Researchers say they've identified more genes that may be part of the underlying causes of autism, raising the number linked to autism spectrum disorder from nine to 33.

That's the result of an analysis of several uncommon genetic differences seen in more than 14,000 DNA samples gathered from parents, affected autistic children and a control group of unrelated individuals.

The largest such sampling to date has yielded evidence suggesting just tiny differences in as many as 1,000 suspected risk genes may contribute to the development of autism, researchers are reporting in two new studies published in the journal Nature.

The genes focused on in the studies can apparently affect the formation of the brain's nerve networks and control the operation of synapses, crucial structures involved in communication between brain cells, they say.

"This makes sense because typical development of brain cells require intricate coordination among thousands of genes and appropriate communication between cells to ensure development of the brain -- the most complicated organ in the human body," says Kathryn Roeder of Carnegie Mellon University, a co-author of one of the studies.

Autism spectrum disorder, seen in one out of 68 children in the United States, can affect social interactions, most especially communications, and other behaviors with different levels of severity.

While the studies increased the number of definitive autism genes to 33, an additional 70 are considered possible ASD genes, the researchers say.

While small but rare genetic differences in the identified genes were discovered to represent a comparatively large increase in a person's risk of developing autism, many more changes in other genes can add smaller amounts of risk, they found.

The combination of gene variations both common and rare holds the key to understanding autism, the researchers suggest.

"While we have very strong findings in these genetic analyses, newfound genetic discoveries must next be moved into molecular, cell and animal studies to realize future benefits for families," says Joseph D. Buxbaum of the Mount Sinai School of Medicine in New York, a co-author author of the Nature study.

"A study like this creates an industry for years to come, with labs worldwide checking the brain changes linked to each new genetic finding, and searching for drugs to counter them," he said.

The studies have provided the clearest theoretical picture yet of the way in which a number of genetic changes can come together to have an impact on the brains of children suffering from autism, the researchers said.