Genotoxicity

In genetics, genotoxicity describes the property of chemical agents that damages the genetic information within a cell causing mutations, which may lead to cancer. While genotoxicity is often confused with mutagenicity, all mutagens are genotoxic, whereas not all genotoxic substances are 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 of the organism or germ cells to be passed on to future generations. Cells prevent expression of the genotoxic mutation by either DNA repair or apoptosis; however, the damage may not always be fixed leading to mutagenesis. In genetics, genotoxicity describes the property of chemical agents that damages the genetic information within a cell causing mutations, which may lead to cancer. While genotoxicity is often confused with mutagenicity, all mutagens are genotoxic, whereas not all genotoxic substances are 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 of the organism or germ cells to be passed on to future generations. Cells prevent expression of the genotoxic mutation by either DNA repair or apoptosis; however, the damage may not always be fixed leading to mutagenesis. 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. 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. 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 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 was inappropriately incorporated into the DNA sequence opposite of the 8-oxo-G base. Therefore, these lesions predominately contain G-->T transversions. 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. Another example of a genotoxic substance causing DNA damage are pyrrolizidine alkaloids (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 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 and hepatocytes. 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,'. 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. 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 and genetic changes; the types of mutations detected are frame shifts and base substitutions. The purpose of in vitro 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 clastogenic or aneugenic 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 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 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.