DNA damage and repair are complex topics with certain details necessary and unavoidable to cover the subject. The BFT audience is comprised of readers with sophisticated science backgrounds and others for whom only the “bottom line” is of interest i.e. what to do to protect their skin. There’s something here for everyone.
DNA DAMAGE TAKES A TOLL…AND IT’S NEVER GOOD
Experimental animals with genetic deficiencies in DNA repair often show decreased life span and increased incidence of cancer. Mice deficient in certain repair mechanisms get lymphoma and infections more often, and, as a consequence, have shorter lifespans. In similar manner, rodents deficient in a key repair and transcription protein that unwinds DNA helices have premature onset of aging-related diseases and consequent shortening of lifespan.
If the rate of DNA damage exceeds the capacity of the cell to repair it, the accumulation of errors can overwhelm the cell and result in early senescence, apoptosis, or cancer. Inherited diseases associated with faulty DNA repair functioning result in premature aging, increased sensitivity to carcinogens, and correspondingly increased cancer risk. On the other hand, organisms with enhanced DNA repair systems exhibit remarkable resistance to the double-strand break-inducing effects of radioactivity, likely due to enhanced efficiency of DNA repair especially.
It’s not possible, and it’s not good to “get used to the damage.” There is a price to pay, sooner or later.
DNA REPAIR IS CRITICAL TO CELL HEALTH AND TO CELL SURVIVAL
Cells exposed to ionizing radiation, ultraviolet light or chemicals are prone to acquire multiple DNA lesions and double-strand breaks. Moreover, DNA damaging agents can damage other biomolecules such as proteins, carbohydrates, lipids, and RNA.
The global response to damage is an act directed toward the cells’ own preservation and triggers multiple pathways of macromolecular repair, lesion bypass, tolerance, or apoptosis. DNA repair is essential for cell vitality, cell survival and cancer prevention, yet cells’ ability to patch up damaged DNA declines with age for reasons not fully understood.
GENERAL TYPES OF DNA DAMAGE
Endogenous damage occurs as a result of the mere fact we are living organisms oxidizing fuel (food) to create cellular energy. This occurs within the mitochondria of the cells throughout our bodies. The biochemical processes produce reactive oxygen species (ROS) as normal metabolic byproducts. The process of creating energy within our mitochondria results in cascades of molecules “stealing” electrons from one another, ultimately donating electrons to the elemental oxygen taken in from the atmosphere. ROS lead to spontaneous DNA mutations.
Exogenous damage is caused by external agents such as UV light radiation from the sun, x-rays and gamma rays, certain plant toxins, human made chemicals, and viruses.
SPECIFIC KINDS OF ENDOGENOUS DNA DAMAGE
- oxidation of bases within the DNA helix and generation of DNA strand interruptions caused by reactive oxygen species
- alkylation of bases (attachment of hydrocarbon sidechains, most common methylation)
- hydrolysis of bases, such as deamination, depurination, and depyrimidination
- adduct formation – a DNA adduct is a segment of DNA bound to a cancer-causing chemical. Adducts may be single or multiple.
- mismatch of bases, due to errors in DNA replication, in which the wrong DNA base is stitched into place in a newly forming DNA strand, or a DNA base is skipped over or mistakenly inserted.
SPECIFIC KINDS OF ENDOGENOUS DNA DAMAGE
- UV-B light causes crosslinking between adjacent cytosine and thymine bases creating pyrimidine dimers. This is called direct DNA damage.
- UV-A light creates mostly free radicals. The damage caused by free radicals is called indirect DNA damage.
- Ionizing radiation such as that created by radioactive decay or in cosmic rays causes breaks in DNA strands. Intermediate-level ionizing radiation may induce irreparable DNA damage (leading to replicational and transcriptional errors needed for neoplasia or may trigger viral interactions) leading to pre-mature aging and cancer.
- Thermal disruption at elevated temperature increases the rate of depurination (loss of purine bases from the DNA backbone) and single-strand breaks.
- Industrial chemicals such as vinyl chloride and hydrogen peroxide, and environmental chemicals such as polycyclic aromatic hydrocarbons found in smoke, soot and tar create a huge diversity of DNA adducts.
NUCLEAR VERSUS MITOCHONDRIAL DAMAGE
In human cells, DNA is found in two cellular locations — inside the nucleus and inside the mitochondria. Nuclear DNA (nDNA) exists as chromatin during non-replicative stages of the cell cycle and is condensed into aggregate structures known as chromosomes during cell division. In either state the DNA is highly compacted and wound up around bead-like proteins called histones.
Whenever a cell needs to express the genetic information encoded in its nDNA the required chromosomal region is unraveled, genes located therein are expressed, and then the region is condensed back to its resting conformation.
CELLULAR RESPONSES TO DNA DAMAGE
Senescence and apoptosis
Senescence, an irreversible process in which the cell no longer divides, is a protective response to the shortening of the chromosome ends (telomeres). The telomeres are long regions of repetitive noncoding DNA that cap chromosomes and undergo partial degradation each time a cell undergoes division. Senescence in cells may serve as a functional alternative to apoptosis (programmed cell death) in cases where the physical presence of a cell for spatial reasons is required by the organism.
Unregulated cell division can lead to the formation of a malignant tumor which is potentially lethal to an organism. Therefore, senescence and apoptosis are considered to be part of a strategy of protection against cancer.
It is important to distinguish between DNA damage and mutation, the two major types of error in DNA. DNA damage and mutation are fundamentally different. Damage results in physical abnormalities in the DNA, such as single- and double-strand breaks, and polycyclic aromatic hydrocarbon adducts. DNA damage can be recognized by enzymes, and thus can be correctly repaired if redundant information, such as the undamaged sequence in the complementary DNA strand or in a homologous chromosome, is available for copying.
If a cell retains DNA damage, transcription of a gene can be prevented, and thus translation into a protein will also be blocked. Cellular replication may also be blocked or the cell may die.
In contrast to DNA damage, a mutation is a change in the base sequence of the DNA. A mutation cannot be recognized by enzymes once the base change is present in both DNA strands, and thus a mutation cannot be repaired. At the cellular level, mutations can cause alterations in protein function and regulation. Mutations are replicated when the cell replicates. In a population of cells, mutant cells will increase or decrease in frequency according to the effects of the mutation on the ability of the cell to survive and reproduce.
MECHANISMS OF DNA REPAIR
Depending on the type of damage inflicted on the DNA’s double helical structure, a variety of repair strategies have evolved to restore lost information. If possible, cells use the unmodified complementary strand of the DNA as a template to recover the original information. Without access to a template, cells use an error-prone recovery mechanism known as translesion synthesis as a last resort.
Damage to DNA alters the spatial configuration of the helix, and such alterations can be detected by the cell. Once damage is localized, specific DNA repair molecules bind at or near the site of damage, inducing other molecules to bind and form a complex that enables the actual repair to take place.
1. Single-strand damage
When only one of the two strands of a double helix has a defect, the other strand can be used as a template to guide the correction of the damaged strand. In order to repair damage to one of the two paired molecules of DNA, there exist a number of excision repair mechanisms that remove the damaged nucleotide and replace it with an undamaged nucleotide complementary to that found in the undamaged DNA strand.
- Base excision repair repairs damage to a single nitrogenous base by deploying enzymes called glycosylases. These enzymes remove a single base. Enzymes called AP endonucleases nick the damaged DNA backbone and removes the damaged region and then correctly synthesizes the new strand using the complementary strand as a template.
- Nucleotide excision repair occurs with bulky helix-distorting DNA damage, typically from UV light. Damaged regions are removed a three-step process of recognition of damage, excision of damaged DNA, and re-synthesis of removed DNA.
- Mismatch repair systems are present in essentially all cells to correct errors that are not corrected by proofreading. These systems consist of at least two proteins. One detects the mismatch, and the other recruits an endonuclease that cleaves the newly synthesized DNA strand close to the region of damage.
2. Double-strand breaks
Double-strand breaks, in which both strands in the double helix are severed, are particularly hazardous to the cell because they can lead to genome rearrangements. The cell will die in the next mitosis or in some rare instances, mutate. Three mechanisms exist to repair double-strand breaks (DSBs): non-homologous end joining (NHEJ), microhomology-mediated end joining (MMEJ), and homologous recombination.
We told you this is complex stuff!
THE VERDICT ON TOPICAL DNA REPAIR ENZYMES: POSITIVE BENEFIT? POSITIVELY!
Our skin cells have built-in protective mechanisms that use DNA repair enzymes, which are molecules that prompt chemical changes to repair UV damage. Scientists can now encapsulate such enzymes in lipid envelopes (liponanosomes) so that they can be topically applied, allowing them to penetrate the skin and supplement its natural protective abilities. Topically applied DNA repair enzymes can reverse free radical damage caused by UV exposure and decrease the number of gene errors (mutations) that can lead to cancers. Although these enzymes are not FDA approved for the prevention of skin cancer, evidence suggests they can prevent precancerous lesions such as actinic keratoses.
Supportive articles are below.
Photoprotection by topical DNA repair enzymes: molecular correlates of clinical studies. Photochem Photobiol. 1999 Feb;69(2):136-40.
A new approach to photoprotection is to repair DNA damage after UV exposure. This can be accomplished by delivery of a DNA repair enzyme with specificity to UV-induced cyclobutane pyrimidine dimers into skin by means of specially engineered liposomes. Treatment of DNA-repair-deficient xeroderma pigmentosum patients or skin cancer patients with T4N5 liposome lotion containing such DNA repair liposomes increases the removal of DNA damage in the first few hours after treatment. In these studies, a DNA repair effect was observed in some patients treated with heat-inactivated enzyme. Unexpectedly, it was discovered that the heat-inactivated T4 endonuclease V enzyme refolds and recovers enzymatic activity. These studies demonstrate that measurements of molecular changes induced by biological drugs are useful adjuvants to clinical studies.
Reversal of DNA Damage in the Skin with DNA Repair Liposomes. Toxicology Volume 193, Issues 1–2, 15 November 2003, Pages 3-34
UV radiation damages DNA within skin cells. This is not a problem so long as there is efficient repair. With excessive, chronic exposure to sunlight, the repair pathway in skin cells becomes overwhelmed allowing for photoproducts to persist and be passed on in subsequent generations of cells, leading to errors in DNA replication, ultimately leading to some cancers.DNA damage caused by UV radiation is in constant repair with an intrinsic repair pathway.
This pathway consists of five steps including the following: 1) recognition of DNA lesion, 2) incision of the damaged strand on both sides of the lesion, 3) removal of the damaged oligonucleotide (DNA material), 4) synthesis of a patch, and 5) insertion of the patch.The importance of this pathway in protecting against skin cancer is illustrated by considering individuals with the hereditary disorder xeroderma pigmentosum (XP). Individuals with XP have a mutation in one of several genes involved in the repair pathway, which involves the products of at least 20 genes. XP patients have a 2000-fold increased risk of skin cancer.
DNA repair enzymes: an important role in skin cancer prevention and reversal of photodamage–a review of the literature. J Drugs Dermatol. 2015 Mar;14(3):297-303.
The incidence of skin cancer continues to increase annually despite preventative measures. Non-melanoma skin cancer affects more than 1,000,000 people in the United States every year.1 The current preventative measures, such as sunscreens and topical antioxidants, have not shown to be effective in blocking the effects of UV radiation based on these statistics. The level of antioxidants contained in the majority of skin creams is not sufficient to majorly impact free radical damage. Sunscreens absorb only a portion of UV radiation and often are not photostable. In this review article, we present the novel use of exogenous DNA repair enzymes and describe their role in combating photocarcinogenesis and photoaging. Topical application of these enzymes serves to supplement intrinsic DNA repair mechanisms. The direct repair of DNA damage by endogenous repair enzymes lessens rates of mutagenesis and strengthens the immune response to tumor cells. However, these innate mechanisms are not 100% efficient. The use of exogenous DNA repair enzymes presents a novel way to supplement intrinsic mechanisms and improve their efficacy. Several DNA repair enzymes critical to the prevention of cutaneous malignancies have been isolated and added to topical preparations designed for skin cancer prevention. These DNA repair enzymes maximize the rate of DNA repair and provide a more efficient response to carcinogenesis.
Skin photoprotection by natural polyphenols: anti-inflammatory, antioxidant and DNA repair mechanisms. Archives of Dermatological Research. March 2010, Volume 302, Issue 2, pp 71–83
Epidemiological, clinical and laboratory studies have implicated solar ultraviolet (UV) radiation in various skin diseases including, premature aging of the skin and melanoma and non-melanoma skin cancers. Chronic UV radiation exposure-induced skin diseases or skin disorders are caused by the excessive induction of inflammation, oxidative stress and DNA damage, etc. The use of chemopreventive agents, such as plant polyphenols, to inhibit these events in UV-exposed skin is gaining attention. Chemoprevention refers to the use of agents that can inhibit, reverse or retard the process of these harmful events in the UV-exposed skin. A wide variety of polyphenols or phytochemicals, most of which are dietary supplements, have been reported to possess substantial skin photoprotective effects. This review article summarizes the photoprotective effects of some selected polyphenols, such as green tea polyphenols, grape seed proanthocyanidins, resveratrol, silymarin and genistein, on UV-induced skin inflammation, oxidative stress and DNA damage, etc., with a focus on mechanisms underlying the photoprotective effects of these polyphenols. The laboratory studies conducted in animal models suggest that these polyphenols have the ability to protect the skin from the adverse effects of UV radiation, including the risk of skin cancers. It is suggested that polyphenols may favorably supplement sunscreens protection and may be useful for skin diseases associated with solar UV radiation-induced inflammation, oxidative stress and DNA damage.
Topical application of preparations containing DNA repair enzymes prevents ultraviolet-induced telomere shortening and c-FOS proto-oncogene hyperexpression in human skin: an experimental pilot study. J Drugs Dermatol. 2013 Sep;12(9):1017-21.
The exposure to ultraviolet radiation (UVR) is one of the most important risk factors for skin aging and increases the risk of malignant transformation. Telomere shortening and an altered expression of the proto-oncogene c-FOS are among the key molecular mechanisms associated with photoaging and tumorigenesis. Photolyase from A. nidulans and endonuclease from M. luteus are xenogenic DNA repair enzymes which can reverse the molecular events associated with skin aging and carcinogenosis caused by UVR exposure. Therefore, the purpose of this study was to investigate whether the topical application of preparations containing DNA repair enzymes may prevent UVR-induced acute telomere shortening and FOS gene hyperexpression in human skin biopsies. Twelve volunteers (Fitzpatrick skin types I and II) were enrolled for this experimental study, and six circular areas (10 mm diameter) were marked out on the nonexposed lower back of each participant. One site was left untreated (site 1: negative control), whereas the remaining five sites (designated sites 2-6) were exposed to solar-simulated UVR at 3 times the MED on four consecutive days. Site 2 received UVR only (site 2: positive control), whereas the following products were applied to sites 3-6, respectively: vehicle (moisturizer base cream; applied both 30 minutes before and immediately after each irradiation; site 3); a traditional sunscreen (SS, SPF 50) 30 minutes before irradiation and a vehicle immediately after irradiation (site 4); a SS 30 minutes before irradiation and an endonuclease preparation immediately after irradiation (site 5); a SS plus photolyase 30 minutes before irradiation and an endonuclease preparation immediately after irradiation (site 6). Skin biopsies were taken 24 h after the last irradiation. The degree of telomere shortening and c-FOS gene expression were measured in all specimens. Strikingly, the combined use of a SS plus photolyase 30 minutes before irradiation and an endonuclease preparation immediately after irradiation completely abrogated telomere shortening and c-FOS gene hyperexpression induced by the experimental irradiations. We conclude that the topical application of preparations containing both photolyase from A. nidulans and endonuclease from M. luteus may be clinically useful to prevent skin aging and carcinogenesis by abrogating UVR-induced telomere shortening and c-FOS gene hyperexpression.
Topical DNA Repair Enzyme May Prevent Skin Cancer. Oncology, May 1, 2001; Vo.10; Issue
In a phase III clinical trial of 30 patients with xeroderma pigmentosum, use of a topically applied DNA repair enzyme (T4N5 liposome lotion) for 1 year reduced the incidence of basal cell carcinoma by 30% and actinic keratoses by 68%, compared with placebo. The lotion contains a bacterial DNA repair enzyme, T4 endonuclease V, encapsulated in a pH-sensitive engineered liposome for delivery into living cells. When delivered intracellularly, this enzyme increases the repair rate of sunlight-induced DNA damage in cells.
A PLETHORA OF DNA REPAIR ENZYMES ARE COMMERCIALLY AVAILABLE
The list of DNA enzymes commercially available is long, with surprising specificity on the precise chemical reaction each enzyme is capable of enhancing.
- coli Endonuclease III (Catalog # 4045-01K-EB; 4045-05K-EB)
Specificity: Releases bases damaged by UV light, ionizing radiation, osmium tetroxide, or acid. Catalyzes the excision of cis- and trans- thymine glycol, 5,6-dihydrothymine, 5,6-dihydroxydihydrothymine, 5-hydroxy-5-methylhydantoin, 6-hydroxy-5,6-dihydropyrimidines, 5-hydroxycytosine and 5-hydroxyuracil, 5-hydroxy-6-hydrothymine, 5,6-dihydrouracil, 5-hydroxy-6-hydrouracil, AP sites, uracil glycol, methyltartronylurea, alloxan, and urea.
Research Application: Radiation, oxidation, and UV damage
- coli Fpg (Catalog # 4040-100-EB)
Specificity: Catalyzes the excision of multiple forms of damaged bases, including 8-oxoguanine, 5-hydroxycytosine, 5-hydroxyuracil, aflatoxin-bound imidazole ring-opened guanine, imidazole ring-opened N-2-aminofluorene-C8-guanine, and open ring forms of 7-methylguanine.
Research Application: Radiation and oxidative damage
- coli MutY DNA Glycosylase (Catalog # 4000-500-EB)
Specificity: Acts with Fpg to prevent the potentially mutagenic consequences of 8-oxo-dG lesions. Catalyzes the removal of adenine from 8-oxo-dG:A mis-pairs to initiate base excision repair.
Research Application: 8-oxo-dG:A mismatches and oxidative damage
- Human OGG1 (Catalog # 4130-100-EB) Specificity: Catalyzes the removal of 8-oxoguanine and DNA-containing formamidopyrimidine moieties. 8-oxoguanine mis-pairs with adenine during replication and gives rise to G:C to T:A transversions. Research Application: Radiation and oxidative damage
- PARP High Specific Activity(Catalog # 4668-100-01; 4668-500-01) Specificity: Modifies proteins post-translationally by poly ADP ribosylation using nicotinamide adenine dinucleotide (NAD) as a substrate. Poly ADP ribosylation of proteins occurs by the addition of an ADP-ribose to glutamic acid residues. Research Application: Assessment of damaged DNA, measurement of PARP inhibition, identification of PARP inhibitors or activators
- thermoautotrophicum TDG (Catalog # 4070-500-EB) Specificity: Recognizes T:G mismatches in duplex DNA and cleaves the strand with the T, while the opposite strand is not cleaved. It also recognizes G:G mismatches if at least one of the neighboring bases is an adenine or a thymine and nicks one strand or the other.
Research Application: T:G, G:G mismatches and deamination of 5-methyl-cytosine
Below: a list of enzymes commercially available from one vendor only.