We recently came across a comprehensive review in Nature Review Drug Discovery by Ling, Q. et al., which provides in-depth insights into AAV-based gene therapy, particularly for neurological disorders. In this article, we’d like to share a concise summary of the key points.

 

Introduction

The review begins by introducing in vivo and ex vivo gene therapy approaches for neurological disorders. In vivo methods directly deliver therapeutic genes, saving time and broadly targeting the central nervous system (CNS). Ex vivo therapies, on the other hand, modify cells outside the body but face immune responses and longer timelines. Both methods encompass gene silencing, gene editing, and gene supplementation.

 

AAV as the Preferred Vector

AAV (Adeno-Associated Virus) is highlighted as the preferred vector for in vivo gene therapy due to its superior safety profile. Over 300 clinical trials employ AAV for gene transfer, with a substantial focus on neurological disorders. The industry strives to enhance AAV-based therapy’s success by improving tissue targeting, cell specificity, intracellular trafficking, overall potency, and CNS transduction efficiency.

 

CNS Gene Supplementation

The review particularly emphasizes CNS gene supplementation through AAV to rectify problematic genes and enhance neurological conditions. AAV9 is noted as the preferred serotype for broad CNS gene transfer, exemplified by the FDA-approved Zolgensma therapy for SMA (spinal muscular atrophy) and trials for MPSIIIA (Mucopolysaccharidosis type III). Other serotypes, like AAVrh.10, show promise, especially in a GM1 gangliosidosis trial involving cerebrospinal fluid (CSF) delivery. The article also highlights the importance of considering various administration routes, such as intravenous and intrathecal lumbar puncture, and localized therapies for specific conditions like Parkinson’s and AADC deficiency gene therapies.

 

Designing Preclinical Studies

The author suggests key factors for designing preclinical AAV gene therapy studies, including dose, administration routes, timing, durability, immune responses, and animal models. These considerations may vary depending on whether the disease is cell-autonomous or non-cell-autonomous. Timeliness is especially critical in the context of neurodegenerative diseases to prevent irreversible damage. The duration of treatment effectiveness can differ among various cell types. The selection of an administration route can influence both the therapy’s efficacy and the occurrence of adverse events, with options including intraparenchymal, systemic, and intracerebrospinal fluid routes. Early intervention typically results in more favorable outcomes. Effective management of immune responses, including those induced by AAV, is imperative and may necessitate the use of immunosuppressive strategies. Ensuring the suitability of animal models that accurately replicate human disease phenotypes is essential for precise preclinical assessments and successful translation to clinical application.

 

Challenges and Innovations

Challenges in the field are addressed in this review, with a focus on CNS gene supplementation for primary mitochondrial diseases and neurodevelopmental disorders. The potential for gene supplementation to restore mitochondrial function is explored and supported by promising preclinical studies. The authors’ laboratory has recently published encouraging proof-of-concept preclinical studies on AAV gene therapy for SURF1-related Leigh syndrome. Additionally, investigations into SLC25A46-related mitochondrial disorders, which may result in conditions like Charcot–Marie–Tooth type 2A neuropathy, Leigh syndrome, and optic atrophy, have demonstrated that early systemic administration of AAV can ameliorate neurological phenotypes in mouse models.

 

Gene Therapy for Neurodevelopmental Disorders

Despite their early onset and uncertain potential for reversibility, neurodevelopmental disorders are being explored with innovative approaches. These disorders often involve genes sensitive to dosage and tightly regulated, requiring precise expression of various isoforms. Possible solutions include self-regulated gene supplementation therapies, gene promoters, or inventive designs like the Kozak sequence for modulating alternative start codons. For example, self-regulating AAV1–BDNF gene therapy improved Prader–Willi syndrome symptoms in mice, while a gene promoter in an epilepsy preclinical study reduced neuronal excitability and alleviated symptoms in both mouse and human cortical spheroid models. In spite of AAV’s limited genome capacity, manipulating the Kozak sequence enabled the creation of two UBE3A protein isoforms in close-to-endogenous proportions, effectively improving motor learning and reducing seizure phenotypes in mice with Angelman syndrome.

 

Safety Concerns and Mitigation

AAV gene therapy often requires higher doses due to its lower transduction rate, potentially causing toxicity. This review discusses safety concerns, including hepatotoxicity and neurological issues. TMA (thrombotic microangiopathy), likely triggered by complement activation, can affect various organs, but detecting it early is challenging. Biomarkers and reduced-dose localized administration may help prevent TMA. Additionally, DRG (dorsal root ganglion) pathology, possibly linked to transgene overexpression, and inflammation from specific brain region administration associated with local overexpression and needle tracks require attention. Effective strategies to address these concerns are vital for safe AAV gene therapy.

 

Future Directions

The review concluded that advancements in AAV gene therapy primarily center around enhancing precision, efficacy, and safety. This is achieved through capsid engineering, involving random peptide insertion into the capsid, and modifying transgene cassettes to incorporate cell-specific promoters or enhancers. Further control over transgene expression is achieved by incorporating specific miRNA binding sites into the recombinant AAV, allowing for the silencing of the transgene in off-target cells. Additionally, codon optimization is utilized to enhance protein expression efficiency. These combined improvements contribute to the overall success of AAV gene therapy, particularly when targeting the central nervous system (CNS). The future of neurological gene therapy holds great promise, driven by advancements in technology, precision medicine, immune management, and early intervention and diagnosis, making gene therapy safer and more effective for various neurological conditions.

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Reference
Ling, Q., Herstine, J.A., Bradbury, A. et al. AAV-based in vivo gene therapy for neurological disorders. Nat Rev Drug Discov (2023). [https://doi.org/10.1038/s41573-023-00766-7](https://doi.org/10.1038/s41573-023-00766-7)

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