What is Genotoxicity?
A genotoxin is a substance that permanently alters DNA (mutagenicity) or alters a cell’s ability to regulate DNA structure and content. Mutation or damage to DNA may or may not permanently change its content or structure, depending on several factors, including DNA repair, metabolism, apoptosis, and oxidative stress defence mechanisms.
Importance of Testing.
While generally regulated as a toxicity endpoint in its own right, genotoxicity is a set of modes of action that can lead to adverse outcomes such as cancer, inherited mutations and diseases, developmental toxicity and ageing. As such, testing for potential genotoxic properties is often one of the first things a toxicologist will do to rule out safety hazards or to shape a risk assessment for a new chemical or product.
However, chemically induced genotoxicity should be put into context. Spontaneously occurring DNA lesions are a common event, with thousands of lesions present in the average cell’s DNA at any given time. This tells us that our bodies are very effective at repairing this damage! In this context, several substances are known to increase mutation, and ultimately tumorigenesis, in animal and epidemiological studies above this repaired level, sometimes at a very low dose and exposure levels.
Whilst there is emerging evidence that some genotoxic substances (including some very potent ones) have a threshold of effect, the prevailing dogma for many years has been to assume that genotoxic substances, particularly those that can induce DNA mutations, either lack a no-effect threshold or that it would be so low as to have little practical utility in assuring safe exposure. This necessitates demonstrating that a chemical does not have genotoxic liability or that its exposure can be safely managed if such liability exists.
The good news is that several genotoxicity tests are available, and many of these can be conducted in vitro, reducing animal use in testing. However, as there are three key classes of genotoxic substances, no single study can detect all types of damage – a combination or battery of studies must be used.
Because in vitro studies don’t always correctly predict in vivo outcomes and because they have been designed to be oversensitive to detecting potential genotoxicity (i.e., the emphasis is on not missing genotoxic effects), in vivo, studies are conducted for chemicals where human or environmental exposure is considered to be high and/or persistent or to see if an in vitro effect is expressed in an in vivo scenario.
Again, no single in vivo study predicts all types of genotoxicity, so particularly in the context of following up an in vitro positive response, the correct in vivo study needs to be selected based on in vitro study findings (e.g. an in vivo gene mutation transgenic rodent study to follow-up an in vitro Ames positive mutation response).
Types of Genotoxic Effects
- Point Mutations: Changes in individual DNA bases.
- Clastogens: Cause chromosome breaks and structural changes.
- Aneugens: Lead to abnormal chromosome numbers.
Here are some examples of different types of genotoxic effects:
- Point mutations. These are typically changes induced at the level of an individual DNA base. However, addition or deletion in a base could lead to a frameshift, which may alter several genes on that chromosome. An example of a substance that causes point mutations is sodium azide.
- These cause DNA double-strand breaks, typically by binding directly with DNA and forming DNA adducts or chemically cross-linking strands of the DNA helix. This results in either disrepair (inducing DNA mutation) or the loss of sections of a chromosome during cell division. Mitomycin C is a good example of a clastogen.
- Aneugens cause whole chromosomes to be lost or gained by cells during cell division by disrupting the proper segregation of chromosomes during mitosis and meiosis. Such missegregation effects (aneuploidy) are linked to cancer and heritable diseases resulting from chromosomal abnormalities within germ cells, such as Down’s Syndrome. Vinblastine is an example of an aneugen.
- Indirect genotoxins include substances that produce reactive oxygen species, topoisomerase inhibitors, certain classes of kinase inhibitors, or substances that deplete cellular oxidative defences. Such materials may induce mutational, clastogenic, or aneugenic effects.
Testing Strategies
- In Vitro Tests: Initial screening for potential genotoxicity.
- In Vivo Tests: Follow-up to confirm in vitro results and assess real-world effects.
Gentronix Services
At Gentronix, we offer a range of in vitro and, soon, in vivo genotoxicity studies that can detect and categorise genotoxic substances, allowing you to identify genotoxic liabilities so you can either eliminate them or restrict exposure to these substances accordingly. We support research across pharmaceutical, agrochemical, food, fragrance, cosmetic and personal care sectors, utilising both early discovery screening and regulatory guideline testing approaches to assess genotoxic potential.
Within this series of articles, we will focus on each class of genotoxicity (point mutations, clastogens and aneugens) and detail how we can help you detect them with appropriate test systems, follow up adverse results to investigate potential mechanisms and help support the management of risk.
