Oxidative stress and DNA damage in white matter lesions of the human ageing brain

White matter lesions (WML), identified as hyperintensities on T2-weighted magnetic resonance images (MRI) in the ageing brain, are associated with dementia and depression in the elderly. Ischaemia may contribute to their pathogenesis but the exact role of glial cell pathology remains unclear.
Recent studies have concluded that oxidative stress is present in high levels in the deep subcortical white matter lesions when compared to periventricular white matter lesions.
The current study investigates the hypothesis that oxidative DNA damage contributes to the pathogenesis of WML, specifically the deep subcortical WM (DSCL). Oxidative DNA damage was investigated in WML and control WM, both from cases with WML (referred to as lesional controls) and without WML derived from the MRC-Cognitive Function and Ageing Study. Lesions were previously identified using post mortem MRI. 8-hydroxy-2’-deoxyguanosine (8-OHdG) was detected by immunohistochemisty and nuclear expression quantified. Double staining was performed to colocalise 8-OHdG with markers for specific cell type (e.g. CD68 for microglia). Expression of Malonaldehyde (MDA) (marker of lipid peroxidation), gamma histone H2AX (ɣH2AX) and DNA dependent protein kinase (DNA-PK) (markers of DNA damage response) were quantified by Western Blotting. -galactosidase and p16 were used to detect induction of cellar senescence as a downstream effect of persistent DNA damage response. QPCR array was carried out using whole tissue RNA extracts to measure differences in expression of key senescence and DNA damage response genes.
Both WML and lesional control WM showed significantly elevated level of DNA oxidation than control WM, whilst WML and lesional controls did not differ. Persistent DNA damage response was detected using MDA, ɣH2AX and DNA-PK antibodies which activated senescence pathways demonstrated in galactosidase activity as well as p16, p21 and p53 as other indicators of cellular senescence. Key genes involved in DNA damage and senescence pathways were highly expressed in CL tissue.
Oxidised DNA is up regulated in ageing WM in different levels and may contribute to pathogenesis of WML. The similarity in the level of oxidative DNA damage in lesional control WM and WML suggests that oxidative damage is widespread in WM in cases with lesions indicating that WML are associated with general WM damage. DNA damage potentially activates cellular senescence as well as cell cycle check proteins, particularly in astrocytes, in aged WM and WML.