p53 mutations experimentally induced by 8-methoxypsoralen plus UVA (PUVA) differ from those found in human skin cancers in PUVA-treated patients

8-Methoxypsoralen (8-MOP) plus UVA irradiation (PUVA therapy) has been used for the treatment of psoriasis, PUVA therapy has been associated with an i ncreased risk of developing skin squamous cell carcinoma (SCC), In order to determine the PUVA-induced p53 mutation spectrum, a yeast expression vecto r harbouring a human wild-type p53 cDNA was incubated with 8-MOP, and UVA i rradiated in vitro. PUVA-damaged and undamaged DNA was transfected into a y east strain containing the ADE2 gene regulated by a p53-responsive promoter , An 8-MOP concentration-dependent decrease in survival and increase in mut ant frequency were observed. At a fixed 8-MOP concentration, survival decre ased and mutant frequency increased as UVA irradiation increased. Eleven mu tant clones contained 11 mutations: 10 were single base pair substitutions, the remaining one being a complex mutation. All eight T:A-targeted mutatio ns were at 5'-TpA sites, hallmark mutations of PUVA mutagenesis, Through a rigorous statistical test, the PUVA-induced p53 mutation spectrum appears t o differ significantly (P < 0.0002) from that observed in SCC in PUVA-treat ed patients, The present work demonstrates that a specific PUVA-induced mut ational fingerprint could be obtained and recognized on human p53 cDNA, Thi s result may suggest that PUVA therapy can be a risk factor for the develop ment of SCC in psoriasis patients through a mechanism not involving the ind uction of p53 mutations.