Base Excision Repair: Mechanisms, Steps, and Clinical Significance

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Introduction to Base Excision Repair

What is Base Excision Repair?

Base excision repair (BER) is the DNA repair pathway responsible for removing small, non-helix-distorting base lesions from the genome. These lesions arise from endogenous cellular processes such as oxidation, deamination, and alkylation, as well as from exogenous agents like ionizing radiation and certain chemotherapeutic drugs. BER operates on damaged bases that do not significantly bend or kink the DNA double helix, distinguishing it from nucleotide excision repair (NER), which handles bulky, helix-distorting adducts such as those caused by ultraviolet light. The pathway is initiated by a class of enzymes called DNA glycosylases that cleave the N-glycosidic bond between the damaged base and the deoxyribose sugar, creating an apurinic/apyrimidinic (AP) site. This AP site is then processed by downstream enzymes that remove the sugar-phosphate remnant, synthesize a short repair patch, and seal the nick.

The fundamental importance of BER is underscored by its conservation from bacteria to humans. In mammalian cells, BER is estimated to repair tens of thousands of lesions per cell per day, primarily oxidative damage such as 8-oxoguanine (8-oxoG) and deaminated bases like uracil. The pathway is constitutively active but can be upregulated in response to increased oxidative stress. Unlike homologous recombination or mismatch repair, BER does not require a sister chromatid or a template strand for strand discrimination; it operates on double-stranded DNA and uses the complementary strand as a template for repair synthesis.

Types of DNA Damage Handled by BER

BER substrates include a chemically diverse set of lesions. The most common are:

• Oxidized bases: 8-oxoguanine (8-oxoG), thymine glycol, and formamidopyrimidines (Fapy lesions)
• Deaminated bases: uracil (from cytosine deamination), hypoxanthine (from adenine deamination), and xanthine (from guanine deamination)
• Alkylated bases: 3-methyladenine, 7-methylguanine, and O⁶-methylguanine (though the latter is often repaired by direct reversal via MGMT)
• Base mismatches involving damaged bases: such as 8-oxoG:A pairs
• Abasic sites: AP sites themselves, which can arise spontaneously or as BER intermediates

The common feature of these lesions is that they are small, do not distort the DNA helix significantly, and are recognized by specific glycosylases that flip the damaged base out of the helix into their active site. This "base flipping" mechanism is a hallmark of BER initiation.

The BER Pathway: Step-by-Step

The BER pathway proceeds through five coordinated steps: damage recognition, base removal, AP site incision, gap filling, and ligation. These steps are tightly coupled, and in many cases the enzymes involved physically associate to form a "repairosome" complex that channels the intermediate products from one enzyme to the next without releasing free AP sites or nicked DNA into the cellular milieu.

Step 1: DNA Glycosylase Action

DNA glycosylases scan the genome for damaged bases by a process of facilitated diffusion along the DNA backbone. Upon encountering a lesion, the glycosylase bends the DNA and flips the damaged base out of the double helix into a deep pocket in the enzyme's active site. The enzyme then catalyzes hydrolysis of the N-glycosidic bond, releasing the free damaged base and leaving an AP site—a deoxyribose sugar lacking its base.

There are two functional classes of glycosylases:

1. Monofunctional glycosylases (e.g., UNG, MPG, MUTYH) possess only glycosylase activity. They cleave the glycosidic bond and dissociate, leaving an intact AP site for downstream processing by AP endonuclease 1 (APE1).

1. Bifunctional glycosylases (e.g., OGG1, NTHL1, NEIL1) possess both glycosylase activity and an associated AP lyase activity. After removing the damaged base, they cleave the phosphodiester backbone 3′ to the AP site via a β-elimination reaction, creating a single-strand break with a 3′ α,β-unsaturated aldehyde and a 5′ phosphate. Some NEIL family members perform β,δ-elimination, generating a 3′ phosphate.

The reaction typically proceeds at 37°C in physiological buffer conditions (approximately 50 mM Tris-HCl, pH 7.5, 50 mM KCl, 1 mM EDTA, 1 mM DTT). The glycosylase reaction is rapid, with turnover numbers ranging from 0.1 to 10 min⁻¹ depending on the enzyme and substrate.

Step 2: AP Endonuclease and AP Lyase

The AP site generated by monofunctional glycosylases is processed by APE1, the major human AP endonuclease. APE1 hydrolyzes the phosphodiester bond 5′ to the AP site, generating a single-strand break with a 3′ hydroxyl group and a 5′ deoxyribose phosphate (dRP) moiety. This 5′ dRP group must be removed before DNA polymerase can fill the gap.

For bifunctional glycosylases, the AP lyase activity already creates a strand break, but the 3′ end is blocked—either with a 3′ unsaturated aldehyde (from β-elimination) or a 3′ phosphate (from β,δ-elimination). APE1 also processes these blocked 3′ ends, removing the damaged sugar remnant and generating a clean 3′ hydroxyl group suitable for DNA synthesis.

The resulting single-strand break with a 5′ dRP and 3′ OH is the substrate for the next step. APE1 is a magnesium-dependent enzyme, requiring 1–10 mM MgCl₂ for optimal activity, and functions at physiological pH (7.4–7.6).

Step 3: Repair Synthesis and Ligation

DNA polymerase β (Pol β) is the primary polymerase in short-patch BER. It performs two functions:

1. dRP lyase activity: Pol β removes the 5′ dRP moiety using its N-terminal lyase domain, generating a clean 5′ phosphate.
2. DNA synthesis: Pol β adds one nucleotide (in short-patch BER) using the undamaged complementary strand as a template.

Pol β is a small (39 kDa) enzyme that fills gaps of 1–6 nucleotides. Its dRP lyase activity is rate-limiting and requires the 5′ dRP to be in a specific conformation. If the dRP is oxidized or otherwise modified, it becomes resistant to Pol β's lyase activity, and the repair pathway switches to long-patch BER (discussed below).

Finally, DNA ligase IIIα (LIG3) seals the nick. LIG3 forms a stable complex with XRCC1 (X-ray repair cross-complementing protein 1), a scaffold protein that coordinates the BER enzymes. The ligation reaction requires ATP (or NAD⁺ in bacteria) and magnesium, and proceeds through a covalent ligase-adenylate intermediate. The final product is a fully repaired DNA duplex with no residual damage.

Key Enzymes and Proteins in BER

DNA Glycosylases

The human genome encodes at least 11 DNA glycosylases, each with specificity for particular types of damage:

Each glycosylase recognizes its substrate through a combination of shape complementarity and hydrogen bonding in the active site pocket. For example, OGG1 has a narrow pocket that excludes undamaged guanine but accommodates the oxidized 8-oxoG lesion, while UNG has a highly selective pocket that excludes thymine and cytosine but accepts uracil.

AP Endonuclease 1 (APE1)

APE1 is a multifunctional protein. Its primary role in BER is the incision of AP sites, but it also functions as a redox regulator of transcription factors such as AP-1, NF-κB, and p53. The endonuclease activity of APE1 is essential for viability—knockout of APE1 in mice is embryonic lethal, and haploinsufficiency leads to increased mutation rates and sensitivity to oxidative stress.

APE1 recognizes AP sites by scanning for the absence of a base and the resulting conformational flexibility of the sugar. It kinks the DNA and inserts residues into the AP site to verify the absence of a base before catalyzing hydrolysis 5′ to the lesion. The enzyme has a strong preference for AP sites in double-stranded DNA and is inhibited by high concentrations of monovalent salts (>150 mM NaCl).

DNA Polymerase β and XRCC1

Pol β is the smallest eukaryotic DNA polymerase (39 kDa) and is dedicated to BER. It consists of two domains: an N-terminal lyase domain (8 kDa) and a C-terminal polymerase domain (31 kDa). The polymerase domain is a member of the X family of polymerases and is characterized by its ability to fill short gaps (1–6 nucleotides) with high fidelity.

XRCC1 is a scaffold protein that physically interacts with Pol β, LIG3, APE1, and PARP1 (poly(ADP-ribose) polymerase 1). It has no enzymatic activity but serves to organize the BER machinery and facilitate substrate channeling. XRCC1 also contains a BRCT domain that mediates protein-protein interactions and is important for the recruitment of BER enzymes to sites of damage. XRCC1 knockout mice are embryonic lethal, highlighting its essential role.

DNA Ligase III/XRCC1 Complex

DNA ligase IIIα exists in two isoforms: a nuclear form (LIG3α) that binds XRCC1, and a mitochondrial form (LIG3β) that lacks the XRCC1 interaction domain. The LIG3-XRCC1 complex is the major ligase for short-patch BER in the nucleus. LIG3 catalyzes the formation of a phosphodiester bond between the 3′ hydroxyl and 5′ phosphate at the nick, using ATP as an energy source.

The ligation reaction proceeds in three steps: (1) activation of the ligase by ATP, forming a covalent ligase-AMP intermediate; (2) transfer of AMP to the 5′ phosphate at the nick, activating it; and (3) nucleophilic attack by the 3′ hydroxyl, forming the phosphodiester bond and releasing AMP. The entire reaction requires magnesium ions and occurs at physiological pH.

Short-Patch vs. Long-Patch BER

BER operates through two sub-pathways that differ in the length of the repair patch synthesized and the enzymes involved.

Short-Patch BER (SP-BER)

Short-patch BER is the predominant pathway, accounting for approximately 80–90% of BER events. It involves the incorporation of a single nucleotide and requires only four enzymatic activities: a glycosylase, APE1, Pol β, and LIG3/XRCC1.

The pathway is initiated by a monofunctional glycosylase, followed by APE1 incision, Pol β dRP lyase removal of the 5′ dRP, Pol β nucleotide incorporation, and LIG3 sealing. The entire process is highly coordinated, with the enzymes forming a complex that channels intermediates. SP-BER is efficient for simple lesions where the 5′ dRP is unmodified and can be processed by Pol β's lyase activity.

Long-Patch BER (LP-BER)

Long-patch BER is used when the 5′ dRP is resistant to Pol β's lyase activity—for example, when it is oxidized or reduced. In this pathway, Pol β (or Pol δ/ε) synthesizes 2–13 nucleotides, displacing the damaged strand to create a flap structure. The flap is then cleaved by flap endonuclease 1 (FEN1), which recognizes the 5′ flap junction and cleaves it, releasing the displaced oligonucleotide.

LP-BER requires additional proteins, including PCNA (proliferating cell nuclear antigen), which acts as a processivity clamp for the polymerases, and replication factor C (RFC), which loads PCNA onto the DNA. The final ligation is performed by LIG1 (DNA ligase I), which is more processive than LIG3 and can seal longer gaps.

The choice between SP-BER and LP-BER is influenced by the cell cycle phase, with LP-BER favored during S phase when PCNA is abundant, and by the chemical nature of the lesion.

Regulation and Coordination with Other Pathways

Cell Cycle Regulation

BER activity is modulated throughout the cell cycle. In G1 phase, BER is primarily short-patch and operates at a basal level. During S phase, both SP-BER and LP-BER are active, with LP-BER becoming more prominent due to the availability of PCNA and replication-associated polymerases. In G2/M, BER activity is reduced, possibly to prevent interference with chromosome condensation and segregation.

The cell cycle regulation of BER is mediated by post-translational modifications. For example, APE1 is phosphorylated by cyclin-dependent kinases (CDKs), which enhances its endonuclease activity. Pol β is phosphorylated by protein kinase C (PKC), which increases its processivity. XRCC1 is phosphorylated by casein kinase 2 (CK2), which promotes its interaction with LIG3 and PARP1.

Interaction with Nucleotide Excision Repair and Mismatch Repair

BER does not operate in isolation. There is significant crosstalk between BER and other repair pathways, particularly nucleotide excision repair (NER) and mismatch repair (MMR). For a detailed comparison of these pathways, see Base Excision Repair vs Nucleotide and Base Excision Repair vs Mismatch.

• BER and NER: While BER handles small lesions and NER handles bulky adducts, there is overlap. Some oxidized lesions, such as cyclopurines, are repaired by transcription-coupled NER rather than BER. Conversely, BER can process certain NER substrates if they are small enough. The Nucleotide Excision Repair NER pathway is the primary defense against UV-induced photoproducts, but BER provides backup for oxidative damage that occurs as a secondary effect of UV exposure.

• BER and MMR: Mismatch repair corrects replication errors that escape proofreading, but it also plays a role in repairing oxidative damage. The MUTYH glycosylase, which removes adenine mispaired with 8-oxoG, is part of the BER pathway but is often considered a bridge between BER and MMR because the 8-oxoG:A mismatch resembles a replication error. The Mismatch Repair vs Excision Repair comparison highlights how these pathways cooperate to maintain genomic stability.

• BER and PARP1: PARP1 is a DNA damage sensor that binds to single-strand breaks and AP sites. Upon binding, PARP1 synthesizes poly(ADP-ribose) chains on itself and target proteins, which serves as a signal to recruit BER enzymes. PARP1 also directly interacts with XRCC1 and Pol β, facilitating their recruitment to damage sites. The Base Excision Repair Pathway is therefore intimately linked to PARP1 signaling.

Methods to Study Base Excision Repair

In Vitro BER Assays

In vitro assays use purified recombinant proteins or cell extracts to reconstitute BER on defined DNA substrates. The most common approach is the oligonucleotide-based assay, where a short duplex DNA (typically 30–50 bp) containing a single lesion is incubated with purified enzymes or cell extract.

A typical reaction contains:

• 10–50 nM lesion-containing duplex DNA
• 50–100 nM glycosylase (if using purified enzymes)
• 10–50 nM APE1
• 10–50 nM Pol β
• 10–50 nM LIG3/XRCC1
• Reaction buffer: 50 mM HEPES-KOH (pH 7.5), 50 mM KCl, 5 mM MgCl₂, 1 mM DTT, 0.1 mg/mL BSA, 1 mM ATP
• Incubation at 37°C for 15–60 minutes

Products are analyzed by denaturing polyacrylamide gel electrophoresis (PAGE), where the repaired product (full-length DNA) is separated from intermediates (nicked DNA, AP site-containing DNA). Quantification is achieved by radiolabeling the DNA at the 5′ end with ³²P or by using fluorescent labels.

Cell-Based Repair Assays

Cell-based assays measure BER activity in living cells. The host cell reactivation assay involves transfecting cells with a plasmid containing a lesion in a reporter gene (e.g., luciferase or GFP). If BER is active, the lesion is repaired, and the reporter gene is expressed. The level of expression correlates with BER efficiency.

Another approach is the single-cell gel electrophoresis (comet) assay, which measures DNA strand breaks. Cells are embedded in agarose, lysed, and subjected to electrophoresis. Damaged DNA migrates further, forming a "comet tail." By treating cells with a glycosylase (e.g., OGG1) after lysis, the assay can be adapted to detect specific oxidative lesions.

Genetic Models and Knockout Mice

Knockout mouse models have been instrumental in understanding BER. Key findings include:

• UNG knockout: Viable but shows increased mutation frequency at immunoglobulin genes, demonstrating UNG's role in somatic hypermutation.
• OGG1 knockout: Viable but accumulates 8-oxoG in the genome and shows increased spontaneous mutation frequency.
• APE1 knockout: Embryonic lethal, demonstrating the essential role of APE1 in development.
• Pol β knockout: Embryonic lethal, with defects in neurogenesis.
• XRCC1 knockout: Embryonic lethal, with increased oxidative damage in the developing nervous system.

These models have revealed that while individual glycosylases are dispensable for viability (due to redundancy), the core BER enzymes (APE1, Pol β, XRCC1, LIG3) are essential.

Clinical Significance and BER Defects

BER and Cancer

Defects in BER are associated with increased cancer risk, primarily through the accumulation of mutations. The most well-characterized example is MUTYH-associated polyposis (MAP), an autosomal recessive condition caused by biallelic mutations in the MUTYH gene. MAP is characterized by multiple colorectal adenomas and a high risk of colorectal cancer. The mechanism involves failure to remove adenine mispaired with 8-oxoG, leading to G:C → T:A transversions in tumor suppressor genes such as APC and KRAS.

Other BER defects linked to cancer include:

• OGG1 polymorphisms: Some variants are associated with increased lung cancer risk, particularly in smokers.
• APE1 polymorphisms: The D148E variant has been associated with altered DNA repair capacity and increased cancer risk in some populations.
• Pol β overexpression: In contrast to loss-of-function mutations, overexpression of Pol β is observed in 30% of human cancers and is associated with increased mutagenesis, likely due to aberrant gap filling.

BER in Neurodegenerative Diseases

The brain has a high metabolic rate and generates significant oxidative stress, making BER particularly important in neurons. Defects in BER are associated with several neurodegenerative conditions:

• Ataxia with oculomotor apraxia type 1 (AOA1): Caused by mutations in APTX (aprataxin), a protein that interacts with XRCC1 and processes abortive ligation intermediates. Patients exhibit progressive cerebellar ataxia and peripheral neuropathy.
• Spinocerebellar ataxia with axonal neuropathy (SCAN1): Caused by mutations in TDP1 (tyrosyl-DNA phosphodiesterase 1), which processes 3′ blocking lesions. Patients show cerebellar atrophy and sensorimotor neuropathy.
• Alzheimer's disease: Post-mortem brain tissue from Alzheimer's patients shows reduced BER activity and increased oxidative DNA damage, suggesting that BER dysfunction contributes to neurodegeneration.

Therapeutic Targeting of BER

BER is an attractive target for cancer therapy because cancer cells often have elevated BER activity to cope with increased oxidative stress. Inhibiting BER can sensitize cancer cells to DNA-damaging agents:

• PARP inhibitors (e.g., olaparib, niraparib) are the most successful BER-related therapeutics. They are approved for BRCA-mutant ovarian and breast cancers. PARP inhibitors trap PARP1 on DNA, creating lesions that require BER for repair. In BRCA-deficient cells, which lack homologous recombination, the trapped PARP1 lesions become lethal.

• APE1 inhibitors are in preclinical development. Compounds such as methoxyamine and lucanthone have shown promise in sensitizing cancer cells to alkylating agents.

• Pol β inhibitors are being explored, though none have reached clinical trials. The challenge is achieving selectivity for cancer cells over normal cells.

Common Pitfalls and Exam Tips

Misconceptions About BER

1. "BER repairs double-strand breaks": This is incorrect. BER repairs single-strand lesions and AP sites. Double-strand breaks are repaired by homologous recombination or non-homologous end joining.

1. "All glycosylases are bifunctional": No. Monofunctional glycosylases (e.g., UNG, MPG) only remove the base, leaving an intact AP site. Bifunctional glycosylases (e.g., OGG1, NTHL1) also cleave the backbone.

1. "APE1 creates the AP site": No. APE1 acts on AP sites created by glycosylases or by spontaneous depurination. It does not remove bases.

1. "Pol β synthesizes long patches": Pol β is primarily a short-patch polymerase. Long-patch synthesis involves Pol δ/ε and requires PCNA.

1. "BER only repairs oxidative damage": BER repairs oxidative, deamination, and alkylation damage. It is not limited to oxidation.

1. "The 5′ dRP is always removed by APE1": No. APE1 creates the 5′ dRP by incising the AP site. The dRP is removed by Pol β's lyase activity in SP-BER or by FEN1 in LP-BER.

Key Points to Remember

• BER is initiated by DNA glycosylases that flip and remove damaged bases.
• Monofunctional glycosylases generate AP sites; bifunctional glycosylases also cleave the backbone.
• APE1 is the central enzyme that processes AP sites and blocked 3′ ends.
• Pol β is the major polymerase in SP-BER and also has dRP lyase activity.
• XRCC1 is a scaffold that coordinates BER enzymes.
• SP-BER replaces one nucleotide; LP-BER replaces 2–13 nucleotides and requires FEN1.
• PARP1 is a damage sensor that recruits BER enzymes.
• MUTYH mutations cause MAP, a colorectal cancer predisposition syndrome.
• PARP inhibitors exploit BER defects in BRCA-mutant cancers.

Practice Questions

1. Question: A patient has a homozygous mutation in MUTYH. What type of DNA damage would accumulate, and what is the likely clinical consequence? Answer: 8-oxoG:A mispairs would accumulate, leading to G:C → T:A transversions. The clinical consequence is MUTYH-associated polyposis with high colorectal cancer risk.

1. Question: Why does APE1 knockout cause embryonic lethality while OGG1 knockout does not? Answer: APE1 is essential for processing all AP sites and blocked 3′ ends in BER, making it indispensable. OGG1 is one of several glycosylases with redundant functions; other glycosylases can partially compensate for its loss.

1. Question: A researcher treats cells with a drug that inhibits Pol β's lyase activity. What would be the effect on BER? Answer: The 5′ dRP would not be removed, blocking short-patch BER. The pathway would shift to long-patch BER, where FEN1 removes the dRP-containing flap.

Frequently Asked Questions

What is base excision repair?

Base excision repair (BER) is a DNA repair pathway that removes small, non-helix-distorting base lesions caused by oxidation, deamination, or alkylation. It is initiated by DNA glycosylases that remove the damaged base, followed by AP site incision, gap filling, and ligation. The Base Excision Repair BER pathway is essential for maintaining genomic stability against endogenous damage.

What are the steps of base excision repair?

The steps are: (1) damage recognition and base removal by a DNA glycosylase, (2) AP site incision by APE1 (or AP lyase activity of bifunctional glycosylases), (3) removal of the 5′ dRP moiety, (4) gap filling by DNA polymerase (primarily Pol β), and (5) nick sealing by DNA ligase (LIG3/XRCC1 in SP-BER or LIG1 in LP-BER). The Base Excision Repair Mechanism is detailed in the pathway description above.

What is the difference between base excision repair and nucleotide excision repair?

BER handles small, non-helix-distorting lesions and replaces 1–13 nucleotides. NER handles bulky, helix-distorting lesions (e.g., UV photoproducts) and replaces 24–32 nucleotides. BER uses glycosylases for damage recognition, while NER uses a multi-protein complex (XPC, TFIIH, XPA) that recognizes helical distortion. BER is primarily for endogenous damage, while NER handles environmental damage. See Base Excision Repair vs Nucleotide for a detailed comparison.

What is an example of a DNA lesion repaired by base excision repair?

8-oxoguanine (8-oxoG), a common oxidative lesion, is repaired by OGG1-initiated BER. Uracil, arising from cytosine deamination, is repaired by UNG-initiated BER. 3-methyladenine, an alkylation lesion, is repaired by MPG-initiated BER.

What is the meaning of base excision repair?

The term "base excision" refers to the excision (removal) of a damaged base from the DNA backbone. This is distinct from "nucleotide excision," where an entire nucleotide (base + sugar + phosphate) is removed as part of an oligonucleotide fragment. The Excision Repair umbrella encompasses both BER and NER.

What is the role of AP endonuclease in base excision repair?

APE1 (AP endonuclease 1) cleaves the phosphodiester backbone 5′ to an AP site, generating a 3′ hydroxyl and a 5′ deoxyribose phosphate. It also processes blocked 3′ ends created by bifunctional glycosylases. Without APE1, AP sites would accumulate and block replication and transcription.

What happens if base excision repair is defective?

Defective BER leads to accumulation of DNA damage, increased mutation frequency, and genomic instability. Clinically, this manifests as cancer predisposition (e.g., MUTYH-associated polyposis), neurodegeneration (e.g., AOA1, SCAN1), and developmental defects. Complete loss of core BER enzymes (APE1, Pol β, XRCC1) is embryonic lethal in mice.

Key Takeaways

• BER is the primary defense against small base lesions from oxidation, deamination, and alkylation, processing tens of thousands of lesions per cell per day.
• The pathway is initiated by lesion-specific DNA glycosylases that flip and remove damaged bases, creating AP sites.
• APE1 is the central enzyme that processes AP sites and blocked ends, generating substrates for Pol β.
• Short-patch BER replaces one nucleotide and is the predominant pathway; long-patch BER replaces 2–13 nucleotides and is used when the 5′ dRP is modified.
• XRCC1 acts as a scaffold coordinating BER enzymes, while PARP1 serves as a damage sensor and recruitment signal.
• BER defects cause cancer predisposition (MUTYH-associated polyposis), neurodegeneration (AOA1, SCAN1), and are embryonic lethal when core enzymes are absent.
• PARP inhibitors, which exploit BER defects, are approved cancer therapeutics for BRCA-mutant tumors.
• BER is regulated by cell cycle phase, post-translational modifications, and crosstalk with NER and MMR pathways.

Related Topics

• Splice a Broken Brake Line
• DNA Repair
• Double Strand Break Repair
• Double Stranded Break Repair

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