Histone deacetylase inhibition enhances adenoviral vector transduction in inner ear tissue

Gene transfer in inner ear cells: a challenging race

Recent advances in human genomics led to the identification of numerous defective genes causing deafness, which represent novel putative therapeutic targets. Future gene-based treatment of deafness resulting from genetic or acquired sensorineural hearing loss may include strategies ranging from gene therapy to antisense delivery. For successful development of gene therapies, a minimal requirement involves the engineering of appropriate gene carrier systems. Transfer of exogenous genetic material into the mammalian inner ear using viral or non-viral vectors has been characterized over the last decade. The nature of inner ear cells targeted, as well as the transgene expression level and duration, are highly dependent on the vector type, the route of administration and the strength of the promoter driving expression. This review summarizes and discusses recent advances in inner ear gene-transfer technologies aimed at examining gene function or identifying new treatment for inner ear disorders.

Adeno-associated virus-mediated gene delivery into the scala media of the normal and deafened adult mouse ear

Murine models are ideal for studying cochlear gene transfer, as many hearing loss-related mutations have been discovered and mapped within the mouse genome. However, because of the small size and delicate nature, the membranous labyrinth of the mouse is a challenging target for the delivery of viral vectors. To minimize injection trauma, we developed a procedure for the controlled release of adeno-associated viruses (AAVs) into the scala media of adult mice. This procedure poses minimal risk of injury to structures of the cochlea and middle ear, and allows for near-complete preservation of low and middle frequency hearing. In this study, transduction efficiency and cellular specificity of AAV vectors (serotypes 1, 2, 5, 6 and 8) were investigated in normal and drug-deafened ears. Using the cytomegalovirus promoter to drive gene expression, a variety of cell types were transduced successfully, including sensory hair cells and supporting cells, as well as cells in the auditory nerve and spiral ligament. Among all five serotypes, inner hair cells were the most effectively transduced cochlear cell type. All five serotypes of AAV vectors transduced cells of the auditory nerve, though serotype 8 was the most efficient vector for transduction. Our findings indicate that efficient AAV inoculation (via the scala media) can be performed in adult mouse ears, with hearing preservation a realistic goal. The procedure we describe may also have applications for intra-endolymphatic drug delivery in many mouse models of human deafness.

Cellular targeting for cochlear gene therapy

Gene therapy has considerable potential for the treatment of disorders of the inner ear. Many forms of inherited hearing loss have now been linked to specific locations in the genome, and for many of these the genes and specific mutations involved have been identified. This information provides the basis for therapy based on genetic approaches. However, a major obstacle to gene therapy is the targeting of therapy to the cells and the times that are required. The inner ear is a very complex organ, involving dozens of cell types that must function in a coordinated manner to result in the formation of the ear, and in hearing. Mutations that result in hearing loss can affect virtually any of these cells. Moreover, the genes involved are active during particular times, some for only brief periods of time. In order to be effective, gene therapy must be delivered to the appropriate cells, and at the appropriate times. In many cases, it must also be restricted to these cells and times. This requires methods with which to target gene therapy in space and time. Cell-specific gene promoters offer the opportunity to direct gene therapy to a desired cell type. Moreover, conditional promoters allow gene expression to be turned off and on at desired times. Theoretically, these technologies offer a mechanism by which to deliver gene therapy to any cell, at any given time. This chapter will examine the potential for such targeting to deliver gene therapy to the inner ear in a precisely controlled manner.