Autism and Genes : Part 2

by National Institute of Health

(Page 2 of 4)

Why study genes to learn about autism?

Past research links autism and genes. For example:

Studies of twins with autism - Scientists have studied autism in both identical twins - who are genetically the same - and fraternal twins - who are genetically similar, but not the same. When identical twins have autism, both have autism more than 60 percent of the time, depending on the criteria used. When fraternal twins have autism, both have autism between 0 percent and 6 percent of the time. If genes were not involved in autism, the rate of autism would be the same for both types of twins.

Family studies of autism - Studies of family histories show that the chances a brother or sister of someone who has autism will also have autism is between 2 percent and 8 percent, which is much higher than in the general population. Also, some of the autismlike symptoms, such as delays in language development, occur more often in parents and adult brothers and sisters of people with autism than in families who have no members or relatives with ASDs. Because members of the same family are more likely to share genes, something about these genes' sequences appears to be related to autism.

Diagnosable disorders and autism - In about 5 percent of autism cases, another single-gene disorder, chromosome disorder, or developmental disorder is also present. This type of co-occurrence helps researchers who are trying to pinpoint the genes involved in autism. Similar disorders or conditions with similar symptoms may have similar genetic beginnings. In cases of one disorder commonly occurring with another, it could be that one is actually a risk factor for the other. This kind of information can provide clues to what actually happens in autism. For example, many people with ASDs also have epilepsy, a condition marked by seizures. If scientists can understand what happens in epilepsy, they may also find clues to what happens in autism.

Based on these and other findings, scientists have long felt that there was a likely link between genes and autism.

But, how symptoms of ASDs affect family members and the wide variety of symptoms in ASDs tell researchers that they aren't looking for just one gene. So, even when scientists find the genes involved in autism, their work will be just beginning. They will still have to uncover what roles the genes play in the condition.

How do researchers look for the genes involved in autism?

Scientists generally use a combination of methods to find candidate genes - genes likely to be involved in autism.

Screen the whole genome.

A genome is all the genetic material in a person's cells - their DNA, their genes, and their chromosomes. Usually, researchers screen the genome of a family or a set of families that has more than one member with an ASD, to look for common features and differences. They look for so-called links between those diagnosed with ASDs and the genes within these families. Using "marker" locations - genes whose position in the genome is known - researchers can narrow down the location of genes involved in ASD. If a gene involved with autism is close to a particular marker, scientists can identify this gene by mapping it in relation to the known markers.

Conduct cytogenetic studies.

In a cytogenetic study, researchers stain chromosomes with a dye and then look at them under a microscope. The dye creates light and dark bands that are unique to each chromosome. Researchers compare the resulting bands of two people with autism, of one person with autism and one relative, or of one person with autism and one person not affected by ASD. These comparisons can point out similarities and differences between regions on the chromosome, which researchers can then study further based on the traits of that specific region.

Examine linkage ratios.

Researchers use this approach to to find hot spots - areas on chromosomes that may contain genes involved in autism. Hot spots are like neighborhoods on the chromosome where the genes involved in autism might "reside." In many cases, genes in the same area of a chromosome are tightly connected to one another, and this connection is hard to break. If the connection is present in more people than you might expect by chance, the situation is called linkage disequilibrium. Linkage disequilibrium helps researchers narrow their genetic search to find the spot in the chromosomal neighborhood where the gene might be.

Evaluate genes based on their known functions.

In some cases, researchers already know what the normal function of a specific gene or genes is. If that specific function is abnormal or incomplete in autism, researchers can look at the genes controlling that function in a person with autism to see what is different or missing. Or, they can look at what kinds of medications are useful in correcting or controlling that function to reduce the symptoms of autism. They can then study the chemical pathways that these medications effect to see what step might be changed or missing in autism. Once they've found the pathway or the change, they can look at the genes that control these features for more information about autism. This approach is known as genetic association analysis.

About the Author

NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.