How are doughnuts to humans like “royal jelly” to bees? Biologist Shirley Tilghman explains
The same process that determines why certain bees become queen bees while others with the exact same DNA become worker bees also plays a role in how doughnuts eaten by a pregnant woman may influence whether her child becomes obese.
The reason for both is linked to the “Wild and Wacky World of Epigenetics,” the title of molecular biologist Shirley Tilghman’s talk at the Edward E. Hatcher Science on Saturday lecture at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) on Jan. 16. Tilghman, a Princeton University professor and the president of Princeton University for 11 years until 2012, gave the second lecture in the nine-week free lecture at PPPL. (A downloadable schedule is available here: http://www.pppl.gov/education/science-education/programs/ronald-e-hatcher-science-saturday-lecture-series.)
“The field of epigenetics is a new way of thinking about how genes are passed from one generation to the next,” Tilghman said.
Almost 500 people attended the lecture with an overflow crowd in the cafeteria and several watching from the lobby. Among them was PPPL Director Stewart Prager who came with his wife, Karen. “The talk was a homerun hit,” Prager said. “As a physicist, I actually learned something about genetics!”
The field of epigenetics has changed dramatically in the past decade, Tilghman said. Researchers now have a better understanding, for example, of how identical bees differentiate. A worker bee is much smaller than the queen, it can’t produce offspring, and it lives just three to six weeks. But a queen bee with the same DNA is much larger, produces hundreds of thousands of bee progeny, and lives for two years.
The reason for the differences, Tilghman said, is due to a factor outside the creatures’ genetics that programs their development called “epigenetics.” A larva that is destined to become a queen is fed large amounts of something called “royal jelly” that causes the growing queen to develop differently than the worker bees, which are fed “worker jelly.”
The same effect can be seen in humans where the “royal jelly” is doughnuts or other fatty foods that could affect a developing fetus and cause it to grow into an obese child or adult. “What’s happening here is exactly what’s happening with the bees and the ants and that is an environmental exposure,” Tilghman said. “The embryo is sharing a placenta as the mother is eating that sugar and fat and the embryo is responding.” This also creates a vicious cycle that could repeat itself over generations, she added.
Tilghman recounted how Gregor Mendel, the founder of genetics, used pea plants producing yellow and green peas to predict inheritance. (Mendel found the first generation always had yellow seeds and the second generation had a consistent three to one ratio of yellow to green seeds). Epigenetics, Tilghman said, explains what happens when the outcome of similar cross-pollination isn’t consistent with Mendel’s principles. For example when the maize plant, which is usually purple, is crossed with a green maize plant, 100 percent of the offspring are green. The coloration is not due to a gene mutation but instead is caused by a failure of the purple pigment gene to switch on. This failure to switch on or off is epigenetics.
This switching on and off of certain genes also causes some diseases, Tilghman said. The vast majority of diseases are genetic diseases that are caused by a gene mutation directly inherited from both parents. But epigenetic diseases do not involve a mutation in the DNA sequence but rather a change in how the gene is packaged, Tilghman said. Some DNA sequences are packaged in a way that allows the gene to be expressed, which means it is open and can be switched on, Tilghman explained. Others are packaged in such a way that they are closed and cannot switch on. “That’s really at the heart of what epigenetics is all about,” Tilghman said.
Tilghman cited the example of Beckwith-Wiedemann syndrome (BWS), which is only passed on through mothers and is silent in fathers. In this disease, the maternal gene that creates the insulin, which stimulates growth, fails to shut off as it normally would. As a result, the child gets two active copies of the gene. Children with BWS are born larger than other children and have an increased risk of cancer, especially during childhood. Tilghman said.
Epigenetics has also led to “a paradigm shift thinking about cancer,” Tilghman said. While most cancer is caused by mutations, epigenetic misprogramming seems to be involved in a number of cancers involving tumors. The cells are misprogrammed to produce a potent growth factor that helps tumors to reproduce quickly. On the other hand, the tumors will also reproduce rapidly if genes that suppress tumors are silent.
Tilghman aimed much of her talk at high school students in the audience. Chris Resch, an AP biology teacher at Montgomery High School, said the lecture demonstrated how useful scientific research is to his students. “It shows them that science isn’t in a vacuum,” he said. “It shows them science is actually applicable to real life.”
Ruth Levy, from Plainsboro, a regular at the lectures, said she thoroughly enjoyed Tilghman’s lecture. “She was incredible,” Levy said. “I like her attitude, I like her caring, and I like her ability to have a human quality. This makes the learning much more vital.”
The Science on Saturday lectures are held each Saturday at PPPL at 9:30 a.m. but seats fill up quickly, so plan to get there early. They can also be streamed live from home at https://mediacentral.princeton.edu/id/1_wdp1m3et. You can view archives of the lectures at http://www.pppl.gov/sos-listing. If Science on Saturday is canceled due to inclement weather or other emergency, an announcement will be posted on the PPPL website at pppl.gov and a message will be left on the Science on Saturday Hotline, (609) 243-2121.
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