Genotoxins

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.

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