A new genetic analysis of people with autism and their relatives has uncovered 18 genes associated with the disorder.
People with autism often had dozens of mutations that may have caused their symptoms — an average of 73 unique mutations, the team at Autism Speaks found.
Some of the mutations might be affected by medications, the researchers reported in the journal Nature Neuroscience.
The study adds to the considerable evidence that autism is a condition caused by genetics, and also adds to a growing body of evidence that each person with autism has his or her own pattern of DNA changes.
"It's noteworthy that we're still finding new autism genes, let alone 18 of them, after a decade of intense focus," said Mathew Pletcher, vice president for genomic discovery at Autism Speaks.
"With each new gene discovery, we're able to explain more cases of autism, each with its own set of behavioral effects and many with associated medical concerns."
Dr. Stephen Scherer of Toronto's Hospital for Sick Children and colleagues studied the DNA of more than 5,000 people in 2,066 families with children diagnosed with autism spectrum disorders, including 2,600 affected children.
They did what's known as a genome-wide association study — a deep dive looking at all the DNA in a person's cells, and how it's different from another person's.
The 18 genes they identified have not been not previously linked with autism, but they are all involved in brain cell communications. There are many different mutations affecting the genes, the researchers said.
There are also genetic changes that don't affect genes, but that are found in stretches of DNA — once called junk DNA — that affect the activation of genes.
"In fact, the genetic predisposition toward autism spectrum disorder may be different for almost every individual," the team wrote.
The same team found in 2015 that even siblings with autism have different genetic mutations from one another. This new study broadens the number of people they looked at.
Autism spectrum disorder can range from the mild social awkwardness, including Asperger's syndrome, to profound mental retardation, debilitating repetitive behaviors and an inability to communicate. There's no cure, but experiments with early treatment suggest it can help.
Autism is becoming more and more common among U.S. kids, and researchers don't quite understand why. One survey by the Centers for Disease Control and Prevention showed 2 percent of U.S. children have been diagnosed with an autism spectrum disorder — one in 68 kids.
Another survey found it had been diagnosed in one in 45 kids.
Studies also show it's clear that genetic mutations are responsible for many, if not most cases of autism. Infections during pregnancy also play a role, and it is possible that certain genetic mutations make children more susceptible to brain changes caused by infections in the womb.
One study found 33 new genes associated with autism, and researchers think they may eventually find 1,000 mutations.
It's already known that kids with autism have larger-than-normal brains. One hypothesis is that the growing brain of a child with autism doesn't "prune" unneeded connections properly, and the resulting overgrowth of nerve connections sends the brain into overdrive.
Another study published Monday found excess cerebrospinal fluid may also be a pointer to a child with autism. The symptoms often do not become clear until a child is a toddler or older, but researchers hope early treatment may help prevent some of the more severe symptoms.
In the gene study, the researchers found that all of the DNA changes affected brain cells or communication, and many of them might be affected by drugs.
"Sixteen genes contained subdomains that could be targeted by pharmaceutical intervention and seven contained subdomains for which specific drug-gene interactions are known," they wrote. For example, people with certain mutations could try drugs known as allosteric modulators of GABA receptors — a class that includes diazepam, also known as Valium; the sleeping pill Ambien and barbiturates.
Autism Speaks is making the data from the project, called MSSNG (it's the missing 'I's' that matter), available freely.
"Through its research platform on the Google Cloud, Autism Speaks is making all of MSSNG's fully sequenced genomes directly available to researchers free of charge, along with analytic tools. In the coming weeks, the MSSNG team will be uploading an additional 2,000 fully sequenced autism genomes, bringing the total to (more than) 7,000."