DNA Damage Increases Secreted Aß40 and Aß42 in Neuronal Progenitor Cells: Relevance to Alzheimer's Disease.

BACKGROUND: Recent studies suggest a strong association between neuronal DNA damage, elevated levels of amyloid-ß (Aß), and regions of the brain that degenerate in Alzheimer's disease (AD). OBJECTIVE: To investigate the nature of this association, we tested the hypothesis that extensive DNA damage leads to an increase in Aß40 and Aß42 generation. METHODS: We utilized an immortalized human neuronal progenitor cell line (NPCs), ReN VM GA2. NPCs or 20 day differentiated neurons were treated with hydrogen peroxide or etoposide and allowed to recover for designated times. Sandwich ELISA was used to assess secreted Aß40 and Aß42. Western blotting, immunostaining, and neutral comet assay were used to evaluate the DNA damage response and processes indicative of AD pathology. RESULTS: We determined that global hydrogen peroxide damage results in increased cellular Aß40 and Aß42 secretion 24 h after treatment in ReN GA2 NPCs. Similarly, DNA double strand break (DSB)-specific etoposide damage leads to increased Aß40 and Aß42 secretion 2 h and 4 h after treatment in ReN GA2 NPCs. In contrast, etoposide damage does not increase Aß40 and Aß42 secretion in post-mitotic ReN GA2 neurons. CONCLUSION: These findings provide evidence that in our model, DNA damage is associated with an increase in Aß secretion in neuronal progenitors, which may contribute to the early stages of neuronal pathology in AD.

Transcription-coupled homologous recombination after oxidative damage.

Oxidative DNA damage induces genomic instability and may lead to mutagenesis and carcinogenesis. As severe blockades to RNA polymerase II (RNA POLII) during transcription, oxidative DNA damage and the associated DNA strand breaks have a profoundly deleterious impact on cell survival. To protect the integrity of coding regions, high fidelity DNA repair at a transcriptionally active site in non-dividing somatic cells, (i.e., terminally differentiated and quiescent/G0 cells) is necessary to maintain the sequence integrity of transcribed regions. Recent studies indicate that an RNA-templated, transcription-associated recombination mechanism is important to protect coding regions from DNA damage-induced genomic instability. Here, we describe the discovery that G1/G0 cells exhibit Cockayne syndrome (CS) B (CSB)-dependent assembly of homologous recombination (HR) factors at double strand break (DSB) sites within actively transcribed regions. This discovery is a challenge to the current dogma that HR occurs only in S/G2 cells where undamaged sister chromatids are available as donor templates.