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.
DNA crosslinking also has useful merit in chemotherapy and targeting cancerous cells for apoptosis, [1] as well as in understanding how proteins interact with DNA.
Many characterized crosslinking agents have two independently reactive groups within the same molecule, each of which is able to bind with a nucleotide residue of DNA. These agents are separated based upon their source of origin and labeled either as exogenous or endogenous. Exogenous crosslinking agents are chemicals and compounds, both natural and synthetic, that stem from environmental exposures such as pharmaceuticals and cigarette smoke or automotive exhaust. Endogenous crosslinking agents are compounds and metabolites that are introduced from cellular or biochemical pathways within a cell or organism.
Crosslinking Agent | Alkylating Agent | Crosslink Structure | Preferential Target Sequence |
Nitrogen Mustard | Intrastrand; 5'-GC | ||
Cisplatin | Intrastrand; 5'-GC Interstrand; 5'-GNG | ||
Carmustine (BNCU) | Interstand; GC basepair | ||
Mitomycin C | Interstrand; 5'-GC | ||
Psoralen | Interstand; 5'-TA | ||
Malondialdehyde | Interstrand; 5'-GC, 5'-CG | ||
Oxidative Stress (and Radiation) | Intrastrand; d(GpT) | ||
Nitrous Acid | Interstrand; 5'-CG |
Crosslinked DNA is repaired in cells by a combination of enzymes and other factors from the nucleotide excision repair (NER) pathway, homologous recombination, and the base excision repair (BER) pathway. To repair interstrand crosslinks in eukaryotes, a 3’ flap endonuclease from the NER, XPF-ERCC1, is recruited to the crosslinked DNA, where it assists in ‘unhooking’ the DNA by cleaving the 3’ strand at the crosslink site. The 5’ strand is then cleaved, either by XPF-ERCC1 or another endonuclease, forming a double-strand break (DSB), which can then be repaired by the homologous recombination pathway. [17]
The repair of interstrand crosslinks requires both nucleotide excision repair and homologous recombinational repair. [18] . The Werner syndrome (WRN) helicase and the BRCA1 protein collaborate as part of a cellular response to repair DNA interstrand crosslinks. [18] In vitro, BRCA1 interacts directly with WRN to stimulate WRN helicase activity. [18]
DNA crosslinks generally cause loss of overlapping sequence information from the two strands of DNA. Therefore, accurate repair of the damage depends on retrieving the lost information from an undamaged homologous chromosome in the same cell. Retrieval can occur by pairing with a sister chromatid produced during a preceding round of replication. In a diploid cell retrieval may also occur by pairing with a non-sister homologous chromosome, as occurs especially during meiosis.[ citation needed ] Once pairing has occurred, the crosslink can be removed and correct information introduced into the damaged chromosome by homologous recombination.
Cleavage of the bond between a deoxyribose sugar in DNA's sugar-phosphate backbone and its associated nucleobase leaves an abasic site in double stranded DNA. These abasic sites are often generated as an intermediate and then restored in base excision repair. However, if these sites are allowed to persist, they can inhibit DNA replication and transcription. [19] Abasic sites can react with amine groups on proteins to form DNA-protein crosslinks or with exocyclic amines of other nucleobases to form interstrand crosslinks. To prevent interstrand or DNA-protein crosslinks, enzymes from the BER pathway tightly bind the abasic site and sequester it from nearby reactive groups, as demonstrated in human alkyladenine DNA glycosylase (AAG) and E. coli 3-methyladenine DNA glycosylase II (AlkA). [20] in vitro evidence demonstrated that the Interstand Cross-Links induced by abasic site (DOB-ICL) is a replication-blocking and highly miscoding lesion. Compared to several other TLS pols examined, pol η is likely to contribute to the TLS-mediated repair of the DOB-ICL in vivo. [21] By using O6-2'-deoxyguanosine-butylene-O6-2'-deoxyguanosine (O6-dG-C4-O6-dG) DNA lesions which is a chemically stable structure, the bypassing activity of several DNA polymerases had been investigated and the results demonstrated that pol η exhibited the highest bypass activity; however, 70% of the bypass products were mutagenic containing substitutions or deletions. The increase in the size of unhooked repair intermediates elevates the frequency of deletion mutation. [22]
Treatment of E. coli with psoralen-plus-UV light (PUVA) produces interstrand crosslinks in the cells’ DNA. Cole et al. [23] and Sinden and Cole [24] presented evidence that a homologous recombinational repair process requiring the products of genes uvrA, uvrB, and recA can remove these crosslinks in E. coli. This process appears to be quite efficient. Even though one or two unrepaired crosslinks are sufficient to inactivate a cell, a wild-type bacterial cell can repair and therefore recover from 53 to 71 psoralen crosslinks. Eukaryotic yeast cells are also inactivated by one remaining crosslink, but wild type yeast cells can recover from 120 to 200 crosslinks. [25]
DNA-protein crosslinking can be caused by a variety of chemical and physical agents, including transition metals, ionizing radiation, and endogenous aldehydes, in addition to chemotherapeutic agents. [26] Similar to DNA crosslinking, DNA-protein crosslinks are lesions in cells that are frequently damaged by UV radiation. The UV's effect can lead to reactive interactions and cause DNA and the proteins that are in contact with it to crosslink. These crosslinks are very bulky and complex lesions. They primarily occur in areas of the chromosomes that are undergoing DNA replication and interfere with cellular processes.
The advancement in structure-identification methods has progressed, and the addition in the ability to measure interactions between DNA and protein is a requirement to fully understand the biochemical processes. The structure of DNA-protein complexes can be mapped by photocrosslinking, which is the photoinduced formation of a covalent bond between two macromolecules or between two different parts of one macromolecule. The methodology involves covalently linking a DNA-binding motif of the target sequence-specific DNA-binding protein with a photoactivatable crosslinking agent capable of reacting with DNA nucleotides when exposed to UV. This method provides information on the interaction between the DNA and protein in the crosslink. [27]
DNA repair pathways can result in the formation of tumor cells. Cancer treatments have been engineered using DNA cross-linking agents to interact with nitrogenous bases of DNA to block DNA replication. These cross-linking agents have the ability to act as single-agent therapies by targeting and destroying specific nucleotides in cancerous cells. This result is stopping the cycle and growth of cancer cells; because it inhibits specific DNA repair pathways, this approach has a potential advantage in having fewer side effects. [28]
In humans, the leading cause of cancer deaths worldwide is lung cancer, including non small cell lung carcinoma (NSCLC) which accounts for 85% of all lung cancer cases in the United States. [29] Individuals with NSCLC are often treated with therapeutic platinum compounds (e.g. cisplatin, carboplatin or oxaliplatin) (see Lung cancer chemotherapy) that cause interstrand DNA crosslinks. Among individuals with NSLC, low expression of the breast cancer 1 gene ( BRCA1) in the primary tumor has correlated with improved survival after platinum-containing chemotherapy. [30] [31] This correlation implies that low BRCA1 in the cancer, and the consequent low level of DNA repair, causes vulnerability of the cancer to treatment by the DNA crosslinking agents. High BRCA1 may protect cancer cells by acting in the homologous recombinational repair pathway that removes the damages in DNA introduced by the platinum drugs. The level of BRCA1 expression is potentially an important tool for tailoring chemotherapy in lung cancer management. [30] [31]
Clinical chemotherapeutics can induce enzymatic and non-enzymatic DNA-protein crosslinks. An example of this induction is with platinum derivatives, such as cisplatin and oxaliplatin. They create non-enzymatic DNA-protein crosslinks through non-specific crosslinking of chromatin-interacting proteins to DNA. Crosslinking is also possible in other therapeutic agents by either stabilizing covalent DNA–protein reaction intermediates or by creating a pseudosubstrate, which traps the enzyme on DNA. Camptothecin derivatives, such as irinotecan and topotecan, target and trap specific DNA topoisomerase 1 (TOP1) by intercalating within the enzyme–DNA interface. Because the toxicity of these drugs depends on TOP1 trapping, cellular sensitivity to these compounds depends directly on TOP1 expression levels. As a result, the function of these drugs is to serve as enzyme poisons rather than inhibitors. This can be applied to treat tumor cells by utilizing TOP 2 enzyme poisons. [32]
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.
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.
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.
In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural polymers.
Cisplatin is a chemical compound with formula cis-[Pt(NH3)2Cl2]. It is a coordination complex of platinum that is used as a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, cervical cancer, bladder cancer, head and neck cancer, esophageal cancer, lung cancer, mesothelioma, brain tumors and neuroblastoma. It is given by injection into a vein.
Nitrogen mustards (NMs) are cytotoxic organic compounds with the bis(2-chloroethyl)amino ((ClC2H4)2NR) functional group. Although originally produced as chemical warfare agents, they were the first chemotherapeutic agents for treatment of cancer. Nitrogen mustards are nonspecific DNA alkylating agents.
Psoralen is the parent compound in a family of naturally occurring organic compounds known as the linear furanocoumarins. It is structurally related to coumarin by the addition of a fused furan ring, and may be considered as a derivative of umbelliferone. Psoralen occurs naturally in the seeds of Psoralea corylifolia, as well as in the common fig, celery, parsley, West Indian satinwood, and in all citrus fruits. It is widely used in PUVA treatment for psoriasis, eczema, vitiligo, and cutaneous T-cell lymphoma; these applications are typically through the use of medications such as Methoxsalen. Many furanocoumarins are extremely toxic to fish, and some are deposited in streams in Indonesia to catch fish.
Busulfan is a chemotherapy drug in use since 1959. It is a cell cycle non-specific alkylating antineoplastic agent, in the class of alkyl sulfonates. Its chemical designation is 1,4-butanediol dimethanesulfonate.
Carmustine, sold under the brand name BiCNU among others, is a medication used mainly for chemotherapy. It is a nitrogen mustard β-chloro-nitrosourea compound used as an alkylating agent.
DNA repair protein RAD51 homolog 1 is a protein encoded by the gene RAD51. The enzyme encoded by this gene is a member of the RAD51 protein family which assists in repair of DNA double strand breaks. RAD51 family members are homologous to the bacterial RecA, Archaeal RadA, and yeast Rad51. The protein is highly conserved in most eukaryotes, from yeast to humans.
In molecular genetics, a DNA adduct is a segment of DNA bound to a cancer-causing chemical. This process could lead to the development of cancerous cells, or carcinogenesis. DNA adducts in scientific experiments are used as biomarkers of exposure. They are especially useful in quantifying an organism's exposure to a carcinogen. The presence of such an adduct indicates prior exposure to a potential carcinogen, but it does not necessarily indicate the presence of cancer in the subject animal.
An alkylating antineoplastic agent is an alkylating agent used in cancer treatment that attaches an alkyl group (CnH2n+1) to DNA.
Satraplatin is a platinum-based antineoplastic agent that was under investigation as a treatment of patients with advanced prostate cancer who have failed previous chemotherapy. It has not yet received approval from the U.S. Food and Drug Administration. First mentioned in the medical literature in 1993, satraplatin is the first orally active platinum-based chemotherapeutic drug; other available platinum analogues—cisplatin, carboplatin, and oxaliplatin—must be given intravenously.
DNA excision repair protein ERCC-1 is a protein that in humans is encoded by the ERCC1 gene. Together with ERCC4, ERCC1 forms the ERCC1-XPF enzyme complex that participates in DNA repair and DNA recombination.
ERCC4 is a protein designated as DNA repair endonuclease XPF that in humans is encoded by the ERCC4 gene. Together with ERCC1, ERCC4 forms the ERCC1-XPF enzyme complex that participates in DNA repair and DNA recombination.
E3 ubiquitin-protein ligase FANCL is an enzyme that in humans is encoded by the FANCL gene.
At its simplest, the adductome is the totality of chemical adducts that are present in particular cellular macromolecules such as DNA, and RNA, or proteins found within the organism. These adducts can detrimentally alter the chemical properties of these macromolecules and are therefore also referred to as damage. Adducts may arise as a consequence of the chemical reaction between a given "physicochemical agent to which an organism is exposed across the lifespan". These physicochemical agents can be exogenous in origin, and include ionizing and non-ionizing radiation, the diet, lifestyle factors, pollution, and xenobiotics. They made damage the macromolecules directly, or indirectly e.g., some xenobiotic substances require metabolism of the xenobiotic to produce a chemically reactive metabolite which can then form a covalent bond with the endogenous macromolecule. Agents that damage macromolecules can also arise from endogenous sources, such as reactive oxygen species that are a side product of normal respiration, leading to the formation of oxidatively damaged DNA etc., or other reactive species e.g., reactive nitrogen, sulphur, carbon, selenium and halogen species.
Platinum-based antineoplastic drugs are chemotherapeutic agents used to treat cancer. Their active moieties are coordination complexes of platinum. These drugs are used to treat almost half of people receiving chemotherapy for cancer. In this form of chemotherapy, commonly used drugs include cisplatin, oxaliplatin, and carboplatin, but several have been proposed or are under development. Addition of platinum-based chemotherapy drugs to chemoradiation in women with early cervical cancer seems to improve survival and reduce risk of recurrence.
Naturally occurring means the damage happens within the cell as a result of normal metabolic processes, without external factors like UV radiation or cigarette smoke causing the damage.
FANCD2/FANCI-associated nuclease 1 (KIAA1018) is an enzyme that in humans is encoded by the FAN1 gene. It is a structure dependent endonuclease. It is thought to play an important role in the Fanconi Anemia (FA) pathway.
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