Retinitis Pigmentosa

Abstract

Retinitis pigmentosa (RP) is the name given to a heterogeneous group of genetic diseases sharing a common phenotype. Traditionally, RP is a progressive rod–cone dystrophy characterized by nyctalopia, peripheral visual field loss, and loss of the rod electroretinogram (ERG) response. In the past 20 years, the number of genes known to cause retinal diseases has grown to over 172 (Source RetNet: http://www.sph.uth.tmc.edu/Retnet/sum-dis.htm#D-graph). Ironically, as our knowledge of the genetic basis for RP has expanded, our understanding has become both clearer and more muddled. Prior to the identification of specific genes, the classification of RP was based solely on the pattern of inheritance, age of onset, and presence of particular clinical features. Our greater understanding of the genetic basis for RP has illustrated the complexity of incorporating genetic information with our prior clinical classifications. We know now for example that seemingly distinct clinical diseases, such as RP and pattern dystrophy, may be different phenotypes of the same mutated gene, RDS/peripherin, even among individuals in the same pedigree. Likewise, depending on the specific mutation, the same gene can result in different phenotypes or patterns of inheritance. For example, mutations of the rhodopsin gene can produce congenital stationary night blindness (CSNB), autosomal dominant RP (adRP), or autosomal recessive RP (arRP). Finally, although most of the different types of RP are monogenic, patients who are compound heterozygous for two independent genes, like RDS/peripherin and ROM, may develop a digenic form of RP. These examples illustrate the complexity of attempting to unify a clinical classification with a genetic diagnosis. For example, how would one classify a patient homozygous for ABCA4 mutations but showing the clinical features of RP? Does this patient have RP or Stargardt disease? The answer to this conundrum is that genetic testing will supplement, not supplant, clinical diagnosis. As genetic testing becomes more available and comprehensive, new genetic information will need to be incorporated within the context of clinical diagnosis. The rapid progress in the field of genetics, or science in general, has accelerated how quickly a textbook becomes out of date. With this in mind, the goals of this chapter are to summarize the clinical and electrophysiologic features of RP, to summarize the growing list of genes causing nonsyndromic RP, and to review in more detail the most common genes associated with adRP, arRP, and X-linked RP (XLRP).