UV-Induced DNA Damage and Repair Mechanisms
Introduction
The relationship between ultraviolet radiation and DNA represents one of the most consequential molecular interactions in dermatology. Every moment of sun exposure initiates a cascade of photochemical reactions within the genome, creating lesions that—if unrepaired—can drive the development of skin cancer. Yet humans have evolved remarkably sophisticated repair systems that can identify and correct most UV-induced damage, maintaining genomic integrity across billions of cell divisions.
Understanding UV-induced DNA damage is essential not only for comprehending photocarcinogenesis but also for appreciating the therapeutic mechanisms of phototherapy, the pathophysiology of DNA repair disorders like xeroderma pigmentosum, and the molecular basis of sunscreen protection.
Chemistry of UV-Induced DNA Lesions
Direct DNA Photodamage
Direct photodamage occurs when DNA absorbs UV photons directly, primarily affecting the pyrimidine bases (thymine and cytosine).
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Cyclobutane Pyrimidine Dimers (CPDs)
CPDs are the most abundant UV-induced DNA lesions, formed by covalent bonding between adjacent pyrimidines on the same DNA strand.
Formation Chemistry:
- UV photon absorption by pyrimidine base (peak 260-280 nm)
- Excitation of π electrons to π* orbital
- Formation of cyclobutane ring between C5-C6 carbons of adjacent pyrimidines
- Creation of a stable four-membered ring structure
Types of CPDs:
| Type | Frequency | Mutagenic Potential |
|---|---|---|
| TT dimers | Most common | Moderate |
| TC dimers | Common | High (C→T transitions) |
| CT dimers | Common | High (C→T transitions) |
| CC dimers | Less common | Very high (CC→TT signature) |
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(6-4) Photoproducts
The (6-4) photoproduct is the second major type of direct UV damage, formed by covalent bonding between the C6 carbon of one pyrimidine and the C4 carbon of the adjacent pyrimidine.
Characteristics:
- More helix-distorting than CPDs
- More readily recognized by repair machinery
- More mutagenic if unrepaired
- Can photoisomerize to Dewar valence isomers under UVB exposure
Dewar Isomers:
- Formed by UVB-induced photoisomerization of (6-4)PPs
- Less helix-distorting
- Slower repair kinetics
- Important in chronic sun exposure scenarios
Comparison of Major Direct Photoproducts
| Feature | CPD | (6-4) Photoproduct | Dewar Isomer |
|---|---|---|---|
| Abundance | ~75-80% of lesions | ~20-25% of lesions | Variable (secondary) |
| Action spectrum peak | 270 nm | 320 nm | Formed from (6-4)PP |
| Helix distortion | Moderate (35° bend) | Severe (44° bend) | Less severe |
| Repair rate | Slower | Faster | Slower |
| Mutagenic potential | High | Very high | High |
| UV signature | C→T, CC→TT | CC→TT | Variable |
| NER recognition | Slower | Faster | Variable |
Indirect DNA Damage: Oxidative Mechanisms
Role of Reactive Oxygen Species
While UVB causes primarily direct DNA damage, UVA exerts most of its genotoxic effects through the generation of reactive oxygen species (ROS).
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8-Oxoguanine: Signature of Oxidative Stress
8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) is the most prevalent and best-studied oxidative DNA lesion.
| Property | Details |
|---|---|
| Formation | Guanine + hydroxyl radical or singlet oxygen |
| Abundance | ~10,000 lesions/cell/day (normal oxidative metabolism) |
| Mutagenic effect | G→T transversions (mispairing with adenine) |
| Repair | Base excision repair (OGG1 glycosylase) |
| Biomarker | Urinary 8-oxo-dG = systemic oxidative stress marker |
| Clinical relevance | Elevated in skin cancers, photoaging |
Additional Oxidative Lesions
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UV Signature Mutation
C→T and CC→TT Transitions
The hallmark of UV-induced mutagenesis is the C→T transition mutation, particularly at dipyrimidine sequences. This "UV signature" is pathognomonic of sun-induced skin cancer.
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Why C→T at Dipyrimidines?
- CPDs form preferentially at pyrimidine dimers (TC, CC, CT, TT)
- Cytosine within a CPD undergoes accelerated deamination to uracil
- During replication, uracil pairs with adenine instead of guanine
- Result: C→T transition mutation
Cancer Genome Evidence
Analysis of skin cancer genomes reveals overwhelming evidence of UV causation:
| Cancer Type | UV Signature Mutations | Evidence |
|---|---|---|
| Cutaneous SCC | 70-80% C→T at dipyrimidines | Very strong UV signature |
| Cutaneous Melanoma | 70-90% in sun-exposed sites | Strong but variable |
| Basal Cell Carcinoma | 50-70% C→T/CC→TT | Strong UV signature |
| Merkel Cell Carcinoma | Variable (virus + UV) | Mixed etiology |
| Acral/Mucosal Melanoma | Low C→T frequency | Non-UV etiology |
DNA Repair Pathways
The cell possesses multiple, overlapping DNA repair systems to address the diverse spectrum of UV-induced damage.
Overview of Repair Pathways
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Nucleotide Excision Repair (NER)
NER is the primary pathway for repairing bulky, helix-distorting lesions like CPDs and (6-4)PPs. It operates through two sub-pathways:
Global Genome NER (GG-NER)
- Scans entire genome for helix-distorting lesions
- Initiated by damage recognition by XPC-RAD23B complex
- Important for preventing mutations in non-transcribed regions
Transcription-Coupled NER (TC-NER)
- Repairs lesions in actively transcribed DNA strands
- Triggered by stalled RNA polymerase II at lesion site
- Faster than GG-NER for transcribed genes
- Deficient in Cockayne syndrome
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NER Proteins and Their Clinical Associations
| Protein | Gene | Function | Disease When Defective |
|---|---|---|---|
| XPA | XPA | Damage verification | XP (severe) |
| XPB | ERCC3 | 3'→5' helicase (TFIIH) | XP, XP/CS, TTD |
| XPC | XPC | GG-NER damage recognition | XP (skin cancer prone) |
| XPD | ERCC2 | 5'→3' helicase (TFIIH) | XP, XP/CS, TTD |
| XPE | DDB2 | GG-NER cofactor | XP (mild) |
| XPF | ERCC4 | 5' endonuclease | XP, XP/CS, Fanconi-like |
| XPG | ERCC5 | 3' endonuclease | XP, XP/CS |
| CSA | ERCC8 | TC-NER | Cockayne syndrome |
| CSB | ERCC6 | TC-NER | Cockayne syndrome |
| TTD-A | GTF2H5 | TFIIH stabilization | Trichothiodystrophy |
Base Excision Repair (BER)
BER handles oxidative DNA damage and small base modifications that don't significantly distort the helix.
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Key DNA Glycosylases
| Glycosylase | Substrate | Primary Lesions |
|---|---|---|
| OGG1 | 8-oxo-dG:C | 8-oxoguanine |
| NTH1 | Oxidized pyrimidines | Thymine glycol, 5-OH-cytosine |
| NEIL1/2/3 | Ring-opened purines | FapyG, FapyA |
| UNG | Uracil | Deaminated cytosine |
| MUTYH | Adenine:8-oxo-dG | Misincorporated adenine |
| MPG/AAG | Alkylated bases | 3-methyladenine |
DNA Damage Response Signaling
ATR-CHK1 Pathway
UV-induced DNA damage activates the ATR (ATM and Rad3-related) kinase pathway, which coordinates cell cycle arrest, DNA repair, and apoptosis.
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p53 Guardian Role
p53 is the "guardian of the genome," playing a central role in the UV damage response:
| p53 Function | Mechanism | Outcome |
|---|---|---|
| Cell cycle arrest | p21 (CDKN1A) induction | Time for repair |
| DNA repair enhancement | XPC, DDB2 upregulation | Improved NER |
| Apoptosis | BAX, PUMA induction | Elimination of damaged cells |
| Senescence | Permanent growth arrest | Prevention of damaged cell proliferation |
| Mutated in cancer | Loss of tumor suppression | Uncontrolled proliferation |
Clinical Pearl: p53 mutations are found in >50% of cutaneous SCCs and BCCs, often bearing the UV signature (C→T at dipyrimidines in the TP53 gene itself).
Translesion Synthesis: Error-Prone Bypass
When DNA repair fails and replication fork stalls at a UV lesion, the cell has a last-resort mechanism: translesion synthesis (TLS).
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DNA Polymerase η (eta): CPD Specialist
Polymerase η is encoded by the POLH gene and is specifically adapted for accurate bypass of CPDs:
| Feature | Details |
|---|---|
| Specificity | TT dimers (accurate), TC/CT/CC (less accurate) |
| Accuracy | Inserts two adenines opposite TT dimer (correct!) |
| Clinical significance | POLH mutations cause XP-variant (XP-V) |
| XP-V phenotype | Skin cancer prone despite normal NER |
| Mechanism in XP-V | Other TLS polymerases (error-prone) substitute |
Clinical Disorders of UV Response
DNA Repair Deficiency Syndromes
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Xeroderma Pigmentosum: A Natural Experiment
XP provides a striking demonstration of UV damage accumulation when repair is defective:
| Complementation Group | Gene | Pathway | Cancer Risk | Neurodegeneration |
|---|---|---|---|---|
| XP-A | XPA | NER core | 10,000× | Yes (severe) |
| XP-B | ERCC3 | TFIIH helicase | 10,000× | Variable |
| XP-C | XPC | GG-NER recognition | 10,000× | No |
| XP-D | ERCC2 | TFIIH helicase | 10,000× | Variable |
| XP-E | DDB2 | GG-NER cofactor | 10× | No |
| XP-F | ERCC4 | 5' endonuclease | 1,000× | Variable |
| XP-G | ERCC5 | 3' endonuclease | 10,000× | Variable |
| XP-V | POLH | Translesion synthesis | 5-10× | No |
Quantifying DNA Damage
Laboratory Methods
| Method | Principle | Lesion Detected | Sensitivity |
|---|---|---|---|
| ELISA | Antibody recognition | CPDs, 6-4PPs | Moderate |
| Comet assay | DNA strand breaks | SSBs, alkali-labile sites | High |
| γH2AX foci | Phosphorylated histone | DSBs | Very high |
| LC-MS/MS | Mass spectrometry | 8-oxo-dG, CPDs | Quantitative |
| Slot blot | Antibody + membrane | CPDs, 6-4PPs | Semi-quantitative |
| Host cell reactivation | Plasmid repair assay | NER capacity | Functional |
| Unscheduled DNA synthesis | Radiolabel incorporation | NER activity | Functional |
Clinical Assessment of Repair Capacity
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Therapeutic Implications
Enhancing DNA Repair
| Strategy | Mechanism | Status |
|---|---|---|
| Photolyase creams | CPD photoreversal (enzyme) | Available (OTC) |
| T4 endonuclease V | T4N5 in liposomes | Investigational |
| Antioxidants | Reduce oxidative damage | Limited evidence |
| Nicotinamide | NAD+ precursor (enhances ATP for repair) | Some clinical data |
| Sirtuins activators | Enhance NER components | Preclinical |
Photolyase: Reversing Damage with Light
Photolyases are enzymes found in many organisms (but not placental mammals) that directly reverse CPDs using visible light energy.
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Clinical Application: Topical photolyase-containing creams (from Anacystis nidulans or Thermus thermophilus) are marketed in Europe for reduction of actinic keratoses and possibly photoaging.
Summary: DNA Damage Timeline
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Key Clinical Pearls
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Cross-References
- Volume 03, Chapter 1.1: Physics of UV Radiation - Wavelength-specific damage
- Volume 03, Chapter 3: Photoprotection - Damage prevention
- Volume 05: Immunology - UV immunosuppression
- Volume 19: Genodermatoses - XP, CS, TTD
- Volume 22: Oncology - Skin cancer pathogenesis
References
- Cadet J, Douki T. Formation of UV-induced DNA damage contributing to skin cancer development. Photochem Photobiol Sci 2018;17:1816-1841.
- Schärer OD. Nucleotide excision repair in eukaryotes. Cold Spring Harb Perspect Biol 2013;5:a012609.
- DiGiovanna JJ, Kraemer KH. Shining a light on xeroderma pigmentosum. J Invest Dermatol 2012;132:785-796.
- Brash DE. UV signature mutations. Photochem Photobiol 2015;91:15-26.
- Marteijn JA, et al. Understanding nucleotide excision repair and its roles in cancer and ageing. Nat Rev Mol Cell Biol 2014;15:465-481.
How to Cite
Cutisight. "UV Induced Lesions." Encyclopedia of Dermatology [Internet]. 2026. Available from: https://cutisight.com/education/volume-03-skin-reactions-and-interactions/01-photobiology/02-dna-damage-repair/01-uv-induced-lesions
This is an open-access resource. Please cite appropriately when using in academic or clinical work.