Secreted ‘STUB’ Gene Therapy Shows Promise in Rat Model

Marisa Wexler, MS avatar

by Marisa Wexler, MS |

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A novel gene therapy that codes for a version of the UBE3A protein that can be secreted by cells outperformed a gene therapy coding unmodified UBE3A protein in a rat model of Angelman syndrome, a new study reports.

The study, “Improving Gene Therapy for Angelman Syndrome with Secreted Human UBE3A,” was published in Neurotherapeutics.

Angelman syndrome is caused by mutations in a gene called UBE3A, which provides instructions for making the UBE3A protein. Theoretically, delivering a healthy version of this gene to cells in a patient’s brain could allow the production of functional UBE3A protein, thus lessening symptoms. This is the basic concept of gene therapy for Angelman syndrome.

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In prior work, scientists have developed gene therapies that utilize viral vectors to carry a healthy copy of UBE3A to cells. Adeno-associated virus (AAV), which does not cause illness in people and is easily manipulated in a laboratory, is commonly used for gene therapy vectors.

A limitation of AAV-based therapies is that, by design, each viral vector delivers a copy of the gene to one single cell. This means that there are likely to be many cells that don’t end up receiving a copy of the gene.

Here, a team of scientists at Morsani College of Medicine at the University of South Florida set out to overcome this limitation by creating an AAV-based gene therapy coding for a version of the UBE3A protein that could be “shared” by cells.

To do this, the researchers attached two signaling sequences to the UBE3A protein: one sequence that allows the protein to be secreted by the cell, and a second sequence that lets other cells take up the protein.

“This allows the vector to create cellular ‘protein factories’ from transduced cells in which the protein will be created and secreted,” the researchers wrote. In theory, this could allow the gene therapy to deliver functional UBE3A protein even to cells that are missed by the viral vector.

The team dubbed its novel version of the UBE3A protein “STUB,” short for “secreted TAT UBE3A.” (TAT refers to the signaling sequences.)

“Through the addition of secretion and cell-penetrating peptides, this novel STUB construct can increase the effectiveness of treatment beyond locally transduced neurons by supplying the replacement of UBE3A to cells that were not transduced with the viral administration,” the researchers wrote.

After some initial tests to ensure that the gene therapy delivered the protein as designed and that STUB functioned similarly to unmodified UBE3A protein, the scientists tested their gene therapy in a rat model of Angelman syndrome. For comparison, they also tested a gene therapy encoding the unmodified version of UBE3A protein. Both were administered via injections into the rats’ brains.

Results showed that both therapies normalized electrical activities in the rats’ brains, but the gene therapy delivering STUB had a more potent effect.

Untreated Angelman rats show marked abnormalities in the rotarod test and hind limb clasping test (both measures of motor function). Whereas gene therapy encoding unmodified UBE3A had little effect on these abnormalities, treatment with STUB-coding gene therapy led to a marked normalization on both tests, though scores were still different from those of rats without Angelman syndrome.

Both STUB and unmodified UBE3A gene therapies normalized scores on fear conditioning testing (which measures memory), though not to levels seen in wild-type rats.

“The STUB vector also showed additional improvements over the hUBE3A [unmodified human UBE3A] vector, with improved recovery of behavioral deficits in the [Angelman] rat model. This rescue suggests that secretion of hUBE3A adds an additional enhancement to the vector design above native hUBE3A,” the scientists concluded.