Zebrafish models of human Usher syndrome

Gene Discovery:

Human Usher syndrome, the most frequent cause of deaf blindness, is characterized by congenital deafness due to loss of sensory hair cells and progressive retinal degeneration. Approximately 1 in 6000 Americans has Usher syndrome. Although at least fourteen different chromosomal loci have been linked to Usher syndrome, only eleven of the genes have been identified to date. Identification of the missing Usher genes is crucial for diagnosis and patient counseling. The eleven known genes encode a surprisingly broad range of different types of proteins and the functions of these proteins are poorly understood. Our research contributes to both these areas, gene discovery and protein function.

Continued gene discovery is a critically important component of patient care and counseling. All children born in the United States are tested for hearing at birth. Of those who fail, a relatively high percentage will later develop Usher syndrome. Parents of deaf newborn children receive counseling about their options, including cochlear implants or raising the child with sign language in the deaf community. This decision could be significantly influenced by knowing that the child will later go blind, because sign language will eventually prove inadequate for effective communication. Studies of children who received cochlear implants at various ages show that early implants, by age six months, are significantly more effective for subsequent language development than implants at later ages. These observations point to a serious inadequacy in the current patient care system, because there are no known behavioral or physiological tests that can detect vision defects due to Usher syndrome at birth or in young infants. Vision loss in Usher syndrome progresses slowly and is not usually apparent until puberty or later. Genotyping is currently the only means of identifying young Usher children, and genotyping microarrays and next generation sequencing protocols are being developed for this purpose. However, genotyping can detect mutations only in the genes that are tested. Thus, it is of utmost importance to discover the remaining, as yet unknown Usher genes. Our studies in zebrafish have validated a new Usher gene (Ebermann et al., 2010) that has been added to clinical genotyping tests.

Gene Function:

The other main aspect of our Usher studies is basic research into understanding what the diverse proteins encoded by the Usher genes normally do and what goes wrong in the disease. Previous studies of Usher proteins have concentrated on their potential functions in the region of the photoreceptor connecting cilium and the stereocilia of mechanosensory hair cells in the inner ear. In hair cells, Usher proteins are thought to stabilize the growth and orientation of stereocilia during development, whereas Usher proteins in the region of the connecting cilium may contribute to the trafficking of molecular cargo from the photoreceptor inner segment to the outer segment. Usher proteins have also been localized at the ribbon synapses of these sensory cells, but their potential functions in this region were previously unknown.

We have also studied the Usher 1C gene (USH1C), which encodes a protein that has been proposed to function as a key scaffolding molecule to which most other Usher proteins can bind. Patients with mutations in USH1C exhibit a severe Usher pathology with profound congenital hearing impairment, vestibular dysfunction, and clinically appreciable retinal degeneration in childhood or early adolescence. Mouse models of USH1C, with mutations in the mouse Ush1c gene, exhibit profound deafness but fail to show notable retinal defects.

We developed two zebrafish models for USH1C, both of which exhibit a strong deafblindness phenotype at early developmental stages. Young fish with depleted ush1c function have penetrant and specific visual defects as well as compromised hair cell development resulting in hearing and balance impairment. Detailed analyses of the resulting phenotype confirmed a requirement of the Ush1c protein in hair cell morphogenesis, and have implicated Müller glial cells in supporting harmonin-mediated ribbon synapse stability and function. Usher syndrome has traditionally not been considered a developmental disease of vision. The results of our research, however, demonstrated that visual defects can be detected from the onset of vision in zebrafish with depletedush1c function, thus providing a rationale for early diagnosis of Usher syndrome visual defects in humans (Phillips et al., 2011). These studies have paved the way toward development of early diagnostic procedures for young Usher patients.

Therapies:

By analyzing interactions among the Usher proteins, we discovered that they preassmble into a multi-molecular complex at the time of there synthesis in the endoplasmic reticulum (ER). Mutations in Usher genes result in failure of the complex to form, as well as intracellular trafficking defects. This triggers the unfolded protein response leading to ER stress and cell death. Thus, ER stress may be the proximal cause of retinal and inner ear hair cell degeneration (Blanco-Sanchéz et al., 2014).

Recent studies of other neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and some retinopathies have implicated ER stress as a probable cause of cell death. The link between ER stress and apoptosis raises the possibility that therapeutics being developed for the treatment of these other neurodegenerative diseases will be helpful in managing the progression of symptoms in Usher syndrome patients. Although hearing defects are typically congenital in Usher syndrome, due to defects in the mechanoreceptors, mechanosensory hair cells ultimately die. In all forms of Usher syndrome, vision loss is progressive and photoreceptors degenerate over decades. Treatments that delay or reduce cell loss will provide time to patients, while therapies that address the defects can be developed and applied to the remaining, non-degenerating cell populations.