Relationship between UV-induced mutant p53 patches and skin tumours, analysed by mutation spectra and by induction kinetics in various DNA-repair-deficient mice

Abstract

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.

Dietary Immunosuppressants Do Not Enhance UV-Induced Skin Carcinogenesis, and Reveal Discordance between p53-Mutant Early Clones and Carcinomas

“…As the hotspot mutations at codons 270 or 275 comprise about half of the mutations in the p53 gene, the mutational frequency suggests that maximally 20% of the cells in the skin would harbor mutated p53 (if cells harbored more than one mutation in similar percentages as mut-p53 cell clusters, the fraction would come down to about 14%; refs. 29,35). These results are in agreement with a recent deep sequencing study of human skin from middle-aged individuals revealing that persistent p53 mutations had accumulated in 14% of the epidermal cells (36).…”

“…Numbers of mut-p53 cell clusters had always correlated with the rate of tumor development in previous studies (5,29,44). Mutp53 cell clusters develop as a clonal expansion of a mutated keratinocyte.…”

Mutation, clonal fitness and field change in epithelial carcinogenesis

“…Consistent with earlier studies, most of the mutations observed were cytosine (C)-to-thymidine (T) base pair changes and CC-to-TT double base pair changes, signatures of ultraviolet (UV) light [32][33][34]. The majority of TP53 mutations stabilize the protein, enabling the mutant cells to be detected by immunostaining, which does not detect the far lower levels of wild-type p53 [35][36][37][38]. Staining of sun-exposed human epidermis reveals frequent (30 per cm 2 ) TP53 immunoreactive clones that range widely in size from 30 to 3000 cells [35,39].…”

Stochastic fate ofp53-mutant epidermal progenitor cells is tilted toward proliferation by UV B during preneoplasia

“…Our analysis explains the full range of clone fate data, unlike the Frontier model, which only provides a good fit to the size distribution of large clones. Yet, although the exponential growth of small PMCs will not affect the tissue, if left unchecked, such behavior would result in many PMCs generating tumors without acquiring additional mutations (30). How is the expansion of PMCs slowed?…”