Non-syndromic retinal ciliopathies: translating gene discovery into therapy

Alejandro Estrada-Cuzcano 0 1 2 Ronald Roepman 0 1 2 Frans P.M. Cremers 1 2 Anneke I. den Hollander 0 1 2 3 Dorus A. Mans 1 2 0 Institute for Genetic and Metabolic Diseases 1 Nijmegen Centre for Molecular Life Sciences 2 Department of Human Genetics 3 Department of Ophthalmology, Radboud University Nijmegen Medical Centre , Nijmegen, The Netherlands Homozygosity mapping and exome sequencing have accelerated the discovery of gene mutations and modifier alleles implicated in inherited retinal degeneration in humans. To date, 158 genes have been found to be mutated in individuals with retinal dystrophies. Approximately one-third of the gene defects underlying retinal degeneration affect the structure and/or function of the 'connecting cilium' in photoreceptors. This structure corresponds to the transition zone of a prototypic cilium, a region with increasing relevance for ciliary homeostasis. The connecting cilium connects the inner and outer segments of the photoreceptor, mediating bi-directional transport of phototransducing proteins required for vision. In fact, the outer segment, connecting cilium and associated basal body, forms a highly specialized sensory cilium, fully dedicated to photoreception and subsequent signal transduction to the brain. At least 21 genes that encode ciliary proteins are implicated in non-syndromic retinal dystrophies such as cone dystrophy, cone - rod dystrophy, Leber congenital amaurosis (LCA), macular degeneration or retinitis pigmentosa (RP). The generation and characterization of vertebrate retinal ciliopathy animal models have revealed insights into the molecular disease mechanism which are indispensable for the development and evaluation of therapeutic strategies. Gene augmentation therapy has proven to be safe and successful in restoring long-term sight in mice, dogs and humans suffering from LCA or RP. Here, we present a comprehensive overview of the genes, mutations and modifier alleles involved in non-syndromic retinal ciliopathies, review the progress in dissecting the associated retinal disease mechanisms and evaluate gene augmentation approaches to antagonize retinal degeneration in these ciliopathies. - INTRODUCTION Up to just a few years ago, identifying the genetic basis of an inherited human disease was a long journey, requiring many years of work in the ascertainment of families for linkage analysis, followed by fine mapping of the locus and, finally, sequencing of candidate genes one by one until the likely causative gene was identified. Until now, at least 158 genes (RetNet; http://www.sph.uth.tmc.edu/retnet/) have been associated with several types of inherited retinal dystrophies, and the arrival of new genetic technologies such as singlenucleotide polymorphism (SNP) microarrays and nextgeneration sequencing (NGS) have increased the pace of disease gene identification. For example, during the past year (2011 2012), the use of SNP arrays and/or NGS have enabled the identification of 10 new retinal dystrophy genes. Remarkably, four of these genes [C8orf37 (1), IFT140 (2), MAK (3,4) and TMEM237 (5)] encode ciliary proteins, which underlines the importance of disrupted ciliary processes in the pathogenesis of retinal dystrophies. The primary cilium functions as the antenna of the cell, allowing the effective transduction of various forms of sensory information from the extracellular environment. The cilium is present on many different types of cells throughout the human body, and the importance of this organelle in biology and in medicine has been highlighted by the growing number of genetic diseases that have been linked to primary cilium defects. The connecting cilium at the base of the photoreceptor outer segment corresponds to the transition zone of a prototypic cilium, a region with increasing relevance for ciliary homeostasis. The connecting cilium connects the inner and outer segments of the photoreceptor cell, mediating bi-directional transport of phototransducing proteins required for vision. In fact, the outer segment, connecting cilium and associated basal body, forms a highly specialized sensory cilium, fully dedicated to photoreception and subsequent signal transduction to the brain. Ciliary databases, such as CiliaProteome (6) or Cildb (7), have integrated bioinformatic, genomic and proteomic ciliary data from numerous highthroughput studies. Of the 158 identified genes implicated in retinal dystrophies, at least 53 (33%) encode proteins that localize to the cilium and/or basal body of the cell. Mutations in these ciliary genes can lead to a wide range of clinical features involving the eye, kidney, heart, liver, central nervous system, adipose tissue, gonads and bones. Various syndromes have been defined based on the combination of clinical features involving these organs (8,9). Syndromic ciliopathies include Alstrom syndrome (ALMS, MIM 203800), Bardet Biedl syndrome (BBS, MIM 209900), Ellis van Creveld syndrome (EVC, MIM 225500), asphyxiating thoracic dystrophy (ATD, MIM 208500), Joubert syndrome (JBTS, MIM 213300), McKusick Kaufman syndrome (MKKS, MIM 236700), Meckel Gruber syndrome (MKS, MIM 249000), orofaciodigital syndrome type 1 (OFD1, MIM 311200), Senior Lken syndrome (SLSN, MIM 266900) and Usher syndrome type 2A (USH2A, MIM 276901). These syndromes are genetically heterogeneous, involving mutations in a large number of genes. They can show considerable clinical and genetic overlap, as mutations in a number of these genes can cause various syndromes. Retinal dystrophy can present as one of the clinical features of these syndromes, but is more often present as an isolated disease without additional features. One-third of these nonsyndromic retinal dystrophies involve a defect in a ciliary protein. This group of retinal dystrophies, the non-syndromic retinal ciliopathies, is the scope of this review. NON-SYNDROMIC RETINAL CILIOPATHIES Non-syndromic retinal ciliopathies can have various clinical presentations, depending on the type of photoreceptors that are primarily affected by the disease. Retinitis pigmentosa (RP, MIM 268000) is the most common inherited retinal degeneration with an estimated worldwide prevalence of 1:4,000 individuals (10). RP is initially characterized by rod photoreceptor dysfunction, giving rise to night blindness, followed by progressive midperipheral vision loss, tunnel vision, and at advanced stages when cones also are affected, eventually can progress to blindness. The disease is genetically heterogeneous and displays all Mendelian patterns of inheritance. In addition, some cases have been linked to mitochondrial mutations or displayed digenic inheritance (11,12). Cone dystrophy (CD) is a progressive cone disorder with an estimated prevalence of 1:30,000 to 1:40,000 (13). Patients have normal cone function initially, but show visual loss and color vision disturbance in the first or second decade (14). Macular abnormalities can be present, and the optic nerv (...truncated)