Longitudinal Study to Assess the Quantitative Use of Fundus Autofluorescence for Monitoring Disease Progression in Choroideremia

C1QTNF5 missense variant causing autosomal dominant gyrate atrophy-like choroidal dystrophy

INTRODUCTION

It was recently proposed that the c.538C>G; p.(Q180E) missense variant in the C1QTNF5 gene leads to autosomal dominant gyrate atrophy-like choroidal dystrophy (adGALCD). AdGALCD has a phenotypic overlap with other chorioretinal dystrophies but exhibits not the typical characteristics of late-onset retinal degeneration (L-ORD). Here, we report a 70-year-old female simplex patient with adGALCD.

METHODS

Comprehensive ophthalmic examinations included visual field testing and multimodal retinal imaging with optical coherence tomography (OCT), short-wavelength-, and near-infrared fundus autofluorescence. Genetic testing was performed using whole exome sequencing.

RESULTS

The patient presented with bilateral progressive concentric visual field defects and photophobia for about 8 years. Best corrected visual acuity was 20/40 in the right and 20/32 in the left eye, respectively. On fundus examination, symmetric areas of large peripheral and peripapillary chorioretinal atrophy with foveal sparing were observed with corresponding visual field defects. Fundus autofluorescence imaging showed a central island of preserved autofluorescence surrounded by an extinguished signal in the atrophic areas. OCT imaging showed preserved central retinal pigment epithelium and ellipsoid zone with peripapillary and peripheral atrophy of both layers. Genetic testing revealed a heterozygous missense variant p.(Q180E) in the C1QTNF5 gene.

DISCUSSION

We confirm that the c.538C>G; p.(Q180E) missense variant in the C1QTNF5 gene may lead to adGALCD.

Ranked Importance of Visual Function Outcome Measures in Choroideremia Clinical Trials

Purpose

Clinical trials of novel therapies for choroideremia require robust and clinically meaningful visual function outcome measures. Best-corrected visual acuity (BCVA) is mostly insensitive to changes in disease state, until late stages, and hence also to potential therapeutic gains after gene therapies. While the insensitivity of BCVA as an effective outcome measure is common wisdom, its low importance has not been rigorously demonstrated in the literature. This work uses statistical techniques to rank the relative importance of potential functional outcome measures in choroideremia.

Methods

Retrospective dominance analysis was performed on data from a longitudinal interventional clinical trial performed at the Oxford Eye Hospital. Functional data from the untreated eye were analyzed and correlated with an anatomic measure of disease progression in the form of blue fundus autofluorescence area of the surviving outer retinal island. Each functional measure was then ranked in terms of variable importance.

Results

Microperimetry was the functional measure ranking first in terms of variable importance, followed by time since baseline visit, Pelli-Robson contrast sensitivity, high spatial frequency contrast sensitivity function, and low luminance visual acuity. Early Treatment Diabetic Screening chart BCVA under standard lighting conditions was ranked lowest among the panel of test modalities.

Conclusions

Clinical trials in choroideremia for early and mid-stage disease would be justified in moving away from using standard BCVA as a clinical trial outcome measure as we have shown its sensitivity to be relatively low compared to microperimetry. We have also demonstrated how functional measures rank in terms of importance.

Choroideremia: The Endpoint Endgame

Choroideremia is an X-linked retinal degeneration resulting from the progressive, centripetal loss of photoreceptors and choriocapillaris, secondary to the degeneration of the retinal pigment epithelium. Affected individuals present in late childhood or early teenage years with nyctalopia and progressive peripheral visual loss. Typically, by the fourth decade, the macula and fovea also degenerate, resulting in advanced sight loss. Currently, there are no approved treatments for this condition. Gene therapy offers the most promising therapeutic modality for halting or regressing functional loss. The aims of the current review are to highlight the lessons learnt from clinical trials in choroideremia, review endpoints, and propose a future strategy for clinical trials.

Female carriers of X-linked inherited retinal diseases - Genetics, diagnosis, and potential therapies

Inherited retinal diseases (IRDs) are a group of heterogeneous conditions that cause progressive vision loss, typically due to monogenic mutations. Female carriers of X-linked IRDs have a single copy of the disease-causing gene, and therefore, may exhibit variable clinical signs that vary from near normal retina to severe disease and vision loss. The relationships between individual genetic mutations and disease severity in X-linked carriers requires further study. This review summarises the current literature surrounding the spectrum of disease seen in female carriers of choroideremia and X-linked retinitis pigmentosa. Various classification systems are contrasted to accurately grade retinal disease. Furthermore, genetic mechanisms at the early embryonic stage are explored to potentially explain the variability of disease seen in female carriers. Future research in this area will provide insight into the association between genotype and retinal phenotypes of female carriers, which will guide in the management of these patients. This review acknowledges the importance of identifying which patients may be at high risk of developing severe symptoms, and therefore should be considered for emerging treatments, such as retinal gene therapy.

Choroideremia: molecular mechanisms and therapies

Choroideremia (CHM) is a monogenic X-linked chorioretinal dystrophy affecting the photoreceptors, retinal pigment epithelium (RPE), and choroid; it is caused by mutations involving the CHM gene. CHM is characterized by night blindness in early childhood, progressing to peripheral visual field loss and eventually to complete blindness from middle age. CHM encodes the ubiquitously expressed Rab escort protein 1 (REP1), which is responsible for prenylation of Rab proteins and is essential for intracellular trafficking of vesicles. In this review we explore the role of REP1 in the retina and its newly discovered systemic manifestations, and discuss the therapeutic strategies for tackling this disease, including the outcomes from recent clinical trials.