Cell-free Xenopus egg extracts for studying DNA damage response pathways

APE1 recruits ATRIP to ssDNA in an RPA-dependent and -independent manner to promote the ATR DNA damage response

Cells have evolved the DNA damage response (DDR) pathways in response to DNA replication stress or DNA damage. In the ATR-Chk1 DDR pathway, it has been proposed that ATR is recruited to RPA-coated single-stranded DNA (ssDNA) by direct ATRIP-RPA interaction. However, it remains elusive how ATRIP is recruited to ssDNA in an RPA-independent manner. Here, we provide evidence that APE1 directly associates ssDNA to recruit ATRIP onto ssDNA in an RPA-independent fashion. The N-terminal motif within APE1 is required and sufficient for the APE1-ATRIP interaction in vitro and the distinct APE1-ATRIP interaction is required for ATRIP recruitment to ssDNA and the ATR-Chk1 DDR pathway activation in Xenopus egg extracts. In addition, APE1 directly associates with RPA70 and RPA32 via two distinct motifs. Taken together, our evidence suggests that APE1 recruits ATRIP onto ssDNA in an RPA-dependent and -independent manner in the ATR DDR pathway.

APE2: catalytic function and synthetic lethality draw attention as a cancer therapy target

AP endonuclease 2 (APE2, APEX2 or APN2) is an emerging critical protein involved in genome and epigenome integrity. Whereas its catalytic function as a nuclease in DNA repair is widely accepted, recent studies have elucidated the function and mechanism of APE2 in the immune response and DNA damage response. Several genome-wide screens have identified APE2 as a synthetic lethal target for deficiencies of BRCA1, BRCA2 or TDP1 in cancer cells. Due to its overexpression in several cancer types, APE2 is proposed as an oncogene and could serve as prognostic marker of overall survival of cancer treatment. However, it remains to be discovered whether and how APE2 catalytic function and synthetic lethality can be modulated and manipulated as a cancer therapy target. In this review, we provide a current understanding of alterations and expression of APE2 in cancer, the function of APE2 in the immune response, and mechanisms of APE2 in ATR/Chk1 DNA damage response. We also summarize the role of APE2 in DNA repair pathways in the removal of heterogenous and complexed 3'-termini and MMEJ. Finally, we provide an updated perspective on how APE2 may be targeted for cancer therapy and future directions of APE2 studies in cancer biology.

Species variations in XRCC1 recruitment strategies for FHA domain-containing proteins

DNA repair scaffolds XRCC1 and XRCC4 utilize a phosphopeptide FHA domain binding motif (FBM) of the form Y-x-x-pS-pT-D-E that supports recruitment of three identified FHA domain-containing DNA repair proteins: polynucleotide kinase/phosphatase (PNKP), aprataxin (APTX), and a third protein, APLF, that functions as a scaffold in support of non-homologous end joining (NHEJ). Mammalian dimeric XRCC4 is able to interact with two of these proteins at any given time, while monomeric XRCC1 binds only one. However, sequence analysis indicates that amphibian and teleost XRCC1 generally contain two FHA binding motifs. X1-FBM1, is similar to the single mammalian XRCC1 FBM and probably functions similarly. X1-FBM2, is more similar to mammalian XRCC4 FBM; it is located closer to the XRCC1 BRCT1 domain and probably is less discriminating among its three likely binding partners. Availability of an additional PNKP or APTX recruitment motif may alleviate the bottleneck that results from using a single FBM motif for recruitment of multiple repair factors. Alternatively, recruitment of APLF by X1-FBM2 may function to rescue a misdirected or unsuccessful SSB repair response by redirecting the damaged DNA to the NHEJ pathway, - a need that results from the ambiguity of the PARP1 signal regarding the nature of the damage. Evaluation of XRCC4 FBMs in acanthomorphs, which account for a majority of the reported teleost sequences, reveals the presence of an additional XRCC4-like paralog, distinct from other previously described members of the XRCC4 superfamily. The FBM is typically absent in acanthomorph XRCC4, but present in the XRCC4-like paralog. Modeling suggests that XRCC4 and its paralog may form homodimers or XRCC4-XRCC4-like heterodimers.

Tools for Decoding Ubiquitin Signaling in DNA Repair

The maintenance of genome stability requires dedicated DNA repair processes and pathways that are essential for the faithful duplication and propagation of chromosomes. These DNA repair mechanisms counteract the potentially deleterious impact of the frequent genotoxic challenges faced by cells from both exogenous and endogenous agents. Intrinsic to these mechanisms, cells have an arsenal of protein factors that can be utilised to promote repair processes in response to DNA lesions. Orchestration of the protein factors within the various cellular DNA repair pathways is performed, in part, by post-translational modifications, such as phosphorylation, ubiquitin, SUMO and other ubiquitin-like modifiers (UBLs). In this review, we firstly explore recent advances in the tools for identifying factors involved in both DNA repair and ubiquitin signaling pathways. We then expand on this by evaluating the growing repertoire of proteomic, biochemical and structural techniques available to further understand the mechanistic basis by which these complex modifications regulate DNA repair. Together, we provide a snapshot of the range of methods now available to investigate and decode how ubiquitin signaling can promote DNA repair and maintain genome stability in mammalian cells.

APE2 Is a General Regulator of the ATR-Chk1 DNA Damage Response Pathway to Maintain Genome Integrity in Pancreatic Cancer Cells

The maintenance of genome integrity and fidelity is vital for the proper function and survival of all organisms. Recent studies have revealed that APE2 is required to activate an ATR-Chk1 DNA damage response (DDR) pathway in response to oxidative stress and a defined DNA single-strand break (SSB) in Xenopus laevis egg extracts. However, it remains unclear whether APE2 is a general regulator of the DDR pathway in mammalian cells. Here, we provide evidence using human pancreatic cancer cells that APE2 is essential for ATR DDR pathway activation in response to different stressful conditions including oxidative stress, DNA replication stress, and DNA double-strand breaks. Fluorescence microscopy analysis shows that APE2-knockdown (KD) leads to enhanced γH2AX foci and increased micronuclei formation. In addition, we identified a small molecule compound Celastrol as an APE2 inhibitor that specifically compromises the binding of APE2 but not RPA to ssDNA and 3′-5′ exonuclease activity of APE2 but not APE1. The impairment of ATR-Chk1 DDR pathway by Celastrol in Xenopus egg extracts and human pancreatic cancer cells highlights the physiological significance of Celastrol in the regulation of APE2 functionalities in genome integrity. Notably, cell viability assays demonstrate that APE2-KD or Celastrol sensitizes pancreatic cancer cells to chemotherapy drugs. Overall, we propose APE2 as a general regulator for the DDR pathway in genome integrity maintenance.

Approaches for the sensitive detection of rare base and prime editing events

Precision chemistry entailing user-directed nucleotide substitutions and template-specified repair can be facilitated by base editing and prime editing, respectively. Recently, the diversification of adenine, cytosine, and prime editor variants obliges a considered, high-throughput evaluation of these tools for optimized, end-point applications. Herein, we outline novel, cost-effective and scalable approaches for the rapid detection of base editing and prime editing outcomes using gel electrophoresis. For base editing, we exploit primer mismatch amplification (SNP genotyping) for the gel-based detection of base editing efficiencies as low as 0.1%. For prime editing, we describe a one-pot reaction combining polymerase chain reaction (PCR) amplification of the target region with restriction digestion (restriction fragment length polymorphism; RFLP). RFLP enables the rapid detection of insertion or deletion events in under 2.5 h from genomic DNA extraction. We show that our method of SNP genotyping is amenable to both endogenous target loci as well as transfected, episomal plasmid targets in BHK-21 cells. Next, we validate the incidence of base and prime editing by describing Sanger sequencing and next-generation sequencing (NGS) workflows for the accurate validation and quantification of on-target editing efficiencies. Our workflow details three different methods for the detection of rare base and prime editing events, enabling a tiered approach from low to high resolution that makes use of gel electrophoresis, Sanger sequencing, and NGS.

DNA Damage Response in Xenopus laevis Cell-Free Extracts

The DNA damage response (DDR) is a coordinated cellular response to a variety of insults to the genome. DDR initiates the activation of cell cycle checkpoints preventing the propagation of damaged DNA followed by DNA repair, which are both critical in maintaining genome integrity. Several model systems have been developed to study the mechanisms and complexity of checkpoint function. Here we describe the application of cell-free extracts derived from Xenopus eggs as a model system to investigate signaling from DNA damage, modulation of DNA replication, checkpoint activation, and ultimately DNA repair. We outline the preparation of cell-free extracts, DNA substrates, and their subsequent use in assays aimed at understanding the cellular response to DNA damage. Cell-free extracts derived from the eggs of Xenopus laevis remain a robust and versatile system to decipher the biochemical steps underlying this essential characteristic of all cells, critical for genome stability.

Function and molecular mechanisms of APE2 in genome and epigenome integrity

APE2 is a rising vital player in the maintenance of genome and epigenome integrity. In the past several years, a series of studies have shown the critical roles and functions of APE2. We seek to provide the first comprehensive review on several aspects of APE2 in genome and epigenome integrity. We first summarize the distinct functional domains or motifs within APE2 including EEP (endonuclease/exonuclease/phosphatase) domain, PIP box and Zf-GRF motifs from eight species (i.e., Homo sapiens, Mus musculus, Xenopus laevis, Ciona intestinalis, Arabidopsis thaliana, Schizosaccharomyces pombe, Saccharomyces cerevisiae, and Trypanosoma cruzi). Then we analyze various APE2 nuclease activities and associated DNA substrates, including AP endonuclease, 3'-phosphodiesterase, 3'-phosphatase, and 3'-5' exonuclease activities. We also examine several APE2 interaction proteins, including PCNA, Chk1, APE1, Myh1, and homologous recombination (HR) factors such as Rad51, Rad52, BRCA1, BRCA2, and BARD1. Furthermore, we provide insights into the roles of APE2 in various DNA repair pathways (base excision repair, single-strand break repair, and double-strand break repair), DNA damage response (DDR) pathways (ATR-Chk1 and p53-dependent), immunoglobulin class switch recombination and somatic hypermutation, as well as active DNA demethylation. Lastly, we summarize critical functions of APE2 in growth, development, and diseases. In this review, we provide the first comprehensive perspective which dissects all aspects of the multiple-function protein APE2 in genome and epigenome integrity.

A non-canonical role for the DNA glycosylase NEIL3 in suppressing APE1 endonuclease-mediated ssDNA damage

The DNA glycosylase NEIL3 has been implicated in DNA repair pathways including the base excision repair and the interstrand cross-link repair pathways via its DNA glycosylase and/or AP lyase activity, which are considered canonical roles of NEIL3 in genome integrity. Compared with the other DNA glycosylases NEIL1 and NEIL2, Xenopus laevis NEIL3 C terminus has two highly conserved zinc finger motifs containing GRXF residues (designated as Zf-GRF). It has been demonstrated that the minor AP endonuclease APE2 contains only one Zf-GRF motif mediating interaction with single-strand DNA (ssDNA), whereas the major AP endonuclease APE1 does not. It appears that the two NEIL3 Zf-GRF motifs (designated as Zf-GRF repeat) are dispensable for its DNA glycosylase and AP lyase activity; however, the potential function of the NEIL3 Zf-GRF repeat in genome integrity remains unknown. Here, we demonstrate evidence that the NEIL3 Zf-GRF repeat was associated with a higher affinity for shorter ssDNA than one single Zf-GRF motif. Notably, our protein-protein interaction assays show that the NEIL3 Zf-GRF repeat but not one Zf-GRF motif interacted with APE1 but not APE2. We further reveal that APE1 endonuclease activity on ssDNA but not on dsDNA is compromised by a NEIL3 Zf-GRF repeat, whereas one Zf-GRF motif within NEIL3 is not sufficient to prevent such activity of APE1. In addition, COMET assays show that excess NEIL3 Zf-GRF repeat reduces DNA damage in oxidative stress in Xenopus egg extracts. Together, our results suggest a noncanonical role of NEIL3 in genome integrity via its distinct Zf-GRF repeat in suppressing APE1 endonuclease-mediated ssDNA breakage.

Distinct roles of XRCC1 in genome integrity in Xenopus egg extracts

Oxidative DNA damage represents one of the most abundant DNA lesions. It remains unclear how DNA repair and DNA damage response (DDR) pathways are co-ordinated and regulated following oxidative stress. While XRCC1 has been implicated in DNA repair, it remains unknown how exactly oxidative DNA damage is repaired and sensed by XRCC1. In this communication, we have demonstrated evidence that XRCC1 is dispensable for ATR-Chk1 DDR pathway following oxidative stress in Xenopus egg extracts. Whereas APE2 is essential for SSB repair, XRCC1 is not required for the repair of defined SSB and gapped plasmids with a 5'-OH or 5'-P terminus, suggesting that XRCC1 and APE2 may contribute to SSB repair via different mechanisms. Neither Polymerase beta nor Polymerase alpha is important for the repair of defined SSB structure. Nonetheless, XRCC1 is important for the repair of DNA damage following oxidative stress. Our observations suggest distinct roles of XRCC1 for genome integrity in oxidative stress in Xenopus egg extracts.

APE1 senses DNA single-strand breaks for repair and signaling

DNA single-strand breaks (SSBs) represent the most abundant type of DNA damage. Unrepaired SSBs impair DNA replication and transcription, leading to cancer and neurodegenerative disorders. Although PARP1 and XRCC1 are implicated in the SSB repair pathway, it remains unclear how SSB repair and SSB signaling pathways are coordinated and regulated. Using Xenopus egg extract and in vitro reconstitution systems, here we show that SSBs are first sensed by APE1 to initiate 3'-5' SSB end resection, followed by APE2 recruitment to continue SSB end resection. Notably, APE1's exonuclease activity is critical for SSB repair and SSB signaling pathways. An APE1 exonuclease-deficient mutant identified in somatic tissue from a cancer patient highlighted the significance of APE1 exonuclease activity in cancer etiology. In addition, APE1 interacts with APE2 and PCNA, although PCNA is dispensable for APE1's exonuclease activity. Taken together, we propose a two-step APE1/APE2-mediated mechanism for SSB end resection that couples DNA damage response with SSB repair in a eukaryotic system.

Integrative biology of native cell extracts: a new era for structural characterization of life processes

Advances in electron microscopy have provided unprecedented access to the structural characterization of large, flexible and heterogeneous complexes. Until recently, cryo-electron microscopy (cryo-EM) has been applied to understand molecular organization in either highly purified, isolated biomolecules or in situ. An emerging field is developing, bridging the gap between the two approaches, and focuses on studying molecular organization in native cell extracts. This field has demonstrated its potential by resolving the structure of fungal fatty acid synthase (FAS) at 4.7 Å [Fourier shell correlation (FSC) = 0.143]; FAS was not only less than 50% enriched, but also retained higher-order binders, previously unknown. Although controversial in the sense that the lysis step might introduce artifacts, cell extracts preserve aspects of cellular function. In addition, cell extracts are accessible, besides cryo-EM, to modern proteomic methods, chemical cross-linking, network biology and biophysical modeling. We expect that automation in imaging cell extracts, along with the integration of molecular/cell biology approaches, will provide remarkable achievements in the study of closer-to-life biomolecular states of pronounced biotechnological and medical importance. Such steps will, eventually, bring us a step closer to the biophysical description of cellular processes in an integrative, holistic approach.

APE2 promotes DNA damage response pathway from a single-strand break

As the most common type of DNA damage, DNA single-strand breaks (SSBs) are primarily repaired by the SSB repair mechanism. If not repaired properly or promptly, unrepaired SSBs lead to genome stability and have been implicated in cancer and neurodegenerative diseases. However, it remains unknown how unrepaired SSBs are recognized by DNA damage response (DDR) pathway, largely because of the lack of a feasible experimental system. Here, we demonstrate evidence showing that an ATR-dependent checkpoint signaling is activated by a defined plasmid-based site-specific SSB structure in Xenopus HSS (high-speed supernatant) system. Notably, the distinct SSB signaling requires APE2 and canonical checkpoint proteins, including ATR, ATRIP, TopBP1, Rad9 and Claspin. Importantly, the SSB-induced ATR DDR is essential for SSB repair. We and others show that APE2 interacts with PCNA via its PIP box and preferentially interacts with ssDNA via its C-terminus Zf–GRF domain, a conserved motif found in >100 proteins involved in DNA/RNA metabolism. Here, we identify a novel mode of APE2–PCNA interaction via APE2 Zf–GRF and PCNA C-terminus. Mechanistically, the APE2 Zf–GRF–PCNA interaction facilitates 3′-5′ SSB end resection, checkpoint protein complex assembly, and SSB-induced DDR pathway. Together, we propose that APE2 promotes ATR–Chk1 DDR pathway from a single-strand break.

Methods for Studying DNA Single-Strand Break Repair and Signaling in Xenopus laevis Egg Extracts

DNA single-strand breaks (SSBs) are the most common type of DNA lesions as they are generated approximately 10,000 times per mammalian cell each day. Unrepaired SSBs compromise DNA replication and transcription programs, leading to genome instability, and have been implicated in many diseases including cancer. In this chapter, we introduce methods to study the ATR-Chk1 DNA damage response (DDR) pathway and DNA repair pathway in response to a site-specific, defined SSB plasmid in Xenopus laevis egg extracts. This experimental system can be applied in future studies to reveal many aspects of the molecular mechanisms of SSB repair and signaling in eukaryotes.

Xenopus Models of Cancer: Expanding the Oncologist's Toolbox

The use of the Xenopus model system has provided diverse contributions to cancer research, not least because of the striking parallels between tumour pathogenesis and early embryo development. Cell cycle regulation, signalling pathways, and cell behaviours such as migration are frequently perturbed in cancers; all have been investigated using Xenopus, and these developmental events can additionally act as an assay for drug development studies. In this mini-review, we focus our discussion primarily on whole embryo Xenopus models informing cancer biology; the contributions to date and future potential. Insights into tumour immunity, oncogene function, and visualisation of vascular responses during tumour formation have all been achieved with naturally occurring tumours and induced-tumour-like-structures in Xenopus. Finally, as we are now entering the era of genetically modified Xenopus models, we can harness genome editing techniques to recapitulate human disease through creating embryos with analogous genetic abnormalities. With the speed, versatility and accessibility that epitomise the Xenopus system, this new range of pre-clinical Xenopus models has great potential to advance our mechanistic understanding of oncogenesis and provide an early in vivo model for chemotherapeutic development.

Single-Strand Break End Resection in Genome Integrity: Mechanism and Regulation by APE2

DNA single-strand breaks (SSBs) occur more than 10,000 times per mammalian cell each day, representing the most common type of DNA damage. Unrepaired SSBs compromise DNA replication and transcription programs, leading to genome instability. Unrepaired SSBs are associated with diseases such as cancer and neurodegenerative disorders. Although canonical SSB repair pathway is activated to repair most SSBs, it remains unclear whether and how unrepaired SSBs are sensed and signaled. In this review, we propose a new concept of SSB end resection for genome integrity. We propose a four-step mechanism of SSB end resection: SSB end sensing and processing, as well as initiation, continuation, and termination of SSB end resection. We also compare different mechanisms of SSB end resection and DSB end resection in DNA repair and DNA damage response (DDR) pathways. We further discuss how SSB end resection contributes to SSB signaling and repair. We focus on the mechanism and regulation by APE2 in SSB end resection in genome integrity. Finally, we identify areas of future study that may help us gain further mechanistic insight into the process of SSB end resection. Overall, this review provides the first comprehensive perspective on SSB end resection in genome integrity.

Probing the biology of cell boundary conditions through confinement of Xenopus cell-free cytoplasmic extracts

Cell-free cytoplasmic extracts prepared from Xenopus eggs and embryos have for decades provided a biochemical system with which to interrogate complex cell biological processes in vitro. Recently, the application of microfabrication and microfluidic strategies in biology has narrowed the gap between in vitro and in vivo studies by enabling formation of cell-size compartments containing functional cytoplasm. These approaches provide numerous advantages over traditional biochemical experiments performed in a test tube. Most notably, the cell-free cytoplasm is confined using a two- or three-dimensional boundary, which mimics the natural configuration of a cell. This strategy enables characterization of the spatial organization of a cell, and the role that boundaries play in regulating intracellular assembly and function. In this review, we describe the marriage of Xenopus cell-free cytoplasm and confinement technologies to generate synthetic cell-like systems, the recent biological insights they have enabled, and the promise they hold for future scientific discovery.

TALENs and CRISPR/Cas9 fuel genetically engineered clinically relevant Xenopus tropicalis tumor models

The targeted nuclease revolution (TALENs, CRISPR/Cas9) now allows Xenopus researchers to rapidly generate custom on-demand genetic knockout models. These novel methods to perform reverse genetics are unprecedented and are fueling a wide array of human disease models within the aquatic diploid model organism Xenopus tropicalis (X. tropicalis). This emerging technology review focuses on the tools to rapidly generate genetically engineered X. tropicalis models (GEXM), with a focus on establishment of genuine genetic and clinically relevant cancer models. We believe that due to particular advantageous characteristics, outlined within this review, GEXM will become a valuable alternative animal model for modeling human cancer. Furthermore, we provide perspectives of how GEXM will be used as a platform for elucidation of novel therapeutic targets and for preclinical drug validation. Finally, we also discuss some future prospects on how the recent expansions and adaptations of the CRISPR/Cas9 toolbox might influence and push forward X. tropicalis cancer research.

Xenopus egg extract: A powerful tool to study genome maintenance mechanisms

DNA repair pathways are crucial to maintain the integrity of our genome and prevent genetic diseases such as cancer. There are many different types of DNA damage and specific DNA repair mechanisms have evolved to deal with these lesions. In addition to these repair pathways there is an extensive signaling network that regulates processes important for repair, such as cell cycle control and transcription. Despite extensive research, DNA damage repair and signaling are not fully understood. In vitro systems such as the Xenopus egg extract system, have played, and still play, an important role in deciphering the molecular details of these processes. Xenopus laevis egg extracts contain all factors required to efficiently perform DNA repair outside a cell, using mechanisms conserved in humans. These extracts have been used to study several genome maintenance pathways, including mismatch repair, non-homologous end joining, ICL repair, DNA damage checkpoint activation, and replication fork stability. Here we describe how the Xenopus egg extract system, in combination with specifically designed DNA templates, contributed to our detailed understanding of these pathways.