Platypus Study Answers Questions About Gene Imprinting
By F. B. Dunn, JNCI
28 February 1997: Can a platypus get cancer? It was a moot question until recent research revealed that the oddball egg-layer shares a propitious trait with people. This genetic feature lowers the risk for cancer, challenges the notion that mice make the best cancer models, and may even have implications in human cloning.
It all began when radiation oncologist Randy Jirtle, Ph.D., of Duke University Medical Center, Durham, N.C., became interested in a gene for the receptor of insulin-like growth factor-II (IGF2R; also called M6P/IGF2R because the receptor binds the protein mannose-6-phosphate as well). Jirtle and his team identified IGF2R, important in prenatal and postnatal growth and development, as a tumor suppressor gene in liver cancer in 1995.
In that report, published in Nature Genetics, the team wrote that 70% of the hepatic tumors studied displayed IGF2R mutations. The team went on to find IGF2R abnormalities in breast, colon, lung, and other cancers.
When Jirtle began delving into the gene’s cancer connection, around 1991, he heard that it was one of the first human genes found to be imprinted. "At the time, I’ll admit I didn’t have a clue what that meant," said Jirtle. "But when I started reading about it, it was fascinating."
Discovered in 1984, the phenomenon of gene imprinting could have driven Mendel from his pea patch. The monk’s painstaking research proved that plants inherit genetic traits from both parents. But with imprinted genes, the offspring display traits from just one. To date, between 40 and 50 imprinted genes have been reported in mice and people. Estimates suggest there may be 500 or more. Of the ones found to date, nearly all help control early development.
As the field of imprinting expanded, so did a debate about IGF2R. In mice, there was no question that it was imprinted. But in people there were conflicting reports. A few articles claimed that the gene was imprinted in perhaps 50% of the population.
"That was very problematic," said Jirtle. "If a lot of people were imprinted, you would predict them to be exquisitely susceptible to tumor formation."
Like most genes, most tumor suppressors carry two active alleles. If one gets knocked out by a mutation, the other can still confer protection. But if IGF2R were imprinted, a single mutation to the active allele would silence it completely. Papers from Constantin Polychronakos, M.D., from Montreal Children’s Hospital Research Institute at McGill University presented evidence that IGF2R imprinting problems were indeed a factor in Wilms’ tumor.
But Jirtle was not convinced, especially after considering the relatively crude techniques available for studying imprinting. He directed his laboratory to help resolve the debate. Using a more precise technique, the team found no evidence of imprinting in 75 human fetuses and 12 full-term placentas. Because every other imprinted gene found in the mouse was also imprinted in people, and because knockout mice with no IGF2R suffered gross overgrowth abnormalities, Jirtle’s interest peaked. He wanted to know when mice and men split.
After collecting tissue samples from a Noah’s Ark of mammals and birds—with the help of Andrew Hoffman, Ph.D., of Stanford University School of Medicine, and with tissue from the Duke University Primate Center—he and his team concluded that IGF2R imprinting appeared about 175 million years ago, when live-bearing mammals split from the monotremes—egg-laying mammals—on the evolutionary tree. The opossum, the most primitive live-bearer, carries the imprint, while the monotreme platypus does not.
Further, he found that sheep, pigs, cows, mice, and rats carried the imprint; primates, from the tiny tree shrew through lemurs on to people, did not. "Mother nature took care of this overgrowth problem [caused by faulty IGF2R imprinting] 70 million years ago," said Jirtle, when primates split from other mammals.
Why? Jirtle’s pet theory is that primate mothers ran into a fetal overgrowth problem. As animals got smarter and brains became larger, pelvises stayed the same size. "If you look at humans," he said, "we’re very close to not being able to deliver. Dying during childbirth was common until recently."
In early primates then, fetuses that lost the imprinting, perhaps through some epigenetic mutation, would have had twice as much IGF2R as normal, preventing overgrowth. Those that kept the imprint would perish during birth. Natural selection would take care of the rest. "There’s no way to prove it," said Jirtle with a chuckle.
One consequence of the find is that cancer models in mice may be less informative than currently thought. Since humans have two functioning IGF2R genes, but mice have only one because of imprinting, it is reasonable to think that humans should be more resistant to cancer-causing agents. But possible carcinogens are usually tested on mice. Jirtle and partner Keith Killian, Ph.D., are working on a "better" mouse model with two working copies of IGF2R.
But the most inflammatory consequence of our lost imprinting involves cloning. The Duke press release announcing the research claimed that the people may be easier to clone than lower animals. Specifically, the loss of IGF2R imprinting should make people less susceptible to large offspring syndrome (LOS), which afflicts 30% to 40% of clones, said Jirtle.
These unfortunate animals tend to be larger than average at birth, with deformed, enlarged, or underdeveloped organs. Support comes from an article by Dolly cloner Ian Wilmut, Ph.D., published earlier this year in Nature Genetics. It showed that loss of IGF2R leads to LOS in sheep fetuses.
Jirtle’s announcement to the media ruffled the feathers of both Wilmut and Rudolf Jaenisch, Ph.D., of the Whitehead Institute at the Massachusetts Institute of Technology. "I thought it was totally ridiculous," Jaenisch said. "He argues that this is a key gene for cloning. That is nonsense—it’s not a key gene."
In his many mouse cloning experiments, Jaenisch said he has not seen any problems with IGF2R. Jaenisch has also said that it is foolhardy to "focus on gene imprinting. It’s part of the problem, but not all of it." Jirtle agreed that there could be other genetic problems that lead to overgrowth and other cloning difficulties.
Jaenisch was critical of what he sees as Jirtle’s manipulation of the media. He’s worried that those who want to clone humans—people he calls "nuts"—will point to Jirtle’s research and say that it gives them the green light.
Jirtle’s response: "It doesn’t mean we’re at green, but we’re not at red either. We’re at amber."