Researchers have developed short gene promoters — structures that act like switches to turn on genes — that may boost the effectiveness of gene therapies targeting the central nervous system.
These promoters allow for the delivery of larger genes into the central nervous system while keeping them active for longer, potentially improving the efficacy of future gene therapies for Parkinson’s and other neurodegenerative diseases.
The study, “Small Alphaherpesvirus Latency-Associated Promoters Drive Efficient and LongTerm Transgene Expression in The Central Nervous System,” was published in the journal Molecular Therapy: Methods & Clinical Development.
Gene therapy is a promising therapeutic approach for neurodegenerative diseases, such as Parkinson’s. Its aim is to deliver genes to nerve cells in the brain that can promote the growth and survival of neurons or lessen the aggregation, or clumping, of alpha-synuclein protein — a major component of Lewy bodies found in the brains of Parkinson’s patients.
To deliver these genes, scientists rely on an adeno-associated virus (AAV) — a common, naturally occurring virus — modified to be safe and not cause disease. Moreover, in recent years, AAVs have been engineered to get around the blood-brain barrier, a thin membrane that prevents many substances from passing from the bloodstream into the central nervous system (CNS; the brain and spinal cord).
Genes delivered by AAVs include a sequence called a promoter that is key to turning on genes once they’ve been delivered to the targeted brain cells. However, a major limitation of AAVs is their limited capacity to carry genetic information of larger sizes.
To overcome this limitation, researchers at the Princeton Neuroscience Institute engineered three shorter promoters for use in gene therapy, saving space for AAVs to carry larger or multiple genes.
The new promoters were designed by adopting attributes of promoters found in another group of viruses — the herpes virus — that can persist for years as a chronic infection in the nervous system.
These promoters are also more resistant to repression or inactivation processes than common promoters. This means the genes activated with these promoters become active for longer periods after a single administration. Additionally, these smaller promoters can not only work with AAV delivery but also with other viral and non-viral gene delivery systems.
“These new promoters will allow us to deliver larger genes or multiple small genes,” which can “remain active for as long as they are needed,” Esteban A. Engel, PhD, a researcher in viral neuroengineering at Princeton and the study’s lead author, said in a university news story.
Researchers first showed that their new promoters were able to activate a gene that emitted fluorescence once activated and was delivered by AAV into nerve cells grown in the lab. Importantly, fluorescence — indicative of gene activity — was detected for 90 days in these nerve cells.
The team then tested their promoters’ ability to turn on this gene in the CNS of mice when delivered via AAV directly into the bloodstream.
Samples of mouse brain and spinal cord collected at 30 and 190 days post-delivery were positive for fluorescence across all promoters tested. Moreover, the level of fluorescence was similar to that of a larger, control promoter across different areas of the brain. These included the brain’s cortex; hippocampal formation (an area that contains the hippocampus and related structures and plays a role in learning, memory, and spatial navigation); and the cerebellum, a region of the brain that plays an important role in motor control.
In other brain regions, including the striatum (a brain region involved in voluntary movement control), the thalamus, midbrain, motor, and sensory areas, one of the shorter promoters was more potent than the control promoter.
Researchers found that the fluorescence mediated by the new promoters was detected in more than 88% of neurons but not in glial cells — nerve cells that support and protect neurons. The new promoters also led to a broad and stable activation of fluorescence throughout the spinal cord.
Overall, the new promoters “may be useful for the treatment of genetic CNS diseases after one-time viral-vector administration,” the researchers wrote.
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