One-shot gene remedy for Angelman syndrome exhibits promise in mice | Spectrum
Big boost: A potential new gene therapy for Angelman syndrome results in expression of UBE3A throughout the mouse brain within days of a single injection.
Courtesy Matt Judson / UNC
Injection of a potential new gene therapy for Angelman syndrome prevents many of the key features of neurological development, according to early tests in mice.
“While additional pharmacology and safety studies are required, our viral vector may provide transformative therapeutic relief with a single dose,” says lead researcher Benjamin Philpot, professor of neuroscience at the University of North Carolina at Chapel Hill.
Angelman syndrome, which affects about 1 in 20,000 children, is associated with significant developmental delays and is often associated with autism. It is caused by mutations or deletions in the maternal copy of the UBE3A gene, which encodes a protein that helps regulate levels of other important proteins.
There are no specific treatments for Angelman syndrome, but several gene therapies are under development. One in clinical trials requires repeated injections into the spine and has shown serious side effects at high doses. These therapies all aim to restore UBE3A function in neurons. One challenge, however, is that neurons produce multiple variants or “isoforms” of the UBE3A protein that vary slightly in length; in mice, for example, neurons form two isoforms in a ratio of about four short forms for each long one.
In contrast to other gene therapies, the new short and long forms of the UBE3A protein produce almost the same ratio as in mouse neurons. Such proportions “could be important for therapeutic efficacy,” says Eric Levine, a professor of neuroscience at the University of Connecticut at Farmington, who was not involved in this study.
The key to achieving this isoform ratio was careful design of DNA sequences known as Kozak sequences that help control the production of the proteins, says Philpot.
“What’s particularly cool is that you can use a single vector to introduce two different protein isoforms with roughly precise proportions,” says Stormy Chamberlain, an adjunct professor of genetics and genomics at the University of Connecticut who was not involved in the research.
Regulatory gene: UBE3A, which encodes a protein that helps regulate levels of other important proteins, is expressed in mouse neurons (green).
Courtesy Matt Judson / UNC
Natural ratio:
Philpot and his colleagues developed their optimized version of UBE3A and integrated it into a virus to transport it in neurons. They injected this vector into cavities called ventricles in the brains of newborn mice that lack the maternal copy of UBE3A and modeled features such as those seen in people with Angelman syndrome, including seizures.
The treatment resulted in UBE3A expression throughout the brain within days of injection, the researchers found. The treated mice behaved like wild-type mice with regard to digging, digging and nesting and performed similarly in tests for motor learning, such as mastering balancing on a rotating bar.
Treated mice also became less prone to experimentally induced epileptic seizures and experienced fewer disorders of the hippocampus, the memory center of the brain, which often cause such seizures. Untreated mice developed the usual Angelman-like impairments. The scientists detailed their findings in the Journal of Clinical Investigation: Insight last month.
While the viral vector used in this study is not ideal for gene therapy in humans, “one shouldn’t put too much emphasis on this choice,” says Chamberlain. This vector was designed for proof-of-principle experiments in mice; Future research could use gene delivery methods that are better suited for use in humans, she notes.
And although mice have two main isoforms of the UBE3A protein, humans have three – a short form and a long form, which can be seen mainly in the nucleus, and a long form, which is mainly found in the cytosol, the fluid that makes up most of it inside the cell, says Ben Distel, associate professor of neuroscience at Erasmus University Medical Center in Rotterdam, the Netherlands, who was not involved in the study.
The new vector did not produce this cytosolic isoform, which makes up a small part of the UBE3A protein in a cell and whose function is still unclear. “Ideally, you would like to express all three human isoforms at their correct level, although this can be technically very difficult,” says Distel.
The new gene therapy often led to UBE3A overexpression in the hippocampus, Philpot and his colleagues found. Overexpression of the gene has been linked to the autism-related condition dup15q syndrome. However, this overexpression was unusual in other important brain structures and did not appear to affect the behavior of the animals.
“We haven’t seen any signs of adverse overexpression in our experiments, although we will continue to investigate and test this,” says Philpot. “A common concern across all gene therapies is long-term safety, but we have not seen any adverse effects in mice up to 6 months of age, the longest we have studied to date.”
Future research should test large animals and try to get therapeutic DNA into only enough brain cells necessary to optimize effects, says Elizabeth Berry-Kravis, professor of Pediatrics and Neurological Sciences at Rush University Medical Center in Chicago, Illinois, who was not involved in learning. Further work should also better quantify how much UBE3A protein is produced per cell in order to avoid problems with its overexpression, says Chamberlain.
“There are many therapeutic options for Angelman syndrome under development,” she says. “Families have a lot to offer, but there is still a lot of work and a lot to do before they can be tried out on people.”
Quote this article: https://doi.org/10.53053/WXZU8323
