The symptoms of attention deficit/hyperactivity disorder (ADHD) and autism have many things in common; they are both common psychiatric diagnosis of children, they are heritable conditions, and there is significant overlap in the known genetic causes of these neurodevelopment issues. Scientists have now identified seven genetic variants that increase the risk of one of these disorders, but not the other. These specific, small changes in the sequences of genes, or genetic variants, could help clinicians make more accurate diagnosis for patients and their families. The findings have been reported in Nature Genetics.
"We have succeeded in identifying both shared genetic risk variants and genetic variants that differentiate the two developmental disorders," said study co-author Professor Anders Børglum of Aarhus University and iPSYCH, Denmark's largest psychiatric research endeavor. We are now starting to understand the biological mechanisms that underlie the development of ADHD and autism, as well as something totally new - the processes that make one of the disorders more likely to occur, noted Børglum.
The variants that were identified in this work are thought to impact neurons in the brain, how the brain develops, and communication. Some of the variants found in this study can influence the cognitive functions of carriers of the variant in the general population as well. For example, some variants that only increase autism risk can also raise cognitive function in carriers of that variant. Other variants that specifically increase ADHD risk, however, can reduce cognitive functioning in carriers.
This study also revealed one genetic variant that increases autism risk while also reducing the brain volume of carriers in one specific region. A complementary variant, however, increases ADHD risk while also enlarging the brain region's volume.
This research is the first to ever show that people who are affected by both ADHD and autism are at genetic risk of either diagnosis, but people with either ADHD or autism tend to only carry genetic variants for a single condition.
So this can mean that people with both diagnoses have the same genetic burden from ADHD factors as people who were diagnosed only with ADHD, while also carrying the load of autism genetic factors that people with only autism have. "So it makes very good biological sense that some people have both diagnoses," Børglum explained.
This work could improve diagnostic criteria for both disorders. Only a few years ago, ADHD could not technically be diagnosed in an autism patient because of diagnostic criteria, noted Børglum. This study has shown how patients can be monitored, especially those who are diagnosed with autism but also have genetic variants that indicate a risk of ADHD. Autism tends to be diagnosed before ADHD, but ADHD symptoms could also be "drowned out" by autism symptoms, and ADHD may not be diagnosed even when it's occurring, added Børglum.
This study has shown that there is an actual difference in biology when people have one disorder, compared with when a person has both ADHD and autism.
While it is only the first step, this work "...is therefore a strong biological argument for the revised diagnostic guidelines, e.g. in the American Diagnosis and Classification system for Mental Disorders (DSM-5), where it is now possible for the same person to receive both diagnoses," said Børglum.
Sources: Aarhus University, Nature Genetics