This spring, White and his colleagues announced that they had bred a new, see-through, zebrafish. The animals’ transparent skin gives scientists a clear view of developmental processes, such as tumor growth, in real time — making it a valuable tool for White and other researchers.
Zebrafish, which have been used to model human disease since the 1930s, are excellent research subjects: They’re small and hardy, able to withstand even dirty rivers polluted with human and industrial waste, and they breed in great numbers.
Also, fish embryos develop outside the body, which makes them ideal for studying early development. And zebrafish stay naturally transparent for the first few weeks of life, a quality that has proved to be critical for studying changes occurring inside a live body.
After the first weeks of life, though, the skin of the fish starts to darken, eventually becoming opaque. White, who was tracking the growth of melanoma, or skin cancer, inside of the fish, was frustrated that the window of time for observation was shuttered so soon.
“I thought, if I could visualize what’s going on inside the animal, I could identify the changes occurring when a tumor gets started,” White recalled. “By the time one of my patients presents with cancer, the tumor is already a billion cells. Could we identify the earliest stage? Could we create a fish that’s transparent?”
It turned out that he could. Zebrafish skin contains three possible types of pigments–reflective, black, and yellow. White mated a male from black-spotted breed with no reflective pigment, called “roy orbison,” with a female from a breed called “nacre” — the French word for pearl — that had no black pigment. The offspring, with only yellow pigment in its skin, looked clear.
It took about a year of cross-breeding to develop the Casper fish. But one day, the researchers saw it swimming around in its tank, about six weeks old and still see-through.
The news and excitement spread quickly through the lab, and then began making its way through the zebrafish research community.
“We were completely in awe that it worked quite as easily as it did,” said Anna Sessa, a lab technician and co-author of the zebrafish study, published in the journal Cell Stem Cell in February.
“It’s as clear as day,” Sessa said. “You can see the heart, aorta, spine, gills. And under a microscope you can see just about everything.”
The Casper model has been awash in attention since the February paper was published. Now, other scientists are increasingly using the Casper fish to research a range of subjects such as blood vessels, brain tissue and intestinal development.
Since February, White has already had about 50 requests from scientists, many of them abroad, who want to use the Casper embryo so they can grow their own fish for research.
Nathan Lawson, associate professor of the Program of Gene Function and Expression at the University of Massachusetts Medical School, is one of those scientists. He’s growing Casper fish to study angiogenesis, the growth of blood vessels. The new Casper line has allowed him to expand his research from embryos to adult animals.
“We need to start to transition to adult disease models in the fish,” Lawson said. “The Casper fish is one important tool that will allow us to get there.”
White now uses the fish to better understand the process by which a tumor evolves from a small clump of cells incurable cancer.
He takes a tumor from one fish, minces it up into isolated cells, and then treats it with chemicals, before implanting the cells into the flank of a Casper fish. Sometimes, he’ll also transplant the tumor and then treat the whole fish with chemicals. The goal is to find a chemical combination that slows or halts tumor development.
Every few day, he gives the fish an anesthetic that makes it sleepy, and puts it under the microscope to observe the tumor.
“A lot of these chemicals tell us about the biology of how tumors form,” he said. “If a chemical interferes with a pathway, it tells us about melanoma.”
He is also studying metastasis, the spread of cancer to other areas of the body.
“The question I’m trying to get at is how cancer can go from a tiny little ball of cells to becoming a widespread tumor that we can’t cure,” White said. “I’m trying to figure out, how can we block that from happening?”
He suspects that one way to block metastasis is to target what he calls the “seed cells” of a tumor, cells that some researchers believe continually re-grow regardless of chemotherapy and other treatment. Melanoma cells, his research also shows, undergo a sort of “homing” process. Even when the cells are implanted in the belly area of the fish, they tend to travel back toward the skin, suggesting that tumors don’t spread randomly.
“Why do tumor cells seem to be comfortable growing in certain places,” he said. “This tells you there has to be a complicated interaction between tumors and the rest of the body.”
Scientists know little about this homing process of tumor metastasis. It’s like a black box in the research, White said.
“Casper gives me a great opportunity to study that,” he added. “Where do cells go? Where do they naturally go? And can I come up with strategies to stop that from happening?”