculated single-particle track standard DNA damage data

Introduction

We developed a new method that drastically speeds up radiobiological Monte Carlo radiation-track-structure (MC-RTS) calculations on a cell-by-cell basis.

Methods

The technique is based on random sampling and superposition of single-particle track (SPT) standard DNA damage (SDD) files from a "pre-calculated" data library, constructed using the RTS code TOPAS-nBio, with "time stamps" manually added to incorporate dose-rate effects. This time-stamped SDD file can then be input into MEDRAS, a mechanistic kinetic model that calculates various radiation-induced biological endpoints, such as DNA double-strand breaks (DSBs), misrepairs and chromosomal aberrations, and cell death. As a benchmark validation of the approach, we calculated the predicted energy-dependent DSB yield and the ratio of direct-to-total DNA damage, both of which agreed with published in vitro experimental data. We subsequently applied the method to perform a superfast cell-by-cell simulation of an experimental in vitro system consisting of neuroendocrine tumor cells uniformly incubated with 177Lu.

Results and discussion

The results for residual DSBs, both at 24 and 48 h post-irradiation, are in line with the published literature values. Our work serves as a proof-of-concept demonstration of the feasibility of a cost-effective "in silico clonogenic cell survival assay" for the computational design and development of radiopharmaceuticals and novel radiotherapy treatments more generally.

Comparison of Multiple Strategies for Precision Transgene Knock-In in Gallus gallus Genome via Microhomology-Mediated End Joining

Precision exogenous gene knock-in is an attractive field for transgenic Gallus gallus (chicken) generation. In this article, we constructed multiple Precise Integration into Target Chromosome (PITCh) plasmid systems mediated by microhomology-mediated end-joining (MMEJ) for large-fragment integration in DF-1 cells and further assess the possibility of GAPDH (glyceraldehyde-3-phosphate dehydrogenase) as a genomic safe harbor for chickens. We designed three targeted sgRNAs for the all-in-one plasmid at the 3'UTR of GAPDH near the stop codon. The donor-plasmid-carrying microhomology arms correspond to sgRNA and EGFP fragments in the forward and reverse directions. MMEJ-mediated EGFP insertion can be efficiently expressed in DF-1 cells. Moreover, the differences between the forward and reverse fragments indicated that promoter interference does affect the transfection efficiency of plasmids and cell proliferation. The comparison of the 20 bp and 40 bp microhomology arms declared that the short one has higher knock-in efficiency. Even though all three different transgene insertion sites in GAPDH could be used to integrate the foreign gene, we noticed that the G2-20R-EGFP cell reduced the expression of GAPDH, and the G3-20R-EGFP cell exhibited significant growth retardation. Taken together, G1, located at the 3'UTR of GAPDH on the outer side of the last base of the terminator, can be a candidate genomic safe harbor (GSH) loci for the chicken genome. In addition, deleted-in-azoospermia-like (DAZL) and actin beta (ACTB) site-specific gene knock-in indicated that MMEJ has broad applicability and high-precision knock-in efficiency for genetically engineered chickens.

Dose-Dependent Shift in Relative Contribution of Homologous Recombination to DNA Repair after Low-LET Ionizing Radiation Exposure: Empirical Evidence and Numerical Simulation

Understanding the relative contributions of different repair pathways to radiation-induced DNA damage responses remains a challenging issue in terms of studying the radiation injury endpoints. The comparative manifestation of homologous recombination (HR) after irradiation with different doses greatly determines the overall effectiveness of recovery in a dividing cell after irradiation, since HR is an error-free mechanism intended to perform the repair of DNA double-strand breaks (DSB) during S/G2 phases of the cell cycle. In this article, we present experimentally observed evidence of dose-dependent shifts in the relative contributions of HR in human fibroblasts after X-ray exposure at doses in the range 20-1000 mGy, which is also supported by quantitative modeling of DNA DSB repair. Our findings indicate that the increase in the radiation dose leads to a dose-dependent decrease in the relative contribution of HR in the entire repair process.

Live cell single-molecule imaging to study DNA repair in human cells

The genetic material in human cells is continuously exposed to a wide variety of insults that can induce different DNA lesions. To maintain genomic stability and prevent potentially deleterious genetic changes caused by DNA damage, mammalian cells have evolved a number of pathways that repair specific types of DNA damage. These DNA repair pathways vary in their accuracy, some providing high-fidelity repair while others are error-prone and are only activated as a last resort. Adding additional complexity to cellular mechanisms of DNA repair is the DNA damage response which is a sophisticated a signaling network that coordinates repair outcomes, cell-cycle checkpoint activation, and cell fate decisions. As a result of the sheer complexity of the various DNA repair pathways and the DNA damage response there are large gaps in our understanding of the molecular mechanisms underlying DNA damage repair in human cells. A key unaddressed question is how the dynamic recruitment of DNA repair factors contributes to repair kinetics and repair pathway choice in human cells. Methodological advances in live cell single-molecule imaging over the last decade now allow researchers to directly observe and analyze the dynamics of DNA repair proteins in living cells with high spatiotemporal resolution. Live cell single-molecule imaging combined with single-particle tracking can provide direct insight into the biochemical reactions that control DNA repair and has the power to identify previously unobservable processes in living cells. This review summarizes the main considerations for experimental design and execution for live cell single-molecule imaging experiments and describes how they can be used to define the molecular mechanisms of DNA damage repair in mammalian cells.

Long-read sequence analysis of MMEJ-mediated CRISPR genome editing reveals complex on-target vector insertions that may escape standard PCR-based quality control

CRISPR genome editing is a powerful tool for elucidating biological functions. To modify the genome as intended, it is essential to understand the various modes of recombination that can occur. In this study, we report complex vector insertions that were identified during the generation of conditional alleles by CRISPR editing using microhomology-mediated end joining (MMEJ). The targeting vector contained two loxP sequences and flanking 40-bp microhomologies. The genomic regions corresponding to the loxP sequences were cleaved with Cas9 in mouse embryonic stem cells. PCR screening for targeted recombination revealed a high frequency of bands of a larger size than expected. Nanopore sequencing of these bands revealed complex vector insertions mediated not only by MMEJ but also by non-homologous end joining and homologous recombination in at least 17% of the clones. A new band appeared upon improving the PCR conditions, suggesting the presence of unintentionally modified alleles that escape standard PCR screening. This prompted us to characterize the recombination of each allele of the genome-edited clones using heterozygous single nucleotide polymorphisms, leading to confirmation of the presence of homozygous alleles. Our study indicates that careful quality control of genome-edited clones is needed to exclude complex, unintended, on-target vector insertion.

High-LET radiation induces large amounts of rapidly-repaired sublethal damage

There is agreement that high-LET radiation has a high Relative Biological Effectiveness (RBE) when delivered as a single treatment, but how it interacts with radiations of different qualities, such as X-rays, is less clear. We sought to clarify these effects by quantifying and modelling responses to X-ray and alpha particle combinations. Cells were exposed to X-rays, alpha particles, or combinations, with different doses and temporal separations. DNA damage was assessed by 53BP1 immunofluorescence, and radiosensitivity assessed using the clonogenic assay. Mechanistic models were then applied to understand trends in repair and survival. 53BP1 foci yields were significantly reduced in alpha particle exposures compared to X-rays, but these foci were slow to repair. Although alpha particles alone showed no inter-track interactions, substantial interactions were seen between X-rays and alpha particles. Mechanistic modelling suggested that sublethal damage (SLD) repair was independent of radiation quality, but that alpha particles generated substantially more sublethal damage than a similar dose of X-rays, [Formula: see text]. This high RBE may lead to unexpected synergies for combinations of different radiation qualities which must be taken into account in treatment design, and the rapid repair of this damage may impact on mechanistic modelling of radiation responses to high LETs.

Gene Therapy for Paediatric Homozygous Familial Hypercholesterolaemia

The clinical outcome for children and adolescents with homozygous familial hypercholesterolaemia (HoFH) can be devastating, and treatment options are limited in the presence of a null variant. In HoFH, atherosclerotic risk accumulates from birth. Gene therapy is an appealing treatment option as restoration of low-density lipoprotein receptor (LDLR) gene function could provide a cure for HoFH. A clinical trial using a recombinant adeno-associated vector (rAAV) to deliver LDLR DNA to adult patients with HoFH was recently completed; results have not yet been reported. However, this treatment strategy may face challenges when translating to the paediatric population. The paediatric liver undergoes substantial growth which is significant as rAAV vector DNA persists primarily as episomes (extra-chromosomal DNA) and are not replicated during cell division. Therefore, rAAV-based gene addition treatment administered in childhood would likely only have a transient effect. With over 2,000 unique variants in LDLR, a goal of genomic editing-based therapy development would be to treat most (if not all) mutations with a single set of reagents. For a robust, durable effect, LDLR must be repaired in the genome of hepatocytes, which could be achieved using genomic editing technology such as clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 and a DNA repair strategy such as homology-independent targeted integration. This review discusses this issue in the context of the paediatric patient group with severe compound heterozygous or homozygous null variants which are associated with aggressive early-onset atherosclerosis and myocardial infarction, together with the important pre-clinical studies that use genomic editing strategies to treat HoFH in place of apheresis and liver transplantation.

DNA double-strand break end synapsis by DNA loop extrusion

DNA double-strand breaks (DSBs) occur every cell cycle and must be efficiently repaired. Non-homologous end joining (NHEJ) is the dominant pathway for DSB repair in G1-phase. The first step of NHEJ is to bring the two DSB ends back into proximity (synapsis). Although synapsis is generally assumed to occur through passive diffusion, we show that passive diffusion is unlikely to produce the synapsis speed observed in cells. Instead, we hypothesize that DNA loop extrusion facilitates synapsis. By combining experimentally constrained simulations and theory, we show that a simple loop extrusion model constrained by previous live-cell imaging data only modestly accelerates synapsis. Instead, an expanded loop extrusion model with targeted loading of loop extruding factors (LEFs), a small portion of long-lived LEFs, and LEF stabilization by boundary elements and DSB ends achieves fast synapsis with near 100% efficiency. We propose that loop extrusion contributes to DSB repair by mediating fast synapsis.

Computational modeling and a Geant4-DNA study of the rejoining of direct and indirect DNA damage induced by low energy electrons and carbon ions

PURPOSE

DNA double-strand breaks (DSBs) created by ionizing radiations are considered as the most detrimental lesion, which could result in the cell death or sterilization. As the empirical evidence gathered from the cellular and molecular radiation biology has demonstrated significant correlations between the initial and lasting levels of DSBs, gaining knowledge into the DSB repair mechanisms proves vital. Much effort has been invested into understanding the mechanisms triggering the repair and processes engaged after irradiation of cells. Given a mechanistic model, we performed - to our knowledge - the first Monte Carlo study of the expected repair kinetics of carbon ions and electrons using on the one hand Geant4-DNA simulations of electrons for benchmarking purposes and on the other hand quantifying the influence of direct and indirect damage. Our objective was to calculate the DSB repair rates using a repair mechanism for G1 and early S phases of the cell cycle in conjunction with simulations of the DNA damage.

MATERIALS AND METHODS

Based on Geant4-DNA simulations of DSB damage caused by electrons and carbon ions - using a B-DNA model and a water sphere of 3 μm radius resembling the mean size of human cells - we derived the kinetics of various biochemical repair processes.

RESULTS

The overall repair times of carbon ions increased with the DSB complexity. Comparison of the DSB complexity (DSBc) and repair times as a function of carbon-ion energy suggested that the repair time of no specific fraction of DSBs could solely be explained as a function of DSB complexity.

CONCLUSION

Analysis of the carbon-ion repair kinetics indicated that, given a fraction of DSBs, decreasing the energy would result in an increase of the repair time. The disagreements of the calculated and experimental repair kinetics for electrons could, among others, be due to larger damage complexity predicted by simulations or created actually by electrons of comparable energies to x-rays. They are also due to the employed repair mechanisms, which introduce no inherent dependence on the radiation type but make direct use of the simulated DSBs.

Characterization of homozygous Foxn1 mutations induced in rat embryos by different delivery forms of Cas9 nuclease

BACKGROUND

The Cas9 nuclease is delivered in the form of either Cas9 protein or mRNA along with CRISPR guide RNA (gRNA: dual-crRNA:tracrRNA or chimeric single-guide RNA) or in a plasmid package encoding both Cas9 and the CRISPR gRNA.

METHODS AND RESULTS

We directly compared the efficiency of producing rat blastocysts with homozygous mutations of the Foxn1 locus by pronuclear injection of Cas9 in the form of protein, mRNA, or plasmid DNA. For highly efficient production of rat blastocysts with homozygous Foxn1 mutations, pronuclear injection of Cas9 protein at 60 ng/µl was likely optimal. While blastocyst harvest in the mRNA groups was higher than those in the protein and plasmid DNA groups, genotype analysis showed that 63.6%, 8.7-20.0%, and 25.0% of the analyzed blastocysts were homozygous mutants in the protein, mRNA, and plasmid DNA groups, respectively. The high efficiency of producing homozygous mutant blastocysts in the 60 ng/µl protein group may be associated with primary genome editing being initiated before the first cleavage. In most cases, homozygous mutations at the target Foxn1 locus are triggered by deletion and repair via nonhomologous end joining or microhomology-mediated end joining. Deletion downstream of the Cas9 break site was more likely than deletion in the upstream direction.

CONCLUSIONS

The Cas9 nuclease in protein form, when coinjected with the CRISPR gRNA (ribonucleoprotein) into a rat zygote pronucleus, can access the target genome site and induce double-strand breaks promptly, resulting in the efficient production of homozygous mutants.

Future directions for adrenal insufficiency: cellular transplantation and genetic therapies

Primary adrenal insufficiency occurs in 1 in 5-7000 adults. Leading aetiologies are autoimmune adrenalitis in adults and congenital adrenal hyperplasia (CAH) in children. Oral replacement of cortisol is lifesaving, but poor quality of life, repeated adrenal crises and dosing uncertainty related to lack of a validated biomarker for glucocorticoid sufficiency, persists. Adrenocortical cell therapy and gene therapy may obviate many of the shortcomings of adrenal hormone replacement. Physiological cortisol secretion regulated by pituitary adrenocorticotropin, could be achieved through allogeneic adrenocortical cell transplantation, production of adrenal-like steroidogenic cells from either stem cells or lineage conversion of differentiated cells, or for CAH, gene therapy to replace or repair a defective gene. The adrenal cortex is a high turnover organ and thus failure to incorporate progenitor cells within a transplant will ultimately result in graft exhaustion. Identification of adrenocortical progenitor cells is equally important in gene therapy where new genetic material must be specifically integrated into the genome of progenitors to ensure a durable effect. Delivery of gene editing machinery and a donor template, allowing targeted correction of the 21-hydroxylase gene, has the potential to achieve this. This review describes advances in adrenal cell transplants and gene therapy that may allow physiological cortisol production for children and adults with primary adrenal insufficiency.

Opportunities and challenges with CRISPR-Cas mediated homologous recombination based precise editing in plants and animals

We summarise recent advancements to achieve higher homologous recombination based gene targeting efficiency in different animals and plants. The genome editing has revolutionized the agriculture and human therapeutic sectors by its ability to create precise, stable and predictable mutations in the genome. It depends upon targeted double-strand breaks induction by the engineered endonucleases, which then gets repaired by highly conserved endogenous DNA repair mechanisms. The repairing could be done either through non-homologous end joining (NHEJ) or homology-directed repair (HDR) pathways. The HDR-based editing can be applied for precise gene targeting such as insertion of a new gene, gene replacement and altering of the regulatory sequence of a gene to control the existing protein expression. However, HDR-mediated editing is considered challenging because of lower efficiency in higher eukaryotes, thus, preventing its widespread application. This article reviews the recent progress of HDR-mediated editing and discusses novel strategies such as cell cycle synchronization, modulation of DNA damage repair factors, engineering of Cas protein favoring HDR and CRISPR-Cas reagents delivery methods to improve efficiency for generating knock-in events in both plants and animals. Further, multiplexing of described methods may be promising towards achieving higher donor template-assisted homologous recombination efficiency at the target locus.

Modeling of DNA Damage Repair and Cell Response in Relation to p53 System Exposed to Ionizing Radiation

Repair of DNA damage induced by ionizing radiation plays an important role in the cell response to ionizing radiation. Radiation-induced DNA damage also activates the p53 system, which determines the fate of cells. The kinetics of repair, which is affected by the cell itself and the complexity of DNA damage, influences the cell response and fate via affecting the p53 system. To mechanistically study the influences of the cell response to different LET radiations, we introduce a new repair module and a p53 system model with NASIC, a Monte Carlo track structure code. The factors determining the kinetics of the double-strand break (DSB) repair are modeled, including the chromosome environment and complexity of DSB. The kinetics of DSB repair is modeled considering the resection-dependent and resection-independent compartments. The p53 system is modeled by simulating the interactions among genes and proteins. With this model, the cell responses to low- and high-LET irradiation are simulated, respectively. It is found that the kinetics of DSB repair greatly affects the cell fate and later biological effects. A large number of DSBs and a slow repair process lead to severe biological consequences. High-LET radiation induces more complex DSBs, which can be repaired by slow processes, subsequently resulting in a longer cycle arrest and, furthermore, apoptosis and more secreting of TGFβ. The Monte Carlo track structure simulation with a more realistic repair module and the p53 system model developed in this study can expand the functions of the NASIC code in simulating mechanical radiobiological effects.

Speciation and adaptation research meets genome editing

Understanding the genetic basis of reproductive isolation and adaptive traits in natural populations is one of the fundamental goals in evolutionary biology. Genome editing technologies based on CRISPR-Cas systems and site-specific recombinases have enabled us to modify a targeted genomic region as desired and thus to conduct functional analyses of target loci, genes and mutations even in non-conventional model organisms. Here, we review the technical properties of genome editing techniques by classifying them into the following applications: targeted gene knock-out for investigating causative gene functions, targeted gene knock-in of marker genes for visualizing expression patterns and protein functions, precise gene replacement for identifying causative alleles and mutations, and targeted chromosomal rearrangement for investigating the functional roles of chromosomal structural variations. We describe examples of their application to demonstrate functional analysis of naturally occurring genetic variations and discuss how these technologies can be applied to speciation and adaptation research. This article is part of the theme issue 'Genetic basis of adaptation and speciation: from loci to causative mutations'.

Harnessing DSB repair to promote efficient homology-dependent and -independent prime editing

Prime editing recently emerged as a next-generation approach for precise genome editing. Here we exploit DNA double-strand break (DSB) repair to develop two strategies that install precise genomic insertions using an SpCas9 nuclease-based prime editor (PEn). We first demonstrate that PEn coupled to a regular prime editing guide RNA (pegRNA) efficiently promotes short genomic insertions through a homology-dependent DSB repair mechanism. While PEn editing leads to increased levels of by-products, it can rescue pegRNAs that perform poorly with a nickase-based prime editor. We also present a small molecule approach that yields increased product purity of PEn editing. Next, we develop a homology-independent PEn editing strategy, which installs genomic insertions at DSBs through the non-homologous end joining pathway (NHEJ). Lastly, we show that PEn-mediated insertions at DSBs prevent Cas9-induced large chromosomal deletions and provide evidence that continuous Cas9-mediated cutting is one of the mechanisms by which Cas9-induced large deletions arise. Altogether, this work expands the current prime editing toolbox by leveraging distinct DNA repair mechanisms including NHEJ, which represents the primary pathway of DSB repair in mammalian cells.

Prime editing is a next-generation approach for precision genome engineering. Here the authors design a nuclease-based prime editor that leverages DNA repair pathways for targeted genomic insertions.

CRISPR/Cas System and Factors Affecting Its Precision and Efficiency

The diverse applications of genetically modified cells and organisms require more precise and efficient genome-editing tool such as clustered regularly interspaced short palindromic repeats/CRISPR-associated protein (CRISPR/Cas). The CRISPR/Cas system was originally discovered in bacteria as a part of adaptive-immune system with multiple types. Its engineered versions involve multiple host DNA-repair pathways in order to perform genome editing in host cells. However, it is still challenging to get maximum genome-editing efficiency with fewer or no off-targets. Here, we focused on factors affecting the genome-editing efficiency and precision of CRISPR/Cas system along with its defense-mechanism, orthologues, and applications.

Employing CRISPR/Cas Technology for the Improvement of Potato and Other Tuber Crops

New breeding technologies have not only revolutionized biological science, but have also been employed to generate transgene-free products. Genome editing is a powerful technology that has been used to modify genomes of several important crops. This review describes the basic mechanisms, advantages and disadvantages of genome editing systems, such as ZFNs, TALENs, and CRISPR/Cas. Secondly, we summarize in detail all studies of the CRISPR/Cas system applied to potato and other tuber crops, such as sweet potato, cassava, yam, and carrot. Genes associated with self-incompatibility, abiotic-biotic resistance, nutrient-antinutrient content, and post-harvest factors targeted utilizing the CRISPR/Cas system are analyzed in this review. We hope that this review provides fundamental information that will be useful for future breeding of tuber crops to develop novel cultivars.

A Mechanistic DNA Repair and Survival Model (Medras): Applications to Intrinsic Radiosensitivity, Relative Biological Effectiveness and Dose-Rate

Variations in the intrinsic radiosensitivity of different cells to ionizing radiation is now widely believed to be a significant driver in differences in response to radiotherapy. While the mechanisms of radiosensitivity have been extensively studied in the laboratory, there are a lack of models which integrate this knowledge into a predictive framework. This paper presents an overview of the Medras model, which has been developed to provide a mechanistic framework in which different radiation responses can be modelled and individual responses predicted. This model simulates the repair of radiation-induced DNA damage, incorporating the overall kinetics of repair and its fidelity, to predict a range of biological endpoints including residual DNA damage, mutation, chromosome aberration, and cell death. Validation of this model against a range of exposure types is presented, including considerations of varying radiation qualities and dose-rates. This approach has the potential to inform new tools to deliver mechanistic predictions of radiation sensitivity, and support future developments in treatment personalization.

Mechanistic Modelling of Slow and Fast NHEJ DNA Repair Pathways Following Radiation for G0/G1 Normal Tissue Cells

Mechanistic in silico models can provide insight into biological mechanisms and highlight uncertainties for experimental investigation. Radiation-induced double-strand breaks (DSBs) are known to be toxic lesions if not repaired correctly. Non-homologous end joining (NHEJ) is the major DSB-repair pathway available throughout the cell cycle and, recently, has been hypothesised to consist of a fast and slow component in G0/G1. The slow component has been shown to be resection-dependent, requiring the nuclease Artemis to function. However, the pathway is not yet fully understood. This study compares two hypothesised models, simulating the action of individual repair proteins on DSB ends in a step-by-step manner, enabling the modelling of both wild-type and protein-deficient cell systems. Performance is benchmarked against experimental data from 21 cell lines and 18 radiation qualities. A model where resection-dependent and independent pathways are entirely separated can only reproduce experimental repair kinetics with additional restraints on end motion and protein recruitment. However, a model where the pathways are entwined was found to effectively fit without needing additional mechanisms. It has been shown that DaMaRiS is a useful tool when analysing the connections between resection-dependent and independent NHEJ repair pathways and robustly matches with experimental results from several sources.

Genome editing in the human liver: Progress and translational considerations

Liver-targeted genome editing offers the prospect of life-long therapeutic benefit following a single treatment and is set to rapidly supplant conventional gene addition approaches. Combining progress in liver-targeted gene delivery with genome editing technology, makes this not only feasible but realistically achievable in the near term. However, important challenges remain to be addressed. These include achieving therapeutic levels of editing, particularly in vivo, avoidance of off-target effects on the genome and the potential impact of pre-existing immunity to bacteria-derived nucleases, when used to improve editing rates. In this chapter, we outline the unique features of the liver that make it an attractive target for genome editing, the impact of liver biology on therapeutic efficacy, and disease specific challenges, including whether the approach targets a cell autonomous or non-cell autonomous disease. We also discuss strategies that have been used successfully to achieve genome editing outcomes in the liver and address translational considerations as genome editing technology moves into the clinic.

CRISPR/Cas-based precision genome editing via microhomology-mediated end joining

Gene editing and/or allele introgression with absolute precision and control appear to be the ultimate goals of genetic engineering. Precision genome editing in plants has been developed through various approaches, including oligonucleotide-directed mutagenesis (ODM), base editing, prime editing and especially homologous recombination (HR)-based gene targeting. With the advent of CRISPR/Cas for the targeted generation of DNA breaks (single-stranded breaks (SSBs) or double-stranded breaks (DSBs)), a substantial advancement in HR-mediated precise editing frequencies has been achieved. Nonetheless, further research needs to be performed for commercially viable applications of precise genome editing; hence, an alternative innovative method for genome editing may be required. Within this scope, we summarize recent progress regarding precision genome editing mediated by microhomology-mediated end joining (MMEJ) and discuss their potential applications in crop improvement.

Targeted Knock-in of a Fluorescent Protein Gene into the Chicken Vasa Homologue Locus of Chicken Primordial Germ Cells using CRIS-PITCh Method

In chickens, primordial germ cells (PGCs) are effective targets for advanced genome editing, including gene knock-in. Although a long-term culture system has been established for chicken PGCs, it is necessary to select a gene-editing tool that is efficient and precise for editing the PGC genome while maintaining its ability to contribute to the reproductive system. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) and CRISPR-mediated precise integration into the target chromosome (CRIS-PITCh) methods are superior as the donor vector is easier to construct, has high genome editing efficiency, and does not select target cells, compared to the homologous recombination method, which has been conventionally used to generate knock-in chickens. In this study, we engineered knock-in chicken PGCs by integrating a fluorescent protein gene cassette as a fusion protein into the chicken vasa homolog (CVH) locus of chicken PGCs using the CRIS-PITCh method. The knock-in PGCs expressed the fluorescent protein in vitro and in vivo, facilitating the tracking of PGCs. Furthermore, we characterized the efficiency of engineering double knock-in cell lines. Knock-in cell clones were obtained by limiting dilution, and the efficiency of engineering double knock-in cell lines was confirmed by genotyping. We found that 82% of the analyzed clones were successfully knocked-in into both alleles. We suggest that the production of model chicken from the knock-in PGCs can contribute to various studies, such as the elucidation of the fate of germ cells and sex determination in chicken.

Roadmap for metal nanoparticles in radiation therapy: current status, translational challenges, and future directions

This roadmap outlines the potential roles of metallic nanoparticles (MNPs) in the field of radiation therapy. MNPs made up of a wide range of materials (from Titanium, Z = 22, to Bismuth, Z = 83) and a similarly wide spectrum of potential clinical applications, including diagnostic, therapeutic (radiation dose enhancers, hyperthermia inducers, drug delivery vehicles, vaccine adjuvants, photosensitizers, enhancers of immunotherapy) and theranostic (combining both diagnostic and therapeutic), are being fabricated and evaluated. This roadmap covers contributions from experts in these topics summarizing their view of the current status and challenges, as well as expected advancements in technology to address these challenges.

Live cell imaging and proteomic profiling of endogenous NEAT1 lncRNA by CRISPR/Cas9-mediated knock-in

In mammalian cells, long noncoding RNAs (lncRNAs) form complexes with proteins to execute various biological functions such as gene transcription, RNA processing and other signaling activities. However, methods to track endogenous lncRNA dynamics in live cells and screen for lncRNA interacting proteins are limited. Here, we report the development of CERTIS (CRISPR-mediated Endogenous lncRNA Tracking and Immunoprecipitation System) to visualize and isolate endogenous lncRNA, by precisely inserting a 24-repeat MS2 tag into the distal end of lncRNA locus through the CRISPR/Cas9 technology. In this study, we show that CERTIS effectively labeled the paraspeckle lncRNA NEAT1 without disturbing its physiological properties and could monitor the endogenous expression variation of NEAT1. In addition, CERTIS displayed superior performance on both short- and long-term tracking of NEAT1 dynamics in live cells. We found that NEAT1 and paraspeckles were sensitive to topoisomerase I specific inhibitors. Moreover, RNA Immunoprecipitation (RIP) of the MS2-tagged NEAT1 lncRNA successfully revealed several new protein components of paraspeckle. Our results support CERTIS as a tool suitable to track both spatial and temporal lncRNA regulation in live cells as well as study the lncRNA-protein interactomes.

Ionizing Radiation and Complex DNA Damage: Quantifying the Radiobiological Damage Using Monte Carlo Simulations

Ionizing radiation is a common tool in medical procedures. Monte Carlo (MC) techniques are widely used when dosimetry is the matter of investigation. The scientific community has invested, over the last 20 years, a lot of effort into improving the knowledge of radiation biology. The present article aims to summarize the understanding of the field of DNA damage response (DDR) to ionizing radiation by providing an overview on MC simulation studies that try to explain several aspects of radiation biology. The need for accurate techniques for the quantification of DNA damage is crucial, as it becomes a clinical need to evaluate the outcome of various applications including both low- and high-energy radiation medical procedures. Understanding DNA repair processes would improve radiation therapy procedures. Monte Carlo simulations are a promising tool in radiobiology studies, as there are clear prospects for more advanced tools that could be used in multidisciplinary studies, in the fields of physics, medicine, biology and chemistry. Still, lot of effort is needed to evolve MC simulation tools and apply them in multiscale studies starting from small DNA segments and reaching a population of cells.

Sharpening the Molecular Scissors: Advances in Gene-Editing Technology

The ability to precisely modify human genes has been made possible by the development of tools such as meganucleases, zinc finger nucleases, TALENs, and CRISPR/Cas. These now make it possible to generate targeted deletions, insertions, gene knock outs, and point variants; to modulate gene expression by targeting transcription factors or epigenetic machineries to DNA; or to target and modify RNA. Endogenous repair mechanisms are used to make the modifications required in DNA; they include non-homologous end joining, homology-directed repair, homology-independent targeted integration, microhomology-mediated end joining, base-excision repair, and mismatch repair. Off-target effects can be monitored using in silico prediction and sequencing and minimized using Cas proteins with higher accuracy, such as high-fidelity Cas9, enhanced-specificity Cas9, and hyperaccurate Cas9. Alternatives to Cas9 have been identified, including Cpf1, Cas12a, Cas12b, and smaller Cas9 orthologs such as CjCas9. Delivery of gene-editing components is performed ex vivo using standard techniques or in vivo using AAV, lipid nanoparticles, or cell-penetrating peptides. Clinical development of gene-editing technology is progressing in several fields, including immunotherapy in cancer treatment, antiviral therapy for HIV infection, and treatment of genetic disorders such as β-thalassemia, sickle cell disease, lysosomal storage disorders, and retinal dystrophy. Here we review these technological advances and the challenges to their clinical implementation.

Microhomology-based CRISPR tagging tools for protein tracking, purification, and depletion

Work in yeast models has benefitted tremendously from the insertion of epitope or fluorescence tags at the native gene locus to study protein function and behavior under physiological conditions. In contrast, work in mammalian cells largely relies on overexpression of tagged proteins because high-quality antibodies are only available for a fraction of the mammalian proteome. CRISPR/Cas9-mediated genome editing has recently emerged as a powerful genome-modifying tool that can also be exploited to insert various tags and fluorophores at gene loci to study the physiological behavior of proteins in most organisms, including mammals. Here we describe a versatile toolset for rapid tagging of endogenous proteins. The strategy utilizes CRISPR/Cas9 and microhomology-mediated end joining repair for efficient tagging. We provide tools to insert 3×HA, His₆FLAG, His₆-Biotin-TEV-RGSHis₆, mCherry, GFP, and the auxin-inducible degron tag for compound-induced protein depletion. This approach and the developed tools should greatly facilitate functional analysis of proteins in their native environment.

MODELLING DSB REPAIR KINETICS FOR DNA DAMAGE INDUCED BY PROTON AND CARBON IONS

Proton and carbon therapy are the main choices of particle therapy for cancer treatment. Particle dose distribution is superior to conventional photon therapy dose distribution due to Bragg peak. However, the basic biology of cellular damage and cell death is not well understood. The aim of this work is to present a mechanistic model of double strand break (DSB) repair that predicts the repair kinetics of damage induced by particles employed in cancer therapy. Monte Carlo Track Damage Simulation (MCDS) was employed to model DNA damage. The frequency of DSB and SSB was computed for proton and carbon ions. DSBs were subjected to repair model to calculate the repair kinetics. Two distinct DSB repair models dependent on the cell cycle were proposed. The DSB repair model contains non-homologous end joining (NHEJ), homologous recombination (HR) and back up non-homologous end joining (B-NHEJ) repair processes. The DSB complexity results in the switch in the repair pathway from NHEJ to a slower process that starts with DSB end resection. DSB end resection in early S and G1 phases of the cell cycle enhances the B-NHEJ repair pathway, while in late S and G2 phases of the cell cycle promotes HR repair pathway. The repair model was transformed to a set of nonlinear differential equations. The model calculates the overall repair kinetics and protein temporal repair activity at the site of damage. The damage and repair model provides a detailed mechanistic understanding of all processes that are involved in the damage induction and repair. The number of DSB and their complexity increase as the particle energy decreases due to the proximity of particle interactions in water. The repair kinetics show a biphasic behaviour that is due to the NHEJ fast repair of simple type DSB and HR slow repair of complex type DSB.

Mechanistic Modelling of Radiation Responses

Radiobiological modelling has been a key part of radiation biology and therapy for many decades, and many aspects of clinical practice are guided by tools such as the linear-quadratic model. However, most of the models in regular clinical use are abstract and empirical, and do not provide significant scope for mechanistic interpretation or making predictions in novel cell lines or therapies. In this review, we will discuss the key areas of ongoing mechanistic research in radiation biology, including physical, chemical, and biological steps, and review a range of mechanistic modelling approaches which are being applied in each area, highlighting the possible opportunities and challenges presented by these techniques.

Modeling the interplay between DNA-PK, Artemis, and ATM in non-homologous end-joining repair in G1 phase of the cell cycle

Modeling a biological process equips us with more comprehensive insight into the process and a more advantageous experimental design. Non-homologous end joining (NHEJ) is a major double-strand break (DSB) repair pathway that occurs throughout the cell cycle. The objective of the current work is to model the fast and slow phases of NHEJ in G1 phase of the cell cycle following exposure to ionizing radiation (IR). The fast phase contains the major components of NHEJ; Ku70/80 complex, DNA-dependent protein kinase catalytic subunit (DNA-PKcs), and XLF/XRCC4/ligase IV complex (XXL). The slow phase in G1 phase of the cell cycle is associated with more complex lesions and involves ATM and Artemis proteins in addition to the major components. Parameters are mainly obtained from experimental data. The model is successful in predicting the kinetics of DSB foci in 13 normal, ATM-deficient, and Artemis-deficient mammalian fibroblast cell lines in G1 phase of the cell cycle after exposure to low doses of IR. The involvement of ATM provides the model with the potency to be connected to different signaling pathways. Ku70/80 concentration and DNA-binding rate as well as XXL concentration and enzymatic activity are introduced as the best targets for affecting NHEJ DSB repair process. On the basis of the current model, decreasing concentration and DNA binding rate of DNA-PKcs is more effective than inhibiting its activity towards the Artemis protein.

Clinically relevant nanodosimetric simulation of DNA damage complexity from photons and protons

Relative Biological Effectiveness (RBE), the ratio of doses between radiation modalities to produce the same biological endpoint, is a controversial and important topic in proton therapy. A number of phenomenological models incorporate variable RBE as a function of Linear Energy Transfer (LET), though a lack of mechanistic description limits their applicability. In this work we take a different approach, using a track structure model employing fundamental physics and chemistry to make predictions of proton and photon induced DNA damage, the first step in the mechanism of radiation-induced cell death. We apply this model to a proton therapy clinical case showing, for the first time, predictions of DNA damage on a patient treatment plan. Our model predictions are for an idealised cell and are applied to an ependymoma case, at this stage without any cell specific parameters. By comparing to similar predictions for photons, we present a voxel-wise RBE of DNA damage complexity. This RBE of damage complexity shows similar trends to the expected RBE for cell kill, implying that damage complexity is an important factor in DNA repair and therefore biological effect.

Relative Biological Effectiveness (RBE) is a controversial and important topic in proton therapy. This work uses Monte Carlo simulations of DNA damage for protons and photons to probe this phenomenon, providing a plausible mechanistic understanding.

The road less traveled: strategies to enhance the frequency of homology-directed repair (HDR) for increased efficiency of CRISPR/Cas-mediated transgenesis

Non-homologous end joining (NHEJ), and to a lesser extent, the error-free pathway known as homology-directed repair (HDR) are cellular mechanisms for recovery from double-strand DNA breaks (DSB) induced by RNA-guided programmable nuclease CRISPR/Cas. Since NHEJ is equivalent to using a duck tape to stick two pieces of metals together, the outcome of this repair mechanism is prone to error. Any out-of-frame mutations or premature stop codons resulting from NHEJ repair mechanism are extremely handy for loss-of-function studies. Substitution of a mutation on the genome with the correct exogenous repair DNA requires coordination via an error-free HDR, for targeted transgenesis. However, several practical limitations exist in harnessing the potential of HDR to replace a faulty mutation for therapeutic purposes in all cell types and more so in somatic cells. In germ cells after the DSB, copying occurs from the homologous chromosome, which increases the chances of incorporation of exogenous DNA with some degree of homology into the genome compared with somatic cells where copying from the identical sister chromatid is always preferred. This review summarizes several strategies that have been implemented to increase the frequency of HDR with a focus on somatic cells. It also highlights the limitations of this technology in gene therapy and suggests specific solutions to circumvent those barriers.

The road less traveled: strategies to enhance the frequency of homology-directed repair (HDR) for increased efficiency of CRISPR/Cas-mediated transgenesis

Non-homologous end joining (NHEJ), and to a lesser extent, the error-free pathway known as homology-directed repair (HDR) are cellular mechanisms for recovery from double-strand DNA breaks (DSB) induced by RNA-guided programmable nuclease CRISPR/Cas. Since NHEJ is equivalent to using a duck tape to stick two pieces of metals together, the outcome of this repair mechanism is prone to error. Any out-of-frame mutations or premature stop codons resulting from NHEJ repair mechanism are extremely handy for loss-of-function studies. Substitution of a mutation on the genome with the correct exogenous repair DNA requires coordination via an error-free HDR, for targeted transgenesis. However, several practical limitations exist in harnessing the potential of HDR to replace a faulty mutation for therapeutic purposes in all cell types and more so in somatic cells. In germ cells after the DSB, copying occurs from the homologous chromosome, which increases the chances of incorporation of exogenous DNA with some degree of homology into the genome compared with somatic cells where copying from the identical sister chromatid is always preferred. This review summarizes several strategies that have been implemented to increase the frequency of HDR with a focus on somatic cells. It also highlights the limitations of this technology in gene therapy and suggests specific solutions to circumvent those barriers. [BMB Reports 2018; 51(9): 437-443].

CRISPR/Cas9-based genome engineering of zebrafish using a seamless integration strategy

Numerous feasible methods for inserting large fragments of exogenous DNA sequences into the zebrafish genome have been developed, as has genome editing technology using programmable nucleases. However, the coding sequences of targeted endogenous genes are disrupted, and the expression patterns of inserted exogenous genes cannot completely recapitulate those of endogenous genes. Here we describe the establishment of a novel strategy for endogenous promoter-driven and microhomology-mediated end-joining-dependent integration of a donor vector using clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated (Cas) 9. We successfully integrated mCherry into the final coding sequence of targeted genes to generate seamless transgenic zebrafish lines with high efficiency. This novel seamless transgenesis technique not only maintained the integrity of the endogenous gene but also did not disrupt the function of targeted gene. Therefore, our microhomology-mediated end-joining-mediated transgenesis strategy may have broader applications in gene therapy. Moreover, this novel seamless gene-editing strategy in zebrafish provides a valuable new transgenesis technique, which was driven by endogenous promoters and in vivo animal reporter modes for translational medicine. It is expected to be a standard gene-editing technique in the field of zebrafish, leading to some important breakthroughs for studies in early embryogenesis.-Luo, J.-J., Bian, W.-P., Liu, Y., Huang, H.-Y., Yin, Q., Yang, X.-J., Pei, D.-S. CRISPR/Cas9-based genome engineering of zebrafish using a seamless integration strategy.

High doses of CRISPR/Cas9 ribonucleoprotein efficiently induce gene knockout with low mosaicism in the hydrozoan Clytia hemisphaerica through microhomology-mediated deletion

Targeted mutagenesis using CRISPR/Cas9 technology has been shown to be a powerful approach to examine gene function in diverse metazoan species. One common drawback is that mixed genotypes, and thus variable phenotypes, arise in the F0 generation because incorrect DNA repair produces different mutations amongst cells of the developing embryo. We report here an effective method for gene knockout (KO) in the hydrozoan Clytia hemisphaerica, by injection into the egg of Cas9/sgRNA ribonucleoprotein complex (RNP). Expected phenotypes were observed in the F0 generation when targeting endogenous GFP genes, which abolished fluorescence in embryos, or CheRfx123 (that codes for a conserved master transcriptional regulator for ciliogenesis) which caused sperm motility defects. When high concentrations of Cas9 RNP were used, the mutations in target genes at F0 polyp or jellyfish stages were not random but consisted predominantly of one or two specific deletions between pairs of short microhomologies flanking the cleavage site. Such microhomology-mediated (MM) deletion is most likely caused by microhomology-mediated end-joining (MMEJ), which may be favoured in early stage embryos. This finding makes it very easy to isolate uniform, largely non-mosaic mutants with predictable genotypes in the F0 generation in Clytia, allowing rapid and reliable phenotype assessment.

Production of microhomologous-mediated site-specific integrated LacS gene cow using TALENs

Gene editing tools (Zinc-Finger Nucleases, ZFN; Transcription Activator-Like Effector Nucleases, TALEN; and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated (Cas)9, CRISPR-Cas9) provide us with a powerful means of performing genetic engineering procedures. A combinational approach that utilizes both somatic cell nuclear transfer (SCNT) and somatic cell gene editing facilitates the generation of genetically engineered animals. However, the associated research has utilized markers and/or selected genes, which constitute a potential threat to biosafety. Microhomologous-mediated end-joining (MMEJ) has showed the utilization of micro-homologous arms (5-25 bp) can mediate exogenous gene insertion. Dairy milk is a major source of nutrition worldwide. However, most people are not capable of optimally utilizing the nutrition in milk because of lactose intolerance. Sulfolobus solfataricus β-glycosidase (LacS) is a lactase derived from the extreme thermophilic archaeon Sulfolobus solfataricus. Our finally aim was to site-specific integrated LacS gene into cow's genome through TALEN-mediated MMEJ and produce low-lactose cow. Firstly, we constructed TALENs vectors which target to the cow's β-casein locus and LacS gene expression vector which contain TALEN reorganization sequence and micro-homologous arms. Then we co-transfected these vectors into fetal derived skin fibroblasts and cultured as monoclone. Positive cell clones were screened using 3' junction PCR amplification and sequencing analysis. The positive cells were used as donors for SCNT and embryo transfer (ET). Lastly, we detected the genotype through PCR of blood genomic DNA. This resulted in a LacS knock-in rate of 0.8% in TALEN-treated cattle fetal fibroblasts. The blastocyst rate of SCNT embryo was 27%. The 3 months pregnancy rate was 20%. Finally, we obtained 1 newborn cow (5%) and verified its genotype. We obtained 1 site-specific marker-free LacS transgenic cow. It provides a basis to solve lactose intolerance by gene engineering breeding. This study also provides us with a new strategy to facilitate gene knock-ins in livestock using techniques that exhibit improved biosafety and intuitive methodologies.

In vivo gene correction with targeted sequence substitution through microhomology-mediated end joining

Genome editing technology using programmable nucleases has rapidly evolved in recent years. The primary mechanism to achieve precise integration of a transgene is mainly based on homology-directed repair (HDR). However, an HDR-based genome-editing approach is less efficient than non-homologous end-joining (NHEJ). Recently, a microhomology-mediated end-joining (MMEJ)-based transgene integration approach was developed, showing feasibility both in vitro and in vivo. We expanded this method to achieve targeted sequence substitution (TSS) of mutated sequences with normal sequences using double-guide RNAs (gRNAs), and a donor template flanking the microhomologies and target sequence of the gRNAs in vitro and in vivo. Our method could realize more efficient sequence substitution than the HDR-based method in vitro using a reporter cell line, and led to the survival of a hereditary tyrosinemia mouse model in vivo. The proposed MMEJ-based TSS approach could provide a novel therapeutic strategy, in addition to HDR, to achieve gene correction from a mutated sequence to a normal sequence.

In vivo genome editing thrives with diversified CRISPR technologies

Prokaryotic type II adaptive immune systems have been developed into the versatile CRISPR technology, which has been widely applied in site-specific genome editing and has revolutionized biomedical research due to its superior efficiency and flexibility. Recent studies have greatly diversified CRISPR technologies by coupling it with various DNA repair mechanisms and targeting strategies. These new advances have significantly expanded the generation of genetically modified animal models, either by including species in which targeted genetic modification could not be achieved previously, or through introducing complex genetic modifications that take multiple steps and cost years to achieve using traditional methods. Herein, we review the recent developments and applications of CRISPR-based technology in generating various animal models, and discuss the everlasting impact of this new progress on biomedical research.

Development of a Reporter System to Explore MMEJ in the Context of Replacing Large Genomic Fragments

Common genome-editing strategies are either based on non-homologous end joining (NHEJ) or, in the presence of a template DNA, based on homologous recombination with long (homology-directed repair [HDR]) or short (microhomology-mediated end joining [MMEJ]) homologous sequences. In the current study, we aim to develop a model system to test the activity of MMEJ after CRISPR/Cas9-mediated cleavage in cell culture. Following successful proof of concept in an episomally based reporter system, we tested template plasmids containing a promoter-less luciferase gene flanked by microhomologous sequences (mhs) of different length (5, 10, 15, 20, 30, and 50 bp) that are complementary to the mouse retinitis pigmentosa GTPase regulator (RPGR)-ORF15, which is under the control of a CMV promoter stably integrated into a HEK293 cell line. Luciferase signal appearance represented successful recombination events and was highest when the mhs were 5 bp long, while longer mhs revealed lower luciferase signal. In addition, presence of Csy4 RNase was shown to increase luciferase signaling. The luciferase reporter system is a valuable tool to study the input of the different DNA repair mechanisms in the replacement of large DNA sequences by mhs.

Radiation-Induced Cardiovascular Disease: Mechanisms and Importance of Linear Energy Transfer

Radiation therapy (RT) in the form of photons and protons is a well-established treatment for cancer. More recently, heavy charged particles have been used to treat radioresistant and high-risk cancers. Radiation treatment is known to cause cardiovascular disease (CVD) which can occur acutely during treatment or years afterward in the form of accelerated atherosclerosis. Radiation-induced cardiovascular disease (RICVD) can be a limiting factor in treatment as well as a cause of morbidity and mortality in successfully treated patients. Inflammation plays a key role in both acute and chronic RICVD, but the underling pathophysiology is complex, involving DNA damage, reactive oxygen species, and chronic inflammation. While understanding of the molecular mechanisms of RICVD has increased, the growing number of patients receiving RT warrants further research to identify individuals at risk, plans for prevention, and targets for the treatment of RICVD. Research on RICVD is also relevant to the National Aeronautics and Space Administration (NASA) due to the prevalent space radiation environment encountered by astronauts. NASA's current research on RICVD can both contribute to and benefit from concurrent work with cell and animal studies informing radiotoxicities resulting from cancer therapy. This review summarizes the types of radiation currently in clinical use, models of RICVD, current knowledge of the mechanisms by which they cause CVD, and how this knowledge might apply to those exposed to various types of radiation.

A Simple and Universal System for Gene Manipulation in Aspergillus fumigatus: In Vitro-Assembled Cas9-Guide RNA Ribonucleoproteins Coupled with Microhomology Repair Templates

Tackling the multifactorial nature of virulence and antifungal drug resistance in A. fumigatus requires the mechanistic interrogation of a multitude of genes, sometimes across multiple genetic backgrounds. Classical fungal gene replacement systems can be laborious and time-consuming and, in wild-type isolates, are impeded by low rates of homologous recombination. Our simple and universal CRISPR-Cas9 system for gene manipulation generates efficient gene targeting across different genetic backgrounds of A. fumigatus. We anticipate that our system will simplify genome editing in A. fumigatus, allowing for the generation of single- and multigene knockout libraries. In addition, our system will facilitate the delineation of virulence factors and antifungal drug resistance genes in different genetic backgrounds of A. fumigatus.

CRISPR (clustered regularly interspaced short palindromic repeat)-Cas9 is a novel genome-editing system that has been successfully established in Aspergillus fumigatus. However, the current state of the technology relies heavily on DNA-based expression cassettes for delivering Cas9 and the guide RNA (gRNA) to the cell. Therefore, the power of the technology is limited to strains that are engineered to express Cas9 and gRNA. To overcome such limitations, we developed a simple and universal CRISPR-Cas9 system for gene deletion that works across different genetic backgrounds of A. fumigatus. The system employs in vitro assembly of dual Cas9 ribonucleoproteins (RNPs) for targeted gene deletion. Additionally, our CRISPR-Cas9 system utilizes 35 to 50 bp of flanking regions for mediating homologous recombination at Cas9 double-strand breaks (DSBs). As a proof of concept, we first tested our system in the ΔakuB (ΔakuB^ku80) laboratory strain and generated high rates (97%) of gene deletion using 2 µg of the repair template flanked by homology regions as short as 35 bp. Next, we inspected the portability of our system across other genetic backgrounds of A. fumigatus, namely, the wild-type strain Af293 and a clinical isolate, A. fumigatus DI15-102. In the Af293 strain, 2 µg of the repair template flanked by 35 and 50 bp of homology resulted in highly efficient gene deletion (46% and 74%, respectively) in comparison to classical gene replacement systems. Similar deletion efficiencies were also obtained in the clinical isolate DI15-102. Taken together, our data show that in vitro-assembled Cas9 RNPs coupled with microhomology repair templates are an efficient and universal system for gene manipulation in A. fumigatus.

IMPORTANCE Tackling the multifactorial nature of virulence and antifungal drug resistance in A. fumigatus requires the mechanistic interrogation of a multitude of genes, sometimes across multiple genetic backgrounds. Classical fungal gene replacement systems can be laborious and time-consuming and, in wild-type isolates, are impeded by low rates of homologous recombination. Our simple and universal CRISPR-Cas9 system for gene manipulation generates efficient gene targeting across different genetic backgrounds of A. fumigatus. We anticipate that our system will simplify genome editing in A. fumigatus, allowing for the generation of single- and multigene knockout libraries. In addition, our system will facilitate the delineation of virulence factors and antifungal drug resistance genes in different genetic backgrounds of A. fumigatus.

Homology-mediated end joining-based targeted integration using CRISPR/Cas9

Targeted integration of transgenes can be achieved by strategies based on homologous recombination (HR), microhomology-mediated end joining (MMEJ) or non-homologous end joining (NHEJ). The more generally used HR is inefficient for achieving gene integration in animal embryos and tissues, because it occurs only during cell division, although MMEJ and NHEJ can elevate the efficiency in some systems. Here we devise a homology-mediated end joining (HMEJ)-based strategy, using CRISPR/Cas9-mediated cleavage of both transgene donor vector that contains guide RNA target sites and ∼800 bp of homology arms, and the targeted genome. We found no significant improvement of the targeting efficiency by the HMEJ-based method in either mouse embryonic stem cells or the neuroblastoma cell line, N2a, compared to the HR-based method. However, the HMEJ-based method yielded a higher knock-in efficiency in HEK293T cells, primary astrocytes and neurons. More importantly, this approach achieved transgene integration in mouse and monkey embryos, as well as in hepatocytes and neurons in vivo, with an efficiency much greater than HR-, NHEJ- and MMEJ-based strategies. Thus, the HMEJ-based strategy may be useful for a variety of applications, including gene editing to generate animal models and for targeted gene therapies.