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Potential Treatments for Genetic Hearing Loss in Humans: Current Conundrums

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Potential Treatments for Genetic Hearing Loss in Humans: Current Conundrums Gene Therapy (2015) 22, 603–609 © 2015 Macmillan Publishers Limited All rights reserved 0969-7128/15 www.nature.com/gt REVIEW Potential treatments for genetic hearing loss in humans: current conundrums R Minoda, T Miwa, M Ise and H Takeda Genetic defects are a major cause of hearing loss in newborns. Consequently, hearing loss has a profound negative impact on human daily living. Numerous causative genes for genetic hearing loss have been identified. However, presently, there are no truly curative treatments for this condition. There have been several recent reports on successful treatments in mice using embryonic gene therapy, neonatal gene therapy and neonatal antisense oligonucleotide therapy. Herein, we describe state-of-the-art research on genetic hearing loss treatment through gene therapy and discuss the obstacles to overcome in curative treatments of genetic hearing loss in humans. Gene Therapy (2015) 22, 603–609; doi:10.1038/gt.2015.27 INTRODUCTION NORMAL INNER EAR ANATOMY AND DEVELOPMENT Sensorineural hearing loss (SNHL) is the most common congenital The inner ear has two basic functions: hearing, which occurs in the disease in humans. The incidences of profound SNHL at birth in cochlea, and balancing, which occurs in the semicircular canals the United Kingdom and United States are 133 per 100,000 and and vestibule. The cochlea is divided into three compartments: 186 per 100,000 births, respectively.1 A genetic defect is the most the scala vestibule, scala tympani and scala media. The scala common cause of hearing loss at birth and in childhood. More media, a part of the endolymphatic space, contains the organ of than half of all neonates with SNHL have inherited hearing loss. Corti (OC). The OC contains three cell populations: inner hair cells Approximately 70% of hereditary hearing loss cases are non- (IHCs), outer hair cells (OHCs) and supporting cells (SCs) (Figure 1). syndromic and ~ 30% are syndromic SNHL.2,3 Hereditary forms of Hair cells have stereocilia that emerge from their apical surface. fl genetic SNHL are as follows: 80% autosomal recessive, ~ 20% Receptor potentials, generated by de ection of the stereocilia autosomal dominant, ~ 1% X linked and ⩾ 1% via mitochondrial within the IHCs, induce neurotransmitter release at the synaptic ends.15 Therefore, sound waves are transmitted via the outer and inheritance.4,5 Autosomal dominant SNHL often takes a post- middle ear to the inner ear fluid in the cochlea and transduced to lingual and progressive form, whereas autosomal recessive SNHL 5 electrical signals via IHCs. These signals are subsequently takes a prelingual form. The most common genetic cause of non- transmitted to the brain via efferent neurons and perceived syndromic SNHL is a mutation within the GJB2 gene, which 6–8 as sound. encodes connexin 26 (Cx26). The second most frequent cause The mammalian inner ear and its sensory neurons develop from of non-syndromic SNHL is a mutation in the GJB6 gene, the otic placode, a thickened patch of head ectoderm.16–18 8,9 which encodes Cx30. The most common form of syndromic Subsequently, one otocyst (per side) is formed by invagination of SNHL is Pendred syndrome, which is associated with mutations the otic placode at the level of the hindbrain at 4 weeks gestation within the solute carrier family 26 (SLC26A4) gene.10,11 Mutations in humans19 and embryonic day 9.5 (E9.5) in mice. Soon after, in mitochondrial DNA can also induce syndromic and non- formation of the otocyst and neuroblasts delaminate from the syndromic hearing loss.5,12 There are many reported causative ventral region of the otocyst. These neuroblasts will coalesce genes of genetic hearing loss in addition to those described adjacent to the developing inner ear and begin to form the above.4 statoacoustic ganglion.20 By E12.5 in mice, the positions of the Although numerous causative genes for genetic hearing loss developing sensory patches, which form from a single common – have been identified, there are no truly curative treatments for this patch in the otocyst, can be identified.20 22 The cochlear part of condition yet. At present, treatments for SNHL only include the otocyst then begins to elongate into a spiral structure. The two hearing aids and/or cochlear implants. While hearing aids and and one-half turns of the coiled cochlea are not completed until 19 cochlear implants are helpful treatments to compensate for 25 weeks gestation in humans, whereas the mouse cochlear hearing loss, they do not restore hearing to normal levels. There is duct has completed three-quarters of one turn around E13.5. still an urgent need for truly curative treatments. Recently, there have been several reports on successful treatment of genetic hearing loss caused by Cx30, vesicular glutamate transporter 3 A THEOREM OF GENETIC HEARING LOSS TREATMENTS (VGLUT3) and Usher syndrome 1c (USH1C) gene mutations, which During development, appropriate spatiotemporal control of gene are all known genetic causes of hearing loss in humans.4,13,14 expression is necessary for normal development of the inner ear. Departments of Otolaryngology-Head and Neck Surgery, Kumamoto University, Graduate School of Medicine, Kumamoto City, Kumamoto, Japan. Correspondence: Dr R Minoda, Departments of Otolaryngology-Head and Neck Surgery, Kumamoto University, Graduate School of Medicine, 1-1-1 Honjo Chuoku, Kumamoto City, Kumamoto 860-0811, Japan. E-mail: [email protected] Received 16 December 2014; revised 24 January 2015; accepted 12 February 2015; accepted article preview online 17 March 2015; advance online publication, 9 April 2015 Potential genetic hearing loss treatments R Minoda et al 604 Data from neonatal hearing screening tests have demonstrated that the majority of SNHL patients can be detected using this method.5 Indeed, hearing loss caused by Cx26 mutations, the most common genetic cause of non-syndromic SNHL, also usually presents with a congenital onset.24 Therefore, embryonic treat- ments are inevitable if the treatment must be administered before hearing loss phenotype occurrence in such congenital genetic hearing loss patients. One important issue regarding embryonic treatments is the need to treat embryonic inner ears in the maternal uterus, which is technically feasible, but not facile. Thus, experiments targeting embryonic mouse inners ears are very complex and involved. Because of this intricateness, most animal studies on genetic hearing loss treatments have not targeted embryonic inner ears, but rather neonatal mouse inner ears. When we use rodents, particularly mice, to study genetic hearing loss treatments, we need to be aware of differences in the development and maturation of auditory functions between rodents and humans. Auditory function initiates around postnatal day 13 (P13) in mice, but at 20 weeks gestation in humans.25,26 Figure 1. Transverse section image of the cochlea. The adult Even if an effective treatment is administered during the mouse mammalian cochlea is divided into three compartments: the scala neonatal period, the respective neonatal treatment would likely vestibule, scala tympani and scala media. This image represents a be ineffective in humans, as mouse neonatal treatment equates to cross-section of the scala media, which contains the OC. The OC human embryonic treatment. contains three cell populations: IHCs, OHCs and SCs. The two types When considering treatment for genetic hearing loss caused by of auditory hair cells (IHCs and OHCs) have critical roles as gain-of-function mutations, treatments in which new molecular mechanoelectrical transducers for hearing. Auditory hair cells are covered by the tectorial membrane. The stria vascularis, located in functions are suppressed through RNA interference or degradation the lateral wall of the scala media, is responsible for the secretion of of the mutated gene product would likely be successful. As K+ into the endolymph and for production of the endocochlear mentioned previously, timing is an important factor for determin- potential. ing the simplicity of a treatment and subsequent methods effective for loss-of-function mutations. This relationship in loss- of-function mutations is generally the same as those in gain-of- During this process, expression of a gene begins; subsequently, function mutations. One difference between loss-of-function and the gene expression gradually becomes more widespread, and gain-of-function mutations is that postnatal treatments may be reaches a spatiotemporal maximum. Disruption of this process by more feasible in cases of genetic SNHL caused by a gain-of- a genetic mutation during inner ear development can cause function mutation, which typically occurs postlingually;5 specifi- genetic hearing loss. There are two major classes of genetic cally, gain-of-function mutations usually do not present as hearing mutations: loss-of-function and gain-of-function. In loss-of- loss during the neonatal period. Additionally, neonatal hearing function mutations, the most common form, the protein product screening test data have demonstrated that ~ 15% of preschool of a gene is either missing, non-functional or reduced in level. children with SNHL show progressive hearing loss.27 In such These are typically recessive mutations, because a wild-type allele genetic SNHL patients who do not present with hearing loss can usually compensate for the non-functional allele. In contrast, during the neonatal period, neonatal treatment may be more the altered gene product takes on a new molecular function in feasible for
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