DNA damage and its links to neurodegeneration

Abstract

The integrity of our genetic material is under constant attack from numerous endogenous and exogenous agents. The consequences of a defective DNA damage response are well studied in proliferating cells, especially with regards to the development of cancer, yet its precise roles in the nervous system are relatively poorly understood. Here we attempt to provide a comprehensive overview of the consequences of genomic instability in the nervous system. We highlight the neuropathology of congenital syndromes that result from mutations in DNA repair factors and underscore the importance of the DNA damage response in neural development. In addition, we describe the findings of recent studies, which reveal that a robust DNA damage response is also intimately connected to aging and the manifestation of age-related neurodegenerative disorders such as Alzheimer's disease and amyotrophic lateral sclerosis.

Figure 1. The single strand break repair pathway

It is not clear whether BER and TOP1-mediated SSBs actually require a sensing step; however, direct breaks are detected by polyADP ribose polymerase 1 (PARP1). Various activities collaborate to generate 3′ OH and 5′P ends that are compatible for ligation. For instance, 3′ phosphoglycolate, and 3′ phosphate and 5′ OH intermediates generated from ROS-mediated sugar disintegration, and products of abortive TOP1 reactions are variously processed by APE1 PNKP, and TDP1, respectively. Occasionally, failure of ligation can result in the formation of a 5′AMP-associated SSB, which is processed by APTX. Like in BER, any gap-filling synthesis is mediated by polβ and nicks are sealed by either XRCC1/LIG3 or FEN1/LIG1.

Figure 2. The NHEJ pathway of DNA double strand break repair

KU70/KU80 heterodimer binds to the broken DNA ends and recruits the catalytic subunit of the DNA-dependent protein kinase, DNA-PKcs. Activation of DNA-PKcs allows end-processing proteins such as the ARTEMIS, to access the broken DNA ends and DNA polymerase (i.e. pol μ and pol λ) to fill in the gap. The XRCC4/LIG4 complex is then recruited to promote religation.

Figure 3. Chromatin modifications in the DDR

The formation of DNA DSBs triggers various chromatin modifications, including poly-ADP-ribosylation mediated by PARP1; histone acetylation/deacetylation mediated by HAT such as p300, MOF and TIP60, and HDAC, such as HDAC1, SIRT1 and SIRT6; and ATM-dependent H2AX phosphorylation and RNF8/RNF168 mediated H2A ubiquitination. ATP-dependent chromatin remodelers can slide, exchange or evict histone dimers or octamers. The consequences of these modifications are depicted, with details provided in the text. H2AX containing nucleosomes are shown in orange.

Figure 4. The Lock, Loosen, Load Model of chromatin dynamics

DNA damage triggers chromatin compaction in the vicinity of damaged sites that allows the broken DNA ends to be stabilized or “locked” and to inhibit transcription in regions adjacent to damaged DNA. This transient state is quickly followed by chromatin relaxation (“Loosen”) that allows a plethora of DNA repair proteins to be recruited (Loaded) to the sites of damage.

Figure 5. The consequences of DNA damage in aging and neurodegeneration

(Left) Erroneous repair of DNA damage can lead to the formation of mutations, which are irreversible and perturb tissue homeostasis in the nervous system by essentially promoting the formation of mosaics. Occasionally, mutations could occur in DNA repair factors (such as FUS, see text) and this can manifest in profound neurodegeneration (red arrow). (Middle) In contrast, although reversible, the accumulation of unrepaired lesions due to decreased DNA repair activities can block the transcription of genes encoding for critical neural functions and downregulate their activity, leading to cognitive decline. (Right) DNA damage also affects the epigenetic landscape. DNA damage-induced epigenetic changes can accrue over time as “epimutations” and affect gene expression. In addition, the redistribution of epigenetic modulators, such as SIRT1, can trigger global changes to the chromatin architecture, leading to large-scale transcriptional deregulation of their normally repressed targets, such as major satellite repetitive DNA.