Early p53-positive foci as indicators of tumor risk in ultraviolet-exposed hairless mice: kinetics of induction, effects of DNA repair deficiency, and p53 heterozygosity

p53 mutations appear to be early events in skin carcinogenesis induced by chronic UVB irradiation. Clusters of epidermal cells that express p53 in mutant conformation ("p53 positive foci") are easily detected by immunohistochemical staining long before the appearance of skin carcinomas or their precursor lesions. In a hairless mouse model, we determined the dose-time dependency of the induction of these p53+ foci and investigated the relationship with the induction of skin carcinomas. The density of p53+ foci may be a good direct indicator of tumor risk. Hairless SKH1 mice were exposed to either of two regimens of daily UVB (500 or 250 J/m2 broadband UV from Philips TL12 lamps; 54% UVB 280-315 nm). With the high-dose regimen, the average number of p53+ foci in a dorsal skin area (7.2 cm2) increased rapidly from 9.0 +/- 2.1 (SE) at 15 days to 470 +/- 80 (SE) at 40 days. At half that daily dose, the induction of p53+ foci was slower by a factor of 1.49 +/- 0.15, very similar to a previously observed slower induction of squamous cell carcinomas by a factor of 1.54 +/- 0.02. In a double-log plot of the average number of p53 + foci versus time, the curves for the two exposure regimens ran parallel (slope, 3.7 +/- 0.7), similar to the curves for the number of tumors versus time (slope, 6.9 +/- 0.8). The difference in slopes (3.7 versus 6.9) is in line with the contention that more rate-limiting steps are needed to develop a tumor than a p53+ focus. By the time the first tumors appear (around 7-8 weeks with the high daily dose), the dorsal skin contains >100 p53+ foci/cm2. To further validate the density of p53+ foci as a direct measure of tumor risk, we carried out experiments with transgenic mice with an enhanced susceptibility to UV carcinogenesis, homozygous Xpa knockout mice (deficient in nucleotide excision repair) and heterozygousp53 knockout mice (i.a. partially deficient in apoptosis). In both of these cancer-prone strains, the p53+ foci were induced at markedly increased rates, corresponding to increased rates of carcinoma formation. Therefore, the frequency of p53+ foci appears to correlate well with UVB-induced tumor risk.

Differential role of transcription-coupled repair in UVB-induced G2 arrest and apoptosis in mouse epidermis

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 J/m(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 G(2) phase. Our data demonstrate that TCR (and/or restoration of UVB-inhibited transcription) enables damaged cells to progress through S phase and prevents the induction of apoptosis and hyperplasia. G(2) arrest is manifest only under conditions of proficient TCR in combination with deficient GGR, indicating that epidermal cells become arrested in the G(2) phase as a result of persisting damage in their genome.

Impact of global genome repair versus transcription-coupled repair on ultraviolet carcinogenesis in hairless mice

The nucleotide excision repair (NER) system is comprised of two subpathways, i.e., transcription-coupled repair (TCR) and global genome repair (GGR). To establish the relative importance of TCR and GGR for UV effects on the skin, we have used hairless knockout mouse strain lacking either TCR (CSB -/-) or GGR (XPC -/-). In single exposure experiments, we found that CSB -/- mice have a 7-16 times higher susceptibility to sunburn than XPC -/- mice and than heterozygous (+/-) and wild-type (+/+) controls. Exposure to 80 J/m2 UV radiation (i.e., suberythemogenic in CSB -/-) on 10 consecutive days gives rise to epidermal hyperplasia in CSB -/- and XPC -/-, whereas repair-proficient controls do not show epidermal hyperplasia from these exposures. In addition, CSB -/- mice develop marked parakeratosis, whereas XPC -/- mice and controls do not. Under continued exposure to this daily dose, squamous cell carcinomas appear in CSB -/-, XPC -/-, and in the control groups, whereas only in the CSB -/- animals is a fairly high number of benign papillomas also found. The median latency time of squamous cell carcinomas (diameters > or = 1 mm) is 84 days for the XPC -/- mice, 115 days for the CSB -/- mice, and 234-238 days for the heterozygous and wild-type control groups. These results indicate that GGR is more important than TCR in protection against UV-induced carcinomas of the skin but not against other UV effects such as sunburn, epidermal thickening, scaling of the stratum corneum, and development of papillomas. These results also indicate that GGR capacity may serve as a better predictor for skin cancer susceptibility than sensitivity to sunburn. The relative cancer susceptibilities of GGR- and TCR-deficient skin could well depend on the balance between an increased mutation rate and the presence (in CSB -/-) or lack (in XPC -/-) of a compensatory apoptotic response.

UVB-induced mutagenesis in hairless lambda lacZ-transgenic mice

UVB-induced mutagenesis was studied in hairless 40.6 transgenic mice (MutaMouse), which contain the lambda gt10lacZ shuttle vector as a target for mutagenesis. Mice were exposed at the dorsal side to either single doses of 200, 500, 800, or 1000 J/m2 UVB or to two successive irradiations of either 200 and 800 J/m2 UVB, with intervals of 1, 3, or 5 days, or to 800 and 200 J/m2 UVB with a 5-day interval. At 23 days after the last exposure, lacZ mutant frequencies (MF) were determined in the epidermis. The lacZ MF increased linearly with increasing dose of UVB. The mutagenic effect of two successive irradiations appeared to be additive. The UV-induced mutation spectrum was dominated by G:C --> A:T transitions at dipyrimidine sites. DNA-sequence analysis of spontaneously mutated phages showed a diverse spectrum consisting of insertions, deletions and G:C --> A:T transitions at CpG sites. The results indicate that the hairless lambda lacZ-transgenic mouse is a suitable in vivo model for studying UVB-induced mutations.