From the headlines, you'd think that the new genetic test for autism described today in the journal Pediatrics will give parents of children with autism the answers they so desperately seek. Not so, alas. Although this test identifies more children who have genetic abnormalities associated with autism, it doesn't nail down the cause of about 90 percent of autism cases.
Right now, children suspected of having autism are tested for genetic abnormalities with two tests: the G-banded karyotype, which looks for abnormalities in the chromosomes, and fragile X testing, which looks for a specific variation on the X chromosome. Those two tests find genetic abnormalities in up to 5 percent of children with autism. In the new study, researchers used a newer test, chromosomal microarray analysis, to identify variations in much smaller chunks of DNA. That test turned up genetic abnormalities in about 7 percent of people with autism spectrum disorders, compared with about 2 percent of people tested with the karyotype method. The researchers, affiliated with institutions that are part of the Autism Consortium of Boston, point to that 5 percentage point difference and argue that the newer genetic test should be used widely in diagnosing children with autism.
However, even with the better genetic test, about 90 percent of children diagnosed with autism will have no known genetic cause of their condition. We simply don't yet have tests capable of finding the subtle genetic cues that play a role in autism, alhtough children with more severe forms of autism or mental retardation are more likely to have identifiable genetic abnormalities.
"Undoubtedly, autism has a genetic basis," says Robert Marion, director of clinical genetics at Montefiore Medical Center in New York and author of Genetic Rounds. "The fact that we've poured billions of dollars into research and have come up with virtually nothing that is really helpful tells me that the mechanism involved in causing autism is different than the standard single-gene mutation" that people think of as causing a genetic disorder. Indeed, Marion and other researchers think that the genetic causes of autism lurk in the 95 percent of human DNA that doesn't code for proteins but rather controls how and when genes are switched on and off. Scientists are just starting to figure out how that epigenetic process works. And since many of the genetic variations identified in people with autism appear to be unique to that person, it will take a lot more testing and research to figure out cause and effect. "Autism is not one syndrome or one condition," Marion says. "It's a common pathway that is led to by a number of problems that affect the way the brain works." At this point, he says, we know something is "screwed up" but not how or why.
So, the more precise genetic tests can be helpful to a small number of parents, sparing the family time and worry en route to a speedier diagnosis. But for the other 90 percent of autistic kids, their parents can only wait for answers.
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