Clusters of p53 immunopositive epidermal keratinocytes (so-called p53 patches, clones or foci) are found in sun or ultraviolet (UV) light-exposed skin. We investigated to what extent these p53 patches are genuine precursors of skin carcinomas in chronically irradiated hairless (SKH1) mice. The mutation spectra of exons 5-8 of the p53 gene of laser-micro-dissected mutant p53 patches and carcinomas were therefore compared. The mutations we found were mainly UV-signature mutations (C-->T and CC-->TT at dipyrimidine sites) located at known hotspots. No significant differences were found between both spectra, indicating that all p53 patches harbour mutations with which they could progress to carcinomas. To examine whether these p53 patches can be used as tumour risk indicators, we made an extensive comparison of the induction kinetics of these patches and carcinomas in genetically modified mice with various defects in nucleotide excision repair (NER), i.e. xeroderma pigmentosum A (Xpa), Xpc and Cockayne syndrome B (Csb) and wild-type mice. In this aforementioned order, the mouse strains developed both p53 patches and carcinomas in the course of daily exposure to 40 J/m(2) UV. Hence, the order in which the NER-deficient mice developed patches was predictive of the order in which they developed tumours. The induction kinetics of the patches in Xpc-deficient mice differed notably from the others: there was a stationary phase (days 13-41) where the numbers were limited to 5-10 patches per mouse before an explosive increase which ran parallel to the other groups. The chance that a p53 patch progresses to carcinoma is relatively small (estimated at 1 out of 8300-40,000/individual when the first tumour appears), but our results are strongly indicative of a causal relationship between p53 patches and carcinomas.

Nucleotide excision repair (NER), apoptosis, and cell-cycle regulation are major defense mechanisms against the carcinogenic effects of UVB light. NER eliminates UVB-induced DNA photolesions via two subpathways: global genome repair (GGR) and transcription-coupled repair (TCR). Defects in NER result in the human disorders xeroderma pigmentosum (XP) and Cockayne syndrome (CS), displaying severe UV sensitivity and in the case of XP, cancer proneness. We investigated the impact of deficiencies in NER subpathways on apoptosis, hyperplasia, and cell cycle progression in the epidermis of UVB-exposed CS group B (Csb ؊/؊ ) mice (no TCR), XP group C (Xpc ؊/؊ ) mice (no GGR), and XP group A (Xpa ؊/؊ ) mice (no TCR and no GGR). On UVB treatment (250 J͞m 2 ), Xpa ؊/؊ and Csb ؊/؊ mice revealed an extensive apoptotic response in the skin, a blockage of cell cycle progression of epidermal cells, and strong hyperplasia. Interestingly, the absence of this apoptotic response in the skin of wild-type and Xpc ؊/؊ mice coincided with the ability of epidermal cells to enter the S phase. However, only epidermal cells of Xpc ؊/؊ mice subsequently became arrested in the G2 phase. Our data demonstrate that TCR (and͞or restoration of UVB-inhibited transcription) enables damaged cells to progress through S phase and prevents the induction of apoptosis and hyperplasia. G2 arrest is manifest only under conditions of proficient TCR in combination with deficient GGR, indicating that epidermal cells become arrested in the G2 phase as a result of persisting damage in their genome.

Rat postimplantation whole-embryo culture (WEC) is a promising alternative test for the assessment of developmental toxicity. Toxicogenomic-based approaches may improve the predictive ability of the WEC model by providing a means to identify compound-specific mechanistic responses associated with embryotoxicity in vivo. Furthermore, alterations in gene expression may serve as a sensitive, objective, and robust marker, which precedes the observation of classical developmental toxicity endpoints in time. In this study, in combination with morphological developmental assessments, we studied transcriptomic responses associated with four distinct teratogens (caffeine [CAF], methylmercury [MM], monobutyl phthalate, and methoxyacetic acid) after 4 h of exposure, well before apparent embryotoxicity in WEC. We evaluated gene expression changes associated with similar levels of induced morphological embryotoxicity for each teratogen (as determined by total morphological score), evaluating for functional enrichment and quantitative changes in response. Concentrations selected for each of the four teratogens used induced a number of common effects on embryonic development (neural tube closure and optic/otic system). Despite inducing common morphological effects, our analysis suggests limited overlap in terms of toxicogenomic response at the gene expression level and at the level of biological processes across all four test chemicals. Many unique responses associated with each chemical correlated with previously hypothesized modes of developmental toxicity. For example, alterations in developmental signaling and cholesterol metabolism were observed with MM and CAF, respectively. This initial study suggests that distinct chemically induced toxicogenomic responses precede morphological effects in WEC and that these responses are relevant with mechanisms of toxicity previously observed in vivo.

The stratum corneum and DNA repair do not completely protect keratinocytes from ultraviolet B. A third defense prevents cells with DNA photoproducts from becoming precancerous mutant cells: apoptosis of ultraviolet-damaged keratinocytes ("sunburn cells"). As signals for ultraviolet-induced apoptosis, some studies implicate DNA photoproducts in actively transcribed genes; other studies implicate non-nuclear signals. We traced and quantitated the in vivo DNA signal through several steps in the apoptosis-signaling pathway in haired mice. Homozygous inactivation of Xpa, Csb, or Xpc nucleotide excision repair genes directed the accumulation of DNA photoproducts to specific genome regions. Repair-defective Xpa-/- mice were 7-10-fold more sensitive to sunburn cell induction than wild-type mice, indicating that 86-90% of the ultraviolet B signal for keratinocyte apoptosis involved repairable photoproducts in DNA; the remainder involves unrepaired DNA lesions or nongenomic targets. Csb-/- mice, defective only in excising photoproducts from actively transcribed genes, were as sensitive as Xpa-/-, indicating that virtually all of the DNA signal originates from photoproducts in active genes. Conversely, Xpc-/- mice, defective in repairing the untranscribed majority of the genome, were as resistant to apoptosis as wild type. Sunburn cell formation requires the Trp53 tumor suppressor protein; 90-96% of the signal for its induction in vivo involved transcribed genes. Mdm2, which regulates the stability of Trp53 through degradation, was induced in vivo by low ultraviolet B doses but was suppressed at erythemal doses. DNA photoproducts in actively transcribed genes were involved in approximately 89% of the Mdm2 response.

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