Connexins and Syndromic Sensorineural Deafness

Our work is focused on investigating the mechanistic basis of cochlear dysfunction in syndromic deafness caused by missense mutations in the GJB2 gene that encodes the human connexin 26 (Cx26) gap junction (GJ) protein. Mutations in GJB2 are one of the most common causes of inherited, non-syndromic deafness in the human population. A subset of Cx mutations leads to syndromes in which deafness is accompanied by a heterogeneous array of cutaneous manifestations. Keratitis-Ichthyosis-Deafness (KID) syndrome is one of the more severe syndromes associated with GJB2 mutations and is characterized by profound, pre-lingual sensorineural hearing loss, vascularizing keratitis, skin lesions that can be fatal due to uncontrollable sepsis and predisposition to squamous cell carcinomas. GJs, which are formed by the docking of two, so-called hemichannels (HCs), one from each of two contacting cells, are abundant between keratinocytes and between cochlear support cells and serve as pathways for direct intercellular electrical and chemical signaling. However, it is now evident that undocked Cx26 HCs can function, thereby providing a signaling role across the plasma membrane. Our principal hypothesis is that the pathogenesis of KID syndrome is the result of a new type of channelopathy, specifically mediated by Cx26 HCs that function aberrantly leading to cell dysfunction and even cell death. We use a combination of molecular, biophysical and imaging approaches to investigate the mechanisms by which hemichannels are dysfunctional in KID syndrome. We have identified a number of aberrant HC properties including altered permeability, impaired regulation by extracellular Ca2+ and pH and shifted voltage-dependent gating. Our current focus is on altered permeability to Ca2+ and ATP, two important signaling molecules in cochlea and skin. To that end we are examining the effects of expressing hCx26 mutants in exogenous expression systems and in support cells of the Organ of Corti using cochlear tissue explants. Parallel efforts are aimed at developing a mouse model for KID syndrome using a proof-of-principle 2-plasmid, Tet-On inducible expression system in cochlea developed to express mutant GJB2 transgenes in keratinocytes. Finally we plan to screen for selective blockers of Cx26 HCs, initially using a small library of compounds enriched in known ion channel pharmacophores. Lead compounds will be followed-up through medicinal chemistry approaches to increase affinity and selectivity.