Anti-clarin-1 AAV-delivered ribozyme induced apoptosis in the mouse cochlea

Advances and challenges in adeno-associated viral inner-ear gene therapy for sensorineural hearing loss

There is growing attention and effort focused on treating the root cause of sensorineural hearing loss rather than managing associated secondary characteristic features. With recent substantial advances in understanding sensorineural hearing-loss mechanisms, gene delivery has emerged as a promising strategy for the biological treatment of hearing loss associated with genetic dysfunction. There are several successful and promising proof-of-principle examples of transgene deliveries in animal models; however, there remains substantial further progress to be made in these avenues before realizing their clinical application in humans. Herein, we review different aspects of development, ongoing preclinical studies, and challenges to the clinical transition of transgene delivery of the inner ear toward the restoration of lost auditory and vestibular function.

Clarin-2 is essential for hearing by maintaining stereocilia integrity and function

Hearing relies on mechanically gated ion channels present in the actin-rich stereocilia bundles at the apical surface of cochlear hair cells. Our knowledge of the mechanisms underlying the formation and maintenance of the sound-receptive structure is limited. Utilizing a large-scale forward genetic screen in mice, genome mapping and gene complementation tests, we identified Clrn2 as a new deafness gene. The Clrn2clarinet/clarinet mice (p.Trp4* mutation) exhibit a progressive, early-onset hearing loss, with no overt retinal deficits. Utilizing data from the UK Biobank study, we could show that CLRN2 is involved in human non-syndromic progressive hearing loss. Our in-depth morphological, molecular and functional investigations establish that while it is not required for initial formation of cochlear sensory hair cell stereocilia bundles, clarin-2 is critical for maintaining normal bundle integrity and functioning. In the differentiating hair bundles, lack of clarin-2 leads to loss of mechano-electrical transduction, followed by selective progressive loss of the transducing stereocilia. Together, our findings demonstrate a key role for clarin-2 in mammalian hearing, providing insights into the interplay between mechano-electrical transduction and stereocilia maintenance.

Emerging Gene Therapies for Genetic Hearing Loss

Gene therapy, or the treatment of human disease using genetic material, for inner ear dysfunction is coming of age. Recent progress in developing gene therapy treatments for genetic hearing loss has demonstrated tantalizing proof-of-principle in animal models. While successful translation of this progress into treatments for humans awaits, there is growing interest from patients, scientists, clinicians, and industry. Nonetheless, it is clear that a number of hurdles remain, and expectations for total restoration of auditory function should remain tempered until these challenges have been overcome. Here, we review progress, prospects, and challenges for gene therapy in the inner ear. We focus on technical aspects, including routes of gene delivery to the inner ear, choice of vectors, promoters, inner ear targets, therapeutic strategies, preliminary success stories, and points to consider for translating of these successes to the clinic.

The cone-dominant retina and the inner ear of zebrafish express the ortholog of CLRN1, the causative gene of human Usher syndrome type 3A

Clarin-1 (CLRN1) is the causative gene in Usher syndrome type 3A, an autosomal recessive disorder characterized by progressive vision and hearing loss. CLRN1 encodes Clarin-1, a glycoprotein with homology to the tetraspanin family of proteins. Previous cell culture studies suggest that Clarin-1 localizes to the plasma membrane and interacts with the cytoskeleton. Mouse models demonstrate a role for the protein in mechanosensory hair bundle integrity, but the function of Clarin-1 in hearing remains unclear. Even less is known of its role in vision, because the Clrn1 knockout mouse does not exhibit a retinal phenotype and expression studies in murine retinas have provided conflicting results. Here, we describe cloning and expression analysis of the zebrafish clrn1 gene, and report protein localization of Clarin-1 in auditory and visual cells from embryonic through adult stages. We detect clrn1 transcripts as early as 24h post-fertilization, and expression is maintained through adulthood. In situ hybridization experiments show clrn1 transcripts enriched in mechanosensory hair cells and supporting cells of the inner ear and lateral line organ, photoreceptors, and cells of the inner retina. In mechanosensory hair cells, Clarin-1 is polarized to the apical cell body and the synapses. In the retina, Clarin-1 localizes to lateral cell contacts between photoreceptors and is associated with the outer limiting membrane and subapical processes emanating from Müller glial cells. We also find Clarin-1 protein in the outer plexiform, inner nuclear and ganglion cell layers of the retina. Given the importance of Clarin-1 function in the human retina, it is imperative to find an animal model with a comparable requirement. Our data provide a foundation for exploring the role of Clarin-1 in retinal cell function and survival in a diurnal, cone-dominant species.

Mouse otocyst transuterine gene transfer restores hearing in mice with connexin 30 deletion-associated hearing loss

Although numerous causative genes for hereditary hearing loss have been identified, there are no fundamental treatments for this condition. Herein, we describe a novel potential treatment for genetic hearing loss. Because mutations or deletions in the connexin (Cx) genes are common causes of profound congenital hearing loss in both humans and mice, we investigated whether gene supplementation therapy using the wild-type Cx gene could cure hearing loss. We first generated inner ear-specific connexin 30 (Cx30)-deficient mice via the transuterine transfer of Cx30-targeted short hairpin RNA (shRNA-Cx30) into otocysts. The inner ear-specific Cx30-deficient mice mimicked homozygous Cx30-deficient mice both histologically and physiologically. Subsequently, we cotransfected the shRNA-Cx30 and the wild-type Cx30 gene. The cotransfected mice exhibited Cx30 expression in the cochleae and displayed normal auditory functions. Next, we performed the transuterine transfer of the wild-type Cx30 gene into the otocysts of homozygous Cx30-deficient mice, thereby rescuing the lack of Cx30 expression in the cochleae and restoring auditory functioning. These results demonstrate that supplementation therapy with wild-type genes can restore postnatal auditory functioning. Moreover, this is the first report to show that Cx-related genetic hearing loss is treatable by in vivo gene therapy.

Effects of p27Kip1- and p53- shRNAs on kanamycin damaged mouse cochlea

AIM: To study the effects of adeno-associated virus (AAV) delivered short hairpin RNAs (shRNAs) on adult CD-1 mouse cochlea damaged by aminoglycoside antibiotic kanamycin.

METHODS: Three different shRNAs were designed (p27 ^Kip1, p53 and p27 ^Kip1+p53) and tested in COS cells. A total of 20 adult CD-1 mice were used in the experiment. Mice were divided into five different groups (four animals/group) depending on the AAV-shRNA construct they received and whether they received kanamycin or not. Saline and AAV-EGFP injected animals were used as controls. All constructs were injected through the round window membrane (RWM) into the cochlea. Cochleae were harvested after 1 mo. Apoptosis was detected with Tunel labeling from paraffin-embedded cochlear tissue sections.

RESULTS: AAV2/2-p27 ^Kip1-shRNA and AAV2/2-p53-shRNA were tested in COS cells. Western blotting analysis confirmed that both constructs silenced their target genes effectively in the cell culture. AAV2/2-shRNA constructs were injected into the cochlea of CD-1 mice through the intact RWM. Cotransduction of individual AAV2/2-shRNAs with AAV2/2-EGFP resulted in EGFP expression in the organ of Corti. Kanamycin treatment had no effect on the expression pattern of the EGFP. AAV2/2-shRNA treated mice (either with p53 or p27^Kip1and p53 together) showed fewer apoptotic hair cells in the cochlea than the control group (P < 0.05; AAV2/2-p53-shRNA vs saline P = 0.00014; AAV2/2-p27+p53-shRNA vs saline P = 0.0011). AAV2/2-p27-shRNA injected cochleae showed no significant difference in the number of apoptotic cells when compared to the saline injected cochleae.

CONCLUSION: Silencing of p53 protein in the kanamycin treated ears may decrease cell death in the organ of Corti.

Localization and expression of clarin-1, the Clrn1 gene product, in auditory hair cells and photoreceptors

The Usher syndrome 3A (CLRN1) gene encodes clarin-1, which is a member of the tetraspanin family of transmembrane proteins. Although identified more than 6 years ago, little is known about its localization or function in the eye and ear. We developed a polyclonal antibody that react with all clarin-1 isoforms and used it to characterize protein expression in cochlea and retina. In the cochlea, we observe clarin-1expression in the stereocilia of P0 mice, and in synaptic terminals present at the base of the auditory hair cells from E18 to P6. In the retina, clarin-1 localizes to the connecting cilia, inner segment of photoreceptors and to the ribbon synapses. RT-PCR from P0 cochlea and P28 retina show mRNAs encoding only isoforms 2 and 3. Western blots show that only isoform 2 is present in protein extracts from these same tissues. We examined clarin-1 expression in the immortomouse-derived hair cell line UB/OC-1. Only isoform 2 is expressed in UB/OC-1 at both mRNA and protein levels, suggesting this isoform is biologically relevant to hair cell function. The protein co-localizes with microtubules and post-transgolgi vesicles. The subcellular localization of clarin-1 in hair cells and photoreceptors suggests it functions at both the basal and apical poles of neurosensoriepithelia.

HOX-GFP and WOX-GFP lentivirus vectors for inner ear gene transfer

CONCLUSION

GFP transgene was expressed in the lining cells of the perilymphatic space. Lentivirus vectors are safe and cause only minimal inflammatory reaction. Transgene products can be delivered into the perilymph by utilizing lentivirus vectors.

OBJECTIVES

To analyze the efficiency and safety of lentiviral vectors HOX-GFP and WOX-GFP in intracochlear gene transfer.

MATERIALS AND METHODS

Lentivirus vectors were tested for their transduction efficiency in vivo in CD-1 mice. Half of the animals were pretreated with kanamycin. Lentivirus vector or saline (1 microl) was injected into the inner ear. All the animals were sacrificed 14 days after the surgery and the cochleae and selected organs were analyzed immunohistochemically.

RESULTS

HOX-GFP and WOX-GFP expression was restricted to the lining cells of the scala tympani and scala vestibuli. No GFP expression was seen in the organ of Corti or the spiral ganglion. Aminoglycoside treatment had no effect on the expression of these vectors. The distant spread of lentivirus vectors was minimal; only the liver of one animal showed some GFP expression. Inflammatory reaction caused by these vectors was mild. Few inflammatory cells were found in the perilymphatic space of the cochlea and in the vestibular organ.

AAV vectors for RNA-based modulation of gene expression

At the post-transcriptional level, gene expression is largely regulated through a network of molecular machines that regulate pre-mRNA maturation integrity, transport, translation and degradation. These processes are based on the formation of nucleoprotein complexes and require the recognition of sequence motifs on the RNA. By masking these targets with complementary RNA sequences forming Watson-Crick base pairing, it is possible to efficiently and specifically impact on the cell phenotype, or to compensate the deleterious effect of mutations. Here we review how the adeno-associated virus technology is being exploited for expressing non-coding RNAs in tissues such as the brain, muscle or liver, in functional genomic studies as well as for the development of novel therapeutic strategies.