SCLERAL PITS IN CHOROIDEREMIA: Implications for Retinal Gene Therapy

CHM mutation spectrum and disease: An update at the time of human therapeutic trials

Choroideremia is an X-linked inherited retinal disorder (IRD) characterized by the degeneration of retinal pigment epithelium, photoreceptors, choriocapillaris and choroid affecting males with variable phenotypes in female carriers. Unlike other IRD, characterized by a large clinical and genetic heterogeneity, choroideremia shows a specific phenotype with causative mutations in only one gene, CHM. Ongoing gene replacement trials raise further interests in this disorder. We describe here the clinical and genetic data from a French cohort of 45 families, 25 of which carry novel variants, in the context of 822 previously reported choroideremia families. Most of the variants represent loss-of-function mutations with eleven families having large (i.e. ≥6 kb) genomic deletions, 18 small insertions, deletions or insertion deletions, six showing nonsense variants, eight splice site variants and two missense variants likely to affect splicing. Similarly, 822 previously published families carry mostly loss-of-function variants. Recurrent variants are observed worldwide, some of which linked to a common ancestor, others arisen independently in specific CHM regions prone to mutations. Since all exons of CHM may harbor variants, Sanger sequencing combined with quantitative polymerase chain reaction or multiplex ligation-dependent probe amplification experiments are efficient to achieve the molecular diagnosis in patients with typical choroideremia features.

Reduced vessel density in deep capillary plexus correlates with retinal layer thickness in choroideremia

Background

To assess retinal layer thickness in choroideremia (CHM) and to reveal its correlation with optical coherence tomography (OCT) angiography (OCTA) findings.

Methods

The study was designed as an observational, cross-sectional clinical series of patients with CHM, which included 14 CHM eyes and 14 age-matched controls. Multimodal imaging included OCT and OCTA. The vessel density (VD) of superficial capillary (SCP), deep capillary (DCP) and choriocapillaris (CC) plexuses was analysed by OCTA. The apparently preserved retinal islet and atrophic regions were investigated separately. Main outcome measures were as follows: best-corrected visual acuity (BCVA), total retinal layers, ganglion cell layer (GCL), inner plexiform layer (IPL), inner nuclear layer (INL), outer plexiform layer (OPL), outer nuclear layer (ONL), ellipsoid zone–retinal pigment epithelium (EZ-RPE) layer, choroidal thickness and VDs of SCP, DCP and of CC.

Results

Mean BCVA was 0.0±0.0 LogMAR in both groups. GCL, ONL, EZ-RPE and choroid were significantly thinned in CHM, particularly in the atrophic region. OPL was unaffected in the apparently preserved islet, whereas INL and IPL were similarly thinned in the atrophic and apparently preserved retina. DCP appeared severely affected in both regions, while CC was only altered in the atrophic retina. Significant correlations were found between OCT and OCTA parameters.

Conclusions

Our study showed severe alterations in both outer and inner retinal layers of patients with CHM. The extended retinal involvement might be the consequence of neuronal and vascular trophic factor reduction produced by the primarily altered RPE and/or secondary Müller glial cell reaction.

Scleral pits represent degeneration around the posterior ciliary arteries and are signs of disease severity in choroideremia

BACKGROUND

Choroideremia is an X-linked recessive condition characterized by progressive chorioretinal degeneration. Recently, peculiar scleral ectasias, termed scleral "pits" and "tunnels," have been described as a novel finding in patients with choroideremia, but little is known regarding their etiology or their evolution over time.

SUBJECTS

This is a retrospective chart review of consecutive patients with molecularly-confirmed choroideremia and related female carriers seen at a university-based tertiary referral center from January 2010 to July 2016. Multimodal imaging was evaluated for the evolution of scleral pits on fundus photography and scleral tunnels on optical coherence tomography (OCT). The presence of scleral pits and tunnels was correlated with markers of disease severity including age, visual acuity, and severity of visual field loss.

RESULTS

Thirty patients (21 affected males, 9 female carriers) were included in the study. Scleral pits were seen in 38.1% (8/21) of affected males and found to occur at insertion sites of the posterior ciliary arteries. Those with scleral pits were older, had poorer visual acuity, and more severe visual field loss than those without (p ≤ 0.05). Scleral tunnels were common (68.4%, 13/19 affected males with available OCT imaging), but no statistically-significant associations with disease severity were seen. The development of new scleral pits and tunnels was observed on longitudinal imaging in 4 and 2 affected males, respectively. No scleral pits or tunnels were visualized in any female carriers.

CONCLUSIONS

Scleral pits represent degeneration around the posterior ciliary arteries and may be useful as clinical markers of disease severity in choroideremia.

Ocular gene therapy for choroideremia: clinical trials and future perspectives

Introduction

Gene therapy offers the potential for targeted replacement of single gene defects in inherited retinal degenerations.

Areas covered

Choroideremia is an X-linked blinding retinal disease resulting from deficiency of the CHM gene product, REP1. The disease represents an ideal target for retinal gene therapy, as it is readily diagnosed in the clinic, relatively homogenous in phenotype and slow progressing, thereby providing a wide therapeutic window for intervention. Ongoing clinical trials of retinal gene therapy for choroideremia using an adeno-associated viral vector have demonstrated safety and early efficacy. We review the clinical characteristics of the disease with a view to interpreting the findings of gene therapy clinical trials and discuss future directions.

Expert commentary

Choroideremia gene therapy has so far demonstrated good safety profile and early functional visual acuity gains in a proportion of trial participants, which appear to be sustained.