Using high-resolution imaging techniques, we observed the following features in a small number of unrelated patients with disease-causing mutations in the peripherin/RDS gene predicted to affect protein structure in the second intradiscal domain. In general, cone spacing was increased significantly throughout macular regions of irregular, heterogeneous AF; however, regions with normal cone spacing surrounded by regions of increased cone spacing were occasionally present (patient 3; D). In addition, cone packing was abnormal throughout regions of heterogeneous AF, demonstrated with Voronoi analysis (; ). Visual sensitivity was reduced in most regions with heterogeneous AF, though visual acuity was well preserved and sensitivity was normal near the fovea in patient 1. OCT measures of retinal thickness demonstrated diffuse loss of outer retinal structures extending from 0.5° eccentricity, with relative preservation of the outer nuclear and inner segment layers at the fovea corresponding to reduced CFS thickness in the study patients compared with healthy subjects. At the fovea, the outer segment and the RPE layers showed hyperreflective profiles; these changes were least pronounced in patient 1, who demonstrated diffuse cone dysfunction on full-field ERG testing, consistent with primary cone degeneration.
These findings suggest that in patients with peripherin/RDS
–associated retinal degeneration, foveal photoreceptor structure and function are preserved within macular regions of increased cone spacing and outer retinal degeneration. Local factors contributing to the observed regional intramacular variability could include differential accumulation of lipofuscin in RPE cells, increased melanin in foveal RPE cells, a protective function of macular pigment, or the lack of rod photoreceptors at the fovea. Our findings suggest that although peripherin/RDS is critical for rod and cone outer segment structure throughout the retina, photoreceptor survival is determined locally by regional factors. This hypothesis is supported by previous reports of greater pericentral than peripheral dysfunction and normal phototransduction in patients with heterozygous mutations in peripherin/RDS
associated with retinal degeneration.15,47
In addition, mice with heterozygous mutations in rds
show retention of cone cell nuclei in the central retina more frequently than rod cell nuclei.48
Our findings suggest that foveal cones may be resistant to photoreceptor degeneration caused by peripherin/RDS
mutations, perhaps implicating local factors such as RPE melanin, lipofuscin accumulation, or macular pigment in the pathogenesis of peripherin/RDS
-associated retinal degeneration. Future studies on macular pigment in patients with in peripherin/RDS
-associated retinal degeneration may help distinguish between these alternative hypotheses.
The foveal sparing observed in patients with peripherin/RDS
-associated retinal degeneration makes visual acuity a poor measure of disease severity and progression. The four patients in the present study had variable degrees of disease severity, ranging from localized macular abnormalities in AF and sensitivity in patient 3 to diffuse photoreceptor dysfunction in patients 1 and 4, but vision was at least 20/30 in at least one eye in all patients. More sensitive indices of disease severity and progression might include foveal or macular sensitivity, color vision, and macular structural measures such as OCT and AOSLO. Foveal preservation may make peripherin/RDS
–associated retinal degeneration an attractive candidate for neuroprotective therapies designed to slow disease progression.49
Clinical trials designed to measure response to experimental therapies may have to study nontraditional outcome measures, including foveal and macular sensitivity and high-resolution structural measures such as those presented here, to most precisely measure safety and efficacy in patients with peripherin/RDS
–associated retinal degeneration because visual acuity is likely to be an insensitive measure in this patient population.
The relationship between hyperreflective cone profiles on both OCT and AOSLO in regions of preserved visual function at or near fixation is worthy of longitudinal study. Perhaps cones progress from normal heterogenous reflective profiles to a diseased state with irregular inner segment/outer segment junction and outer segment/RPE junction layers on SD-OCT, where cones retain function but show increased reflectivity. Progressive degeneration may be associated with reduced reflectivity associated with reduced sensitivity and increased cone spacing, as observed in patient 4 in regions where unambiguous cones were visualized adjacent to regions of RPE atrophy (D).
Some features of clinical retinal degeneration associated with peripherin/RDS
mutations suggest there are similarities to the rds± mouse model, in which the peripherin/RDS heterozygote has unstable disc structure, altered disc shedding, and large phagosomes in the RPE that lead to secondary RPE changes and fundus abnormalities.15,50
In the present study, irregular profiles were observed in the outer segment/RPE junction layer throughout the macula and especially at the fovea in all patients (–C). These hyperreflective profiles were associated with an irregular, disrupted appearance to the outer segment/RPE junction layer within several degrees of the fovea in the macula of each patient. A similar pattern of disrupted inner segment/outer segment layers with foveal preservation was reported by Godara et al34
in a patient with fundus AF characteristics similar to those observed in patients with mutations in peripherin/RDS
, although molecular characterization of the peripherin/RDS
gene was not described in that study. The irregular outer segment layer observed in the present study may represent the human analog of the abnormal outer segment whorl-like structures reported in mice with heterozygous mutations in peripherin/RDS48
and transgenic mice expressing a proline-216-leucine substitution in peripherin/RDS.51
The outer segment/RPE junction layer abnormalities may be attributed to abnormal intracellular interactions or abnormal interactions with the extracellular matrix. In addition, the hyperreflective outer segment/RPE junction layer and cone profiles observed with OCT and AOSLO may represent light wave-guided by cones overlying RPE cells with enlarged phagosomes containing shed abnormal disc outer segments, as have been observed in rds± mice,48
or RPE cells engorged with lipofuscin deposits, as reported in a histopathological study of a patient with pattern dystrophy caused by the Cys213Tyr mutation as in patient 1 in the present study.4
Perhaps enlarged phagosomes or lipofuscin increase scattered light from RPE cells, with secondary increased wave-guiding of overlying cone inner segments. The present study provides functional correlates to the observation of outer segment abnormalities caused by peripherin/RDS
mutations; hyperreflective, abnormal outer segments retain useful function at the fovea, though sensitivity is reduced in association with irregular, disrupted outer segment/RPE junction layer profiles in the macula outside the foveal center.
Light and electron microscopic analysis of a retina from a patient with the Cys213Tyr peripherin/RDS
mutation showed RPE cells distended with lipofuscin and melano-lipofuscin associated with disrupted outer segments and abrupt transition between areas of photoreceptor and RPE atrophy and relatively intact retina.4
Immunohistochemical and electron microscopic evaluation of the peripheral retina of a patient with the R172W peripherin/RDS
mutation showed reduced outer segment length and density and dysplastic outer segments with discs distorted into whorl-like structures.52
A clinicopathological study of a patient whose family was subsequently found to carry the P210R mutation showed focal loss of retinal photoreceptors and pigment migration into the retina with clumping of pigment-laden cells and chorioretinal adhesion at the fovea and paracentral thinning of the pigment epithelium.53
No histologic studies have been reported from patients with maculopathies caused by the G208D peripherin/RDS
Histopathologic specimens studied ex vivo preclude accurate correlation between macular cone structure and function. Measures of outer retinal structure and function on a localized scale, such as that presented in the present study, provide critical insight into the effect of peripherin/RDS mutations on macular photoreceptors and demonstrate significant regional variation within the maculae of affected patients. These findings may yield insight into local factors that affect photoreceptor resistance or sensitivity to degeneration in patients with mutations in peripherin/RDS.