Nucleotide Excision Repair And Base Excision Repair

Introduction

DNA is constantly under attack from both internal metabolic processes and external environmental factors, leading to a variety of lesions that threaten the integrity of the genome. Understanding how NER and BER function not only reveals the elegance of cellular self‑maintenance but also provides insight into numerous genetic disorders and cancer therapies. These pathways operate like molecular detectives, scanning the double helix for different types of damage and orchestrating a precise series of steps to remove and replace the faulty nucleotides. To maintain genetic stability, cells have evolved sophisticated repair mechanisms, two of the most important being nucleotide excision repair (NER) and base excision repair (BER). This article explores the fundamental principles, step‑by‑step processes, key enzymes, and clinical relevance of both repair systems, and answers common questions about their distinctions and importance Not complicated — just consistent. No workaround needed..

Nucleotide Excision Repair (NER)

Overview of NER

NER is a high‑fidelity pathway that excises a short single‑stranded DNA segment (≈24–32 nucleotides) surrounding bulky DNA lesions. Because the damage is large and distorts the helix, NER is particularly suited for repairing UV‑induced pyrimidine dimers, chemical adducts, and interstrand crosslinks. The process is divided into two sub‑pathways: transcription‑coupled NER (TC‑NER), which prioritizes lesions that block RNA polymerase, and global genomic NER (GG‑NER), which scans the entire genome for distortions Surprisingly effective..

Not the most exciting part, but easily the most useful Easy to understand, harder to ignore..

Steps of NER

1. Damage Recognition

   • In GG‑NER, the heterotrimeric complex XPC‑HR23B binds to helix‑distorting lesions.
   • In TC‑NER, the stalled RNA polymerase II recruits CSA and CSB proteins, which signal the repair machinery.

2. Verification and Stabilization

   • XPA verifies the lesion and helps recruit other factors, while TFIIH (containing the helicases XPB and XPD) unwinds DNA around the damage.

3. Incision of Damaged Strand

   • XPG cuts the DNA downstream of the lesion.
   • XPF‑ERCC1 cuts upstream, creating a bubble that will be removed.

4. Excision of Oligonucleotide

   • The damaged oligonucleotide is released, leaving a single‑strand gap.

5. DNA Synthesis and Ligation

   • DNA polymerase δ/ε (or Pol η in some contexts) fills the gap using the complementary strand as a template.
   • DNA ligase I seals the nick, restoring the continuous duplex.

Key Enzymes

• XPC‑HR23B – initial lesion detection in GG‑NER.
• TFIIH (XPB, XPD) – helicase activity that opens the DNA around the damage.
• XPA – verifies damage and coordinates the assembly of the repair complex.
• XPG – endonuclease that makes the 3′ cut.
• XPF‑ERCC1 – endonuclease that makes the 5′ cut.
• DNA polymerases δ/ε – fill the gap with high fidelity.
• DNA ligase I – final ligation step.

Types of Lesions Repaired by NER

• UV‑induced cyclobutane pyrimidine dimers (CPDs) and 6‑4 photoproducts.
• Chemical adducts such as benzo[a]pyrene‑DNA adducts.
• Interstrand crosslinks (often in conjunction with homologous recombination).

Base Excision Repair (BER)

Overview of BER

BER is a short‑patch repair system that targets small, non‑bulky base modifications resulting from oxidative stress, deamination, or alkylation. g.The pathway is essential for removing oxidized bases (e.Unlike NER, BER does not require DNA unwinding; it works locally on a single base or a few nucleotides. , 8‑oxoguanine), deaminated cytosines, and uracil misincorporated into DNA.

Steps of BER

1. Recognition and Initial Removal

   • DNA glycosylases (e.g., OGG1 for 8‑oxoguanine, UNG for uracil) recognize and excise the damaged base, leaving behind an apurinic/apyrimidinic (AP) site.

2. AP Endonuclease Activity

   • APE1 (APEX1) cleaves the DNA backbone 5′ to the AP site, generating a 3′‑OH and 5′‑deoxyribose phosphate (dRP) group.

3. Gap Filling

   • DNA polymerase β (Pol β) performs short‑patch synthesis, adding the correct nucleotide and often removing the dRP via its dRP lyase activity.

4. Ligation

   • DNA ligase III (in complex with XRCC1) seals the nick, completing the repair.

Key Enzymes

• DNA glycosylases – specific for each type of damaged base.
• APE1 – the principal AP endonuclease that creates the single‑strand break.
• DNA polymerase β – the main polymerase for BER, combining polymerase and dRP lyase activities.
• DNA ligase III‑XRCC1 – ligates the final nick.

Types of Lesions Repaired by BER

• Oxidized bases (8‑oxoguanine, 5‑hydroxymethyluracil).
• Deaminated bases (uracil in DNA, hypoxanthine).
• Alkylated bases (3‑methyladenine, 7‑methylguanine).
• Single‑base mismatches resulting from replication errors.

Comparison of NER and BER

Comparison of NER and BER

Sub-pathway Nuances

• Transcription-Coupled NER (TC-NER) vs. Global Genome NER (GG-NER): While both share the core incision and ligation machinery, TC-NER is initiated by stalled RNA Polymerase II and the CSA/CSB complex, prioritizing repair of the transcribed strand of active genes. GG-NER relies on XPC-HR23B for genome-wide surveillance. Defects in TC-NER typically cause developmental/neurological phenotypes (CS), whereas GG-NER defects drive cancer predisposition (XP).
• Short-Patch vs. Long-Patch BER: The default pathway is short-patch (single-nucleotide replacement), driven by Pol β’s intrinsic dRP lyase activity. If the 5′ dRP moiety is resistant to cleavage (e.g., oxidized or reduced abasic sites), the pathway switches to long-patch BER. This involves Pol β/δ/ε displacing a 2–10 nt flap, which is cleaved by FEN1 before ligation by DNA Ligase I.

Interplay and Crosstalk

Although NER and BER are biochemically distinct, they operate within a shared nuclear environment and exhibit significant crosstalk:

1. Lesion Ambiguity: Certain lesions, such as thymine glycol or formamidopyrimidines (Fapy), can be processed by both pathways. The choice often depends on lesion density, chromatin context, and the relative affinity of XPC versus specific glycosylases (e.g., NTHL1, NEIL1).
2. Shared Protein Partners: XRCC1 acts as a scaffold not only for BER (Ligase III, Pol β) but also interacts with NER factors during the repair of oxidative damage in transcribed strands. PARP1 detects strand breaks generated by both pathways (APE1 incision in BER; XPF/XPG incisions in NER) and recruits XRCC1 or XPC, respectively, facilitating handoff.
3. Long-Patch BER and NER Convergence: Long-patch BER utilizes PCNA, FEN1, and DNA Ligase I—the same accessory factors employed during the resynthesis step of NER. This modular reuse of the "replication/repair toolkit" ensures efficiency.
4. Backup Pathways: When NER is overwhelmed or defective (e.g., in XP cells), some bulky adducts may be shunted toward Translesion Synthesis (TLS) polymerases or processed by Homologous Recombination (HR) if they collapse replication forks. Conversely, unrepaired BER intermediates (persistent AP sites or single-strand breaks) can convert to double-strand breaks, engaging HR or Non-Homologous End Joining (NHEJ).

Conclusion

Nucleotide Excision Repair and Base Excision Repair represent two evolutionary masterstrokes in genome maintenance, distinguished primarily by the structural footprint of the damage they correct. NER functions as a versatile "