In CS cells, there are deficiencies in the repair of oxidative DNA damage in the nuclear and mitochondrial DNA, and this may be a major underlying cause of the disease. We found that CSB-deficient cells accumulate oxidized bases, 8-hydroxyguanine and 8-hydroxyadenine, after oxidative stress, consistent with the observation that CSB and oxoguanine DNA glycosylase (OGG1), the major DNA glycosylase for 8-oxoG repair, are in a complex in vivo. We also found that the CSB protein physically interacts with the Nei-like DNA glycosylase, NEIL1, which is also involved in the repair of oxidized bases. This interaction significantly stimulates NEIL1 catalytic activities, both the glycosylase as well as the AP-lyase. The observation that CSB-deficient mice accumulate significantly higher levels of several oxidized DNA bases in brain tissue, including fapyadenine and fapyguanine, supports a role for the CSB protein in the removal of oxidized lesions in vivo. It is notewhorty that Fapy lesions are considered canonical substrates for NEIL1, underscoring the biolgical relevance for this protein interaction. We recently demonstrated that the CSB protein also interacts with PARP1, a protein involved in the early steps of single-strand break repair, and that these two proteins cooperate in the cellular responses to oxidative stress. CSB is a substrate for PARP-1 ribosylation and it is likely that these two proteins function together in the process of base excision. Our results indicate that the CSB protein plays an important role in the repair of oxidative DNA damage and that accumulation of unrepaired lesions, particular in target tissues, like the brain, may be relevant to the CS pathology, which is characterized by severe early onset neurodegeneration. Moreover, we have identified a novel catalytic activity of the CSB protein. Despite having 7 conserved helicase domains (characteristic of the SWI/SNF protein family), the only identified catalytic activity of CSB was ATP hydrolysis. We found that CSB efficiently catalyzes the annealing of two complementary strands of DNA. We are now mapping this novel activity to gain a better understanding of its biological relevance. Repair of 8-oxoG is of special interest since this lesion is believed to be highly mutagenic and accumulates with age. We find that OGG1 interacts with and can be phosphorylated by the cyclin-dependent kinase cdk4. This post-translational modification modulates OGG1 catalytic activity, suggesting a role for signaling pathways in the response to oxidative DNA damage.
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