Loss of base excision repair in aging rat neurons and its restoration by DNA polymerase β

Abstract

Synthetic staggered oligodeoxynucleotide duplexes are formed by annealing a 5′- 32P-labeled 14-mer with four different 21-mers. These duplexes have either a correct or mismatched base pair at 3′-end of the primer. With these model template primers the ability of neuronal extracts, obtained from rats of different ages, to extend the primer to the predicted length was tested. While the neuronal extracts of all ages were able to degrade the 14-mer to shorter lengths, extension of the primers in general and in particular, the mismatched, is achieved only feebly by the young and adult neuronal extracts and undetectable with old neuronal extracts. The possibility of restoring the lost activity by supplementing the neuronal extracts with pure DNA polymerases was examined. Of the three polymerases tested (calf thymus α polymerase, E. coli DNA polymerase I and rat liver DNA polymerase β) only polymerase β gave consistent and encouraging results although the extension was slow and distributive in nature and mismatched primers were extended much less efficiently than the correctly paired primer. However, significantly improved extension, including those of mismatched primers, was achieved by prior removal of mismatched bases in a preincubation with just the neuronal extracts (3′–5′exonuclease activity) followed by extension by the added polymerase β and dNTPs in the presence of Mn 2+ instead of the usual Mg 2+. These results are taken to indicate that the activity of polymerase β in brain cells is compromised with age and that this deficit can be corrected in vitro by the addition of pure recombinant rat liver polymerase β under appropriate conditions.