Washington, Apr 11 (ANI): According to researchers at Stanford University School of Medicine, the two most prevalent forms of genetic mental retardation - Fragile X and Down syndrome may share a common cause.

Although the genetics of the disorders are very different, the end result for the brain seems to be the same, according to Daniel Madison, PhD, associate professor of molecular and cellular physiology.

"It's as if you had every light in your house wired to just one or two switches, rather than having many switches that can be flipped on or off in complex combinations to control the lighting in one room," he said.

In the latest study, Madison and postdoctoral scholar Jesse Hanson, PhD, studied Fragile X syndrome, which is a leading cause of mental retardation in this country. Affected people tend to have learning disabilities, distinct physical characteristics such as enlarged ears and a long face, and such behavioral problems as attention deficit disorder, speech disturbances and unusual responses to various sights or sounds.

Although it's not known why, about one-third of people with Fragile X also develop autism - a much higher percentage than in the general population. This makes Fragile X, which can be studied in mice, the only genetic model for autism.

As the syndrome's name suggests, the responsible gene, called Fmr1, is located on the X chromosome. Because boys have only one X chromosome while girls have two, boys are usually more severely affected when Fmr1 is mutated. Girls are not immune to the condition, however. A phenomenon called X-inactivation, which randomly silences one member of every X chromosome pair, creates a mosaic of affected and unaffected nerve cells in the brain.

In some conditions linked to the X chromosome, such as hemophilia, the normal cells can cover for their useless peers. Not so for an elite corps of brain neurons. Here, where cooperation and communication are a key, a few deadbeats in the mix can be disastrous.

Madison said the findings from this study point researchers in a new direction.

"Until now the emphasis in the field has been on the receiving, or post-synaptic, side of the synapse. But these results unequivocally show that the pre-synaptic cells are the important ones in this defect," he said.

The result paralleled the researchers' earlier finding in the brains of the mice with Down syndrome symptoms; more connections are made by fewer cells.

"We believe that these reduced-complexity networks are the basis for the mental retardation that occurs with both syndromes," Madison said.

If so, the problem is rooted in early development. Synapse formation appears at first to be completely disordered, with connections between neurons making random paths like hairline cracks racing across a breaking sheet of ice. But as the person or animal begins to learn and remember, the well-trodden paths, or cracks, connect in purposeful, yet unique ways.

"No two nerve cells will always be connected in the same way in different people. But populations of cells will develop similar connections as the developing brain practices using its own network. If we can compensate for the synaptic deficiency of the mutant cells, we may begin to start to think about ways to increase the mental capacity of patients with Down syndrome or Fragile X,” said Madison

The study will be published in today’s issue of the Journal of Neuroscience. (ANI)