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In studies of autism and ADHD, it appears genes influence symptoms along a spectrum from very mild symptoms that “normal” people may have to very severe symptoms that highly impaired people will have. In other words, genes seem to determine whether “normal” behaviors can become disease pathologies.
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Severely affected autistic people don’t generally have children, so how can the problem be inherited? Well, parents can pass along what for them were sub-threshold traits that can become more severe in the proper inherited combinations. So children with autism often have parents who actually have mild symptoms but who still are thought of as “normal.”
(On February 19, Constantino and colleague, Richard D. Todd, M.D., Ph.D., speak on “Familiality and genetics of reciprocal social behavior, attention and impulsivity.” The presentation is part of a session called “Man the hunted: The origin and nature of human sociality.” The session runs from 8:30 a.m. to 10 a.m.)
Successful social interactions require the ability to recognize, interpret and respond to interpersonal situations. In extreme forms, deficits in reciprocal social behavior are thought to be the hallmark of the autistic spectrum disorders and of attention deficit/hyperactivity disorder (ADHD). Using epidemiological sampling of families and twins, Constantino and Todd study the family/genetic basis of these behaviors.
Several studies have demonstrated that autism has a strong genetic component. If one child in a family has autism, there’s about a 10 percent chance a sibling also will have some form of autism. To identify genetic factors, Todd and Constantino study families, looking for impairments that usually aren’t considered abnormal but would be thought of as autism or ADHD in their most severe forms.
The Centers for Disease Control and Prevention estimates that one baby in every 250 is born with autism. As many as one 1.5 million Americans are believed to have some form of autism, and that number is on the rise. Epidemiologists estimate the number of autistic Americans could reach 4 million in the next decade.
It’s four times more prevalent in boys than in girls, but autism does not seem to affect any racial, ethnic or social group more than others. It’s also important to keep in mind that autism is not an “all-or-nothing” disorder. According to Constantino, there is a wide range of possibilities between the extremes.
“Although we once believed you either had this condition or you didn’t, we now know that there’s a continuous distribution of autism symptoms from very mild to very severe,” Constantino says.
ADHD diagnoses also are increasing. Experts say approximately 5 percent of school-aged children have some form of ADHD. The most common ADHD symptoms are the inability to sustain attention and impulsive, hyperactive behavior.
“As with autism, we find that ADHD is highly heritable,” Todd says. “We have especially noted genetic factors that appear to be related to ADHD subtypes where severe inattention is the primary problem related to the disorder.”
In both autism spectrum disorders and ADHD, it’s possible for healthy people to have very subtle impairments. They would be considered abnormal only in their most severe forms, but Todd and Constantino believe those behaviors may indicate genetic tendencies that contribute to autism, ADHD or other disorders when they occur in the proper combinations.
They measure such subtle indicators with a diagnostic interview tool called the Social Responsiveness Scale (SRS) that they developed. Studying families in which at least one child is affected, they use the SRS to measure the presence of sub-threshold traits — such as social impairment, language problems or stereotypic, repeated behaviors — that appear to have the same genetic causes as more severe forms of the behaviors that occur in autism. Similar scales are available to measure sub-threshold symptoms of ADHD.
Todd and Constantino say their findings suggest these behaviors have similar genetic structures across the spectrum of human behavior and that uncommon clinical manifestations, such as autism and ADHD, occur because of abnormalities in the same biological pathways involved in normal behavioral. In other words, in an average environment, accepted social behavior is, indeed, the normal response. In contrast, aloof, impulsive and antisocial behaviors are infrequent and occur because of deficits in normal pathways or occur as secondary developmental responses to extreme environments.
Washington University School of Medicine’s full-time and volunteer faculty physicians also are the medical staff of Barnes-Jewish and St. Louis Children’s hospitals. The School of Medicine is one of the leading medical research, teaching and patient care institutions in the nation, currently ranked third in the nation by U.S. News & World Report. Through its affiliations with Barnes-Jewish and St. Louis Children’s hospitals, the School of Medicine is linked to BJC HealthCare.