Autism Spectrum DisorderGenetics

Theories on Genetics and the Presentation of Autism #

Autism spectrum disorder can look wildly different from one child to another. In addition to a broad definition of symptoms, these symptoms can also arise at different times, with some children showing symptoms in infancy or their early toddler years, while others don’t show any symptoms until after they begin attending school. In recent years, scientists have found that there may be multiple different types of autism, driven by unique clusters of genetic variants. While this science is not yet settled, several theories are presented below.

Early/Late Theory #

As noted above, some children’s autism symptoms arise in early childhood, while others show symptoms only later in development. Using a large genetic database, a research group based at the University of Cambridge in the UK has proposed that autism can be divided into two larger groups, defined by early versus late presentation of symptoms.¹

Early: The “early” type of autism tends to include a higher risk of cognitive disability and developmental delay, but a lower risk of co-occurring mental health disorders. These children are often diagnosed before the age of 5.

Late: The “late” type of autism has a much higher risk of co-occurring attention deficit-hyperactivity disorder (ADHD), depression, and other mental health conditions. These children are often diagnosed later in childhood, often in their teen years.

These two groups have distinct genetic profiles, suggesting that these may represent two different conditions with overlapping symptoms.

Genetic Program Theory #

Another group, based at Princeton University, used a similar genetic database to identify distinct types of autism,²which likewise have their own genetic variants, symptoms, and time of symptom onset. Their analysis suggests that there may be four autism clusters with unique biological mechanisms.

Social and/or behavioral group: This group has normal cognitive development, but displays challenges in social function and behavior. They may be more likely to receive diagnoses of ADHD, obsessive compulsive disorder, and other types of mental health conditions. Many children in this group have common autism risk variants. The variants that drive this condition seem to affect neuronal development after birth, which may explain why these children’s symptoms and diagnoses occur later in childhood. In the study, approximately 35% of children with autism fell into this category.

Mixed autism with developmental delay group: Like the “early” group identified by Zhang et al, this group has developmental delays in language and motor skills. They also have mixed autistic traits, meaning that they may display challenges in some but not all areas often associated with autism. These children tend to have rare variants that convey greater autism risk and often cause neuronal development issues prior to birth. In the study, approximately 20% of children with autism fell into this category.

Moderate challenges group: Individuals in this group tend to have milder symptoms than the other categories. While relatively rare and acting early in development, their genetic variants tend to cause fewer or lighter symptoms. Approximately 35% of children with autism fell into this category.

Broadly affected group: Children in this group have the highest levels of symptoms of all the groups, including cognitive impairment, developmental disability, challenges in social function, and more. This group tends to carry rare gene variants with very high autism risk; these genes affect neuronal activity throughout development. Because of their developmental disability and cognitive impairment, these children tend to receive an early diagnosis. Approximately 10% of children with autism fell into this category.