Hearing loss is a common sensory disorder, affecting about 20% of the world population. In around 500 million people, hearing loss has invalidating consequences for private or professional development and leads to reduced ability to communicate, social isolation, cognitive decline and depression. The vast majority of sensorineural hearing loss forms are acquired throughout life, the most prevalent being age-related, which affects about a third of the retired population. Noise-induced and ototoxic hearing loss are more prevalent in the younger population. Current solutions, such as hearing aids, only alleviate symptoms but do not address the underlying causes. This Privat-Docent thesis presents a translational strategy to move beyond symptomatic treatment by targeting molecular and cellular mechanisms underlying acquired forms of hearing loss. It addresses important barriers to the development and clinical translation of inner ear therapeutics, encompassing gene- and drug-based strategies for otoprotection and regeneration, as well as the development of safer and more targeted delivery systems.
Pathogenic genes, involved in mediating ototoxic signalling in cochlear cells, need to be identified and precisely targeted. The first article uncovers a novel mechanism of age-related hearing loss (ARHL) involving NADPH oxidase (NOX)-mediated redox signalling. The study reveals that NOX deficiency in mice leads to downregulation of excitatory neurotransmission genes and confers resistance to glutamate-induced excitotoxicity in auditory neurons. NOX3, a key source of ROS in the cochlea, is identified as central to this pathogenic process, highlighting redox homeostasis and excitatory pathway modulation as therapeutic targets for neuroprotection in ARHL and potentially other neurodegenerative conditions. Translating these insights into therapeutic intervention, the second article details the development of siRNAs targeting NOX3 to prevent sensorineural hearing loss (SNHL). After screening multiple constructs, the most effective siRNA achieved over 60% knockdown of NOX3 in vivo when delivered directly to the inner ear, also preserving hearing thresholds. This establishes siRNA-mediated NOX3 silencing as a viable and safe otoprotective strategy, with ongoing efforts directed at refining delivery methods for clinical application.
As acquired forms of hearing loss often arise from complex aetiologies and the synergy of multiple pathogenic genes, focusing on a single target gene may be suboptimal. The third paper introduces the Therapeutic MiniGene (TMG) technology, enabling efficient and sustained multiplex gene knockdown through the concatenation of optimized synthetic miRNA hairpins constructs. The use of specific spacer sequence between the promoter and miRNA hairpins enhances transcript processing and stability, allowing robust, simultaneous silencing of multiple genes upon single transduction. While siRNA, due to their relatively short half-lives, are particularly suited for acute hearing trauma (e.g., noise exposure or ototoxic treatment), the TMG technology allows for long-lasting multiplex gene silencing and could represent a disruptive approach to address the multigenic aetiology of long-term or progressive hearing loss conditions such as presbycusis.
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