Genetics Study Reveals Possible Link Between Autism and ADHD

By Melissa Lee Phillips Neuroscience for Kids Consultant February 28, 2003 Autism and Attention Deficit Hyperactivity Disorder (ADHD) are two disorders commonly diagnosed in children. Generally, both are first observed in the child's early years, but some symptoms may persist into adulthood. Although a few behaviors, such as inattentiveness and hyperactivity, are often seen in both disorders, many of the behaviors associated with autism are distinct from those of ADHD. Most researchers and doctors previously thought that these two disorders were completely separate from one another and that the behavioral signs were the only way these disorders could be diagnosed. Now that we know more about the genetic makeup of humans, however, scientists can search for the genetic causes for diseases and disorders. One such search, published in the American Journal of Human Genetics, reveals a surprising find: although autism and ADHD may be different behaviorally, there is evidence that they may be caused, in part, by the same gene. Previous studies have suggested that a region on chromosome 16 is likely to contain a gene contributing to the development of autism. Researchers believe that children with an abnormal version of this gene are more likely than other children to develop autism. Dr. Susan Smalley and her fellow scientists at the University of California in Los Angeles searched for regions of the genome that might contain a "risk gene" for ADHD. They performed a genetic "scan" on 277 pairs of siblings, both of whom had been diagnosed with ADHD. This scan allowed them to find areas on specific chromosomes that are likely to contain genes contributing to ADHD. They found that the same area of chromosome 16 that had been implicated in scans looking for autism genes also showed up in their ADHD gene search. This was surprising because the behavioral symptoms of each disorder can be so different. Some scientists think this means that a gene on this chromosome causes the behaviors that are similar in each disorder. Others believe that there are certain genes in the genome that simply predispose people to develop any of a variety of mental disorders. Other factors, such as other genes and environmental influences, may determine which disorder the person develops. Autism and ADHD are most likely caused by the combined effects of many genes and the effects of unknown environmental factors. It is very unlikely that one gene on chromosome 16 will decide entirely whether someone develops either of these disorders. It is also important for other scientists to confirm these initial results. The researchers do not know for sure that the same gene is contributing to both disorders. However, if these results are supported by future studies, they would point to a very interesting -- and as yet unexplained -- link between two previously unlinked disorders.

Reference: Smalley, S.L., Kustanovich,V., Minassian, S.L., Stone, J.L., Ogdie, M.N., McGough, J.J., McCracken, J.T., MacPhie, I.L., Francks, C., Fisher, S.E., Cantor, R.M., Monaco, A.P., and Nelson, S.F. Genetic Linkage of Attention-Deficit/Hyperactivity Disorder on Chromosome 16p13, in a Region Implicated in Autism," Am. J. Hum. Genet., 71:959-963, 2002.