The DNA damage signal for Mdm2 regulation, Trp53 induction, and sunburn cell formation in vivo originates from actively transcribed genes

The stratum corneum and DNA repair do not completely protect keratinocytes from ultraviolet B. A third defense prevents cells with DNA photoproducts f rom becoming precancerous mutant cells: apoptosis of ultraviolet-damaged ke ratinocytes ("sunburn cells"). As signals for ultraviolet-induced apoptosis , some studies implicate DNA photoproducts in actively transcribed genes; o ther studies implicate non-nuclear signals. We traced and quantitated the i n vivo DNA signal through several steps in the apoptosis-signaling pathway in haired mice. Homozygous inactivation of Xpa, Csb, or Xpc nucleotide exci sion repair genes directed the accumulation of DNA photoproducts to specifi c genome regions. Repair-defective Xpa(-/-) mice were 7-10-fold more sensit ive to sunburn cell induction than wild-type mice, indicating that 86-90% o f the ultraviolet B signal for keratinocyte apoptosis involved repairable p hotoproducts in DNA; the remainder involves unrepaired DNA lesions or nonge nomic targets. Csb(-/-) mice, defective only in excising photoproducts from actively transcribed genes, were as sensitive as Xpa(-/-), indicating that virtually all of the DNA signal originates from photoproducts in active ge nes. Conversely, Xpc(-/-) mice, defective in repairing the untranscribed ma jority of the genome, were as resistant to apoptosis as wild type. Sunburn cell formation requires the Trp53 tumor suppressor protein; 90-96% of the s ignal for its induction in vivo involved transcribed genes. Mdm2, which reg ulates the stability of Trp53 through degradation, was induced in vivo by l ow ultraviolet B doses but was suppressed at erythemal doses. DNA photoprod ucts in actively transcribed genes were involved in approximate to 89% of t he Mdm2 response.