Previous Article in Journal
Delayed Diagnosis of a Low-Flow Temporal Arteriovenous Malformation in a Child Presenting with Recurrent Intracerebral Hemorrhage
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
CTN
Volume 9
Issue 3
10.3390/ctn9030032
ctn-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

Implications of AAV Serotypes in Neurological Disorders: Current Clinical Applications and Challenges

by
Sachin Sharma
1,2,*,
Vibhuti Joshi
3 and
Vivek Kumar
4,*
1
Department of Bioscience, Graphic Era University, Dehradun 248001, India
2
Department of Neurosurgery, Cedars-Sinai Medical Center, Los Angeles, CA 90048, USA
3
Department of Biotechnology, Bennett University, Greater Noida 201009, India
4
Michigan Institute of Neuroscience, University of Michigan, Ann Arbor, MI 48103, USA
*
Authors to whom correspondence should be addressed.
Clin. Transl. Neurosci. 2025, 9(3), 32; https://doi.org/10.3390/ctn9030032
Submission received: 31 May 2025 / Revised: 5 July 2025 / Accepted: 14 July 2025 / Published: 15 July 2025
Browse Figures
Review Reports Versions Notes

Abstract

Adeno-associated virus (AAV) vectors have emerged as powerful tools for in vivo gene therapy, enabling long-term transgene expression in targeted tissues with minimal pathogenicity. This review examines the AAV serotypes used in clinical gene therapy trials for neurodegenerative (central nervous system, CNS) diseases, highlighting their tropisms, engineering advances, and translational progress. We discuss how capsid modifications, cell-specific promoters, and novel delivery routes are enhancing AAV tropism and reducing immunogenicity to overcome current limitations. Key clinical trials in neurodegenerative disorders (such as Parkinson’s, Alzheimer’s, and Huntington’s disease) are summarized, including delivery methods (intravenous, intracoronary, intrathecal, etc.) and outcomes. We further outline the regulatory landscape with recent approvals of AAV-based therapies and ongoing efforts to address safety challenges like immune responses and vector dose toxicity. A more translational, forward-looking perspective is adopted to consider combination therapies (e.g., AAV with immune modulation or genome editing) and strategic directions to improve the next generation of AAV vectors. Overall, continued innovation in AAV vector design and delivery, alongside careful clinical evaluation, is accelerating the translation of gene therapies for neurodegenerative diseases.

1. Introduction

Gene therapy, once considered a limited and experimental technique, has now emerged as a clinically viable strategy, with numerous trials underway across a spectrum of diseases, including cardiovascular, neurological, and ophthalmological disease [1,2]. In the past decade alone, over 2000 gene therapy clinical trials have been initiated or completed worldwide. Various viral vector-based platforms have been employed for transgene delivery, including AAV, lentivirus (LV), retrovirus, and herpes simplex virus (HSV) [3,4,5]. While lentiviral vectors have primarily been used for ex vivo gene transfer [6], AAV vectors have demonstrated substantial success in in-vivo gene therapy applications [7,8].
Several AAV-based gene therapy products have received regulatory approval: LUXTURNA (voretigene neparvovec-rzyl) for Leber congenital amaurosis (retinal disease), and Zolgensma (onasemnogene abeparvovec-xioi) for spinal muscular atrophy. More recently, Lexeo therapeutics announced positive outcomes from the first gene therapy targeting the genetic basis of APOE4-associated Alzheimer’s disease (Clinical Trials on Alzheimer’s Disease Conference (CTAD), 2025). The appeal of AAV vectors lies in their ability to selectively transduce a wide array of tissue types depending on the serotype-specific tropism [9,10]. Beyond serological differences, molecular cloning of AAV genes has identified hundreds of naturally occurring AAV variants across multiple species, expanding the toolkit for tailored gene therapy approaches [5,11,12].
In this review, we explore the diversity of AAV serotypes utilized in gene therapy, strategies for vector design and engineering, their limitations, and recent clinical applications, with a particular focus on neurological disorders and the ongoing clinical trials targeting these diseases. While several recent reviews have outlined technical advancements in AAV vector design and gene delivery challenges in central nervous system disorders, the present review uniquely integrates a translational perspective by focusing on emerging clinical trials, personalized gene therapy strategies, and novel AAV modifications such as machine learning-guided capsid optimization and microglial-targeted therapies. By emphasizing future precision medicine approaches and synthesizing both biological barriers and proposed engineering solutions, this review aims to bridge the gap between preclinical innovations and clinical application in neurodegenerative gene therapy.

2. Advancements in AAV Vector Design

One of the major limitations of AAV vectors is their restricted packaging capacity, which allows the expression of transgenes up to only ~4.7 kb. Another significant hurdle is pre-existing immunity, which often leads to the exclusion of patients from clinical trials [13]. Prior exposure to wild-type AAVs or recombinant AAV during in-vivo clinical trials results in the presence of neutralizing antibodies in patient serum, which effectively block capsid-receptor interactions and cellular transduction [14]. A study by Boutin et al. further confirmed the high prevalence of anti-AAV IgG and neutralizing antibodies against common serotypes, including AAV serotypes 1, 2, 5, 6, 8, and 9, thereby complicating the clinical translation of AAV-based therapeutics [15,16].
Complications in recent FDA-approved AAV trials have raised additional safety concerns. Reports of patient deaths were linked to immune responses, off-target effects, and vector integration at non-target sites. High-dose systemic administration of neurotropic AAV9 led to severe neurotoxicity in both peripheral and central nervous systems in preclinical studies using piglets and non-human primates [17,18,19]. While cell-specific promoters, such as photoreceptor-specific ones, were found to be safe in high doses in mouse models, ubiquitous or non-cell-restricted promoters triggered cytotoxicity and inflammation, particularly in the retinal pigmented epithelium [20]. Additionally, single-dose AAV administration may not be sufficient to achieve therapeutic benefit in all monogenic disease mice models, suggesting the need for re-dosing strategies [21].
To overcome these challenges, next-generation capsid engineering has focused on improving immune evasion, targeting efficiency, and production scalability. One widely used strategy involves modifying antigenic capsid residues through site-directed mutagenesis and rational design, enhancing transduction while retaining tissue-tropism. For instance, empty capsids and receptor-binding site mutations were engineered in AAV2 to absorb circulating NAbs, optimizing transgene delivery by adjusting the full-to-empty capsid ratio in the final formulation [22]. Novel capsid variants such as AAV2.15, AAV2.4, and AAVhum.8 have shown promise in preclinical studies. Additionally, AAV.CAP-B10, an engineered capsid, exhibited a shift in tropism toward neurons and away from astrocytes compared to AAV-PHP.eB, as confirmed by single-cell RNA sequencing [23].
The interaction between AAV capsids and cell surface receptors and co-receptors governs their entry and tissue specificity. The availability of high-resolution crystal structures of AAV serotypes (e.g., AAV2) has enabled precise peptide insertions and chemical modifications, enhancing capsid functionality [24,25]. Genetic engineering and chemical conjugation of peptides have been used to increase cellular attachment and permeability, thereby boosting transduction efficiency (Figure 1) [26,27]. Another innovative approach is surface tethering (Figure 1), which modifies surface interactions to optimize biodistribution, clearance, and cellular uptake, offering broader applications than conventional capsid modification strategies [28]. Table 1 summarizes AAV challenges and solutions.
In muscle-targeted gene therapies, high AAV doses are often required for functional benefit but carry risks of toxicity or even lethality [29,30]. To address this, chimeric capsids have been developed—for example, AAV2.5, which incorporates five conserved residues from AAV1 into AAV2, resulting in enhanced muscle transduction. Similarly, inserting a 7-mer peptide between VP1 residues 588–589 in AAV9 significantly improved its delivery efficiency. Traditional selection methods for AAV capsid libraries are often inefficient; to resolve this, a machine learning model known as “Fit4Function” was developed to enable multi-trait capsid optimization [31].
Beyond capsid design, AAV genome engineering also plays a crucial role in improving transgene stability and expression (Figure 1). The AAV genome is flanked by inverted terminal repeats (ITR), each 145 bp in length, which are essential for AAV replication, transcriptional regulation, and genome packaging [32]. Modified vectors that include self-complementary AAV (scAAV) designs—by mutating one of the ITRs—enable faster and more efficient transduction [33]. In retinal ganglion cells, the addition of a Kozak consensus sequence into the vector enhanced eGFP transgene expression, providing further evidence of the potential of genome-level modifications [34]. Furthermore, among the serotypes, AAV8 has demonstrated superior neuronal transduction efficiency, followed closely by AAV2 [35]. However, a key limitation remains the low transduction ratio of neurons compared to glial cells, and non-specific targeting of peripheral tissues due to systemic delivery. To overcome this, indirect strategies—such as localized or retrograde delivery—are being explored for improving CNS-specific gene targeting [21]. Retrograde transport from peripheral muscle injection sites to spinal motor and sensory neurons has shown reasonable efficiency depending on the AAV pseudotype. Additionally, utilizing retrograde transport has been shown to improve symptoms in a mouse model of amyotrophic lateral sclerosis [36]. Anterograde transport has also been utilized for more widespread gene delivery, including thalamocortical and striatonigral circuits, the latter being particularly relevant in Parkinson’s disease. A summary of administration routes, AAV serotypes, and associated disease targets is shown in Figure 2. Technological solutions discussed here are further evaluated against clinical hurdles in later sections.

3. Clinical Translation of AAV Gene Therapies in Neurological Disorders

The advent of gene therapy has brought renewed optimism for treating a wide range of genetic and degenerative disorders. Since the term “gene” was first coined in 1909, the field has progressed from fundamental research to the first clinical application in 1990, involving adenosine deaminase (ADA) gene transfer in patients with ADA deficiency [37]. By March 2023, over 3900 gene therapy clinical trials had been initiated across 46 countries [38], reflecting substantial global investment in this modality. Several neurological diseases, such as Parkinson’s, Alzheimer’s and spinal muscular atrophy—have progressed into clinical trials using AAV-based gene therapies [39]. Recent studies highlight the therapeutic promise of AAV vectors in these disorders, emphasizing the importance of timing, route of delivery, and tropism for therapeutic success. These strategies are visually summarized in Figure 2.
Several clinical trials are currently evaluating intrathecal or intraperitoneal AAV delivery: AAV2-BDNF delivery for Alzheimer’s diseases (NCT05040217), AAV2-GAD for Parkinson’s diseases (NCT05603312) and AAV9-based gene transfer in Spinal muscular atrophy (NCT05089656). A novel variant, AAV-TT (derived from AAV2), exhibits enhanced neurotropism and is under investigation for targeted CNS delivery. For Alzheimer’s disease (AD), a three-pronged approach using AAV has been proposed: 1) Delivery of nerve growth factor, 2) Modulation of neurotropic factors at the blood–brain barrier, 3) Stimulation of synaptic plasticity and memory-associated genes [40]. Table 2 summarizes recently approved pipeline AAV gene therapies for neurological diseases.
Another experimental strategy involves the AAV-mediated delivery of gene telomerase reverse transcriptase to protect neurons from age-related degeneration and extend cellular longevity [41]. Clinical efforts have also focused on APOE targeting (e.g., NCT04748861), though therapeutic outcomes in AD patients remain inconclusive. Zolgensma®, an AAV9-based gene therapy, became the first FDA-approved treatment for SMA [42]. Despite its success, systemic AAV9 administration across the blood–brain barrier (BBB) requires high viral doses, which increases manufacturing costs and raises concerns over immune toxicity and hepatic injury. Indeed, some patients have experienced acute liver toxicity following intravenous Zolgensma delivery, and the systemic administration process remains technically challenging. Currently, additional trials are evaluating beta-galactosidase gene delivery using vectors such as AAV9, AAVhu.68, and AAVrh.10 [5]. In early-phase clinical trials, AAVrh.10 was delivered directly into the cerebrospinal fluid (CSF), leading to increased β-galactosidase activity without adverse effects observed up to one-year post-administration [43]. This demonstrates a successful bench-to-bedside translation of AAV therapy, where the delivery route, timing, and dose are critical determinants of safety and efficacy. Recent clinical trial NCT04737460 investigates these three factors alongside vector type and disease indication [44].
Several early-phase trials are currently underway targeting central nervous system (CNS) disorders: for instance, AAV2-mediated delivery of brain-derived neurotrophic factor (BDNF) is being tested in patients with early Alzheimer’s disease to slow neurodegeneration (NCT05040217). Similarly, glutamic acid decarboxylase gene transfer via AAV is being explored for Parkinson’s disease patients (NCT04228653), and mitochondrial ND2 gene delivery is in evaluation for Leber’s Hereditary Optic Neuropathy (LHON) (NCT02161380). In Huntington’s disease, AAV5-miHTT is being administered to suppress mutant huntingtin expression (NCT04120493). AAV2-RPE65 (NCT00749957), utilizing a recombinant AAV2 vector, has been evaluated in a Phase I/II trial for the treatment of Leber Congenital Amaurosis, a hereditary retinal dystrophy resulting in severe vision loss. Another Phase I/II study, AAV2-CNGA3 (NCT02935517), targets achromatopsia, a condition characterized by color blindness and visual impairment, by delivering corrective CNGA3 gene sequences via AAV2 vectors. Expanding beyond ocular disorders, AAV9-AP4M1 (NCT05518188) is being tested in a Phase I/II trial for Spastic Paraplegia Type 50, a rare neurodegenerative condition marked by progressive lower limb weakness, employing AAV9 for targeted gene delivery. Furthermore, AAV-hSMN1 (NCT06288230) is under Phase I/II evaluation for spinal muscular atrophy, focusing on augmenting SMN1 gene expression using an AAV-based platform. These ongoing clinical programs reflect the growing versatility of AAV vectors across diverse CNS and neuromuscular disease indications.

4. Current Challenges of AAV-Mediated Gene Therapy

Among the delivery systems, adeno-associated virus (AAV) vectors have shown remarkable promise due to their safety profile and long-term gene expression. Despite increasing clinical interest, several major challenges persist. One of the most pressing concerns is the immune response triggered by AAV vectors. The capsid proteins present multiple epitopes that activate T-cell-mediated responses, impeding efficient gene transfer and potentially leading to vector clearance [45,46]. Both humoral and cellular immune reactions can be influenced by factors such as AAV serotype, dosage, route of administration, and the presence of tissue damage [14]. Furthermore, systemic delivery of AAV vectors often results in their preferential accumulation in the liver, reducing target efficiency to the CNS or other intended tissues. In addition, cellular uptake is dependent on the presence of specific receptors, and suboptimal internalization at the target site further hampers therapeutic efficacy [47]. Another limitation is the restricted packaging capacity of AAV vectors, which can accommodate only ~4.7 kb of genetic material—insufficient for larger or dual-gene constructs [48]. Beyond biological challenges, manufacturing hurdles also persist. Issues surrounding vector purity, identity, and stability must be addressed to meet regulatory standards for clinical application [49,50]. Addressing these challenges is essential to advance AAV-based gene therapy from promising experimental treatments to broadly applicable, safe, and effective clinical interventions. Improvements in capsid engineering, immune modulation, vector production, and delivery strategies will be critical to overcome current barriers and ensure successful outcomes in future trials. Finally, it’s important to consider future targets and frontiers for AAV. Neurodegenerative diseases are now firmly in the sight of gene therapy researchers. Within the next decade, we anticipate to see: 1) Stroke gene therapy administered intra-arterially during thrombectomy, delivering neuroprotectants to the at-risk brain tissue. This could be combined with clot retrieval in a one-stop procedure. 2) In the CNS, therapies for frontotemporal dementia, multiple system atrophy, or chronic traumatic encephalopathy, which may involve combinations of anti-aggregant factors and neurotrophins delivered by AAV.

5. Discussion

The adeno-associated virus vector has emerged as a powerful tool for targeted gene delivery, playing a pivotal role in advancing gene therapy and enabling successful clinical trials in neurological disorders. Enhancing the precision of AAV-based delivery systems, researchers have incorporated targeting ligands—such as peptides, aptamers, and monoclonal antibodies—onto AAV capsids to achieve cell-type-specific transduction in complex tissues. However, a major limitation remains the host immune response against AAVs, which can compromise vector efficacy and restrict patient eligibility in clinical settings. To address these challenges, capsid engineering strategies have been employed to alter tropism, reduce immune recognition, introduce novel receptor-binding affinities, and improve transduction efficiency. Additionally, these modifications also improve the scalability in AAV production, which is crucial for clinical translation. Cutting-edge technologies such as high-throughput screening and Isotag-based purification methods are accelerating the discovery of novel capsid variants with improved immune evasion profiles. Collectively, these innovations contribute to a next-generation AAV platform, aligning with the goals of precision medicine and opening new avenues for translational research and immune-modulating therapeutic strategies.
AAV9 is particularly well-suited for targeting neurodegenerative tissues due to its strong tissue-tropism and proven ability to cross the blood-brain barriers, as demonstrated in several preclinical animals’ studies [51]. Consequently, both wild-type AAV9 and engineered AAV variants are being widely explored in preclinical models of neurological diseases [52]. One promising and rapidly evolving application of AAV-based gene therapy lies in correcting microglial dysfunction, which is increasingly recognized as a central contributor to the pathogenesis of numerous neurodegenerative diseases. Microglia, the resident immune cells of the central nervous system (CNS), play a critical role not only in immune surveillance but also in synaptic pruning, neuronal maintenance, and tissue homeostasis. Dysregulated or “reactive” microglia have been implicated in a range of CNS pathologies, including Alzheimer’s disease, Parkinson’s disease, frontotemporal dementia, and ALS, where their pro-inflammatory phenotypes exacerbate neurodegeneration and contribute to disease progression.
At the molecular level, the colony stimulating factor-1 receptor (CSF1R) is indispensable for microglial development, survival, and maturation. Mutations in the CSF1R gene are causative for Hereditary Diffuse Leukoencephalopathy with Spheroids (HDLS), a devastating microglial leukodystrophy characterized by demyelination, white matter loss, and progressive cognitive decline. AAV-mediated gene replacement of CSF1R—or modulation of its signaling axis—represents a highly rational strategy for rescuing microglial deficits in HDLS and related conditions.
Beyond CSF1R, other microglia-specific genes, such as P2RY12, SALL1, and CX3CR1 are being explored as functional targets. These genes are involved in regulating microglial identity, communication with neurons, and responses to injury. Targeted delivery of gene therapy vectors to modulate these genes could allow for cell-type–specific modulation of microglial behavior, potentially reprogramming pathogenic microglia into a neuroprotective phenotype [53]. While AAV vectors have traditionally shown poor tropism for microglia, recent advances in capsid engineering have led to the development of microglia-optimized AAVs. Notably, two engineered capsids, AAV-cMG.QRP and AAV-cMG.WPP, have demonstrated exceptional transduction efficiencies of 55% and 75%, respectively, in primary mouse microglia cultures. These vectors were developed through directed evolution approaches that select for enhanced CNS penetration and microglial uptake—breaking a long-standing barrier in gene delivery to this elusive cell type [54].
The clinical translation of microglial-targeting AAVs holds transformative potential. For example, in Alzheimer’s disease, reprogramming microglia to clear amyloid plaques or suppress chronic inflammation could slow disease progression. In other contexts, microglial delivery of neurotrophic factors, anti-inflammatory cytokines, or RNA-based regulators (e.g., siRNA or miRNA) could be deployed to counteract pathology-specific mechanisms. Furthermore, the ability to selectively target microglia minimizes off-target transgene expression and may reduce systemic toxicity, a critical consideration in the design of CNS gene therapies.
Collectively, these advancements underscore the growing relevance of microglial modulation in neurotherapeutics and highlight how next-generation AAV vectors can enable precision targeting of both neuronal and non-neuronal cell types. As the field continues to explore the cellular diversity of the brain, microglia stand out as not just passive responders, but as active, druggable targets in the gene therapy landscape.

6. Future Directions

Emerging techniques such as capsid shuffling, machine learning-guided capsid design, and multi-trait optimization platforms are poised to substantially improve the specificity, immune evasion, and transduction efficiency of AAV vectors. These approaches allow for the systematic engineering of capsids that are better suited for complex tissue environments such as the CNS, while reducing off-target effects and immunogenicity.
In parallel, advances in targeted delivery technologies are helping to address longstanding challenges associated with efficient AAV biodistribution and tissue penetration. Ligand-directed targeting, where capsids are modified to display peptides or antibodies with affinity for tissue-specific receptors (e.g., transferrin receptors or integrins), can significantly improve delivery to the brain, retina, or spinal cord. Nanoparticle-AAV hybrid systems are another promising approach, allowing co-packaging or surface tethering of AAV vectors onto lipid or polymeric nanoparticles to enhance their cellular uptake, promote endosomal escape, or modulate biodistribution. Such formulations also offer the potential to carry adjuvants or co-therapies—such as immunomodulatory agents—that can improve transduction or reduce host responses.
Additionally, intranasal AAV delivery has gained attention as a non-invasive and highly translational method to bypass the blood–brain barrier (BBB). By targeting the olfactory epithelium and trigeminal nerves, this route allows for CNS entry with minimal systemic exposure, offering a practical delivery method for gene therapies targeting neurodegenerative and neurodevelopmental disorders.
Beyond delivery, novel promoter engineering is being used to refine cell-type specificity and avoid off-target expression. Coupled with strategies for immune tolerance induction—such as transient immunosuppression or use of regulatory T-cell–activating capsids—these improvements are expected to expand patient eligibility and reduce adverse events in future trials.
The concept of personalized gene therapy is also gaining momentum. Rather than a “one-vector-fits-all” paradigm, future therapies may involve tailoring the capsid serotype to each patient’s pre-existing immunity profile or customizing the transgene payload, such as delivering patient-specific antisense constructs or allele-targeted therapeutic proteins. Although individualized AAV vector manufacturing remains a logistical and regulatory challenge, innovations in modular, on-demand vector production platforms and point-of-care biomanufacturing may bring this vision closer to clinical reality.
Taken together, these innovations signal a transition toward more precision-engineered, safe, and scalable AAV platforms, capable of addressing the unmet needs in the treatment of complex neurological and neurodegenerative disorders.

7. Conclusions

AAV vectors have firmly established themselves as a versatile and potent gene delivery platform. Clinical trials in neurodegenerative diseases, though in relatively early phases compared to ocular or hematologic disorders, have shown proof-of-mechanism that gene therapy can alter disease biology (improving motor scores, or biomarker trajectories). Significant challenges remain–ensuring long-term safety, managing immune reactions, and demonstrating clear clinical efficacy in larger trials–but the trajectory is optimistic. The coming years are likely to bring the first approved AAV therapies for common diseases like Alzheimer’s, which would be paradigm-shifting in medicine. Even partial success (slowing disease progression) in these could greatly improve patient quality of life. Moreover, the knowledge gained in these domains will feedback to benefit all gene therapy efforts, creating safer and more effective vectors. With continued interdisciplinary effort and careful attention to translational hurdles, AAV-mediated gene therapy is poised to become an integral part of the future therapeutic armamentarium for neurodegenerative diseases, offering hope for conditions that have long been without disease-modifying treatments.

Author Contributions

Conceptualization, S.S. and V.J.; Writing—original draft preparation, S.S., V.J. and V.K.; Writing—review and editing, S.S., V.J. and V.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

Authors would like to thank Director of Research–Graphic Era University and Department Head Biotechnology–Graphic Era University, Dehradun, India for their continuous support and help during the preparation of this manuscript.

Conflicts of Interest

The authors have declared that research conducted during the preparation of this article does not hold any commercial and financial relationships that could be construed as a potential conflict of interest. The authors also declare that there are no other relevant disclosures to discuss in the manuscript.

References

  1. Hastie, E.; Samulski, R.J. Adeno-Associated Virus at 50: A Golden Anniversary of Discovery, Research, and Gene Therapy Success—A Personal Perspective. Hum. Gene Ther. 2015, 26, 257–265. [Google Scholar] [CrossRef]
  2. Rose, J.A.; Hoggan, M.D.; Shatkin, A.J. Nucleic Acid from an Adeno-Associated Virus: Chemical and Physical Studies. Proc. Natl. Acad. Sci. USA 1966, 56, 86–92. [Google Scholar] [CrossRef]
  3. Bijlani, S.; Pang, K.M.; Sivanandam, V.; Singh, A.; Chatterjee, S. The Role of Recombinant AAV in Precise Genome Editing. Front. Genome Ed. 2022, 3, 799722. [Google Scholar] [CrossRef]
  4. Issa, S.S.; Shaimardanova, A.A.; Solovyeva, V.V.; Rizvanov, A.A. Various AAV Serotypes and Their Applications in Gene Therapy: An Overview. Cells 2023, 12, 785. [Google Scholar] [CrossRef]
  5. Wang, D.; Tai, P.W.L.; Gao, G. Adeno-Associated Virus Vector as a Platform for Gene Therapy Delivery. Nat. Rev. Drug Discov. 2019, 18, 358–378. [Google Scholar] [CrossRef]
  6. Naldini, L. Ex Vivo Gene Transfer and Correction for Cell-Based Therapies. Nat. Rev. Genet. 2011, 12, 301–315. [Google Scholar] [CrossRef]
  7. Grimm, D.; Kay, M. From Virus Evolution to Vector Revolution: Use of Naturally Occurring Serotypes of Adeno-Associated Virus (AAV) as Novel Vectors for Human Gene Therapy. Curr. Gene Ther. 2003, 3, 281–304. [Google Scholar] [CrossRef]
  8. Hu, C.; Lipshutz, G.S. AAV-Based Neonatal Gene Therapy for Hemophilia A: Long-term Correction and Avoidance of Immune Responses in Mice. Gene Ther. 2012, 19, 1166–1176. [Google Scholar] [CrossRef]
  9. Burdett, T.; Nuseibeh, S. Changing Trends in the Development of AAV-based Gene Therapies: A Meta-Analysis of Past and Present Therapies. Gene Ther. 2023, 30, 323–335. [Google Scholar] [CrossRef]
  10. Naso, M.F.; Tomkowicz, B.; Perry, W.L.; Strohl, W.R. Adeno-Associated Virus (AAV) as a Vector for Gene Therapy. BioDrugs 2017, 31, 317–334. [Google Scholar] [CrossRef]
  11. Calcedo, R.; Vandenberghe, L.H.; Gao, G.; Lin, J.; Wilson, J.M. Worldwide Epidemiology of Neutralizing Antibodies to Adeno-associated Viruses. J. Infect. Dis. 2009, 199, 381–390. [Google Scholar] [CrossRef]
  12. Schnepp, B.C.; Jensen, R.L.; Chen, C.-L.; Johnson, P.R.; Clark, K.R. Characterization of Adeno-Associated Virus Genomes Isolated from Human Tissues. J. Virol. 2005, 79, 14793–14803. [Google Scholar] [CrossRef]
  13. Kruzik, A.; Fetahagic, D.; Hartlieb, B.; Dorn, S.; Koppensteiner, H.; Horling, F.M.; Scheiflinger, F.; Reipert, B.M.; Rosa, M. Prevalence of Anti-Adeno-Associated Virus Immune Responses in International Cohorts of Healthy Donors. Mol. Ther. Methods Clin. Dev. 2019, 14, 126–133. [Google Scholar] [CrossRef]
  14. Hamilton, B.A.; Wright, J.F. Challenges Posed by Immune Responses to AAV Vectors: Addressing Root Causes. Front. Immunol. 2021, 12, 675897. [Google Scholar] [CrossRef]
  15. Boutin, S.; Monteilhet, V.; Veron, P.; Leborgne, C.; Benveniste, O.; Montus, M.F.; Masurier, C. Prevalence of Serum IgG and Neutralizing Factors against Adeno-Associated Virus (AAV) Types 1, 2, 5, 6, 8, and 9 in the Healthy Population: Implications for Gene Therapy Using AAV Vectors. Hum. Gene Ther. 2010, 21, 704–712. [Google Scholar] [CrossRef]
  16. Fu, H.; Meadows, A.S.; Pineda, R.J.; Kunkler, K.L.; Truxal, K.V.; McBride, K.L.; Flanigan, K.M.; McCarty, D.M. Differential Prevalence of Antibodies Against Adeno-Associated Virus in Healthy Children and Patients with Mucopolysaccharidosis III: Perspective for AAV-Mediated Gene Therapy. Hum. Gene Ther. Clin. Dev. 2017, 28, 187–196. [Google Scholar] [CrossRef]
  17. Hinderer, C.; Katz, N.; Buza, E.L.; Dyer, C.; Goode, T.; Bell, P.; Richman, L.K.; Wilson, J.M. Severe Toxicity in Nonhuman Primates and Piglets Following High-Dose Intravenous Administration of an Adeno-Associated Virus Vector Expressing Human SMN. Hum. Gene Ther. 2018, 29, 285–298. [Google Scholar] [CrossRef]
  18. Stone, D.; Aubert, M.; Jerome, K.R. Adeno-Associated Virus Vectors and Neurotoxicity-Lessons from Preclinical and Human Studies. Gene Ther. 2023, 32, 60–73. [Google Scholar] [CrossRef]
  19. Stone, D.; Aubert, M.; Jerome, K.R. Breaching the Blood-Brain Barrier: AAV Triggers Dose-Dependent Toxicity in the Brain. Mol. Ther. Methods Clin. Dev. 2023, 31, 101105. [Google Scholar] [CrossRef]
  20. Xiong, W.; Wu, D.M.; Xue, Y.; Wang, S.K.; Chung, M.J.; Ji, X.; Rana, P.; Zhao, S.R.; Mai, S.; Cepko, C.L. AAV Cis-Regulatory Sequences Are Correlated with Ocular Toxicity. Proc. Natl. Acad. Sci. USA 2019, 116, 5785–5794. [Google Scholar] [CrossRef]
  21. Brommel, C.M.; Cooney, A.L.; Sinn, P.L. Adeno-Associated Virus-Based Gene Therapy for Lifelong Correction of Genetic Disease. Hum. Gene Ther. 2020, 31, 985–995. [Google Scholar] [CrossRef]
  22. Mingozzi, F.; Anguela, X.M.; Pavani, G.; Chen, Y.; Davidson, R.J.; Hui, D.J.; Yazicioglu, M.; Elkouby, L.; Hinderer, C.J.; Faella, A.; et al. Overcoming Preexisting Humoral Immunity to AAV Using Capsid Decoys. Sci. Transl. Med. 2013, 5, 194ra92. [Google Scholar] [CrossRef]
  23. Brown, D.; Altermatt, M.; Dobreva, T.; Chen, S.; Wang, A.; Thomson, M.; Gradinaru, V. Deep Parallel Characterization of AAV Tropism and AAV-Mediated Transcriptional Changes via Single-Cell RNA Sequencing. Front. Immunol. 2021, 12, 730825. [Google Scholar] [CrossRef]
  24. Agbandje-McKenna, M.; Kleinschmidt, J. AAV Capsid Structure and Cell Interactions. In Methods in Molecular Biology; Humana Press: Clifton, NJ, USA, 2011; Volume 807, pp. 47–92. [Google Scholar]
  25. Xie, Q.; Bu, W.; Bhatia, S.; Hare, J.; Somasundaram, T.; Azzi, A.; Chapman, M.S. The Atomic Structure of Adeno-Associated Virus (AAV-2), a Vector for Human Gene Therapy. Proc. Natl. Acad. Sci. USA 2002, 99, 10405–10410. [Google Scholar] [CrossRef]
  26. Lam, A.K.; Frabutt, D.; Li, L.; Xiao, W. Chemical Modifications of the Capsid for Redirecting and Improving the Efficacy of Adeno-Associated Virus Vectors. Hum. Gene Ther. 2021, 32, 1433–1438. [Google Scholar] [CrossRef]
  27. Pham, Q.; Glicksman, J.; Chatterjee, A. Chemical Approaches to Probe and Engineer AAV Vectors. Nanoscale 2024, 16, 13820–13833. [Google Scholar] [CrossRef]
  28. Zhou, X.; Liu, J.; Xiao, S.; Liang, X.; Li, Y.; Mo, F.; Xin, X.; Yang, Y.; Gao, C. Adeno-Associated Virus Engineering and Load Strategy for Tropism Modification, Immune Evasion and Enhanced Transgene Expression. Int. J. Nanomed. 2024, 19, 7691–7708. [Google Scholar] [CrossRef]
  29. Bourg, N.; Hong, A.V.; Lostal, W.; Jaber, A.; Guerchet, N.; Tanniou, G.; Bordier, F.; Bertil-froidevaux, E.; Georger, C.; Daniele, N.; et al. Co-Administration of Simvastatin Does Not Potentiate the Benefit of Gene Therapy in the mdx Mouse Model for Duchenne Muscular Dystrophy. Int. J. Mol. Sci. 2022, 23, 2016. [Google Scholar] [CrossRef]
  30. Israeli, D.; Cosette, J.; Corre, G.; Amor, F.; Poupiot, J.; Stockholm, D.; Montus, M.; Gjata, B.; Richard, I. An AAV-SGCG Dose-Response Study in a γ-Sarcoglycanopathy Mouse Model in the Context of Mechanical Stress. Mol. Ther. Methods Clin. Dev. 2019, 13, 494–502. [Google Scholar] [CrossRef]
  31. Eid, F.E.; Chen, A.T.; Chan, K.Y.; Huang, Q.; Zheng, Q.; Tobey, I.G.; Pacouret, S.; Brauer, P.P.; Keyes, C.; Powell, M.; et al. Systematic Multi-Trait AAV Capsid Engineering for Efficient Gene Delivery. Nat. Commun. 2024, 15, 1–14. [Google Scholar] [CrossRef]
  32. Asaad, W.; Volos, P.; Maksimov, D.; Khavina, E.; Deviatkin, A.; Mityaeva, O.; Volchkov, P. AAV Genome Modification for Efficient AAV Production. Heliyon 2023, 9, 15071. [Google Scholar] [CrossRef]
  33. Li, C.; Samulski, R.J. Engineering Adeno-Associated Virus Vectors for Gene Therapy. Nat. Rev. Genet. 2020, 21, 255–272. [Google Scholar] [CrossRef]
  34. Nieuwenhuis, B.; Laperrousaz, E.; Tribble, J.R.; Verhaagen, J.; Fawcett, J.W.; Martin, K.R.; Williams, P.A.; Osborne, A. Improving Adeno-Associated Viral (AAV) Vector-Mediated Transgene Expression in Retinal Ganglion Cells: Comparison of Five Promoters. Gene Ther. 2023, 30, 503–519. [Google Scholar] [CrossRef]
  35. Duan, D. Systemic Delivery of Adeno-Associated Viral Vectors. Curr. Opin. Virol. 2016, 21, 16–25. [Google Scholar] [CrossRef]
  36. Tosolini, A.P.; Sleigh, J.N. Intramuscular Delivery of Gene Therapy for Targeting the Nervous System. Front. Mol. Neurosci. 2020, 13, 129. [Google Scholar] [CrossRef]
  37. Arabi, F.; Mansouri, V.; Ahmadbeigi, N. Gene Therapy Clinical Trials, Where Do We Go? An Overview. Biomed. Pharmacother. 2022, 153, 113324. [Google Scholar] [CrossRef]
  38. Ginn, S.L.; Mandwie, M.; Alexander, I.E.; Edelstein, M.; Abedi, M.R. Gene Therapy Clinical Trials Worldwide to 2023—An Update. J. Gene Med. 2024, 26, e3721. [Google Scholar] [CrossRef]
  39. Peng, J.; Zou, W.W.; Wang, X.L.; Zhang, Z.G.; Huo, R.; Yang, L. Viral-mediated gene therapy in pediatric neurological disorders. World J. Pediatr. WJP 2024, 20, 533–555. [Google Scholar] [CrossRef]
  40. Tordo, J.; O’Leary, C.; Antunes, A.S.L.M.; Palomar, N.; Aldrin-Kirk, P.; Basche, M.; Bennett, A.; D’Souza, Z.; Gleitz, H.; Godwin, A.; et al. A Novel Adeno-Associated Virus Capsid with Enhanced Neurotropism Corrects a Lysosomal Transmembrane Enzyme Deficiency. Brain J. Neurol. 2018, 141, 2014–2031. [Google Scholar] [CrossRef]
  41. Xu, L.; Yao, S.; Ding, Y.E.; Xie, M.; Feng, D.; Sha, P.; Tan, L.; Bei, F.; Yao, Y. Designing and Optimizing AAV-Mediated Gene Therapy for Neurodegenerative Diseases: From Bench to Bedside. J. Transl. Med. 2024, 22, 866. [Google Scholar] [CrossRef]
  42. Reilly, A.; Yaworski, R.; Beauvais, A.; Schneider, B.L.; Kothary, R. Long Term Peripheral AAV9-SMN Gene Therapy Promotes Survival in a Mouse Model of Spinal Muscular Atrophy. Hum. Mol. Genet. 2024, 33, 510–519. [Google Scholar] [CrossRef]
  43. Bey, K.; Deniaud, J.; Dubreil, L.; Joussemet, B.; Cristini, J.; Ciron, C.; Hordeaux, J.; Le Boulc’h, M.; Marche, K.; Maquigneau, M.; et al. Intra-CSF AAV9 and AAVrh10 Administration in Nonhuman Primates: Promising Routes and Vectors for Which Neurological Diseases? Molecular Therapy. Methods Clin. Dev. 2020, 17, 771. [Google Scholar] [CrossRef]
  44. Jensen, T.L.; Gøtzsche, C.R.; Woldbye, D.P.D. Current and Future Prospects for Gene Therapy for Rare Genetic Diseases Affecting the Brain and Spinal Cord. Front. Mol. Neurosci. 2021, 14, 695937. [Google Scholar] [CrossRef] [PubMed]
  45. Madsen, D.; Cantwell, E.R.; O’Brien, T.; Johnson, P.A.; Mahon, B.P. Adeno-Associated Virus Serotype 2 Induces Cell-Mediated Immune Responses Directed against Multiple Epitopes of the Capsid Protein VP1. J. Gen. Virol. 2009, 90 Pt 11, 2622–2633. [Google Scholar] [CrossRef] [PubMed]
  46. Wu, T.L.; Li, H.; Faust, S.M.; Chi, E.; Zhou, S.; Wright, F.; High, K.A.; Ertl, H.C.J. CD8+ T Cell Recognition of Epitopes within the Capsid of Adeno-Associated Virus 8-Based Gene Transfer Vectors Depends on Vectors’ Genome. Mol. Ther. 2014, 22, 42–51. [Google Scholar] [CrossRef] [PubMed]
  47. Pillay, S.; Meyer, N.L.; Puschnik, A.S.; Davulcu, O.; Diep, J.; Ishikawa, Y.; Jae, L.T.; Wosen, J.E.; Nagamine, C.M.; Chapman, M.S.; et al. An Essential Receptor for Adeno-Associated Virus Infection. Nature 2016, 530, 108–112. [Google Scholar] [CrossRef]
  48. Wu, Z.; Yang, H.; Colosi, P. Effect of Genome Size on AAV Vector Packaging. Mol. Ther. 2010, 18, 80–86. [Google Scholar] [CrossRef]
  49. Jiang, Z.; Dalby, P.A. Challenges in Scaling up AAV-Based Gene Therapy Manufacturing. Trends Biotechnol. 2023, 41, 1268–1281. [Google Scholar] [CrossRef]
  50. Tustian, A.D.; Bak, H. Assessment of Quality Attributes for Adeno-Associated Viral Vectors. Biotechnol. Bioeng. 2021, 118, 4186–4203. [Google Scholar] [CrossRef]
  51. Castle, M.J.; Turunen, H.T.; Vandenberghe, L.H.; Wolfe, J.H. Controlling AAV Tropism in the Nervous System with Natural and Engineered Capsids. Methods Mol. Biol. 2016, 1382, 133–149. [Google Scholar] [CrossRef]
  52. Zhou, L.; Wang, Y.; Xu, Y.; Zhang, Y.; Zhu, C. Advances in AAV-Mediated Gene Replacement Therapy for Pediatric Monogenic Neurological Disorders. Mol. Ther. Methods Clin. Dev. 2024, 32, 101357. [Google Scholar] [CrossRef]
  53. Zhou, L.; Wang, Y.; Xu, Y.; Zhang, Y.; Zhu, C. A Comprehensive Review of AAV-Mediated Strategies Targeting Microglia for Therapeutic Intervention of Neurodegenerative Diseases. J. Neuroinflamm. 2024, 21, 232. [Google Scholar] [CrossRef]
  54. Lin, R.; Zhou, Y.; Yan, T.; Wang, R.; Li, H.; Wu, Z.; Zhang, X.; Zhou, X.; Zhao, F.; Zhang, L.; et al. Directed Evolution of Adeno-Associated Virus for Efficient Gene Delivery to Microglia. Nat. Methods 2022, 19, 976–985. [Google Scholar] [CrossRef]
Ctn 09 00032 g001
Figure 1. Schematic illustration of various capsid engineering approaches used to enhance adeno-associated virus (AAV) function for gene therapy. Shown are methods such as site-directed mutagenesis, peptide insertion, and directed evolution to improve tissue tropism, immune evasion, and transduction efficiency. The figure also highlights innovations in genome engineering—such as self-complementary AAVs (scAAV), Kozak sequence optimization, and full-to-empty capsid ratio modulation—to increase transgene expression, stability, and delivery efficacy across neurological tissues. This figure was created using BioRender.com.
Figure 1. Schematic illustration of various capsid engineering approaches used to enhance adeno-associated virus (AAV) function for gene therapy. Shown are methods such as site-directed mutagenesis, peptide insertion, and directed evolution to improve tissue tropism, immune evasion, and transduction efficiency. The figure also highlights innovations in genome engineering—such as self-complementary AAVs (scAAV), Kozak sequence optimization, and full-to-empty capsid ratio modulation—to increase transgene expression, stability, and delivery efficacy across neurological tissues. This figure was created using BioRender.com.
Ctn 09 00032 g001
Ctn 09 00032 g002
Figure 2. Overview of delivery routes for AAV-based gene therapies targeting central nervous system (CNS) disorders. Left panel shows anatomical routes including intravenous, intrathecal, intraparenchymal, and intranasal delivery. Right panel summarizes AAV serotypes and associated gene therapy applications across neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, spinal muscular atrophy, and frontotemporal dementia. This visualization also includes delivery methods used in clinical trials (e.g., AAV2 via intraparenchymal injection) and the specific AAV serotypes employed for each disorder, underscoring the importance of route and serotype selection for therapeutic success. This figure was created using BioRender.com.
Figure 2. Overview of delivery routes for AAV-based gene therapies targeting central nervous system (CNS) disorders. Left panel shows anatomical routes including intravenous, intrathecal, intraparenchymal, and intranasal delivery. Right panel summarizes AAV serotypes and associated gene therapy applications across neurodegenerative disorders such as Alzheimer’s disease, Parkinson’s disease, spinal muscular atrophy, and frontotemporal dementia. This visualization also includes delivery methods used in clinical trials (e.g., AAV2 via intraparenchymal injection) and the specific AAV serotypes employed for each disorder, underscoring the importance of route and serotype selection for therapeutic success. This figure was created using BioRender.com.
Ctn 09 00032 g002
Table 1. Summary of AAV challenges and solutions.
Table 1. Summary of AAV challenges and solutions.
ChallengesSolutions
Pre-existing ImmunityCapsid Engineering, Decoy Capsids
Limited Packaging CapacityDual Vector Systems, Mini-genes
Off-target EffectsTissue-specific Promoters, Targeted Delivery
Dose-related ToxicityOptimized Dosing, Immune Modulation
Table 2. Approved and Pipeline AAV Gene Therapies for Neurological Diseases.
Table 2. Approved and Pipeline AAV Gene Therapies for Neurological Diseases.
Therapy NameIndicationAAV SerotypeStage
Upstaza®Aromatic L-Amino Acid Decarboxylase (AADC) DeficiencyAAV2EMA Approved
Luxturna®Leber Congenital Amaurosis (LCA2)AAV2FDA Approved
Zolgensma®
(NCT03505099)		Spinal Muscular Atrophy (SMA)AAV9FDA Approved
AAV2-BDNF (NCT05040217)Early Alzheimer’s DiseaseAAV2Phase I
AAV2-GAD (NCT04228653)Parkinson’s DiseaseAAV2Phase I/II
AAV2-RPE65 (NCT00749957)Leber Congenital AmaurosisAAV2Phase I/II
AAV2- CNGA3 (NCT02935517)AchromatopsiaAAV2Phase I/II
AAV5-miHTT (NCT04120493)Huntington’s DiseaseAAV5Phase I/II
AAV9-beta-gal (NCT04737460)GM1 GangliosidosisAAV9, AAVrh.10Phase I/II
AAV.PGRN (NCT06064890)Frontotemporal Dementia (FTD-GRN)AAV9Phase I/II
AAV9-AP4M1 (NCT05518188)Spastic Paraplegia Type 50AAV9Phase I/II
AAV-hSMN1 (NCT06288230)Spinal Muscular AtrophyAAVPhase I/II
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Sharma, S.; Joshi, V.; Kumar, V. Implications of AAV Serotypes in Neurological Disorders: Current Clinical Applications and Challenges. Clin. Transl. Neurosci. 2025, 9, 32. https://doi.org/10.3390/ctn9030032

AMA Style

Sharma S, Joshi V, Kumar V. Implications of AAV Serotypes in Neurological Disorders: Current Clinical Applications and Challenges. Clinical and Translational Neuroscience. 2025; 9(3):32. https://doi.org/10.3390/ctn9030032

Chicago/Turabian Style

Sharma, Sachin, Vibhuti Joshi, and Vivek Kumar. 2025. "Implications of AAV Serotypes in Neurological Disorders: Current Clinical Applications and Challenges" Clinical and Translational Neuroscience 9, no. 3: 32. https://doi.org/10.3390/ctn9030032

APA Style

Sharma, S., Joshi, V., & Kumar, V. (2025). Implications of AAV Serotypes in Neurological Disorders: Current Clinical Applications and Challenges. Clinical and Translational Neuroscience, 9(3), 32. https://doi.org/10.3390/ctn9030032

Article Metrics

Back to TopTop