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Advanced late-onset retinitis pigmentosa with dominant-acting D477G RPE65 mutation is responsive to oral synthetic retinoid therapy

Dec 1, 2020

Paul F Kenna, Marian M Humphries, Anna-Sophia Kiang, Philippe Brabet, Laurent Guillou, Ema Ozaki, Matthew Campbell, G Jane Farrar, Robert Koenekoop, Pete Humphries | BMJ Open Ophthalmology | 2020 | 5:e000462 | doi: 10.1136/bmjophth-2020-00046


Abstract

Objectives: No therapeutic interventions are currently available for autosomal dominant retinitis pigmentosa (adRP). An RPE65 Asp477Gly transition associates with late-onset adRP, reduced RPE65 enzymatic activity being one feature associated with this dominant variant. Our objective: to assess whether in a proof-of-concept study, oral synthetic 9 cis-retinyl acetate therapy improves vision in such advanced disease.


Methods and analysis: A phase 1b proof-of-concept clinical trial was conducted involving five patients with advanced disease, aged 41-68 years. Goldmann visual fields (GVF) and visual acuities (VA) were assessed for 6-12 months after 7-day treatment, patients receiving consecutive oral doses (40 mg/m2) of 9-cis-retinyl acetate, a synthetic retinoid replacement.


Results: Pathological effects of D477G variant were preliminarily assessed by electroretinography in mice expressing AAV-delivered D477G RPE65, by MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxyme- thoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium] assays on RPE viability and enzyme activity in cultured cells. In addition to a mild dominant effect reflected in reduced electroretinographics in mice, and reduced cellular function in vitro, D477G exhibited reduced enzymatic RPE65 activity in vitro. In patients, significant improvements were observed in GVF from baseline ranging from 70% to 200% in three of five subjects aged 67-68 years, with largest improvements at 7-10 months. Of two GVF non-responders, one had significant visual acuity improvement (5-15 letters) from baseline after 6 months.


Conclusion: Families with D477G variant have been identified in Ireland, the UK, France, the USA and Canada. Effects of single 7-day oral retinoid supplementation lasted at least 6 months, possibly giving visual benefit throughout remaining life in patients with advanced disease, where gene therapy is unlikely to prove beneficial.


Introduction

Mutations in genes encoding enzymatic components of the retinoid cycle, reducing the availability of 11-cis-retinal, have been encountered in recessive forms of inherited retinal degenerations (IRDs), retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA).1 In principle, administration of 11-cis-retinal would assist in preventing retinal degeneration in those IRDs where mutations within genes of the retinoid cycle are present. While 11-cis-retinal is unstable and not suitable for pharmacological use, 9-cis-retinal can be used as a replacement, forming iso-rhodopsin which retains function in photon capture and which has been shown, following intravitreal inoculation, to improve vision in dogs with null mutations within the RPE65 gene.2


QLT091001 (9-cis-retinyl acetate) is a late-stage synthetic retinoid replacement therapy being developed for inherited retinal diseases in humans (Retinagenix). It is a prodrug, converted by hydrolysis in vivo to 9-cis-retinol, which is then converted to 9-cis-retinal, thus restoring the key biochemical component of the retinoid cycle. QLT091001 treatment has previously been shown to result in vision improvement (VA) and/or visual field (VF) in subjects with LCA or RP due to recessive mutations in either RPE65 or LRAT genes.3 4 The mutation, D477G in exon 13 of the RPE65 gene is the only mutation within this gene so far identified manifesting autosomal dominant transmission.5 Families displaying this dominant mutation have now been identified in Ireland, the UK, France, the USA and Newfoundland (Professor J. Green, personal communication).5–7


Knock-in mice expressing D477G allele have deficits in retinal structure and function, with disruption of the visual cycle, indicative of decreased RPE65 enzymatic activity. Shin et al reported slower regeneration of 11-cis-retinaldehyde with lower electroretinographic (ERG) a-wave recovery following photobleaching, indicating delayed dark adaptation.8 Choi et al conclude from X-ray crystallographic structural analysis that a possible mechanism for photoreceptor degeneration may involve D477G RPE65 cellular toxicity.9 They also observed retinyl ester accumulation and slower regeneration of 11-cis-retinal after photobleaching in knock-in mice aged 9 months, and slow thinning of the retinal outer nuclear layer accompanied by decline in scotopic and photopic ERG, indicating loss of cone and rod photoreceptor function. Our in vitro data confirm a reduced enzymatic activity of the variant of approximately 25%. However, aberrant splicing of D477G transcripts has recently been suggested as an alternative mechanism contributing to reduced enzymatic activity and ultimately photoreceptor degeneration.10 Hence, in vitro data may not accurately reflect the reduction in enzymatic activity occurring in vivo. Nevertheless, it can be strongly inferred that a component of the disease process in this very slowly progressing dominant retinopathy is limited availability of chromophore 11-cis-retinal and that oral synthetic retinoid therapy may be extendable in improving remaining vision in patients with this genetic subtype of disease, where, as yet, no means of prevention are available for any form of dominant RP.


We report here further observations on the molecular pathology of the disease relevant to a pilot proof-of-concept study in which effects of a single 1 week course of oral retinoid therapy were assessed, and where VFs were significantly improved in three of five patients with advanced disease over periods of 6 months to 1 year. While previous studies on recessive LCA and RP have indicated maximum increases in VA at 2 months following retinoid treatment, responders in the current study were older (average age 62 years) and showed improvements over longer periods lasting over 6–12 months, which we suggest is a possible reflection of the time taken for regeneration of outer segments in remaining photoreceptors in these patients.



 

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