Innovations in Hearing and Vision Restoration Through Gene Therapy

This article was originally published on November 5, 2024 on Advances in Motion.

In This Article

• Mass Eye and Ear researcher Artur A. Indzhykulian, MD, PhD focuses his group's research efforts on the proteins essential for maintaining stereocilia in sensory hair cells, aiming to develop gene therapies for conditions like Usher syndrome.
• Usher syndrome causes hearing, vision, and balance loss, with genetic mutations in hair cells disrupting sensory signal conversion into an electrical signal in the ear and retina.
• Adeno-associated virus vectors are crucial for gene therapy due to their safety and specificity, though they face challenges such as limited capacity to deliver genetic material.
• Organoids, while limited in complexity, offer a promising platform for testing gene therapies in a human context.
• Cross-disciplinary collaboration at Mass General Brigham accelerates gene and cell therapy research by rapidly translating laboratory discoveries to clinical practice.

Indzhykulian: Our research focuses on understanding the proteins that form and maintain stereocilia in sensory hair cells. These proteins are essential for preserving the integrity of the hair cell stereocilia bundle, which is vital for the hearing function. Some of these proteins form the hair cell mechanotransduction complex, which converts sound into electrical signals that the brain can interpret. Mutations in these proteins often result in deafness or conditions like Usher syndrome, a severe genetic disorder affecting hearing, vision and balance. By employing cutting-edge techniques such as electrophysiology, optical microscopy, and electron microscopy, we investigate the roles of these proteins in the hair cell complex. Our ultimate goal is to use that knowledge to develop gene therapy approaches to treat Usher syndrome, targeting hearing, balance, and vision impairments.

Q: What is Usher syndrome, how do hair cells function in the inner ear, and what impact do genetic mutations have on hearing and balance?

Indzhykulian: Usher syndrome is a genetic condition that leads to loss of hearing, vision, and balance. The sensory cells of the inner ear, called hair cells, have hair-like projections called stereocilia that detect sound waves and translate them into electrical signals for the brain to process. Similar hair cells in the vestibular system are responsible for balance, detecting forces from head movement or acceleration rather than sound. Genetic mutations affecting these cells can disrupt their function, leading to varying degrees of hearing loss and balance disorders, often including complete loss of these senses in severe cases. In the retina, these proteins help to maintain photoreceptor structure and function, and their deficit leads to vision loss.

Q: What are the key considerations when designing gene therapy vectors for targeting specific cell types and how can these be generalized to other cell types?

Indzhykulian: In designing gene therapy vectors, two critical factors must be considered: their efficacy and safety. The goal is to maximize the delivery of the therapeutic genetic cargo to the target cells while minimizing immune responses and off-target effects. The FDA has already approved adeno-associated viruses (AAVs) for several gene therapies, which are often used for their safety profile and ability to target non-dividing cells like inner ear sensory hair cells. AAV vectors don't integrate the therapeutic DNA into the host cell's genome, which reduces the risk of long-term complications. Different subtypes of viral capsids can enhance the vector's ability to target specific cell types, improving both efficacy and safety. In addition, tissue-specific and cell-specific promoters, like those designed to specifically express proteins in hair cells, enable precise targeting, reducing unintended, so-called off-target effects in other tissues or cell types.

Q: Can you explain how AAV vectors can improve gene delivery and therapies? What are some of the challenges?

Indzhykulian: While AAVs have become a valuable tool for gene therapy, they do have limitations. For instance, large-scale production is costly, and achieving high transfection efficiency in given cell types can be challenging. Another significant challenge is AAV's limited capacity to carry genetic material, which is limited to only about 5,000 nucleotides. This constraint means the therapeutic gene, along with its necessary regulatory elements, must fit within this small space. To address this, researchers sometimes use miniaturized versions of target genes or divide the genetic "cargo" sequence across multiple AAV vectors. Despite these challenges, AAV vectors remain a powerful option due to their safety and specificity.

Q: Can you discuss the potential for using organoid models in preclinical evaluations of gene therapies beyond the inner ear, and what are the advantages and limitations?

Indzhykulian: Organoids offer an exciting platform for preclinical evaluations because they mimic the 3D architecture and function of human organs. They can be derived from patient cells, enabling the study of patient-specific genetic mutations and disease progressions. This is particularly useful for testing gene therapies in a more realistic, human-relevant context compared to traditional cell culture systems or testing in animal models.

However, organoids have some limitations. They typically represent early-stage tissue development and may not capture the full complexity of mature organs. Additionally, they cannot model systemic interactions within the body, such as immune responses or off-target effects in other organs. Despite these limitations, organoids offer an invaluable tool for testing human-specific gene therapies, though they must be used alongside other models to fully evaluate safety and efficacy. An ongoing project in the laboratory, in collaboration with experts in the area of developing inner ear organoid systems, is to test in human organoid hair cells a gene therapy solution we have previously developed and shown to work in the mouse inner ear. Once shown to work in the mouse, testing the therapy in human inner ear organoids brings the therapy a step closer to the bedside.

Q: How do the principles of gene therapy for Usher syndrome translate to other genetic disorders affecting sensory cells in different parts of the body?

Indzhykulian: The strategies used for treating Usher syndrome can be applied to a wide range of sensory disorders. Many of the principles behind gene therapy—such as vector design, targeting specific cell types, and ensuring therapeutic efficacy—are broadly applicable across different conditions. Usher syndrome, in particular, offers a unique opportunity to learn lessons that can be transferred across various sensory systems and to other treatments. Since it affects both the inner ear and the eye, which are distinct environments with their own enclosed fluid systems, gene therapies must be tailored to function in each system. By addressing these challenges, we establish a versatile framework that can be adapted for therapies targeting other sensory cells in various parts of the body.

Q: How do you foresee advancements in mini-gene technology influencing the development of gene therapies for other genetic conditions?

Indzhykulian: Advances in structure-guided, rational protein engineering approaches we implemented in our mini-gene technology offer two significant benefits for future gene therapies. First, they enable treatment for a broader range of genetic disorders by allowing large genes, which would otherwise exceed the packaging capacity of viral vectors, to be shortened and delivered efficiently. This opens new possibilities for treating diseases previously beyond the reach of AAV-based gene therapy. Second, as mini-genes are developed and tested, they provide valuable insights into which parts of proteins are most essential for function. This knowledge allows us to refine and optimize therapeutic proteins, improving our ability to target and treat new genetic conditions.

Q: What role do collaborative efforts across Mass General Brigham play in advancing the field of gene and cell therapy, and how can researchers from different specializations best work together?

Indzhykulian: The collaborative environment at Mass General Brigham is crucial for advancing gene and cell therapy research. Close partnerships between scientists and clinicians in a hospital-based setting accelerate the translation of discoveries from the lab to clinical practice. This dynamic interaction ensures that our basic research-inspired translational work directly addresses the real-world challenges faced by patients and healthcare providers. By maintaining close ties to the clinical environment, we can rapidly adapt our work to improve patient care. The cross-disciplinary collaboration at MGB and across the greater Boston area fosters innovation and drives the development of therapies that can make a tangible impact in clinical settings.