The Opportunities and Challenges of Gene Therapy for Treatment of Inherited Forms of Vision and Hearing Loss.

Inherited forms of blindness and deafness are highly prevalent and severe conditions that significantly impact the lives of millions of people worldwide. The lack of therapeutic options for these conditions poses a major socioeconomic burden. Over the last decades, gene therapy has proven to be a life changing treatment for hereditary and acquired forms of diseases, and extensive preclinical investigation in animal models of both retinal and inner ear disorders has highlighted promising translational opportunities for these disorders too. This led to dozens of clinical trials investigating the efficiency of gene therapy-based approaches, with some of the products for retinal conditions successfully reaching phase III of development or even market authorization. However, challenges remain for the use of gene therapy, which are related to both the features of the delivery vehicles currently available and characteristics of the retinal and inner ear disorders targeted. Therefore, further developments in gene therapy platforms' design, including exploitation of novel technologies such as genome editing, RNA-targeted therapies, and optogenetics, are actively ongoing, driving the field forward. In this study, we review the ongoing applications and achievements of gene therapy for treatment of inherited forms of blindness and deafness as well as the developments that are being pursued in the field to overcome the current limitations.

Genetic Medicine for Hearing Loss: OTOF as Exemplar.

Millions of people worldwide have disabling hearing loss because one of their genes generates an incorrect version of some specific protein the ear requires for hearing. In many of these cases, delivering the correct version of the gene to a specific target cell within the inner ear has the potential to restore cochlear function to enable high-acuity physiologic hearing. Purpose: In this review, we outline our strategy for the development of genetic medicines with the potential to treat hearing loss. We will use the example of otoferlin gene (OTOF)-mediated hearing loss, a sensorineural hearing loss due to autosomal recessive mutations of the OTOF gene.

Dopaminergic innervation of the mouse inner ear: evidence for a separate cytochemical group of cochlear efferent fibers.

Immunostaining mouse cochleas for tyrosine hydroxylase (TH) and dopamine beta-hydroxylase suggests that there is a rich adrenergic innervation throughout the auditory nerve trunk and a small dopaminergic innervation of the sensory cell areas. Surgical cuts in the brainstem confirm these dopaminergic fibers as part of the olivocochlear efferent bundle. Within the sensory epithelium, TH-positive terminals are seen only in the inner hair cell area, where they intermingle with other olivocochlear terminals expressing cholinergic markers (vesicular acetylcholine transporter; VAT). Double immunostaining suggests little colocalization of TH and VAT; quantification of terminal volumes suggests that TH-positive fibers constitute only 10-20% of the efferent innervation of the inner hair cell area. Immunostaining of mouse brainstem revealed a small population of TH-positive cells in and around the lateral superior olive. Consistent with cochlear projections, double staining for the cholinergic marker acetylcholinesterase suggested that TH-positive somata are not cholinergic and vice versa. All observations are consistent with the view that a small dopaminergic subgroup of lateral olivocochlear neurons 1) projects to the inner hair cell area, 2) is distinct from the larger cholinergic group projecting there, and 3) may correspond to lateral olivocochlear "shell" neurons described by others (Warr et al. [1997] Hear. Res 108:89-111).

Formulations for trans-tympanic antibiotic delivery.

We have developed a drug delivery system for prolonged trans-tympanic antibiotic delivery from a single dose administration. Increased permeability to ciprofloxacin of the intact tympanic membrane (TM) was achieved by chemical permeation enhancers (CPEs--bupivacaine, limonene, sodium dodecyl sulfate); this was also seen by CPEs contained within a hydrogel (poloxamer 407) to maintain the formulation at the TM. The CPE-hydrogel formulation had minimal effects on auditory thresholds and tissue response in vivo. CPE-hydrogel formulations have potential for ototopical delivery of ciprofloxacin for the treatment of acute otitis media (AOM) and other middle ear diseases.

Effect of chemical permeation enhancers on nerve blockade.

Chemical permeation enhancers (CPEs) have the potential to improve access of local anesthetics to the nerve, thereby improving nerve block performance. We assessed the effects of six CPEs on nerve blockade from tetrodotoxin (TTX) and from bupivacaine. Each of the six surfactants, representing three CPE subgroups (anionic, cationic, and nonionic surfactants) was coinjected with TTX or bupivacaine at the sciatic nerve of Sprague-Dawley rats. Myotoxicity of CPEs, alone and with TTX, was assessed in vitro in C2C12 myotubes and in vivo via histological analysis. All enhancers produced marked concentration-dependent improvements in the frequency and duration of block with TTX but not bupivacaine. An in vitro toxicity assay showed a wide range of CPE myotoxicity, but in vivo histological assessment showed no signs of muscle or nerve damage at concentrations of CPEs that produced a half-maximal increase in the duration of block of TTX (except in the case of the cationic surfactant DDAB). This study demonstrates that CPEs can provide marked prolongation of nerve blockade from TTX but not bupivacaine, without apparent local tissue toxicity. These results may enhance the clinical applicability of TTX for prolonged-duration local anesthesia.