Removal of oxygen free-radical-induced 5 ',8-purine cyclodeoxynucleosides from DNA by the nucleotide excisicon-repair pathway in human cells

Exposure of cellular DNA to reactive oxygen species generates several class es of base lesions, many of which are removed by the base excision-repair p athway. However, the lesions include purine cyclodeoxynucleoside formation by intramolecular crosslinking between the C-8 position of adenine or guani ne and the 5' position of 2-deoxyribose. This distorting form of DNA damage , in which the purine is attached by two covalent bonds to the sugar-phosph ate backbone, occurs as distinct diastereoisomers, It was observed here tha t both diastereoisomers block primer extension by mammalian and microbial r eplicative DNA polymerases, using DNA with a site-specific purine cyclodeox ynucleoside residue as template, and consequently appear to be cytotoxic le sions. Plasmid DNA containing either the 5'R or 5'S form of 5',8-cyclo-2-de oxyadenosine was a substrate for the human nucleotide excision-repair enzym e complex, The R diastereoisomer was more efficiently repaired than the S i somer, No correction of the lesion by direct damage reversal or base excisi on repair was detected. Dual incision around the lesion depended on the cor e nucleotide excision-repair protein XPA. In contrast to several other type s of oxidative DNA damage, purine cyclodeoxynucleosides are chemically stab le and would be expected to accumulate at a slow rate over many years in th e DNA of nonregenerating cells from xeroderma pigmentosum patients. High le vels of this form of DNA damage might explain the progressive neurodegenera tion seen in XPA individuals.