If scientists worry about the applicability of mouse models to human disease, could anything possibly be learned from fish?
MDA grantee Jeffrey Guyon, a biochemist at Children’s Hospital in Boston, thinks so and has been studying zebrafish with muscle disease since 2001.
"We picked fish because they’re small relative to mice, and they reproduce in large numbers," Guyon says. "A female fish can lay between 100 and 250 eggs every week. Another advantage of the fish is that the egg is so accessible. You can just inject it to insert genes or block genes; you don’t have to put it into another fish, as you do in the mouse. You just inject it and it’s ready to go."
|Zebrafish with muscular dystrophy are curved, and their swim bladders dont inflate.|
Guyon says that, while there are obvious differences between fish and humans, it’s important to note the similarities.
"When I look for genes that cause muscular dystrophy in a fish, I take all the genes in humans that we know are associated with different forms of MD, and we look in the fish, and we can find most of them," he says.
Finding that a drug works in zebrafish with MD isn’t the last step before human trials, he notes; the fish are a screen to see which drugs warrant testing in mice.
"You use the fish to identify things and to do those experiments that you can’t do in the mouse or that are very difficult to do in the mouse," he says. "After you find that information, you go to the mouse and validate that it’s true and refine your findings."
Among the hundreds of fish Guyon has, he checks frequently for any that look like they might have a muscular dystrophy, a screen that’s as simple as looking at them with the naked eye. In contrast to normal zebrafish, MD-affected zebrafish are curved, have uninflated swim bladders, and spend a lot of time on the bottom of their containers. They also refract light differently.
After Guyon has identified some fish with MD, he plans to expose each of them to a solution of about five chemicals. "You don’t need to know how these chemicals work," he says. "You just look for a fish that gets better. If you find a fish that gets better, then one of those five chemicals is a potential drug target."
Guyon says he can easily breed enough fish to screen 5,000 to 10,000 chemicals, out of which he expects to find five to 20 compounds that will go on to tests in mice.
Another phase of the research will test the ability of various types of cells to undergo successful transplantation into zebrafish muscles. "We can transplant marked cells at the 2-day-old stage," he says. "The fish are transparent, so you know exactly where you’re injecting the cells." Within days, he says, "you can tell whether or not those cells engrafted into muscle, and what the percentage of the engrafted cells is."
People have done screens like this using mice, but, Guyon says, "It just takes a lot more resources and a lot more time." Needless to say, those are commodities everyone wants to save.