Huntington’s disease is a case in point. Current rodent models do not capture key aspects of the disorder, in part because of differences between their brains and those of humans. People with Huntington’s typically display abnormal movement, especially the writhing movements called chorea; they also tend to develop dementia or even psychosis. But rodents don’t have the types of neurons, the number of synapses, or the types of neural relay centers that are crucial to human motor control, so researchers can’t see exactly how potential treatments will affect these systems. A team led by Steve Goldman, a professor of neurology and neurosurgery at the University of Rochester Medical Center, has developed a possible therapy that uses stem cells to regenerate a type of neuron lost in Huntington’s disease. It works in mice, he says. But it’s not something that’s going to be tested in humans until the researchers can try it out in primates first, because the relevant anatomy in mice is just too different.
Transgenic primates could also prove extremely helpful as disease models for ALS and Alzheimer’s. By using them instead of mice, Morrison predicts, “we’d get a much more faithful model of the degeneration you see in humans.” For instance, efforts to develop antibodies against amyloids–protein deposits that typically develop in the brains of Alzheimer’s patients–seemed promising in mice but failed in humans. “My guess is that if we’d had a really good primate intermediate, we would’ve been better informed,” he says.
To be sure, creating transgenic primates raises tricky ethical issues, especially if the new genes come from humans. The concern is that researchers might challenge the boundaries between humans and other species by inadvertently creating an animal with cognitive abilities such as rational thinking or moral reflection–a creature that would necessarily deserve a greater degree of respect than a typical lab animal, says Robert Streiffer, a bioethicist at the University of Wisconsin at Madison. The idea of experimenting on such an animal would probably strike both researchers and the public as unacceptable.
That situation seems like a distant possibility at the moment, Streiffer says. Still, monkeying with the germlines of primates does cross a new line and deserves careful scrutiny. For one thing, it could open the door to similar engineering in humans. Historically, ethicists have distinguished between introducing new genes into tissues like the liver or pancreas and altering egg cells, sperm cells, or embryos; the latter type of modification, which could be passed on to a recipient’s offspring, has not been performed in humans. Critics typically point to the more frivolous possibilities–say, parents who might wish to give future generations a gene for strength or height. Still, when it comes to some serious diseases like Huntington’s or certain mitochondrial disorders, germline genetic treatment could turn out to be the best or even the only option. “Are we willing to delay the possible discovery of treatments for terrible illnesses because we want to draw a line in the sand?” asks Mark Rothstein, a bioethicist at the University of Louisville. “I’m not willing to do that.”
It is worth keeping in mind that these disorders are devastating. And for the most part, they are currently untreatable. Given that reality, the promise represented by the baby marmosets should be given ample opportunity to grow up.
Amanda Schaffer is a science and medical columnist for Slate and a contributor to the New York Times.