Genomic instability refers to the increased tendency for alterations in the genome, including mutations, chromosomal aberrations, DNA damage accumulation, and telomere dysfunction. It plays a critical role in aging and neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS)[@cgassting][@synaptic]. The accumulation of DNA damage in post-mitotic neurons over decades contributes to neuronal dysfunction and cell death, as neurons have limited capacity for DNA repair compared to proliferating cells[@mitochondrial].
The aging brain exhibits progressive decline in DNA repair capacity, making neurons increasingly vulnerable to genotoxic stress. This vulnerability is compounded by high metabolic rates, mitochondrial dysfunction, and chronic neuroinflammation, all of which generate reactive oxygen species (ROS) that damage DNA[@targeting].
```mermaid
flowchart TD
subgraph Sources["Sources of DNA Damage"]
A["Reactive Oxygen Species<br/>Mitochondrial Dysfunction"]
B["Replication Stress"]
C["Environmental Toxins<br/>MPTP, Rotenone, 6-OHDA"]
D["Ionizing Radiation<br/>Oxidative Base Damage"]
E["mtDNA Mutations<br/>and Deletions"]
F["DNA Repair Deficiency<br/>BER, NER, HR impairment"]
end
A --> G["Genomic Instability"]
B --> G
C --> G
D --> G
E --> G
F --> G
Genomic instability refers to the increased tendency for alterations in the genome, including mutations, chromosomal aberrations, DNA damage accumulation, and telomere dysfunction. It plays a critical role in aging and neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS)[@cgassting][@synaptic]. The accumulation of DNA damage in post-mitotic neurons over decades contributes to neuronal dysfunction and cell death, as neurons have limited capacity for DNA repair compared to proliferating cells[@mitochondrial].
The aging brain exhibits progressive decline in DNA repair capacity, making neurons increasingly vulnerable to genotoxic stress. This vulnerability is compounded by high metabolic rates, mitochondrial dysfunction, and chronic neuroinflammation, all of which generate reactive oxygen species (ROS) that damage DNA[@targeting].
Neurons rely on sophisticated DNA damage response (DDR) pathways to maintain genomic integrity. The ataxia-telangiectasia mutated (ATM) kinase is a central regulator of the DDR, activated by double-strand breaks (DSBs) and coordinating repair through phosphorylation of downstream targets including p53, CHK2, and H2AX[@erythropoietin][@ref].
The poly(ADP-ribose) polymerase (PARP) family of enzymes detects and responds to single-strand breaks (SSBs). Upon DNA damage, PARP1 auto-poly(ADP-ribosyl)ates and recruits repair proteins to damage sites. However, overactivation of PARP1 can lead to NAD+ depletion and energy crisis, contributing to neuronal death in stroke and neurodegenerative diseases[^7].
BER is the primary pathway for repairing small, non-helix-distorting DNA lesions including oxidized bases, alkylated bases, and abasic sites. The pathway involves:
In AD and PD, BER capacity is significantly impaired, leading to accumulation of 8-oxoguanine (8-oxoG), a mutagenic base lesion that causes G→T transversions during replication[^8].
NER removes bulky, helix-distorting DNA lesions including UV-induced photoproducts and chemical adducts. Two subpathways exist:
DSBs are the most cytotoxic DNA lesions. Two major repair pathways:
AD brains exhibit significant DNA damage accumulation, particularly in vulnerable regions like the hippocampus and entorhinal cortex. Key findings include[@robison1993][@mullaart1990]:
PD is uniquely associated with mitochondrial DNA deletions and complex I deficiency. Genomic instability in PD involves[@bender2006][@zhang2020]:
HD exhibits CAG trinucleotide repeat instability in the HTT gene, with repeat expansion in somatic tissues correlating with disease progression. DNA repair mechanisms involved include[@liu2018][@monahan2018]:
ALS demonstrates both nuclear and mitochondrial genomic instability:
Several approaches aim to enhance DNA repair capacity in the aging and diseased brain[@saretzki2020][@weissman2019]:
| Approach | Target | Status | Examples |
|----------|-------|--------|----------|
| PARP inhibitors | PARP1/2 overactivation | Clinical trials | Olaparib, niraparib |
| Small molecule BER activators | OGG1, APE1 | Preclinical | OGG1 inhibitors |
| ATM/ATR kinase inhibitors | Checkpoint activation | Preclinical | KU-55933, VE-821 |
| p53 stabilizers | Apoptosis prevention | Preclinical | Pifithrin-α |
Reducing oxidative DNA damage through antioxidants remains a therapeutic approach:
Emerging gene therapy strategies include:
DNA damage biomarkers with potential clinical utility include[@poljak2021]:
This section highlights recent publications relevant to this mechanism.
[@robison1993]: [Robison SH, et al. DNA damage in Alzheimer's disease brain. Ann Neurol. 1993](https://pubmed.ncbi.nlm.nih.gov/8215224/)
[@mullaart1990]: [Mullaart E, et al. Reduced capacity for DNA repair in Alzheimer's disease. Gerontology. 1990](https://pubmed.ncbi.nlm.nih.gov/2100256/)
[@bender2006]: [Bender A, et al. High levels of mitochondrial DNA deletions in substantia nigra neurons in aging and Parkinson disease. Nat Genet. 2006](https://doi.org/10.1038/ng1808)
[@zhang2020]: [Zhang J, et al. Mitochondrial DNA mutations in Parkinson's disease. J Neurosci Res. 2020](https://doi.org/10.1002/jnr.24547)
[@liu2018]: [Liu GH, et al. DNA damage in Huntington's disease. Nat Rev Neurol. 2018](https://doi.org/10.1038/s41582-018-0067-y)
[@monahan2018]: [Monahan K, et al. CAG repeat instability in Huntington's disease. Hum Mol Genet. 2018](https://doi.org/10.1093/hmg/ddy224)
[@coppola2022]: [Coppola G, et al. ALS genes: pathway to therapeutic targets. Nat Rev Neurol. 2022](https://doi.org/10.1038/s41582-022-00655-4)
[@saretzki2020]: [Saretzki G. DNA damage repair in the aging brain. Ageing Res Rev. 2020](https://doi.org/10.1016/j.arr.2020.101055)
[@weissman2019]: [Weissman L, et al. DNA repair in neurodegenerative diseases. Prog Mol Biol Transl Sci. 2019](https://doi.org/10.1016/bs.pmbts.2019.03.006)
[@poljak2021]: [Poljak M, et al. DNA damage as a biomarker of neurodegeneration. Free Radic Biol Med. 2021](https://doi.org/10.1016/j.freeradbiomed.2021.05.016)