Retinitis pigmentosa: unfolding its mystery

Retinitis pigmentosa affects 50,000-100,000 people in the United States and about 1.5 million people worldwide. Patients usually report impaired adaptation, night blindness, and loss of mid-peripheral visual field in adolescence. As the condition progresses, they lose far-peripheral visual field and eventually lose central vision as well. Some patients have become blind as early as age 30. The majority are legally blind by age 60, with a central visual field diameter of less than 20?. Findings on ophthalmoscopy include intraretinal pigment around the mid-peripheral retina, for which this condition is named. Histopathologic examinations of autopsy eyes with advanced stages have shown that loss of vision is due to degeneration of both rod and cone photoreceptor cells (1, 2). Retinitis pigmentosa can be detected in early life by electroretinographic testing. Patients with early stages of this disease have electroretinograms (ERGs) that are reduced in amplitude with delays in their temporal aspects (Fig. 1). ERG amplitudes become smaller as the disease progresses. Abnormal ERGs have been detected in asymptomatic children in some cases a decade before diagnostic changes are seen on routine ocular examination. Individuals, age 6 and older, with normal ERGs and a family history of retinitis pigmentosa have not been observed to develop retinitis pigmentosa at a later time (2, 3). The common forms of retinitis pigmentosa have yielded to treatment with vitamin A supplementation. In a randomized, controlled trial, the course of retinal degeneration as monitored by the ERG was slower on average among adult patients taking 15,000 international units of vitamin A daily, whereas the course appeared to be hastened by supplementation with 400 international units daily of vitamin E. The mechanism by which vitamin A supplementation slows the progression of retinitis pigmentosa is not known. Vitamin E may have an adverse effect on this condition by reducing the amount of vitamin A reaching the eye, as serum vitamin A levels were observed to be significantly lower in patients taking vitamin E (4). Retinitis pigmentosa can be inherited by an autosomal dominant, autosomal recessive, X-linked, or digenic mode (5). Substantial genetic heterogeneity has been observed in this condition, with over 20 chromosomal loci mapped (6, 7). Mutations have been identified in seven genes (5, 8-15). Four of these genes encode proteins in the rod phototransduction cascade-namely rhodopsin, the a and 13 subunits of rod cGMP phosphodiesterase, and the rod cGMP cation-gated channel protein a subunit. Two of these genes encode proteins involved in maintaining photoreceptor outer segment disc structure-namely peripherin/RDS and rod outer segment membrane protein 1. Mutations in the gene encoding myosin Vlla have been found in a form of retinitis pigmentosa with associated profound congenital deafness (Usher syndrome, type I). Mutations in these seven genes together account for about 20-25% of cases of retinitis pigmentosa in the United States. Mutations in the rhodopsin gene account for about 10% of cases in the United States and, therefore, represent the most common cause of retinitis pigmentosa for which a molecular genetic basis is known. More than 70 mutations have been found in the rhodopsin gene; most are missense mutations altering a single amino acid in the rhodopsin molecule (16-18). Differing severity of disease at a given age has been observed both within and between families, even among patients with the same gene defect (3, 19). The reason for variable clinical expression among patients of comparable age with the same gene defect is not known. The mechanism by which a rhodopsin gene defect expressed in rods leads to cone photoreceptor cell death also is not understood. Opsin is a seven-transmembrane segment protein with 348 amino acid residues (Fig. 2, Upper) (20, 21). It is well known that opsin binds to its vitamin A-derived chromophore, 11cis-retinal, via a Schiff-base linkage at the Lys-296 residue to form rhodopsin. Opsin is normally folded with the first (helix A) and seventh (helix G) transmembrane segments in proximity to form a pocket for 11-cis-retinal (Fig. 2, Lower). Change in the conformation of rhodopsin in response to light initiates the phototransduction cascade with consequent hydrolysis of cGMP. The decline in cGMP results in closure of the cGMP-gated channels and hyperpolarization of rod photoreceptor cells. The normal functioning of rhodopsin depends on its proper folding and binding to 11-cis-retinal (22-25). In two papers on the structure and function of rhodopsin in these Proceedings (26, 27), Khorana and coworkers report precise methods for separation and characterization of correctly folded and misfolded rhodopsin expressed in cultured COS cells containing synthetic mutant opsin genes. In the first paper, Liu and coworkers (26) judiciously select two rhodopsin mutants, P23H and G188R, known to cause retinitis pigmentosa in humans, as well as two site-specific mutants, D19OA and AY191-Y192, that would be expected to affect the folding of opsin. The proteins expressed from the P23H and D19OA mutants partially regenerated the rhodopsin chromophore with 11-cis-retinal and were mixtures of the correctly folded (retinal-binding) and misfolded (non-retinal-binding) opsins. The proteins expressed from G188R and AY191-Y192 were composed of totally misfolded non-retinal-binding opsins. They suggest that most, if not all, of the point mutations in the intradiscal domain result in partial or complete misfolding of rhodopsin and that misfolded opsins lead to less compact outer-segment disc structure. In the second paper, Garriga and coworkers (27) study mutations in transmembrane helix C (designated as III in Fig. 2) that would be expected to affect the cysteine disulfide bond that is critical for proper folding of opsin. The mutant opsins showed abnormal dissociation of opsin from 11-cis-retinal in response to light and destabilized breakdown products of rhodopsin with reduced transducin activation. They conclude that folding in the transmembrane domain is coupled to that in the intradiscal domain. It is well known that a normal rod photoreceptor, schematically represented in Fig. 3, sheds about 10% of its outer segment discs at its apex after light onset and renews a corresponding amount of outer segment discs at its base over the course of each day (29). Patients with retinitis pigmentosa and rhodopsin P23H have shown an abnormal rod ERG diurnal rhythm; rod ERG sensitivities are abnormally reduced