12 January 2001
Me Tarzan, You Genetically Modified
by Kate Melville
A baby monkey carrying an extra bit of DNA--described in the 12 January 2001 issue of the journal, Science--may suggest a way to speed new treatments for a host of disabling human conditions, from diabetes and breast cancer to Parkinson's and HIV.
Named ANDi, backwards for "inserted DNA," the active, healthy rhesus monkey received an extra gene while he was still an unfertilized egg, making him the world's first genetically modified non-human primate.
Born on 2 October, 2000, "ANDi is robust and plays normally with his two roommates," reported Science author Gerald Schatten of the Oregon Regional Primate Research Center at the Oregon Health Sciences University in Portland, Oregon.
ANDi's modified DNA consists only of a simple marker gene, designed to be easily identified within his genetic blueprint. But, the same modification method should lead to other laboratory animals carrying genes associated with specific medical conditions, Schatten said.
"We could just as easily introduce, for example, an Alzheimer's gene, to accelerate the development of a vaccine for that disease," said Schatten, who last year reported the first monkey successfully cloned by embryo splitting (see Science, 14 January 2000). "In this way, we hope to bridge the scientific gap between transgenic mice and humans. We could also get better answers from fewer animals, while accelerating the discovery of cures through molecular medicine."
The latest experiment, by lead Science author Anthony W.S. Chan, Schatten and others, was no easy task: Some 224 eggs were modified and then fertilized to produce 40 embryos and five pregnancies, which resulted in three live births.
To demonstrate their genetic modification method, the Oregon team--including K.Y. Chong, C. Martinovich, and C. Simerly--added a marker gene directly to a mother monkey's egg, or oocyte. The additional gene was carried into the egg by a non-infectious or "pseudotyped" viral carrier system called a vector, often used in human gene-therapy investigations.
Because viral vectors are programmed to bind with cell surfaces, even when they're neutralized, the imposter virus containing the new bit of DNA quickly latched onto the outside of the monkey egg. As it moved through the egg's surface to reach the interior, the carrier molecules were shed, leaving behind the new genetic material: A marker gene called GFP because it expresses a green fluorescent protein, making it easy to detect. This gene was reverse-transcribed and copied into a double strand of DNA, which entered the mother's chromosomes.
Modified eggs were fertilized by injection with a father monkey's sperm, producing 40 embryos. After the embryos were transferred to 20 surrogates, five pregnancies resulted. Three healthy male babies were born, while two infants were stillborn, and another pregnancy didn't develop.
Of the healthy infants, only ANDi demonstrated successful transgene integration.
His extra DNA was determined by amplifying and analyzing bits of DNA and RNA, carefully extracted from the inside of his cheek, his hair, and cells in his urine, as well as studies of his placenta and birth cord. To further assess their genetic modification system, the researchers also examined tissue from the stillborn monkeys, to identify the GFP gene product through fluorescence tests.
Tests on the two monkeys lost at birth proved crucial because ANDi himself does not exhibit fluorescence-perhaps because the inserted protein is expressed in such small quantities, or because expression won't begin until he is older.
Stem cells and gene therapy show promise for eradicating many devastating diseases, according to Schatten. "Monkeys like ANDi and Tetra, a cloned monkey, will quickly but safely help us determine if innovative therapies are safe and effective," he said. "It may soon be possible to introduce markers monitored by non-invasive techniques, such as MRI [magnetic resonance imaging] or PET [positron emission tomography], to discover the developmental events that lead to diseases like diabetes, heart disease and even mental illnesses."
ANDi and his surrogate mother, as well as Tetra--the female monkey cloned last year at the Oregon Regional Primate Research Center--remain healthy, Schatten said.