Genotoxicity is the
property
Property is a system of rights that gives people legal control of valuable things, and also refers to the valuable things themselves. Depending on the nature of the property, an owner of property may have the right to consume, alter, share, r ...
of chemical agents that damage the genetic information within a cell causing
mutations, which may lead to
cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
. While genotoxicity is often confused with
mutagenicity
In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer i ...
, all mutagens are genotoxic, but some genotoxic substances are not mutagenic. The alteration can have direct or indirect effects on the DNA: the induction of mutations, mistimed event activation, and direct DNA damage leading to mutations. The permanent, heritable changes can affect either
somatic cells
A somatic cell (from Ancient Greek σῶμα ''sôma'', meaning "body"), or vegetal cell, is any biological cell forming the body of a multicellular organism other than a gamete, germ cell, gametocyte or undifferentiated stem cell. Such cells compo ...
of the organism or
germ cells
Germ or germs may refer to:
Science
* Germ (microorganism), an informal word for a pathogen
* Germ cell, cell that gives rise to the gametes of an organism that reproduces sexually
* Germ layer, a primary layer of cells that forms during embryo ...
to be passed on to future generations.
Cells prevent expression of the genotoxic mutation by either
DNA repair
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 da ...
or
apoptosis; however, the damage may not always be fixed leading to
mutagenesis
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 lab ...
.
To
assay for genotoxic molecules, researchers assay for DNA damage in cells exposed to the toxic substrates. This DNA damage can be in the form of single- and double-strand breaks, loss of excision repair, cross-linking, alkali-labile sites, point mutations, and structural and numerical chromosomal aberrations.
The compromised integrity of the genetic material has been known to cause cancer. As a consequence, many sophisticated techniques including Ames Assay, ''in vitro'' and ''in vivo'' Toxicology Tests, and Comet Assay have been developed to assess the chemicals' potential to cause DNA damage that may lead to cancer.
Mechanism
![Transitions_and_transversions](https://upload.wikimedia.org/wikipedia/commons/0/0d/Transitions_and_transversions.svg)
The genotoxic substances induce damage to the genetic material in the cells through interactions with the DNA sequence and structure. For example, the transition metal
chromium interacts with DNA in its high-valent oxidation state so to incur DNA lesions leading to
carcinogenesis
Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abno ...
. The metastable oxidation state Cr(V) is achieved through reductive activation. The researchers performed an experiment to study the interaction between DNA with the carcinogenic chromium by using a Cr(V)-Salen complex at the specific oxidation state.
The interaction was specific to the
guanine
Guanine () ( symbol G or Gua) is one of the four main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine (uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside is c ...
nucleotide in the genetic sequence. In order to narrow the interaction between the Cr(V)-Salen complex with the guanine base, the researchers modified the bases to 8-oxo-G so to have site specific oxidation. The reaction between the two molecules caused DNA lesions; the two lesions observed at the modified base site were guanidinohydantoin and spiroiminodihydantoin. To further analyze the site of lesion, it was observed that polymerase stopped at the site and
adenine
Adenine () ( symbol A or Ade) is a nucleobase (a purine derivative). It is one of the four nucleobases in the nucleic acid of DNA that are represented by the letters G–C–A–T. The three others are guanine, cytosine and thymine. Its deri ...
was inappropriately incorporated into the DNA sequence opposite of the 8-oxo-G base. Therefore, these lesions predominately contain G-->T
transversions
Transversion, in molecular biology, refers to a point mutation in DNA in which a single (two ring) purine ( A or G) is changed for a (one ring) pyrimidine ( T or C), or vice versa. A transversion can be spontaneous, or it can be caused by io ...
. High-valent chromium is seen to act as a carcinogen as researchers found that "the mechanism of damage and base oxidation products for the interaction between high-valent chromium and DNA... are relevant to ''in vivo'' formation of DNA damage leading to cancer in chromate-exposed human populations".
Consequently, it shows how high-valent chromium can act as a carcinogen with 8-oxo-G forming
xenobiotics
A xenobiotic is a chemical substance found within an organism that is not naturally produced or expected to be present within the organism. It can also cover substances that are present in much higher concentrations than are usual. Natural compo ...
.
Another example of a genotoxic substance causing DNA damage are
pyrrolizidine alkaloids
Pyrrolizidine alkaloids (PAs), sometimes referred to as necine bases, are a group of naturally occurring alkaloids based on the structure of pyrrolizidine. Pyrrolizidine alkaloids are produced by plants as a defense mechanism against insect he ...
(PAs). These substances are found mainly in plant species and are poisonous to animals, including humans; about half of them have been identified as genotoxic and many as tumorigenic. The researchers concluded from testing that when metabolically activated, "PAs produce DNA adducts, DNA cross-linking, DNA breaks, sister chromatid exchange, micronuclei, chromosomal aberrations, gene mutations, and chromosome mutations ''in vivo'' and ''in vitro''."
The most common mutation within the genes are G:C --> T:A tranversions and tandem base substitution. The pyrrolizidine alkaloids are mutagenic ''in vivo'' and ''in vitro'' and, therefore, responsible for the carcinogenesis prominently in the liver.
Comfrey
''Symphytum'' is a genus of flowering plants in the borage family, Boraginaceae, known by the common name comfrey (pronounced ). There are 59 recognized species.WFO (2022): Symphytum L. Published on the Internet; http://www.worldfloraonline.org/ ...
is an example of a plant species that contains fourteen different PAs. The active metabolites interact with DNA to cause DNA damage, mutation induction, and cancer development in liver
endothelial cells
The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel ...
and
hepatocytes
A hepatocyte is a cell of the main parenchymal tissue of the liver. Hepatocytes make up 80% of the liver's mass.
These cells are involved in:
* Protein synthesis
* Protein storage
* Transformation of carbohydrates
* Synthesis of cholesterol, ...
. The researchers discovered in the end that the "comfrey is mutagenic in liver, and PA contained in comfrey appear to be responsible for comfrey-induced toxicity and tumor induction,".
Test techniques
The purpose of genotoxicity testing is to determine if a substrate will influence genetic material or may cause cancer. They can be performed in bacterial, yeast, and mammalian cells.
With the knowledge from the tests, one can control early development of vulnerable organisms to genotoxic substances.
Bacterial Reverse Mutation Assay
The Bacterial Reverse Mutation Assay, also known as the
Ames Assay, is used in laboratories to test for gene mutation. The technique uses many different bacterial strains in order to compare the different changes in the genetic material. The result of the test detects the majority of genotoxic
carcinogens
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
and genetic changes; the types of mutations detected are
frame shifts and base substitutions.
''in vitro'' toxicology testing
The purpose of ''
in vitro
''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...
'' testing is to determine whether a substrate, product, or environmental factor induces genetic damage. One technique entails cytogenetic assays using different mammalian cells.
The types of
aberrations detected in cells affected by a genotoxic substance are chromatid and chromosome gaps, chromosome breaks, chromatid deletions, fragmentation, translocation, complex rearrangements, and many more. The
clastogen
A clastogen is a mutagenic agent that disturbs normal DNA related processes or directly causes DNA strand breakages, thus causing the deletion, insertion, or rearrangement of entire chromosome sections. These processes are a form of mutagenesis ...
ic or
aneugen An aneugen is a substance that causes a daughter cell to have an abnormal number of chromosomes or aneuploidy. A substance's aneugenicity reflects its ability to induce aneuploidy. Unlike clastogens, aneugenic events do not damage the physical stru ...
ic effects from the genotoxic damage will cause an increase in frequency of structural or numerical aberrations of the genetic material.
This is similar to the
micronucleus test and chromosome aberration assay, which detect structural and numerical chromosomal aberrations in mammalian cells.
In a specific mammalian tissue, one can perform a mouse
lymphoma
Lymphoma is a group of blood and lymph tumors that develop from lymphocytes (a type of white blood cell). In current usage the name usually refers to just the cancerous versions rather than all such tumours. Signs and symptoms may include enla ...
TK+/- assay to test for changes in the genetic material.
Gene mutations are commonly point mutations, altering only one base within the genetic sequence to alter the ensuing transcript and amino acid sequence; these point mutations include base substitutions, deletions, frame-shifts, and rearrangements. Also,
chromosomes' integrity may be altered through chromosome loss and clastogenic
lesions
A lesion is any damage or abnormal change in the tissue of an organism, usually caused by disease or trauma. ''Lesion'' is derived from the Latin "injury". Lesions may occur in plants as well as animals.
Types
There is no designated classifi ...
causing multiple gene and multilocus deletions. The specific type of damage is determined by the size of the colonies, distinguishing between genetic mutations (mutagens) and chromosomal aberrations (clastogens).
The
SOS/umu assay test evaluates the ability of a substance to induce DNA damage; it is based on the alterations in the induction of the SOS response due to DNA damage. The benefits of this technique are that it is a fast and simple method and convenient for numerous substances. These techniques are performed on water and wastewater in the environment.
''in vivo'' testing
The purpose for ''
in vivo
Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...
'' testing is to determine the potential of DNA damage that can affect chromosomal structure or disturb the
mitotic apparatus that changes chromosome number; the factors that could influence the genotoxicity are ADME and DNA repair. It can also detect genotoxic agents missed in ''in vitro'' tests. The positive result of induced chromosomal damage is an increase in frequency of micronucelated PCEs.
A
micronucleus
Micronucleus is the name given to the small nucleus that forms whenever a chromosome or a fragment of a chromosome is not incorporated into one of the daughter nuclei during cell division. It usually is a sign of genotoxic events and chromosomal i ...
is a small structure separate from the nucleus containing nuclear DNA arisen from DNA fragments or whole chromosomes that were not incorporated in the daughter cell during mitosis. Causes for this structure are mitotic loss of acentric chromosomal fragments (clastogenicity), mechanical problems from chromosomal breakage and exchange, mitotic loss of chromosomes (aneugenicity), and apoptosis. The micronucleus test ''in vivo'' is similar to the ''in vitro'' one because it tests for structural and numerical chromosomal aberrations in mammalian cells, especially in rats' blood cells.
Comet assay
Comet assays are one of the most common tests for genotoxicity. The technique involves lysing cells using detergents and salts. The DNA released from the lysed cell is electrophoresed in an agarose gel under neutral pH conditions. Cells containing DNA with an increased number of double-strand breaks will migrate more quickly to the anode. This technique is advantageous in that it detects low levels of DNA damage, requires only a very small number of cells, is cheaper than many techniques, is easy to execute, and quickly displays results. However, it does not identify the mechanism underlying the genotoxic effect or the exact chemical or chemical component causing the breaks.
Cancer
Genotoxic effects such as deletions, breaks and/or rearrangements can lead to cancer if the damage does not immediately lead to cell death. Regions sensitive to breakage, called
fragile sites, may result from genotoxic agents (such as pesticides). Some chemicals have the ability to induce fragile sites in regions of the chromosome where
oncogenes
An oncogene is a gene that has the potential to cause cancer. In tumor cells, these genes are often mutated, or expressed at high levels. are present, which could lead to carcinogenic effects. In keeping with this finding, occupational exposure to some mixtures of pesticides are positively correlated with increased genotoxic damage in the exposed individuals. DNA damage is not uniform in its severity across populations because Individuals vary in their ability to activate or detoxify genotoxic substances, which leads to variability in the incidence of cancer among individuals. The difference in ability to detoxify certain compounds is due to individuals’ inherited
polymorphisms of genes involved in the metabolism of the chemical. Differences may also be attributed to individual variation in efficiency of DNA repair mechanisms
The
metabolism
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...
of some chemicals results in the production of
reactive oxygen species
In chemistry, reactive oxygen species (ROS) are highly reactive chemicals formed from diatomic oxygen (). Examples of ROS include peroxides, superoxide, hydroxyl radical, singlet oxygen, and alpha-oxygen.
The reduction of molecular oxygen () p ...
(ROS), which is a possible mechanism of genotoxicity. This is seen in the metabolism of
arsenic
Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, ...
, which produces
hydroxyl radicals
The hydroxyl radical is the diatomic molecule . The hydroxyl radical is very stable as a dilute gas, but it decays very rapidly in the condensed phase. It is pervasive in some situations. Most notably the hydroxyl radicals are produced from the ...
, which are known to cause genotoxic effects.
Similarly, ROS have been implicated in genotoxicity caused by particles and fibers. Genotoxicity of nonfibrous and fibrous particles is characterized by high production of ROS from
inflammatory cells.
Genotoxic chemotherapy
Genotoxic chemotherapy is the treatment of cancer with the use of one or more genotoxic drugs. The treatment is traditionally part of
standardized regime. By utilizing the destructive properties of genotoxins treatments aims to induce DNA damage into cancer cells. Any damage done to a cancer is passed on to descendent cancer cells as
proliferation continues. If this damage is severe enough, it will induce cells to undergo
apoptosis.
Risks
A drawback of treatment is that many genotoxic drugs are effective on cancerous cells and normal cells alike. Selectivity of a particular drug's action is based on the sensitivity of the cells themselves. So, while rapidly dividing cancer cells are particularly sensitive to many drug treatments, often normal functioning cells are affected.
Another risk of treatment is that, in addition to being genotoxic, many of the drugs are also
mutagenic
In genetics, a mutagen is a physical or chemical agent that permanently changes genetic material, usually DNA, in an organism and thus increases the frequency of mutations above the natural background level. As many mutations can cause cancer i ...
and
cytotoxic. So the effects of these drugs are not limited to just DNA damage. In addition, some of these drugs that are meant to treat cancers are also
carcinogens
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
themselves, raising the risk of secondary cancers, such as
leukemia
Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ...
.
Different treatments
This table depicts different genotoxic-based cancer treatments along with examples.
See also
*
Cancer
Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumors, which do not spread. Possible signs and symptoms include a lump, abnormal b ...
*
Carcinogen
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive subs ...
*
Carcinogenesis
Carcinogenesis, also called oncogenesis or tumorigenesis, is the formation of a cancer, whereby normal cells are transformed into cancer cells. The process is characterized by changes at the cellular, genetic, and epigenetic levels and abno ...
*
Carcinogenicity
A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...
*
Mutagen
*
Mutagenesis
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 lab ...
References
Further reading
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{{Genotoxicity
Mutation
Carcinogenesis