High mutation frequency at Ha-ras exons 1-4 in squamous cell carcinomas from PUVA-treated psoriasis patients

Clinical follow-up studies have revealed that PUVA-treated patients are at increased risk of skin cancer, particularly squamous cell carcinoma (SCC). However, since psoralen and UVA (PUVA) is not only a potent mutagen and car cinogen but also an immunosuppressor, and since other (co)carcinogenic fact ors often exist in psoriasis patients, the exact causes and mechanisms of P UVA-associated SCC are still not completely understood. In order to fill th is gap the tools of molecular epidemiology are being used to study the SCC mutational spectra of p53 and Ha-ras, two of the most commonly mutated gene s in human cancers. A previous mutation analysis revealed that SCC in PUVA- treated patients often carried mutated p53 genes and that many of the mutat ions had the UV fingerprint (i.e. C-->T or CC-->TT transitions at dipyrimid ine sites). In the present study DNA-sequencing analysis revealed a total o f 18 Ha-ras missense or nonsense mutations at exons 1-4 in 13 of 17 SCC (76 %) from 8 of 11 (73%) PUVA-treated psoriasis patients. Six of the 18 mutati ons (33%) were of UV-fingerprint type (C-->T transitions), five (28%) were at 5'-TpG sites (i.e. potential psoralen-binding sites and thus potentially caused by PUVA) and seven were of other type (39%), including six G:C-->T: A transversions at hotspot codon 12. In addition, in the case of 6 of the 1 1 subjects (55%) both tumor and normal skin samples contained a T:A-->C:G b ase change at codon 27 (a 5'-ATT site), a change previously hypothesized to be a possible silent Ha-ras polymorphism at one allele. When we compared t he present Ha-ras mutation spectrum with the p53 mutation spectrum from a p revious study of the samples, we found that approximately half of the tumor s harbored mutations in both Ha-ras and p53. Together, our results indicate that Ha-ras mutations are present in a large proportion of PUVA-associated SCC and that UVB, PUVA and other agents may induce Ha-ras mutations and ac t together with p53 in the formation of SCC in psoriasis patients.