New study pinpoints how a specific cellular mechanism drives autism and suggests an existing drug could help patients with UBE3A-related conditions.
In late 2015, genetic studies identified more than 1,000 gene mutations associated with autism, but the cellular pathways linking those mutations to the disorder were largely unknown. Researchers at the UNC School of Medicine have now identified how a single autism-linked mutation disables a molecular regulatory switch in the gene UBE3A, producing a hyperactive enzyme that disrupts brain development and contributes to autism.
Published in the journal Cell, the study demonstrates that UBE3A is regulated by phosphorylation: when a phosphate group is added to UBE3A, the enzyme is turned off, and removing that phosphate can turn it back on. This on/off mechanism allows tight control of UBE3A activity during normal neuronal development. The research team led by Mark Zylka, PhD, associate professor of cell biology and physiology, discovered that a specific autism-associated missense mutation destroys this phosphorylation control. The broken switch leaves UBE3A permanently active, which in turn alters synapse development in the brain.
“Genetic evidence points to roughly a thousand genes linked to autism, meaning mutations in many different places can lead to the disorder. We’ve shown how one of these mutations operates at a molecular level,” said Zylka, senior author of the paper and a member of the UNC Neuroscience Center.
The experiments combined human cell lines and mouse models. Using cells from the Simons Simplex Collection, the team sequenced genes from an affected child and the child’s parents. The mutation was de novo: it was present in the child but not in the parents. Cells from the child demonstrated hyperactive UBE3A because the phosphorylation site that normally regulates the enzyme was disrupted.
When the research team introduced the child’s mutation into an animal model, they observed an abnormal increase in dendritic spine formation on neurons. Excess dendritic spines have been linked to altered synaptic connectivity in autism, supporting the conclusion that hyperactive UBE3A can drive atypical neural development.
This finding aligns with prior genetic evidence: duplication of chromosome region 15q — which includes UBE3A and other genes — is a common genomic alteration in individuals with autism and is known as Dup15q syndrome. Too much UBE3A activity had been suspected to contribute to autism in such cases, but the new study reveals a direct molecular mechanism that can produce UBE3A hyperactivity even without chromosomal duplication.
Importantly, the team identified protein kinase A (PKA) as the enzyme responsible for adding the phosphate group to UBE3A at residue T485, thereby inhibiting UBE3A’s ubiquitin ligase activity. This insight points to potential therapeutic strategies because drugs that modulate PKA activity already exist. In laboratory tests, two known compounds substantially reduced UBE3A activity in neurons, suggesting it may be possible to lower UBE3A activity therapeutically in Dup15q or other UBE3A-hyperactivity cases to restore more normal enzyme levels in the brain.
One of the compounds tested, rolipram, was previously evaluated in clinical trials for depression but discontinued for side effects. Given the serious seizure risk and sudden unexpected death in epilepsy that can accompany Dup15q syndrome, Zylka and colleagues note that reexamining drugs that increase PKA activity—at carefully controlled, lower doses—might offer symptom relief where benefits outweigh risks. Future work using animal models of Dup15q will be important to test whether this pharmacological approach can safely and effectively normalize UBE3A activity and improve neurological outcomes.
The study also connected this regulatory mechanism to Angelman syndrome, a distinct neurodevelopmental disorder caused by loss of UBE3A function that leads to developmental delay, seizures, motor problems, and limited speech. Zylka’s team observed that several Angelman-linked mutations cluster near the phosphorylation region of UBE3A and disrupt the enzyme’s function or stability. Those mutations effectively eliminate UBE3A activity, which contrasts with the hyperactivation seen in the autism-linked mutation. Recognizing mutations that impair UBE3A stability could improve diagnosis and understanding of Angelman syndrome.
Other co-authors of the study include Ben Philpot, PhD, William Snider, MD, and Klaus Hahn, PhD, as well as graduate student Janet Berrios and Jason Newbern, PhD. The work was funded by the National Institutes of Health, the Angelman Syndrome Foundation, The Foundation for Angelman Syndrome Therapeutics, and Autism Speaks.
Source: Mark Derewicz – UNC
Image Credit: The image is in the public domain
Original Research: Abstract for “An Autism-Linked Mutation Disables Phosphorylation Control of UBE3A” by Jason J. Yi, Janet Berrios, Jason M. Newbern, William D. Snider, Benjamin D. Philpot, Klaus M. Hahn, and Mark J. Zylka in Cell. Published online August 6, 2015. doi:10.1016/j.cell.2015.06.045
Abstract
An Autism-Linked Mutation Disables Phosphorylation Control of UBE3A
Highlights
• PKA phosphorylates UBE3A at T485 and inhibits UBE3A ubiquitin ligase activity
• An autism-linked UBE3A T485A missense mutation disrupts this phosphorylation regulation
• The T485A mutation hyperactivates UBE3A and increases synapse formation in vivo
Summary
Loss of UBE3A causes Angelman syndrome, while increased UBE3A copy number or activity is associated with autism, indicating that precise control of UBE3A ubiquitin ligase activity is essential for normal brain development. The study shows that PKA phosphorylates UBE3A at residue T485 outside the catalytic domain to inhibit its activity toward itself and other substrates. An autism-linked de novo T485A missense mutation blocks this phosphorylation, producing enhanced UBE3A activity in vitro, accelerated substrate turnover in patient-derived cells, and excessive dendritic spine development in vivo. These results identify PKA as an upstream regulator of UBE3A and implicate excessive UBE3A activity and consequent synaptic dysregulation in autism pathogenesis.
“An Autism-Linked Mutation Disables Phosphorylation Control of UBE3A” by Jason J. Yi, Janet Berrios, Jason M. Newbern, William D. Snider, Benjamin D. Philpot, Klaus M. Hahn, and Mark J. Zylka in Cell. Published online August 6, 2015. doi:10.1016/j.cell.2015.06.045