Marmosets whose skin glows green under ultraviolet light may take on a big role in the study of aging and neurodegenerative diseases in humans. A Japanese team has introduced the gene for green fluorescent protein (GFP), a widely used biological marker, into the world's tiniest monkeys and has found the gene in the animals' next generation. Getting an introduced gene to pass from parent to offspring is a first in a nonhuman primate and is a step toward breeding marmoset colonies with human disorders.
Researchers have engineered mice, rats, pigs, and other animals to carry the mutant genes that cause various human diseases. But these animals aren't similar enough to people to effectively model human aging and neurodegenerative diseases. There has been some success in transferring genes to monkeys (ScienceNOW, 19 May 2008), but those transgenic animals either died young or failed to pass the genes on to their offspring.
In the new study, Hideyuki Okano, a neuroscientist at Keio University School of Medicine in Tokyo, and colleagues turned to marmosets. At about 20 centimeters tall, the creatures are smaller than other monkeys and less expensive to handle and house. Marmosets are also more fertile: Females mature in 12 to 18 months and can bear 4 to 6 offspring a year. Still, Okano's team had to overcome a couple of hurdles.
The first was tweaking standard gene-transfer procedures. To add a new gene to a fertilized egg, researchers typically inject a virus carrying that gene into the space between the egg and a protective membrane surrounding it; the virus then transfers the gene into the egg cells. Okano and his team made this process more efficient by placing the egg and its membrane in a chemical soup that caused the egg to shrink, creating a void within the membrane that they could then fill with more virus particles carrying the GFP gene.
The second improvement hinged on good timing. Once a gene is added to a fertilized egg, the resulting transgenic embryos must then be quickly placed in the wombs of surrogate monkeys who are at an optimal time in their menstrual cycles. Researchers have found that they can control the reproductive cycles of marmosets more effectively than those of other monkeys. That meant Okano's team could relatively easily have an adequate number of surrogates ready to receive embryos at just the right time.
The researchers injected the gene for GFP into 91 marmoset embryos. They then implanted these embryos into 50 surrogate mothers, some of whom received multiple embryos. Four of the surrogates produced a total of five live offspring, all carrying the GFP gene as shown by exposing the skin to ultraviolet light, which causes it to glow, and by using more sophisticated genetic methods to confirm the presence of the gene. One of those marmosets grew up, apparently healthy, and eventually produced a baby also carrying the GFP gene, Okano's team reports in tomorrow's Nature.
In theory, researchers can now repeat the experiment, adding human disease genes such as ones causing Parkinson's and amyotrophic lateral sclerosis (also known as Lou Gehrig's disease). "This represents a step forward in the development of [animal] models more appropriate for human disease," says Thaddeus Golos, a reproductive biologist at the Wisconsin National Primate Research Center in Madison. The marmoset has limitations, however. Rhesus macaques and other monkeys are genetically more similar to humans in ways that may be important for studying metabolic and endocrine disorders, Golos says. And Okano points out that the virus his team used in this experiment can carry only relatively small genes into the embryos. As such, the technique will have to be modified for larger genes, such as the one that causes muscular dystrophy.