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Some Down syndrome problems reversed in mice

Down syndrome is a genetic disease resulting from a chromosomal abnormality. People with Down syndrome inherit all or part of an extra copy of chromosome 21.Darryl Leja, NHGRI

Down syndrome is the most common genetic cause of intellectual disability. People born with the syndrome have deficits in learning and memory. They also have an increased risk of developing Alzheimer’s disease by middle age.

Down syndrome is caused by an extra copy of chromosome 21. Scientists haven’t understood how the extra chromosome causes many of the brain and health problems associated with Down syndrome. One idea is that alterations of overall protein levels in the brain may affect learning and memory.

Researchers led by Drs. Mauro Costa-Mattioli from Baylor College of Medicine and Peter Walter from the University of California, San Francisco set out to examine how the disorder alters proteins in the brain. They used a mouse model of Down syndrome called Ts65Dn as well as brain tissue and cells from people with Down syndrome.

The team focused on a cell signaling pathway called the integrated stress response (ISR). The ISR turns on to help protect the cell from dangerous conditions. One of its effects is to reduce protein production in the cell. The study was funded in part by NIH’s National Institute of Neurological Disorders and Stroke (NINDS), National Human Genome Research Institute (NHGRI), and National Cancer Institute (NCI). Results were published on November 15, 2019, in Science.

Compared with normal mice, protein production was reduced by about 40% in the hippocampus region in the brains of Ts65Dn mice. The hippocampus plays a central role in learning and memory formation. Further experiments showed that the ISR was persistently active in the hippocampus of the Ts65Dn mice.

To assess whether Down syndrome may also involve these changes, the team measured ISR activity in samples taken from brains and cells donated by people with Down syndrome. The samples had increased ISR activity as well.

ISR activity can be controlled by different enzymes. In the Ts65Dn mice, the team found, the increase in ISR activity in the hippocampus was driven by a specific enzyme called PKR. When they inhibited PKR—either genetically or with drugs—the mice had normal protein production levels. They were also able to perform similarly to normal mice on standard memory and behavior tests.

Further experiments revealed that blocking PKR in the brains of Down syndrome mice improved communication between nerve cells—a process required for memory formation.

“The cell is constantly monitoring its own health. When something goes wrong, the cell responds by making less protein, which is usually a sound response to cellular stress. But you need protein synthesis for higher cognitive functions, so when protein synthesis is reduced, you get a pathology of memory formation,” says Walter.

“More than 10 years ago we discovered that the ISR serves as a molecular switch for the protein synthesis needed for long-term memory formation,” Costa-Mattioli explains. “In this study, we found that the switch is off in Down syndrome. More importantly, turning the switch back on in these mice reverses their long-term memory deficits.”

Targeting the ISR with drugs may be a strategy for reducing the cognitive deficits associated with Down syndrome. More research is needed to understand whether such an approach might be feasible in people.

—by Sharon Reynolds