One of the most important steps in genotyping is gel electrophoresis. After this step, it is important that the gel is visualized. This process is typically accomplished by staining the DNA. While some stains allow the direct visualization of DNA bands, many require the gel to be exposed to UV light. One of the most common reagents used to stain DNA is ethidium bromide (EtBr, or ethidium). EtBr is fluorescent, and intercalates itself into DNA. However, one of the most common warnings associated with EtBr is that it is a mutagen. While in small doses it is useful, it must be handled with care and disposed of as a mutagen. Larger doses have the potential to cause cancer. Or do they?
There have not been any documented cases of cancer caused directly by ethidum bromide exposure. In fact, the chemical has actually been used in veterinary medicine. EtBr was first developed to treat African Sleeping Sickness and is still used to treat particularly nasty cases of the disease in cattle. Furthermore, the compound seems to be most dangerous after it has interacted with enzymes. In other words, accidentally spilling buffer made with ethidium on yourself is unlikely to cause any disastrous effects, as long as it is washed off soon after (eating or drinking it might, but why are you eating or drinking in the lab?). However, the lethal dose of ethidium is 1.5g/kg in rats. This does seem like a small number, indicating that a low dose of ethidium can have lethal effects. With the average weight of a human (~62kg, with variance based on sex and location), this translates to a lethal dose of ethidium being much higher than anything observed in a lab.
So, where did this myth come from? Likely simple observation. DNA visualization with EtBr is effective, yet simplistic; it isn’t difficult to translate these results to an increased likelihood of cell mutations. After all, generally molecules that intercalate themselves into cellular DNA are mutagenic. To be honest, the UV light that is used to visualize EtBr also has the potential to be mutagenic, and very little fuss is raised about it. While EtBr, like any laboratory chemical, should be handled with care, there is little evidence to suggest it is the mutagen that it’s made out to be.
Editor: Olivia Vo