Crazy Postdoc Update

KTVU reports that the Benchum Liu (the accused postdoc) will plead not guilty to poisoning his labmate Mei Cao.  Interestingly, Liu does not deny adding ethidum bromide (EtBr) to Cao’s water (he even participated in a videotaped reenactment for police), but will be arguing that EtBr is not a poison.  My last post talked about EtBr as a carcinogen, and this is certainly what I’ve been taught my whole scientific career. It’s also extremely believable because a) the flat, planar rings make it a great intercalating agent, and b) I know it intercalates because I use EtBr routinely to visualize DNA.  That said, there are some caveats on why it might not be harmful to humans – first off there’s the issue of delivery – merely ingesting a small molecule like EtBr doesn’t guarantee it gets into your cells (in fact propidium iodide, chemically very similar to EtBr, only stains dead cells). Whether EtBr can effect your cells will  be dictated by the ability of EtBr to diffuse across the membrane (or taken up  via protein transporters) and from there it still has to get into the nucleus, all the while remaining soluble.

So what’s known about EtBr in living systems?  Well according to Wikipedia, its used to treat Trypanosomosis (a parasite) in cattle.  That suggests it’s not so bad, as the cattle presumably survive the treatment and (I assume) can be used as food sources for humans.  Well, then the question becomes how does the amount of EtBr that Liu gave to Cao compare to what cattle get?  Rosie Redfield at the RRResearch Blog, writing about common misconceptions about EtBr, did the the following calculation:

“The recommended dose for cattle is 1mg/kg body weight (up to 50mg/kg has been used in mice). Compare this with the 0.25 – 1 microgram/ml used in molecular biology (previous error corrected – thanks, anonymous commenter). A 50kg researcher would need to drink 50 liters of gel-staining solution to get even the non-toxic dose used in cattle.”

For reference, I use about 1 ul (10^-6 L)  of EtBr per experiment (diluted in 50 mL buffer) and have never seen more than 20 mL of EtBr just sitting around (having 50 L of it would be strange to say the least). So does Liu have a case?  I would say that evidence suggests that EtBr WILL cause DNA mutations, but EtBr probably DOESN’T get into living cells at even high concentrations.  What does that mean?  Cao will probably be okay – in fact the National Toxicology Program (NTP) report on EtBr has a puzzling description of it as “Bitter tasting dark red crystals from alcohol (Budavari, 1989)”, suggesting Cao is not the first person to digest EtBr. That still doesn’t change the fact that Liu did expose her to a potential carcinogen that has very stringent disposal guidelines in his lab, and I’m betting his lab is going to get fined for improper disposal at the very least.  It’s also important to note that the evidence in cattle suggests EtBr won’t harm humans, BUT there hasn’t been a carefully controlled study to show that.  Furthermore, there IS evidence that it can act as a mutagen once inside a cell, so given a choice between ingesting it and not ingesting it a reasonable person would choose the latter. I don’t know what this means legally, but it looks like the case has taken yet another unexpected turn, and this time I was able to learn something along the way.

Tags: crazy postdoc, EtBr, UCSF, University of California