Expression of a mammalian DNA photolyase confers light-dependent repair activity and reduces mutations of UV-irradiated shuttle vectors in xeroderma pigmentosum cells

UV Protection in the Cornea: Failure and Rescue

Simple Summary

The sun is a deadly laser, and its damaging rays harm exposed tissues such as our skin and eyes. The skin’s protection and repair mechanisms are well understood and utilized in therapeutic approaches while the eye lacks such complete understanding of its defenses and therefore often lacks therapeutic support in most cases. The aim here was to document the similarities and differences between the two tissues as well as understand where current research stands on ocular, particularly corneal, ultraviolet protection. The objective is to identify what mechanisms may be best suited for future investigation and valuable therapeutic approaches.

Abstract

Ultraviolet (UV) irradiation induces DNA lesions in all directly exposed tissues. In the human body, two tissues are chronically exposed to UV: the skin and the cornea. The most frequent UV-induced DNA lesions are cyclobutane pyrimidine dimers (CPDs) that can lead to apoptosis or induce tumorigenesis. Lacking the protective pigmentation of the skin, the transparent cornea is particularly dependent on nucleotide excision repair (NER) to remove UV-induced DNA lesions. The DNA damage response also triggers intracellular autophagy mechanisms to remove damaged material in the cornea; these mechanisms are poorly understood despite their noted involvement in UV-related diseases. Therapeutic solutions involving xenogenic DNA-repair enzymes such as T4 endonuclease V or photolyases exist and are widely distributed for dermatological use. The corneal field lacks a similar set of tools to address DNA-lesions in photovulnerable patients, such as those with genetic disorders or recently transplanted tissue.

Fluorescently-labelled CPD and 6-4PP photolyases: new tools for live-cell DNA damage quantification and laser-assisted repair

UV light induces cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4) photoproducts (6-4PPs), which can result in carcinogenesis and aging, if not properly repaired by nucleotide excision repair (NER). Assays to determine DNA damage load and repair rates are invaluable tools for fundamental and clinical NER research. However, most current assays to quantify DNA damage and repair cannot be performed in real time. To overcome this limitation, we made use of the damage recognition characteristics of CPD and 6-4PP photolyases (PLs). Fluorescently-tagged PLs efficiently recognize UV-induced DNA damage without blocking NER activity, and therefore can be used as sensitive live-cell damage sensors. Importantly, FRAP-based assays showed that PLs bind to damaged DNA in a highly sensitive and dose-dependent manner, and can be used to quantify DNA damage load and to determine repair kinetics in real time. Additionally, PLs can instantly reverse DNA damage by 405 nm laser-assisted photo-reactivation during live-cell imaging, opening new possibilities to study lesion-specific NER dynamics and cellular responses to damage removal. Our results show that fluorescently-tagged PLs can be used as a versatile tool to sense, quantify and repair DNA damage, and to study NER kinetics and UV-induced DNA damage response in living cells.

Transfection of Human Keratinocytes with Nucleoside-Modified mRNA Encoding CPD-Photolyase to Repair DNA Damage

In vitro-synthesized mRNA containing nucleoside modifications has great therapeutical potential to transiently express proteins with physiological importance. One such protein is photolyase which rapidly removes UV-induced DNA damages, but this enzyme is absent in humans. Here, we apply a novel mRNA-based platform to achieve functional nonhuman photolyase production in cultured human keratinocytes. Transfection of nucleoside-modified mRNA encoding photolyase leads to accelerated repair of DNA photolesions in human keratinocytes.

Transfection of pseudouridine-modified mRNA encoding CPD-photolyase leads to repair of DNA damage in human keratinocytes: a new approach with future therapeutic potential

UVB irradiation induces harmful photochemical reactions, including formation of Cyclobutane Pyrimidine Dimers (CPDs) in DNA. Accumulation of unrepaired CPD lesions causes inflammation, premature ageing and skin cancer. Photolyases are DNA repair enzymes that can rapidly restore DNA integrity in a light-dependent process called photoreactivation, but these enzymes are absent in humans. Here, we present a novel mRNA-based gene therapy method that directs synthesis of a marsupial, Potorous tridactylus, CPD-photolyase in cultured human keratinocytes. Pseudouridine was incorporated during in vitro transcription to make the mRNA non-immunogenic and highly translatable. Keratinocytes transfected with lipofectamine-complexed mRNA expressed photolyase in the nuclei for at least 2days. Exposing photolyase mRNA-transfected cells to UVB irradiation resulted in significantly less CPD in those cells that were also treated with photoreactivating light, which is required for photolyase activity. The functional photolyase also diminished other UVB-mediated effects, including induction of IL-6 and inhibition of cell proliferation. These results demonstrate that pseudouridine-containing photolyase mRNA is a powerful tool to repair UVB-induced DNA lesions. The pseudouridine-modified mRNA approach has a strong potential to discern cellular effects of CPD in UV-related cell biological studies. The mRNA-based transient expression of proteins offers a number of opportunities for future application in medicine.

Evolution of mutation rates: phylogenomic analysis of the photolyase/cryptochrome family

Photoreactivation, one of the first DNA repair pathways to evolve, is the direct reversal of premutagenic lesions caused by ultraviolet (UV) irradiation, catalyzed by photolyases in a light-dependent, single-enzyme reaction. It has been experimentally shown that photoreactivation prevents UV mutagenesis in a broad range of species. In the absence of photoreactivation, UV-induced photolesions are repaired by the more complex and much less efficient nucleotide excision repair pathway. Despite their obvious beneficial effects, several lineages, including placental mammals, lost photolyase genes during evolution. In this study, we ask why photolyase genes have been lost in those lineages and discuss the significance of these losses in the context of the evolution of the genomic mutation rates. We first perform an extensive phylogenomic analysis of the photolyase/cryptochrome family, to assess what species lack each kind of photolyase gene. Then, we estimate the ratio of nonsynonymous to synonymous substitution rates in several groups of photolyase genes, as a proxy of the strength of purifying natural selection, and we ask whether less evolutionarily constrained photolyase genes are more likely lost. We also review functional data and compare the efficiency of different kinds of photolyases. We find that eukaryotic photolyases are, on average, less evolutionarily constrained than eubacterial ones and that the strength of natural selection is correlated with the affinity of photolyases for their substrates. We propose that the loss of photolyase genes in eukaryotic species may be due to weak natural selection and may result in a deleterious increase of their genomic mutation rates. In contrast, the loss of photolyase genes in prokaryotes may not cause an increase in the mutation rate and be neutral in most cases.

Reduced efficiency and increased mutagenicity of translesion DNA synthesis across a TT cyclobutane pyrimidine dimer, but not a TT 6-4 photoproduct, in human cells lacking DNA polymerase eta

Xeroderma pigmentosum variant (XPV) patients carry germ-line mutations in DNA polymerase eta (poleta), a major translesion DNA synthesis (TLS) polymerase, and exhibit severe sunlight sensitivity and high predisposition to skin cancer. Using a quantitative TLS assay system based on gapped plasmids we analyzed TLS across a site-specific TT CPD (thymine-thymine cyclobutane pyrimidine dimer) or TT 6-4 PP (thymine-thymine 6-4 photoproduct) in three pairs of poleta-proficient and deficient human cells. TLS across the TT CPD lesion was reduced by 2.6-4.4-fold in cells lacking poleta, and exhibited a strong 6-17-fold increase in mutation frequency at the TT CPD. All targeted mutations (74%) in poleta-deficient cells were opposite the 3'T of the CPD, however, a significant fraction (23%) were semi-targeted to the nearest nucleotides flanking the CPD. Deletions and insertions were observed at a low frequency, which increased in the absence of poleta, consistent with the formation of double strand breaks due to defective TLS. TLS across TT 6-4 PP was about twofold lower than across CPD, and was marginally reduced in poleta-deficient cells. TLS across TT 6-4 PP was highly mutagenic (27-63%), with multiple mutations types, and no significant difference between cells with or without poleta. Approximately 50% of the mutations formed were semi-targeted, of which 84-93% were due to the insertion of an A opposite the template G 5' to the 6-4 PP. These results, which are consistent with the UV hyper-mutability of XPV cells, highlight the critical role of poleta in error-free TLS across CPD in human cells, and suggest a potential involvement, although minor, of poleta in TLS across 6-4 PP under some conditions.

CPDs and 6-4PPs play different roles in UV-induced cell death in normal and NER-deficient human cells

Ultraviolet (UV) light generates two major DNA lesions: cyclobutane pyrimidine dimers (CPDs) and pyrimidine-(6-4)-pyrimidone photoproducts (6-4PPs), but the specific participation of these two lesions in the deleterious effects of UV is a longstanding question. In order to discriminate the precise role of unrepaired CPDs and 6-4PPs in UV-induced responses triggering cell death, human fibroblasts were transduced by recombinant adenoviruses carrying the CPD-photolyase or 6-4PP-photolyase cDNAs. Both photolyases were able to prevent UV-induced apoptosis in cells deficient for nucleotide excision repair (NER) to a similar extent, while in NER-proficient cells UV-induced apoptosis was prevented only by CPD-photolyase, with no effects observed when 6-4PPs were removed by the specific photolyase. These results strongly suggest that both CPDs and 6-4PPs contribute to UV-induced apoptosis in NER-deficient cells, while in NER-proficient cells, CPDs are the only lesions responsible for UV-killing, probably due to the rapid repair of 6-4PPs by NER. As a consequence, the difference in skin photosensitivity, including carcinogenesis, of most of the xeroderma pigmentosum patients and of normal people is probably not only a quantitative aspect, but depends on the type of DNA damage induced by sunlight and its rate of repair.

Identification and expression of the gene product encoding a CPD photolyase from Dunaliella salina

Ultraviolet light induces photoproducts, cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs), in cellular DNA, which cause cytotoxic and genotoxic effects on the cells. Cells have several DNA repair mechanisms to repair the damage and to maintain genetic information of the cells. Photoreactivation is one of the DNA repair mechanism to remove UV-induced DNA damage from cellular DNA catalyzed by photolyase under visible light. Two types of photolyase, CPD photolyase and (6-4) photolyase, are specific for CPDs and for (6-4)PPs. We have isolated a gene product encoding CPD photolyase, named PHR2, from Dunaliella salina which is a kind of unicellular alga. Sequence analysis showed that PHR2 encodes a protein that has 529 amino acids and is similar to other Class II CPD photolyase. The complementation assay of the photoreactivation deficiency of the Escherichia coli SY2 by PHR2 cDNA showed a significant increase in survival rate when cells were irradiated with UV-C. Real-time PCR analysis indicated that the transcription of PHR2 was induced by UV-C, white light, high salinity, and H(2)O(2).

Photolyases: capturing the light to battle skin cancer

Photolyases comprise efficient enzymes to remove the major UV-induced DNA lesions, cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs). While photolyases are present in all three kingdoms of life (i.e., bacteria, prokaryotes and eukaryotes), placental mammals appear to have lost these enzymes when they diverted from marsupials during evolution. Consequently, man and mice have to rely solely on the more complex and, for these lesions, less efficient nucleotide excision repair (NER) system. To assess the relative contribution of CPDs and 6-4PPs to the cytotoxic and genotoxic effects of the UV component of sunlight, we have recently generated a comprehensive set of transgenic mice expressing CPD and/or 6-4PP photolyases. Here, we discuss the use of photolyase transgenic mice as effective tools to study the adverse effects of UV irradiation.

Status and applications of genomic resources for the gray, short-tailed opossum, Monodelphis domestica, an American marsupial model for comparative biology

Owing to its small size, favourable reproductive characteristics, and simple husbandry, the gray, short-tailed opossum, Monodelphis domestica, has become the most widely distributed and intensively utilised laboratory-bred research marsupial in the world today. This article provides an overview of the current state and future projections of genomic resources for this species and discusses the potential impact of this growing resource base on active research areas that use M. domestica as a model system. The resources discussed include: fully arrayed, bacterial artificial chromosome (BAC) libraries; an expanding linkage map; developing full-genome BAC-contig and chromosomal fluorescence in situ hybridisation maps; public websites providing access to the M. domestica whole-genome-shotgun sequence trace database and the whole-genome sequence assembly; and a new project underway to create an expressed-sequence database and microchip expression arrays for functional genomics applications. Major research areas discussed span a variety of genetic, evolutionary, physiologic, reproductive, developmental, and behavioural topics, including: comparative immunogenetics; genomic imprinting; reproductive biology; neurobiology; photobiology and carcinogenesis; genetics of lipoprotein metabolism; developmental and behavioural endocrinology; sexual differentiation and development; embryonic and fetal development; meiotic recombination; genome evolution; molecular evolution and phylogenetics; and more.

CPD-photolyase adenovirus-mediated gene transfer in normal and DNA-repair-deficient human cells

Cyclobutane pyrimidine dimers (CPDs) are the most frequent and deleterious lesions generated in the mammalian genome after UV-C irradiation. The persistence of these lesions in DNA can be toxic and mutagenic, and also represents a specific signal to apoptosis. To investigate the CPDs repair in situ and consequent UV-induced apoptosis in human cells, we generated a recombinant adenovirus vector containing the gene encoding a CPD-photolyase-EGFP fusion protein (Adphr-EGFP). Adphr-EGFP-infected cells are proficient in photorepair, which prevents apoptotic cell death in comparison with samples kept in the dark, indicating that the fusion protein is functional in CPD recognition and removal. By using local UV irradiation, foci of the photolyase fusion protein were observed in UV-damaged areas of the nuclei in colocalization with NER enzymes. Phr-EGFP migration to CPD sites and redistribution after photorepair was followed, and shown to present similar kinetics in normal or DNA-repair-deficient cells. To our knowledge, this is the first report of an investigation of CPDs repair in situ employing a CPD-photolyase-EGFP enzyme. The Adphr-EGFP vector can be an informative tool to investigate the repair and cellular consequences of UV-induced lesions in primary human cells.

Skin cancer: lights on genome lesions

Sunlight generates skin damage mainly by inducing DNA lesions in epidermal cells. The recent development of transgenic mice expressing specific photolyases has identified cyclobutane pyrimidine dimers as the major player in ultraviolet-induced damage, including skin cancer.

UV light-induced DNA damage and tolerance for the survival of nucleotide excision repair-deficient human cells

DNA damage can cause cell death unless it is either repaired or tolerated. The precise contributions of repair and tolerance mechanisms to cell survival have not been previously evaluated. Here we have analyzed the cell killing effect of the two major UV light-induced DNA lesions, cyclobutane pyrimidine dimers (CPDs) and 6-4 pyrimidine-pyrimidone photoproducts (6-4PPs), in nucleotide excision repair-deficient human cells by expressing photolyase(s) for light-dependent photorepair of either or both lesions. Immediate repair of the less abundant 6-4PPs enhances the survival rate to a similar extent as the immediate repair of CPDs, indicating that a single 6-4PP lesion is severalfold more toxic than a CPD in the cells. Because UV light-induced DNA damage is not repaired at all in nucleotide excision repair-deficient cells, proliferation of these cells after UV light irradiation must be achieved by tolerance of the damage at replication. We found that RNA interference designed to suppress polymerase zeta activity made the cells more sensitive to UV light. This increase in sensitivity was prevented by photorepair of 6-4PPs but not by photorepair of CPDs, indicating that polymerase zeta is involved in the tolerance of 6-4PPs in human cells.

Photorepair of RNA polymerase arrest and apoptosis after ultraviolet irradiation in normal and XPB deficient rodent cells

Cyclobutane pyrimidine dimers (CPDs) are directly involved in signaling for UV-induced apoptosis in mammalian cells. Failure to remove these lesions, specially those located at actively expressing genes, is critical, as cells defective in transcription coupled repair have increased apoptotic levels. Thus, the blockage of RNA synthesis by lesions is an important candidate event triggering off active cell death. In this work, wild-type and XPB mutated Chinese hamster ovary (CHO) cells expressing a marsupial photolyase, that removes specifically CPDs from the damaged DNA, were generated, in order to investigate the importance of this lesion in both RNA transcription blockage and apoptotic induction. Photorepair strongly recovers RNA synthesis in wild-type CHO cell line, although the resumption of transcription is decreased in XPB deficient cells. This recovery is accompanied by the prevention of cells entering into apoptosis. These results demonstrate that marsupial photolyase has access to CPDs blocking RNA synthesis in vivo, and this may be affected by the presence of a mutated XPB protein.

Cyclobutane pyrimidine dimers are responsible for the vast majority of mutations induced by UVB irradiation in mammalian cells

The most prevalent DNA lesions induced by UVB are the cyclobutane pyrimidine dimers (CPDs) and the pyrimidine (6-4) pyrimidone photoproducts ((6-4)PPs). It has been a long standing controversy as to which of these photoproduct is responsible for mutations in mammalian cells. Here we have introduced photoproduct-specific DNA photolyases into a mouse cell line carrying the transgenic mutation reporter genes lacI and cII. Exposure of the photolyase-expressing cell lines to photoreactivating light resulted in almost complete repair of either CPDs or (6-4)PPs within less than 3 h. The mutations produced by the remaining, nonrepaired photoproducts were scored. The mutant frequency in the cII gene after photoreactivation by CPD photolyase was reduced from 127 x 10(-5) to 34 x 10(-5) (background, 8-10 x 10(-5)). Photoreactivation with (6-4) photolyase did not lower the mutant frequency appreciably. In the lacI gene the mutant frequency after photoreactivation repair of CPDs was reduced from 148 x 10(-5) to 28 x 10(-5) (background, 6-10 x 10(-5)). Mutation spectra obtained with and without photoreactivation by CPD photolyase indicated that the remaining mutations were derived from background mutations, unrepaired CPDs, and other DNA photopoducts including perhaps a small contribution from (6-4)PPs. We conclude that CPDs are responsible for at least 80% of the UVB-induced mutations in this mammalian cell model.

Cyclin G1 is involved in G2/M arrest in response to DNA damage and in growth control after damage recovery

Cyclin G1 is one of the target genes of the transcription factor p53, and is induced in a p53-dependent manner in response to DNA damage. Although cyclin G1 has been implicated in a range of biological phenomena, its precise function remains unclear. Here we present an analysis of the physiological role of cyclin G1 using mice homozygous for a targeted disruption of the cyclin G1 gene. In order to clarify the role of cyclin G1 in the p53 pathway, downstream events such as apoptosis, cell growth and cell cycle checkpoint control were analysed in thymocytes and embryonic fibroblasts derived from cyclin G1-disrupted mice. No difference was detected in induction of apoptosis between mouse embryo fibroblasts (MEFs) derived from cyclin G1+/+ and cyclin G1-/- mice. Following irradiation, cyclin G1-/- MEFs proliferated more slowly and reached lower cell densities in culture dishes than cyclin G1+/+ MEFs. Analysis of cell survival showed that cyclin G1-/- MEFs were about twice as sensitive as cyclin G1+/+ MEFs to gamma radiation or UV radiation. Cyclin G1-/- mice were more sensitive to gamma radiation than wild-type mice. Flow cytometeric analysis revealed that the number of cyclin G1-/- MEFs in G2/M phase after irradiation was reduced by 50% relative to cyclin G1+/+ MEFs. Our results demonstrate that cyclin G1 plays roles in G2/M arrest, damage recovery and growth promotion after cellular stress.

Xeroderma pigmentosum p48 gene enhances global genomic repair and suppresses UV-induced mutagenesis

UV-damaged DNA-binding activity (UV-DDB) is deficient in some xeroderma pigmentosum group E individuals due to mutation of the p48 gene, but its role in DNA repair has been obscure. We found that UV-DDB is also deficient in cell lines and primary tissues from rodents. Transfection of p48 conferred UV-DDB to hamster cells, and enhanced removal of cyclobutane pyrimidine dimers (CPDs) from genomic DNA and from the nontranscribed strand of an expressed gene. Expression of p48 suppressed UV-induced mutations arising from the nontranscribed strand, but had no effect on cellular UV sensitivity. These results define the role of p48 in DNA repair, demonstrate the importance of CPDs in mutagenesis, and suggest how rodent models can be improved to better reflect cancer susceptibility in humans.