A gene mutation that shrinks brain cells may be responsible for the mental retardation of Down’s syndrome, U.S. researchers reported on Wednesday.

The finding suggests there may be a way to interfere with or even reverse the additional mental decline often seen as people with Down’s syndrome age. The finding, published in the journal Neuron, also may apply to Alzheimer’s disease, the researchers said.

“If we can decrease the expression of this gene we may be able to provide something more than supportive care to people with Down’s syndrome,” said neurologist Dr. William Mobley of the Stanford University School of Medicine and Lucile Packard Children’s Hospital in California. Reducing gene expression turns down the activity of a gene.

Down’s syndrome is the most frequent genetic cause of mental retardation and affects one out of 800 babies. It is caused when there is an extra copy of chromosome 21, making three instead of two.

It causes learning difficulties and sometimes childhood heart disease and leukemia. Most people with Down’s syndrome develop Alzheimer’s disease by the age of 40.

The researchers worked with genetically engineered mice to find the gene, which is called App - short for amyloid precursor protein. Mutations are known to cause early-onset Alzheimer’s disease in otherwise healthy people.

Like people with Down’s, the mice had three abnormal copies of the App gene. When the researchers deleted the third copy of App in the mice, the animals became more normal.

“We’re now investigating ways in which we might be able to turn down App expression,” Mobley said in a statement.

“It’s not even necessary to turn it off completely. All we need to do is to reduce it by one third, from 150 percent of normal back down to 100 percent,” said Dr.Ahmad Salehi, who led the study.

The researchers stressed that deleting the third copy of App did not restore the mice to normal, so other genes must also affect the brain decline.

“First we need to figure out at a molecular level how App works in Down’s syndrome,” Mobley said. “Then we need to examine other genes that might be involved and test possible compounds in mouse and human cells. If we are able to do all that, we might begin to think of helping children and adults with Down’s syndrome to develop and age more normally.”