PROJECT SUMMARY. Individuals with Down syndrome (DS), the genetic condition caused by trisomy 21 (T21), display widespread auditory dysfunction, with current estimates indicating that ~80% of this population may have one or more forms of hearing loss. Despite the obvious importance of hearing for neurodevelopment, speech development, learning, cognition, social integration, inclusion in the workforce, and overall well-being, the mechanisms by which an extra copy of chromosome 21 causes hearing loss are unknown. Therefore, additional research in this area has potential for transformative discoveries that could improve multiple dimensions of development, health, and quality of life in this population. To fulfill this unmet need and transform the field, we have assembled a multi- disciplinary team of synergistic expertise to develop the following Specific Aims: 1. To complete deep phenotyping of peripheral auditory and vestibular function in DS. We will complete a deep characterization of auditory and vestibular function in a cohort of 300 participants via behavioral and physiologic assessments including air and bone conduction behavioral audiometry, otoacoustic emissions, standard tympanometry, wideband acoustic immittance, and auditory brainstem response testing. We will also administer a battery of vestibular function assessments and complete a detailed annotation of health histories, with an emphasis on co-occurring conditions and previous use of ototoxic medicines. 2. To identify dysregulated biological processes underlying hearing loss in DS. Paired to efforts in Aim 1, we will collect biospecimens, including blood samples and nasopharyngeal swabs, to complete both hypothesis- driven and hypotheses-generating analyses of underlying mechanisms, including analyses of the transcriptome, proteome, metabolome, neurodegeneration and neuroinflammation markers, and variations in the microbiome. These datasets will enable the identification of novel biosignatures associated with different etiologies of hearing loss and vestibular deficits. 3. To define cause-effect relationships between gene triplication and hearing loss in a mouse model. We will complete deep phenotyping of hearing loss in a mouse model of DS that displays this phenotype, including imaging studies to monitor for structural differences, wideband acoustic immittance measurements, distortion product otoacoustic emissions, tests of vestibular function, and measurements of otitis media. These studies will be complemented by assays not possible or very difficult in humans, such as single cell transcriptomics and cochlear endolymphatic potentials. Lastly, we will define the preclinical value of potential pharmacological interventions and narrow down the genomic region(s) driving hearing loss in mice. Funding of this proposal will enable a multidisciplinary team of experts to address a major knowledge gap in the field, with clear potential for transformative discoveries that would benefit tho...