Pseudo-fovea formation after gene therapy for RPE65-LCA

Abstract

Purpose: The purpose of this study was to evaluate fixation location and oculomotor characteristics of 15 patients with Leber congenital amaurosis (LCA) caused by RPE65 mutations (RPE65-LCA) who underwent retinal gene therapy.

Methods: Eye movements were quantified under infrared imaging of the retina while the subject fixated on a stationary target. In a subset of patients, letter recognition under retinal imaging was performed. Cortical responses to visual stimulation were measured using functional magnetic resonance imaging (fMRI) in two patients before and after therapy.

Results: All patients were able to fixate on a 1° diameter visible target in the dark. The preferred retinal locus of fixation was either at the anatomical fovea or at an extrafoveal locus. There were a wide range of oculomotor abnormalities. Natural history showed little change in oculomotor abnormalities if target illuminance was increased to maintain target visibility as the disease progressed. Eleven of 15 study eyes treated with gene therapy showed no differences from baseline fixation locations or instability over an average of follow-up of 3.5 years. Four of 15 eyes developed new pseudo-foveas in the treated retinal regions 9 to 12 months after therapy that persisted for up to 6 years; patients used their pseudo-foveas for letter identification. fMRI studies demonstrated that preservation of light sensitivity was restricted to the cortical projection zone of the pseudo-foveas.

Conclusions: The slow emergence of pseudo-foveas many months after the initial increases in light sensitivity points to a substantial plasticity of the adult visual system and a complex interaction between it and the progression of underlying retinal disease. The visual significance of pseudo-foveas suggests careful consideration of treatment zones for future gene therapy trials. (ClinicalTrials.gov number, NCT00481546.).

Keywords: gene therapy; nystagmus; retinal degeneration.

Copyright 2015 The Association for Research in Vision and Ophthalmology, Inc.

Figures

Figure 1

Range of nystagmus features recorded monocularly under direct infrared retinal imaging in dark-adapted patients with RPE65-LCA in a dark room viewing a stationary stimulus brighter than the visibility threshold for the eye being recorded. Results from a representative normal subject (A) are compared to those of patients (P)15, P11, P1, P5, and P7, respectively (B–F). For each subject, 10-second epochs of eye movement data during fixation to a large visible target are shown in spatial and spatiotemporal coordinates. Spatial distribution of fixation clouds are shown on standard circles (radii at 1.65°, 5°, and 10°) representing the macular region centered on the anatomical foveal depression. Spatiotemporal distribution of eye movements are shown on chart records for x and y directions; up is nasal retina for x and superior retina for y. All results are presented as equivalent right eyes for comparability. Horizontal gray lines on the charts show the locations of the anatomical fovea. Dots between x and y traces represent the timing of the high-frequency eye movements used to calculate nystagmus frequency. I, inferior; N, nasal; S, superior; T, temporal retina. (G–I) Representative fixation records from P3, P8, and P9 show individual differences in nystagmus waveforms as a function of the visibility of the target in an otherwise darkened ambient environment. Low-frequency components have been removed from the long duration x-y chart records (but not from the fixation clouds shown on standard circles) in order to better demonstrate nystagmus characteristics as the eyes drift away from the anatomical fovea upon extinction of the visible target. The break in axis for P9 represents a period of 25 to 80 seconds.

Figure 2

Long-term serial evaluation of fixation location and instability in control (A, B) and study (C, D) eyes of RPE65-LCA patients. (A) Mean fixation location over a 10-second epoch in 14 untreated control eyes (control eye of P10 could not be imaged due to severe keratoconus) along the horizontal (x) and vertical (y) axes in terms of the distance from the anatomical fovea. Open symbols indicate eyes that retained extrafoveal fixation. (B) Fixation instability (upper) and its change from the first visit (lower) in control eyes. Both fixation location and instability show no significant changes over the duration of serial follow-up. Data are plotted as a function of age at the time of the recording. (C) Fixation location in 10 of the 15 study eyes fixating foveally at baseline and post treatment. Baseline (BL) refers to two or three measurements performed before the treatment in the study eye. (D) Fixation instability (upper) and its change from baseline (lower) in 10 of the 15 study eyes. Data are plotted as a function of time after gene therapy.

Figure 3

Emergence and persistence of pseudo-fovea in four study eyes of RPE65-LCA patients at baseline and at selected times after gene therapy treatment. Fixation clouds are shown for high and low illuminance targets for each patient; gray x represents no perception of the low illuminance target. Standard circles represent 1.65°, 5°, and 10° eccentricity from the anatomical fovea; some of the standard circles are supplemented with an arc at 20° eccentricity. All eyes are shown as the right eye equivalent for comparability. N, nasal retina; T, temporal retina.

Figure 4

Cortical projection sites of treatment show preserved luminance responses. (A, B) The visual cortex response to stimulation with a wide-field, flickering stimulus of medium luminance (−3 log) was obtained using BOLD fMRI from patients P2 and P10, several years before and after treatment (B) and from nine normally sighted control subjects (A). The amplitude of neural response (averaged across V1, V2, and V3) is projected onto the visual field (log eccentricity scale). The left side of the plot contains data from the right hemisphere of the brain and thus combines input from the nasal retina of the left eye and temporal retina of the right eye. A uniform amplitude of BOLD response across visual field representation was seen for the control population. For P2 and P10, macular responses are present at 2 and 5 years pretreatment (respectively), but peripheral responses are markedly attenuated. The black outline indicates the visual field location of the treated retinal area. In the responses measured at 6 and 3 years post treatment (for P2 and P10, respectively), there has been further loss of macular responses, with the exception of the cortical sites corresponding to the treated area of retina. (C, D) Each plot provides the amplitude of BOLD fMRI response averaged across V1, V2, and V3, as a function of variation in stimulus luminance. Inset in each plot indicates the visual field projection of the cortical region examined. For the control subjects, a steadily increasing cortical response to luminance can be seen (C). For P2 and P10, the cortical projection of the isoeccentric, untreated portion of retina ([D], upper row) was examined pre (red) and post treatment (green). A marked decrement in response between the two time points is seen. In contrast, within the treatment projection zone ([D], bottom row) responses at post treatment (green) are preserved despite the elapse of nearly 8 years.