| RAD18 Gene | |
|---|---|
| RAD18 E3 Ubiquitin Protein Ligase | |
| Gene Symbol | RAD18 |
| Full Name | RAD18 E3 Ubiquitin Protein Ligase |
| Chromosomal Location | 9q31.1 |
| NCBI Gene ID | [56852](https://www.ncbi.nlm.nih.gov/gene/56852) |
| OMIM | [605360](https://www.omim.org/entry/605360) |
| Ensembl ID | [ENSG00000028203](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000028203) |
| UniProt ID | [Q9NS91](https://www.uniprot.org/uniprotkb/Q9NS91/entry) |
| Protein Class | E3 Ubiquitin Ligase |
| Pathway | [DNA Damage Response](/mechanisms/dna-repair) |
RAD18 (RAD18 E3 Ubiquitin Protein Ligase) is a key DNA damage response protein that plays a critical role in maintaining genome stability through translesion DNA synthesis (TLS). The gene encodes an E3 ubiquitin ligase that partners with RAD6A and RAD6B to catalyze the monoubiquitination of PCNA, a process essential for lesion bypass during DNA replication[1]. This function is particularly important in post-mitotic neurons, which cannot rely on homologous recombination for DNA repair due to their non-dividing state[2].
The RAD18-RAD6 complex represents a fundamental mechanism by which cells tolerate DNA lesions that would otherwise block replication forks. In neurons, where DNA damage accumulates over a lifetime due to oxidative metabolism, environmental exposures, and normal cellular processes, the proper functioning of this pathway is crucial for maintaining genomic integrity and preventing neurodegeneration[3].
The RAD18 protein contains several key structural features that enable its function in DNA damage tolerance:
The crystal structure of the RAD18-RAD6 complex has revealed that RAD18 serves as a scaffold that positions RAD6 for optimal ubiquitin transfer to PCNA lysine residues[4:1]. This structural arrangement allows for efficient monoubiquitination of PCNA at stalled replication forks.
The primary function of RAD18 in translesion DNA synthesis involves:
This polymerase switch mechanism allows cells to continue DNA replication despite encountering damaged bases, preventing replication fork collapse and double-strand break formation.
Beyond translesion synthesis, RAD18 also participates in homologous recombination repair pathways. The protein has been shown to:
RAD18 interacts with and is regulated by several key DNA damage response proteins:
RAD18 expression is highest in:
In neurons, RAD18 localizes to:
RAD18 and the translesion DNA synthesis pathway have emerging roles in Alzheimer's disease pathogenesis:
DNA damage accumulation: Alzheimer's disease brains show evidence of increased DNA damage, including strand breaks, oxidized bases, and telomere attrition[8]. The RAD18-mediated TLS pathway may be overwhelmed in neurons attempting to deal with this damage load.
Amyloid-beta toxicity: Studies suggest that amyloid-beta peptides can induce DNA damage in neurons, potentially activating RAD18-mediated repair pathways. However, chronic activation may lead to pathway exhaustion and neuronal death[3:1].
Tau pathology: Hyperphosphorylated tau protein, a key component of neurofibrillary tangles, has been shown to sequester DNA repair proteins, potentially compromising RAD18 function in affected neurons.
In Parkinson's disease, RAD18 may play important roles:
Oxidative stress: The substantia nigra dopaminergic neurons are particularly susceptible to oxidative damage due to dopamine metabolism and mitochondrial dysfunction. This creates high demand for DNA repair pathways including RAD18-mediated TLS.
Alpha-synuclein toxicity: Alpha-synuclein aggregation, the hallmark of Lewy bodies, may interfere with DNA repair machinery. Studies suggest that alpha-synuclein can interact with DNA repair proteins and potentially impair RAD18 function.
Mitochondrial DNA damage: While RAD18 primarily acts on nuclear DNA, there is emerging evidence for mitochondrial TLS pathways that may involve RAD18 homologs.
In ALS, DNA repair deficits contribute to motor neuron degeneration:
Oxidative DNA damage: Motor neurons face high metabolic demands and are exposed to reactive oxygen species, requiring robust DNA damage tolerance mechanisms.
C9orf72 toxicity: The hexanucleotide repeat expansion in C9orf72, a common genetic cause of familial ALS, may lead to R-loop formation and increased replication stress, potentially overwhelming RAD18-mediated repair[9].
Ataxia-telangiectasia: While not directly involving RAD18, the ATM deficiency in this disorder highlights the critical importance of DNA repair in neuronal survival.
Xeroderma pigmentosum: Patients with XP have defects in nucleotide excision repair and show extreme neurodegeneration, underscoring how DNA repair defects lead to neuronal loss.
Mouse models lacking functional Rad18 show:
Cell culture studies have demonstrated:
The RAD18 pathway represents a potential therapeutic target for neurodegenerative diseases:
Small molecule activators: Compounds that enhance RAD18 activity or PCNA ubiquitination could improve DNA damage tolerance in neurons
Gene therapy: Viral vector delivery of RAD18 to increase expression in vulnerable neuronal populations
Combination approaches: Enhancing RAD18 function alongside other DNA repair pathways (BER, NER) may provide synergistic benefits
RAD18 expression and activity levels may serve as:
Kunkel TA, et al. Eukaryotic translesion polymerases and their roles and regulation in DNA damage tolerance. DNA Repair. 2009. ↩︎
Bijers TP, et al. DNA damage response and repair in neurodegeneration and aging. Advances in Experimental Medicine and Biology. 2019. ↩︎
Madhav K, et al. DNA repair deficits in Alzheimer's disease and related dementias. Acta Neuropathologica Communications. 2019. ↩︎ ↩︎
Takizawa Y, et al. Structures of the RAD18-RAD6 ring complex and implications for translesion DNA synthesis. Nature Structural & Molecular Biology. 2010. ↩︎ ↩︎
Sale JE. Translesion DNA polymerases. Cold Spring Harbor Perspectives in Biology. 2013. ↩︎
Yang Y, et al. Rad18 mediates DNA damage signaling and checkpoint activation. Molecular and Cellular Biology. 2010. ↩︎
Gupta R, et al. DNA damage tolerance and genome instability in neurons. Frontiers in Molecular Neuroscience. 2018. ↩︎
Kruman II, et al. DNA strand breaks, DNA repair deficits, and neuronal death in stroke and Alzheimer's disease. Cerebral Cortex. 2004. ↩︎
Kathuria A, et al. DNA damage signaling in neurodegenerative diseases: mechanisms and therapeutic potential. Neuropharmacology. 2018. ↩︎
Maison D, et al. Translesion DNA synthesis in the brain: maintaining genomic stability in neurons. DNA Repair. 2020. ↩︎