Role of DNA Repair Deficiency in Cancer Development

The DNA is constantly under attack from endogenous and exogenous damaging agents. The damaged DNA must be repaired quickly to avoid genomic instability and to prevent the occurrence of a malignant transformation. Once a lesion is detected, the DNA repair mechanism initiates and replaces the structurally altered base or any other abnormality. The cell repair mechanisms include direct reversal, excision repair (base excision repair [BER] and nucleotide excision repair [NER]), mismatch repair (MMR), homologous recombination repair (HR) and non-homologous end joining (NHEJ). Unrepaired DNA could lead to mutation, cell death or cancer. This review will discuss how the defects in DNA repair play a vital role in cancer initiation, development and progression.

Construction of a Secondary Enclosure for UVB Irradiation of Mice

UV irradiation is commonly used in murine models of skin cancers. Despite the popularity of using UVB rays to model photocarcinogenesis in animals, there is a lack of standardization in the secondary enclosures used to administer radiation. An appraisal of the literature also shows a general lack of details regarding the materials and procedures utilized in the fabrication of such enclosures. We present in this study a detailed overview of the construction of a UVB exposure chamber that successfully induces lesions in hairless mice. A standardized protocol for producing a UVB enclosure may reduce methodological variation in future studies seeking to investigate photocarcinogenesis in animals.

Targeting MC1R depalmitoylation to prevent melanomagenesis in redheads

Some genetic melanocortin-1 receptor (MC1R) variants responsible for human red hair color (RHC-variants) are consequently associated with increased melanoma risk. Although MC1R signaling is critically dependent on its palmitoylation primarily mediated by the ZDHHC13 protein-acyl transferase, whether increasing MC1R palmitoylation represents a viable therapeutic target to limit melanomagenesis in redheads is unknown. Here we identify a specific and efficient in vivo strategy to induce MC1R palmitoylation for therapeutic benefit. We validate the importance of ZDHHC13 to MC1R signaling in vivo by targeted expression of ZDHHC13 in C57BL/6J-MC1R^RHC mice and subsequently inhibit melanomagenesis. By identifying APT2 as the MC1R depalmitoylation enzyme, we are able to demonstrate that administration of the selective APT2 inhibitor ML349 treatment efficiently increases MC1R signaling and represses UVB-induced melanomagenesis in vitro and in vivo. Targeting APT2, therefore, represents a preventive/therapeutic strategy to reduce melanoma risk, especially in individuals with red hair.

Melanocortin-1 receptor is a palmitoylated protein and variants of the receptor are associated with red hair colour and susceptibility to melanoma. Here, the authors describe a method to enhance the palmitoylation of the receptor, which can inhibit melanomagenesis in mice.

Brahma deficiency in keratinocytes promotes UV carcinogenesis by accelerating the escape from cell cycle arrest and the formation of DNA photolesions

BACKGROUND

Ultraviolet radiation (UVR) is the principal cause of keratinocyte skin cancers. Previous work found that levels of the chromatin remodelling protein, Brahma (Brm), are diminished during the progression from actinic keratoses to cutaneous squamous cell carcinomas in humans, and its loss in UV-irradiated mouse skin causes epidermal hyperplasia and increased tumour incidence.

METHODS

The skins of mice and mouse and human keratinocytes deficient in Brm were exposed to UVR and evaluated for cell cycle progression and DNA damage response.

OBJECTIVE

To identify the mechanisms by which loss of Brm contributes to UVR-induced skin carcinogenesis.

RESULTS

In both mouse keratinocytes and HaCaT cells, Brm deficiency led to an increased cell population growth following UVR exposure compared to cells with normal levels of Brm. Cell cycle analysis using a novel assay showed that Brm-deficient keratinocytes entered cell cycle arrest normally, but escaped from cell cycle arrest faster, enabling them to begin proliferating earlier. In mouse keratinocytes, Brm primarily affected accumulation in G₀/G₁-phase, whereas in HaCaT cells, which lack normal p53, accumulation in G₂/M-phase was affected. Brm-deficient keratinocytes in mouse skin and human cell cultures also had higher levels of UVR-induced cyclobutane pyrimidine dimer photolesions. These effects occurred without any compensatory increase in DNA repair or cell death to remove photolesions or the cells that harbor them from the keratinocyte population.

CONCLUSION

The loss of Brm in keratinocytes exposed to UVR enables them to resume proliferation while harboring DNA photolesions, leading to an increased fixation of mutations and, consequently, increased carcinogenesis.

Application of dynamic thermal imaging in a photocarcinogenesis mouse model

INTRODUCTION

In clinical practice and experimental settings, cutaneous premalignant and malignant lesions are commonly diagnosed by histopathological biopsy. However, this technique is invasive and results in functional or cosmetic defects. Dynamic thermal imaging is a non-invasive technique that quantifies the infra-red (IR) radiation emitted by a subject after the introduction of external thermal stimuli (such as heat or cold).

METHODS

Forty hairless albino (Crl:SKH1-hr) mice were randomised to the control group or the experimental group. The experimental group was regularly irradiated with artificial ultraviolet. Clinical photographs, immunohistochemical staining and dynamic thermal imaging results of both groups were obtained.

RESULTS

As photocarcinogenesis proceeded, faster thermal recovery to basal temperature after heat stimuli was significant on dynamic thermal imaging. With histopathological correlations, it was possible to differentiate normal, premalignant and malignant cutaneous lesions according to thermal imaging results. CD 31 staining analysis showed that increased vasculature was the key change responsible for different thermal imaging results among photocarcinogenesis steps.

CONCLUSIONS

Dynamic thermal imaging is useful to differentiate normal, premalignant and malignant cutaneous lesions. Increased vasculature is the key change responsible for different thermal imaging results.

Pathogenesis of Skin Carcinomas and a Stem Cell as Focal Origin

UV radiation in sunlight has long been recognized as the main exogenous cause of skin carcinomas. We present a brief historical perspective on the progress in understanding the pathogenesis of skin carcinomas, and recent advances. Sun-exposed skin carries numerous UV-related mutations, and skin carcinomas rank among the tumors with the highest mutational loads. In this multitude of mutations only a few are crucial in driving the tumor. Some are known from hereditary (skin) cancer syndromes and other recurrent ones have been validated in transgenic mice. Considering the continuous renewal of the epidermis, the question arises whether the lifelong residing stem cells are the main targets in skin carcinogenesis, a multistep process that would require ample time to evolve. Therefore, classic quiescent stem cells have been studied as potential tumor-initiating cells, as well as more recently discovered actively dividing stem cells (either Lgr5+ or Lgr6+). Interesting differences have emerged between experimental UV and two-stage chemical carcinogenesis, e.g., the latter appears to originate from follicular stem cells, in contrast to the former.

Lgr6+ stem cells and their progeny in mouse epidermis under regimens of exogenous skin carcinogenesis, and their absence in ensuing skin tumors

Lgr6+ cells have been identified as a novel class of proliferating (Ki67+) stem cells in mouse epidermis. We investigated their response to UV exposure in Lgr6-EGFP-Ires-CreERT2/R26R-LacZ haired and hairless mice and whether they become initiating cells of UV- or chemically induced skin tumors. UV overexposure erased Lgr6+ cells (EGFP+) from the interfollicular epidermis (IFE), but - as after wounding - they apparently repopulated the IFE from the hair follicles. Under sub-sunburn chronic UV exposure, Lgr6+ cells and their progeny (LacZ+ after pulse of tamoxifen) diminished strongly in the IFE. Although the inter-tumoral IFE clearly showed Lgr6 progeny, none of the UV- or chemically induced tumors (n = 22 and 41, respectively) appeared to be clonal expansions of Lgr6+ stem cells; i.e. no Lgr6+ cells or progeny in the proliferating tumor bulk. In checking for promoter methylation we found it to occur stochastically for the EGFP-Cre cassette. Lgr6 mRNA measured by qPCR was found to be diminished in skin tumors (also in UV tumors from wt type mice). The ratio of Lgr6/Ki67 was significantly reduced, pointing at a loss of Lgr6+ cells from the proliferative pool. Our data show that Lgr6+ cells are not major tumor-initiating cells in skin carcinogenesis.

Lgr5+ stem cells and their progeny in mouse epidermis under regimens of exogenous skin carcinogenesis, and their absence in ensuing skin tumors

Actively proliferating Lgr5+ skin stem cells are found deep in the hair follicle (HF). These cells renew the HF and drive its expansion in anagen phase. Their long residence and continuous mitotic activity make them prime candidates to transform into skin tumor-initiating cells. This was investigated by subjecting Lgr5-EGFP-Ires-CreERT2/R26R-LacZ mice (haired and hairless) to chemical and UV carcinogenic regimens. In the course of these regimens Lgr5+ cells (EGFP+) remained exclusively located in HFs, and in deep-seated cysts of hairless skin. In haired mice, progeny of Lgr5+ stem cells (LacZ+ after a pulse of tamoxifen) appeared in the interfollicular epidermis upon UV-induced sunburn and in TPA-induced hyperplasia. In hairless mice the progeny remained located in deep-seated cysts and in HF remnants. Progeny in hairless skin was only detected interfollicularly at a late stage, in between outgrowing tumors. Lgr5+ stem cells were absent in the ultimate tumor masses, and no tumor appeared to be a (clonal) expansion of Lgr5+ cells (52 tumors with tamoxifen at the start of carcinogenesis, 42 tumors with tamoxifen late during tumor outgrowth). In contrast to CD34/K15+ quiescent bulge stem cells, actively proliferating Lgr5+ stem cells do therefore not appear to be tumor drivers in experimental skin carcinogenesis.

Fractionation of a tumor-initiating UV dose introduces DNA damage-retaining cells in hairless mouse skin and renders subsequent TPA-promoted tumors non-regressing

Sunburns and especially sub-sunburn chronic UV exposure are associated with increased risk of squamous cell carcinomas (SCCs). Here we focus on a possible difference in tumor initiation from a single severe-sunburn dose (on day 1, 21 hairless mice) and from an equal dose fractionated into very low sub-sunburn doses not causing any (growth-promoting) epidermal hyperplasia (40 days daily exposure, n=20). From day 47 all mice received 12-O-Tetradecanoylphorbol-13-acetate (TPA) applications (2x/wk) for 20 weeks to promote tumor development within the lifetime of the animals. After the sub-sunburn regimen sparse DNA damage-retaining basal cells (quiescent stem cells, QSCs) remained in the non-hyperplastic epidermis. These cells were forced to divide by TPA. After discontinuation of TPA tumors regressed and disappeared in the ‘sunburn group’ but persisted and grew in the ‘sub-sunburn group’ (0.06 vs 2.50 SCCs and precursors ≥4mm/mouse after 280 days, p=0.03). As the tumors carried no mutations in p53, H/K/N-Ras and Notch1/2, these ‘usual suspects' were not involved in the UV-driven tumor initiation. Although we could not selectively eliminate QSCs (unknown phenotype) to establish causality, our data suggest that forcing specifically DNA damage-retaining QSCs to divide – with high mutagenic risk - gives rise to persisting (mainly ‘in situ’) skin carcinomas.

Sex differences and pathology status correlated to the toxicity of some common carcinogens in experimental skin carcinoma

The increased susceptibility of men as compared to women to develop different types of cancer, including skin cancer, is well known; however, the mechanisms involved in this process are still a matter of debate. This study aimed to obtain animal models of photo-chemically-induced skin carcinogenesis by exposure to ultraviolet radiation B (UVB) coupled with topical applications of a tumor initiator (7,12-dimethylbenz(a)anthracene, DMBA) and a tumor promoter (12-O-tetradecanoylphorbol-13-acetate, TPA) in order to characterize the gender disparities regarding the skin lesions developed by the female and male SKH-1 hairless mice included in this study. Histopathological analysis confirmed the presence of malignant lesions in both cases, in female and male mice, following chronic exposure (24 weeks) to the noxious effects of the carcinogens applied, whereas the tumors in male mice had a more severe histological grade. In addition, tumor incidence, size and multiplicity were higher in male mice than in female mice.

"Eruptive Lentiginosis" in a Patient With Vitiligo and Inflammatory Bowel Disease Treated With Azathioprine

Multiple lentiginosis are seen in many multisystemic diseases and during the course of many treatment schemes in the area of application of topical substances, PUVA, or more disseminated reactions in the case of systemic drugs. We report a case of a 67-year-old man with multiple comorbidities including vitiligo, and a recent diagnosis of inflammatory bowel disease, who developed millimeter-size, circular, brown macules in photoexposed areas both affected and not affected by vitiligo while was taken azathioprine, which disappeared after drug withdrawal. Biopsy showed groups of apoptotic keratinocytes, basal hyperpigmentation, and slight dermal inflammation. The authors describe "eruptive lentiginosis" as an adverse event of azathioprine not previously described in the literature.

Role of vitamin D3 in modulation of ΔNp63α expression during UVB induced tumor formation in SKH-1 mice

ΔNp63α, a proto-oncogene, is up-regulated in non-melanoma skin cancers and directly regulates the expression of both Vitamin D receptor (VDR) and phosphatase and tensin homologue deleted on chromosome ten (PTEN). Since ΔNp63α has been shown to inhibit cell invasion via regulation of VDR, we wanted to determine whether dietary Vitamin D₃ protected against UVB induced tumor formation in SKH-1 mice, a model for squamous cell carcinoma development. We examined whether there was a correlation between dietary Vitamin D₃ and ΔNp63α, VDR or PTEN expression in vivo in SKH-1 mice chronically exposed to UVB radiation and fed chow containing increasing concentrations of dietary Vitamin D₃. Although we observed differential effects of the Vitamin D₃ diet on ΔNp63α and VDR expression in chronically irradiated normal mouse skin as well as UVB induced tumors, Vitamin D₃ had little effect on PTEN expression in vivo. While low-grade papillomas in mice exposed to UV and fed normal chow displayed increased levels of ΔNp63α, expression of both ΔNp63α and VDR was reduced in invasive tumors. Interestingly, in mice fed high Vitamin D₃ chow, elevated levels of ΔNp63α were observed in both local and invasive tumors but not in normal skin suggesting that oral supplementation with Vitamin D₃ may increase the proliferative potential of skin tumors by increasing ΔNp63α levels.

Grp1-associated scaffold protein regulates skin homeostasis after ultraviolet irradiation

Grp1-associated scaffold protein (Grasp), the product of a retinoic acid-induced gene in P19 embryonal carcinoma cells, is expressed primarily in brain, heart, and lung of the mouse. We report herein that Grasp transcripts are also found in mouse skin in which the Grasp gene is robustly induced following acute ultraviolet-B (UVB) exposure. Grasp(-/-) mice were found to exhibit delayed epidermal proliferation and a blunted apoptotic response after acute UVB exposure. Immunohistochemical analyses revealed that the nuclear residence time of the tumor suppressor protein p53 was reduced in Grasp(-/-) mice after UVB exposure. Taken together, our results suggest that a physiological role of Grasp may be to regulate skin homeostasis after UVB exposure, potentially by influencing p53-mediated apoptotic responses in skin.

Photoprotective effects of Romanian propolis on skin of mice exposed to UVB irradiation

We aimed at investigating the antioxidant, antiinflamatory, antiapoptotic and antigenotoxic effects of a Romanian Propolis (RP) extract in two concentrations (RP1 3 mg, respectively RP2 1.5 mg polyphenols/cm(2)), topically administered, either prior to or after UVB exposure, in a Swiss mouse model. Our results showed that both concentrations of RP extract, independent of the time of administration, significantly attenuated the malondialdehyde (MDA) formation and restored glutathione peroxidase (GPx) activity. However, the 8-hydroxy-2-deoxy-guanosine (8-oxo-dG), nitric oxide (NO) and glutathione (GSH) levels were not influenced by UVB exposure and RP treatment. Interleukin (IL)-6 levels were significantly decreased by RP treatment, both before and after UVB-exposure. RP2 extract, in both regimens, significantly reduced the epidermal hyperplasia and dermal inflammation, whereas RP1 pre-treatment diminished only the dermal inflammation. The effect of our RP extract in terms of reduction of sunburn cell formation and of activated caspase-3 and TUNEL-positive cells was observed in both subsets of the experiment, RP2 having a slightly better protective effect as compared to RP1. The antigenotoxic effect of RP was demonstrated by significantly reduced cyclobutane pyrimidine dimers (CPDs) formation. Our results suggest that RP extract might be a potential chemopreventive candidate by modulation of multiple UVB-induced signaling pathways in skin.

Tracking cells in their native habitat: lineage tracing in epithelial neoplasia

For tumours to develop, mutations must disrupt tissue homeostasis in favour of deregulated proliferation. Genetic lineage tracing has uncovered the behaviour of proliferating cells that underpins the maintenance of epithelial tissues and the barriers that are broken in neoplastic transformation. In this Review, we focus on new insights revealed by quantifying the behaviour of normal, preneoplastic and tumour cells in epithelia in transgenic mice and consider their potential importance in humans.

Chronic low dose UV exposure and p53 mutation: tilting the odds in early epidermal preneoplasia?

PURPOSE

This review addresses how mutation of the TP53 gene (p53) and ultraviolet light alter the behavior of normal progenitor cells in early epidermal preneoplasia.

CONCLUSIONS

Cancer is thought to evolve from single mutant cells, which expand into clones and ultimately into tumors. While the mutations in malignant lesions have been studied intensively, less is known about the earliest stages of preneoplasia, and how environmental factors may contribute to drive expansion of mutant cell clones. Here we review the evidence that ultraviolet radiation not only creates new mutations but drives the exponential growth of the numerous p53 mutant clones found in chronically exposed epidermis. Published data is reconciled with a new paradigm of epidermal homeostasis which gives insights into the behavior of mutant cells. We also consider the reasons why so few mutant cells progress into tumors and discuss the implications of these findings for cancer prevention.