Molecular nature of ultraviolet B light-induced deletions in the murine epidermis

Depletion of the stratospheric ozone layer leads to an increase in ambient UV loads, which are expected to raise skin cancer incidences, Tumor develop ment in the skin could be a multistep process in which various genetic alte rations, such as point mutations and deletions, occur successively. Here, w e demonstrate that UVB irradiation efficiently induces deletions in the epi dermis using a novel transgenic mouse, gpt delta. In this mouse model, dele tions in lambda DNA integrated in the chromosome are preferentially selecte d as Spi(-) (sensitive to P2 interference) phages, which can then be subjec ted to molecular analysis. The mice were exposed to UVB at single doses of 0.3, 0.5, 1,0, 1,5, and 2.0 kJ/m(2). After 4 weeks, A phage was rescued fro m the genomic DNA of the epidermis by in vitro packaging reactions, The mut ant frequencies of Spi(-) with large deletions in the epidermis increased > 15-fold at a UVB dose of 0.5 kJ/m(2) over the control. Molecular sizes of most of the large deletions were > 1000 bp, More than one-half of the large deletions occurred between short direct-repeat sequences from 1 to 6 bp, a nd the remainder had flush ends, In the unirradiated mouse, almost all of t he Spi- mutants were I-bp frameshifts in runs of identical bases. These res ults suggest that UVB irradiation induces deletions in the murine epidermis , and most of the deletions are generated through end-joining of double str and breaks in DNA.