Impact of Oxidative DNA Damage and the Role of DNA Glycosylases in Neurological Dysfunction.

The human brain requires a high rate of oxygen consumption to perform intense metabolic activities, accounting for 20% of total body oxygen consumption. This high oxygen uptake results in the generation of free radicals, including reactive oxygen species (ROS), which, at physiological levels, are beneficial to the proper functioning of fundamental cellular processes. At supraphysiological levels, however, ROS and associated lesions cause detrimental effects in brain cells, commonly observed in several neurodegenerative disorders. In this review, we focus on the impact of oxidative DNA base lesions and the role of DNA glycosylase enzymes repairing these lesions on brain function and disease. Furthermore, we discuss the role of DNA base oxidation as an epigenetic mechanism involved in brain diseases, as well as potential roles of DNA glycosylases in different epigenetic contexts. We provide a detailed overview of the impact of DNA glycosylases on brain metabolism, cognition, inflammation, tissue loss and regeneration, and age-related neurodegenerative diseases based on evidence collected from animal and human models lacking these enzymes, as well as post-mortem studies on patients with neurological disorders.

Alkyladenine DNA glycosylase associates with transcription elongation to coordinate DNA repair with gene expression.

Base excision repair (BER) initiated by alkyladenine DNA glycosylase (AAG) is essential for removal of aberrantly methylated DNA bases. Genome instability and accumulation of aberrant bases accompany multiple diseases, including cancer and neurological disorders. While BER is well studied on naked DNA, it remains unclear how BER efficiently operates on chromatin. Here, we show that AAG binds to chromatin and forms complex with RNA polymerase (pol) II. This occurs through direct interaction with Elongator and results in transcriptional co-regulation. Importantly, at co-regulated genes, aberrantly methylated bases accumulate towards the 3'end in regions enriched for BER enzymes AAG and APE1, Elongator and active RNA pol II. Active transcription and functional Elongator are further crucial to ensure efficient BER, by promoting AAG and APE1 chromatin recruitment. Our findings provide insights into genome stability maintenance in actively transcribing chromatin and reveal roles of aberrantly methylated bases in regulation of gene expression.

Diverse functions of DNA glycosylases processing oxidative base lesions in brain.

Endogenous and exogenous oxidative agents continuously damage genomic DNA, with the brain being particularly vulnerable. Thus, preserving genomic integrity is key for brain health and neuronal function. Accumulation of DNA damage is one of the causative factors of ageing and increases the risk of a wide range of neurological disorders. Base excision repair is the major pathway for removal of oxidized bases in the genome and initiated by DNA glycosylases. Emerging evidence suggest that DNA glycosylases have non-canonical functions important for genome regulation. Understanding canonical and non-canonical functions of DNA glycosylases processing oxidative base lesions modulating brain function will be crucial for the development of novel therapeutic strategies.

APIM-Mediated REV3L⁻PCNA Interaction Important for Error Free TLS Over UV-Induced DNA Lesions in Human Cells.

Proliferating cell nuclear antigen (PCNA) is essential for the organization of DNA replication and the bypass of DNA lesions via translesion synthesis (TLS). TLS is mediated by specialized DNA polymerases, which all interact, directly or indirectly, with PCNA. How interactions between the TLS polymerases and PCNA affects TLS specificity and/or coordination is not fully understood. Here we show that the catalytic subunit of the essential mammalian TLS polymerase POLζ, REV3L, contains a functional AlkB homolog 2 PCNA interacting motif, APIM. APIM from REV3L fused to YFP, and full-length REV3L-YFP colocalizes with PCNA in replication foci. Colocalization of REV3L-YFP with PCNA is strongly reduced when an APIM-CFP construct is overexpressed. We also found that overexpression of full-length REV3L with mutated APIM leads to significantly altered mutation frequencies and mutation spectra, when compared to overexpression of full-length REV3L wild-type (WT) protein in multiple cell lines. Altogether, these data suggest that APIM is a functional PCNA-interacting motif in REV3L, and that the APIM-mediated PCNA interaction is important for the function and specificity of POLζ in TLS. Finally, a PCNA-targeting cell-penetrating peptide, containing APIM, reduced the mutation frequencies and changed the mutation spectra in several cell lines, suggesting that efficient TLS requires coordination mediated by interactions with PCNA.

p38 MAPK signaling and phosphorylations in the BRCT1 domain regulate XRCC1 recruitment to sites of DNA damage.

XRCC1 is a scaffold protein involved in base excision repair and single strand break repair. It is a phosphoprotein that contains more than 45 phosphorylation sites, however only a few of these have been characterized and connected to specific kinases and functions. Mitogen activated protein kinases (MAPK) are mediators of cellular stress responses, and here we demonstrate that p38 MAPK signaling is involved in phosphorylation of XRCC1 and regulation of recruitment to oxidative stress. Inhibition of p38 MAPK caused a marked pI shift of XRCC1 towards a less phosphorylated state. Inhibition of p38 also increased the immediate accumulation of XRCC1 at site of DNA damage in a poly(ADP)-ribose (PAR) dependent manner. These results suggest a link between PARylation, p38 signaling and XRCC1 recruitment to DNA damage. Additionally, we characterized two phosphorylation sites, T358 and T367, located within, or close to, the phosphate-binding pocket of XRCC1, which is important for interaction with PAR. Mutation of these sites impairs recruitment of XRCC1 to DNA damage and binding to PARP1/PAR. Collectively, our data suggest that phosphorylation of T358 and T367 and p38 signaling are important for proper regulation of XRCC1 recruitment to DNA damage and thereby avoidance of potential toxic and mutagenic BER-intermediates.