Novel RDH12 sequence variations in Leber congenital amaurosis

Natural History and Genotype-Phenotype Correlations in RDH12-Associated Retinal Degeneration

Mutations in retinol dehydrogenase 12 (RDH12) cause a severe early-onset retinal degeneration, for which there is no treatment. RDH12 is involved in photoreceptor retinoid metabolism and is a potential target for gene therapy, which has been successful in treating RPE65-associated LCA. RDH12-associated retinal degeneration is particularly devastating due to early macular atrophy, which will likely impact therapeutic outcomes. Defining the unique features and natural history of disease associated with RDH12 mutations is a critical first step in developing treatments. The purpose of this review is to aggregate and summarize the body of literature on phenotypes in RDH12-associated retinal degeneration to help map the natural history of disease and identify phenotypic milestones in disease progression. The results reveal a severe blinding disorder with onset in early childhood and frequent retention of reduced yet useful vision until adolescence. The severity is associated with genotype in some cases. Distinct phenotypic features include macular atrophy followed by bone spicule pigment early in life, in contrast to other forms of LCA which often have a relatively normal fundus appearance in childhood despite severe visual dysfunction. Formal natural history studies are needed to define milestones in disease progression and identify appropriate outcome measures for future therapy trials.

Retinol dehydrogenase 12 (RDH12): Role in vision, retinal disease and future perspectives

Retinol dehydrogenase 12 (RDH12) is an NADPH-dependent retinal reductase, which is expressed in the inner segments of the photoreceptors. It functions as part of the visual cycle, which is a series of enzymatic reactions required for the regeneration of the visual pigment, and has also been implicated in detoxification of lipid peroxidation products. Mutations in RDH12 have been linked to Leber congenital amaurosis (LCA) and autosomal dominant retinitis pigmentosa. A number of in-vitro studies have shown that mutations in RDH12 result in little or no enzyme activity. Knockout mouse models however do not recapitulate the severe phenotype observed in patients, resulting in a limited understanding of the disease mechanisms. With gene replacement and small molecule drugs emerging for inherited retinal dystrophies, herein we provide a review of RDH12 structure, its role in vision and the current understanding of disease mechanisms linked to clinical phenotype to support therapeutic development.

Development of a Gene Therapy Vector for RDH12-Associated Retinal Dystrophy

Early-onset severe retinal dystrophy (EOSRD) is a genetically heterogeneous group of diseases resulting in serious visual disability in children. A significant number of EOSRD cases, often diagnosed as Leber congenital amaurosis (LCA13), are associated with mutations in the gene encoding retinol dehydrogenase 12 (RDH12). RDH12 is a member of the enzyme family of short-chain dehydrogenases/reductases. In the retina, RDH12 plays a critical role in reducing toxic retinaldehydes generated by visual cycle activity that is required for the light response of the photoreceptor cells. Individuals with RDH12 deficiency exhibit widespread retinal degeneration impacting both rods and cones. Although Rdh12-deficient (Rdh12^-/-) mice do not exhibit retinal degeneration, functional deficits relevant to visual cycle function can be demonstrated. In the present study, we describe the development and preclinical testing of a recombinant adeno-associated viral (rAAV) vector that has the potential for use in treating EOSRD due to RDH12 mutations. Wild-type and Rdh12^-/- mice that received a subretinal injection of rAAV2/5 carrying a human RDH12 cDNA driven by a human rhodopsin-kinase promoter exhibited transgene expression that was stable, correctly localized, and did not cause retinal toxicity. In addition, administration of the vector reconstituted retinal reductase activity in the retinas of Rdh12^-/- mice and decreased susceptibility to light damage associated with Rdh12 deficiency, thus demonstrating potential therapeutic efficacy in an animal model that does not exhibit a retinal degeneration phenotype. These findings support further efforts to develop gene replacement therapy for individuals with RDH12 mutations.

RDH12 Mutations Cause a Severe Retinal Degeneration With Relatively Spared Rod Function

Purpose

To describe the retinal phenotype of pediatric patients with mutations in the retinol dehydrogenase 12 (RDH12) gene.

Methods

Twenty-one patients from 14 families (ages 2-17 years) with RDH12-associated inherited retinal degeneration (RDH12-IRD) underwent a complete ophthalmic exam and imaging with spectral domain optical coherence tomography (SD-OCT) and near infrared and short-wavelength fundus autofluorescence. Visual field extent was measured with Goldmann kinetic perimetry, visual thresholds with dark-adapted static perimetry or with dark-adapted chromatic full-field stimulus testing (FST) and transient pupillometry.

Results

Visual acuity ranged from 20/40 to light perception. There was parafoveal depigmentation or atrophic maculopathies accompanied by midperipheral intraretinal pigment migration. SD-OCT revealed foveal thinning in all patients and detectable but thinned outer nuclear layer (ONL) at greater eccentricities from the fovea. Photoreceptor outer segment (POS) signals were only detectable in small pockets within the central retina. Measurable kinetic visual fields were limited to small (<5-10°) central islands of vision. Electroretinograms were reported as undetectable or severely reduced in amplitude. FST sensitivities to a 467 nm stimulus were rod-mediated and reduced on average by ∼2.5 log units. A thinned central ONL colocalized with severely reduced to nondetectable cone-mediated sensitivities. Pupillometry confirmed the psychophysically measured abnormalities.

Conclusions

RDH12-IRD causes an early-onset, retina-wide disease with particularly severe central retinal abnormalities associated with relatively less severe rod photoreceptor dysfunction, a pattern consistent with an early-onset cone-rod dystrophy. Severely abnormal POS but detectable ONL in the pericentral and peripapillary retina suggest these regions may become targets for gene therapy.

Phenotype-genotype correlation with Sanger sequencing identified retinol dehydrogenase 12 (RDH12) compound heterozygous variants in a Chinese family with Leber congenital amaurosis

BACKGROUND

Leber congenital amaurosis (LCA) is a group of clinically and genetically heterogeneous retinal dystrophy. To date, 22 genes are known to be responsible for LCA, and some specific phenotypic features could provide significant prognostic information for a potential genetic etiology. This study is to identify gene variants responsible for LCA in a Chinese family using direct Sanger sequencing, with the help of phenotype-genotype correlations.

METHODS

A Chinese family with six members including two individuals affected with LCA was studied. All patients underwent a complete ophthalmic examination. Based on phenotype-genotype correlation, direct Sanger sequencing was performed to identify the candidate gene on all family members and normal controls. Targeted next-generation sequencing was used to exclude other known LCA genes.

RESULTS

By Sanger sequencing, we identified two novel missense variants in the retinol dehydrogenase 12 (RDH12) gene: a c.164C>A transversion predicting a p.T55K substitution, and a c.535C>G transversion predicting a p.H179D substitution. The two affected subjects carried both RDH12 variants, while their parents and offspring carried only one of heterozygous variants, showing complete cosegregation of the variants. The compound heterozygous variants were not present in 600 normal controls. Besides, the RDH12 variants were confirmed by targeted next-generation sequencing.

CONCLUSIONS

The RDH12 compound heterozygous variants might be the cause of the LCA family. Our study adds to the molecular spectrum of RDH12-related retinopathy and offers an effective example of the power of phenotype-genotype correlations in molecular diagnosis of LCA.

Longitudinal clinical course of three Japanese patients with Leber congenital amaurosis/early-onset retinal dystrophy with RDH12 mutation

PURPOSE

To report the longitudinal clinical course of three Japanese patients from two families with Leber congenital amaurosis/early-onset retinal dystrophy (LCA/EORD), and the results of next-generation DNA sequences on them.

PATIENTS AND METHODS

The patients were three Japanese children: a 4-year-old girl, a 6-year-old boy, and a 3-year-old girl. Patients 1 and 2 were siblings, and patient 3 was from an unrelated family. Standard ophthalmic examinations including perimetry, electroretinography, optical coherence tomography, and ultrasonography were performed on each patient. The patients were observed for 28, 16, and 10 years. Whole exomes of the patients and their non-symptomatic parents were analyzed using a next-generation sequence technique.

RESULTS

The decimal visual acuity varied between 0.07 and 0.6 at the initial visit and decreased to counting finger to hand motion in their teens. Funduscopy showed diffuse retinal and macular degeneration. During the follow-up period, a posterior staphyloma developed and the macular area became atrophic. Patient 1 developed cataracts in her early twenties. Genetic analysis revealed a homozygous A126V substitution in the RDH12 gene in all patients.

CONCLUSIONS

The three patients with LCA/EORD had a progressive decrease of their vision with the formation of a posterior staphyloma. This is the first report of Japanese patients with LCA/EORD with a RDH12 mutation.