Xeroderma pigmentosum patients with a defect in the nucleotide-excision repair gene XPA are characterized by, for example, a > 1,000-fold higher risk of developing sunlight-induced skin cancer. Nucleotide-excision repair (NER) is involved in the removal of a wide spectrum of DNA lesions. The XPA protein functions in a pre-incision step, the recognition of DNA damage. To permit the functional analysis of the XPA gene in vivo, we have generated XPA-deficient mice by gene targeting in embryonic stem cells. The XPA-/-mice appear normal, at least until the age of 13 months. XPA-/-mice are highly susceptible to ultraviolet (UV)-B-induced skin and eye tumours and to 7,12-dimethylbenz[a]anthracene (DMBA)-induced skin tumours. We conclude that the XPA-deficient mice strongly mimic the phenotype of humans with xeroderma pigmentosum.

A mouse model for the nucleotide excision repair disorder Cockayne syndrome (CS) was generated by mimicking a truncation in the CSB(ERCC6) gene of a CS-B patient. CSB-deficient mice exhibit all of the CS repair characteristics: ultraviolet (UV) sensitivity, inactivation of transcription-coupled repair, unaffected global genome repair, and inability to resume RNA synthesis after UV exposure. Other CS features thought to involve the functioning of basal transcription/repair factor TFIIH, such as growth failure and neurologic dysfunction, are present in mild form. In contrast to the human syndrome, CSB-deficient mice show increased susceptibility to skin cancer. Our results demonstrate that transcription-coupled repair of UV-induced cyclobutane pyrimidine dimers contributes to the prevention of carcinogenesis in mice. Further, they suggest that the lack of cancer predisposition in CS patients is attributable to a global genome repair process that in humans is more effective than in rodents.

High levels of the p53 protein are immunohistochemically detectable in a majority of human nonmelanoma skin cancers and UVB-induced murine skin tumors. These increased protein levels are often associated with mutations in the conserved domains of the p53 gene. To investigate the timing of the p53 alterations in the process of UVB carcinogenesis, we used a well defined murine model (SKH:HR1 hairless mice) in (1), and from animal studies it appeared that the UVB part of the solar spectrum is the most carcinogenic (2). This has been substantiated by detection of mutations in the p53 tumor-suppressor gene in human SCCs (3) and basal cell carcinomas (4) that are characteristic for UVB radiation: i.e., mainly C T transitions at dipyrimidine sites among which are CC TT tandem mutations. There are indications that p53 is involved in the earliest stages of human nonmelanoma skin cancer. Recently, it has been reported that p53 mutations are already present in a benign precursor of SCC, actinic keratosis (5), and in skin adjacent to basal cell carcinomas (6). Furthermore, it has been shown that CC --TT tandem mutations in the p53 gene are detectable in biopsies from nonneoplastic skin of sun-exposed sites from Australian skin cancer patients (7).The suspected causal relationship between chronic UV exposure and p53 mutation and their relation to tumor forThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. mation can most directly and unequivocally be established in an animal model in which UV exposure is the only well controlled carcinogenic agent. A robust model is the SKH:HR1 hairless mouse for which the relationship between UVB exposure and carcinogenic response is well established (8, 9) and for which the pathogenesis of UVB-induced SCC shows close similarities with that of human SCC (10).Under physiological circumstances, the wild-type p53 protein has a very short half-life and is present in such small quantities that it is not immunohistochemically detectable (11,12). There are different pathways that lead to accumulation of the p53 protein up to immunohistochemically detectable levels. (i) DNA damage gives rise to a temporary accumulation of the wild-type p53 protein resulting in an arrest of the cell cycle assumed to prevent replication of damaged DNA (13). (ii) Missense mutations in the p53 gene in general lead to a dramatic increase in half-life of the p53 protein (11). In contrast to the transient accumulation of wild-type p53, the latter can lead to a constitutively high p53 level in the cell. We have recently reported that >75% of UVB-induced murine skin carcinomas show immunostaining with the p53-specific polyclonal antiserum CM-5, which was primarily confined to the proliferative compartments of the tumors. A substantial part of the p53-positive staining was associated with point mutations in the conserved domains of the p53 gene (14). Subsequ...

The sun-sensitive form of the severe neurodevelopmental, brittle hair disorder trichothiodystrophy (TTD) is caused by point mutations in the essential XPB and XPD helicase subunits of the dual functional DNA repair/basal transcription factor TFIIH. The phenotype is hypothesized to be in part derived from a nucleotide excision repair defect and in part from a subtle basal transcription deficiency accounting for the nonrepair TTD features. Using a novel gene-targeting strategy, we have mimicked the causative XPD point mutation of a TTD patient in the mouse. TTD mice reflect to a remarkable extent the human disorder, including brittle hair, developmental abnormalities, reduced life span, UV sensitivity, and skin abnormalities. The cutaneous symptoms are associated with reduced transcription of a skin-specific gene strongly supporting the concept of TTD as a human disease due to inborn defects in basal transcription and DNA repair.

It is generally presumed that xeroderma pigmentosum (XP) patients are extremely sensitive to developing UV erythema, and that they have a more than 1000-fold increased skin cancer risk. Recently established mouse models for XP can be employed to investigate the mechanism of these increased susceptibilities. In line with human data, both XPA and XPC knockout mice have been shown to have an increased susceptibility to UVB induced squamous cell carcinomas. In XPA knockouts, nucleotide excision repair of UV induced DNA photolesions is completely defective (i.e., both global genome repair and transcription coupled repair are defective). We determined the strand specific removal of cyclobutane pyrimidine dimers and pyrimidine [6-4] pyrimidone photoproducts from the p53 gene in cells from XPC knockout mice and wild-type littermates. Analogous to human XPC cells, embryonic fibroblasts from XPC knockout mice are only capable of performing transcription coupled repair of DNA photolesions. We show that these XPC knockout mice, in striking contrast to XPA knockout mice, do not have a lower minimal erythema/edema dose than their wild-type littermates. Hence, defective global genome repair appears to lead to skin cancer susceptibility, but does not influence the sensitivity to acute effects of UVB radiation, such as erythema and edema. The latter phenomena thus relate to the capacity to perform transcription coupled repair, which suggests that blockage of RNA synthesis is a key event in the development of UV erythema and edema.