Uracil-DNA glycosylase (UNG) is a critical DNA repair enzyme that protects the genome from uracil incorporation and deamination. The UNG gene encodes the primary enzyme responsible for removing uracil residues from DNA, initiating the base excision repair (BER) pathway. This page covers the gene structure, protein function, and its specific roles in neurodegenerative diseases including Alzheimer's Disease and Parkinson's Disease.
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{{- infobox
| name = UNG
| image =
| caption = UNG DNA repair enzyme
| gene_symbol = UNG
| gene_name = Uracil-DNA glycosylase
| chromosome = 12
| locus = 12q24.1
| ncbi_gene_id = 7306
| omim_id = 191525
| ensembl_id = ENSG00000166200
| uniprot_id = P13051
| encoded_protein = UNG Protein
}}
The UNG gene (Uracil-DNA glycosylase) encodes a DNA repair enzyme that removes uracil residues from DNA. This gene is crucial for maintaining genome integrity through the base excision repair (BER) pathway. UNG deficiency has been implicated in neurodegenerative diseases due to accumulated DNA damage. The enzyme acts as a front-line defense against the most common form of DNA damage—uracil incorporation—and its activity is essential for preventing mutations that can lead to cellular dysfunction and death.
| Symbol | UNG |
|---|
| Full Name | Uracil-DNA Glycosylase |
|---|
| Chromosomal Location | 12q24.1 |
|---|
| NCBI Gene ID | [7306](https://www.ncbi.nlm.nih.gov/gene/7306) |
|---|
| OMIM | [191525](https://www.omim.org/entry/191525) |
|---|
| Ensembl | ENSG00000166200 |
|---|
| UniProt | [P13051](https://www.uniprot.org/uniprot/P13051) |
|---|
| Gene Family | Uracil-DNA glycosylase family, DNA glycosylase superfamily I |
|---|
¶ Protein Structure and Function
UNG belongs to the family of DNA glycosylases, enzymes that recognize and remove damaged bases from DNA. Specifically, it is a uracil-DNA glycosylase that catalyzes the hydrolysis of the N-glycosidic bond between the uracil base and the deoxyribose sugar, releasing free uracil and creating an abasic site (AP site) in the DNA backbone. This reaction initiates the base excision repair (BER) pathway.
The UNG protein contains several key structural elements:
- N-terminal domain: Contains the catalytic residues and DNA-binding interface
- Active site: Conserved motifs involved in uracil recognition and catalytic activity
- DNA-binding groove: Shaped to accommodate both single-stranded and double-stranded DNA
- Lesion recognition loop: Flexible region that scans DNA for uracil residues
The crystal structure of UNG reveals a base-flipping mechanism where the uracil base rotates out of the DNA helix into the enzyme's active site, enabling catalytic cleavage. This mechanism is conserved across uracil-DNA glycosylases from bacteria to humans.
UNG catalyzes the removal of uracil through a base-catalyzed hydrolysis reaction:
- Uracil recognition: The enzyme scans DNA and detects uracil by hydrogen bonding patterns that differ from normal bases
- Base flipping: The uracil base rotates out of the helix into the active site
- Glycosidic bond cleavage: Nucleophilic attack by a water molecule, activated by a catalytic cysteine residue
- AP site creation: The resulting abasic site is handed off to AP endonuclease (APE1) for subsequent processing
UNG efficiently removes:
- Deaminated cytosine (U): The most common substrate, arising from spontaneous cytosine deamination to uracil
- Incorporated uracil: dUTP misincorporated during DNA replication
- Uracil in various sequence contexts: Active on single-stranded and double-stranded DNA
The base excision repair (BER) pathway is the primary mechanism for repairing small, non-helix-distorting DNA lesions. UNG initiates this pathway as the damage-specific enzyme that recognizes and removes uracil. The complete BER process involves:
- Damage recognition: UNG identifies uracil in DNA
- Base removal: UNG cleaves the N-glycosidic bond, releasing uracil
- AP site processing: AP endonuclease (APE1) cleaves the phosphodiester backbone 5' to the AP site
- DNA strand scission: DNA polymerase β removes the deoxyribose phosphate
- DNA synthesis: DNA polymerase β fills in the correct nucleotide
- DNA ligation: DNA ligase seals the nick
UNG function extends to both nuclear and mitochondrial DNA compartments:
- Nuclear UNG: Maintains genomic integrity during replication and transcription
- Mitochondrial UNG (UNG2): Variant isoform with mitochondrial targeting sequence, crucial for repairing mitochondrial DNA (mtDNA) that is particularly vulnerable to oxidative damage
The mitochondrial isoform is essential for preventing mtDNA mutation accumulation, which is especially relevant in energy-demanding tissues like the brain.
When UNG activity is reduced, backup DNA glycosylases can partially compensate:
- SMUG1: Single-strand-selective monofunctional uracil-DNA glycosylase
- TDG: Thymine-DNA glycosylase, removes uracil and other base lesions
- MBD4: Methyl-CpG binding domain protein 4
These enzymes provide redundancy in uracil removal but have different substrate preferences and tissue distributions.
UNG exhibits broad tissue expression with notable patterns:
- High expression: Lymphoid tissues (spleen, thymus, bone marrow), testis, gastrointestinal tract
- Moderate expression: Liver, kidney, lung, brain
- Cell-type specific: Higher activity in proliferating cells
In the brain, UNG is expressed in:
UNG expression in the brain varies by region:
- Cerebral cortex: Moderate expression in pyramidal neurons
- Hippocampus: High expression in CA1-CA3 regions and dentate gyrus
- Cerebellum: Purkinje cells show distinctive UNG activity
- Substantia nigra: Dopaminergic neurons express UNG, relevant to Parkinson's Disease
- Basal ganglia: Variable expression patterns
UNG plays a significant role in Alzheimer's Disease pathogenesis through multiple mechanisms:
- DNA damage accumulation: Impaired UNG activity leads to increased uracil accumulation in neuronal DNA
- Oxidative stress: Amyloid-beta (Aβ) and tau pathology generate oxidative stress that overwhelms DNA repair capacity
- Neuronal vulnerability: Post-mitotic neurons cannot dilute DNA damage through cell division, making efficient repair essential
- Epigenetic alterations: UNG deficiency may affect DNA methylation patterns through altered dUTP/dTTP ratios
- Mitochondrial dysfunction: Reduced UNG activity in mitochondria contributes to mtDNA mutation accumulation
Evidence from studies:
- Reduced UNG activity in AD brain tissue
- Elevated uracil levels in AD neuronal DNA
- Correlation between DNA repair capacity and cognitive decline
UNG dysfunction contributes to Parkinson's Disease through:
- Mitochondrial DNA damage: Dopaminergic neurons are particularly vulnerable to mtDNA damage due to high oxidative stress
- Impaired BER capacity: Age-related decline in UNG activity accelerates neuronal loss
- Alpha-synuclein interaction: Alpha-synuclein aggregation may impair DNA repair machinery including UNG
- Environmental toxins: MPTP and other PD-inducing toxins generate oxidative DNA damage that overwhelms repair systems
Key mechanisms:
- Reduced UNG expression in substantia nigra neurons
- Accumulation of mtDNA mutations in PD patients
- Correlation between DNA repair capacity and disease progression
¶ Aging and Cognitive Decline
UNG function declines with age:
- Decreased UNG expression and activity in aged brain
- Accumulation of uracil in genomic and mitochondrial DNA
- Reduced capacity for DNA repair in neurons
- Contributes to age-related cognitive decline and neurodegeneration
UNG mutations or deficiency increase cancer susceptibility:
- Hyper-IgM syndrome: UNG deficiency in humans leads to immunodeficiency with increased cancer risk
- Lymphomas and leukemias: Accumulated mutations drive malignant transformation
- Solid tumors: Various carcinomas show UNG dysregulation
UNG interacts with multiple proteins in the DNA repair machinery:
| Partner |
Interaction Type |
Function |
| APE1 |
Direct binding |
AP site processing in BER |
| DNA Polymerase β |
Direct binding |
Gap-filling DNA synthesis |
| XRCC1 |
Direct binding |
Scaffold protein in BER |
| DNA Ligase III |
Direct binding |
Nick sealing |
| PNKP |
Direct binding |
5'-phosphate processing |
| SMUG1 |
Functional redundancy |
Backup uracil removal |
Strategies to improve UNG function in neurodegeneration:
- Gene therapy: Viral vector delivery of UNG to neurons
- Small molecule activators: Compounds that enhance UNG activity
- Substrate analogs: dUTPase inhibitors to reduce uracil incorporation
- Antioxidant therapy: Reduce oxidative DNA damage burden
- DNA repair enhancement: Combine UNG modulation with other BER components
- Mitochondrial targeting: Mitochondria-specific UNG delivery
- BBB penetration: Achieving therapeutic concentrations in the brain
- Selectivity: Avoiding effects on rapidly dividing cells
- Timing: Intervention likely needs to occur early in disease course
- UNG Protein - Protein product
- APE1 - AP endonuclease 1, next step in BER
- PARP1 - Poly(ADP-ribose) polymerase 1, DNA damage sensor
- XRCC1 - DNA repair scaffold protein