A research team at Seoul National University has developed a gene therapy that accurately corrects the representative mutation of the MPZL2 gene, which causes hereditary hearing loss, and successfully demonstrated the effectiveness of restoring hearing with a single injection.

This is the first study worldwide to demonstrate the therapeutic potential of the Adenine Base Editor (ABE) gene editing tool at the preclinical level for hereditary hearing loss mutations, thereby suggesting the possibility of a “one-and-done” gene therapy.

The Seoul National University Hospital (SNUH) announced on Monday that its research team has developed a humanized mouse model with mutations in the MPZL2 gene, which causes hereditary hearing loss, and confirmed the effects of hearing restoration and auditory cell recovery by applying its own gene editing tool.

The research team was led by Professor Lee Sang-yeon of the Department of Otolaryngology and Professor Bae Sang-su of the Department of Biochemistry at Seoul National University College of Medicine. It also included Jeong So-hyang, a student in the Brain Science Collaborative Program, and Koo Han-sol, a student in the Tumor Biology Collaborative Program.

The c.220C>T mutation in the MPZL2 gene is one of the primary causes of DFNB11-type sensorineural hereditary hearing loss, occurring at a high frequency in East Asian populations and observed in approximately 10% of all patients with hereditary hearing loss. This mutation is a “nonsense mutation” where the CAG codon encoding glutamine is replaced by the stop codon TAG, leading to the cessation of MPZL2 protein production and causing rapid hearing loss after adolescence, which progressively worsens to severe hearing loss. However, no fundamental treatment for this condition has been available to date.

The research team analyzed a cohort of 1,437 families with hereditary hearing loss from Korean and international sources, confirming that the c.220C>T mutation is the primary cause, and developed a humanized mouse model mimicking this mutation. They then developed the latest adenine base editing gene scissors (ABE8eWQ-SpRY) to enhance correction efficiency and accuracy, establishing a strategy to precisely correct the mutant base to the normal sequence.

This gene editing tool operates by converting adenine (A) to guanine (G) without cutting the DNA double helix, making it a next-generation gene editing technology that causes minimal cellular damage and achieves high target accuracy.

The research team loaded this gene scissors into AAV-ie, a modified vector of adeno-associated virus (AAV), and delivered it into the inner ear of mice via a single injection through the round window of the cochlea. AAV-ie exhibits high delivery efficiency and stability in auditory cells, and the gene scissors, guided by guide RNA (sgRNA), precisely reach the mutated site and correct the adenine base, thereby correcting the disease to a normal guanine.

The therapeutic effect was objectively measured using auditory brainstem response (ABR) and double-pulse otoacoustic emission (DPOAE) tests.

As a result, a hearing improvement of 20–30 dB was observed across all frequency bands in the treatment group, and this effect persisted beyond week 20. This is considered a significant result, supporting the feasibility and therapeutic efficacy of in vivo gene correction technology for the inner ear.

Tissue analysis also revealed a significant increase in the survival rate of outer hair cells (OHCs) and supporting cells (SCs), which are auditory cells, as well as a clear recovery of the cochlear tissue structure. The research team explained that this suggests gene correction contributes not only to the function of auditory cells but also to histological restoration.

Additionally, to evaluate the accuracy and safety of the treatment, the research team utilized the Cas-OFFinder program at the DNA level to predict off-target correction possibilities and verified the absence of off-targets through GUIDE-seq analysis.

Analysis of actual cochlear tissue sequencing revealed no off-target editing or off-target effects. RNA-seq analysis at the RNA level also confirmed the absence of off-target RNA editing, thereby demonstrating that the genetic scissors used in this study possess both high target accuracy and in vivo safety.

This study is significant as it has practically demonstrated the potential of precision gene therapy by accurately targeting a genetic mutation causing hereditary hearing loss that is prevalent in East Asian populations and correcting it using genetically engineered scissors developed in-house,” Professor Bae said.

Professor Lee Sang-yeon said, “While hearing loss treatment has been limited to hearing aids or cochlear implants, we anticipate that this study will open new avenues for applying customized gene therapy to patients with hearing loss caused by clear genetic causes. We will continue to strive to translate these findings into actual patient treatments through further preclinical and clinical studies.”