The gene signature approach, which Boston Children's Hospital has licensed exclusively to
Southborough, Mass.), can potentially diagnose autism far more often than the genetic tests currently available. Those tests look for variety of autism-related mutations—from small "spelling" changes to lost or extra copies of a gene or genes (known as copy number variants) to wholesale chromosome abnormalities—but together, the known mutations account for fewer than 20 percent of autism cases.
"It's clear that no single mutation or even a single pathway is responsible for all cases," says Kohane. "By looking at this 55-gene signature, which can capture disruptions in multiple pathways at once, we can say with about 70 percent accuracy, 'this child does not have autism,' or 'this child could be at risk,' putting him at the head of the queue for early intervention and evaluation. And we can do it relatively inexpensively and quickly."
Previous gene-expression studies on blood samples have been limited by study size and by the challenge of obtaining well-matched control groups, Kohane notes.
The 55 genes whose expression was altered also suggest more than one path to what we know as autism. Based on their genetic signatures, subjects with ASDs clustered into four subgroups marked by changes in different biological pathways:
- Synaptic pathways, specifically long-term potentiation pathways (essential for memory and learning) and neurotrophic pathways (signaling neurons to survive, develop and grow)
- Immune/inflammatory pathways
"In our sample, about half of the autism cases had some sort of alteration on immune pathways, synaptic pathways, or both," says Kong.
Adding to the neuro-immune theory of autism
Most current theories of autism focus on disordered synapses (connections between brain cells). But, citing previous studies in the medical literature, Kohane speculates that brain development in autism may be impaired by abnormal immune responses to infections and other stressors, during infancy or prenatally. For example, chemokines (inflammatory chemical messengers that recruit immune cells to fight infections) have been
to be overexpressed in some neurocognitive disorders
and animal studies have found that prenatal infections are associated with autism-like features in offspring.
"We know that if a mother has rheumatoid arthritis or the father has type 1 diabetes, there is an increased risk of autism in the child," says Kohane. "
have also noted an increase in autoimmune disease, just as autism is increasing."
of brain tissue from patients with autism
have found significant activation of immune cells in the cerebral cortex and cerebellum, and
recent research at Boston Children's Hospital
has shown that the same molecules used by the immune system also play a role in the development and functioning of the brain.
"One can imagine that an infectious assault during pregnancy could increase chemokines into the brain," Kohane says. "Or there may be something intrinsic to the brain that causes it to generate an inflammatory reaction. Genetic makeup may set up vulnerability in a subgroup of infants, but there may be some environmental factor that triggers the disease in that subgroup."