The researchers found that almost 70% of the studied siblings had different risk genes for the condition.
The study, published in the journal Nature Medicine and led by Dr. Stephen Scherer of The Hospital for Sick Children in Ontario, Canada, questions whether autism is typically inherited or a result of other factors.
Autism affects around 1 in 68 children in the US and is almost five times more common among boys than girls.
Numerous studies have suggested that autism can be inherited. According to the Centers for Disease Control and Prevention (CDC), research has shown that if one identical twin has autism, the other twin has a 36-95% chance of developing the condition.
What is more, studies have shown that parents who have a child with autism have a 2-18% chance of having another child with the condition.
'Each child with autism is like a snowflake - unique from each other'
For their study, Dr. Scherer and his team conducted whole genome sequencing on 85 families with two siblings who had been diagnosed with autism.
As opposed to standard genetic testing, whole genome sequencing assesses the entire DNA make-up of an organism's genome at one time.
The results of the analysis revealed that 69.4% of siblings carried different autism-related genetic mutations. What is more, the researchers found that these siblings displayed more variability in autism symptoms than the 30.6% of siblings who did share the same autism-risk genes.
These findings indicate that among siblings who both have autism, the disorder may be caused by gene mutations that have "developed sporadically."
"Although it is possible that other undetected or uncharacterized combinations of rare variants not considered in this study or other common variants may be contributing," the researchers say, "many autism-risk genes implicated in these families are considered on their own to be sufficiently penetrant to cause autism-relevant phenotypes."
Medical News Today asked Dr. Scherer if these findings came as a surprise. "Initially this surprised us," he replied, "but when we thought more about it, based on what we know, it seems to make sense. Lots of different genes are involved [in autism] - more than 100 - and based on all of the different potential combinations of events, it would be more likely for different genes to be involved than the same gene."
"We knew that there were many differences in autism, but our recent findings firmly nail that down. We believe that each child with autism is like a snowflake - unique from the other."
Findings show importance of whole genome sequencing
Dr. Scherer says the findings also suggest clinicians should go beyond searching for suspected autism-related genes, as is normally done using diagnostic genetic testing.
"What often happens now is that if there is an older child with an autism diagnosis and the gene involved is known, in future children - for reasons of genetic counseling - they often just sequence the known gene to look for mutations in the siblings or other family members," he told us. "Our story shows you need to go deeper than that and preferably sequence the entire genome."
The data in this study form a part of the MSSNG project - a collaboration between Autism Speaks, who funded the study, and Google. The project aims to produce a database of at least 10,000 autism genomes openly available for global research, with the aim of improving understanding of autism and aiding the development of personalized treatments for the condition.
Study co-author Robert Ring, chief science officer at Autism Speaks, says making such data accessible to researchers worldwide is "breaking down barriers in a way never done before."
"As always, our goal at Autism Speaks is to accelerate scientific discovery that will ultimately improve the lives of individuals with autism at home and around the world," he adds.
MNT recently reported on a study published in The Lancet Psychiatry, in which researchers claim video-based therapy may improve autism-related behavior in infants at risk of the condition.