ORLANDO — Scientists have made major strides in gene therapy, and experts convened to share their insights on gene therapy development and challenges at the annual meeting of the American Epilepsy Society during a session called “Recent Advances Gene Therapies for the Epilepsies: A Preclinical Perspective.”
Four types of gene therapy
Suzanne Paradis, PhD, cofounder and president of Severin Therapeutics Inc., initiated the session, giving the audience an overview of the four types of gene therapy — the first being gene replacements, where a copy of the gene is added back. The second type of therapy, transcriptional enhancement, entails upregulating an endogenous copy of the gene.
“Both gene replacement and transcriptional enhancement can prove effective in treating monogenetic genetic disorders,” she said.
The third type is transcriptional enhancement, which upregulates an endogenous copy of the gene.
Generalizable gene therapies, the fourth type of gene therapy, involve adding a gene that bypasses either or both ictogenesis and seizure propagation.
As it stands, of the nearly 30 gene therapies currently marketed for neurological disorders, only four are indicated for central nervous system (CNS) disorders. Of the four currently approved by the FDA for seizures, onasemnogene abeparvovec-xioi (Zolgensma) is the only one that truly targets the CNS.
“Developing treatment that targets the CNS requires several important considerations,” Dr. Paradis said. “These include the right model system, appropriate delivery method, a product that can cross the blood-brain barrier (BBB) and target neurons, and the durability of transgene expression.”
Epilepsy May Be Amenable to Gene Therapy
To illustrate these principles, Meghan Eller, a PhD candidate at the University of Texas Southwestern in Dallas, shared research on potential new gene therapies that might one day become effective options in treating CNS diseases.
She spoke on viral-mediated gene delivery, specifically by employing adeno-associated virus (AAV) treatment in this arena.
“We capitalized on the ability of viruses to infect genetic materials,” she told the audience. “Viruses are naturally designed to infect cells and deliver genetic material.”
The viruses have three components that make them attractive. One of three viruses is typically used for this work — adenoviruses, lentiviruses, or AAV. The virus type used may be dictated by the gene of interest, meaning whether the gene is expressed, knocked down, or edited. Lastly, several regulatory elements are required; these are the promoter, polyadenylation signal, and the regulatory binding sites necessary for transcription.
“More recent technologies are CRISPR for gene editing, and with promoter, we can control the specific cell type in which gene will be expressed,” Ms. Eller explained.
Regulatory binding sites within a binding site allow regulation within an endogenous transgene.
“AAV genome is naturally single-stranded, but we can introduce a mutation to form a self-complementary cassette,” she said.
Using AAV as a vector for gene delivery has several advantages. First and foremost, it is easy to engineer. Moreover, it can infect dividing and non-dividing cells. It also exhibits long-lasting expression and has a low immune response. In addition, the AAV virion particle has demonstrated activity on cells found in numerous organs, including those of the lymph nodes, adrenal glands, kidneys, various muscle tissue, retinal cells, and digestive system as well as the CNS.
Yet, for all its benefits, the AAV comes with some limitations. For example, it carries as preexisting immunity and exhibits lost expression in dividing cells.
Another important drawback is its package size constraints, as many genes do not fall within its 2.4 kb self-complementary of 4.8 kb single-stranded packaging capacity.
For her research, Ms. Eller and colleagues took into account several considerations for therapy development. The appropriate route helps ensure the therapy reaches critical regions of the brain and that there is adequate expression in the periphery. The immune response becomes important regarding the body’s reaction to non-self proteins — a property, which, at times, can be modified based on dose. Thirdly, expression level and cell type expression can affect the therapy’s activity. In addition, a small amount of the vector will be incorporated into the host DNA.
The fact that AAV can cross the BBB allows for intravenous delivery; however, it limits brain transduction.
“Gene therapy may not be as effective if the delivery window is missed or there is significant neuron loss,” Ms. Eller said.
She stressed the importance of determining the minimal dose necessary for therapeutic benefit to minimize dose-related toxicity. She also distinguished when and why one might choose one type of gene therapy over another, using gene addition to help illustrate her point.
“Gene addition is the most important approach when there is a monogenic gene,” she said. “SLC13A5 and SLC6A1 are examples where gene addition is effective.”
Modulation of ion channels can help the delivery of therapeutic. Such is the case for NaV1.1 and Kv1.1. Finally, AAV can enhance the delivery of therapeutic proteins, as seen with Sema4D and neuropeptide Y.
Ms. Eller explained how the path to developing a gene therapy as an investigational new drug mirrors those historically traveled in conventional drug development to some extent. Preclinical studies offer proof of concept by determining efficacy, dosing, and toxicity in small animals such as mice. From there, studies progress to the pre-IND state by exploring pharmacology and clinical trial design while further investigating toxicity. FDA and regulatory approval require addressing safety concerns and establishing therapeutic benefit, at which point the therapy progresses to the fourth and final stage: clinical trials. During this stage, investigators monitor dosage and safety while evaluating efficacy.Optimal transgene expression regulation requires scientists to create an environment that gives rise to the perfect level of transgene expression. Otherwise, too little protein will result in no therapeutic benefit, while too much protein can become toxic.
Ms. Eller presented her work on investigating whether the reduction of Scn8a is therapeutic, given that epileptogenic Scn8a mutations increase neuronal firing. She treated both the control and Scn8a mice with antisense oligonucleotides (ASO), which depresses neuronal activity. Upon comparing the effects in ASO-treated mice to control, she found that long-term downregulation of Scn8a (50%) prevents seizures and increases survival — regardless of whether ASO therapy was initiated before or during seizure onset.
Additional studies exploring novel and potential gene therapies for epilepsy are on the horizon.
Dr. Paradis is an employee of Severin Therapeutics Inc. Ms Eller has no relevant disclosures.
