Introduction: The process of hearing involves a series of events. The energy of sound is captured by the outer ear and further transferred through the external auditory canal to the middle ear. In the middle ear, sound waves are converted into movements of the tympanic membrane and the ossicles, thereby amplifying the pressure so that it is sufficient to cause movement of the cochlear fluid. The traveling wave within the cochlea leads to depolarization of the inner ear hair cells that, in turn, release the neurotransmitter glutamate. Thereby, the spiral ganglion neurons are activated to transfer the signals via the auditory pathway to the primary auditory cortex. This complex combination of mechanosensory and physiological mechanisms involves many distinct types of cells, the function of which are impacted by numerous proteins, including those involved in ion channel activity, signal transduction and transcription.Deafness or hearing loss in many cases is inherited in families and in some cases the gene causing the disease is known and the ability to predict, prevent or prevent new cases in the family with genetic tests is provided.Genetic deafness is divided into two main types, syndromic and non-syndromic. In the first type (syndromic), in addition to deafness or hearing loss, there are other symptoms and disorders. In the second or non-syndromic case, deafness or hearing loss is the only sign of the disease and is not accompanied by other complications and symptoms. Of course, it should be noted that the inability to speak (dumbness) is the result of deafness and is seen in both groups of patients and the cause is the lack of learning speech skills through listening. The earlier deafness is diagnosed and treated, the less likely it is to be able to speak.Most cases of non-syndromic deafness are permanent and result from damage to the inner ear. The inner ear consists of three parts: the cochlea, the auditory nerves, and the semicircles of balance. Deafness associated with the inner ear is also called sensory nerve.Deafness can also be caused by middle ear disorders, in which case it is called conductive deafness. The middle ear is the location of the hase bones, which are very delicate and thin, which are responsible for transmitting sound.Some deficiencies such as the DFN3 form are associated with disorders in both mid-middle and interior ears. This is a case of a mixed hearing impairment. The deaf and deficit of the hearing is complicated and so many genes have been reported by the researchers.Some of these genes and their performance in deafness are not completely clear. Some genes only create a deaf of denial and some other types. Some of these genes create a syndrome or non-syndromeic type. The genetic deafness in some families is still uncertain, and genetic science has not yet been able to identify all deafness factors.GJB2 gene mutations are a common factor in non-syndrometic prediction derivations. This gene produces protein-like protein 26. The next gene called GJB6 also produces the protein-protein 30th gene and gene mutations in both of these genes cause deafness.Another type of deafness, DNF3, is caused by the mutation in the POU3F4 gene-dependent gene, and it can not be a good movement of one middle ear bones.The mutations of mitochondrial genes (cell metabolism organs), such as the MT-RNR1 and MT-TS1 gene, have been reported in deaf and low hearing patients. Especially with mutations in the MT-RNR1 gene exposed to deafness due to the use of some antibiotics.
Methods: Gene therapy is a medical field which focuses on the genetic modification of cells to produce a therapeutic effect or the treatment of disease by repairing or reconstructing defective genetic material.The first attempt at modifying human DNA was performed in 1980 by Martin Cline, but the first successful nuclear gene transfer in humans, approved by the National Institutes of Health, was performed in May 1989.The first therapeutic use of gene transfer as well as the first direct insertion of human DNA into the nuclear genome was performed by French Anderson in a trial starting in September 1990. It is thought to be able to cure many genetic disorders or treat them over time.The concept of gene therapy is to fix a genetic problem at its source. If, for instance, in an (usually recessively) inherited disease a mutation in a certain gene results in the production of a dysfunctional protein, gene therapy could be used to deliver a copy of this gene that does not contain the deleterious mutation, and thereby produces a functional protein. This strategy is referred to as gene replacement therapy.
The delivery of DNA into cells can be accomplished by multiple methods. The two major classes are recombinant viruses (sometimes called biological nanoparticles or viral vectors) and naked DNA or DNA complexes (non-viral methods).
Viruses :
In order to replicate, viruses introduce their genetic material into the host cell, tricking the host's cellular machinery into using it as blueprints for viral proteins. Retroviruses go a stage further by having their genetic material copied into the genome of the host cell. Scientists exploit this by substituting a virus's genetic material with therapeutic DNA. (The term 'DNA' may be an oversimplification, as some viruses contain RNA, and gene therapy could take this form as well.) A number of viruses have been used for human gene therapy, including retroviruses, adenoviruses, herpes simplex, vaccinia, and adeno-associated virus. Like the genetic material (DNA or RNA) in viruses, therapeutic DNA can be designed to simply serve as a temporary blueprint that is degraded naturally or (at least theoretically) to enter the host's genome, becoming a permanent part of the host's DNA in infected cells.
Non-viral methods present certain advantages over viral methods, such as large scale production and low host immunogenicity. However, non-viral methods initially produced lower levels of transfection and gene expression, and thus lower therapeutic efficacy. Newer technologies offer promise of solving these problems, with the advent of increased cell-specific targeting and subcellular trafficking control.
Non-viral :
Methods for non-viral gene therapy include the injection of naked DNA, electroporation, the gene gun, sonoporation, magnetofection, the use of oligonucleotides, lipoplexes, dendrimers, and inorganic nanoparticles.
Vector options to introduce gene therapeutics into the inner ear
Generally, different concepts are available for gene therapy of hereditary hearing loss. To substitute for the function of a defective gene, an intact copy can be introduced into the relevant cells of the inner ear. Gene suppression, for example through expression of an shRNA that targets the transcript of the mutated gene to prevent its translation, can serve to eliminate dominant-negative effects that may interfere with proper cellular function even if an intact gene copy is provided. Finally, gene correction utilizing gene editing based on designer nuclease systems allows the specific removal of PV, thereby also keeping the natural regulation of gene expression via the physiologic promoter and chromatin environment.Common to all different gene therapy strategies is the requirement for efficient transfer technologies to equip the target cells with expression units for the intact gene or miRNA, for shRNAs, or for the gene editing components. The complex 3D architecture and defined arrangement of the specific cell types inside the cochlea (see Figure 1) excludes ex-vivo cell manipulation and restricts treatment options to in vivo delivery systems. This is in contrast to other organ systems, such as the hematopoietic system, where stem cells can be extracted and re-infused into the patient upon ex-vivo gene therapy. Viruses have evolutionarily co-evolved with their hosts and, as such, have developed specialized mechanisms to enter their target species and cell type(s). Therefore, viral vectors appear to be ideal vehicles to deliver genetic information to the cochlea. Furthermore, the different compartments in the cochlea are filled with lymph, which allows for the distribution of injected viral vectors throughout the cochlea via this intracochlear fluid, while spread to other organs is theoretically limited to the enclosed organ system of the inner ear.Several parameters are important for the success of viral vector-based gene therapy approaches in the cochlea: (1) The vector volume that can be administered is limited. The outer wall of the inner ear is rigid, so that injection of too high vector volumes would increase the pressure and cause hydraulic trauma. Standard injection volumes are 1 µL in mice and are estimated to be 10–30 µL in humans. Thus, high-titer vector preparations are required to allow delivery in a small volume. One advantage for gene therapy application to the inner ear is that the total number of cells present in the cochlea is low as compared to other gene therapy-relevant organ systems, so that a comparably low number of vector particles should suffice to achieve clinical benefit. (2) The endocochlear potential as a result of the different ion compositions of perilymph and endolymph is an important prerequisite for proper functioning of the hearing cascade. Thus, the buffer used to deliver vector preparations should be compatible with inner ear fluids and cell types. (3) Optimal delivery routes to administer viral vectors to the cochlea need to be investigated (Figure 3), and vector distribution and dissemination from the site of injection need to be characterized. (4) Pre-existing immunity to vector components, such as the capsid, or to transferred genes might limit gene transfer and/or expression efficiency, or cause local inflammation.Currently, three main viral vector systems have emerged for inner ear gene therapy: (1) lentiviral (LV) vectors, (2) adenoviral vectors (AdV), and (3) adeno-associated virus (AAV) vectors. Each of these were tested in in vitro transduction experiments using cell lines, dissociated primary tissue and cochlear explants and were also characterized in vivo in rodent models. Due to space limitations, we will primarily focus on LV and AAV vectors.In contrast to AdV and AAV vector platforms, LV vectors stably anchor their genomic information into the host cell’s genome. While this feature is of great advantage when targeting dividing cells – guaranteeing stable, long-term gene addition and transmission to daughter cells – non-integrating vectors have a superior safety profile. Many of the specialized and treatment relevant otic cell types, such as hair cells (HC) and spiral ganglion neurons (SGN), are post-mitotic and thus compatible with non-integrating vector systems. Nevertheless, although naturally integration-competent, LV vectors can be rendered integration-deficient, e. g. upon catalytic inactivation of the viral integrase enzyme, creating so-called non-integrating LV vectors. Although so far only the integrating LV vectors have been tested in the context of otic gene therapy settings.
Results: After identifying the defective gene that led to the deafness, using a suitable vector that carries the healthy gene, injects it into the defective cells to correct the defective gene.Gene therapy can involve not only the insertion of a transgene through efficient viral transduction, but also silencing of a dominant negative allele through miRNA or siRNAs. Off-target effects will be minimized through enhancing the specificity of therapy. Next-generation CRISPR-Cas systems will be harnessed for precise disruption and editing of DNA or RNA for each patient.
Conclusion: Gene therapy is an emerging therapy in the treatment of genetic diseases. This article describes how deafness can be partially controlled using gene therapy. Of course, this requires time to use the right vector to transfer a healthy gene instead of a defective one.Hearing loss is an attractive model for gene therapy approaches as multiple causative monogenetic defects have been identified.The inner ear can be locally targeted with established surgical approaches.Gene replacement and gene correction, e. g. gene editing strategies, can be used to treat hearing loss due to recessive and dominant gene variants.