Developmental pediatrician Paul Wang is Autism Speaks' senior vice president and head of medical research. We asked Dr. Wang to elaborate on the recent study on a surplus of synapses and autism.
The study on brain synapses in autism, led by Drs. Guomei Tang, David Sulzer, and colleagues, has been receiving a lot of attention. This study helps us to understand the differences in brain structure that are found in autism. This new research actually fits very well with other recent work, which shows that individuals with autism have differences in brain connectivity. That is, different areas of the brain do not coordinate with each other in the same way as they do in other persons. Dr. Tang and Sulzer’s research gives an explanation for this atypical connectivity, because synapses are the physical connections through which brain cells communicate with each other.
An interesting feature of brain development is that the number of synapses actually decreases as children grow older. Toddlers and pre-schoolers have more synapses than teenagers, and those “extra” synapses are supposed to get “pruned” away as the years go by. Tang and Sulzer’s work shows that in autism, the number of synapses is normally high early in life, but fails to decrease in the usual way. As a result, teenagers with autism end up with more synapses than is typical.
Drs. Tang and Sulzer, and their team, also studied a particular mouse that is considered a research model for autism. This mouse has a mutation in the gene for tuberous sclerosis, which increases the risk for autism in people, and the mouse also has too many synapses. The researchers treated their mice with a drug (rapamycin) that decreased the number of synapses and improved social behaviors.
Rapamycin (also called sirolimus) is used in human patients with organ transplants to prevent rejection. Unfortunately, it can have severe side effects, including suppressing the immune system, lung inflammation, and risk for diabetes. Because of these side effects, it cannot be recommended for children with autism. But it is worth knowing that a very similar drug, called everolimus, is actually being studied in patients with tuberous sclerosis, a genetic disorder that causes tumors, cognitive impairments, and increases risk for autism.
Scientists will now be hot on the trail of new drugs that are safer, and that help to restore normal pruning of synapses. But we don’t know whether synaptic pruning is a problem in only a small number of patients with autism, or a larger number. Research will need to address that question, too. And even if a drug can normalize pruning and behavior in mice, it may not have the same effects in people with autism. That question can only be answered by direct testing, in clinical drug trials.
As a final note, I’d like to join Drs. Tang and Sulzer in thanking the families who showed incredible generosity by donating the brains of their departed children for research. We are hopeful that the insights made possible by their act of kindness have put us on the trail of future treatments for autism.