GO:0006287 base-excision repair, gap-filling: Mechanism, Genes, Functions and Research Methods
Research-grade guide for scientists and biopharma professionals
Key Takeaways
• GO:0006287 (base-excision repair, gap-filling) is the biological process in which the damaged DNA strand is repaired by the combined action of an AP endonuclease that removes a few bases and a DNA polymerase that synthesizes a short patch in the 5' to 3' direction using the undamaged strand as template.
• DNA polymerase beta (POLB) is the primary gap-filling enzyme in mammalian base excision repair (BER), but other polymerases can substitute under certain conditions.
• Gap-filling is a critical step in BER, ensuring that the single-nucleotide or short-patch repair is completed before DNA ligation seals the nick.
• Defects in gap-filling can lead to accumulation of DNA breaks, genomic instability, and are associated with cancer and neurodegenerative diseases.
• Active DNA demethylation involves BER gap-filling to replace oxidized methylcytosine derivatives, linking this process to epigenetic regulation and cell fate specification.
• Experimental models for studying gap-filling include knockout cell lines, point mutants of POLB, and biochemical reconstitution assays using purified proteins [1,7,8].
Description
Base-excision repair (BER) is a major DNA repair pathway that removes small, non-helix-distorting base lesions caused by oxidation, alkylation, deamination, or hydrolysis. Within BER, the gap-filling step (GO:0006287) is essential for restoring the integrity of the DNA strand after the damaged base has been excised. This process involves the coordinated action of an apurinic/apyrimidinic (AP) endonuclease, which incises the DNA backbone and removes a few nucleotides, and a DNA polymerase that fills the resulting gap by synthesizing a complementary patch using the undamaged strand as a template. The gap-filling step is highly conserved from bacteria to humans and is critical for maintaining genomic stability. In mammalian cells, DNA polymerase beta (POLB) is the main enzyme responsible for gap-filling during single-nucleotide BER, but other polymerases such as POLG, POLK, and POLN can also participate under certain contexts. The choice of polymerase and the size of the repair patch (short-patch vs. long-patch) depend on the type of lesion and cellular conditions. Defects in gap-filling can result in persistent DNA breaks, mutagenesis, and cell death, highlighting its importance in cancer, aging, and neurological disorders. Recent studies have expanded the role of BER gap-filling beyond canonical DNA repair. For example, active DNA demethylation requires BER gap-filling to replace 5-methylcytosine derivatives, thereby influencing gene expression and cell fate decisions. Additionally, viral and bacterial systems have provided insights into the evolutionary conservation of this process. Understanding the molecular mechanisms of gap-filling is therefore crucial for developing therapeutic strategies that target DNA repair in diseases such as cancer.
base-excision repair, gap-filling At A Glance
| GO ID | GO:0006287 |
|---|---|
| GO term | base-excision repair, gap-filling |
| Ontology | biological_process |
| Synonym | none |
| Major function | Fills the gap left after excision of a damaged base by a DNA polymerase using the undamaged strand as template |
| Parent term | base-excision repair (GO:0006284) |
| Related enzymes | AP endonuclease (APEX1), DNA polymerase beta (POLB), DNA ligase III (LIG3) |
| Pathway context | Short-patch and long-patch base excision repair |
| Conservation | Highly conserved from bacteria to humans |
What Is GO:0006287?
GO:0006287, base-excision repair, gap-filling, is defined as the repair of the damaged DNA strand by the combined action of an apurinic endonuclease that degrades a few bases on the damaged strand and a polymerase that synthesizes a 'patch' in the 5' to 3' direction, using the undamaged strand as a template. This process is a sub-step of base-excision repair and ensures that the DNA strand is restored after removal of a damaged base.
Why Is base-excision repair, gap-filling Important in Cell Biology?
Gap-filling is a critical step in base-excision repair because it restores the correct DNA sequence after removal of a damaged base. Without efficient gap-filling, single-strand breaks persist, leading to replication fork collapse, double-strand breaks, and genomic instability. This process is essential for maintaining cellular homeostasis and preventing mutations that can drive cancer and other diseases. Moreover, gap-filling is involved in active DNA demethylation, linking DNA repair to epigenetic regulation and cell fate specification. Therefore, understanding the mechanisms and regulation of gap-filling has broad implications for cancer biology, neurobiology, and aging research.
• Maintains genomic stability by completing base-excision repair after damaged base removal.
• Prevents accumulation of cytotoxic and mutagenic single-strand breaks.
• Plays a role in active DNA demethylation and epigenetic reprogramming.
• Defects in gap-filling are associated with cancer predisposition and neurodegeneration.
• DNA polymerase beta (POLB) is a key gap-filling enzyme and a potential drug target in cancer therapy.
• Gap-filling mechanisms are conserved across species, facilitating model organism studies.
• Biochemical reconstitution assays allow detailed mechanistic studies of gap-filling.
• Environmental toxicants can compromise gap-filling and ligation steps, leading to repair failure.
What Happens During base-excision repair, gap-filling?
Excision of the damaged base and AP site formation
In simple terms: First, the damaged base is removed, leaving an empty spot called an AP site.
Base-excision repair is initiated by a DNA glycosylase that recognizes and removes a damaged base, creating an apurinic/apyrimidinic (AP) site. This step is lesion-specific and involves glycosylases such as OGG1 for oxidized guanine or UNG for uracil. The resulting AP site is a substrate for the next step.
AP endonuclease incision and gap generation
In simple terms: An enzyme cuts the DNA backbone at the AP site and removes a few bases, creating a gap.
AP endonuclease (APEX1 in humans) incises the DNA backbone 5' to the AP site, generating a 3'-hydroxyl and a 5'-deoxyribose phosphate (dRP) terminus. In short-patch BER, the dRP group is removed by the lyase activity of DNA polymerase beta, leaving a single-nucleotide gap. In long-patch BER, additional nucleotides are displaced, creating a longer gap.
Gap-filling by DNA polymerase
In simple terms: A DNA polymerase fills the gap by adding the correct nucleotides using the undamaged strand as a guide.
DNA polymerase beta (POLB) is the primary gap-filling enzyme in mammalian short-patch BER. It adds the missing nucleotide(s) in the 5' to 3' direction, using the complementary strand as a template. POLB also possesses dRP lyase activity, removing the 5'-dRP group. In long-patch BER, replicative polymerases such as POL delta/epsilon or POL beta may perform strand displacement synthesis. Other polymerases like POL lambda, POL iota, and POL kappa can also participate in gap-filling under specific conditions.
DNA ligation and completion of repair
In simple terms: Finally, an enzyme seals the nick to restore the continuous DNA strand.
After gap-filling, the remaining nick is sealed by DNA ligase III (LIG3) in complex with XRCC1 for short-patch BER, or by DNA ligase I (LIG1) for long-patch BER. This step completes the repair process, restoring the original DNA sequence. Defects in ligation can lead to persistent breaks and genomic instability.
Coordination and channeling in BER
In simple terms: The enzymes work together in a coordinated way to avoid releasing dangerous intermediates.
Biochemical studies have shown that BER proteins can channel substrates, meaning that the product of one step is directly passed to the next enzyme without diffusing away. This channeling is particularly important for gap-filling and ligation, as it prevents the accumulation of toxic intermediates like single-strand breaks. POLB and LIG3 interact physically, facilitating efficient repair.
Key Genes Involved in GO:0006287 base-excision repair, gap-filling
The following genes and proteins are key players in base-excision repair gap-filling, as supported by published literature.
| Gene | Major Role | Research Relevance |
|---|---|---|
| POLB | DNA polymerase beta; main gap-filling enzyme in short-patch BER | Knockout causes sensitivity to alkylating agents; point mutants affect catalytic activity |
| APEX1 | AP endonuclease; incises AP sites and generates gap | Knockout is embryonic lethal; regulates BER initiation |
| LIG3 | DNA ligase III; seals nick after gap-filling in short-patch BER | Defects lead to accumulation of breaks; interacts with XRCC1 |
| XRCC1 | Scaffold protein; coordinates BER enzymes including POLB and LIG3 | Mutations associated with cancer and neurological disorders |
| OGG1 | DNA glycosylase; removes oxidized guanine | Deficiency linked to cancer and aging |
| UNG | Uracil-DNA glycosylase; removes uracil from DNA | Knockout affects antibody diversity and DNA repair |
| MUTYH | DNA glycosylase; removes adenine opposite oxidized guanine | Mutations cause MUTYH-associated polyposis |
| NEIL1 | DNA glycosylase; removes oxidized pyrimidines | Deficiency leads to metabolic syndrome and cancer |
| PARP1 | Poly(ADP-ribose) polymerase; involved in BER regulation | Inhibitors used in cancer therapy; synthetic lethality with BRCA mutations |
| FEN1 | Flap endonuclease; involved in long-patch BER | Deficiency causes genomic instability |
| PCNA | Proliferating cell nuclear antigen; processivity factor for long-patch BER | Required for POL delta/epsilon-mediated gap-filling |
| POLG | Mitochondrial DNA polymerase; gap-filling in mitochondrial BER | Mutations cause mitochondrial diseases |
| POLK | DNA polymerase kappa; can perform gap-filling in specific contexts | Involved in lesion bypass and BER backup |
| POLN | DNA polymerase nu; gap-filling in alternative BER | May contribute to repair in specific tissues |
| POLI | DNA polymerase iota; gap-filling in alternative BER | Potential backup polymerase |
| POLK | DNA polymerase kappa; gap-filling in alternative BER | Potential backup polymerase |
| LIG1 | DNA ligase I; seals nick in long-patch BER | Deficiency causes immune deficiency and growth retardation |
| TDP1 | Tyrosyl-DNA phosphodiesterase 1; processes stalled topoisomerase I-DNA complexes | Defects cause spinocerebellar ataxia with axonal neuropathy |
How Is base-excision repair, gap-filling Regulated?
Base-excision repair gap-filling is regulated at multiple levels. The expression and activity of POLB can be modulated by post-translational modifications, including phosphorylation and ubiquitination. APEX1 activity is regulated by redox status and protein-protein interactions. PARP1 activation at DNA damage sites promotes the recruitment of BER factors, including POLB and LIG3. Additionally, the choice between short-patch and long-patch BER is influenced by the type of lesion, cell cycle stage, and availability of repair proteins. Environmental toxicants can impair gap-filling and ligation, leading to repair deficiency.
base-excision repair, gap-filling and Human Disease
| Gene | Disease / Biology | Potential Experimental Model |
|---|---|---|
| POLB | Cancer, alkylating agent sensitivity | POLB knockout cell lines, point mutants |
| MUTYH | MUTYH-associated polyposis (colorectal cancer) | MUTYH knockout mice, organoids |
| TDP1 | Spinocerebellar ataxia with axonal neuropathy | TDP1 knockout neurons, iPSC-derived models |
| XRCC1 | Cancer, neurological disorders | XRCC1 mutant cell lines, knock-in mice |
| APEX1 | Cancer, neurodegeneration | APEX1 knockdown/knockout cells |
Cancer and genomic instability
Defects in BER gap-filling can lead to accumulation of mutations and genomic instability, which are hallmarks of cancer. For example, mutations in POLB have been found in various cancers, and reduced POLB activity is associated with increased sensitivity to alkylating agents. MUTYH mutations cause MUTYH-associated polyposis, a hereditary colorectal cancer syndrome. Additionally, PARP inhibitors exploit defects in BER to induce synthetic lethality in BRCA-mutated cancers.
Neurodegeneration
Neurons are particularly vulnerable to DNA damage due to high metabolic activity and limited proliferative capacity. Defects in BER gap-filling have been implicated in neurodegenerative diseases such as Alzheimer's disease and amyotrophic lateral sclerosis. For instance, mutations in TDP1 cause spinocerebellar ataxia with axonal neuropathy, and reduced BER capacity is observed in aging brains.
Epigenetic regulation and development
Active DNA demethylation involves BER gap-filling to replace oxidized 5-methylcytosine derivatives. This process is essential for cell fate specification and embryonic development. Disruption of gap-filling during demethylation can affect gene expression programs and lead to developmental abnormalities.
From base-excision repair, gap-filling-Related Genes to Experimental Models
| Research Question | Suitable Model |
|---|---|
| What is the role of POLB in gap-filling? | POLB knockout cell lines (e.g., HeLa, HEK293) |
| How do point mutations in POLB affect catalytic activity? | POLB point-mutant knock-in cells |
| Does gap-filling contribute to active DNA demethylation? | Knock-in of tagged POLB, APEX1; reporter assays |
| What is the impact of gap-filling defects on cancer therapy? | Overexpression of POLB in cancer cell lines; xenograft models |
| How is gap-filling coordinated with ligation? | Biochemical reconstitution with purified proteins |
| What is the evolutionary conservation of gap-filling? | Mimiviral BER reconstitution |
How to Study the base-excision repair, gap-filling Process
| Method | What It Measures | Typical Application |
|---|---|---|
| Biochemical reconstitution | Gap-filling activity, dRP lyase activity, ligation | Mechanistic studies of POLB and APEX1 |
| Comet assay | DNA single-strand breaks | Assessing BER capacity in cells |
| RNA-seq | Gene expression changes | Transcriptional response to DNA damage |
| Whole-genome sequencing | Mutational signatures | Identifying BER deficiency in tumors |
| CRISPR knockout | Loss-of-function phenotypes | Determining gene essentiality in BER |
| CRISPR knock-in | Tagged protein localization | Studying protein interactions in gap-filling |
| Cell extract assay | BER activity in vitro | Monitoring BER in model organisms |
| Mass spectrometry | Protein-protein interactions | Identifying BER complex components |
Biochemical reconstitution assays
Biochemical reconstitution using purified BER proteins (e.g., POLB, APEX1, LIG3, XRCC1) allows detailed mechanistic studies of gap-filling. These assays can measure polymerase activity, dRP lyase activity, and ligation efficiency. They are also used to study channeling between enzymes.
Cell-based DNA repair assays
Cell extracts can be used to monitor BER gap-filling using defined DNA substrates containing AP sites or single-strand breaks. For example, Chlamydomonas reinhardtii cell extracts have been used to monitor BER. Comet assays and alkaline unwinding can measure overall BER capacity.
CRISPR-based knockout and knock-in models
CRISPR/Cas9 can generate knockout cell lines for POLB, APEX1, LIG3, and other BER genes to study their roles in gap-filling. Point mutations can be introduced to dissect catalytic domains. Knock-in of tagged proteins allows localization and interaction studies.
Next-generation sequencing and bioinformatics
RNA-seq can measure expression changes in BER genes upon DNA damage. Whole-genome sequencing can identify mutations arising from gap-filling defects. Bioinformatics tools can analyze mutational signatures associated with BER deficiency.
How CRISPR Can Be Used to Study GO:0006287 base-excision repair, gap-filling
Knockout
CRISPR knockout of POLB, APEX1, or LIG3 can reveal their essential roles in gap-filling. For example, POLB knockout cells are viable but sensitive to alkylating agents, indicating that backup polymerases can partially compensate. Knockout of APEX1 is embryonic lethal in mice, underscoring its critical function.
Point Mutation
Point mutations in the catalytic domains of POLB (e.g., D256A, E295K) can abolish gap-filling activity without affecting protein stability. These mutants are valuable for dissecting the enzymatic mechanism and for studying the consequences of specific repair defects.
Knock-in
Knock-in of tagged POLB (e.g., GFP or FLAG) allows live-cell imaging and immunoprecipitation to study its dynamics and interactions during gap-filling. Knock-in of disease-associated mutations can model human disorders.
Overexpression
Overexpression of POLB or other gap-filling enzymes can be used to study their effects on DNA repair capacity, mutagenesis, and drug sensitivity. For example, overexpression of POLB in cancer cells may increase resistance to alkylating agents.
How EDITGENE Supports base-excision repair, gap-filling Research
Researchers studying base-excision repair, gap-filling-related genes often need to determine whether a candidate gene is causally involved in the repair process, how specific mutations affect enzyme activity, and whether targeting these genes can sensitize cancer cells to therapy. EDITGENE provides comprehensive CRISPR-based services to address these questions.
Contact EDITGENE today to design your custom CRISPR model for base-excision repair, gap-filling research.
Frequently Asked Questions About base-excision repair, gap-filling
What is base-excision repair gap-filling?
Base-excision repair gap-filling (GO:0006287) is the step in BER where an AP endonuclease removes a few bases and a DNA polymerase fills the gap using the undamaged strand as a template.
What genes are involved in base-excision repair gap-filling?
Key genes include POLB, APEX1, LIG3, XRCC1, and various DNA glycosylases [1,6].
Which enzyme is the main gap-filling polymerase in humans?
DNA polymerase beta (POLB) is the primary gap-filling enzyme in short-patch BER.
How does gap-filling differ between short-patch and long-patch BER?
Short-patch BER involves a single nucleotide gap filled by POLB, while long-patch BER involves 2-10 nucleotides and uses replicative polymerases.
What diseases are associated with defects in gap-filling?
Defects are linked to cancer, neurodegeneration, and developmental disorders.
Can CRISPR be used to study gap-filling?
Yes, CRISPR knockout, knock-in, and point mutation models are widely used to study gap-filling genes.
What is the role of POLB in gap-filling?
POLB fills single-nucleotide gaps and removes 5'-dRP groups during short-patch BER.
How is gap-filling regulated?
It is regulated by protein-protein interactions, post-translational modifications, and cellular signaling.
What methods are used to measure gap-filling activity?
Biochemical reconstitution, comet assays, and cell extract assays are commonly used [5,7].
Why is gap-filling important for epigenetic regulation?
Gap-filling is required for active DNA demethylation, which affects gene expression and cell fate.
Conclusion
Base-excision repair gap-filling (GO:0006287) is a fundamental biological process that ensures the completion of DNA repair after damaged base removal. Its mechanisms are highly conserved and involve a coordinated interplay between AP endonucleases, DNA polymerases, and ligases. Defects in gap-filling contribute to cancer, neurodegeneration, and developmental disorders, making it a critical area of research. Advances in CRISPR-based models and biochemical assays continue to unravel the complexities of this process, offering potential therapeutic targets for a range of diseases.
References
- 1. Beard WA et al.. 2019. DNA polymerase beta and other gap-filling enzymes in mammalian base excision repair.. Enzymes 45:1-26 PMID: 31627875
- 3. Wang D et al.. 2022. Active DNA demethylation promotes cell fate specification and the DNA damage response.. Science 378(6623):983-989 PMID: 36454826
- 4. Çağlayan M et al.. 2015. Reprint of "Oxidant and environmental toxicant-induced effects compromise DNA ligation during base excision DNA repair".. DNA Repair (Amst) 36:86-90 PMID: 26596511
- 5. Morales-Ruiz T et al.. 2018. Monitoring base excision repair in Chlamydomonas reinhardtii cell extracts.. DNA Repair (Amst) 65:34-41 PMID: 29547780
- 6. Kim YJ et al.. 2012. Overview of base excision repair biochemistry.. Curr Mol Pharmacol 5(1):3-13 PMID: 22122461
- 7. Çağlayan M. 2020. Pol β gap filling, DNA ligation and substrate-product channeling during base excision repair opposite oxidized 5-methylcytosine modifications.. DNA Repair (Amst) 95:102945 PMID: 32853828
- 8. Lad SB et al.. 2023. Biochemical Reconstitution of the Mimiviral Base Excision Repair Pathway.. J Mol Biol 435(17):168188 PMID: 37380013
