January 15, 2002

 

 Genetically engineering a pet? History shows it won't be easy - January 15, 2002

Posted on January 1, 2002

 

David Avner, M.D., an emergency room resident in Syracuse, N.Y, has always wanted a cat, but he is allergic, just like 5 to 10 percent of the American population. The young physician, hasn't given up hope, however, that he will own a cat one day. He has founded Transgenic Pets Inc., a company that will try to genetically engineer a cat that doesn't cause itchy eyes and runny noses.

History
Although engineering a pet might seem objectionable to some people, it is certainly nothing new, and modifying animals to suit a particular need is even older. "The truth is we have been selecting characteristics ever since we first started domesticating plants and animals," says Dr. Alex Livingston, dean of the Western College of Veterinary Medicine, University of Saskatchewan.

"The process previously was merely taking advantage of spontaneous mutations and selectively breeding for characteristics. Right now, with genetic engineering, we can do it in a more rapid and targeted fashion."

A transgenic animal is one whose genome has been altered by the introduction of foreign DNA. Scientists created the first transgenic animals to advance basic biomedical research, genetically modifying laboratory rats, mice, rabbits, and monkeys to give them characteristics that mimic diseases. Studying these animals helped researchers understand the mechanisms of various diseases, including how genes go awry and cause cancer.

Next, scientists began modifying the genomes of cows, sheep, and goats so that they could be used as "pharmaceutical factories." The researchers introduced genes into animals so that they would produce protein-based agents in their milk. Some goats, for example, have been modified to carry the gene for antithrombin III, a blood protein that can prevent blood from clotting in people. Scientists can purify the protein out of a goat's milk, and it can be used, for example, during heart surgery. Research continues and attempts are under way to genetically modify the organs of animals, such as pigs, for possible transplantation into humans.

But medicine isn't the only market to feel the impact of transgenic animals; the food industry has entered the arena. Genetically engineered salmon, for example, grow much faster than regular salmon after the insertion of a growth hormone gene.

And now, Dr. Avner hopes the pet industry can get in on the action. Whereas some people hope to clone their pet to allow a second chance for "Toto," the allergen-free cat would be the first transgenic pet.

Eliminating the culprit
The Syracuse company's special cats will lack the ability to produce Fel d1, a protein secreted by glands in a cat's skin. This protein is responsible for many people's allergies, and Dr. Avner hopes that knocking out the gene that induces it will create a cat whose owners are sniffle-free.

To achieve his goal, he has contracted with Xiangzhong Yang, PhD, a professor of animal science and head of the Transgenic Animal Facility at the University of Connecticut's Biotechnology Center. Dr. Yang will use a technique involving nuclear transfer (cloning). After growing skin cells from a cat in a test tube, the researchers will turn off both copies of the gene that makes the allergenic protein by replacing them with defective copies. These allergen-free cells will be fused with egg cells that have had their genetic material removed. The eggs will grow into an allergen-free embryo, which will then be implanted into a surrogate cat. Kittens born from these surrogate mothers will, hopefully, be allergen-free, and, as adults, can be bred to produce more allergen-free cats.

Although research is in the early stages and the company is still raising funds for the project, the first cats could be on the market as early as 2003. The cats will be spayed or neutered to protect the company's profits and will cost between $750 and $1,000.

According to Matthew Wheeler, PhD, a professor of animal sciences at the University of Illinois at Urbana-Champaign, nuclear transfer is used less than ten percent of the time to make transgenic animals. "Different techniques you use to make transgenic animals have specific advantages and disadvantages," Dr. Wheeler said. "You have to balance what you are trying to do in choosing a technique."

Dr. Yang chose to use nuclear transfer because it allows him to target and change a specific gene as well as produce large numbers of animals. The question of success is still up in the air. "Gene targeting in mice is efficient, but gene targeting in cattle is not as efficient," Dr. Wheeler said. "I don't know how efficient nuclear transfer in a cat would be. I don't think anyone has reported that yet."

The hurdles
In creating his cat, Dr. Yang will struggle to avoid the various pitfalls that researchers have faced since they first started creating transgenic mice in the 1970s. In addition to trying to alter an animal that has been little studied in the field of transgenics, Dr. Yang faces the difficulty of trying to stop protein production.

"It's more difficult to make an animal stop producing something than start producing something," explained Se-Jin Lee, M.D., Ph.D., an assistant professor of molecular biology and genetics at Johns Hopkins School of Medicine. Genes often have more than one function, he says. Oftentimes, you think a gene is involved in one particular function, but most of the time, the animal with the gene removed has either unexpected adverse effects or behaves identically to an animal with the gene.

Dr. Livingston says that the literature abounds with examples of transgenic animals with unexpected ramifications. In one study, published 1997 in Nature, researchers genetically engineered a strain of mice to lack a prostacyclin receptor. Prostacyclin is a chemical that keeps blood from clotting, and researchers were hoping to study stroke and cardiovascular ailments with their newly created mouse model. When the researchers engineered their mice, however, they found that the chemical had another function, a role in pain perception.

"Nobody had connected the prostacyclin compound with anything to do with pain but suddenly they found that these mice had a totally different response to pain," Dr. Livingston said. "You dicker with one thing and you don't really know what the ramifications are going to be throughout the whole body system."

Effects can be seen on many levels. Cows that have been genetically modified to have larger udders produce more milk, but have the burden of carrying more stress on their limbs and may have more limb injuries. Some cloned animals have been born with a variety of cardiopulmonary ailments.

Dr. Avner says that he doesn't think there will be any unexpected effects. "This particular protein is well characterized and is only found in glands in the skin," Dr. Avner explained. "They haven't found it in any other organ." Although the protein also has a role in making the skin moist, he continued, a few other proteins also perform this task, but without causing allergies. If the other proteins can pick up the slack, redundancy in this scenario will be beneficial.

Redundancy could also work against Transgenic Pets, however, if more than one gene creates the allergenic protein. Researchers saw this type of scenario when they attempted to create an estrogen receptor knockout mouse in the mid 1990s. The researchers expected that the animals wouldn't be fertile because estrogen is required for normal reproductive function, but this didn't happen. "It turns out that there was a second estrogen receptor. that nobody knew about, and the animal was perfectly healthy," said Dr. Wheeler.

Not for everyone
If Transgenic Pets Inc. succeeds, their cat will be added to the list of transgenic success stories. Ninety-five percent of people who are allergic to cats are allergic to Fel d1 and many of these people will have the choice of becoming a pet owner. It won't, however, help everyone. Between 10 and 20 percent of people who are allergic to cats are allergic to two other proteins, cat albumin and cystatin. Martin Chapman, PhD, an allergy specialist at the University of Virginia, explains, "Getting rid of Fel d1 could reduce the problem, but not eliminate it."