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Interstrand crosslinks (ICLs) are highly toxic DNA lesions that prevent transcription and replication by inhibiting DNA strand separation. Agents that induce ICLs were one of the earliest, and are still the most widely used, forms of chemotherapeutic drug. Only recently, however, have we begun to understand how cells repair these lesions. Important insights have come from studies of individuals with Fanconi anaemia (FA), a rare genetic disorder that leads to ICL sensitivity. Understanding how the FA pathway links nucleases, helicases and other DNA-processing enzymes should lead to more targeted uses of ICL-inducing agents in cancer treatment and could provide novel insights into drug resistance.
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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 (such as proteins).
Crosslinking generally involves covalent bonds that join two polymer chains. The term curing refers to the crosslinking of thermosetting resins, such as unsaturated polyester and epoxy resin, and the term vulcanization is characteristically used for rubbers.[1] When polymer chains are crosslinked, the material becomes more rigid. The mechanical properties of a polymer depend strongly on the cross-link density. Low cross-link densities increase the viscosities of polymer melts. Intermediate cross-link densities transform gummy polymers into materials that have elastomeric properties and potentially high strengths. Very high cross-link densities can cause materials to become very rigid or glassy, such as phenol-formaldehyde materials.[2]
In one implementation, unpolymerized or partially polymerized resin is treated with a crosslinking reagent. In vulcanization, sulfur is the cross-linking agent. Its introduction changes rubber to a more rigid, durable material associated with car and bike tires. This process is often called sulfur curing. In most cases, cross-linking is irreversible, and the resulting thermosetting material will degrade or burn if heated, without melting. Chemical covalent cross-links are stable mechanically and thermally. Therefore, cross-linked products like car tires cannot be recycled easily. A class of polymers known as thermoplastic elastomers rely on physical cross-links in their microstructure to achieve stability, and are widely used in non-tire applications, such as snowmobile tracks, and catheters for medical use. They offer a much wider range of properties than conventional cross-linked elastomers because the domains that act as cross-links are reversible, so can be reformed by heat. The stabilizing domains may be non-crystalline (as in styrene-butadiene block copolymers) or crystalline as in thermoplastic copolyesters.
In contrast to chemical cross-links, physical cross-links are formed by weaker interactions. For example, sodium alginate gels upon exposure to calcium ions, which form ionic bonds that bridge between alginate chains.[5] Polyvinyl alcohol gels upon the addition of borax through hydrogen bonding between boric acid and the polymer's alcohol groups.[6][7] Other examples of materials which form physically cross-linked gels include gelatin, collagen, agarose, and agar agar.
Crosslinking is often measured by swelling tests. The crosslinked sample is placed into a good solvent at a specific temperature, and either the change in mass or the change in volume is measured. The more crosslinking, the less swelling is attainable. Based on the degree of swelling, the Flory Interaction Parameter (which relates the solvent interaction with the sample), and the density of the solvent, the theoretical degree of crosslinking can be calculated according to Flory's Network Theory.[8] Two ASTM standards are commonly used to describe the degree of crosslinking in thermoplastics. In ASTM D2765, the sample is weighed, then placed in a solvent for 24 hours, weighed again while swollen, then dried and weighed a final time.[9] The degree of swelling and the soluble portion can be calculated. In another ASTM standard, F2214, the sample is placed in an instrument that measures the height change in the sample, allowing the user to measure the volume change.[10] The crosslink density can then be calculated.
Lignin is a highly crosslinked polymer that comprises the main structural material of higher plants. A hydrophobic material, it is derived from precursor monolignols. Heterogeneity arises from the diversity and degree of crosslinking between these lignols.
Intrastrand DNA crosslinks have strong effects on organisms because these lesions interfere with transcription and replication. These effects can be put to good use (addressing cancer) or they can be lethal to the host organism. The drug cisplatin functions by formation of intrastrand crosslinks in DNA.[11] Other crosslinking agents include mustard gas, mitomycin, and psoralen.[12]
In proteins, crosslinks are important in generating mechanically stable structures such as hair and wool, skin, and cartilage. Disulfide bonds are common crosslinks.[13] Isopeptide bond formation is another type of protein crosslink.
The process of applying a permanent wave to hair involves the breaking and reformation disulfide bonds. Typically a mercaptan such as ammonium thioglycolate is used for the breaking. Following this, the hair is curled and then "neutralized". The neutralizer is typically an acidic solution of hydrogen peroxide, which causes new disulfide bonds to form, thus permanently fixing the hair into its new configuration.
Compromised collagen in the cornea, a condition known as keratoconus, can be treated with clinical crosslinking.[14]In biological context crosslinking could play a role in atherosclerosis through advanced glycation end-products (AGEs), which have been implicated to induce crosslinking of collagen, which may lead to vascular stiffening.[15]
CubeSat Laser Infrared CrosslinK (CLICK) mission will demonstrate technology to advance the state of the art in communications between small spacecraft as well as the capability to gauge their relative distance and location. CLICK is comprised of two sequential missions:
The CLICK technology will provide crosslink communication for small spacecraft that could enable NASA science missions that involve constellations or swarms of small spacecraft. High rate crosslinks are needed when communication nodes within small spacecraft must rapidly exchange or route significant amounts of data to other spacecraft or to the ground. Rapid exchange of observational data can enable onboard image processing that fuses images from multiple spacecraft to look for indicators that would trigger action or additional high-priority observations. NASA science missions involving multiple small spacecraft that form a constellation or swarm may carry one or more moderate to high-resolution imaging or scientific sensors that require high date rate processing and transmission.
The CLICK mission is managed and funded by the Small Spacecraft Technology (SST) program within the Space Technology Mission Directorate. The SST expands U.S. capability to execute unique missions through rapid development and in space demonstration of capabilities for small spacecraft applicable to exploration, science, and the commercial space sector. The SST will enable new mission architectures through the use of small spacecraft with goals to expand their reach to new destinations and challenging new environments.
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DNA interstrand crosslinks (ICLs) may arise following exposure to environmental mutagens, and are potently toxic when induced in large numbers by chemotherapeutic drugs. ICL-based chemotherapy is one of the most widely used forms of cancer treatment, particularly in the treatment of leukaemias.
Fanconi anaemia (FA) is a disorder that results in sensitivity to ICLs, which is caused by the mutation of one of at least 14 different genes. Although their functions have not been completely elucidated, there is substantial evidence to suggest that these genes, including the BRCA2 (also known as FANCD1) breast and ovarian cancer tumour suppressor, participate in a common pathway of ICL repair.
Homologous recombination (HR)-mediated repair of ICLs is promoted by the FA pathway. FA cells that are exposed to ICL-inducing agents, or chronically treated wild-type cells, may use alternative pathways of repair that lead to deleterious genetic aberrations such as radial chromosomes.
The modulation of ICL repair could improve chemotherapy outcomes. For example, the dose-limiting toxicities of ICLs mostly affect the blood system, so increasing the ability to repair ICLs in blood cells could prevent anaemia phenotypes. Alternatively, targeted downregulation of ICL repair in tumours could improve ICL-mediated tumour killing.
This research was supported by a grant to A.J.D. and S.C.W. from the Fanconi Anaemia Research Fund. Work in S.C.W.'s laboratory is supported by the European Research Council, the Louis-Jeantet foundation, the Breast Cancer Campaign, the Swiss Bridge Foundation and Cancer Research UK.
A structure thought to result from the fusion of the broken arms of non-homologous chromosomes. Such chromosomes cannot be properly segregated in most cells, resulting in either chromosome breakage or a failure in cell division.
(TLS). A DNA damage tolerance process that allows replication past DNA lesions. If the normal replicative polymerase cannot insert a base owing to damage in the template strand, it is often replaced by a lower fidelity translesion polymerase. 152ee80cbc
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