Detrimental effects of UV-B radiation in a xeroderma pigmentosum-variant cell line

The accurate bypass of pyrimidine dimers by DNA polymerase eta contributes to ultraviolet-induced mutagenesis

Human xeroderma pigmentosum variant (XP-V) patients are mutated in the POLH gene, responsible for encoding the translesion synthesis (TLS) DNA polymerase eta (Pol eta). These patients suffer from a high frequency of skin tumors. Despite several decades of research, studies on Pol eta still offer an intriguing paradox: How does this error-prone polymerase suppress mutations? This review examines recent evidence suggesting that cyclobutane pyrimidine dimers (CPDs) are instructional for Pol eta. Consequently, it can accurately replicate these lesions, and the mutagenic effects induced by UV radiation stem from the deamination of C-containing CPDs. In this model, the deamination of C (forming a U) within CPDs leads to the correct insertion of an A opposite to the deaminated C (or U)-containing dimers. This intricate process results in C>T transitions, which represent the most prevalent mutations detected in skin cancers. Finally, the delayed replication in XP-V cells amplifies the process of C-deamination in CPDs and increases the burden of C>T mutations prevalent in XP-V tumors through the activity of backup TLS polymerases.

Current state of knowledge of human DNA polymerase eta protein structure and disease-causing mutations

POLη, encoded by the POLH gene, is a crucial protein for replicating damaged DNA and the most studied specialized translesion synthesis polymerases. Mutations in POLη are associated with cancer and the human syndrome xeroderma pigmentosum variant, which is characterized by extreme photosensitivity and an increased likelihood of developing skin cancers. The myriad of structural information about POLη is vast, covering dozens of different mutants, numerous crucial residues, domains, and posttranslational modifications that are essential for protein function within cells. Since POLη is key vital enzyme for cell survival, and mutations in this protein are related to aggressive diseases, understanding its structure is crucial for biomedical sciences, primarily due to its similarities with other Y-family polymerases and its potential as a targeted therapy-drug for tumors. This work provides an up-to-date review on structural aspects of the human POLη: from basic knowledge about critical residues and protein domains to its mutant variants, posttranslational modifications, and our current understanding of therapeutic molecules that target POLη. Thus, this review provides lessons about POLη's structure and gathers critical discussions and hypotheses that may contribute to understanding this protein's vital roles within the cells.

Chemical Composition and Protective Effect of Young Barley (Hordeum vulgare L.) Dietary Supplements Extracts on UV-Treated Human Skin Fibroblasts in In Vitro Studies

Young barley seems to be a promising material for use as nutricosmetic due to the presence of many biologically active compounds. The aim of this study was to evaluate the effect of Hordeum vulgare L. extracts on human skin fibroblasts exposed to ultraviolet radiation B (UVB) radiation. Analysis of the chemical composition showed a predominance of 9,12,15-octadecatrienoic acid. The quality assessment showed that young barley preparations have high total polyphenolic content (TPC) and favourable total antioxidant status (TAS). They also contain antioxidant elements such as zinc, copper, and selenium. Furthermore, the analyzed products were found to be safe in terms of toxic elements (lead, cadmium and mercury) and lack of cytotoxic effect of young barley extracts on cells. In vitro bioactivity assays showed that young barley extract increased the survival rate and accelerated the migration of fibroblasts in research models with UVB radiation. The application of both extracts caused an increase in DNA biosynthesis, and in the number of cells arrested in S phase. Moreover, an inhibitory effect of the tested extracts on the expression of matrix metalloproteinase 2 (MMP-2) and matrix metalloproteinase 9 (MMP-9) was observed. The results indicate that young barley extracts, due to protective as well as restorative effect, could potentially be used in the production of nutricosmetics and skin care products.

Cytotoxicity and Mutagenicity of Narrowband UVB to Mammalian Cells

Phototherapy using narrowband ultraviolet-B (NB-UVB) has been shown to be more effective than conventional broadband UVB (BB-UVB) in treating a variety of skin diseases. To assess the difference in carcinogenic potential between NB-UVB and BB-UVB, we investigated the cytotoxicity via colony formation assay, genotoxicity via sister chromatid exchange (SCE) assay, mutagenicity via hypoxanthine phosphoribosyltransferase (HPRT) mutation assay, as well as cyclobutane pyrimidine dimer (CPD) formation and reactive oxygen species (ROS) generation in Chinese hamster ovary (CHO) and their NER mutant cells. The radiation dose required to reduce survival to 10% (D₁₀ value) demonstrated BB-UVB was 10 times more cytotoxic than NB-UVB, and revealed that NB-UVB also induces DNA damage repaired by nucleotide excision repair. We also found that BB-UVB more efficiently induced SCEs and HPRT mutations per absorbed energy dosage (J/m²) than NB-UVB. However, SCE and HPRT mutation frequencies were observed to rise in noncytotoxic dosages of NB-UVB exposure. BB-UVB and NB-UVB both produced a significant increase in CPD formation and ROS formation (p < 0.05); however, higher dosages were required for NB-UVB. These results suggest that NB-UVB is less cytotoxic and genotoxic than BB-UVB, but can still produce genotoxic effects even at noncytotoxic doses.

Whole-exome sequencing reveals the impact of UVA light mutagenesis in xeroderma pigmentosum variant human cells

UVA-induced mutagenesis was investigated in human pol eta-deficient (XP-V) cells through whole-exome sequencing. In UVA-irradiated cells, the increase in the mutation frequency in deficient cells included a remarkable contribution of C>T transitions, mainly at potential pyrimidine dimer sites. A strong contribution of C>A transversions, potentially due to oxidized bases, was also observed in non-irradiated XP-V cells, indicating that basal mutagenesis caused by oxidative stress may be related to internal tumours in XP-V patients. The low levels of mutations involving T induced by UVA indicate that pol eta is not responsible for correctly replicating T-containing pyrimidine dimers, a phenomenon known as the ‘A-rule’. Moreover, the mutation signature profile of UVA-irradiated XP-V cells is highly similar to the human skin cancer profile, revealing how studies involving cells deficient in DNA damage processing may be useful to understand the mechanisms of environmentally induced carcinogenesis.

Cooperation and interplay between base and nucleotide excision repair pathways: From DNA lesions to proteins

Base and nucleotide excision repair (BER and NER) pathways are normally associated with removal of specific types of DNA damage: small base modifications (such as those induced by DNA oxidation) and bulky DNA lesions (such as those induced by ultraviolet or chemical carcinogens), respectively. However, growing evidence indicates that this scenario is much more complex and these pathways exchange proteins and cooperate with each other in the repair of specific lesions. In this review, we highlight studies discussing the involvement of NER in the repair of DNA damage induced by oxidative stress, and BER participating in the removal of bulky adducts on DNA. Adding to this complexity, UVA light experiments revealed that oxidative stress also causes protein oxidation, directly affecting proteins involved in both NER and BER. This reduces the cell’s ability to repair DNA damage with deleterious implications to the cells, such as mutagenesis and cell death, and to the organisms, such as cancer and aging. Finally, an interactome of NER and BER proteins is presented, showing the strong connection between these pathways, indicating that further investigation may reveal new functions shared by them, and their cooperation in maintaining genome stability.

Diagnosis of eight groups of xeroderma pigmentosum by genetic complementation using recombinant adenovirus vectors

Because patients with xeroderma pigmentosum (XP) must avoid ultraviolet (UV) light from an early age, an early diagnosis of this disorder is essential. XP is composed of seven genetic complementation groups, XP-A to -G, and a variant type (XP-V). To establish an easy and accurate diagnosis of the eight disease groups, we constructed recombinant adenoviruses that expressed one of the XP cDNA. When fibroblasts derived from patients with XP-A, -B, -C, -D, -F or -G were infected with the adenovirus expressing XPA, XPB, XPC, XPD, XPF or XPG, respectively, and UV-C at 5-20 J/m² was irradiated, cell viability was clearly recovered by the corresponding recombinant adenoviruses. In contrast, XP-E and XP-V cells were not significantly sensitive to UV irradiation and were barely complemented by the matched recombinant adenoviruses. However, co-infection of Ad-XPA with Ad-XPE increased survival rate of XP-E cells after UV-C exposure. When XP-V cell strains, including one derived from a Japanese patient, were infected with Ad-XPV, exposed to UV-B and cultured with 1 mmol/L of caffeine, flow cytometry detected a characteristic decrease in the S phase in all the XP-V cell strains. From these results, the eight groups of XP could be differentiated by utilizing a set of recombinant adenoviruses, indicating that our procedure provides a convenient and correct diagnostic method for all the XP groups including XP-E and XP-V.

Minimal detection of nuclear mutations in XP-V and normal cells treated with oxidative stress inducing agents

Elevated levels of reactive oxygen species (ROS) can be induced by exposure to various chemicals and radiation. One type of damage in DNA produced by ROS is modification of guanine to 7,8-dihydro-8-oxoguanine (8-oxoG). This particular alteration to the chemistry of the base can inhibit the replication fork and has been linked to mutagenesis, cancer, and aging. In vitro studies have shown that the translesion synthesis polymerase, DNA polymerase η (pol η), is able to efficiently bypass 8-oxoG in DNA. In this study, we wanted to investigate the mutagenic effects of oxidative stress, and in particular 8-oxoG, in the presence and absence of pol η. We quantified levels of oxidative stress, 8-oxoG levels in DNA, and nuclear mutation rates. We found that most of the 8-oxoG detected were localized to the mitochondrial DNA, opposed to the nuclear DNA. We also saw a corresponding lack of mutations in a nuclear-encoded gene. This suggests that oxidative stress' primary mutagenic effects are not predominantly on genomic DNA.