Caroline Schneider, 2011 MS Student, University of Wisconsin Madison
(Previously published online with the University of Wisconsin Madison: Grow – Wisconsin’s magazine for the life sciences, Fall 2023)
(Image Source: Michael P. King)
Through a first-of-its-kind study, CALS animal scientists show us how our diets could modify the DNA of our grandchildren — and beyond.
“You are what you eat.” We’ve all heard this old adage before. But during pregnancy, maternal nutrition can have a large impact on a baby, as well, suggesting you’re also what your mother eats. Now, new research from the Department of Animal and Dairy Sciences is taking that one step further by showing that you may in fact be what your grandparents — or even great grandparents — ate.
It might be time for a new adage. Professor Hasan Khatib and colleagues in his lab have spent several years studying how a specialized diet in one generation of animals might affect those that come after. More specifically, they’re looking at the DNA of animals — sheep, in this case — to see if changes get passed on to their offspring, their offspring’s offspring, and so on. However, they’re not examining the sequence of the DNA, the order in which nucleotides are arranged. Instead, they’re focused on the epigenetics of the animals.
“When we say epigenetics, we’re talking about something beyond the genes, beyond the sequence, that dictates what the genes do,” explains Khatib. “These aren’t changes or mutations in the sequence; they are small methyl groups (molecular structures) that get added to the DNA and can turn a gene on or off.”
Epigenetics acts as a mediator between the environment and genes. For example, identical twins may have different phenotypes, or observable traits (such as birth weight, personality, and susceptibility to disease), even though they have the same gene sequences. That’s because environmental conditions can alter their epigenetics. Khatib and his colleagues aim to document the transmission of these epigenetic changes caused by environmental cues from one generation to another, a phenomenon called transgenerational epigenetic inheritance.
“Transgenerational epigenetic inheritance, or TEI, means that you have an environmental factor that affects one generation,” Khatib says. “That effect is then passed to subsequent generations that were not exposed to the original environmental factor themselves.”
Research has shown that TEI is at play in some organisms. In plants and in C. elegans (a roundworm often used as a research model), it has been documented through more than 20 generations. But in mammals, there is less evidence — and there’s even disagreement about whether it exists.
Khatib wanted to take a methodical and thoughtful approach to determine if TEI occurs in mammals, and he decided to use sheep as his model organism. Camila Braz, who was a postdoctoral researcher in his lab and is now an assistant professor at the University of Illinois Urbana-Champaign, worked with Khatib on the project. They also enlisted the help of Todd Taylor, the research program manager at UW’s Arlington Agricultural Research Station.
Taylor grew up around sheep in Wyoming and has more than 20 years of experience managing research animals at Arlington. Over the last five years, he has helped Khatib and his team breed and study sheep to determine if they could find evidence of TEI. Experimental design was extremely important for this project since direct comparison of epigenetics between sheep fed a specialized diet and those fed a control diet would be the best way to show TEI. So, Khatib and Taylor turned to Polypay sheep.
“Polypay sheep have a lot of twins, triplets, and even quadruplets,” says Taylor. “They were developed in the 60s to increase the number of lambs born each year. They were specifically selected to be more prolific, and that works perfectly for Dr. Khatib’s studies, since he needs twins to compare.”
Researchers planned to feed the males (rams) the specialized diet. But questions remained. What should that diet be, and how could they give it to the sheep? Khatib decided to feed the treatment group a diet supplemented with methionine, an essential amino acid. Methionine is used to add methyl groups to DNA, so it influences the epigenetics of the animal.
Finding the best way to feed the sheep took some trial and error. At first, Taylor fed the sheep individually to ensure each one ate the supplemented diet; but with so many sheep, the process became too labor intensive. So, he devised a feeding container 18 grow with multiple sections that would allow only one sheep in the pen to eat from each compartment. This way, every sheep would get their own portion of the
special diet.
“The methionine diet is just a small amount, so we make sure that those pieces of food are right at the top,” explains Taylor. “They pretty much get the methionine in the first bite. And we know each sheep in the treatment group is getting it because they each eat from their own section.”
Another piece of the feeding puzzle was timing. Khatib and his team wanted to affect the germ cells of the sheep — in males, the sperm — so they fed the rams from weaning to puberty, before the point at which the sperm cells start developing. Only the “zero” generation (F0, those fed the diet) and the first generation (F1, derived from F0 germ cells) would be exposed to the methionine diet. If any epigenetic changes were seen in the second generation of sheep (F2, the grandchildren of the rams fed methionine), that would be evidence of TEI, since those offspring had no exposure to the specialized diet.
And that’s just what Khatib and his colleagues found. In a study published last spring in the Proceedings of the National Academy of Sciences Nexus, the research team presented evidence of epigenetic changes due to the methionine diet out to the third generation (F3, the greatgrandchildren). Searching for those changes is quite the effort. Khatib’s lab used a technique called whole genome bisulfite sequencing to look at every single cytosine in the DNA sequence to see which had methyl groups attached (i.e., were “methylated”). Cytosine is one of four nucleobases found in DNA, and it’s a common site for methylation.
“More than 100 cytosines that were methylated due to diet in the treated generation were also methylated in the F1 and F2 generations,” explains Khatib. “The consistent trends of these methylated cytosines across three generations is a strong indication of TEI.”
Documenting those epigenetic changes, however, was just one half of what Khatib wanted to accomplish. He also wanted to know if those modifications on the DNA would result in changes in the observable traits of the sheep, the phenotypes. Jessica Townsend, a graduate student in Khatib’s lab, travels to Arlington often to work with the sheep and help Taylor take the measurements they need.
“Recently, we were looking for phenotypes in close to 90 rams,” says Townsend. “We take weights, measure the scrotal circumference, and then perform ultrasound for the loin muscle depth.” When the researchers looked at scrotal circumference, a measure of testicular size and often an indirect measure of function, they found that the circumference was reduced through the F2 generation in the animals fed methionine. They also found that loin muscle depth, a phenotype related to growth of an animal, was lower in the F2 generation. These data suggested an influence of diet on male reproductive and growth traits in subsequent generations of sheep that were not fed the methionine diet.
With the publication of this comprehensive study, Khatib and his team have shown evidence of TEI in mammals. They are working to learn more about the phenomenon through other studies, namely feeding ewes methionine and discovering how the diet might affect embryo development. And, if TEI is shown to occur in humans, Khatib is also interested in the social implications.
“TEI covers basic scientific questions as well as applied questions,” says Khatib. “In humans, it would intersect with social responsibility, thinking about how what we eat affects others, as well as social justice, as we consider how those in war zones or experiencing trauma might pass epigenetic changes on to subsequent generations. There’s so much to consider, and these studies will advance our understanding of the relationships between nature and nurture.”