Article by Kenneth Gadow, PhD Advances Knowledge of Genetic Links Between ADHD and Autism Spectrum Disorder


Kenneth Gadow, PhD reported that a genetic variant in the serotonin transporter (5-HTT) gene is associated with symptoms of ADHD and autism spectrum disorder (ASD) in children diagnosed with ASD. His findings were published in Progress in Neuro-Pharmacology & Biological Psychiatry.

Previously, Dr. Gadow and his colleagues found high rates of ADHD in children with ASD, and the symptoms of ADHD in these children often led to impairment in social and academic functioning. The high rate of co-occurrence between ASD and ADHD has led to speculation as to whether the two disorders are related in some way. Building on prior research implicating the serotonin system in a variety of neuropsychiatric syndromes, Dr. Gadow and his colleagues examined the association of the high-expressing long allele and low-expressing short allele of the 5-HTT gene with severity of ADHD and ASD symptoms in 118 children between the ages of 4 and 14 years who had been diagnosed with ASD. He also analyzed the relationship between the variants and the symptoms of ASD.

After sorting the children into genotype groups, Dr. Gadow found that those with the low-expressing (short) allele (which is associated with lower levels of 5-HTT) were significantly more likely to be rated by their mothers as having more severe symptoms of ADHD (hyperactivity and impulsivity) than children with the high-expressing (long) allele. Other researchers have found an association between the low-expressing (short) allele and ADHD in children who do not have ASD. Conversely, children with the high-expressing allele had more severe social deficits than the low-expressing allele group. They also found that the degree to which parents and teachers agreed about the severity of the child’s problems depended on the child’s genotype.

Although his study could not definitively answer the question of whether ADHD is an epiphenomenon of ASD, a co-occurring disorder, or a unique syndrome within the ASD phenotype, his identification of a genetic variant associated with the severity of symptoms of both ADHD and ASD supports the notion that the two disorders are in fact interrelated in some way. The study also provides one potential explanation for the overlap among symptoms of ADHD and ASD and suggests a possible mechanism to explain their co-occurrence.