Three hours after a particular poster session began at the 2007 Society for Toxicology meeting, the line to see Courtney Kozul's poster still wrapped around the room, and she had collected 90 business cards. Clearly, her findings on the effects of diet on gene expression in mice were of interest.
In 2006, under the guidance of Joshua Hamilton at the Dartmouth Medical School, Kozul set out to determine the baseline level of arsenic in standard lab mouse chow. Hamilton's group had found that arsenic in drinking water disrupts hormones and can contribute to cancer, diabetes, and cardiovascular disease. In light of the US Environmental Protection Agency's decision to lower the federal limit for arsenic in drinking water from 50 parts per billion (ppb) to 10 ppb, Kozul wanted to study the effects of much smaller arsenic doses. There are no limits for arsenic levels in food, and trace amounts can be found in wine (made from grapes sprayed with arsenic-containing pesticides) or seafood (especially finned fish).
But to measure the effect of small doses of arsenic, Kozul first needed to know the arsenic baseline, or how much the animals encountered daily. Kozul started with the standard lab rodent chow—Purina LRD5001. To her surprise, it contained a whopping 360 ppb of arsenic—36 times more than the EPA recommended for drinking water. Total exposure depends on how much food and water the animals consume, of course, but the findings got Kozul thinking. "Can you imagine people looking at arsenic exposure and exposing their animals to 10 ppb when they're only looking at a change of 400–410 ppb? They won't be able to discern a difference," and their findings would suggest that low levels of arsenic had no effect.
To find out what effect this baseline exposure might have on gene expression, Kozul set up a new experiment: One group of mice ate the standard diet. A second group of mice were fed a purified, well-defined diet (with every ingredient accounted for and minimal variability between batches) from the American Society for Nutritional Sciences, which contained less than 20 ppb of arsenic.
"The diets are pretty much different by night and day," says David Robbins, a nutritionist at Harlan, the company that produces the purified diet. The purified diet contains refined, human-grade food, each formulation mixed by hand by nutritionists. Standard diets contain an assortment of ingredients, and sometimes animal proteins such as fishmeal, the likely source of arsenic. Kozul also found the standard chow contained cadmium, chromium, copper, lead, and zinc, at concentration levels ranging from 10 and 100 fold greater than the purified feed. The differences show in the cost: the purified diet can be 15 times more expensive than the standard.
Kozul did a microarray analysis of 20,000 mouse genes in the liver and lung of the two groups and found in some cases, a 40-fold higher level of gene expression in the mice on the standard diet, especially in genes involved in xenobiotic and glutathione metabolism. (Previous nutritional studies had shown the effect of diet on single genes or metabolism, specifically, never large-scale gene expression.) Kozul presented her results at the 2007 meeting, and published them last year (Chemico-Biologic Interact, 173:129–40, 2008). Purina declined to comment on the paper's findings.
"It was quite a convincing study; a little bit of a surprise at the magnitude of the things being observed," says Richard Brennan, director of toxicology at GeneGo, whose company analyzes gene expression and proteomics. "I thought 'wow, this really is important to know'." The study showed that the standard diet concealed the effects of increased arsenic levels in drinking water—effects that were clearly observable in the mice eating the purified diet. "You're masking the levels of the effects of low level exposure to a compound," adds Brennan.
Kozul now only orders a custom-formulated chow recipe for her mice, and she's uncovered some surprising effects of arsenic, even at concentrations of only 10ppb: In particular, gene expression is altered and some mice show an impaired immune response to infection.
Kozul's findings appear to have had an impact. At talks during the 2008 Society for Toxicology meeting, she noticed members of the audience asking speakers what type of chow their rodents were eating. "People will control for other factors and not really think of diet," says Kozul. "The type of diet you use affects the baseline, and the results you see," she says.