Single‐strand conformation polymorphism (SSCP) and heteroduplex analysis (HA) are popular electrophoretic methods for the identification of sequences. The principle reasons for the popularity of these two methods are their technical simplicity and their relatively high sensitivity for the detection of mutations. Here we review the theory and practice of SSCP and HA, including the factors contributing to the sensitivity of mutation detection. For SSCP analysis, these factors include: choice of gel matrix, electrophoretic conditions, presence of neutral additives, fragment size, and G+C content. For HA, the principle factors influencing sensitivity are the gel matrix and the identity of the base mismatch.

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Individuals suffering from psoriasis are treated with a combination of psoralen and UVA radiation, commonly referred to as "PUVA" therapy. Epidemiologic studies have shown that PUVA therapy is a risk factor for skin cancer in psoriasis patients. Although PUVA treatment induces skin cancer in laboratory animals, it is unknown whether the increased incidence of skin cancer reported in PUVA-treated psoriasis patients is due to the carcinogenic effects of PUVA or due to other factors such as UVB. Because UV and PUVA induce different types of DNA damage resulting in unique types of p53 mutation, we investigated whether skin cancers from PUVA-treated psoriasis patients have PUVA-type or UV-type p53 mutations. Analysis of 17 squamous cell carcinomas (SCCs) from Austrian PUVA-treated patients revealed a total of 25 p53 mutations in 11 SCCs. A majority of p53 mutations occurred at 5'TpG sites. Although previous studies have shown that 5'TpA sites are the primary targets for PUVA mutagenesis, substitutions at 5'TpG sites are also quite common. Interestingly, a sizable portion of p53 mutations detected were C-->T or CC-->TT transitions, characteristic of UV-induced mutations. Because some psoriasis patients had substantial exposure to UVB before PUVA therapy and because the light sources used in PUVA therapy contained small but significant wavelengths in the UVB region, it is possible that the C-->T and CC-->TT transitions detected in SCCs from PUVA-treated patients were induced by UVB. Nonetheless, our results indicate that both PUVA and UVB may play a role in the development of skin cancer in Austrian psoriasis patients who undergo PUVA therapy.

A combination of psoralens and ultraviolet A radiation is widely used to treat psoriasis. Long-term, high-dose exposure to psoralen + ultraviolet A is associated with an increased risk of nonmelanoma skin cancer, particularly squamous cell carcinoma. In this study, we used p53 mutations as a molecular marker to determine the separate contributions of psoralen + ultraviolet A and other ultraviolet exposures, such as ultraviolet B for skin cancer development in psoralen + ultraviolet A-treated psoriasis patients. The results indicated that of 69 tumors analyzed, 37 (54%) tumors had one or more p53 mutations. Of 37 tumors with mutations, 17 (46%) tumors had only ultraviolet-type mutations, two (5%) tumors had only psoralen + ultraviolet A-type mutations, and 18 (49%) tumors had both types of mutations. Interestingly, psoralen + ultraviolet A-type p53 mutations were more frequent than ultraviolet type in tumors arising in patients with high-dose exposure to psoralen + ultraviolet A. Field cancerization and tumor heterogeneity appeared to occur frequently in the same patient and even in the same tumor. This study's data suggest that psoralen + ultraviolet A-induced p53 mutations may play an important part in the development of nonmelanoma skin cancer in psoralen + ultraviolet A-treated patients, but these mutations are likely to act in concert with the effects of other carcinogenic exposures, particularly ultraviolet B, in the development of skin cancer.

A combination of psoralen and ultraviolet A radiation (PUVA) is widely used in the treatment of psoriasis. However, PUVA treatment increases the risk of developing skin cancer in psoriasis patients and induces skin cancer in mice. Since the DNA damage induced by PUVA is quite different from that induced by UV, we investigated whether PUVA-induced mouse skin cancers display carcinogenspecific mutations in the p53 tumor suppressor gene. The results indicated that 10 of 13 (77%) PUVA-induced skin tumors contained missense mutations predominantly at exons 6 and 7. In contrast, tumor-adjacent, PUVA-exposed skin from tumor-bearing animals did not exhibit p53 mutation in exons 4-8. Interestingly, about 40% of all mutations in PUVA-induced skin tumors occurred at 5'-TA sites, and an equal number of mutations occurred at one base flanking 5'-TA or 5'-TAT sites. Since PUVA induces DNA cross-links exclusively at these sites and since UV "signature" mutations were rarely detected in PUVA-induced skin cancers, we can conclude that PUVA acts as a carcinogen by inducing unique PUVA signature mutations in p53. This finding may have implications for identifying the etiology of skin cancer in psoriasis patients who have undergone PUVA therapy.