Residency · Residency · Dermatology

Ultraviolet Radiation and Photocarcinogenesis

Introduction

Ultraviolet radiation is the most significant modifiable environmental risk factor for skin cancer, the most common malignancy in humans. Understanding the physics of UV radiation, its molecular effects on DNA and cellular signaling, the mechanisms of DNA repair, and the multistep process of photocarcinogenesis is essential for dermatologists who manage the consequences of UV exposure daily and counsel patients on prevention.

Physics of Ultraviolet Radiation

The UV Spectrum

UVC (100 to 280 nm) has the highest energy but is absorbed by the ozone layer and does not reach the Earth's surface under normal conditions; it finds application in germicidal settings. UVB (280 to 320 nm) is the primary cause of sunburn (erythema) and direct DNA damage, accounting for approximately 5% of terrestrial UV, with intensity varying significantly by season, time of day, and latitude. UVA (320 to 400 nm) penetrates deeper into the dermis and is the primary driver of photoaging and immunosuppression, accounting for approximately 95% of terrestrial UV. UVA is relatively constant throughout the year and penetrates window glass. UVA is further divided into UVA2 (320 to 340 nm), which overlaps with UVB in biologic effects, and UVA1 (340 to 400 nm), which has the deepest penetration and generates reactive oxygen species. Emerging evidence suggests that visible light (400 to 700 nm) and infrared radiation contribute to skin aging and pigmentation, particularly in darker skin types.

Factors Affecting UV Exposure

UV exposure is influenced by time of day (peak UVB intensity between 10 AM and 4 PM), latitude and altitude (greater UV at lower latitudes and higher altitudes), cloud cover (up to 80% of UV penetrates clouds), and reflection from snow (80%), sand (15 to 25%), and water (10%). Ozone depletion is particularly consequential: each 1% decrease in ozone increases UVB exposure by 2%.

UV-Induced DNA Damage

Direct DNA Damage (UVB-Predominant)

UVB is directly absorbed by DNA bases, primarily pyrimidines. Cyclobutane pyrimidine dimers (CPDs) are the most common UV-induced DNA lesion, formed by covalent bonds between adjacent pyrimidines (primarily thymine-thymine, but also thymine-cytosine). 6-4 photoproducts (6-4 PPs) are the second most common lesion, less frequent but more mutagenic than CPDs. Both cause characteristic UV signature mutations: C-to-T and CC-to-TT transitions at dipyrimidine sites, which are virtually pathognomonic for UV mutagenesis.

Indirect DNA Damage (UVA-Predominant)

UVA generates reactive oxygen species (ROS) through photosensitization reactions with endogenous chromophores (porphyrins, flavins, melanin). ROS produce 8-oxo-7,8-dihydroguanine (8-oxoG), the most common oxidative DNA lesion, which causes G-to-T transversion mutations. Notably, UVA also generates CPDs (more than previously recognized), potentially contributing to direct mutagenesis.

<image>Molecular diagram showing UV-induced DNA damage: UVB photons being absorbed by adjacent pyrimidines forming cyclobutane pyrimidine dimers and 6-4 photoproducts, alongside UVA-mediated reactive oxygen species generation causing 8-oxoguanine lesions, with the resulting UV signature mutations (C-to-T transitions and G-to-T transversions) indicated</image>

DNA Repair Mechanisms

Nucleotide Excision Repair (NER)

NER is the primary pathway for repairing CPDs and 6-4 PPs. It operates through two sub-pathways: global genome NER (GG-NER), which repairs damage throughout the genome and is initiated by the XPC-RAD23B damage recognition complex, and transcription-coupled NER (TC-NER), which preferentially repairs damage on the transcribed strand of active genes, initiated when RNA polymerase II stalls at a lesion. The repair process involves damage recognition, unwinding (XPB/XPD helicases in the TFIIH complex), incision (XPF-ERCC1 and XPG endonucleases), gap filling (DNA polymerase), and ligation. Xeroderma pigmentosum (XP) is an autosomal recessive deficiency in NER genes (XPA through XPG) in which patients develop skin cancers at more than 1000-fold the normal rate and before age 10, serving as the prototypic disease demonstrating UV-induced carcinogenesis.

Base Excision Repair (BER)

BER repairs oxidative DNA damage (8-oxoG) and small base modifications, initiated by 8-oxoguanine glycosylase (OGG1).

Mismatch Repair (MMR)

MMR corrects mismatched base pairs, and its deficiency is associated with Muir-Torre syndrome (sebaceous neoplasms plus visceral malignancies).

UV-Induced Immunosuppression

UV radiation is a potent local and systemic immunosuppressant. Cis-urocanic acid, UV-isomerized from trans-UCA in the stratum corneum, activates Th2 responses and regulatory T cells. Langerhans cell depletion occurs as UV causes apoptosis and migration of epidermal Langerhans cells (antigen-presenting cells), reducing immune surveillance. Regulatory T-cell induction promotes CD4+CD25+FoxP3+ Treg cells that suppress anti-tumor immunity. Prostaglandin E2 (PGE2) and IL-10 are immunosuppressive cytokines produced by UV-damaged keratinocytes. UV-induced immunosuppression allows transformed keratinocytes to escape immune detection and proliferate, representing a critical step in photocarcinogenesis. This mechanism also explains the increased skin cancer risk in immunosuppressed patients such as transplant recipients.

Multistep Photocarcinogenesis

The p53 Pathway

TP53 is the most commonly mutated gene in UV-induced skin cancers. UV-induced CPDs in TP53 cause C-to-T signature mutations at dipyrimidine sites. Under normal conditions, p53 responds to DNA damage by inducing cell cycle arrest (G1 checkpoint), DNA repair, or apoptosis (sunburn cells). Sunburn cells are apoptotic keratinocytes with dense eosinophilic cytoplasm, representing p53-mediated elimination of UV-damaged cells. Loss of p53 function allows UV-damaged cells to survive and accumulate additional mutations. p53 clones are microscopic clusters of p53-mutant keratinocytes in chronically sun-exposed skin that serve as precursors to actinic keratoses and SCC.

Specific Pathways in Skin Cancer

Basal Cell Carcinoma (BCC)

BCC is driven by Hedgehog pathway activation through UV-induced mutations in PTCH1 (Patched 1, a Hedgehog pathway inhibitor) or activating mutations in SMO (Smoothened), leading to constitutive Hedgehog signaling. UV signature mutations in PTCH1 are found in over 90% of BCCs. TP53 mutations are present in 50 to 60% of BCCs but play a secondary role.

Squamous Cell Carcinoma (SCC)

TP53 is the primary driver of SCC, with UV signature mutations found in over 90% of cutaneous SCCs. NOTCH1/NOTCH2 loss-of-function mutations are found in 75% of SCCs and disrupt keratinocyte differentiation. RAS pathway mutations occur in a subset. The progression model follows: UV damage leads to p53 clones, then actinic keratosis, then invasive SCC.

Melanoma

Melanoma has a more complex relationship with UV than keratinocyte carcinomas. The BRAF V600E mutation is the most common driver (50 to 60% of cutaneous melanomas), found in intermittently sun-exposed sites and induced by UVA radiation. Intermittent, intense UV exposure (blistering sunburns, especially in childhood) is a stronger risk factor than chronic cumulative exposure. NRAS mutations (20 to 25%) are the second most common driver. NF1 mutations are associated with chronic sun-damaged skin melanoma. C-to-T signature mutations found in the melanoma genome confirm UV as a direct mutagen.

<image>Diagram illustrating the multistep process of photocarcinogenesis: UV radiation causing DNA damage (CPDs), failure of DNA repair in NER-deficient or p53-mutant cells, clonal expansion of p53-mutant keratinocytes, progression through actinic keratosis to squamous cell carcinoma, with parallel pathway showing Hedgehog pathway mutation leading to basal cell carcinoma</image>

Photoaging (Dermatoheliosis)

Chronic UV exposure causes premature skin aging distinct from intrinsic (chronologic) aging. UVA is the primary driver due to deeper dermal penetration. Features include coarse wrinkling, solar lentigines, telangiectasias, leathery texture, mottled pigmentation, and solar elastosis (accumulation of degraded elastin in the dermis). The mechanism involves UV activation of MMP-1 (collagenase), MMP-3, and MMP-9 through AP-1 transcription factor induction, degrading dermal collagen and elastin while simultaneously reducing collagen synthesis by inhibiting TGF-beta/Smad signaling. Solar elastosis on histopathology is pathognomonic, showing basophilic, amorphous elastic material replacing normal dermal collagen, with a Grenz zone of uninvolved papillary dermis above the elastotic material.

Photoprotection

Sunscreen should be broad-spectrum (UVA plus UVB), SPF 30 or higher, applied 15 minutes before exposure and reapplied every 2 hours. Inorganic (physical) filters such as zinc oxide and titanium dioxide reflect and scatter UV. Organic (chemical) filters such as avobenzone, octocrylene, and ecamsule absorb UV. SPF measures UVB protection only; the PA+++ or Broad Spectrum label indicates UVA protection. Protective clothing with UPF-rated fabrics, wide-brimmed hats, and sunglasses provides additional protection. Behavioral measures include seeking shade and avoiding peak UV hours (10 AM to 4 PM). Nicotinamide (vitamin B3, 500 mg twice daily) reduces UV-induced immunosuppression and enhances NER, demonstrating a 23% reduction in new NMSC in the ONTRAC trial. Oral polypodium leucotomos extract has modest evidence for reducing UV-induced erythema. Tanning beds are classified as a Group 1 carcinogen by the WHO/IARC and should be avoided.

Key Clinical Pearls

UV signature mutations (C-to-T transitions at dipyrimidine sites) are the molecular fingerprint of UV-induced carcinogenesis and are found in the vast majority of keratinocyte carcinomas. UVA contributes to carcinogenesis through ROS generation and direct CPD formation, making broad-spectrum sunscreen with adequate UVA protection critical. Xeroderma pigmentosum is the definitive demonstration that NER deficiency leads to UV-induced skin cancer, with these patients developing their first cancers before age 10. p53 clones in chronically sun-damaged skin are the precursors of actinic keratoses and SCC and are present in clinically normal-appearing sun-exposed skin. Nicotinamide (500 mg twice daily) is a safe, evidence-based chemoprevention strategy for high-risk patients with multiple prior NMSCs.

References

1. Brash DE. UV signature mutations. Photochem Photobiol. 2015;91(1):15-26.
2. Narayanan DL, Saladi RN, Fox JL. Ultraviolet radiation and skin cancer. Int J Dermatol. 2010;49(9):978-986.
3. Chen AC, Martin AJ, Choy B, et al. A phase 3 randomized trial of nicotinamide for skin-cancer chemoprevention. N Engl J Med. 2015;373(17):1618-1626.
4. D'Orazio J, Jarrett S, Amaro-Ortiz A, Scott T. UV radiation and the skin. Int J Mol Sci. 2013;14(6):12222-12248.

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