Mutagenicity of a bi-stranded clustered DNA lesion containing (5'S) or (5'R) 5',8-cyclo-2'-deoxyAdenosine in Escherichia coli model.

Ionizing radiation induces DNA damage, including characteristic clusters and tandem lesions e.g., 5',8-cyclo-2'-deoxyPurines (cdPus). Clustered DNA Lesions (CDL) defined as 2 or more lesions within 1-2 helical turns resulting from a single radiation track contribute to the harmful effects of radiation. Moreover, the presence of CDL and cdPus in human DNA may decrease the efficiency of the DNA repair mechanisms, which in consequence may lead to, e.g., carcinogenesis. This preliminary study showed the mutagenic potential of CDL containing dU on one strand and 5',8-cyclo-2'-deoxyAdenosine (cdA) on a complementary strand separated by up to 4 bp. Mutagenicity was determined using Escherichia coli reporter assay and 40-mer model ds-oligonucleotides with CDL. Mutation frequencies were determined to be significantly higher for CDL than for single isolated lesions (cdA or dU placed only in one strand). The results demonstrated that the dU lesion located on the opposite DNA strand separated by 0 or 1 bp from cdA led to severe mutagenicity. The most frequent mutations observed comprised point deletions and transitions. Oligonucleotides with CDL containing ScdA/RcdA demonstrated even up to 100% mutation rate. Interestingly, increasing the distance between lesions within CDL to 4 bp led to full recovery of the correct sequence of ds-oligonucleotides, indicating an efficient repair process. The results obtained with the bacterial model are in agreement with previous in vitro studies on eukaryotic models. The high mutagenicity and/or inhibited repair process of clusters with lesions located in close proximity provides additional verification of the previously presented trends describing how the distance between cdPu and dU affects DNA repair processes.