UVB-induced mutations in human key gatekeeper genes governing signalling pathways and consequences for skin tumourigenesis

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Abstract
The UVB component of the solar spectrum induces DNA lesions that, in the absence of error-free DNA repair, may give rise during DNA replication to mutations in caretaker and gatekeeper genes. The DNA repair genes are the best candidates for caretaker genes as exemplified by the human hereditary xeroderma pigmentosum (XP) syndrome. Cultured XP cells are hypermutable after UVB irradiation. This increased mutation frequency is also found in gatekeeper genes, which govern signalling pathways implicated in the control of cellular proliferation, differentiation and survival of human epidermal keratinocytes. We describe and discuss the role of mutated gatekeeper genes in five specific signalling pathways which have been implicated in skin carcinogenesis. The pathways we focus on in this review are: (i) P16INK4A-CDK4/6-RB; (ii) P14ARF-HDM2-P53; (iii) Sonic hedgehog (SHH)/GLI; (iv) WNT/β-catenin; and (v) Bone Morphogenetic Protein (BMP)/SMAD. 70–80% of XP skin cancers exhibit one or several mutations in the P53, PTCH-1, SMO or CDKN2A genes, the type and frequency of mutated genes being different between squamous cell (SCCs) and basal cell carcinomas (BCCs). In XP cancers, the typically UVB-induced CC to TT tandem transitions represent approximately 60% of total mutations compared to 10–15% in skin tumours from DNA repair-proficient patients. Acquired activation of the pathways described herein can alter proliferation and differentiation of keratinocytes, allowing a damaged cell to replicate and give rise to mutated daughter cells, then eventually to the development of the carcinogenic process following clonal selection.