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(6–4) Photolyases ((6–4) PLs) are ubiquitous photoenzymes that use the energy of sunlight to catalyze the repair of carcinogenic UV-induced DNA lesions, pyrimidine(6–4)pyrimidone photoproducts. To repair DNA, (6–4) PLs must first undergo so-called photoactivation, in which their excited flavin adenine dinucleotide (FAD) cofactor is reduced in one or two steps to catalytically active FADH − via a chain of three or four conserved tryptophan residues, transiently forming FAD •− /FADH − ⋯ TrpH •+ pairs separated by distances of 15 to 20 Å. Photolyases and related photoreceptors cryptochromes use a plethora of tricks to prevent charge recombination of photoinduced donor–acceptor pairs, such as chain branching and elongation, rapid deprotonation of TrpH •+ or protonation of FAD •− . Here, we address Arabidopsis thaliana (6–4) PL ( At 64) photoactivation by combining molecular biology, in vivo survival assays, static and time-resolved spectroscopy and computational methods. We conclude that At 64 photoactivation is astonishingly efficient compared to related proteins—due to two factors: exceptionally low losses of photoinduced radical pairs through ultrafast recombination and prevention of solvent access to the terminal Trp 3 H •+ , which significantly extends its lifetime. We propose that a highly conserved histidine residue adjacent to the 3rd Trp plays a key role in Trp 3 H •+ stabilization.

Deamination of DNA bases can create missense mutations predisposing humans to cancer and also interfere with other basic molecular genetic processes; this deamination generates deoxyinosine from deoxyadenosine. In Escherichia coli , the highly conserved endonuclease V is involved in alternative excision repair that removes deoxyinosine from DNA. However, its exact activities and roles in humans are unknown. Here we characterize the FLJ35220 protein, the human homologue of E. coli endonuclease V, hEndoV as a ribonuclease specific for inosine-containing RNA. hEndoV preferentially binds to RNA and efficiently hydrolyses the second phosphodiester bond located 3′ to the inosine in unpaired inosine-containing ssRNA regions in dsRNA. It localizes to the cytoplasm in cells. The ribonuclease activity is promoted by Tudor staphylococcal nuclease and detected on inosine-containing dsRNA created by the action of adenosine deaminases acting on RNA. These results demonstrate that hEndoV controls the fate of inosine-containing RNA in humans. In Escherichia coli , the highly conserved enzyme endonuclease V has a role in DNA repair. Here the authors show that human endonuclease V is an inosine 3' endoribonuclease and that Tudor Staphylococcal nuclease enhances this activity, suggesting a role for human endonuclease V in RNA metabolism.

In mammalian global genomic nucleotide excision repair, UV-DDB plays a central role in recognizing DNA lesions, such as 6-4 photoproducts and cyclobutane pyrimidine dimers, within chromatin. In the present study, we perform cryo-electron microscopy analyses coupled with chromatin-immunoprecipitation to reveal that the cellular UV-DDB binds to UV-damaged DNA lesions in a chromatin unit, the nucleosome, at a position approximately 20 base-pairs from the nucleosomal dyad in human cells. An alternative analysis of the in vitro reconstituted UV-DDB-cyclobutane pyrimidine dimer nucleosome structure demonstrates that the DDB2 subunit of UV-DDB specifically recognizes the cyclobutane pyrimidine dimer lesion at this position on the nucleosome. We also determine the structures of UV-DDB bound to DNA lesions at other positions in purified cellular human nucleosomes. These cellular and reconstituted UV-DDB-nucleosome complex structures provide important evidence for understanding the mechanism by which UV lesions in chromatin are recognized and repaired in mammalian cells. UV-DDB is a protein that plays a key role in recognizing DNA lesions. Here, the authors determine the cryo-EM structure of UV-DDB bound to UV-damaged chromatin in human cells, identifying a nucleosome binding site.

Deamination of bases is a form of DNA damage that occurs spontaneously via the hydrolysis and nitrosation of living cells, generating hypoxanthine from adenine. E. coli endonuclease V (eEndoV) cleaves hypoxanthine-containing double-stranded DNA, whereas human endonuclease V (hEndoV) cleaves hypoxanthine-containing RNA; however, hEndoV in vivo function remains unclear. To date, hEndoV has only been examined using hypoxanthine, because it binds closely to the base located at the cleavage site. Here, we examined whether hEndoV cleaves other lesions (e.g., AP site, 6-methyladenine, xanthine) to reveal its function and whether 2′-nucleoside modification affects its cleavage activity. We observed that hEndoV is hypoxanthine-specific; its activity was the highest with 2′-OH modification in ribose. The cleavage activity of hEndoV was compared based on its base sequence. We observed that it has specificity for adenine located on the 3′-end of hypoxanthine at the cleavage site, both before and after cleavage. These data suggest that hEndoV recognizes and cleaves the inosine generated on the poly A tail to maintain RNA quality. Our results provide mechanistic insight into the role of hEndoV in vivo.

Carcinogens often generate mutable DNA lesions that contribute to cancer and aging. However, the chemical structure of tumorigenic DNA lesions formed by acetaldehyde remains unknown, although it has long been considered an environmental mutagen in alcohol, tobacco, and food. Here, we identify an aldehyde-induced DNA lesion, forming an intrastrand crosslink between adjacent guanine bases, but not in single guanine bases or in other combinations of nucleotides. The GG intrastrand crosslink exists in equilibrium in the presence of aldehyde, and therefore it has not been detected or analyzed in the previous investigations. The newly identified GG intrastrand crosslinks might explain the toxicity and mutagenicity of acetaldehyde in DNA metabolism.

Background In vivo genome editing offers a long-term therapeutic approach for monogenic diseases by directly modifying genetic sequences. However, its application to non-monogenic, noncommunicable diseases, which are the leading causes of global mortality, remains limited due to the lack of well-defined genetic targets. Methods We developed an in vivo genome-editing approach to introduce a gene encoding the glucagon-like peptide-1 (GLP-1) receptor agonist Exendin-4, modified with a secretion signal peptide. Mice with obesity and pre-diabetic conditions received a single administration of genome editing. Blood Exendin-4 levels, food intake, body weight, and metabolic parameters were monitored over several months. Results Here we show that in vivo genome editing enables sustained Exendin-4 secretion from liver cells, leading to prolonged elevation of Exendin-4 levels in the bloodstream. Treated mice exhibited reduced food intake, attenuated weight gain, and improved glucose metabolism and insulin sensitivity without detectable adverse effects. Conclusions This study demonstrates that a single administration of genome editing can achieve sustained therapeutic peptide secretion, providing a potential strategy for treating complex diseases without defined genetic causes. Hirose et al. demonstrate a genome editing-based strategy to treat obesity and pre-diabetes, complex diseases without a defined genetic cause. A single in vivo knock-in of a secretion-engineered Exendin-4 gene into the liver enables sustained peptide release, reducing body weight and improving glucose metabolism in mice. Plain language summary Many diseases, such as diabetes and obesity, are not caused by a single change in a person’s DNA so are difficult to treat with regular gene therapy, a treatment type that modifies DNA. In this study, we used genome editing, a tool that makes precise and lasting changes to DNA. We used it to help the body produce and release a beneficial molecule called Exendin-4, which controls appetite and blood sugar. In mice, just one treatment led to long-term improvements in weight and blood sugar, without noticeable side effects. This study suggests that genome editing could be used to create lasting treatments for complex diseases, potentially reducing the need for frequent medication.

Nucleotide excision repair (NER) is a versatile DNA repair pathway, which can remove an extremely broad range of base lesions from the genome. In mammalian global genomic NER, the XPC protein complex initiates the repair reaction by recognizing sites of DNA damage, and this depends on detection of disrupted/destabilized base pairs within the DNA duplex. A model has been proposed that XPC first interacts with unpaired bases and then the XPD ATPase/helicase in concert with XPA verifies the presence of a relevant lesion by scanning a DNA strand in 5′-3′ direction. Such multi-step strategy for damage recognition would contribute to achieve both versatility and accuracy of the NER system at substantially high levels. In addition, recognition of ultraviolet light (UV)-induced DNA photolesions is facilitated by the UV-damaged DNA-binding protein complex (UV-DDB), which not only promotes recruitment of XPC to the damage sites, but also may contribute to remodeling of chromatin structures such that the DNA lesions gain access to XPC and the following repair proteins. Even in the absence of UV-DDB, however, certain types of histone modifications and/or chromatin remodeling could occur, which eventually enable XPC to find sites with DNA lesions. Exploration of novel factors involved in regulation of the DNA damage recognition process is now ongoing.

Nucleotide excision repair (NER) removes helix-destabilizing adducts including ultraviolet (UV) lesions, cyclobutane pyrimidine dimers (CPDs), and pyrimidine (6–4) pyrimidone photoproducts (6–4PPs). In comparison with CPDs, 6–4PPs have greater cytotoxicity and more strongly destabilizing properties of the DNA helix. It is generally believed that NER is the only DNA repair pathway that removes the UV lesions as evidenced by the previous data since no repair of UV lesions was detected in NER-deficient skin fibroblasts. Topoisomerase I (TOP1) constantly creates transient single-strand breaks (SSBs) releasing the torsional stress in genomic duplex DNA. Stalled TOP1-SSB complexes can form near DNA lesions including abasic sites and ribonucleotides embedded in chromosomal DNA. Here we show that base excision repair (BER) increases cellular tolerance to UV independently of NER in cancer cells. UV lesions irreversibly trap stable TOP1-SSB complexes near the UV damage in NER-deficient cells, and the resulting SSBs activate BER. Biochemical experiments show that 6–4PPs efficiently induce stable TOP1-SSB complexes, and the long-patch repair synthesis of BER removes 6–4PPs downstream of the SSB. Furthermore, NER-deficient cancer cell lines remove 6–4PPs within 24 h, but not CPDs, and the removal correlates with TOP1 expression. NER-deficient skin fibroblasts weakly express TOP1 and show no detectable repair of 6–4PPs. Remarkably, the ectopic expression of TOP1 in these fibroblasts led them to completely repair 6–4PPs within 24 h. In conclusion, we reveal a DNA repair pathway initiated by TOP1, which significantly contributes to cellular tolerance to UV-induced lesions particularly in malignant cancer cells overexpressing TOP1.

In vivo genome editing holds great therapeutic potential for treating monogenic diseases by enabling precise gene correction or addition. However, improving the efficiency of delivery systems remains a key challenge. In this study, we investigated the use of lipid nanoparticles (LNPs) for in vivo knock-in of ectopic DNA. Our in vitro experiments demonstrated that the homology-independent targeted integration (HITI)-mediated genome-editing method achieved significantly higher knock-in efficiency at the Alb locus in hepatic cells compared to the traditional homology-directed repair (HDR)-mediated approach. By optimizing LNP composition and administration routes, we successfully achieved HITI-mediated GFP knock-in (2.1–2.7%) in the livers of mice through intravenous delivery of LNP-loaded genome editing components. Notably, repeated intravenous dosing led to a twofold increase in liver GFP knock-in efficiency (4.3–7.0%) compared to a single dose, highlighting the potential for cumulative genome editing effects. These findings provide a solid foundation for the use of LNPs in in vivo knock-in strategies, paving the way for future genome-editing therapies.

Mismatched base pairs, produced by nucleotide misincorporation by DNA polymerase, are repaired by the mismatch repair (MMR) pathway to maintain genetic integrity. We have developed a method for the fluorescence detection of the cellular MMR ability. A mismatch, which would generate a stop codon in the mRNA transcript unless it was repaired, was introduced into the gene encoding the enhanced green fluorescent protein (EGFP) in an expression plasmid. When MMR-proficient HeLa cells were transformed with this plasmid, the production of active EGFP was observed by fluorescence microscopy. It was assumed that the nick required to initiate the MMR pathway was produced non-specifically in the cells. In contrast, fluorescence was not detected for three types of MMR-deficient cells, LoVo, HCT116, and DLD-1, transformed with the same plasmid. In addition, the expression of a red fluorescent protein gene was utilized to avoid false-negative results. This simple fluorescence method may improve the detection of repair defects, as a biomarker for cancer diagnosis and therapy.