Drugs Could Reverse Down Syndrome Symptoms : Discovery News
The Stanford University study, led by physician and neuroscientist Ahmad Salehi, was conducted on mice genetically engineered to develop a rodent version of Down syndrome.
In humans, the condition -- also known as trisomy 21 -- results from the presence of an extra copy of chromosome 21. (Chromosomes typically come in pairs.) The additional copy of the chromosome adds between 130 and 300 extra genes, which are responsible for characteristics of Down syndrome-like cognitive impairment.
Cognitive impairment in Down syndrome is the result of a breakdown in the function of the hippocampus. This region of the brain is responsible for contextual learning, or gaining and applying knowledge in real-world situations. Recalling the location of certain items is a prime example.
"Remembering where your keys are isn't just based on navigation," Salehi told Discovery News. "It also includes the sound the keys made when you placed them down and all of the other little bits of information involved."
The hippocampus pulls all of these disparate sensations and awareness of our bodies in relation to our surroundings together to form new memories. When we need our keys again, we draw on these memories to help us find them.
People with Down syndrome may have trouble forming such memories, and Salehi and his team appear to have pinpointed the reason why.
Salehi and his team probed the hippocampi of the mice used in the study and found that the region wasn't functioning correctly due to a lack of the neurotransmitter norepinephrine, a chemical that allows neurons in the hippocampus to form the connections that form contextual memories.
In the study, Salehi and his team found that this dysfunction could be corrected in the mice -- and surprisingly easily. The researchers injected modified mice with a drug that is converted into the norepinephrine within the brain.
Within five hours of injection, the modified mice that previously couldn't learn to fear a tone accompanied by an electric shock could now remember the conditioned response. The modified mice also learned to nest in a novel environment, which is a standard sign of intelligence in the rodents.
After being injected with the drug, L-DOPS, the mice modified to express Down syndrome showed no significant difference from their counterparts in terms of intelligence.
The study was published in the Nov. 18 issue of the journal Science Translational Medicine.
Salehi says he was surprised by how quickly the effects of the cognitive impairment were reversed, but he is cautious about predicting a similar strategy could work on humans.
Salehi says that when the mice were tested again after two weeks, they had returned to their previous levels of intellect. He also points out that what works on mice may not work on humans. In addition, he asserts, Down syndrome is a multifaceted condition; cognitive impairment is just one of its characteristics.
Richard Urbano, a Down syndrome researcher at Vanderbilt University who didn't take part in the Stanford study, said that Salehi's findings could show real promise. Like Salehi, he is cautiously optimistic.
"If this finding held in humans, it would be a really important one," he said. "But just because it works in mice, it doesn't mean it will work in humans."
Drugs that introduce norepinephrine to the brain are already widely available, making Salehi's treatment strategy closer to a reality -- if it can be proven to work.
"If in the future it turns out that this is the right strategy, the good news is we do have access to drugs that already do this," Salehi said