Residual Cone Structure in Achromatopsia: Implications for Gene Therapy

Abstract

 

Purpose

 

Achromatopsia (ACHM) is associated with absent or severely reduced cone function. Central to the success of emerging gene-replacement therapies is identifying patients with residual cone structure despite functional deficits. Adaptive optics (AO) imaging studies have shown that most residual cones have reduced or absent reflectivity, interfering with quantification of cone populations. Here we demonstrate a novel AO imaging method to visualize cones regardless of their waveguided signal.

 

Methods

 

Twenty-three subjects with a clinical diagnosis of ACHM, 13 with confirmed genetic mutations, were imaged using confocal and split-detection AOSLO. Split-detection AOSLO enables visualizing structures using multiply scattered light. These images were captured concurrently, in exact spatial registration with one another. Cones with visible confocal signal were matched to locations of cone inner segments (IS) in split detection images. In regions of mismatch between the two images, cone IS diameters were measured in the split detection images.

 

Results

 

All subjects showed regions of diminished or absent reflectivity as previously reported. Structures seen in three subjects using the split detector method were found to have a mean diameter of 6.0, 6.3, 6.7 and 7.0μm at 0.5, 5, 10 and 15° from the foveal center. The structures nearest the fovea were about twice the size of histologic values of cone IS diameter and change with increasing eccentricity was smaller than in histology. In images acquired with split-detection the location of many IS correspond to regions with no confocal signal (see Figure). IS structure was preserved throughout the retina in these three subjects, including a contiguous mosaic in the fovea. The split detector findings indicate that the regions of diminished reflectivity in the retinas of the 20 other subjects are likely to correspond to locations of cone IS.

 

Conclusions

 

The split-detector AOSLO method allows for observation of a robust population of non-reflecting cones in subjects with ACHM. This ability is important for screening efforts, as emerging gene therapy trials will benefit from objective parameters defining the therapeutic potential of prospective participants.

 

 

Foveal (A,B) and 15 degree temporal (C,D) confocal (A,C) and split detector (B,D) AOSLO images. Dark regions in A and C correspond to cone IS in B and D. Scale bar 50µm.

 

Keywords: 550 imaging/image analysis: clinical • 538 gene transfer/gene therapy • 648 photoreceptors