Nucleotide Excision Repair

Nucleotide Excision Repair: Introduction by Hanspeter Naegeli

Lecture Overview and Instructor

This lecture focuses on Nucleotide Excision Repair (NER), presented by Professor Hanspeter Naegeli from the University of Zurich, Institute of Veterinary Pharmacology & Toxicology.

Curriculum Vitae of Hanspeter Naegeli

Education & Postdoc:
- DVM (Doctor of Veterinary Medicine) from the University of Zurich.
- Postdoctoral research at Stanford University (CA) and Southwestern Medical Center at Dallas (TX).
Current Position: Professor of Toxicology at the University of Zurich.
Research Focus: DNA repair.
Expert Activity: Involved with Swissmedic and the European Food Safety Authority.

Lecture Outline

Background: Chemical Carcinogens, UV Radiation, and Nucleotide Excision Repair

Biological relevance of these topics.
Evolutionary perspectives on DNA repair mechanisms.
Medical insights, particularly concerning skin cancer.

Mechanism of Nucleotide Excision Repair in Humans

Global-genome repair (GGR): Repair pathway that scans the entire genome.
Transcription-coupled repair (TCR): Repair pathway specifically associated with actively transcribed genes.

Pathway to Cancer: Carcinogens and DNA Damage

This section illustrates the cascade leading to cancer from various exposures:

Carcinogens: Sources include tobacco, food, air pollution, textiles, cosmetics, consumer goods, drugs/drug impurities, and solar radiation.
DNA Adducts: These carcinogens induce damage in DNA, forming DNA adducts.
DNA Repair: The cellular machinery attempts to repair these adducts.
Mutations: If DNA repair is insufficient or fails, DNA adducts lead to permanent mutations.
Cancer: Accumulation of mutations can result in cancer.

Genotoxic Carcinogens: Benzo[a]pyrene Example

Benzo[a]pyrene: A well-known genotoxic carcinogen found in sources like cigarette smoke and polluted air.
Biotransformation: It undergoes metabolic activation by cytochrome P450 monooxygenases, specifically CYP1A1 and CYP1B1.
Ultimate Carcinogen: This biotransformation produces Benzo[a]pyrene diol epoxide, a highly reactive compound.
Covalent Binding to DNA: The Benzo[a]pyrene diol epoxide then forms bulky, covalent adducts with DNA bases (e.g., guanine), leading to DNA damage and potential mutations.

UV Radiation and its Spectrum

Sunlight contains various wavelengths, including ultraviolet radiation, classified as:

UV-C: Wavelengths between 100-280 ext{ nm}.
UV-B: Wavelengths between 280-320 ext{ nm}.
UV-A: Wavelengths between 320-400 ext{ nm}.
The full spectrum of sunlight extends up to 700 ext{ nm} (visible light).

Questions on UV Wavelengths

What UV wavelengths successfully reach the Earth's surface?
Which UV wavelengths are commonly utilized in solariums?
To what extent do different UV wavelengths penetrate human skin?
- (Implied answers: UV-A and UV-B reach Earth; solariums use mostly UV-A/some UV-B; UV-A penetrates deeper than UV-B).

DNA Lesions Caused by UV Light

UV radiation induces characteristic DNA lesions:

Cyclobutane Pyrimidine Dimer (CPD):
- Caused by both UV-A and UV-B.
- Involves a covalent bond between two adjacent pyrimidine bases (e.g., thymine-thymine).
- Induces a minor helix disruption, with a 9^ ext{o} bend in the DNA structure.
(6-4) ext{ Photoproduct (6-4PP)}:
- Primarily caused by UV-B.
- Involves a more complex covalent linkage between adjacent pyrimidines.
- Causes a major helix disruption, with a 44^ ext{o} bend in the DNA structure.
Frequency: During summer, an exposed skin cell can accumulate up to 10^5 UV lesions per day.

UV Signature Mutations

UV lesions can lead to specific mutations:

Hydrolytic Deamination: Cytosine (C) residues within CPDs are highly susceptible to hydrolytic deamination.
- The half-life (t_{1/2}) of C in a CPD is drastically reduced to 2 ext{ h} from the normal 30,000 ext{ h}.
- Deamination converts cytosine to uracil (U), which is mispaired with adenine during replication.
Error-prone Bypass: Translesion DNA polymerases (e.g., DNA Pol epsilon, iota, kappa, theta, zeta) often bypass CPDs in an error-prone manner.
- This typically leads to C-to-T transitions in the subsequent replication round.
Error-free Bypass: DNA polymerase eta ( ext{Pol } ext{eta}) can perform error-free bypass of CPDs, preventing mutations.
5-Methylcytosine: The vulnerability of 5-methylcytosine to deamination in UV lesions is also a significant factor, contributing to C-to-T transitions at methylated sites, which are common mutational hotspots in cancer.

Adverse Effects of UV Radiation: Melanoma Rates

UV radiation is a significant risk factor for skin cancer.

Geographical distribution maps show direct correlation between UV exposure levels and rates of melanoma.
Melanoma rates vary across regions, for example, from <5.3 cases to 16.9+ cases per 100,000 people per year, highlighting the impact of UV exposure on public health.

UV-Seeking Behavior and Endorphins

Research demonstrates an

Nucleotide Excision Repair