Retinal Gene Therapy for Usher Syndrome: Current Developments, Challenges, and Perspectives.

Abstract

Usher syndrome (USH) represents a group of genetically heterogenous autosomal recessive disorders, characterized by combined vision and sensorineural hearing loss, and in some cases vestibular dysfunction.1–3 It primarily affects the light-sensitive photoreceptor cells in the retina and the auditory hair cells in the cochlea. Vision loss in all cases is progressive, and manifests as retinitis pigmentosa (RP), characterized by the gradual degeneration of the photoreceptor cells.4 Peripheral rod function is lost first, leading to night blindness and constricted visual fields, followed by the death of cones and severe visual impairment. There are currently no drugs or biological therapies proven to be effective in treating USH syndrome. Cochlear implantation, which bypasses the damaged hair cells and stimulates the primary auditory neurons directly, is an effective approach to alleviate the hearing impairment in patients. However, there is no remedy for the progressive loss of the photoreceptor neurons in the retina, and thus a critical unmet need exists to develop therapeutic strategies to prevent blindness in USH syndrome. The generation of animal models that faithfully mimic the human USH disorder, the timing of gene therapy interventions, the identification of correct target cells and patient selection are key factors in successfully reaching this goal. USH syndrome was named after Dr Charles Howard Usher, a Scottish ophthalmologist who described 69 patients with the disease in 1914.5 As is commonly the case in medicine, the disease was first described much earlier (1858) in brothers suffering from blindness and deafness by von Grafe6 and was further characterized by his student, Liebreich,7 3 years later in a larger population of patients. From its discovery until recently, USH syndrome and its subgroups were delineated by clinical characteristics.8 Now the disease is recognized by symptomology and classified by genotyping. To date, there are at least 10 causative genes that are associated with USH syndrome, summarized in previous reviews.1,2,9 In general, based on its diverse clinical symptoms, particularly the onset and severity of the sensorineural hearing impairment and the presence of vestibular dysfunction, USH syndrome is grouped into 3 clinical subtypes. Type I disease is associated with severe-to-profound prelingual hearing loss, vestibular abnormalities and prepubescent onset of RP symptoms.10,11 USH1 results from mutations in MYO7A (USH1B), HARMONIN (USH1C), CDH23 (USH1D), PCDH15 (USH1F), SANS (USH1G), and CIB2 (USH1J). Patients with type II disease display moderate-to-severe congenital hearing loss without vestibular dysfunction, and account for >50% of all USH cases. Their RP symptoms generally begin in the second decade.12 USH2 is caused by mutations in USH2A (USH2A), ADGRV1 (USH2C), and WHRN (USH2D). Patients with type III disease caused by mutations in CLRN1 (USH3A) gene have postlingual progressive hearing loss, variable vestibular dysfunction, and variable onset of RP that rapidly progresses to legal blindness by the fourth decade of life.13–17 Mutations in the HARS gene (Histidyl-tRNA synthetase) are associated with the ultra-rare form USH3B.18,19 Other genes have been described in cases of atypical USH, including: CEP250 and CEP78, encoding members of the CEP family of centrosome-associated proteins; ESPN, encoding the F-actin cross-linker espin; ARSG, encoding the arylsulfatase G enzyme.20