C&B Notes

Zebrafish to the Rescue

Zebrafish have unique characteristics – transparent eggs and mass production of embryos – that make them ideal laboratory models to study and treat human diseases.  Their use has exploded since the mid-‘90’s, and a recent case is indicative of what they can help accomplish in a short time.

The boy was quickly running out of options. His legs and belly were swelling like sponges, his lungs kept filling with fluid so badly that he needed an oxygen tank, and the extreme measures his physicians had tried worked only temporarily. “He got worse and worse and worse,” said Dr. Hakon Hakonarson, a pediatric lung specialist and director of the Center for Applied Genomics at Children’s Hospital of Philadelphia (CHOP). “He was going to die.” But the boy lived. Born with a rare, complex genetic condition, he owes his life to zebrafish, the nearly transparent creatures that have become the go-to lab animal in countless studies of genetics, development, and disease.

Hakonarson and his colleagues identified the mutation causing the boy’s illness, in which lymphatic vessels proliferate out of control and leak fluid into the lungs and other organs; engineered the mutation into zebrafish; waited for the fish to develop a piscine version of the boy’s disease; tested multiple drugs on the fish; found one that stopped the vessels’ kudzu-like growth; and got permission from federal health officials to try it on their young patient. It worked, they reported in Nature Medicine on Monday, a success that shows “precision medicine” can be expanded well beyond cancers, where it has shown the greatest promise. Much like, say, lung cancer, the child’s illness, central conducting lymphatic anomaly, can be caused by any of several mutations. Each leads down a different biological road to the disease. Only a drug that targets the right road can stop the disease, just as blockading only the roads taken by an invading army can stop it from laying siege to a city.

* * * * *

ARAF mutations had never been linked to lymphatic disorders, however. Maybe it was an innocent bystander unrelated to Daniel’s now-life-threatening illness. To determine that, the scientists genetically engineered zebrafish embryos to carry the mutation. Within five days, the fish had developed a lymphatic system. “That’s one of the nice features of zebrafish,” said CHOP’s Christoph Seiler, who led this part of the study: “They develop lightning-fast.” With the ARAF mutation, what the quarter-inch-long fish developed were lymphatic vessels that grew and grew, just like Daniel’s. It was “proof that this mutation causes overgrowth” of lymphatic vessels, Hakonarson said.

That was scientifically interesting, but the researchers had a dying patient. They gave groups of fish with the ARAF mutation one of 10 different drugs. “We just put it in the water and they take up the drug,” Seiler said. “You don’t need to inject it like you do with a mouse.” All the drugs are “MEK inhibitors,” which hit exactly the biological pathway that Daniel’s ARAF mutation takes the brakes off. The scientists anxiously examined the fish for evidence that the chaotic lymphatic vessel proliferation had stopped but that nothing else had been harmed. They had a winner: trametinib, a melanoma drug that Novartis sells as Mekinist…

With FDA approval, the CHOP team gave Daniel trametinib. Within two months, his breathing improved. At three months, the fluid in his lungs had receded enough that he no longer needed supplemental oxygen. The swelling in his legs disappeared. An MRI showed that his lymphatic vessels reshaped themselves into something close to normal. It was the first time a drug had remodeled an entire organ system. “You try a lot of things in research and only a few work,” Seiler said. “It’s so great to go back into the clinic and treat” on the basis of what the zebrafish revealed. Now 14, Daniel plays basketball, rides his bicycle, and helps coach soccer.

Experts not involved in the study cautioned that one success is suggestive, but not proof, that exome sequencing and targeted therapy will help patients with rare lymphatic anomalies. Still, “this is the first paper to implicate ARAF in vascular malformation and to demonstrate MEK inhibition may be effective,” said Dr. Matt Warman, a medical geneticist at Boston Children’s Hospital. “This is important since several clinicians are currently using MEK inhibitors in other patients whose vascular malformations are due to mutations.”

Referenced In This Post