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.