The condition at the heart of this story is autosomal recessive deafness type 9, known as DFNB9, which is caused by mutations in the OTOF gene and accounts for roughly 2 to 8 percent of hereditary cases of congenital sensorineural hearing loss. What makes DFNB9 uniquely suited to gene replacement is the anatomy of its defect. Unlike many forms of genetic deafness in which the delicate hair cells of the inner ear are missing or degenerated, DFNB9 patients retain structurally intact hair cells. The problem lies instead in the synaptic machinery: otoferlin, the protein encoded by OTOF, is essential for the calcium-triggered release of neurotransmitter at the ribbon synapse, the specialized junction through which hair cells transmit sound-encoded signals to the auditory nerve. In the absence of functional otoferlin, the ear’s microphone still works, but the signal never reaches the brain.

This synaptic bottleneck defines both the pathology and the therapeutic opportunity. Because the hair cells are alive and waiting for their missing protein, restoring even a partial supply of otoferlin could, in principle, reopen the auditory pathway without requiring cell replacement or regeneration. That biological premise has now been tested at scale. Between 2024 and 2026, the research team led by Yilai Shu at Fudan University published a remarkable sequence of clinical studies in The Lancet, Nature Medicine and Nature, progressing from unilateral treatment in the first human trial, to bilateral administration, and finally to a 42-participant multicenter cohort with follow-up extending as long as 2.5 years. Across these studies, the evidence consistently pointed in the same direction: children who had been profoundly deaf from birth began to hear, and in many cases to develop speech and language, after a single surgical delivery of the therapeutic vector.

Getting there required solving formidable technical problems, and the review devotes substantial attention to the vector engineering that made the trials possible. The OTOF gene is enormous by viral-vector standards, far exceeding the roughly 4.7-kilobase packaging capacity of a single AAV particle. The solution was dual-vector technology, in which the otoferlin coding sequence is split across two AAV genomes and reconstituted inside the target cell through mechanisms such as trans-splicing, hybridization of overlapping fragments, or intein-mediated protein trans-splicing. Each strategy carries trade-offs in reconstitution efficiency and fidelity, and much of the preclinical work of the past three years has focused on optimizing which split design delivers the most functional otoferlin to hair cells with the least immunological and toxicological burden.

Equally critical has been the refinement of the delivery system itself. AAV vectors must reach the hair cells of the cochlea, a structure encased in bone and fluid-filled compartments that are notoriously difficult to access without causing additional damage. The review highlights three converging advances: the design of AAV capsids with improved tropism for inner hair cells, the development of hair cell-specific promoters that restrict otoferlin expression to the correct cell type and reduce off-target expression, and minimally invasive inner ear delivery methods, typically via the round window membrane, that allow the vector to perfuse the cochlear duct while preserving residual hearing and cochlear architecture. Together, these innovations moved OTOF gene therapy from animal models into the operating room within a remarkably short window.

The clinical results have been the most consequential development. In the successive Fudan-led trials, treated children showed substantial recovery of auditory function, with many progressing from profound deafness to thresholds compatible with speech perception and, in several cases, spontaneous language development. The safety record has been encouraging: across the cohorts, AAV-mediated OTOF replacement was well tolerated, with no treatment-related serious adverse events reported that undermined the feasibility of the approach. The consistency of benefit across unilateral, bilateral and multicenter designs strengthened the case that the effect was real and reproducible rather than an artifact of a single center or a small sample. The culmination came in 2026, when the United States Food and Drug Administration approved Otarmeni (lunsotogene parvec-cwha), the first OTOF gene therapy, a milestone the review describes as pivotal for the entire field of inner ear medicine.

Yet the review is equally clear about the challenges that remain. The first is the therapeutic window. Gene replacement must occur before auditory pathways and speech circuits have irreversibly reorganized in the absence of input, and the precise age range during which treatment confers maximal benefit has not been fully delineated. Most treated children have been young, reflecting the assumption that earlier is better, but the boundaries of that window, and whether older children or adults with DFNB9 could also benefit, remain open questions. Defining the window precisely will require longer follow-up and careful correlation of treatment age with outcomes, a task that the ongoing trials are only beginning to address.

The second cluster of challenges is industrial and economic. Producing AAV vectors at clinical scale under current Good Manufacturing Practice conditions is a recognized bottleneck for the entire gene therapy field, and the dual-vector requirement of OTOF therapy doubles the dose burden, since each treatment requires two full AAV genomes per target cell. Manufacturing complexity translates directly into cost, and the review notes that the prohibitive price of gene therapy creates accessibility barriers that could leave most of the world’s DFNB9 patients untreated. Because DFNB9 is distributed globally and congenital deafness screening is uneven, the gap between what the science can achieve and what health systems can deliver is likely to be one of the defining tensions of the coming decade.

Long-term safety is the third unresolved issue. With a maximum follow-up of 2.5 years, the clinical evidence base cannot yet speak to durability of hearing restoration or to late-emerging adverse effects such as immune responses to the vector or the reconstituted protein. The review emphasizes that the absence of long-term safety data is a critical gap that only time and continued surveillance can fill. It also points toward the broader horizon of precision intervention: OTOF is a proof of principle, but the same vector engineering, delivery and trial infrastructure now being validated could be adapted to other genetic forms of deafness, each with its own molecular target and technical constraints.

Taken together, the review by Peng, Ren, Yin and their colleagues documents one of the fastest bench-to-bedside journeys in modern medicine. In roughly three years, OTOF gene therapy moved from preclinical vector design through the first unilateral human trial, bilateral treatment and a 42-participant multicenter study, to FDA approval of the first therapy for congenital deafness. The molecular logic was elegant from the start: a structurally intact but synaptically silent inner ear, waiting for a single missing protein. What the trials demonstrated is that, with the right capsid, the right promoter and the right surgical delivery, that protein can be supplied and hearing can begin. The remaining work, defining the therapeutic window, scaling manufacturing, extending safety surveillance and lowering cost, is formidable but tractable, and it will determine whether this new era of precision intervention for congenital deafness reaches the children worldwide who stand to benefit from it.

Subject of Research: AAV-mediated OTOF gene replacement therapy for DFNB9 congenital deafness

Article Title: OTOF gene therapy: from the first human trial to a new era of precision intervention for congenital deafness

Article References: Peng, L.-P., Ren, Y., Liu, Y., Shen, L., Zhu, B.-B., Ban, Y.-Q., Liang, C.-L., Chen, L., He, J.-N., Wei, R.-D., & Yin, S.-H. (2026). OTOF gene therapy: from the first human trial to a new era of precision intervention for congenital deafness. Journal of Translational Medicine. https://doi.org/10.1186/s12967-026-08890-9

Image Credits: AI Generated

DOI: 10.1186/s12967-026-08890-9

Keywords: OTOF, DFNB9, gene therapy, adeno-associated virus, congenital deafness, sensorineural hearing loss, ribbon synapse, clinical trial, FDA approval, AAV vectors, inner ear delivery, clinical translation

The research, led by Franziska Köhler and Sonja Kallendrusch of Leipzig University and the HMU Health and Medical University, together with colleagues in Erfurt and Potsdam, compared two complementary patient-derived culture systems side by side. The first, patient-derived neurospheres (PDNS), are compact, free-floating spheroids grown from dissociated tumor cells under serum-free conditions. They offer a controlled, tumor-cell-intrinsic system in which variables can be manipulated with relative ease. The second, patient-derived tissue slice cultures (PDTC), preserve the tumor’s native architecture: thin, 350-micrometer slices of fresh surgical tissue are cultured at an air–liquid interface, retaining the extracellular matrix, vascular structures, immune cells, and stromal compartments that neurospheres inevitably discard. The team systematically exposed both models to two AAV serotypes, AAV2 and AAV6, each carrying a green fluorescent protein (GFP) reporter gene, and then tracked transduction with live confocal microscopy, quantitative PCR, flow cytometry, and immunofluorescence.

The technical rigor of the workflow is central to the study’s credibility. Thirteen surgical specimens from 25 collected samples ultimately entered the analysis, twelve of them IDH-wildtype glioblastomas under the WHO 2021 classification. Tissue not required for diagnosis was obtained with informed consent and ethical approval. For neurospheres, tumor tissue was mechanically and enzymatically dissociated with Accutase, filtered, and seeded at 100,000 cells per well; after 10 to 15 days, spheroids of 100 to 150 micrometers in diameter were ready for experiments. Slice cultures were punched into 2 to 3 millimeter discs and maintained on membrane inserts. After testing several media formulations, the researchers selected NeuroCult-XF, a serum-free proliferation medium, because it preserved tissue architecture and histopathological features better than serum-containing alternatives or other defined formulations. Blinded pathological review confirmed that fibrillary architecture and even characteristic gemistocytic features—enlarged cell bodies with eccentric nuclei—were retained in both models.

The cellular composition analysis revealed stark differences between the two systems, exactly as the team had hypothesized. Slice cultures retained expression of immune markers such as CD68, the vascular marker CD31, and the hypoxia marker HIF1α in organotypic patterns closely mirroring the original tumors. Neurospheres, by contrast, showed virtually no CD31, scarce and disorganized CD68-positive cells, and only weak HIF1α restricted to larger spheroids. More subtly, sphere-forming conditions selectively enriched stem-like populations: the mean SOX2-positive cell fraction rose from 0.114 in native tissue to 0.328 in PDNS, and Nestin positivity climbed from 0.132 to 0.393, while the mesenchymal marker CD44 fell. This finding matters because it confirms that neurosphere culture is not a neutral act of preservation but an active selection process that reshapes the cellular landscape before any vector is ever introduced.

When the vectors were applied, the serotype differences were unambiguous. In neurospheres, AAV6 drove significantly greater GFP expression than AAV2 across a dose range of 10^6 to 10^9 particles, with significant signal appearing as early as day 2 at higher doses, while AAV2 reached significance only at the highest dose after 5 days. Quantitative PCR corroborated the fluorescence data: five days after exposure to 10^8 particles, PLAT-normalized vector genome abundance averaged 146.7 copies per cell for AAV2 but 1,074.0 for AAV6 in EGF-containing medium. Notably, epidermal growth factor supplementation itself proved to be a critical experimental variable. Because AAV6 is known to use EGFR as a co-receptor for cellular entry, the team deliberately compared EGF-containing and EGF-free conditions. In neurospheres, EGF presence modified both the magnitude and temporal stability of AAV6-mediated GFP expression, and marker-defined analysis showed that EGF deprivation broadened GFP positivity across GFAP-, CD44-, Nestin-, and SOX2-positive populations, whereas under EGF-containing conditions only the Nestin-positive fraction showed significant GFP signal.

Exploratory flow cytometry from two independent experiments added an intriguing layer. GFP positivity was markedly enriched among SOX2-positive cells (40.8 percent versus 7.2 percent in SOX2-negative cells) and among cells triple-positive for CD133, Nestin, and SOX2 (41.0 percent versus 7.5 percent). The authors are careful to stress that these figures, based on limited sample numbers, are descriptive and cannot distinguish preferential vector uptake from differences in population abundance, survival, or promoter activity. Still, the pattern suggests that the most stem-like, aggressive cellular states within a glioblastoma may be among those most efficiently engaged by AAV6—a hypothesis with obvious therapeutic implications if it can be confirmed with larger datasets and mechanistic studies.

The most sobering results came from the tissue slice cultures. Overall GFP readouts were comparable to, or in some cases higher than, those in neurospheres, but interpatient variability was dramatically greater. Some patient specimens transduced robustly; others showed little or no detectable GFP despite identical conditions. Mean fluorescence intensities of 4,989 for AAV2, 4,232 for AAV6, and 6,448 for AAV6 without EGF in slice cultures contrasted with far lower values in matched neurospheres, but the spread between patients was wide enough that no single-patient result could be generalized. The authors argue that this variability should not be dismissed as experimental noise. Instead, it may represent a biologically faithful reflection of the heterogeneity that any real-world gene therapy for glioblastoma would confront. A vector that performs uniformly in a homogeneous spheroid panel may still fail in a substantial fraction of patients, and only multi-patient tissue-based validation can expose that vulnerability before clinical translation.

Spatial analysis added another dimension. In neurospheres, GFP-positive cells clustered predominantly at the spheroid periphery, consistent with diffusion-limited vector transport through densely packed three-dimensional tissue, although the authors caution that GFP fluorescence reflects the downstream consequence of delivery and expression rather than physical particle penetration itself. Slice cultures showed a more homogeneous distribution of fluorescence across the tissue, an effect not associated with preferential vascular localization and therefore likely driven by preserved microarchitecture rather than vascular entry routes. Equally notable was the observation of occasional GFP-positive cells in two samples of adjacent non-neoplastic brain tissue, a preliminary but cautionary signal that off-target transduction of tumor-adjacent cells can occur under experimental conditions, consistent with the broad central nervous system tropism of naturally occurring AAV serotypes.

The study’s self-imposed limitations are candidly acknowledged. Vector dose escalation could not be performed in slice cultures because of limited tissue availability, preventing fully matched comparisons between models. Flow cytometric findings rested on only two experiments. Vector genome quantification cannot distinguish entry, trafficking, persistence, or processing, and GFP expression additionally depends on transcription and translation. Slice cultures themselves, while architecturally faithful, remain ex vivo systems that cannot reproduce systemic immunity, vascular dynamics, or long-term tumor evolution. The significant increase in SOX2 expression observed in neurospheres after AAV6 treatment without EGF likewise supports an association rather than a demonstrated causal effect of vector exposure on tumor-cell state.

What the study ultimately delivers is a framework rather than a verdict. The authors explicitly do not claim that both models are mandatory for every preclinical AAV program, nor that either predicts in vivo performance. Instead, they advocate a question-driven approach: neurospheres, with their lower variability and experimental tractability, are well suited to initial serotype screening and dissection of tumor-cell-intrinsic effects; candidate vectors that pass that filter can then be challenged against panels of slice cultures from multiple patients, where preserved architecture, stromal and vascular compartments, and honest interpatient heterogeneity reveal how performance holds up in tissue that more closely resembles what a surgeon would leave behind in a resection cavity. In a disease where nearly every promising preclinical result has eventually collided with clinical reality, the insistence that model selection itself is a determinant of apparent vector efficacy may prove to be one of the more consequential methodological messages glioblastoma gene therapy has received in years.

Subject of Research: Preclinical evaluation of AAV-mediated gene delivery in patient-derived glioblastoma cultures

Article Title: Patient-derived tissue cultures complement neurospheres for preclinical evaluation of AAV-mediated gene delivery in glioblastoma

Article References: Köhler, F., Hess, K., Koloske, C., Gaunitz, F., Rosahl, S. K., Gerlach, R., & Kallendrusch, S. (2026). Patient-derived tissue cultures complement neurospheres for preclinical evaluation of AAV-mediated gene delivery in glioblastoma. Journal of Neuro-Oncology, 179(3), Article 93. https://doi.org/10.1007/s11060-026-05807-w

Image Credits: AI Generated

DOI: 10.1007/s11060-026-05807-w

Keywords: glioblastoma, AAV vectors, gene therapy, patient-derived models, neurosphere cultures, tissue slice cultures, tumor microenvironment, viral vectors, EGF, transduction, preclinical testing, gene delivery