Hereditary Retinal Degenerations

Abstract

Hereditary retinal diseases are a group of very heterogeneous disorders involving the retinal or choroidal tissue or both. Retinal disorders are classified broadly according to the retinal layers primarily involved and whether they are stationary or progressive. Thus there are the rod-dominated diseases (such as retinitis pigmentosa (RP) and night blindness), cone-dominated disease (such as cone and cone-rod dystrophies and macular degenerations), generalized photoreceptor dystrophies (involve both types of photoreceptor), and vitreoretinal diseases (affecting the vitreous and retina). These entities are further grouped according to whether they are stationary or progressive and, within each of these categories, into syndromic and non-syndromic disorders (Berger et al. 2010). The syndromic retinal dystrophies involve other organs in addition to the retina. Genetics plays a major role in further grouping of these diseases, based on the inheritance mode (autosomal dominant, recessive, X-linked, digenic, mitochondrial, and simplex diseases) and, further, according to their molecular etiologies, depending on the genetic mutations involved. Among the so-called “monogenic” diseases, the diseases which primarily affect the photoreceptors are a predominant group. The disorders that are dealt with in the following sections include two major forms of non-syndromic retinal dystrophy—retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA)—that have overlaps in their underlying genetic bases. The aspects of genetics of RP and LCA are arranged according to the cellular pathways in which various genes function. In addition, a subsequent section on a syndromic form of RP—Usher syndrome—deals with the genetics of the different types of Usher syndrome.