Sunlight is a carcinogen to which everyone is exposed. Its UV component is the major epidemiologic risk factor for squamous cell carcinoma of the skin. Of the multiple steps in tumor progression, those that are sunlight-related would be revealed if they contained mutations specific to UV. In a series of New England and Swedish patients, we find that 14/24 (58%) of invasive squamous cell carcinomas of the skin contain mutations in the p53 tumor suppressor gene, each altering the amino acid sequence. Involvement of UV light in these p53 mutations is indicated by the presence in three of the tumors of a CC --TT double-base change, which is only known to be induced by UV. UV is also implicated by a UV-like occurrence of mutations exclusively at dipyrimidine sites, including a high frequency of C --T substitutions. p53 mutations in internal malignancies do not show these UVspecific mutations. The dipyrimidine specificity also implicates dipyrimidine photoproducts containing cytosine as oncogenic photoproducts. We believe these results identify a carcinogenrelated step in a gene involved in the subsequent human cancer.The frequency of skin cancers induced by sunlight in the United States approaches that of all other cancers combined and is doubling each decade (1-3). Ninety-five percent of these are non-melanoma skin cancers, resulting in one-third as many deaths as melanoma (4).Epidemiology has identified causal agents for many human cancers, including skin cancer (5), and reagents such as retroviruses have revealed genes that become oncogenic when mutated (6). Yet, the events between a human carcinogen and the human tumor mutations are unknown. Several questions central to oncology converge on these missing events. In the case of squamous cell carcinoma of the skin, they include the wavelength oflight, the gene that absorbs the photon, the DNA photoproduct, the contribution of mutagenesis versus systemic effects of sunlight such as immunosuppression, the type of mutation, possible hotspot sequences, and confidence that the genetic alterations observed in a tumor participated in tumor formation.Squamous cell carcinoma of the skin is an ideal cancer for determining which of the multiple steps in tumorigenesis are carcinogen-related for the following reasons.(i) The carcinogen is known; in lightly pigmented individuals of all races, the majority of skin cancers are due to sunlight. Of these, squamous cell carcinoma is more sunlightdependent than basal cell carcinoma or melanoma (1, 7). This carcinogen is physically well-defined, whereas agents such as tobacco smoke are complex mixtures.(ii) UV light produces distinctive mutations, leaving a "signature" in the DNA. Mutations due to direct absorption of UV light by DNA are predominantly C --T transitions at dipyrimidine sites, including CC --TT double-base mutations, in organisms from viral to human (refs. 8-12 and references therein). Because CC -* TT base changes are only known to be caused by UV, their presence identifies UV as the mutagen. The appearance of C...

Genomic analysis of archival tissues fixed in formalin is of fundamental importance in biomedical research, and numerous studies have used such material. Although the possibility of polymerase chain reaction (PCR)-introduced artifacts is known, the use of direct sequencing has been thought to overcome such problems. Here we report the results from a controlled study, performed in parallel on frozen and formalin-fixed material, where a high frequency of nonreproducible sequence alterations was detected with the use of formalin-fixed tissues. Defined numbers of well-characterized tumor cells were amplified and analyzed by direct DNA sequencing. No nonreproducible sequence alterations were found in frozen tissues. In formalin-fixed material up to one mutation artifact per 500 bases was recorded. The chance of such artificial mutations in formalin-fixed material was inversely correlated with the number of cells used in the PCR-the fewer cells, the more artifacts. A total of 28 artificial mutations were recorded, of which 27 were C-T or G-A transitions. Through confirmational sequencing of independent amplification products artifacts can be distinguished from true mutations. However, because this problem was not acknowledged earlier, the presence of artifacts may have profoundly influenced previously reported mutations in formalin-fixed material, including those inserted into mutation databases.

Huge differences in incidence rates of invasive cervical cancer occur among populations. These differences reflect the influences of both etiological environmental factors and removal of precursor lesions detected upon screening. The purposes of this article are (i) to describe similarities and differences in the shapes and magnitudes of age-specific incidence rates of invasive cervical cancer before screening had an effect, (ii) to provide baseline data for further global study of screening effects, and (iii) to provide baseline incidence data for the design of optimal screening programs. To eliminate the impact of screening effects, we have selected age-specific incidence rates from times when and from populations in which screening was insignificant. The selected rates were suitably scaled and compared regarding age at onset of increase in incidence, age at peak incidence, and rate of subsequent decline. Despite a 16-fold difference in incidence rates, all curves had the same basic structure, with an increase to a peak followed by a decline or a plateau. Although all populations but one had an onset around age 25, 7 European countries showed an earlier peak age (mean = 46 vs. 59) and a more rapid decline after the peak than most other populations. The common basic shape of the age-specific incidence curve, overall, suggests a relatively similar development of invasive cervical cancer in different populations. These results illustrate the underlying similarities in the markedly different age-specific incidence rates of invasive cervical cancer. They also provide a basis for studying screening effects and for optimizing screening programs in specific geographic areas.

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