Nucleotide Excision Repair: Protecting the Genome from UV Damage and Mutagens

Our DNA is under constant assault from various environmental factors, including radiation, chemicals, and other mutagens. To maintain genomic integrity, the body relies on specialized mechanisms to identify and fix these errors. Among the most critical of these is nucleotide excision repair (NER), a sophisticated pathway designed to remove bulky DNA lesions that could otherwise lead to mutations or cell death.

While other pathways like Base Excision Repair (BER) handle specific non-bulky lesions and DNA Mismatch Repair (MMR) corrects mismatched Watson-Crick base pairs, NER is uniquely equipped to handle large, structural distortions in the DNA helix. This is particularly vital for repairing damage caused by ultraviolet (UV) light, such as thymine dimers and 6,4-photoproducts.

Diagram of both the TC-NER and GG-NER pathways. The two pathways differ only in initial DNA damage recognition.[1]

How Nucleotide Excision Repair Works

The NER process is a multi-step operation that ensures the correct segment of damaged DNA is removed and replaced. Once the damage is recognized, a short single-stranded DNA segment containing the lesion is excised. The remaining undamaged strand serves as a template for DNA polymerase, which synthesizes a new, complementary sequence. Finally, DNA ligase seals the gap to restore the double-stranded structure.

The NER pathway is divided into two distinct subpathways that differ only in how they initially recognize the damage:

Global Genomic NER (GG-NER): Scans the entire genome for distortions.
Transcription-Coupled NER (TC-NER): Specifically targets damage located in actively transcribed genes.

Schematic depicts binding of proteins involved with GG-NER.[3]

The Dual Incision Process

In eukaryotes, the excision phase is driven by a complex set of proteins. A key player is Transcription Factor II H (TFIIH), which facilitates the unwinding of the DNA. Within the TFIIH complex, the subunits XPD and XPB act as helicases to create a junction between the single-stranded and double-stranded DNA.

The actual cutting, or dual incision, is performed by endonucleases: XPG cuts on the 3' side of the damage, while the XPF–ERCC1 heterodimer cuts on the 5' side. This results in the removal of a single-stranded DNA segment approximately 25 to 30 nucleotides long. These excised oligonucleotides are initially released in complex with TFIIH before being bound by Replication Protein A (RPA).

Schematic depicts binding of proteins involved with TC-NER.[3]

NER in Prokaryotes vs. Eukaryotes

While the fundamental goal of NER is the same across all life, the machinery differs. Prokaryotes utilize Uvr proteins to manage the repair process. In contrast, eukaryotes—including humans—employ a much more complex system involving at least nine major proteins. In humans, deficiencies in these proteins are directly linked to severe genetic disorders.

A schematic representation of models for the nucleotide excision repair pathway controlled by Uvr proteins.[4]

Comparison of Major DNA Excision Repair Pathways
Pathway	Primary Target	Mechanism Summary
Nucleotide Excision Repair (NER)	Bulky adducts (e.g., UV-induced thymine dimers)	Dual incision and replacement of a short DNA segment.
Base Excision Repair (BER)	Specific non-bulky lesions	Removal of damaged bases by specific glycosylases.
Mismatch Repair (MMR)	Mismatched Watson-Crick base pairs	Correction of errors occurring during DNA replication.

Clinical Significance: Disease and Aging

The vital role of NER is most evident when the system fails. Mutations in the genes encoding NER proteins can lead to profound health consequences, often manifesting as increased cancer risk or premature aging.

Genetic Disorders

Two prominent diseases associated with NER dysfunction are:

Xeroderma Pigmentosum (XP): Caused by deficiencies in the GG-NER pathway. Patients face a staggering 1000-fold increase in the risk of skin cancer due to an inability to repair UV damage.
Cockayne Syndrome (CS): Linked to mutations in CSA (ERCC8) or CSB (ERCC6) proteins. This condition is characterized by photosensitivity, intellectual disability, and features of accelerated aging (progeria).
Trichothiodystrophy (TTD): Associated with TC-NER mutations, leading to photosensitivity and physical or mental developmental challenges.

DNA excision pathways work in tandem to repair DNA damage. Unrepaired damage or malfunctioning proteins associated with excision repair could lead to unregulated cell growth and cancer.[6]

Cancer Risk and Genetic Polymorphisms

Even without full deficiency, variations in DNA (polymorphisms) can influence cancer susceptibility. For example, polymorphisms in the XPD (ERCC2) and XPC genes have been correlated with increased risks for skin, breast, and prostate cancers. In clinical settings, certain XPD variants have even been linked to higher relapse rates in colorectal cancer patients following chemotherapy.

Key Facts

NER is essential for repairing bulky DNA damage, such as thymine dimers caused by UV light.
The two subpathways, GG-NER and TC-NER, differ only in their initial damage recognition methods.
TFIIH is a central enzyme complex that unwinds DNA during the repair process.
Deficiencies in NER proteins can cause Xeroderma pigmentosum, increasing skin cancer risk by 1000-fold.
Mutations in genes like ERCC1, XPD, and XPB are linked to premature aging phenotypes.

Frequently Asked Questions

What is the difference between GG-NER and TC-NER?

GG-NER (Global Genomic NER) scans the entire genome for damage, whereas TC-NER (Transcription-Coupled NER) specifically targets damage that occurs in genes currently being transcribed into RNA.

How does UV light cause DNA damage?

UV radiation induces the formation of bulky DNA adducts, most commonly thymine dimers, which distort the structure of the DNA double helix.

Why does NER deficiency lead to cancer?

When NER proteins are defective, the cell cannot repair bulky lesions. These unrepaired lesions can lead to permanent mutations during DNA replication, which may drive unregulated cell growth and cancer.

What is the role of TFIIH in DNA repair?

TFIIH is a key enzyme complex that acts as a helicase to unwind the DNA around the site of the lesion, allowing the repair machinery to access the damaged strand.

Can NER mutations affect aging?

Yes. Germline mutations in several NER-related genes, such as ERCC1 and CSB, are known to cause features of premature aging, including intellectual disability and physical growth issues.