Mutagens- Definition, Types (Physical, Chemical, Biological)

A mutagen is a physical or chemical agent that can cause mutations in DNA and raises their frequency above natural background levels.

  • They may directly damage DNA due to their actions, which most frequently leads to replication errors.
  • Mutations occur randomly, i.e., not directed according to the organism’s requirements. Environmental factors cause most mutations, but they can also be created in the lab using chemicals, radiation, or physical stimuli.
  • These mutagens may be carcinogens because numerous mutations can lead to cancer in animals, though not necessarily in all cases.
  • All mutagens exhibit distinctive mutational signatures, while some chemicals can become mutagenic due to cellular processes.
Mutagens
Mutagens

Mutagens Types

Mutagens can be classified into three major types based on their origin. They are:

  1. Physical mutagen
  2. Chemical mutagen
  3. Biological mutagen

Physical Mutagen

It includes:

  1. High energy radiations 
  2. Increase in temperature

A. Radiation

The radiations which are important in mutagenesis are of two categories. They are:

  1. Ionizing radiations: X-rays and gamma rays; alpha and beta rays; electrons, neutrons, protons, and other fast-moving particles
  2. Non-ionizing radiations: Ultraviolet and visible light.

Ionizing radiations

The process by which ionizing radiations trigger mutation is still mostly unknown. Ionizing radiation causes chromosomal changes such break, deletion, addition, inversion, and translocation by damaging the poly sugar-phosphate backbone of DNA.

The active function of oxygen is anticipated during DNA molecule breaking caused by ionizing radiation because oxygen is necessary for producing H2O2 and HO2, which may cause DNA molecule breakage in irradiated water.

The most common lab sources include cobalt-60 and cesium-137.

Non-ionizing radiations

Ultraviolet (UV) light is a non-ionizing radiation that may cause mutation. 

The most effective wavelength of ultraviolet for inducing mutations is about 2,600 A°. This is a wavelength best absorbed by DNA and at which proteins absorb little energy.

Some ways that UV radiations induce mutations include base deletion, strand breakage, cross-linking, and the formation of nucleotide dimers.

UV radiations are of three types: 

UV-A has a wavelength of 320nm (near-visible range) and is known to cause the dimerization of pyrimidines. This particular form of pyrimidine dimerization alters the DNA structure, preventing the creation of the replication fork during the replication process. Such dimerization could have negative health effects.

UV-B has a wavelength of 290-320nm and is highly lethal to DNA. 

UV-C has a wavelength of 180-290nm, which is majorly absorbed by the ozone layer and is the most lethal and carcinogenic.

B. Temperature 

The rate of all chemical reactions is influenced by temperature. 

A temperature increase of 10°C doubles or triples the rate of mutation.

The thermal stability of DNA and the rate at which other substances react with it are both impacted by temperature, which also destroys the hydrogen and phosphodiester bonds in DNA.

Chemical Mutagen

Many chemical compounds are known to increase the mutability of genes. 

Auerbach and Robson’s experiment with male Drosophila melanogaster in 1947, utilizing mustard gas and related substances such as nitrogen and Sulphur mustards, mustard oil, and chloracetone, was the first to show that chemicals can cause mutations.

Chemical mutagens affect the chromosomal DNA in the following two ways:

  1. Direct gene change
  2. Copy error

Chemical mutagens can be classified into different categories, such as:

  1. Base analogs
  2. Intercalating agents
  3. Metal ions
  4. Alkylating agents

A. Base analogs

  • These substances have structural characteristics with bases like purines and pyrimidines.
  • 5-Bromouracil and aminopurine are the two most prevalent base analogs that are considered to be chemical mutagens.
  • Base analogs are integrated into the DNA structure during replication because of the structural resemblances between these agents and DNA bases.
  • Like adenine, aminopurine can pair up with either C or T to form a base pair (though base pairing with C is rare).
  • Some additional base analogs, including urethane triazine, caffeine (found in coffee, tea, and soft beverages), phenol and carcinogens, acridines (proflavin, etc.), and others, are similarly mutagenic.

B. Intercalating agents

  • Intercalating molecules have a hydrophobic heterocyclic ring structure that resembles the base pair ring structure.
  • These agents embed themselves in the DNA helix, causing interference with transcription, replication, and mutation, most frequently a frameshift mutation.

Some of the common intercalating agents are:

  • Ethidium bromide
  • Proflavine 
  • Acridine orange 
  • Actinomycin D 
  • Daunorubicin

C. Metal ion

  • Reactive oxygen species (ROS) are produced by mineral ions, such as nickel, chromium, cobalt, cadmium, arsenic, chromium, and iron, that lead to DNA hypermethylation. This promotes DNA damage and obstructs the DNA repair process.
  • For many organisms, certain inorganic chemicals like manganese chloride are mutagenic because they bind calcium and interfere with the integrity of the chromosome structure.

D. Alkylating agents

These chemicals cause DNA damage by inducing alkyl groups.

Alkyl group introduction boosts ionization, leading to base-pairing mistakes that eventually cause holes in the DNA strand and have a direct mutagenic effect on the DNA molecule.

Some of the common alkylating agents are: 

  • Nitrous acid
  • Ethylnitrosourea
  • Methylhydrazine
  • Dacarbazine
  • Formaldehyde
  • Vinyl chloride
  • Epoxides
  • Dimethyl and diethyl sulphonate
  • Methyl and ethyl methanesulphonate (MMS and EMS)
  • Nitrosoguanadine (NG) 

However, these substances can be eliminated from the DNA by the depurination process during the DNA repair process.

Biological Mutagen

It includes:

  1. Transposons and Insertion sequences (IS)
  2. Viruses
  3. Bacteria

A. Transposons and Insertion sequences (IS)

  • They are DNA units that carry out the DNA fragment’s self-directed relocation and multiplication.
  • Insertion sequences, or IS, are the shortest transposons (between 10 and 50 base pairs long).
  • Since IS and transposon migrates around the DNA, they are both referred to as jumping genes. The functionality of the genes is disrupted when transposons are inserted into chromosomal DNA.
  • Information for the enzyme transposases, which aids in creating the new transposition location in the DNA, is encoded by both IS and transposons.

Three types of transposons are usually found. They are:

  1. Replicative transposons: The transposons that retain the original locus and translocate its copy.
  2. Conservative transposons: The original transposon translocates itself.
  3. Retrotransposons transpose: These transposons translocate via RNA intermediates.

B. Viruses

  • The insertion of viral DNA into the genome may lead to the disruption of genetic function. 
  • The viruses cause deletions, insertions, and point alterations, including base substitutions.
  • The stimulation of cells’ prone-to-error repair mechanisms is reported to be connected to virus-induced mutagenesis.
  • Rous sarcoma virus has been reported to induce cancer. 

C. Bacteria

  • Reactive oxygen species are produced by some bacteria that cause inflammation, such as Helicobacter pylori, leading to DNA damage and decreased DNA repair.
  • By boosting activation-induced cytidine deaminase (AID), a DNA/RNA editing enzyme that links mutagenesis and cancer, H. pylori impacts genome integrity.

Positive effects of Mutagen

  1. Evolution

The changes in the gene pool that ultimately contributed to the evolution of life over time are caused by mutations.

Natural selection has allowed populations with a variety of mutations to persist, while populations that were unable to adapt to environmental changes or evolve as a result of those changes eventually perish.

One such genetic evolutionary stage involves the development of coat color by insects and animals for camouflage.

  1. Adaptation

Apolipoprotein A1-Milano (or Apo A1M), a mutant protein, is found in a small Italian group. 

Normal Apolipoprotein is the protein responsible for the transportation of cholesterol. 

The mutant form of apolipoprotein possesses antioxidant characteristics in addition to removing cholesterol and dissolving plaques and has aided the Italian population from cardiac diseases.

  1. Antibiotic Resistance in Bacteria

Mutations in many bacteria enable them to endure the presence of antibiotic medications. The mutations result in bacterial strains that are resistant to antibiotics.

Negative Effects of Mutagen

  1. A single missense mutation brings on sickle cell anemia in the germ cell-globin gene at codon 6. With this mutation, valine takes the place of glutamic acid at position 6 in the original protein. Hemoglobin, a protein that transports oxygen, is negatively impacted by this alteration.
  2. Some pesticides, including rotenone, paraquat, and maneb, can cause gene mutations, such as base pair alterations, which may result in neurological illnesses like Parkinson’s.
  3. Retinoblastoma or Retinal tumors in children.
  4. Genetic disorders include Tay-Sachs disease, Phenylketonuria, Color-blindness, and Cystic fibrosis.
Cancer Cell Transformation
Cancer Cell Transformation

Mutagenicity Testing

Identifying compounds that potentially change the genetic makeup of somatic and/or germ cells is the objective of mutagenicity testing, which influences regulatory decisions.

Genetic alterations usually manifest after a long time after exposure to the mutagen, in contrast to the majority of other types of toxicity.

Three endpoints are usually assessed to provide an adequate evaluation of the mutagenic potential. They are:

  1. Gene mutation
  2. Structural chromosome aberrations
  3. Numerical chromosome aberrations

The commonly used in vitro and in vivo studies are:

In Vitro Studies

  1. Bacterial reverse mutation test – Ames test
  2. Mammalian chromosome aberration test
  3. Mammalian cell gene mutation test
  4. UDS in mammalian cells

In Vivo Studies

  1. Rodent dominant lethal mutation test
  2. Mouse heritable translocation assay
  3. Mouse-specific locus test
  4. Sister chromatid exchange (SCE) analysis in spermatogonia
  5. UDS test in testicular cells
  6. Transgenic rodent somatic and germ cell gene mutation assay
  7. Mammalian erythrocyte micronucleus test

Antimutagen

It is an agent capable that can inactivate a mutagen, stopping the mutagen from acting, or otherwise preventing a mutagen’s interaction with DNA. 

It may directly or indirectly trigger, inhibit, or inactivate the enzymes of the DNA repair, recombination, and replication pathways.

The antimutagens can be classified as:

  1. Desmutagens
  2. Bio-antimutagens
  1. Desmutagens

These are compounds that, through enzymatic or chemical interaction, partially or completely render mutagens inactive before the mutagen affects the genes.

  1. Bio-antimutagens

These are true antimutagens, which suppress the mutation after mutagens damage genes. The mutagen-damaged DNA is repaired and replicated by them, which reduces the frequency of mutations.

Mechanism of Antimutagenesis

The major mechanisms of antimutagenesis can be broadly described as under:

  1. Chemical or enzymatic inactivation
  2. Prevention of the formation of active species
  3. Scavenging
  4. Antioxidant free radical scavenging
  1. Chemical or enzymatic inactivation 

Various substances can directly inactivate numerous mutagens since they are reactive and act not only on DNA but also on proteins and enzymes.

Both inducers and inhibitors of cytochrome P-450 enzymes, including indole-3-carbinol, have been linked to antimutagenic and anticarcinogenic activities.

Example:

Isothiocyanates such as benzyl isothiocyanate 

Antioxidants such as 2, 3-tert butyl-4-hydroxy-anisole (BHA).

  1. Prevention of the formation of active species

Numerous genotoxic mutagens or carcinogens need to be metabolically or biologically activated before they may become an electrophilic species (the active species) and interact with DNA.

Although these processes frequently occur in the liver, there is mounting evidence that other organs, particularly the GIT, can also activate metabolism.

Example:

N-nitro compounds frequently result from an interaction between secondary or tertiary amines and nitrite in the stomach.

  1. Scavenging

Several desmutagens can bind to or adsorb dietary mutagens to remove them.

The mutagen often does not change during this phase but cannot interact with DNA.

Example:

Chlorophyllin and some dietary fibers.

  1. Antioxidant and free radical scavenging

Free radicals can harm DNA, lead to mutagenicity and cytotoxicity, and as a result, are crucial to the development of cancer.

reactive oxygen species (ROS) can cause mutations and prevent DNA repair, which inactivates some tumor suppressor genes and results in cancer.

Numerous antimutagenic substances have antioxidant or free radical scavenging properties and can quickly neutralize most free radicals, especially those with a short half-life.

Example: 

OH radical.

Antimutagenic compounds

The major classes of antimutagenic compounds include:

  1. Vitamins: 
  • Vitamins C and E are found to be antimutagenic against doxorubicin-induced chromosomal aberrations.
  • Vitamins A, C, and E were reported to be antimutagenic towards Methyl Azoxy Methanol (MAM) induced mutagenesis in Salmonella Typhimurium strain TA100.
  • When given with a pesticide, vitamin C (ascorbic acid) significantly reduced the frequency of pesticide-induced mutations.
  1. Flavonoids
  • Leuteolin, kaempherol, and other phenolic hydroxyl group-containing flavonoids, as well as all flavones, exhibited antimutagenic properties.
  • Isoflavones and flavonoid glycosides have been found to have potent antimutagenic properties.
  • Hispidulin and hortensin were found to have an antimutagenic impact when tested against 2-amino anthracene, a compound that causes mutations caused by aflatoxin B1. 
  • Citrus juice flavonoids have anticarcinogenic and antimutagenic qualities.
  • Salmonella Typhimurium has been used to investigate the antimutagenic activity of all these flavonoids against various mutagens.
  1. Anthraquinones
  • Aloe barborescence anthraquinones have been found to have antimutagenic properties.
  • Anthrone, acridone, and xanthone, chemicals chemically similar to anthraquinones, exerted antimutagenecity, with anthrone being the most potent example.
  • All naphthaquinones are potent antimutagens.
  1. Phenolic compounds
  • Certain phenolic chemicals, such as ellagic acid, present in strawberries, raspberries, grapes, walnuts, etc., are reported to be antimutagenic.
  • The antimutagenic properties of green tea and black tea have been attributed to substances like epicatechin, (-) epicatechin gallate, (-) epigallocatechins, and (-) epigallocatechin gallate.
  • Green tea’s antimutagenic properties can stop the frequency of sister chromatid exchanges induced by smoking cigarettes.
  • The phenolics present in turmeric and clove, namely curcumin and eugenol, were found to inhibit the mutagenicity produced by direct-acting mutagens such as N-methyl-N’-nitro-N-nitrosoguanidine. 
  • Eugenol inhibits tobacco-induced mutagenesis in the Ames test. 
  1. Carotenoids
  • Carotenoids are known to affect the activation of promutagens. 
  • Antimutagenic activity of β-carotenre, canthaxanthin, β-carotene-8-apo-β-carotenal and 8-apo-β-carotene methyl ester showed a dose-dependent decrease in the mutagenicity.
  1. Diterpenoids
  • Erythroxydiol, a diterpenoid-like compound isolated from Aquillaria agallocha, exhibited antimutagenic and anticancer action.
  • Pulcherrimins A, B, C, and D, four new dibenzoate diterpenes isolated in the roots of Caesalpinia pulcherrima, were discovered to be active in a DNA repair-deficient yeast strain.
  1. Coumarins
  • Antimutagenic compounds such as coumarin, umbelliferone, and 8-methoxypsoralen have been identified from various plant sources.
  • Psoralen from Psoralea corylifolia, imperatorin, and osthol from Selinum monniere can reduce the mutagenicity brought on by benzo[α]pyrene.
  1. Tannins
  • Some tannins’ antioxidative properties are crucial in preventing cellular oxidative damage, such as lipid peroxidation, to their anticarcinogenic and antimutagenic capabilities.
  • Catechin, ellagic acid, and gallic acid have antimutagenic properties against mutagenicity brought on by known mutagens.
  1. Hormonal steroid
  • The synthetic β-oestradiol derivatives ethinyl oestradiol and mestranol, both commonly found in birth control tablets, were both potent mutagenicity inhibitors that acted at nanomolar doses.
  • The hormones testosterone, β-oestradiol, and dicthyl stilbesterol were antimutagenic and co-recombinogenic in experiments employing yeast without an external metabolic activation mechanism.
  1. Saponins
  • Human gut bacteria-introduced ginseng saponin metabolites were discovered to be antigenotoxic against benzo[α]pyrene-induced clastogenecity.
  1. Marine products
  • A variety of mutagens have the potential to be inhibited in their ability to cause mutations in Salmonella Typhimurium by certain secondary metabolites prevalent in marine species.
  • Elatol and obtusol are two antimutagenic substances discovered in a sea hare extract.
  • Using an antimutagenicity assay, the green algae Cymopolia barbata was used to isolate cymobarbatol and 4-isocymobarbatol.
  1. Miscellaneous compounds
  • The organosulfur compounds present in garlic extract with considerable antimutagenic activity include ajoene and one of the allicin’s derivatives.
  • Numerous other diverse phytocompound subgroups, including caffeine, trigonelline, and piperine, have been shown to have antimutagenic activities.
  • When isolated from Visma amazonica, xanthones such euxanthone and 1,5-dihydroxy-8-methoxyxanthone exhibit significant antimutagenic action against 2-aminoanthracene and EMS.
  • In a Salmonella mutation assay, an 80% ethanol extract of lemon grass (Cymbopogon citrates) was discovered to be antimutagenic against several recognized mutagens.
  • Lemongrass essential oil has an antimutagenic effect against lead nitrate and cyclophosphamide-induced micronuclei and chromosomal aberration. This oil is a component of Lemongrass Tea in central and southern India.

Food Products as Antimutagens

  • Dietary desmutagens function as tumor growth inhibitors later in the carcinogenesis process.
  • Some regularly ingested spices and vegetables, including turmeric, mustard, green leafy vegetables, and allium species, have been found to have antimutagenic and anticarcinogenic properties.
  • A human intervention trial using various carotenoids in vegetable products indicated that adding tomato, carrot, or spinach products to the diet significantly reduced the amount of lymphocyte DNA damage.
  • Using the Salmonella Typhimurium strains TA100 and TA1535, the garlic extract inhibited the mutagenicity caused by direct-acting mutagens such N-methyl-N’-nitro-N-nitrosoguanidine and sodium azide.
  • In the DNA repair host-mediated assay and liquid suspension experiment, casein demonstrated a high antimutagenic action in vivo and ex vivo.
  • Yogurt and Guava (Psidium guajava) are reported to be antimutagenic.
  • The mechanism of antimutagenic effects of mushrooms was found to be by direct chemical interaction with the mutagens viz. aflatoxin B1, benzo[α]pyrene, and acridine or inhibition of the activation process in the case of promutagens.
  • Asafoetida and turmeric extracts suppress microsomal activation-dependent mutagenicity of 2-acetamido fluorine.
  • Indian spinach leaf extract, curcumin, and eugenol, phenolics present in turmeric and clove, are reported to be antimutagenic.
  • In the Ames test, alkyl-resorcinols, amphiphilic substances frequently present in wheat grains, showed evidence of antimutagenicity.

References

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    https://www.biologyonline.com/dictionary/mutagen
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  4. National Human Genome Research Institute (NIH). (2022). Mutagen. Accessed from:
    https://www.genome.gov/genetics-glossary/Mutagen
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  6. Sundar R., Jain M, R., & Valani D. (2018). Chapter Ten – Mutagenicity Testing: Regulatory Guidelines and Current Needs. In Mutagenicity: Assays and Applications. Academic Press, Pages 191-228. ISBN 9780128092521. https://doi.org/10.1016/B978-0-12-809252-1.00010-9
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