Protein shows a link between autism and addiction

A specific protein that is associated with autism may play a role in addiction.

In a paper published in the latest issue of the neuroscience journalNeuron, McLean Hospital investigators report that a gene essential for normal brain development, and previously linked to Autism Spectrum Disorders, also plays a critical role in addiction-related behaviors.

"In our lab, we investigate the brain mechanisms behind drug addiction -- a common and devastating disease with limited treatment options," explained Christopher Cowan, PhD, director of the Integrated Neurobiology Laboratory at McLean and an associate professor of Psychiatry at Harvard Medical School. "Chronic exposure to drugs of abuse causes changes in the brain that could underlie the transition from casual drug use to addiction. By discovering the brain molecules that control the development of drug addiction, we hope to identify new treatment approaches."

The Cowan lab team, led by Laura Smith, PhD, an instructor of Psychiatry at Harvard Medical School, used animal models to show that the fragile X mental retardation protein, or FMRP, plays a critical role in the development of addiction-related behaviors. FMRP is also the protein that is missing in Fragile X Syndrome, the leading single-gene cause of autism and intellectual disability. Consistent with its important role in brain function, the team found that cocaine utilizes FMRP to facilitate brain changes involved in addiction-related behaviors.

Cowan, whose work tends to focus on identifying novel genes related to conditions such as autism and drug addiction, explained that FMRP controls the remodeling and strength of connections in the brain during normal development. Their current findings reveal that FMRP plays a critical role in the changes in brain connections that occur following repeated cocaine exposure.

"We know that experiences are able to modify the brain in important ways. Some of these brain changes help us, by allowing us to learn and remember. Other changes are harmful, such as those that occur in individuals struggling with drug abuse," noted Cowan and Smith. "While FMRP allows individuals to learn and remember things in their environment properly, it also controls how the brain responds to cocaine and ends up strengthening drug behaviors. By better understanding FMRP's role in this process, we may someday be able to suggest effective therapeutic options to prevent or reverse these changes."

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Same mutations seen in epilepsy and autism

The same mutations are seen in children with epilepsy and autism indicating that the disorders may be linked.

Mutations in GABRB3, a brain receptor linked to autism, are prevalent in severe childhood epilepsy, according to a study published 12 September inNature¹.

The study also found that many of the spontaneous, or de novo, mutations found in children with epilepsy overlap with those linked to autism and fragile X syndrome.

About one-third of people with autism suffer from epilepsy. This overlap suggests that the two disorders may have a common origin — a theory borne out by examples of shared genetics.

Studies suggest that, biologically, autism is the result of an imbalance between excitatory and inhibitory brain signals, which may also underlie epilepsy.

In the new study, researchers sequenced the protein-coding DNA, or exomes, of 264 children who suffer from either infantile spasms or Lennox-Gastaut syndrome, and their parents. Both disorders are characterized by multiple seizures during sleep, which may lead to cognitive problems.

The researchers found multiple mutations in nine genes in the children with epilepsy that are not present in their parents. In particular, four children have a mutation in GABRB3, a receptor that transmits calming signals to neurons. None of 610 controls or their parents have a mutation in this gene.

A 2009 study found a rare variant of GABRB3 that affects neuronal signaling in 17 of 1,152 families with autism and 1 of 292 controls². 

Overall, the researchers found 277 mutations in the exomes of children with epilepsy that are not present in their parents. Of these, 41 are also seen in children with autism and 64 may be regulated by FMRP, the protein missing in fragile X syndrome.

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