Background

One of the biggest hurdles in designing gene therapies for neurological disorders is the blood-brain barrier (BBB). This barrier, which prevents the entry of foreign substances, does not allow therapeutic gene vectors to pass through. While direct injection of viral vectors into brain tissue has been used to bypass the barrier, this approach carries a high risk of brain tissue damage and has the limitation of only delivering the drug to a localized area. In particular, for intractable diseases such as multiple sclerosis (MS) or lysosomal storage disease (LSD), which are caused by dysfunction of astrocytes or oligodendrocytes, it is necessary to precisely target glial cells, which are distributed throughout the brain. However, existing adeno-associated virus (AAV) vectors have limitations in that they are selectively delivered only to neurons or enter other organs such as the liver, causing off-target side effects.

Key Findings

The research team, led by Professor Steve Goldman of the Center for Translational Neuroscience at the University of Rochester Medical Center, focused on improving the targeting efficiency by genetically modifying the capsid, which is the outer envelope protein of AAV5. The researchers first created a chimeric mouse by transplanting human glial progenitor cells (hGPCs) derived from human stem cells into neonatal mice, replacing the brain's support cells with human cells. Subsequently, several capsid variants with enhanced targeting ability were administered to the cisterna magna of the mice. At this stage, the researchers focused on the glymphatic system, which is responsible for the circulation of cerebrospinal fluid (CSF). When hypertonic saline or mannitol was injected into the animal's blood vessels to induce systemic hypertonicity, the gaps between brain cells widened, and the diffusion rate of the gene vector increased. As a result, the injected variants were able to spread evenly to deep areas of the brain parenchyma. The research team utilized this fluid dynamics pathway to finally select the optimal AAV5 variant capsid that strongly binds only to human hGPCs. The selected vector boasts high selectivity, targeting only human hGPCs and astrocytes and oligodendrocytes derived from them, without affecting surrounding cells or organs.

Significance and Prospects

This study has garnered significant attention in the academic community for opening up the possibility of targeting cells located throughout the brain parenchyma without directly crossing the BBB. In particular, it is highly efficient in delivering therapeutic genes to astrocytes and oligodendrocytes, which have been the most difficult to treat in gene therapy, and is considered to have paved the way for a breakthrough in the treatment of degenerative brain diseases. It is likely to be immediately applied to the treatment of intractable white matter diseases such as MS, which is caused by demyelination, and infantile LSD. In addition, due to the excellent targeting ability of the capsid variant, it is possible to minimize side effects such as liver toxicity that occur during systemic administration. However, there are still some challenges to be overcome before actual clinical application. Unlike mouse models, the human brain is much larger in volume, and the flow of CSF is also more complex, so it is necessary to verify whether the same level of diffusion effect can be achieved. In addition, a thorough safety evaluation is necessary to ensure that the systemic hypertonicity therapy used to increase drug delivery does not burden elderly patients or patients with high intracranial pressure.

Why It Matters

In the clinical setting, it is possible to envision a scenario in which the expression of mutant huntingtin protein, which is distributed throughout the brain of Huntington's disease patients, is suppressed. By injecting a therapeutic AAV into the cisterna magna, where cerebrospinal fluid flows, and administering systemic hypertonicity, gene editing tools can be evenly delivered to human hGPCs and astrocytes in the deep brain without surgical intervention. This will lead to a significant reduction in the risk of brain damage compared to the conventional multi-point local injection method, while maximizing the therapeutic range. Industrially, it is possible to expand into a platform business that designs and produces a large number of AAV capsids tailored to specific neurological diseases by combining artificial intelligence (AI). For example, a new drug development ecosystem can be established in which gene therapies with optimized targeting ability can be rapidly discovered and developed, tailored to the patient's genetic characteristics or the specific glial cell subtype to be targeted.