Cellular imaging demonstrates genetic mosaicism in heterozygous carriers of an X-linked ciliopathy gene

Abstract

X-linked retinitis pigmentosa (XLRP) is the least common genetic type of retinitis pigmentosa; however, it has extremely devastating consequences to patients' activities of daily living. RPGR and RP2 genes expressed in the photoreceptor sensory cilia are predominantly implicated in XLRP; however, the interpretation of genetic mutations and their correlation with clinical phenotypes remain unknown, and the role of these genes in photoreceptor cilia function is not completely elucidated. Therefore, we evaluated structural characteristics in five female obligate carriers of XLRP by using state-of-the-art non-invasive imaging methods, including adaptive optics (AO) scanning laser ophthalmoscopy (SLO). In all five carriers examined, qualitative and quantitative analyses by AO SLO imaging revealed a mosaic pattern of cone disruption, even in the absence of visual symptoms, normal visual acuity and normal macular thickness, on optical coherence tomography and mildly subnormal full-field cone electroretinographic findings. As the technique is sensitive to the level of a single cone, the ability to visualize the cone cells in vivo should be especially useful in other retinal diseases. In addition, further investigation of XLRP carriers may yield insight into how cone structures change over time and ultimately enable understanding of the role of RPGR and RP2 in cone cell survival.

Figure 1

Retinal images of the left eyes of XLRP carrier. (a) Fundus photograph of subject 2 shows fundus TLR within the posterior pole with tigroid changes. (b) Hyperautofluorescences are detected within the area of the TLR. (c) IR image showing an enhanced reflectivity that correlated with the fundus TLR. AF, autofluorescence.

Figure 2

High-resolution SD-OCT scans through the fovea of the five XLRP carriers. SD-OCT images of (a) subject 1, (b) subject 2, (c) subject 3, (d) subject 4 and (e) subject 5. All carriers with TLR have normal retinal microstructure and thickness on SD-OCT images. These SD-OCT images also showed an intact inner ellipsoid band, indicating the presence of photoreceptor IS/OS junctions. Photoreceptor OSs, RPE and Bruch's membrane were also intact in all carriers. The position of the SD-OCT line scan is indicated by the white or black dashed lines in AF or IR images. AF, autofluorescence; RPE, retinal pigment epithelium; SD-OCT, spectral domain optical coherence tomography.

Figure 3

Retinal images of the left eye of subject 1. (a) Fundus photograph shows fundus TLR within the posterior pole, which is more prominent temporal to the macula, and myopic changes with prominence of the choroidal vascular pattern. (b) No abnormal AF is detected on FAF imaging. (c) A montage of AO-SLO images from subject 1 matched with the IR images. (d) A magnified AO-SLO image of the area (nasal 0.5 mm) indicated by the white box in the image of c, showing a relatively regular cone cell appearance. Some morphologically large and/or dysmorphic cone cells are indicated by arrowheads, but it is not that much compared with another area (inferior 0.5 mm) in the same patient. The number of cone cells within the area indicated by the white box was estimated to be 58. (e) A magnified AO-SLO image from an unaffected control at the same location. (f) A magnified AO-SLO image of the area (inferior 0.5 mm) indicated by the white box in the image of c, shows less compact distributions than those with normal controls. Some cone cells show irregular appearance and are of variable asymmetrical sizes and shapes, which are indicated by arrowheads and the cone count is 45 in the white box area. (g) A magnified AO-SLO image from an unaffected control at the same location.

Figure 4

AO-SLO images of carriers of XLRP and controls. Manual counting of the number of cone cells was performed within the area indicated by white box. Some morphologically large and/or abnormal cone cells are indicated by arrowheads. (a) AO-SLO image of subject 2 (temporal 0.5 mm). (b) AO-SLO image of subject 3 (temporal 1.0 mm). (c) AO-SLO image of subject 5 (inferior 0.5 mm). (d) AO-SLO image of subject 4 (inferior 0.5 mm). Presented images of all carriers have several small patch lesions where cones are lost or decreased and residual cone cells appear to be asymmetrical sizes and be of variable shapes. (e, f, g, h) AO-SLO images from an unaffected control at the same location and magnification used for XLRP carriers. The images show normal cone mosaic patterns and no abnormal patch lesions.