The in situ repair kinetics of epidermal thymine dimers and 6-4 photoproducts in human skin types I and II

On the chemistry of sunlight-induced DNA lesions: A perspective on the alkaline chemical-induced reactivities of photo-damaged pyrimidine intra-strand dimers

Photoexcitation of cellular as well as isolated DNAs upon exposure to the UV portion of sunlight or other UV sources can lead to the covalent dimerization of adjacent intra-strand stacked pyrimidine nucleobase rings (i.e., at 5'-Py-p-Py-3' sites). These modifications generate, in mammalian DNA as well as the DNA of all other forms of life, lesions such as cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs); and, in bacterial endospores, spore photoproducts (SPs). Importantly, the lesions formed in higher organisms can lead to disease states including cancer. While the formation, structure, and biological outcomes of pyrimidine dimer lesions have been the focus of much research, less has been known about their fundamental chemical properties until recently. Such an understanding of these lesions may lead to novel means to chemically identify and quantitate their presence in the genome. This review is intended to provide an overview of intra-strand pyrimidine dimer lesions derived from 5'-T-p-T sites with a focus on presenting what is currently known about their individual in vitro alkaline chemical reactivities. Included here are descriptions of investigations of the DNA lesions CPD, 6-4PP, and SP, and, for comparison, the monomeric pyrimidine lesion 5,6-dihydo-2'-deoxyuridine (dHdU). Of interest, the alkaline hydrolyses of these various lesions are all found to be centered on the loss of aromaticity of a lesion Py ring (T) leading to a carbonyl "hot spot," the focal point of initial hydrolytic attack.

Blue light impairs the repair of UVB-induced pyrimidine dimers in a human skin model

In recent years, interest is growing in the biological cutaneous effects of high-energy visible light (400-450 nm). In the present study, we explored the impact of blue light (BL) on the repair of pyrimidine dimers, the major class of premutagenic DNA damage induced by exposure to sunlight. We unambiguously demonstrate that the exposure of in vitro reconstructed human epidermis to environmentally relevant doses of BL strongly decreases the rate of repair of cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts induced by a subsequent UVB irradiation. Using the highly sensitive and specific liquid chromatography-tandem mass spectrometry assay, we did not observe induction of pyrimidine dimers by BL alone. Finally, we showed that application, during the BL exposure step, of a formula containing a new filter, named TriAsorB and affording BL photoprotection, prevented the decrease in DNA repair efficiency. These results emphasize the potential deleterious effects of BL on DNA repair and the interest in providing adequate skin protection against this wavelength range of sunlight.

Differing structures and dynamics of two photolesions portray verification differences by the human XPD helicase

Ultraviolet light generates cyclobutane pyrimidine dimer (CPD) and pyrimidine 6-4 pyrimidone (6-4PP) photoproducts that cause skin malignancies if not repaired by nucleotide excision repair (NER). While the faster repair of the more distorting 6-4PPs is attributed mainly to more efficient recognition by XPC, the XPD lesion verification helicase may play a role, as it directly scans the damaged DNA strand. With extensive molecular dynamics simulations of XPD-bound single-strand DNA containing each lesion outside the entry pore of XPD, we elucidate strikingly different verification processes for these two lesions that have very different topologies. The open book-like CPD thymines are sterically blocked from pore entry and preferably entrapped by sensors that are outside the pore; however, the near-perpendicular 6-4PP thymines can enter, accompanied by a displacement of the Arch domain toward the lesion, which is thereby tightly accommodated within the pore. This trapped 6-4PP may inhibit XPD helicase activity to foster lesion verification by locking the Arch to other domains. Furthermore, the movement of the Arch domain, only in the case of 6-4PP, may trigger signaling to the XPG nuclease for subsequent lesion incision by fostering direct contact between the Arch domain and XPG, and thereby facilitating repair of 6-4PP.

Skin optical properties from 200 to 300 nm support far UV-C skin-safety in vivo

The growing threat of multi-drug resistant pathogens and airborne microbial diseases has highlighted the need to improve or develop novel disinfection methods for clinical environments. Conventional ultraviolet C (UV-C) lamps effectively inactivate microorganisms but are harmful to human skin and eyes upon exposure. The use of new 233 nm far UV-C LEDs as an antiseptic can overcome those limitations. In this research, the light penetration into the skin was elucidated for the UV-C region (<300 nm) by measuring the scattering and absorption of skin layers and inverse Monte Carlo simulation, and further confirmed by the first clinical pilot trial in which healthy volunteers were irradiated with a dose of 60 mJ/cm2 at 233 nm. The radiation is strongly absorbed in the stratum corneum, resulting in minimal skin damage without inducing inflammatory responses. The results suggest that 233 nm far UV-C light emitting diodes (LEDs) could effectively inactivate microorganisms, while being safe and soft for the skin.

Impact of DNA sequences on DNA 'opening' by the Rad4/XPC nucleotide excision repair complex

Rad4/XPC recognizes diverse DNA lesions to initiate nucleotide excision repair (NER). However, NER propensities among lesions vary widely and repair-resistant lesions are persistent and thus highly mutagenic. Rad4 recognizes repair-proficient lesions by unwinding ('opening') the damaged DNA site. Such 'opening' is also observed on a normal DNA sequence containing consecutive C/G's (CCC/GGG) when tethered to Rad4 to prevent protein diffusion. However, it was unknown if such tethering-facilitated DNA 'opening' could occur on any DNA or if certain structures/sequences would resist being 'opened'. Here, we report that DNA containing alternating C/G's (CGC/GCG) failed to be opened even when tethered; instead, Rad4 bound in a 180°-reversed manner, capping the DNA end. Fluorescence lifetime studies of DNA conformations in solution showed that CCC/GGG exhibits local pre-melting that is absent in CGC/GCG. In MD simulations, CGC/GCG failed to engage Rad4 to promote 'opening' contrary to CCC/GGG. Altogether, our study illustrates how local sequences can impact DNA recognition by Rad4/XPC and how certain DNA sites resist being 'opened' even with Rad4 held at that site indefinitely. The contrast between CCC/GGG and CGC/GCG sequences in Rad4-DNA recognition may help decipher a lesion's mutagenicity in various genomic sequence contexts to explain lesion-determined mutational hot and cold spots.

CTCF binding modulates UV damage formation to promote mutation hot spots in melanoma

Somatic mutations in DNA-binding sites for CCCTC-binding factor (CTCF) are significantly elevated in many cancers. Prior analysis has suggested that elevated mutation rates at CTCF-binding sites in skin cancers are a consequence of the CTCF-cohesin complex inhibiting repair of UV damage. Here, we show that CTCF binding modulates the formation of UV damage to induce mutation hot spots. Analysis of genome-wide CPD-seq data in UV-irradiated human cells indicates that formation of UV-induced cyclobutane pyrimidine dimers (CPDs) is primarily suppressed by CTCF binding but elevated at specific locations within the CTCF motif. Locations of CPD hot spots in the CTCF-binding motif coincide with mutation hot spots in melanoma. A similar pattern of damage formation is observed at CTCF-binding sites in vitro, indicating that UV damage modulation is a direct consequence of CTCF binding. We show that CTCF interacts with binding sites containing UV damage and inhibits repair by a model repair enzyme in vitro. Structural analysis and molecular dynamic simulations reveal the molecular mechanism for how CTCF binding modulates CPD formation.

Impact of the Nucleosome Histone Core on the Structure and Dynamics of DNA-Containing Pyrimidine-Pyrimidone (6-4) Photoproduct

The pyrimidine-pyrimidone (6-4) photoproduct (64-PP) is an important photoinduced DNA lesion constituting a mutational signature for melanoma. The structural impact of 64-PP on DNA complexed with histones affects the lesion mutagenicity and repair but remains poorly understood. Here we investigate the conformational dynamics of DNA-containing 64-PP within the nucleosome core particle by atomic-resolution molecular dynamics simulations and multiscale data analysis. We demonstrate that the histone core exerts important mechanical restraints that largely decrease global DNA structural fluctuations. However, the local DNA flexibility at the damaged site is enhanced due to imperfect structural adaptation to restraints imposed by the histone core. If 64-PP faces the histone core and is therefore not directly accessible by the repair protein, the complementary strand facing the solvent is deformed and exhibits higher flexibility than the corresponding strand in a naked, undamaged DNA. This may serve as an initial recognition signal for repair. Our simulations also pinpoint the structural role of proximal residues from the truncated histone tails.

Oxidative Stress and Genotoxicity in Melanoma Induction: Impact on Repair Rather Than Formation of DNA Damage?

Keratinocytes and melanocytes, two cutaneous cell types located within the epidermis, are the origin of most skin cancers, namely carcinomas and melanomas. These two types of tumors differ in many ways. First, carcinomas are almost 10 times more frequent than melanomas. In addition, the affected cellular pathways, the mutated genes and the metastatic properties of the tumors are not the same. This review addresses another specificity of melanomas: the role of photo-oxidative stress. UVA efficiently produces reactive oxygen species in melanocytes, which results in more frequent oxidatively generated DNA lesions than in other cell types. The question of the respective contribution of UVB-induced pyrimidine dimers and UVA-mediated oxidatively generated lesions to mutagenesis in melanoma remains open. Recent results based on next-generation sequencing techniques strongly suggest that the mutational signature associated with pyrimidine dimers is overwhelming in melanomas like in skin carcinomas. UVA-induced oxidative stress may yet be indirectly linked to the genotoxic pathways involved in melanoma through its ability to hamper DNA repair activities.

The 6-4 photoproduct is the trigger of UV-induced replication blockage and ATR activation

The most prevalent human carcinogen is sunlight-associated ultraviolet (UV), a physiologic dose of which generates thousands of DNA lesions per cell, mostly of two types: cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs). It has not been possible, in living cells, to precisely characterize the respective contributions of these two lesion types to the signals that regulate cell cycle progression, DNA replication, and cell survival. Here we coupled multiparameter flow cytometry with lesion-specific photolyases that eliminate either CPDs or 6-4PPs and determined their respective contributions to DNA damage responses. Strikingly, only 6-4PP lesions activated the ATR-Chk1 DNA damage response pathway. Mechanistically, 6-4PPs, but not CPDs, impeded DNA replication across the genome as revealed by microfluidic-assisted replication track analysis. Furthermore, single-stranded DNA accumulated preferentially at 6-4PPs during DNA replication, indicating selective and prolonged replication blockage at 6-4PPs. These findings suggest that 6-4PPs, although eightfold fewer in number than CPDs, are the trigger for UV-induced DNA damage responses.

Structure and mechanism of pyrimidine-pyrimidone (6-4) photoproduct recognition by the Rad4/XPC nucleotide excision repair complex

Failure in repairing ultraviolet radiation-induced DNA damage can lead to mutations and cancer. Among UV-lesions, the pyrimidine–pyrimidone (6-4) photoproduct (6-4PP) is removed from the genome much faster than the cyclobutane pyrimidine dimer (CPD), owing to the more efficient recognition of 6-4PP by XPC-RAD23B, a key initiator of global-genome nucleotide excision repair (NER). Here, we report a crystal structure of a Rad4–Rad23 (yeast XPC-Rad23B ortholog) bound to 6-4PP-containing DNA and 4-μs molecular dynamics (MD) simulations examining the initial binding of Rad4 to 6-4PP or CPD. This first structure of Rad4/XPC bound to a physiological substrate with matched DNA sequence shows that Rad4 flips out both 6-4PP-containing nucleotide pairs, forming an ‘open’ conformation. The MD trajectories detail how Rad4/XPC initiates ‘opening’ 6-4PP: Rad4 initially engages BHD2 to bend/untwist DNA from the minor groove, leading to unstacking and extrusion of the 6-4PP:AA nucleotide pairs towards the major groove. The 5′ partner adenine first flips out and is captured by a BHD2/3 groove, while the 3′ adenine extrudes episodically, facilitating ensuing insertion of the BHD3 β-hairpin to open DNA as in the crystal structure. However, CPD resists such Rad4-induced structural distortions. Untwisting/bending from the minor groove may be a common way to interrogate DNA in NER.

Variable impact of conformationally distinct DNA lesions on nucleosome structure and dynamics: Implications for nucleotide excision repair

The packaging of DNA in nucleosomes presents a barrier for biological transactions including replication, transcription and repair. However, despite years of research, how the DNA is freed from the histone proteins and thereby allows the molecular machines to access the DNA remains poorly understood. We are interested in global genomic nucleotide excision repair (GG-NER). It is established that the histones are obstacles to this process, and DNA lesions are repaired less efficiently in nucleosomes than in free DNA. In the present study, we utilized molecular dynamics simulations to elucidate the nature of the distortions and dynamics imposed in the nucleosome by a set of three structually different lesions that vary in GG-NER efficiencies in free DNA, and in nucleosomes [Shafirovich, Geacintov, et. al, 2019]. Two of these are bulky lesions derived from metabolic activation of the environmental carcinogen benzo[a]pyrene, the 10R (+)-cis-anti-B[a]P-N²-dG and the stereoisomeric 10S (+)-trans-anti-B[a]P-N²-dG, which respectively adopt base-displaced/intercalated and minor groove-aligned conformations in DNA. The third is a non-bulky lesion, the 5'R-8-cyclo-2'-deoxyguanosine cross-link, produced by reactive oxygen and nitrogen species; cyclopurine lesions are highly mutagenic. These adducts are placed near the dyad axis, and rotationally with the lesion-containing strand facing towards or away from the histones. While each lesion has distinct conformational characteristics that are retained in the nucleosome, a spectrum of structural and dynamic disturbances, from slight to substantial, are displayed that depend on the lesion's structure and position in the nucleosome. We hypothesize that these intrinsic structural and dynamic distinctions provide different signals to initiate the cascade of chromatin-opening processes, including acetylation and other post translational modifications, remodeling by ATP-dependent complexes and spontaneous unwrapping that regulate the rate of access to the lesion; this may translate ultimately into varying GG-NER efficiencies, including repair resistance when signals for access are too weak.

Photoprotective and Antigenotoxic Effects of the Flavonoids Apigenin, Naringenin and Pinocembrin

This work evaluated the photoprotective and antigenotoxic effects against ultraviolet B (UVB) radiation of flavonoid compounds apigenin, naringenin and pinocembrin. The photoprotective efficacy of these compounds was estimated using in vitro photoprotection indices, and the antigenotoxicity against UVB radiation was evaluated using the SOS chromotest and an enzymatic (proteinase K/T4 endonuclease V enzyme) comet assay in UV-treated Escherichia coli and human (HEK-293) cells, respectively. Naringenin and pinocembrin showed maximum UV-absorption peak in UVC and UVB zones, while apigenin showed UV-absorption capability from UVC to UVA range. These compounds acted as UV filters reducing UV-induced genotoxicity, both in bacteria and in human cells. The enzymatic comet assay resulted highly sensitive for detection of UVB-induced DNA damage in HEK-293 cells. In this work, the photoprotective potential of these flavonoids was widely discussed.

Formation of UV-induced DNA damage contributing to skin cancer development

UV-induced DNA damage plays a key role in the initiation phase of skin cancer. When left unrepaired or when damaged cells are not eliminated by apoptosis, DNA lesions express their mutagneic properties, leading to the activation of proto-oncogene or the inactivation of tumor suppression genes. The chemical nature and the amount of DNA damage strongly depend on the wavelength of the incident photons. The most energetic part of the solar spectrum at the Earth's surface (UVB, 280-320 nm) leads to the formation of cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (64PPs). Less energetic but 20-times more intense UVA (320-400 nm) also induces the formation of CPDs together with a wide variety of oxidatively generated lesions such as single strand breaks and oxidized bases. Among those, 8-oxo-7,8-dihydroguanine (8-oxoGua) is the most frequent since it can be produced by several mechanisms. Data available on the respective yield of DNA photoproducts in cells and skin show that exposure to sunlight mostly induces pyrimidine dimers, which explains the mutational signature found in skin tumors, with lower amounts of 8-oxoGua and strand breaks. The present review aims at describing the basic photochemistry of DNA and discussing the quantitative formation of the different UV-induced DNA lesions reported in the literature. Additional information on mutagenesis, repair and photoprotection is briefly provided.

Mechanisms of and variables affecting UVR photoadaptation in human skin

Humans have been exposed to solar UV radiation since their appearance on Earth and evolution has enabled most individuals to adapt to this exposure, to some degree. UV radiation produces several deleterious effects in human skin and light-skinned individuals are at greatest risk for both acute and long-term negative effects such as DNA damage, sunburn, immune suppression and skin cancer. The benefits of photoadaptation, which leads to a decreased response after acclimatization, are that humans who have skin that is capable of photoadaptation can work and play in the sun with reduced fear of painful sunburn. However, the effects of photoadaptation on DNA damage and development of skin cancer are quite complex and less well-understood. In this article, we have reviewed the current state of knowledge of UVR photoadaptation in human skin. However, more studies are needed to explore the use of UVR photoadaptation to protect against critical endpoints, such as skin cancer.

Sunscreen applied at ≥ 2 mg cm-2 during a sunny holiday prevents erythema, a biomarker of ultraviolet radiation-induced DNA damage and suppression of acquired immunity

BACKGROUND

Sun protection factor (SPF) is assessed with sunscreen applied at 2 mg cm^-2 . People typically apply around 0·8 mg cm^-2 and use sunscreen daily for holidays. Such use results in erythema, which is a risk factor for skin cancer.

OBJECTIVES

To determine (i) whether typical sunscreen use resulted in erythema, epidermal DNA damage and photoimmunosuppression during a sunny holiday, (ii) whether optimal sunscreen use inhibited erythema and (iii) whether erythema is a biomarker for photoimmunosuppression in a laboratory study.

METHODS

Holidaymakers (n = 22) spent a week in Tenerife (very high ultraviolet index) using their own sunscreens without instruction (typical sunscreen use). Others (n = 40) were given SPF 15 sunscreens with instructions on how to achieve the labelled SPF (sunscreen intervention). Personal ultraviolet radiation (UVR) exposure was monitored electronically as the standard erythemal dose (SED) and erythema was quantified. Epidermal cyclobutane pyrimidine dimers (CPDs) were determined by immunostaining, and immunosuppression was assessed by contact hypersensitivity (CHS) response.

RESULTS

There was no difference between personal UVR exposure in the typical sunscreen use and sunscreen intervention groups (P = 0·08). The former had daily erythema on five UVR-exposed body sites, increased CPDs (P < 0·001) and complete CHS suppression (20 of 22). In comparison, erythema was virtually absent (P < 0·001) when sunscreens were used at ≥ 2 mg cm^-2 . A laboratory study showed that 3 SED from three very different spectra suppressed CHS by around ~50%.

CONCLUSIONS

Optimal sunscreen use prevents erythema during a sunny holiday. Erythema predicts suppression of CHS (implying a shared action spectrum). Given that erythema and CPDs share action spectra, the data strongly suggest that optimal sunscreen use will also reduce CPD formation and UVR-induced immunosuppression.

The UV/Visible Radiation Boundary Region (385-405 nm) Damages Skin Cells and Induces "dark" Cyclobutane Pyrimidine Dimers in Human Skin in vivo

The adverse effects of terrestrial solar ultraviolet radiation (UVR) (~295–400 nm) on the skin are well documented, especially in the UVB region (~295–320 nm). The effects of very long-wave UVA (>380 nm) and visible radiation (≥400 nm) are much less known. Sunscreens have been beneficial in inhibiting a wide range of photodamage, however most formulations provide very little protection in the long wave UVA region (380–400 nm) and almost none from shortwave visible wavelengths (400–420 nm). We demonstrate photodamage in this region for a number of different endpoints including cell viability, DNA damage (delayed cyclobutane pyrimidine dimers), differential gene expression (for genes associated with inflammation, oxidative stress and photoageing) and induction of oxidizing species in vitro in HaCaT keratinocytes and in vivo in human volunteers. This work has implications for phototherapy and photoprotection.

The application of transcriptional benchmark dose modeling for deriving thresholds of effects associated with solar-simulated ultraviolet radiation exposure

Considerable data has been generated to elucidate the transcriptional response of cells to ultraviolet radiation (UVR) exposure providing a mechanistic understanding of UVR-induced cellular responses. However, using these data to support standards development has been challenging. In this study, we apply benchmark dose (BMD) modeling of transcriptional data to derive thresholds of gene responsiveness following exposure to solar-simulated UVR. Human epidermal keratinocytes were exposed to three doses (10, 20, 150 kJ/m² ) of solar simulated UVR and assessed for gene expression changes 6 and 24 hr postexposure. The dose-response curves for genes with p-fit values (≥ 0.1) were used to derive BMD values for genes and pathways. Gene BMDs were bi-modally distributed, with a peak at ∼16 kJ/m² and ∼108 kJ/m² UVR exposure. Genes/pathways within Mode 1 were involved in cell signaling and DNA damage response, while genes/pathways in the higher Mode 2 were associated with immune response and cancer development. The median value of each Mode coincides with the current human exposure limits for UVR and for the minimal erythemal dose, respectively. Such concordance implies that the use of transcriptional BMD data may represent a promising new approach for deriving thresholds of actinic effects. Environ. Mol. Mutagen. 59:502-515, 2018. © 2018 The Authors Environmental and Molecular Mutagenesis published by Wiley Periodicals, Inc. on behalf of Environmental Mutagen Society.

Fractional Sunburn Threshold UVR Doses Generate Equivalent Vitamin D and DNA Damage in Skin Types I-VI but with Epidermal DNA Damage Gradient Correlated to Skin Darkness

Public health guidance recommends limiting sun exposure to sub-sunburn levels, but it is unknown whether these can gain vitamin D (for musculoskeletal health) while avoiding epidermal DNA damage (initiates skin cancer). Well-characterized healthy humans of all skin types (I–VI, lightest to darkest skin) were exposed to a low-dose series of solar simulated UVR of 20%–80% their individual sunburn threshold dose (minimal erythema dose). Significant UVR dose responses were seen for serum 25-hydroxyvitamin D and whole epidermal cyclobutane pyrimidine dimers (CPDs), with as little as 0.2 minimal erythema dose concurrently producing 25-hydroxyvitamin D and CPD. Fractional MEDs generated equivalent levels of whole epidermal CPD and 25-hydroxyvitamin D across all skin types. Crucially, we showed an epidermal gradient of CPD formation strongly correlated with skin darkness (r = 0.74, P < 0.0001), which reflected melanin content and showed increasing protection across the skin types, ranging from darkest skin, where high CPD levels occurred superficially, with none in the germinative basal layer, to lightest skin, where CPD levels were induced evenly across the epidermal depth. People with darker skin can be encouraged to use sub-sunburn UVR-exposure to enhance their vitamin D. In people with lighter skin, basal cell damage occurs concurrent with vitamin D synthesis at exquisitely low UVR levels, providing an explanation for their high skin cancer incidence; greater caution is required.

Meeting Vitamin D Requirements in White Caucasians at UK Latitudes: Providing a Choice

The body gains vitamin D through both oral intake (diet/supplementation) and synthesis in skin upon exposure to ultraviolet radiation (UVR). Sun exposure is the major source for most people even though sun exposure is complex and limited by climate and culture. We aimed to quantify the sun exposure required to meet vitamin D targets year-round and determine whether this can be safely achieved in a simply defined manner in the UK as an alternative to increasing vitamin D oral intake. Data from observation (sun exposure, diet, and vitamin D status) and UVR intervention studies performed with white Caucasian adults were combined with modeled all-weather UVR climatology. Daily vitamin D effective UVR doses (all-weather) were calculated across the UK based on ten-year climatology for pre-defined lunchtime exposure regimes. Calculations then determined the time necessary to spend outdoors for the body to gain sufficient vitamin D levels for year-round needs without being sunburnt under differing exposure scenarios. Results show that, in specified conditions, white Caucasians across the UK need nine minutes of daily sunlight at lunchtime from March to September for 25(OH)D levels to remain ≥25 nmol/L throughout the winter. This assumes forearms and lower legs are exposed June-August, while in the remaining, cooler months only hands and face need be exposed. Exposing only the hands and face throughout the summer does not meet requirements.

Vitamin E inhibits the UVAI induction of "light" and "dark" cyclobutane pyrimidine dimers, and oxidatively generated DNA damage, in keratinocytes

Solar ultraviolet radiation (UVR)-induced DNA damage has acute, and long-term adverse effects in the skin. This damage arises directly by absorption of UVR, and indirectly via photosensitization reactions. The aim of the present study was to assess the effects of vitamin E on UVAI-induced DNA damage in keratinocytes in vitro. Incubation with vitamin E before UVAI exposure decreased the formation of oxidized purines (with a decrease in intracellular oxidizing species), and cyclobutane pyrimidine dimers (CPD). A possible sunscreening effect was excluded when similar results were obtained following vitamin E addition after UVAI exposure. Our data showed that DNA damage by UVA-induced photosensitization reactions can be inhibited by the introduction of vitamin E either pre- or post-irradiation, for both oxidized purines and CPD (including so-called "dark" CPDs). These data validate the evidence that some CPD are induced by UVAI initially via photosensitization, and some via chemoexcitation, and support the evidence that vitamin E can intervene in this pathway to prevent CPD formation in keratinocytes. We propose the inclusion of similar agents into topical sunscreens and aftersun preparations which, for the latter in particular, represents a means to mitigate on-going DNA damage formation, even after sun exposure has ended.

The use of suction blisters to measure sunscreen protection against UVR-induced DNA damage

The formation of DNA photoproducts caused by solar UVR exposure needs to be investigated in-vivo and in particular in order to assess sunscreens' level of protection against solar genotoxicity. The study's purposes were: i) to evaluate if the roof of suction blisters is an appropriate sampling method for measuring photoproducts, and ii) to measure in-vivo sunscreen protection against cyclobutane pyrimidine dimers. Skin areas on the interior forearms of eight healthy volunteers were exposed in-vivo to 2 MED of simulated solar radiation (SSR) and to 15 MED on a sunscreen protected area. After irradiation, six suction blisters were induced and the blister roofs were collected. Analysis of SSR-induced CPDs was performed by two independent methods: a chromatography coupled to mass spectroscopy (HPLC-MS/MS) approach and a 3D-imaging of CPD immunostaining by multiphoton microscopy on floating epidermal sheets. HPLC-MS/MS analyses showed that SSR-unexposed skin presented no CPD dimers, whereas 2 MED SSR-exposed skin showed a significant number of TT-CPD. The sunscreen covered skin exposed to 15 MED appeared highly protected from DNA damage, as the amount of CPD-dimers remained below the detection limit. The multiphoton-immunostaining analysis consistently showed that no CPD staining was observed on the non-SSR-exposed skin. A significant increase of CPD staining intensity and number of CPD-positive cells were observed on the 2 MED SSR-exposed skin. Sunscreen protected skin presented a very low staining intensity and the number of CPD-positive cells remained very close to non-SSR-exposed skin. This study showed that suction blister samples are very appropriate for measuring CPD dimers in-vivo, and that sunscreens provide high protection against UVR-induced DNA damage.

Impact of the Circadian Clock on UV-Induced DNA Damage Response and Photocarcinogenesis

The skin is in constant exposure to various external environmental stressors, including solar ultraviolet (UV) radiation. Various wavelengths of UV light are absorbed by the DNA and other molecules in the skin to cause DNA damage and induce oxidative stress. The exposure to excessive ultraviolet (UV) radiation and/or accumulation of damage over time can lead to photocarcinogenesis and photoaging. The nucleotide excision repair (NER) system is the sole mechanism for removing UV photoproduct damage from DNA, and genetic disruption of this repair pathway leads to the photosensitive disorder xeroderma pigmentosum (XP). Interestingly, recent work has shown that NER is controlled by the circadian clock, the body's natural time-keeping mechanism, through regulation of the rate-limiting repair factor xeroderma pigmentosum group A (XPA). Studies have shown reduced UV-induced skin cancer after UV exposure in the evening compared to the morning, which corresponds with times of high and low repair capacities, respectively. However, most studies of the circadian clock-NER connection have utilized murine models, and it is therefore important to translate these findings to humans to improve skin cancer prevention and chronotherapy.

The impact of skin colour on human photobiological responses

Terrestrial solar ultraviolet radiation (UVR) exerts both beneficial and adverse effects on human skin. Epidemiological studies show a lower incidence of skin cancer in people with pigmented skins compared to fair skins. This is attributed to photoprotection by epidermal melanin, as is the poorer vitamin D status of those with darker skins. We summarize a wide range of photobiological responses across different skin colours including DNA damage and immunosuppression. Some studies show the generally modest photoprotective properties of melanin, but others show little or no effect. DNA photodamage initiates non‐melanoma skin cancer and is reduced by a factor of about 3 in pigmented skin compared with white skin. This suggests that if such a modest reduction in DNA damage can result in the significantly lower skin cancer incidence in black skin, the use of sunscreen protection might be extremely beneficial for susceptible population. Many contradictory results may be explained by protocol differences, including differences in UVR spectra and exposure protocols. We recommend that skin type comparisons be done with solar‐simulated radiation and standard erythema doses or physical doses (J/m²) rather than those based solely on clinical endpoints such as minimal erythema dose (MED).

Photochemistry and Photobiology of the Spore Photoproduct: A 50-Year Journey

Fifty years ago, a new thymine dimer was discovered as the dominant DNA photolesion in UV-irradiated bacterial spores [Donnellan, J. E. & Setlow R. B. (1965) Science, 149, 308-310], which was later named the spore photoproduct (SP). Formation of SP is due to the unique environment in the spore core that features low hydration levels favoring an A-DNA conformation, high levels of calcium dipicolinate that acts as a photosensitizer, and DNA saturation with small, acid-soluble proteins that alters DNA structure and reduces side reactions. In vitro studies reveal that any of these factors alone can promote SP formation; however, SP formation is usually accompanied by the production of other DNA photolesions. Therefore, the nearly exclusive SP formation in spores is due to the combined effects of these three factors. Spore photoproduct photoreaction is proved to occur via a unique H-atom transfer mechanism between the two involved thymine residues. Successful incorporation of SP into an oligonucleotide has been achieved via organic synthesis, which enables structural studies that reveal minor conformational changes in the SP-containing DNA. Here, we review the progress on SP photochemistry and photobiology in the past 50 years, which indicates a very rich SP photobiology that may exist beyond endospores.

A new hair follicle-derived human epidermal model for the evaluation of sunscreen genoprotection

Induction of skin cancer is the most deleterious effect of excessive exposure to sunlight. Accurate evaluation of sunscreens to protect the genome is thus of major importance. In particular, the ability of suncare products to prevent the formation of DNA damage should be evaluated more directly since the Sun Protection Factor is only related to erythema induction. For this purpose, we developed an in vitro approach using a recently characterized reconstituted human epidermis (RHE) model engineered from hair follicle. The relevance of this skin substitute in terms of UV-induced genotoxicity was compared to ex vivo explants exposed to solar-simulated radiation (SSR). The yield of bipyrimidine photoproducts, their rate of repair, and the induction of apoptosis were very similar in both types of skin samples. In order to evaluate the protection afforded by sunscreen against DNA damage, bipyrimidine photoproducts were quantified in tissue models following SSR exposure in the presence or absence of a SPF50+ formula. A rather high DNA protection factor of approximately 20 was found in RHE, very similar to that determined for explants. Thus, RHE is a good surrogate to human skin, and also a convenient and useful tool for investigation of the genoprotection of sunscreens.

Photocarcinogenesis: an epidemiologic perspective on ultraviolet light and skin cancer

Photocarcinogenesis is the result of a complex interplay between ultraviolet radiation, DNA damage, mutation formation, DNA repair, apoptosis, and the immune system. Recent trends show an increase in incidence of both melanoma and nonmelanoma skin cancers. Some individuals have a genetic predisposition toward increased risk for skin cancer, whereas others experience increased risk through ultraviolet exposure and subsequent mutation formation. The initiation and propagation pathways of melanoma and nonmelanoma skin cancers differ but have some elements in common. The increase in incidence of skin cancer has been discovered to vary among age groups and gender.

Rescue of cells from apoptosis increases DNA repair in UVB exposed cells: implications for the DNA damage response

Classically, the nucleotide excision repair (NER) of cyclobutane pyrimidine dimers (CPD) is a lengthy process (t_1/2 > 48 h).

An unexpected deamination reaction after hydrolysis of the pyrimidine (6-4) pyrimidone photoproduct

Pyrimidine (6-4) pyrimidone photoproduct (6-4PP), a common DNA photolesion formed under solar irradiation, was indicated to hydrolyze under strong basic conditions, breaking the N3-C4 bond at the 5'-thymine. The reanalysis of this reaction revealed that the resulting water adduct may not be stable as previously proposed; it readily undergoes an esterification reaction induced by the 5-OH group at 6-4PP to form a five-membered ring, eliminating a molecule of ammonia.

The significance of the Dewar valence photoisomer as a UV radiation-induced DNA photoproduct in marine microbial communities

Induction of pyrimidine dimers in DNA by solar UV radiation has drastic effects on microorganisms. To better define the nature of these DNA photoproducts in marine bacterioplankton and eukaryotes, a study was performed during a cruise along a latitudinal transect in the Pacific Ocean. The frequency of all possible cyclobutane pyrimidine dimers, pyrimidine (6-4) pyrimidone photoproducts (64PPs) and their related Dewar valence isomers (DEWs) was determined by high-performance liquid chromatography-mass spectrometry. Studied samples were bacterioplankton and eukaryotic fractions isolated from sea water either collected before sunrise or exposed to ambient sunlight from sunrise to sunset. Isolated DNA dosimeters were also exposed to daily sunlight for comparison purposes. A first major result was the observation in all samples of large amounts of DEWs, a class of photoproducts rarely considered outside photochemical studies. Evidence was obtained for a major role of UVA in the formation of these photoisomerization products of 64PPs. Considerations on the ratio between the different classes of photoproducts in basal and induced DNA damage suggests that photoenzymatic repair (PER) is an important DNA repair mechanism used by marine microorganisms occupying surface seawater in the open ocean. This result emphasizes the biological role of DEWs which are very poor substrate for PER.

Novel vitamin D compounds and skin cancer prevention

As skin cancer is one of the most costly health issues in many countries, particularly in Australia, the possibility that vitamin D compounds might contribute to prevention of this disease is becoming increasingly more attractive to researchers and health communities. In this article, important epidemiologic, mechanistic and experimental data supporting the chemopreventive potential of several vitamin D-related compounds are explored. Evidence of photoprotection by the active hormone, 1α,25dihydroxyvitamin D3, as well as a derivative of an over-irradiation product, lumisterol, a fluorinated analog and bufalin, a potential vitamin D-like compound, are provided. The aim of this article is to understand how vitamin D compounds contribute to UV adaptation and potentially, skin cancer prevention.

Human erythema and matrix metalloproteinase-1 mRNA induction, in vivo, share an action spectrum which suggests common chromophores

Matrix metalloproteinase 1 (MMP-1) is widely regarded as a biomarker of photoageing. We tested the hypothesis that MMP-1 mRNA expression and erythema share a common action spectrum by comparing the effects of erythemally equivalent doses of UVB, UVA1 and solar simulated radiation (SSR) on acute MMP-1 mRNA expression in whole human skin in vivo. Our results show comparable MMP-1 expression with all three spectra, which supports our hypothesis. The sharing of an action spectrum implies common chromophores, one of which is likely to be DNA. We have previously shown that all spectra that we used readily induce cyclobutane thymine dimers (T<>T) in human epidermis in vivo but we lack quantitative data on damage to dermal DNA. This is important because we do not know if dermal MMP-1 induction occurs via direct damage to the dermis, or indirectly via damage to the epidermis. Our results show that UVB induces about 3 times more T<>T compared with erythemally equivalent doses of UVA1, which is similar to our published epidermal data. This supports previously published work that also implicates an unknown UVA1 chromophore for erythema and MMP-1 induction. However, the distribution of the dermal DNA damage varies considerably with spectrum. In the case of UVB it is primarily in the upper dermis, but with UVA1 it is evenly distributed. Thus, irrespective of chromophores, MMP-1 induction by direct dermal damage by both spectra is possible. The practical conclusions of our data are that the small (<5%) UVB content of solar UVR is likely to be the main cause of photoageing, at least in terms of MMP-1 expression. Furthermore, prevention of erythema by sunscreen use is likely to result in reduced MMP-1 expression.

UVA1 induces cyclobutane pyrimidine dimers but not 6-4 photoproducts in human skin in vivo

UVB readily induces cyclobutane pyrimidine dimers, mainly thymine dimers (TTs), and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) in DNA. These lesions result in "UVB signature mutations" found in skin cancers. We have investigated the induction of TTs and 6-4PPs in human skin in vivo by broadband UVA1, and have compared this with comparable erythemal doses of monochromatic UVB (300 nm). In vitro and ex vivo studies have shown the production of TTs, without 6-4PPs, by UVA1. We show that UVA1 induces TTs, without 6-4PPs, in the epidermis of healthy volunteers in vivo, whereas UVB induced both photoproducts. UVB induced more TTs than UVA1 for the same level of erythema. The level of UVA1-induced TTs increased with epidermal depth in contrast to a decrease that was seen with UVB. UVA1- and UVB-induced TTs were repaired in epidermal cells at a similar rate. The mechanism by which UVA1 induces TTs is unknown, but a lack of intra-individual correlation between our subjects' UVB and UVA1 minimal erythema doses implies that UVA1 and UVB erythema occur by different mechanisms. Our data suggest that UVA1 may be more carcinogenic than has previously been thought.

Assessment of cyclobutane pyrimidine dimers by digital photography in human skin

UV-mediated DNA damage and repair are important mechanisms in research on UV-induced carcinogenesis. UV-induced DNA-damage and repair can be determined by immunohistochemical staining of photoproduct positive nuclei of keratinocytes in the epidermis. We developed a new method of analysing and quantifying thymine dimer (TT-CPD) positive cells in the epidermis. Normal skin of healthy controls was exposed to UVB ex vivo and in vivo. Skin samples were immunohistochemically stained for TT-CPDs. Digital images of the epidermis were quantified for TT-CPDs both visually and digitally. There was a UVB-dose dependent induction of TT-CPDs present in the ex vivo UVB-irradiated skin samples. The linear measurement range of the digital quantification was increased compared to the manual counting. The average 24-hour repair rate of the initiated TT-CPDs elicited by the UVB irradiation at T=0 of the 8 HCs showed a 34% decrease of TT-CPD photoproducts by the manual counting method and a 51% decrease determined by digital counting. The digital quantification method improves immunohistochemical quantification of DNA photo damage. It is more sensitive in measuring the extent of DNA-damage per nucleus.

Simple ultraviolet and high-performance liquid chromatography methods for the evaluation of sunscreen efficacy

BACKGROUND

To prevent DNA damage caused by the ultraviolet (UV) radiation of sunlight, sunscreens are commonly used to protect human skin. Current analysis of sunscreens' effectiveness is done through complicated procedures, including human exposure.

OBJECTIVE

We sought to design a simple system using thymidine-thymidine (TT) dinucleotides to analyze the effectiveness of sunscreens.

METHODS

We can directly analyze sunscreen effectiveness and the formation of TT photolesions simply by using UV spectrophotometry and high-performance liquid chromatography (HPLC). Efficient sunscreen has protective effects against UV irradiation damage.

RESULTS

We have developed a simple method using TT dinucleotide, UV, and HPLC for the analysis of sunscreen effectiveness. Our research indicates that the analytical results from UV are consistent with those of HPLC, which is used to monitor the formation of the TT photolesions. Moreover, both UV and HPLC analyses indicate that TT dinucleotides are better protected against UV damage, using the sunscreens with higher UVB sun protection factor (SPF) value, and that sunscreens with higher SPF lead to reduced photolesion formation. Our UV and HPLC analyses confirm the SPF grading of commercial sunscreens.

LIMITATIONS

In this experiment, only sunscreens were tested. The experiment, therefore, does not apply to other commercial products, such as cosmetic materials that claim UV protection as a secondary benefit.

CONCLUSION

In conclusion, we have established a simple strategy to analyze the effectiveness of sunscreens and the quality of these potential cancer-preventive products.

Stepping up melanocytes to the challenge of UV exposure

Exposure to solar ultraviolet radiation (UV) is the main etiological factor for skin cancer, including melanoma. Cutaneous pigmentation, particularly eumelanin, afforded by melanocytes is the main photoprotective mechanism, as it prevents UV-induced DNA damage in the epidermis. Therefore, maintaining genomic stability of melanocytes is crucial for prevention of melanoma, as well as keratinocyte-derived basal and squamous cell carcinoma. A critical independent factor for preventing melanoma is DNA repair capacity. The response of melanocytes to UV is mediated mainly by a network of paracrine factors that not only activate melanogenesis, but also DNA repair, anti-oxidant, and survival pathways that are pivotal for maintenance of genomic stability and prevention of malignant transformation or apoptosis. However, little is known about the stress response of melanocytes to UV and the regulation of DNA repair pathways in melanocytes. Unraveling these mechanisms might lead to strategies to prevent melanoma, as well as non-melanoma skin cancer.

The effects of topically applied glycolic acid and salicylic acid on ultraviolet radiation-induced erythema, DNA damage and sunburn cell formation in human skin

BACKGROUND

alpha-Hydroxy acids (alphaHAs) are reported to reduce signs of aging in the skin and are widely used cosmetic ingredients. Several studies suggest that alphaHA can increase the sensitivity of skin to ultraviolet radiation. More recently, beta-hydroxy acids (betaHAs), or combinations of alphaHA and betaHA have also been incorporated into antiaging skin care products. Concerns have also arisen about increased sensitivity to ultraviolet radiation following use of skin care products containing beta-HA.

OBJECTIVE

To determine whether topical treatment with glycolic acid, a representative alphaHA, or with salicylic acid, a betaHA, modifies the short-term effects of solar simulated radiation (SSR) in human skin.

METHODS

Fourteen subjects participated in this study. Three of the four test sites on the mid-back of each subject were treated daily Monday-Friday, for a total of 3.5 weeks, with glycolic acid (10%), salicylic acid (2%), or vehicle (control). The fourth site received no treatment. After the last treatment, each site was exposed to SSR, and shave biopsies from all four sites were obtained. The endpoints evaluated in this study were erythema (assessed visually and instrumentally), DNA damage and sunburn cell formation.

RESULTS

Treatment with glycolic acid resulted in increased sensitivity of human skin to SSR, measured as an increase in erythema, DNA damage and sunburn cell formation. Salicylic acid did not produce significant changes in any of these biomarkers.

CONCLUSIONS

Short-term topical application of glycolic acid in a cosmetic formulation increased the sensitivity of human skin to SSR, while a comparable treatment with salicylic acid did not.

From UVs to metastases: modeling melanoma initiation and progression in the mouse

Cutaneous malignant melanoma is highly invasive and capable of metastasizing to distant sites where it is typically resistant to available therapy. While striving to prevent or eradicate melanoma, researchers have two significant advantages not shared by those working on many other cancers. The main environmental etiological agent, UV radiation, is known and melanocytic lesions are excisable for molecular analysis from most stages. Yet knowledge about how UV initiates melanoma has been insufficient to achieve prevention, and the understanding of metastatic mechanisms has been inadequate to reduce mortality. Here, we review the value of melanoma mouse models, focusing on these critical early and late stages.

The protective role of melanin against UV damage in human skin

Human skin is repeatedly exposed to UVR that influences the function and survival of many cell types and is regarded as the main causative factor in the induction of skin cancer. It has been traditionally believed that skin pigmentation is the most important photoprotective factor, as melanin, besides functioning as a broadband UV absorbent, has antioxidant and radical scavenging properties. Besides, many epidemiological studies have shown a lower incidence for skin cancer in individuals with darker skin compared to those with fair skin. Skin pigmentation is of great cultural and cosmetic importance, yet the role of melanin in photoprotection is still controversial. This article outlines the major acute and chronic effects of UVR on human skin, the properties of melanin, the regulation of pigmentation and its effect on skin cancer prevention.

Molecular response of nasal mucosa to therapeutic exposure to broad-band ultraviolet radiation

Ultraviolet radiation (UVR) phototherapy is a promising new treatment for inflammatory airway diseases. However, the potential carcinogenic risks associated with this treatment are not well understood. UV-specific DNA photoproducts were used as biomarkers to address this issue. Radioimmunoassay was used to quantify cyclobutane pyrimidine dimers (CPDs) and (6–4) photoproducts in DNA purified from two milieus: nasal mucosa samples from subjects exposed to intranasal phototherapy and human airway (EpiAirway™) and human skin (EpiDerm™) tissue models. Immunohistochemistry was used to detect CPD formation and persistence in human nasal biopsies and human tissue models. In subjects exposed to broadband ultraviolet radiation, DNA damage frequencies were determined prior to as well as immediately after treatment and at increasing times post-treatment. We observed significant levels of DNA damage immediately after treatment and efficient removal of the damage within a few days. No residual damage was observed in human subjects exposed to multiple UVB treatments several weeks after the last treatment. To better understand the molecular response of the nasal epithelium to DNA damage, parallel experiments were conducted in EpiAirway and EpiDerm model systems. Repair rates in these two tissues were very similar and comparable to that observed in human skin. The data suggest that the UV-induced DNA damage response of respiratory epithelia is very similar to that of the human epidermis and that nasal mucosa is able to efficiently repair UVB induced DNA damage.

Melanin mediated apoptosis of epidermal cells damaged by ultraviolet radiation: factors influencing the incidence of skin cancer

Ultraviolet (UV)-induced skin cancers, including melanomas and basal/squamous cell carcinomas, occur more frequently in individuals with fair skin than in those with dark skin. Melanin plays an important role in protecting the skin against UV radiation and levels of melanin correlate inversely with amounts of DNA damage induced by UV in human skin of different racial/ethnic groups. The objectives of this study are to review recent progress in our understanding of mechanisms underlying differences in cancer incidence in skins of different colors, particularly between Black and White skin. More specifically, we review DNA damage and apoptosis in various types of skin before and after exposure to UV in our human study protocols using a single UV dose, either one minimal erythema dose (MED) or a similar low dose of 180-200 J/m2. Our data and other published reports indicate that several major mechanisms underlie the increased rates of photocarcinogenesis in fair/light skin. First, UV-induced DNA damage in the lower epidermis (including keratinocyte stem cells and melanocytes) is more effectively prevented in darker skin. Second, rates of repair of DNA damage can differ significantly in individuals. Third, UV-induced apoptosis to remove potentially precancerous cells is significantly greater in darker skin. These results suggest that pigmented epidermis is an efficient UV filter and that UV damaged cells are removed more efficiently in darker skin. The combination of decreased DNA damage and more efficient removal of UV-damaged cells may play a critical role in the decreased photocarcinogenesis seen in individuals with darker skin.

Risk estimation of skin damage due to ultrashort pulsed, focused near-infrared laser irradiation at 800 nm

New imaging techniques using near-infrared (NIR) femtosecond lasers (fs-lasers) in multiphoton laser scanning microscopy (MPLSM) have great potential for in vivo applications, particularly in human skin. However, little is known about possible risks. In order to evaluate the risk, a "biological dosimeter" was used. We irradiated fresh human skin samples with both an fs-laser and a solar simulator UV source (SSU). DNA damage introduced in the epidermis was evaluated using fluorescent antibodies against cyclobutane-pyrimidin-dimers (CPDs) in combination with immunofluorescence image analysis. Four fs-irradiation regimes (at 800-nm wavelength) were evaluated differing in laser power and step width of horizontal scans. Fs-irradiation did not give CPDs at 15-mW or 30-mW irradiation power using 10 horizontal scans every 5 microns. CPDs could be seen at 60-mW laser power and 5-microm step size and at 35-mW using 1-micron step width. Quantitative comparison of SSU-induced CPDs showed that the 60-mW laser irradiation regime is comparable to UV-irradiation, giving 0.6 minimal erythemal dose (MED). The 1-micron irradiation regime was comparable to 0.45 MED. Under these experimental conditions, the risk of DNA damage due to fs-laser irradiation on skin is in the range of natural UV-exposure.

Skin DNA photodamage and its biological consequences

It is well established that ultraviolet (UV) radiation from sunlight damages skin cells' DNA. Wavelengths in the UVB range are absorbed by DNA and can induce mutagenic lesions such as pyrimidine dimers. On the other hand, genotoxic effects of solar UVA are mainly mediated by the activation of endogenous photosensitizers resulting in the generation of a local oxidative stress. Exogenous chemicals, such as drugs like psoralens or fluoroquinolones, sometimes amplify UV-induced harmful effects. DNA damage can lead to mutations and genetic instability. This is one of the reasons why sunlight overexposure increases the risk of skin cancer. But DNA photolesions can also be involved in other skin-specific responses to UV radiation: erythema, immunosuppression, and melanogenesis are examples reported in the literature. The aim of this short review is to summarize the general knowledge in the field of UV-induced DNA damage. Besides the biological consequences of DNA photolesions, this article also deals with technologies used for their detection and shows how helpful such approaches can be to assess photoprotection provided by sunscreens.

Differential repair of UVB-induced cyclobutane pyrimidine dimers in cultured human skin cells and whole human skin

Cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are the two main classes of mutagenic DNA damages induced by UVB radiation. Numerous studies have been devoted so far to their formation and repair in human cells and skin. However, the biochemical methods used often lack the specificity that would allow the individual study of each of the four CPDs and 6-4PPs produced at TT, TC, CT and CC dinucleotides. In the present work, we applied an HPLC-mass spectrometry assay to study the formation and repair of CPDs and 6-4PPs photoproducts in primary cultures of human keratinocytes and fibroblasts as well as in whole human skin. We first observed that the yield of dimeric lesions was slightly higher in fibroblasts than in keratinocytes. In contrast, the rate of global repair was higher in the last cell type. Moreover, removal of DNA photoproducts in skin biopsies was found to be slower than in both cultured skin cells. In agreement with previous works, the repair of 6-4PPs was found to be more efficient than that of CPDs in the three types of samples, with no observed difference between the removal of the TT and TC derivatives. In contrast, a significant influence of the nature of the two modified pyrimidines was observed on the repair rate of CPDs. The decreasing order of removal efficiency was the following: C<>T>C<>C>T<>C>T<>T. These data, together with the known intrinsic mutational properties of the lesions, would support the reported UV mutation spectra. A noticeable exception concerns CC dinucleotides that are mutational hotspots with an UV-specific CC to TT tandem mutation, although related bipyrimidine photoproducts are produced in low yields and efficiently repaired.