Pyrimidine dimers are molecular lesions formed from thymine or cytosine bases in DNA via photochemical reactions, commonly associated with direct DNA damage.Ultraviolet light (UV; particularly UVC) induces the formation of covalent linkages between consecutive bases along the nucleotide chain in the vicinity of their carbon–carbon double bonds. The photo-coupled dimers are fluorescent. The dimerization reaction can also occur among pyrimidine bases in dsRNA (double-stranded RNA)—uracil or cytosine. Two common UV products are cyclobutane pyrimidine dimers (CPDs) and 6–4 photoproducts. These premutagenic lesions alter the structure of the DNA helix and cause non-canonical base pairing. Specifically, adjacent thymines or cytosines in DNA will form a cyclobutane ring when joined together and cause a distortion in the DNA. This distortion prevents replication or transcription machinery beyond the site of the dimerization. Up to 50–100 such reactions per second might occur in a skin cell during exposure to sunlight, but are usually corrected within seconds by photolyase reactivation or nucleotide excision repair. In humans, the most common form of DNA repair is nucleotide excision repair (NER). In contrast, organisms such as bacteria can counterintuitively harvest energy from the sun to fix DNA damage from pyrimidine dimers via photolyase activity. If these lesions are not fixed, polymerase machinery may misread or add in the incorrect nucleotide to the strand. If the damage to the DNA is overwhelming, mutations can arise within the genome of an organism and may lead to the production of cancer cells. Uncorrected lesions can inhibit polymerases, cause misreading during transcription or replication, or lead to arrest of replication. It causes sunburn and it triggers the production of melanin. Pyrimidine dimers are the primary cause of melanomas in humans.
Types of dimers
A cyclobutane pyrimidine dimer (CPD) contains a four membered ring arising from the coupling of the two double-bonded carbons of each of the pyrimidines. Such dimers interfere with base pairing during DNA replication, leading to mutations.
A 6–4 photoproduct (6–4 pyrimidine–pyrimidone or 6–4 pyrimidine–pyrimidinone) is an alternate dimer consisting of a single covalent bond between the carbon at the 6 position of one ring and carbon at the 4 position of the ring on the next base. This type of conversion occurs at one third the frequency of CPDs but is more mutagenic.
A third type of lesion is a Dewar pyrimidinone, formed by a reversible isomerization of the 6–4 photoproduct upon further exposure to light.
Mutagenesis
Translesion polymerases frequently introduce mutations at pyrimidine dimers, both in prokaryotes (SOS mutagenesis) and in eukaryotes. Although the thymine-thymine CPDs (thymine dimers) are the most frequent lesions caused by UV light, translesion polymerases are biased toward introduction of As, so that TT dimers are often replicated correctly. On the other hand, any cytosine involved in CPDs is prone to be deaminated, inducing a C to T transition.
DNA repair
Pyrimidine dimers introduce local conformational changes in the DNA structure, which allow recognition of the lesion by repair enzymes. In most organisms (excluding placental mammals such as humans) they can be repaired by photoreactivation. Photoreactivation is a repair process in which photolyase enzymes reverse CPDs using photochemical reactions. In addition, some photolyases can also repair 6-4 photoproducts of UV induced DNA damage. Photolyase enzymes utilize flavin adenine dinucleotide (FAD) as a cofactor in the repair process.
The UV dose that reduces a population of wild-type yeast cells to 37% survival is equivalent (assuming a Poisson distribution of hits) to the UV dose that causes an average of one lethal hit to each of the cells of the population. The number of pyrimidine dimers induced per haploid genome at this dose was measured as 27,000. A mutant yeast strain defective in the three pathways by which pyrimidine dimers were known to be repaired in yeast was also tested for UV sensitivity. It was found in this case that only one or, at most, two unrepaired pyrimidine dimers per haploid genome are lethal to the cell. These findings thus indicate that the repair of thymine dimers in wild-type yeast is highly efficient.
Nucleotide excision repair, sometimes termed "dark reactivation", is a more general mechanism for repair of lesions and is the most common form of DNA repair for pyrimidine dimers in humans. This process works by using cellular machinery to locate the dimerized nucleotides and excise the lesion. Once the CPD is removed, there is a gap in the DNA strand that must be filled. DNA machinery uses the undamaged complementary strand to synthesize nucleotides off of and consequently fill in the gap on the previously damaged strand.
Xeroderma pigmentosum (XP) is a rare genetic disease in humans in which genes that encode for NER proteins are mutated and result in decreased ability to combat pyrimidine dimers that form as a result of UV damage. Individuals with XP are also at a much higher risk of cancer than others, with a greater than 5,000 fold increased risk of developing skin cancers. Some common features and symptoms of XP include skin discoloration, and the formation of multiple tumors proceeding UV exposure.
A few organisms have other ways to perform repairs:
- Spore photoproduct lyase is found in spore-forming bacteria. It returns thymine dimers to their original state.
- Deoxyribodipyrimidine endonucleosidase is found in bacteriophage T4. It is a base excision repair enzyme specific for pyrimidine dimers. It is then able to cut open the AP site.
Another type of repair mechanism that is conserved in humans and other non-mammals is translesion synthesis. Typically, the lesion associated with the pyrimidine dimer blocks cellular machinery from synthesizing past the damaged site. However, in translesion synthesis, the CPD is bypassed by translesion polymerases, and replication and or transcription machinery can continue past the lesion. One specific translesion DNA polymerase, DNA polymerase η, is deficient in individuals with XPD.
Effect of topical sunscreen and effect of absorbed sunscreen
Direct DNA damage is reduced by sunscreen, which also reduces the risk of developing a sunburn. When the sunscreen is at the surface of the skin, it filters the UV rays, which attenuates the intensity. Even when the sunscreen molecules have penetrated into the skin, they protect against direct DNA damage, because the UV light is absorbed by the sunscreen and not by the DNA.Sunscreen primarily works by absorbing the UV light from the sun through the use of organic compounds, such as oxybenzone or avobenzone. These compounds are able to absorb UV energy from the sun and transition into higher-energy states. Eventually, these molecules return to lower energy states, and in doing so, the initial energy from the UV light can be transformed into heat. This process of absorption works to reduce the risk of DNA damage and the formation of pyrimidine dimers. UVA light makes up 95% of the UV light that reaches earth, whereas UVB light makes up only about 5%. UVB light is the form of UV light that is responsible for tanning and burning. Sunscreens work to protect from both UVA and UVB rays. Overall, sunburns exemplify DNA damage caused by UV rays, and this damage can come in the form of free radical species, as well as dimerization of adjacent nucleotides.