Down syndrome is the most common type of chromosomal genetic disorder. One in every 691 babies in the U.S. is born with Down Syndrome, and more than 400,000 Americans are living with the condition. One of the hallmark symptoms of Down syndrome is mild to moderate cognitive delay.
The condition is caused by an extra copy of chromosome 21: The surplus genetic material causes certain pathways in the brain to develop differently and organize less efficiently. Although therapies exist for children with Down syndrome, none of them can reduce the developmental delays.
“If you go back even as recently as 2004, researchers didn't have much of a clue about the mechanisms involved in this developmental disability,” said Michael Harpold, chief scientific officer at the Down Syndrome Research and Treatment Foundation, in an interview with Scientific American. But new discoveries in the past few years have revealed likely genetic culprits for doctors to target.
Scientists describe one of these genetic pathways in a new study published in Science Translational Medicine. They used an existing drug to reverse some of the effects of Down syndrome in mice. This could someday lead to a drug for humans that would improve the mental abilities of people living with the condition and give them greater quality of life.
The Sonic Hedgehog Genes
Johns Hopkins University's Roger Reeves, Ph.D., and his team created mice with a genetic disorder similar to Down syndrome. The mice had extra copies of about half the genes involved in Down syndrome and several symptoms of the syndrome to match. The mice showed difficulty in a number of tests, including a memory test that challenged them to navigate through a water maze and locate a platform in the water.
Like humans with Down Syndrome, the Down’s mice had a smaller-than-normal cerebellum—the brain’s physics calculator—which controls and coordinates movement. Humans with Down syndrome have cerebellums that are only about 60 percent of normal size.
Reeves’ team examined a gene that codes for a protein called sonic hedgehog, so named because removing the gene in fruit flies caused small, pointy spikes to cover their bodies. Sonic hedgehog is most active during fetal development, guiding the placement of the limbs and structures in the brain. Previous research had identified sonic hedgehog as being involved in Down syndrome.
To test its involvement, the team used a drug called SAG, which activates the sonic hedgehog protein pathway. The researchers gave SAG to the Down’s mice just once, on the day they were born. The results were incredible. “We were able to completely normalize growth of the cerebellum through adulthood with that single injection,” Reeves said in a press release.
Although Reeves and his team primarily examined the cerebellum, they also subjected the mice to a barrage of behavioral tests to see how they were affected. “Making the animals, synthesizing the compound, and guessing the right dose were so difficult and time-consuming that we wanted to get as much data out of the experiment as we could,” Reeves said.
The Down’s mice that had been given the drug performed the same way on the water-maze test as normal mice. Not only does Reeves’ finding suggest that SAG could correct some of the cognitive impairments seen in Down syndrome, but it also raises the question of what role the cerebellum plays in physical and spatial memory. Reeves wants to study the effects of SAG further, speculating that the drug might strengthen connections between different areas of the brain.
An Imperfect Solution
There are many barriers to overcome before SAG can be used in humans. First and foremost, sonic hedgehog is involved in functions throughout the body, so scientists would need to develop a variant of the drug that acts only on the brain. Second, supercharging the sonic hedgehog pathway with SAG has been shown to produce cancer in adults, so a more stable form of the drug must be created. Still, Reeves’ research lays the groundwork for these discoveries.
Another problem lies in administering the drug. Mice are born substantially earlier in their development than humans are—a newborn mouse is equivalent to a third-trimester human fetus. The cerebellums of newborn mice are still growing at birth. By the time humans are born, the nerve cells of the brain have already grown and migrated into position. This suggests that the best time to give the drug to babies with Down syndrome might be while they’re still in the womb. This is much more difficult than giving an injection to a newborn, and it might carry risks, including a chance of miscarriage.
Even if these problems can be worked out, the drug alone wouldn’t reverse all of the cognitive and memory-related problems associated with Down syndrome. “Down syndrome is very complex, and nobody thinks there’s going to be a silver bullet that normalizes cognition,” Reeves said. “Multiple approaches will be needed.”