Sublethal doses of beta-amyloid peptide abrogate DNA-dependent protein kinase activity.

Accumulation of DNA damage and deficiency in DNA repair potentially contribute to the progressive neuronal loss in neurodegenerative disorders, including Alzheimer 's disease (AD). In multicellular eukaryotes, Double Strand Breaks (DSBs), the most lethal form of DNA damage, are mainly repaired by non-homologous end joining (NHEJ) pathway, which relies on DNA-PK complex activity. Both the presence of DSBs and a decreased end joining activity have been reported in AD brains but the molecular player causing DNA repair dysfunction is still undetermined. A-beta, a potential proximate effector of neurotoxicity in AD, might exert cytotoxic effects by ROS generation and oxidative stress induction, which may then cause DNA damage. Here we show that in PC12 cells sublethal concentrations of aggregated A-beta 25-35 inhibit DNA-PK kinase activity, compromising DSB repair and sensitizing cells to non lethal oxidative injury. The inhibition of DNA-PK activity is associated with downregulation of the catalytic subunit DNA-PKcs protein levels, caused by oxidative stress and reversed by antioxidant treatment. Moreover, we show that sublethal doses of A-beta 1-42 oligomers enter the nucleus of PC12 cells, accumulate as insoluble oligomeric species and reduce DNA-PK kinase activity, although in the absence of oxidative stress. Overall these findings suggest that A-beta mediates inhibition of DNA-PK-dependent NHEJ pathway contributing to the accumulation of DSBs which, if not efficiently repaired, may lead to the neuronal loss observed in AD.