Pyrimidine dimer

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Formation of thymine dimer lesion in DNA. The photon causes two consecutive bases on one strand to bind together, destroying the normal base-pairing double-strand structure in that area. DNA UV mutation.svg
Formation of thymine dimer lesion in DNA. The photon causes two consecutive bases on one strand to bind together, destroying the normal base-pairing double-strand structure in that area.

Pyrimidine dimers represent molecular lesions originating from thymine or cytosine bases within DNA, resulting from photochemical reactions. [1] [2] These lesions, commonly linked to direct DNA damage, [3] are induced by ultraviolet light (UV), particularly UVC, result in the formation of covalent bonds between adjacent nitrogenous bases along the nucleotide chain near their carbon–carbon double bonds, [4] the photo-coupled dimers are fluorescent. [5] Such dimerization, which can also occur in double-stranded RNA (dsRNA) involving uracil or cytosine, leads to the creation of cyclobutane pyrimidine dimers (CPDs) and 6–4 photoproducts. These pre-mutagenic lesions modify the DNA helix structure, resulting in abnormal non-canonical base pairing and, consequently, adjacent thymines or cytosines in DNA will form a cyclobutane ring when joined together and cause a distortion in the DNA. This distortion prevents DNA replication and transcription mechanisms beyond the dimerization site. [6]

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While up to 100 such reactions per second may transpire in a skin cell exposed to sunlight resulting in DNA damage, they are typically rectified promptly through DNA repair, such as through photolyase reactivation or nucleotide excision repair, with the latter being prevalent in humans. Conversely, certain bacteria utilize photolyase, powered by sunlight, to repair pyrimidine dimer-induced DNA damage. Unrepaired lesions may lead to erroneous nucleotide incorporation by polymerase machinery. Overwhelming DNA damage can precipitate mutations within an organism's genome, potentially culminating in cancer cell formation. [7] Unrectified lesions may also interfere with polymerase function, induce transcription or replication errors, or halt replication. Notably, pyrimidine dimers contribute to sunburn and melanin production, and are a primary factor in melanoma development in humans.

Types of pyrimidine dimers

Cyclobutane dimer (CPD) (left), 6,4-dimer (6-4PP) (right) CPD & 6-4PP thymidine photodimer.svg
Cyclobutane dimer (CPD) (left), 6,4-dimer (6-4PP) (right)

Pyrimidine dimers encompass several types, each with distinct structures and implications for DNA integrity.

Cyclobutane pyrimidine dimer (CPD) is a dimer which features a four-membered ring formed by the fusion of two double-bonded carbons from adjacent pyrimidines. CPDs disrupt the formation of the base pair during DNA replication, potentially leading to mutations. [8] [9] [10]

The 6–4 photoproduct (6–4 pyrimidine–pyrimidone, or 6–4 pyrimidine–pyrimidinone) is an alternate dimer configuration consisting of a single covalent bond linking the carbon at the 6 (C6) position of one pyrimidine ring and carbon at the 4 (C4) position of the adjoining base’s ring. [11] This type of conversion occurs at one third the frequency of CPDs and has a higher mutagenic risk. [12]

A third type of molecular lesion is a Dewar pyrimidinone, resulting from the reversible isomerization of a 6–4 photoproduct under further light exposure. [13]

Mutagenesis

Mutagenesis, the process of mutation formation, is significantly influenced by translesion polymerases which often introduce mutations at sites of pyrimidine dimers. This occurrence is noted both in prokaryotes, through the SOS response to mutagenesis, and in eukaryotes. Despite thymine-thymine CPDs being the most common lesions induced by UV, translesion polymerases show a tendency to incorporate adenines, resulting in the accurate replication of thymine dimers more often than not. Conversely, cytosines that are part of CPDs are susceptible to deamination, leading to a cytosine to thymine transition, thereby contributing to the mutation process. [14]

DNA repair

Melanoma, a type of skin cancer Melanoma.jpg
Melanoma, a type of skin cancer

Pyrimidine dimers introduce local conformational changes in the DNA structure, which allow recognition of the lesion by repair enzymes. [15] In most organisms (excluding placental mammals such as humans) they can be repaired by photoreactivation. [16] 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. [17]

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. [18] The number of pyrimidine dimers induced per haploid genome at this dose was measured as 27,000. [18] 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. [18] 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. [6]

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. [7] 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:

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. [20]

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. [21] 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. [22]

See also

Related Research Articles

Mutagenesis is a process by which the genetic information of an organism is changed by the production of a mutation. It may occur spontaneously in nature, or as a result of exposure to mutagens. It can also be achieved experimentally using laboratory procedures. A mutagen is a mutation-causing agent, be it chemical or physical, which results in an increased rate of mutations in an organism's genetic code. In nature mutagenesis can lead to cancer and various heritable diseases, and it is also a driving force of evolution. Mutagenesis as a science was developed based on work done by Hermann Muller, Charlotte Auerbach and J. M. Robson in the first half of the 20th century.

<span class="mw-page-title-main">DNA polymerase</span> Form of DNA replication

A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create two identical DNA duplexes from a single original DNA duplex. During this process, DNA polymerase "reads" the existing DNA strands to create two new strands that match the existing ones. These enzymes catalyze the chemical reaction

<span class="mw-page-title-main">DNA repair</span> Cellular mechanism

DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA damage, resulting in tens of thousands of individual molecular lesions per cell per day. Many of these lesions cause structural damage to the DNA molecule and can alter or eliminate the cell's ability to transcribe the gene that the affected DNA encodes. Other lesions induce potentially harmful mutations in the cell's genome, which affect the survival of its daughter cells after it undergoes mitosis. As a consequence, the DNA repair process is constantly active as it responds to damage in the DNA structure. When normal repair processes fail, and when cellular apoptosis does not occur, irreparable DNA damage may occur. This can eventually lead to malignant tumors, or cancer as per the two-hit hypothesis.

<span class="mw-page-title-main">Molecular lesion</span> Damage to the structure of a biological molecule

A molecular lesion or point lesion is damage to the structure of a biological molecule such as DNA, RNA, or protein. This damage may result in the reduction or absence of normal function, and in rare cases the gain of a new function. Lesions in DNA may consist of breaks or other changes in chemical structure of the helix, ultimately preventing transcription. Meanwhile, lesions in proteins consist of both broken bonds and improper folding of the amino acid chain. While many nucleic acid lesions are general across DNA and RNA, some are specific to one, such as thymine dimers being found exclusively in DNA. Several cellular repair mechanisms exist, ranging from global to specific, in order to prevent lasting damage resulting from lesions.

Cyclobutane is a cycloalkane and organic compound with the formula (CH2)4. Cyclobutane is a colourless gas and is commercially available as a liquefied gas. Derivatives of cyclobutane are called cyclobutanes. Cyclobutane itself is of no commercial or biological significance, but more complex derivatives are important in biology and biotechnology.

Nuclear DNA (nDNA), or nuclear deoxyribonucleic acid, is the DNA contained within each cell nucleus of a eukaryotic organism. It encodes for the majority of the genome in eukaryotes, with mitochondrial DNA and plastid DNA coding for the rest. It adheres to Mendelian inheritance, with information coming from two parents, one male and one female—rather than matrilineally as in mitochondrial DNA.

<span class="mw-page-title-main">Nucleotide excision repair</span> DNA repair mechanism

Nucleotide excision repair is a DNA repair mechanism. DNA damage occurs constantly because of chemicals, radiation and other mutagens. Three excision repair pathways exist to repair single stranded DNA damage: Nucleotide excision repair (NER), base excision repair (BER), and DNA mismatch repair (MMR). While the BER pathway can recognize specific non-bulky lesions in DNA, it can correct only damaged bases that are removed by specific glycosylases. Similarly, the MMR pathway only targets mismatched Watson-Crick base pairs.

<span class="mw-page-title-main">Photolyase</span> Class of enzymes

Photolyases are DNA repair enzymes that repair damage caused by exposure to ultraviolet light. These enzymes require visible light both for their own activation and for the actual DNA repair. The DNA repair mechanism involving photolyases is called photoreactivation. They mainly convert pyrimidine dimers into a normal pair of pyrimidine bases. Photo reactivation, the first DNA repair mechanism to be discovered, was described initially by Albert Kelner in 1949 and independently by Renato Dulbecco also in 1949.

A postzygotic mutation is a change in an organism's genome that is acquired during its lifespan, instead of being inherited from its parent(s) through fusion of two haploid gametes. Mutations that occur after the zygote has formed can be caused by a variety of sources that fall under two classes: spontaneous mutations and induced mutations. How detrimental a mutation is to an organism is dependent on what the mutation is, where it occurred in the genome and when it occurred.

<span class="mw-page-title-main">Crosslinking of DNA</span> Phenomenon in genetics

In genetics, crosslinking of DNA occurs when various exogenous or endogenous agents react with two nucleotides of DNA, forming a covalent linkage between them. This crosslink can occur within the same strand (intrastrand) or between opposite strands of double-stranded DNA (interstrand). These adducts interfere with cellular metabolism, such as DNA replication and transcription, triggering cell death. These crosslinks can, however, be repaired through excision or recombination pathways.

Postreplication repair is the repair of damage to the DNA that takes place after replication.

<span class="mw-page-title-main">POLI</span> Protein-coding gene in the species Homo sapiens

DNA polymerase iota is an enzyme that in humans is encoded by the POLI gene. It is found in higher eukaryotes, and is believed to have arisen from a gene duplication from Pol η. Pol ι, is a Y family polymerase that is involved in translesion synthesis. It can bypass 6-4 pyrimidine adducts and abasic sites and has a high frequency of wrong base incorporation. Like many other Y family polymerases Pol ι, has low processivity, a large DNA binding pocket and doesn't undergo conformational changes when DNA binds. These attributes are what allow Pol ι to carry out its task as a translesion polymerase. Pol ι only uses Hoogsteen base pairing, during DNA synthesis, it will add adenine opposite to thymine in the syn conformation and can add both cytosine and thymine in the anti conformation across guanine, which it flips to the syn conformation.

<span class="mw-page-title-main">DDB2</span> Protein-coding gene in the species Homo sapiens

DNA damage-binding protein 2 is a protein that in humans is encoded by the DDB2 gene.

<span class="mw-page-title-main">DNA polymerase eta</span> Protein-coding gene in the species Homo sapiens

DNA polymerase eta, is a protein that in humans is encoded by the POLH gene.

Spore photoproduct lyase is a radical SAM enzyme that repairs DNA cross linking of thymine bases caused by UV-radiation. There are several types of thymine cross linking, but SPL specifically targets 5-thyminyl-5,6-dihydrothymine, which is also called spore photoproduct (SP). Spore photoproduct is the predominant type of thymine crosslinking in germinating endospores, which is why SPL is unique to organisms that produce endospores, such as Bacillus subtilis. Other types of thymine crosslinking, such as cyclobutane pyrimidine dimers (CPD) and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs), are less commonly formed in endospores. These differences in DNA crosslinking are a function of differing DNA structure. Spore genomic DNA features many DNA binding proteins called small acid soluble proteins, which changes the DNA from the traditional B-form conformation to an A-form conformation. This difference in conformation is believed to be the reason why dormant spores predominantly accumulate SP in response to UV-radiation, rather than other forms of cross linking. Spores cannot repair cross-linking while dormant, instead the SPs are repaired during germination to allow the vegetative cell to function normally. When not repaired, spore photoproduct and other types of crosslinking can cause mutations by blocking transcription and replication past the point of the crosslinking. The repair mechanism utilizing spore photoproduct lyase is one of the reasons for the resilience of certain bacterial spores.

DNA damage-binding protein or UV-DDB is a protein complex that is responsible for repair of UV-damaged DNA. This complex is composed of two protein subunits, a large subunit DDB1 (p127) and a small subunit DDB2 (p48). When cells are exposed to UV radiation, DDB1 moves from the cytosol to the nucleus and binds to DDB2, thus forming the UV-DDB complex. This complex formation is highly favorable and it is demonstrated by UV-DDB's binding preference and high affinity to the UV lesions in the DNA. This complex functions in nucleotide excision repair, recognising UV-induced (6-4) pyrimidine-pyrimidone photoproducts and cyclobutane pyrimidine dimers.

<span class="mw-page-title-main">Kataegis</span>

In molecular biology, kataegis describes a pattern of localized hypermutations identified in some cancer genomes, in which a large number of highly patterned basepair mutations occur in a small region of DNA. The mutational clusters are usually several hundred basepairs long, alternating between a long range of C→T substitutional pattern and a long range of G→A substitutional pattern. This suggests that kataegis is carried out on only one of the two template strands of DNA during replication. Compared to other cancer-related mutations, such as chromothripsis, kataegis is more commonly seen; it is not an accumulative process but likely happens during one cycle of replication.

DNA Polymerase V is a polymerase enzyme involved in DNA repair mechanisms in bacteria, such as Escherichia coli. It is composed of a UmuD' homodimer and a UmuC monomer, forming the UmuD'2C protein complex. It is part of the Y-family of DNA Polymerases, which are capable of performing DNA translesion synthesis (TLS). Translesion polymerases bypass DNA damage lesions during DNA replication - if a lesion is not repaired or bypassed the replication fork can stall and lead to cell death. However, Y polymerases have low sequence fidelity during replication. When the UmuC and UmuD' proteins were initially discovered in E. coli, they were thought to be agents that inhibit faithful DNA replication and caused DNA synthesis to have high mutation rates after exposure to UV-light. The polymerase function of Pol V was not discovered until the late 1990s when UmuC was successfully extracted, consequent experiments unequivocally proved UmuD'2C is a polymerase. This finding lead to the detection of many Pol V orthologs and the discovery of the Y-family of polymerases.

<span class="mw-page-title-main">PrimPol</span> Protein-coding gene in the species Homo sapiens

PrimPol is a protein encoded by the PRIMPOL gene in humans. PrimPol is a eukaryotic protein with both DNA polymerase and DNA Primase activities involved in translesion DNA synthesis. It is the first eukaryotic protein to be identified with priming activity using deoxyribonucleotides. It is also the first protein identified in the mitochondria to have translesion DNA synthesis activities.

Mutational signatures are characteristic combinations of mutation types arising from specific mutagenesis processes such as DNA replication infidelity, exogenous and endogenous genotoxin exposures, defective DNA repair pathways, and DNA enzymatic editing.

References

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