Scientists working at the University of North Carolina, Chapel Hill reported in the Oct. 21, 2020, issue of Nature on the successful development of a one-time specific sequence-directed gene therapy approach using the combination of AAV with CRISPR technology that successfully prevented the presentation of Angelman syndrome throughout the lifetime of a mouse model.

Lifelong gene therapy has held promise for decades now as one of the only approaches that could possibly address many neurodevelopmental genetic disorders. But even after decades of research, gene therapy still possesses significant risks due to untoward random genomic insertions of vectors that could ultimately cause other genetic disorders.

Meanwhile, it has been known for decades now that adeno-associated virus (AAV) is a particularly powerful potential gene therapy vector because AAV integrates into the genome so well. However, the integration of AAV has always been random and so it inherently comes with significant risk.

This is the first time that a treatment for Angelman syndrome has been shown to correct this neurodevelopmental disorder.

Principal investigator, Mark Zylka, professor of Cell Biology and Physiology in the Neuroscience Center, University of North Carolina, Chapel Hill, told BioWorld Science, "The key really from what we can tell is going early in treatment. So for the animals that have the disorder we can identify them with genotyping. If you catch it early, you can treat them one time and it lasts forever as far as we can tell.

That longevity, he said, "contrasts with treatments that are in development using antisense technologies that usually have to be injected every 4 months or so, which is not ideal for a pediatric disorder that will last a lifetime."

Angelman syndrome is caused by loss of function of the maternal Ube3a allele, while the paternal allele is normally silenced by a very long antisense noncoding RNA known as Ube3a-ATS. Previously in a 2011 Nature publication Zylka and collaborators demonstrated that a class of drugs called topoisomerase inhibitors could reactivate the paternal allele by interfering with Ube3a-ATS. So Zylka knew that if the paternal copy of Ube3a can be turned on, this will provide the possibility of treating the condition.

Topoisomerase inhibitors, which include chemotherapy agents such as irinotecan and doxorubicin, are not a therapeutic option for Angelman syndrome due to their broad-spectrum nature and toxicity. But with the development of CRISPR combined with AAV, the researchers have now developed a tool to precisely hone in on specific regions of the genome.

First, the team screened 250 different RNA guided CRISPR/Cas9 constructs in cell culture until they identified the best one (Spjw33) reactivating the Ube3a-ATS allele. These clones had the good fortune to target Snord115 genes within the large Ube3a-ATS locus. The Snord genes are functionally redundant, with over 100 of them present in both mice and humans.

Ultimately the CRISPR/Cas9 with the cloned RNA guide was used to a specific region of the DNA, where DNA was inserted into the Snord115 gene of the Ube3a-ATS locus. The inserted DNA possessed a polyadenylation signal that caused the premature termination of the Ube3a-ATS noncoding RNA such that it no longer silenced the paternal expression of Ube3a.

With the Ube3a now made in the mouse, it fully developed and no longer presented with any phenotypes resembling Angelman syndrome throughout the life of the animal.

In short, instead of deleting the gene, this approach disrupted the Ube3a-ATS gene by stopping its full production prematurely. Only a small nonfunctioning part of the noncoding RNA was still produced in treated animals.

Earlier is better

The broad implications are that the study proves that Angelman syndrome can be treated and possibly prevented, if it is done early enough.

Previous studies showed that if turning on the paternal copy later, even within just a few days after birth in a mouse, this approach does not prevent Angelman syndrome.

Zylka said, "It is like with a building. You want to make sure the foundation is done correctly. Tons of time is put into the foundation. If there is a problem with the foundation, then when building on top of it, it is very hard and next to impossible to go back and fix the foundation. When the brain is developing, it is the initial foundation upon which the brain is built that is critical and you cannot really go back and fix it. So this study now shows that you can fix the problem if you catch it early enough by administering just a single treatment."

One encouraging result was the lack of gene therapy occurring in the mother. The team injected the vector into the fetus, but no gene therapy was detectable in the mother's liver and brain. Instead, the gene therapy was restricted to only the fetus. This was remarkable and very important since AAV is well known to particularly target the liver.

The technology to identify fetuses with the mutation that causes Angelman syndrome is already available and currently used in hospitals around the world. Techniques like amniocentesis, chorionic villus sampling, and even newer noninvasive technologies involving taking extra blood from the mom can now detect fetal DNA and cells to find out if there are any Angelman syndrome mutations.

However, there has not been a strong incentive to look for Angelman syndrome given that there are no therapeutic options at this point.

Zylka hopes to ultimately test the approach in the clinic. But first-time gene therapy technologies are often only given one shot in clinical trials and safety is of primary concern. So, extensive further research will be necessary to not throw away his shot (Wolter, J.M. et al. Nature 2020, Advanced publication).