Dr. Robert Schultz, director of the Center for Autism Research (CAR) at The Children’s Hospital of Philadelphia (CHOP), says “at a population level” genetics is “unequivocally” the most important cause of autism. Researchers can infer this based on studies of identical twins: About 80 percent of the time, when one twin has it, so does the other.
But since it’s not 100 percent of the time, there has to be something else going on, some other cause.
“That means, for any given kid, we probably often don’t know why,” says Schultz. “We don’t know what we’re looking for. We don’t know what the genes are by and large. We’ve identified about 10-15 percent of the genetic cause ... So we’ve discovered the lowest-hanging fruit. But the most complicated and the vast majority we haven’t discovered yet.”
What makes autism so confounding is it’s a heterogeneous disorder, meaning on a case-by-case basis there can be multiple causes, unlike Down syndrome, which is caused by a single genetic anomaly — the presence of an extra chromosome.
With autism, it could be the genes, the environment, or genes and environment conspiring together.
But the medical community is as unclear on what those environmental circumstances may be — neurotoxicants that damage the nervous system or materials that impinge directly upon DNA — as they are on the genes.
The medical community’s focus currently tilts more heavily toward gene study than environmental factors, in part because identifying the genes that cause autism is thought to be easier. There are roughly 50,000 known neurotoxicants — lead, for example — versus about 25,000 genes in the human body, Schultz says.
“So we’ve got twice as many influences. And those exposures could happen once. You could be exposed to something terrible when you’re 7 months old and you could have it around in your body for a week or a month,” he says. “Now you’re 17 and you’re seeing me. How am I going to know about that? So everyone says, ‘Let’s study the genes.’ There’s fewer of them. They’re always with you.” [emphasis added]
If science can discover the genes causing autism, they can then study how the environment affects those genes. Epigenetics is a field of study that examines how genes express themselves and the outside elements that cause those expressions to be altered. Research in this area could help shed light on how environmental influences may contribute to autism.
Schultz explains: “Basically genes turn on and off across development. Some genes are always on. But a lot of developmental genes that affect the development of the brain are on for a while, and then they’re off, because they’re needed during a certain development period. There’s a lot of things that regulate when they’re on and when they’re off. And the environment can regulate turning genes on and off.”
Sunday, August 28, 2011
The Causation Puzzle
Rob Scott writes at The Gloucester County Times in New Jersey: