Sex Chromosome Clue to Autism
Emily Singer, Salford, England
NewScientist.com
September 15, 2003

Dr. David Skuse of the Institute of Child Health in London has reported the discovery of an area on the X chromosome that may be responsible for the inability to recognize fear in another’s face. It has long been thought that autism has a genetic component, but no one specific gene has yet been identified. Skuse and his colleagues hypothesize that there is a cluster of genes on the X chromosome that is linked to the development of the amygdala, the fear centre of the brain. Other researchers have reported abnormal activity in the amygdala when individuals with autism are given the task of looking at photographs depicting different facial expressions.

The X chromosome is vital to many chemical processes in the body. Women have two X chromosomes, while men have an X and a Y. In women, most of the double copies of the X bearing genes are turned off, but some persist in their "double dose."

Women who have only one X chromosome, a condition known as Turner Syndrome, often get X-linked diseases, which are caused by a single dose of a gene on the X chromosome. X-linked diseases are usually seen in men, who have only one X chromosome. Many of these conditions would be fatal if the genes were in a double dose. Women with Turner Syndrome have higher rates of autism, and researchers have also discovered the interesting fact that both people with autism and women with Turner Syndrome avoid eye contact and that both groups have trouble perceiving fear in another person’s facial expressions.

What makes this even more intriguing is that another region on the X chromosome is related to the severity of autism in an individual. The gene or genes that code for monoamine oxidase (MAO), a chemical that helps regulate the levels of serotonin in the brain, is found on the X chromosome. Serotonin, a neurotransmitter, or chemical messenger in the brain, is found in abnormally high levels in some people with autism and their first degree relatives.

In talking about his discovery, Dr. Skuse stated that, as yet, a specific gene has not been identified, but when it is it can be characterized and its specific function investigated.