Presence of benzo[a]pyrene diol epoxide adducts in target DNA leads to an increase in UV-induced DNA single strand breaks and supF gene mutations

Exposure to DNA damaging agents and mutagens often occurs as combinations o f agents, or as complex mixtures of chemicals. We found that plasmid DNA ad ducted with benzo[a]pyrene diol epoxide (BPDE) was more susceptible to UV-i nduced single strand breaks than was control DNA. To determine whether the increase in DNA damage also applied to mutagenic lesions, the supF gene for ward mutation assay was used to compare mutations induced by BPDE alone, UV B, UVC, BPDE followed by UVB and BPDE followed by UVC. It was found that th e mutation frequency for BPDE + UVB (1167 in 10(4) transformants) was highe r than BPDE alone (12 in 10(4) transformants) or UVB alone (446 in 104 tran sformants), and the mutation frequency for BPDE + UVC (197 in 10(4) transfo rmants) was higher than BPDE alone or UVC alone (26 in 10(4) transformants) . For BPDE + UVB and BPDE + UVC there was a significant increase in plasmid s with multiple mutations. Whilst these indicate error prone repair due to the single strand breaks, the different mutation frequencies in plasmids tr eated to give similar levels of strand breaks suggest other mechanisms for the mutations in plasmids with single mutation events. The spectrum of non- multiple mutations in the two combined treatments included both UV signatur e mutations (GC --> AT as the most common mutation) and BPDE signature muta tions (GC --> TA and GC --> CG as the most common mutations). However, the increase in absolute mutation frequency of BPDE signature mutations between BPDE treatment and BPDE + UV treatment was greater than the increase in ab solute mutation frequency of UV signature mutations, even though the level of BPDE adducts was identical in each case. These results suggest two possi bilities: (i) the BPDE adducts are photoactivated to a more mutagenic lesio n, or (ii) the presence of UV lesions lead to the BPDE adducts becoming mor e mutagenic.