Transgenic primates transmit DNA

Japanese researchers have successfully generated the world's first transgenic primates capable of passing on a foreign gene to their offspring. The feat, linkurl:reported;http://www.nature.com/nature/journal/v459/n7246/abs/nature08090.html in tomorrow's (May 28) issue of __Nature__, should pave the way for more sophisticated models of human disease, though the monkey models still have many hurdles to overcome.

Transgenic marmoset twins Kei and Kou
("keikou" means fluorescence in Japanese)
Image: E. Sasaki / Nature

"This is the first time that we actually can see a transgene integrated into every tissue including the germline [in a primate] and that the transgene has been passed on to the next generation," linkurl:Shoukhrat Mitalipov,;http://www.ohsu.edu/oscc/Shoukhrat_Mitalipov.php a developmental biologist at the Oregon National Primate Research Center (ONPRC) in Beaverton who wrote an accompanying linkurl:commentary;http://www.nature.com/nature/journal/v459/n7246/full/459515a.html to the study, told __The Scientist__. A team led by linkurl:Erika Sasaki;http://www.ips-s.jst.go.jp/e/sakigake/saki_10.html of the Central Institute for Experimental Animals in Kawasaki, Japan, injected viral vectors containing a green fluorescence protein (GFP) transgene into embryos of the common marmoset, a small New World monkey found in the forests of Eastern Brazil. Marmosets have short gestation periods, reach sexual maturity after around a year, and females can have up to 80 babies throughout their lifetimes -- all of which makes the animal more amenable to laborious transgenic technologies than some other primates with slower reproductive rates. Sasaki's team started with 91 transgenic embryos and then implanted 80 of these into 50 surrogate mothers. Seven pregnancies took hold, resulting in five offspring -- including one pair of twins -- born to four mothers, all of which expressed the glowing transgene in several tissue types. What's more, two monkeys showed transgene expression in the germ cells, and one fathered a glowing, healthy marmoset, born last month. Genetic analyses also confirmed the presence of the transgene in this second-generation offspring. Since then, said Sasaki at a press briefing this morning (May 27), three more offspring have been born, although one died shortly after birth. All but one of these four offspring fluoresced under UV light, and further genetic analyses are currently underway to confirm the presence of the transgene. The birth of these transgenic marmosets brings researchers "a step closer" to a useful primate model for studying many human neurological and behavioral conditions, linkurl:Anthony Chan,;http://genetics.emory.edu/FACULTY/faculty_bio_chan.php a geneticist at the Yerkes National Primate Research Center in Atlanta, Georgia, who was not involved in the study, told __The Scientist__. linkurl:Hideyuki Okano,;http://www.coe-stemcell.keio.ac.jp/member/okano.html a neuroscientist at Keio University School of Medicine and a coauthor on the paper, told reporters that the researchers first plan to target Parkinson's disease and amyotrophic lateral sclerosis, though they are also considering building marmoset models of cerebellar ataxia and Huntington's.

A second-generation transgenic marmoset
Image: E. Sasaki / Nature

In 2001, a group led by Chan and linkurl:Gerald Schatten,;http://www.pdc.magee.edu/faculty/schatten.html then at the ONPRC and now at the University of Pittsburgh School of Medicine, created the first transgenic primate -- a linkurl:rhesus macaque named ANDi;http://www.sciencemag.org/cgi/content/abstract/291/5502/309 that carried GFP, but this monkey never passed transgenes to its offspring. linkurl:Last year,;http://www.the-scientist.com/blog/display/54652/ Chan also reported the birth of five linkurl:rhesus macaques with the human Huntington's disease gene,;http://www.nature.com/nature/journal/v453/n7197/abs/nature06975.html though these monkeys have yet to reach sexual maturity. Chan said he's "confident" that these animals will display germline transmission of the Huntington's transgene, but "we'll have to sit patiently and wait until next year" to find out. The major advantage of using marmosets, said Sasaki in the press briefing, is that unlike macaques, they can be propagated very quickly. "We can expand lots of colonies if we establish the transgenic founder marmosets," she said. "It's easy to expand and use these animals for research purposes," because once a stable transgenic population is established, researchers don't have to go through the onerous and time-consuming process of reengineering individual monkeys. There's no reason, however, why the same techniques shouldn't also work for macaques and other Old World primates that are more closely related to humans, she added. Apart from turning to marmosets, the study's main technical achievement, said Mitalipov, was in the method used to get a hold of embryos for genetic manipulation. In addition to embryos obtained from in vitro fertilization (IVF), Sasaki's team obtained embryos produced through natural intercourse and then flushed from the reproductive tracts of mated females -- these natural embryos proved to more efficiently incorporate the transgene than IVF embryos. "Other than that, there hasn't been any technological advancement," he said. Despite the utility of this new primate research model, the protocol used to engineer the marmosets has its limitations, noted Okano in the press briefing. The self-inactivating lentiviral vectors used in the study can only insert small transgenes -- shorter than 10 kilobases of DNA. Plus, because the transgenes insert randomly, they can be hard to screen for if they're not expressed in the correct tissues, and they can activate other potentially harmful genes. "It's not like homologous recombination," the technique commonly used for targeted, site-specific, transgene integration in mice, Okano told reporters. "What we need now is to use gene targeting along with animal cloning techniques to have desirable effects," Mitalipov told __The Scientist__. "That will have enormous applications for deriving genetically engineered primates. Unfortunately, that's still in the future."
**__Related stories:__***linkurl:Monkey model for Huntington disease;http://www.the-scientist.com/blog/display/54652/
[19th May 2008]*linkurl:One child, two fathers -- marmoset-style;http://www.the-scientist.com/news/display/53033/
[27th March 2007]*linkurl:The first transgenic primate;http://www.the-scientist.com/article/display/19383/
[12th January 2001]