Cyclobutane Pyrimidine Dimers Produced with Narrowband UVB Are on Average More Mutagenic than Those with Broadband UVB in Mouse Skin.

Although narrowband UVB (NB-UVB) has replaced broadband UVB (BB-UVB) because of its greater effectiveness in dermatological phototherapy, it is twice as carcinogenic as BB-UVB at an equivalent inflammatory dose. To clarify the basis of the different genotoxicities, we comparatively evaluated the mutagenicities in mouse skin of the two UVB types along with their efficiencies in the formation of cyclobutane pyrimidine dimer (CPD), which is a major mutagenic DNA photolesion specifically produced by UVR. We found that the mutagenicity averaged per single molecule of CPD was 2.

Seasonal Differences in the UVA/UVB Ratio of Natural Sunlight Influence the Efficiency of the Photoisomerization of (6-4) Photoproducts into their Dewar Valence Isomers.

The UVA and UVB components of sunlight can produce three classes of bipyrimidine DNA photolesions [cyclobutane pyrimidine dimers (CPDs), pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) and related Dewar valence isomers (DewarPPs)]. The UVA/UVB ratio of sunlight is high in winter and low in summer in the Northern Hemisphere. Since UVB radiation produces 6-4PPs and UVA radiation converts them into DewarPPs through photoisomerization, it is expected that there may be differences in the photoisomerization of 6-4PPs between summer and winter, although that has never been documented.

Author Correction: Nrf2 contributes to the weight gain of mice during space travel.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Nrf2 contributes to the weight gain of mice during space travel.

Space flight produces an extreme environment with unique stressors, but little is known about how our body responds to these stresses. While there are many intractable limitations for in-flight space research, some can be overcome by utilizing gene knockout-disease model mice. Here, we report how deletion of Nrf2, a master regulator of stress defense pathways, affects the health of mice transported for a stay in the International Space Station (ISS).

Wavelength- and Tissue-dependent Variations in the Mutagenicity of Cyclobutane Pyrimidine Dimers in Mouse Skin.

The cyclobutane pyrimidine dimer (CPD) is a main mutagenic photolesion in DNA produced by UVR. We previously studied the wavelength-dependent kinetics of mutation induction efficiency using monochromatic UVR sources and transgenic mice developed for mutation assay and established the action spectra of UVR mutagenicity in the mouse epidermis and dermis. Here, we further established the action spectra of CPD and pyrimidine(6-4)pyrimidone photoproduct formation in the same tissues and in naked DNA using the same sources and mouse strain.

Roles of the KEAP1-NRF2 system in mammalian skin exposed to UV radiation.

The KEAP1 (Kelch-like ECH-associated protein 1)-NRF2 (NF-E2-related factor 2) system controls the biochemical defense activity against agents toxic to mammals and responds to exogenous and endogenous stressors such as electrophilic and oxidative substances, which can have destructive and genotoxic effects on affected mammalian tissues. Although this system can be activated by various environmental stressors, it remains unclear whether ultraviolet radiation (UVR), which is one of the major environmental agents that has inflammatory and carcinogenic impacts on human skin and eyes, induces NRF2-dependent defense activity. Here, we review the recent progress in the study of the contributions of NRF2 and related factors to protection against UVR.

Mechanistic considerations on the wavelength-dependent variations of UVR genotoxicity and mutagenesis in skin: the discrimination of UVA-signature from UV-signature mutation.

Ultraviolet radiation (UVR) predominantly induces UV-signature mutations, C → T and CC → TT base substitutions at dipyrimidine sites, in the cellular and skin genome. I observed in our in vivo mutation studies of mouse skin that these UVR-specific mutations show a wavelength-dependent variation in their sequence-context preference. The C → T mutation occurs most frequently in the 5'-TCG-3' sequence regardless of the UVR wavelength, but is recovered more preferentially there as the wavelength increases, resulting in prominent occurrences exclusively in the TCG sequence in the UVA wavelength range, which I will designate as a "UVA signature" in this review.

Quantitative analysis of UV photolesions suggests that cyclobutane pyrimidine dimers produced in mouse skin by UVB are more mutagenic than those produced by UVC.

The amount of photolesions produced in DNA after exposure to physiological doses of ultraviolet radiation (UVR) can be estimated with high sensitivity and at low cost through an immunological assay, ELISA, which, however, provides only a relative estimate that cannot be used for comparisons between different photolesions such as cyclobutane pyrimidine dimer (CPD) and pyrimidine(6-4)pyrimidone photoproduct (64PP) or for analysis of the genotoxicity of photolesions on a molecular basis. To solve this drawback of ELISA, we introduced a set of UVR-exposed, calibration DNA whose photolesion amounts were predetermined and estimated the absolute molecular amounts of CPDs and 64PPs produced in mouse skin exposed to UVC and UVB. We confirmed previously reported observations that UVC induced more photolesions in the skin than UVB at the same dose, and that both types of UVR produced more CPDs than 64PPs.

In Vivo Spectrum of UVC-induced Mutation in Mouse Skin Epidermis May Reflect the Cytosine Deamination Propensity of Cyclobutane Pyrimidine Dimers.

Although ultraviolet radiation (UVR) has a genotoxicity for inducing skin cancers, the skin may tolerate UVC component because the epidermal layer prevents this short wavelength range from passing through. Here, UVC genotoxicity for mouse skin was evaluated in terms of DNA damage formation and mutagenicity. UVC induced UVR photolesions and mutations remarkably in the epidermis but poorly in the dermis, confirming the barrier ability of the epidermis against shorter UVR wavelengths.

Remarkable induction of UV-signature mutations at the 3'-cytosine of dipyrimidine sites except at 5'-TCG-3' in the UVB-exposed skin epidermis of xeroderma pigmentosum variant model mice.

The human POLH gene is responsible for the variant form of xeroderma pigmentosum (XP-V), a genetic disease highly susceptible to cancer on sun-exposed skin areas, and encodes DNA polymerase η (polη), which is specialized for translesion DNA synthesis (TLS) of UV-induced DNA photolesions. We constructed polη-deficient mice transgenic with lacZ mutational reporter genes to study the effect of Polh null mutation (Polh(-/-)) on mutagenesis in the skin after UVB irradiation. UVB induced lacZ mutations with remarkably higher frequency in the Polh(-/-) epidermis and dermis than in the wild-type (Polh(+/+)) and heterozygote.

Skin can control solar UVR-induced mutations through the epidermis-specific response of mutation induction suppression.

Skin exposure to solar ultraviolet radiation (UVR) has been a major public concern because of its genotoxicity. We established recently three action spectra of UVR biological effects using inflammation, mutagenicity, and mutation induction suppression (MIS) as indicators to evaluate UVR risk for mammalian skin. MIS is an antigenotoxic epidermis-specific response by which the increase of the mutant frequency (MF) levels off above a certain UVR dose.

Solar-UV-signature mutation prefers TCG to CCG: extrapolative consideration from UVA1-induced mutation spectra in mouse skin.

UVA1 exerts its genotoxicity on mammalian skin by producing cyclobutane pyrimidine dimers (CPDs) in DNA and preferentially inducing solar-UV-signature mutations, C → T base substitution mutations at methylated CpG-associated dipyrimidine (Py-mCpG) sites, as demonstrated previously using a 364 nm laser as a UVA1 source and lacZ-transgenic mice that utilize the transgene as a mutational reporter. In the present study, we confirmed that a broadband UVA1 source induced the same mutation profiles in mouse epidermis as the UVA1 laser, generalizing the previous result from a single 364 nm to a wider wavelength range of UVA1 (340-400 nm). Combined with our previous data on the mutation spectra induced in mouse epidermis by UVB, UVA2 and solar UVR, we proved that the solar-UV-signature mutation is commonly observed in the wavelength range from UVB to UVA, and found that UVA1 induces this mutation more preferentially than the other shorter wavelength ranges.

Action spectrum analysis of UVR genotoxicity for skin: the border wavelengths between UVA and UVB can bring serious mutation loads to skin.

UVR causes erythema, which has been used as a standardized index to evaluate the risk of UVR for human skin. However, the genotoxic significance of erythema has not been elucidated clearly. Here, we characterized the wavelength dependence of the genotoxic and erythematic effects of UVR for the skin by analyzing the induction kinetics of mutation and inflammation in mouse skin using lacZ-transgenic mice and monochromatic UVR sources.

Role of the Msh2 gene in genome maintenance and development in mouse fetuses.

In an attempt to evaluate the roles of the mismatch repair gene Msh2 in genome maintenance and in development during the fetal stage, spontaneous mutations and several developmental indices were studied in Msh2-deficient lacZ-transgenic mouse fetuses. Mutation levels in fetuses were elevated at 9.5 dpc (days post coitum) when compared to wild-type mice, and the level of mutations continued to increase until the fetuses reached the newborn stage.

Fully functional global genome repair of (6-4) photoproducts and compromised transcription-coupled repair of cyclobutane pyrimidine dimers in condensed mitotic chromatin.

During mitosis, chromatin is highly condensed, and activities such as transcription and semiconservative replication do not occur. Consequently, the condensed condition of mitotic chromatin is assumed to inhibit DNA metabolism by impeding the access of DNA-transacting proteins. However, about 40 years ago, several researchers observed unscheduled DNA synthesis in UV-irradiated mitotic chromosomes, suggesting the presence of excision repair.

The mechanisms of UV mutagenesis.

Ultraviolet (UV) light induces specific mutations in the cellular and skin genome such as UV-signature and triplet mutations, the mechanism of which has been thought to involve translesion DNA synthesis (TLS) over UV-induced DNA base damage. Two models have been proposed: "error-free" bypass of deaminated cytosine-containing cyclobutane pyrimidine dimers (CPDs) by DNA polymerase η, and error-prone bypass of CPDs and other UV-induced photolesions by combinations of TLS and replicative DNA polymerases--the latter model has also been known as the two-step model, in which the cooperation of two (or more) DNA polymerases as misinserters and (mis)extenders is assumed. Daylight UV induces a characteristic UV-specific mutation, a UV-signature mutation occurring preferentially at methyl-CpG sites, which is also observed frequently after exposure to either UVB or UVA, but not to UVC.

Effects of calorie restriction on the age-dependent accumulation of mutations in the small intestine of lacZ-transgenic mice.

To understand the effect of calorie restriction on genome maintenance systems, the age-dependent accumulation of mutations in animals maintained on high and low calorie diets was examined using lacZ-transgenic mice. Mice were fed a diet of 95 kcal/w or 65 kcal/w from 2 to 17 months of age. The mutation frequencies in the lacZ gene in epithelial tissues from the small intestine were examined at 12 and 17 months.

Antigenotoxic effects of p53 on spontaneous and ultraviolet light B--induced deletions in the epidermis of gpt delta transgenic mice.

Tumor development in the skin may be a multistep process where multiple genetic alterations occur successively. The p53 gene is involved in genome stability and thus is referred to as "the guardian of the genome." To better understand the antigenotoxic effects of p53 in ultraviolet light B (UVB)-induced mutagenesis, mutations were measured in the epidermis of UVB-irradiated p53(+/+) and p53(-/-) gpt delta mice.

Influences of p53 deficiency on the apoptotic response, DNA damage removal and mutagenesis in UVB-exposed mouse skin.

p53 suppresses the genomic instability provoked by genotoxic agents. Ultraviolet (UV) B induces skin cancers by producing DNA damage and mutations in the skin genome, whereas the skin tissue responds to the UVB insult with cell cycle arrest and apoptosis as well as damage exclusion by DNA repair. To address the p53 contribution to these skin responses in vivo, we analyzed the time course of DNA damage removal, apoptosis induction and hyperplasia in the skin after UVB irradiation in p53-knockout mice.

XPC is involved in genome maintenance through multiple pathways in different tissues.

In an attempt to evaluate the role of the Xpc gene in maintaining genomic stability in vivo under normal conditions, the age-dependent accumulation of spontaneous mutations in different tissues was analyzed in Xpc-deficient lacZ-transgenic mice. Brain, testis, and small intestine revealed no effects from the Xpc-deficiency, whereas liver, spleen, heart, and lung showed an enhanced age-related accumulation of mutations in Xpc-deficient mice. In the spleen, the effect was not obvious at 2 and 12 months of age, but became apparent at 23 months.

Absence of Ku70 gene obliterates X-ray-induced lacZ mutagenesis of small deletions in mouse tissues.

With the goal of understanding the role of non-homologous end-joining repair in the maintenance of genetic information at the tissue level, we studied mutations induced by radiation and subsequent repair of DNA double-strand breaks in Ku70 gene-deficient lacZ transgenic mice. The local mutation frequencies and types of mutations were analyzed on a lacZ gene that had been chromosomally integrated, which allowed us to monitor DNA sequence alterations within this 3.1-kbp region.

UVA1 genotoxicity is mediated not by oxidative damage but by cyclobutane pyrimidine dimers in normal mouse skin.

UVA1 induces the formation of 8-hydroxy-2'-deoxyguanosines (8-OH-dGs) and cyclobutane pyrimidine dimers (CPDs) in the cellular genome. However, the relative contribution of each type of damage to the in vivo genotoxicity of UVA1 has not been clarified. We irradiated living mouse skin with 364-nm UVA1 laser light and analyzed the DNA damage formation and mutation induction in the epidermis and dermis.

Chromatin fine structure of the c-MYC insulator element/DNase I-hypersensitive site I is not preserved during mitosis.

During mitosis in higher eukaryotic cells, transcription is silenced and transcription complexes are absent from promoters in the condensed chromosomes; however, epigenetic information concerning the pattern of expressed and silent genes must be preserved. Recently, it has been reported that CTCF, a major protein in vertebrate insulator elements, remains associated with mitotic chromatin. If the structure of insulators is preserved during mitosis, then it is possible that insulators can function as components or elements of the mechanism involved in the transfer of epigenetic information through the mitotic phase and can help guide the reconstitution of domain structure and nuclear organization after the completion of this phase.

A model for triplet mutation formation based on error-prone translesional DNA synthesis opposite UV photolesions.

A triplet mutation is defined as multiple base substitutions or frameshifts within a three-nucleotide sequence which includes a dipyrimidine sequence. Triplet mutations have recently been identified as a new type of UV-specific mutation, although the mechanism of their formation is unknown. A total of 163 triplet mutations were identified through an extensive search of previously published data on UV-induced mutations, including mutations from skin, skin cancer, and cultured mammalian cells.

Mutation spectrum in UVB-exposed skin epidermis of Xpa-knockout mice: frequent recovery of triplet mutations.

Knockout mutations in both alleles of the Xpa gene give rise to a complete deficiency in nucleotide excision repair (NER) in mammalian cells. We used transgenic mice harboring the lambda-phage-based lacZ mutational reporter gene to study the effect of Xpa null mutation (Xpa(-/-)) on damage induction, repair, and mutagenesis in mouse skin epidermis after UVB irradiation. UVB induced equal amounts of cyclobutane pyrimidine dimers (CPDs) and pyrimidine(6-4)pyrimidone photoproducts (64PPs) in mouse skin epidermis of Xpa(-/-) and wild-type mice.