Novel RDH12 mutations associated with Leber congenital amaurosis and cone-rod dystrophy: biochemical and clinical evaluations

Pseudocoloboma-like maculopathy with biallelic RDH12 missense mutations

BACKGROUND

Hereditary maculopathy is a group of clinically and genetically heterogeneous disorders. With distinctive clinical features, subtypes of macular atrophy may correlate with their genetic defects.

METHODS

Seven patients from six families with adolescent/adult-onset maculopathy were examined in this clinical case series. A detailed medical history and eye examination were performed. Genomic DNA sequencing was performed using whole exome sequencing or direct sequencing of retinol dehydrogenase 12 (RDH12) coding exons.

RESULTS

Seven patients, including one male and six female patients, with pseudocoloboma-like maculopathy had biallelic missense RDH12 mutations. The most common mutant allele found in six of the seven patients was p.Ala269Gly. The average disease onset was at age 19.3 years, and visual acuity ranged from count fingers to 1.0. Most of the patients had mild myopic refraction. Common findings on fundus examination and spectral-domain optical coherence tomography include discrete margins of pseudocoloboma-like macular lesions with variable degrees of chorioretinal atrophy, excavation of retinal tissue and pigmentary changes mainly in the macular area. The electroretinograms were relatively normal to subnormal in all participants.

CONCLUSIONS

Progressive macular degeneration with a relatively normal peripheral retina and subsequent development of a pseudocoloboma-like appearance were the main clinical features in patients with compound heterozygous RDH12 missense mutations. Genetic testing may be crucial for early diagnosis and may play a key role in the development of future treatment strategies.

Clinical and Genetic Analysis of RDH12-Associated Retinopathy in 27 Chinese Families: A Hypomorphic Allele Leads to Cone-Rod Dystrophy

Purpose

The purpose of this study was to elucidate the genetic basis of 2 distinct phenotypes associated with biallelic variants in RDH12.

Methods

Patients with biallelic variants in RDH12 were recruited from our genetic eye clinic. Ocular phenotypes were evaluated. Genotype-phenotype correlations were further clarified using in-house and existing databases.

Results

In total, 22 biallelic RDH12 variants, including 5 novel variants, were identified in 29 patients from 27 families. Two distinct phenotypes were observed: early-onset and generalized retinal dystrophy with severe impairment of rods and cones in 24 patients (82.8%, 24/29), and late-onset cone-rod dystrophy (CORD) with central macular atrophy in 5 patients from 5 unrelated families (17.2%, 5/29), in which a hypomorphic allele (c.806C>G/p.Ala269Gly) was shared by all 5 patients. During follow-up, patients with late-onset CORD were relatively stable and did not progress to the severe form, which was considered to be an independent manifestation of RDH12-associated retinopathy caused by specific genotypes.

Conclusions

The hypomorphic allele is responsible for the unique late-onset CORD in 5 families with recessive RDH12-associated retinopathy, in contrast to the well-known severe and generalized retinopathy. Determining the therapeutic value of interventions may depend on understanding the molecular mechanisms underlying manifestation of this hypomorphic variant only in the central macular region, with relative preservation of the peripheral retina.

Associations Between Fundus Types and Clinical Manifestations in Patients with RDH12 Gene Mutations

To study the associations between RDH12 gene mutations, fundus types, and clinical manifestations. In total, 46 patients with inherited eye diseases caused by RDH12 gene mutations were included in this study. High-throughput chip capture sequencing, Sanger sequencing, and gene panel detection were used to determine that RDH12 was the pathogenic gene. All patients underwent the following detailed ophthalmic examinations: visual acuity, visual field, intraocular pressure, fundus photography, electroretinography, and optical coherence tomography (OCT). Statistical analysis was used to evaluate the clinical phenotype. A total of 32 mutations were identified in 46 patients. The most common mutations were c.437T > A, c.184C > T, and c.524C > T; the corresponding amino acid changes were p.Val146Asp, p.Arg62Ter, and p.Ser175Leu. Of the 46 patients, retinitis pigmentosa (RP) was found in 31 (68.9%); leber congenital amaurosis (LVA) was found in 11 (24.4%); early onset of severe retinal dystrophy (EOSRD) was found in one (2.2%); cone rod dystrophy (CORD) was found in one (2.2%); and Stargardt disease was found in one (2.2%). There was a significant difference in best-corrected visual acuity among patients based on fundus type (p = 0.0124). Linear trend analysis showed that best-corrected visual acuity gradually decreased as the fundus type increased in severity. In addition, there was a significant difference in the incidence of night blindness among patients with different fundus types (p = 0.0429): types I and IV fundi were associated with the highest incidences of night blindness. RDH12 gene mutation can cause serious inherited retinal diseases, which primarily include RP and LCA. Combined with clinical symptoms and fundus types, the progression of the disease can be characterized and used to guide genetic diagnosis and gene therapy.

Pharmaceutical interest of in-silico approaches

The virtual environment within the computer using software performed on the computer is known as in-silico studies. These drugs designing software play a vital task in discovering new drugs in the field of pharmaceuticals. These designing programs and software are employed in gene sequencing, molecular modeling, and in assessing the three-dimensional structure of the molecule, which can further be used in drug designing and development. Drug development and discovery is not only a powerful, extensive, and an interdisciplinary system but also a very complex and time-consuming method. This book chapter mainly focused on different types of in-silico approaches along with their pharmaceutical applications in numerous diseases.

Gene Therapy in Inherited Retinal Diseases: An Update on Current State of the Art

Background: Gene therapy cannot be yet considered a far perspective, but a tangible therapeutic option in the field of retinal diseases. Although still confined in experimental settings, the preliminary results are promising and provide an overall scenario suggesting that we are not so far from the application of gene therapy in clinical settings. The main aim of this review is to provide a complete and updated overview of the current state of the art and of the future perspectives of gene therapy applied on retinal diseases.

Methods: We carefully revised the entire literature to report all the relevant findings related to the experimental procedures and the future scenarios of gene therapy applied in retinal diseases. A clinical background and a detailed description of the genetic features of each retinal disease included are also reported.

Results: The current literature strongly support the hope of gene therapy options developed for retinal diseases. Although being considered in advanced stages of investigation for some retinal diseases, such as choroideremia (CHM), retinitis pigmentosa (RP), and Leber's congenital amaurosis (LCA), gene therapy is still quite far from a tangible application in clinical practice for other retinal diseases.

Conclusions: Gene therapy is an extremely promising therapeutic tool for retinal diseases. The experimental data reported in this review offer a strong hope that gene therapy will be effectively available in clinical practice in the next years.

ISOLATED MACULOPATHY AND MODERATE ROD-CONE DYSTROPHY REPRESENT THE MILDER END OF THE RDH12-RELATED RETINAL DYSTROPHY SPECTRUM

PURPOSE

To describe an isolated maculopathy and an intermediate rod-cone dystrophy phenotype as the milder end of the RDH12-related retinal dystrophy spectrum.

METHODS

Seven patients (17-34 years of age) underwent an extensive ophthalmic workup including psychophysical and electrophysiological testing and multimodal imaging.

RESULTS

Three patients have isolated macular disease. Best-corrected visual acuity (BCVA) ranges from 20/125 to 20/40 with normal visual fields or only limited central, relative scotomata, and normal full-field ERGs. Both optical coherence tomography scans and autofluorescent imaging hint at relatively better-preserved foveal quality initially. An intermediate rod-cone phenotype in four patients is characterized by a central retinal dystrophy extending just beyond the vascular arcades, characteristic peripapillary sparing, and additional scattered atrophic patches. Again, foveal quality is initially better on optical coherence tomography scans. Best-corrected visual acuity ranges from counting fingers to 20/32. Goldmann visual fields vary from central scotomata to severe generalized abnormalities. ERGs range between mild and severe rod-cone dysfunction. Nine distinct RDH12 pathogenic variants, two of which are novel, are identified.

CONCLUSION

The classic phenotype of RDH12-related early-onset retinal dystrophy is expanded to include an isolated maculopathy and intermediate dystrophy phenotype, characterized by its later onset and milder course with a fair visual potential until much later in life, emphasizing the phenotypic heterogeneity of RDH12-related retinopathy.

PHENOTYPIC VARIABILITY OF RECESSIVE RDH12-ASSOCIATED RETINAL DYSTROPHY

PURPOSE

To characterize the phenotypic variability and report the genetic defects in a cohort of Chinese patients with biallelic variants of the retinol dehydrogenase 12 (RDH12) gene.

METHODS

The study included 38 patients from 38 unrelated families with biallelic pathogenic RDH12 variants. Systematic next-generation sequencing data analysis, Sanger sequencing validation, and segregation analysis were used to identify the pathogenic mutations. Detailed ophthalmic examinations, including electroretinogram, fundus photography, fundus autofluorescence and optical coherence tomography, and statistical analysis were performed to evaluate phenotype variability.

RESULTS

Twenty-five different mutations of RDH12 were identified in the 38 families. Six of these variants were novel. Val146Asp was observed at the highest frequency (23.7%), and it was followed by Arg62Ter (14.5%) and Thr49Met (9.2%). Twenty-three probands were diagnosed with early-onset severe retinal dystrophy, 6 with Leber congenital amaurosis, 7 with autosomal recessive retinitis pigmentosa, and 2 with cone-rod dystrophy. Self-reported nyctalopia occurred in about a half of patients (55.3%) and was significantly more common among older patients (P < 0.01). Nyctalopia was not significantly associated with best-corrected visual acuity (P = 0.72), but older patients had significantly greater best-corrected visual acuity loss (P < 0.01). Only 15.8% of the patients had nystagmus, which was significantly more likely to occur among 36.8% of the patients with hyperopia >3D (P < 0.01) and/or in cases of reduced best-corrected visual acuity (P = 0.01), but was not associated with age (P = 0.87).

CONCLUSION

Several high-frequency RDH12 variants were identified in patients with inherited retinal dystrophies, most of which were missense mutations. Variable but characteristic phenotypes of a progressive nature was observed. Overall, the findings indicated that biallelic RDH12 mutations are a common cause of early-onset retinal dystrophy and a rare cause of cone-rod dystrophy.

Novel Heterozygous Deletion in Retinol Dehydrogenase 12 (RDH12) Causes Familial Autosomal Dominant Retinitis Pigmentosa

Mutations in the retinol dehydrogenase 12 (RDH12) gene are primarily associated with Leber congenital amaurosis (LCA) type 13, a severe early onset autosomal recessive retinal dystrophy. Only one family with a heterozygous variant, associated with mild retinitis pigmentosa (RP), has been reported. We report a novel heterozygous variant [(c.759del; p.(Phe254Leufs^∗24)], resulting in a frameshift and premature termination identified in two unrelated individuals with familial autosomal dominant RP. Both heterozygous variants are associated with a late onset RP phenotype, suggesting a possible genotype-phenotype correlation.

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.

Expanding the phenotypic spectrum in RDH12-associated retinal disease

Retinol dehydrogenase 12, RDH12, plays a pivotal role in the visual cycle to ensure the maintenance of normal vision. Alterations in activity of this protein result in photoreceptor death and decreased vision beginning at an early age and progressing to substantial vision loss later in life. Here we describe 11 patients with retinal degeneration that underwent next-generation sequencing (NGS) with a targeted panel of all currently known inherited retinal degeneration (IRD) genes and whole-exome sequencing to identify the genetic causality of their retinal disease. These patients display a range of phenotypic severity prompting clinical diagnoses of macular dystrophy, cone-rod dystrophy, retinitis pigmentosa, and early-onset severe retinal dystrophy all attributed to biallelic recessive mutations in RDH12. We report 15 causal alleles and expand the repertoire of known RDH12 mutations with four novel variants: c.215A > G (p.Asp72Gly); c.362T > C (p.Ile121Thr); c.440A > C (p.Asn147Thr); and c.697G > A (p.Val233Ille). The broad phenotypic spectrum observed with biallelic RDH12 mutations has been observed in other genetic forms of IRDs, but the diversity is particularly notable here given the prior association of RDH12 primarily with severe early-onset disease. This breadth emphasizes the importance of broad genetic testing for inherited retinal disorders and extends the pool of individuals who may benefit from imminent gene-targeted therapies.

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.

Comparative Genomics Reveals the Genetic Mechanisms of Musk Secretion and Adaptive Immunity in Chinese Forest Musk Deer

The Chinese forest musk deer (Moschus berezovskii; FMD) is an artiodactyl mammal and is both economically valuable and highly endangered. To investigate the genetic mechanisms of musk secretion and adaptive immunity in FMD, we compared its genome to nine other artiodactyl genomes. Comparative genomics demonstrated that eight positively selected genes (PSGs) in FMD were annotated in three KEGG pathways that were related to metabolic and synthetic activity of musk, similar to previous transcriptome studies. Functional enrichment analysis indicated that many PSGs were involved in the regulation of immune system processes, implying important reorganization of the immune system in FMD. FMD-specific missense mutations were found in two PSGs (MHC class II antigen DRA and ADA) that were classified as deleterious by PolyPhen-2, possibly contributing to immune adaptation to infectious diseases. Functional assessment showed that the FMD-specific mutation enhanced the ADA activity, which was likely to strengthen the immune defense against pathogenic invasion. Single nucleotide polymorphism-based inference showed the recent demographic trajectory for FMD. Our data and findings provide valuable genomic resources not only for studying the genetic mechanisms of musk secretion and adaptive immunity, but also for facilitating more effective management of the captive breeding programs for this endangered species.

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.

Detailed clinical characterisation, unique features and natural history of autosomal recessive RDH12-associated retinal degeneration

BACKGROUND

Defects in retinol dehydrogenase 12 (RDH12) account for 3.4%-10.5 % of Leber congenital amaurosis and early-onset severe retinal dystrophy (EOSRD) and are a potential target for gene therapy. Clinical trials in inherited retinal diseases have unique challenges, and natural history studies are critical to successful trial design. The purpose of this study was to characterise the natural history of RDH12-associated retinal degeneration.

METHODS

A retrospective chart review was performed in individuals with retinal degeneration and two likely disease-causing variants in RDH12.

RESULTS

57 subjects were enrolled from nine countries. 33 subjects had clinical records available from childhood. The data revealed an EOSRD, with average age of onset of 4.1 years. Macular atrophy was a universal clinical finding in all subjects, as young as 2 years of age. Scotopic and photopic electroretinography (ERG) responses were markedly reduced in all subjects, and a non-recordable ERG was documented as young as 1 year of age. Assessment of visual acuity, visual field and optical coherence tomography revealed severe loss of function and structure in the majority of subjects after the age of 10 years. Widefield imaging in 23 subjects revealed a unique, variegated watercolour-like pattern of atrophy in 13 subjects and sparing of the peripapillary area in 18 subjects.

CONCLUSIONS

This study includes the largest collection of phenotypic data from children with RDH12-associated EOSRD and provides a comprehensive description of the timeline of vision loss in this severe, early-onset condition. These findings will help identify patients with RDH12-associated retinal degeneration and will inform future design of therapeutic trials.

Structural Insights into the Drosophila melanogaster Retinol Dehydrogenase, a Member of the Short-Chain Dehydrogenase/Reductase Family

The 11-cis-retinylidene chromophore of visual pigments isomerizes upon interaction with a photon, initiating a downstream cascade of signaling events that ultimately lead to visual perception. 11-cis-Retinylidene is regenerated through enzymatic transformations collectively called the visual cycle. The first and rate-limiting enzymatic reaction within this cycle, i.e., the reduction of all-trans-retinal to all-trans-retinol, is catalyzed by retinol dehydrogenases. Here, we determined the structure of Drosophila melanogaster photoreceptor retinol dehydrogenase (PDH) isoform C that belongs to the short-chain dehydrogenase/reductase (SDR) family. This is the first reported structure of a SDR that possesses this biologically important activity. Two crystal structures of the same enzyme grown under different conditions revealed a novel conformational change of the NAD⁺ cofactor, likely representing a change during catalysis. Amide hydrogen-deuterium exchange of PDH demonstrated changes in the structure of the enzyme upon dinucleotide binding. In D. melanogaster, loss of PDH activity leads to photoreceptor degeneration that can be partially rescued by transgenic expression of human RDH12. Based on the structure of PDH, we analyzed mutations causing Leber congenital amaurosis 13 in a homology model of human RDH12 to obtain insights into the molecular basis of RDH12 disease-causing mutations.

Conditional Ablation of Retinol Dehydrogenase 10 in the Retinal Pigmented Epithelium Causes Delayed Dark Adaption in Mice

Regeneration of the visual chromophore, 11-cis-retinal, is a crucial step in the visual cycle required to sustain vision. This cycle consists of sequential biochemical reactions that occur in photoreceptor cells and the retinal pigmented epithelium (RPE). Oxidation of 11-cis-retinol to 11-cis-retinal is accomplished by a family of enzymes termed 11-cis-retinol dehydrogenases, including RDH5 and RDH11. Double deletion of Rdh5 and Rdh11 does not limit the production of 11-cis-retinal in mice. Here we describe a third retinol dehydrogenase in the RPE, RDH10, which can produce 11-cis-retinal. Mice with a conditional knock-out of Rdh10 in RPE cells (Rdh10 cKO) displayed delayed 11-cis-retinal regeneration and dark adaption after bright light illumination. Retinal function measured by electroretinogram after light exposure was also delayed in Rdh10 cKO mice as compared with controls. Double deletion of Rdh5 and Rdh10 (cDKO) in mice caused elevated 11/13-cis-retinyl ester content also seen in Rdh5(-/-)Rdh11(-/-) mice as compared with Rdh5(-/-) mice. Normal retinal morphology was observed in 6-month-old Rdh10 cKO and cDKO mice, suggesting that loss of Rdh10 in the RPE does not negatively affect the health of the retina. Compensatory expression of other retinol dehydrogenases was observed in both Rdh5(-/-) and Rdh10 cKO mice. These results indicate that RDH10 acts in cooperation with other RDH isoforms to produce the 11-cis-retinal chromophore needed for vision.

Exome Sequencing Identified a Recessive RDH12 Mutation in a Family with Severe Early-Onset Retinitis Pigmentosa

Retinitis pigmentosa (RP) is the most important hereditary retinal disease caused by progressive degeneration of the photoreceptor cells. This study is to identify gene mutations responsible for autosomal recessive retinitis pigmentosa (arRP) in a Chinese family using next-generation sequencing technology. A Chinese family with 7 members including two individuals affected with severe early-onset RP was studied. All patients underwent a complete ophthalmic examination. Exome sequencing was performed on a single RP patient (the proband of this family) and direct Sanger sequencing on other family members and normal controls was followed to confirm the causal mutations. A homozygous mutation c.437T<A (p.V146D) in the retinol dehydrogenase 12 (RDH12) gene, which encodes an NADPH-dependent retinal reductase, was identified as being related to the phenotype of this arRP family. This homozygous mutation was detected in the two affected patients, but not present in other family members and 600 normal controls. Another three normal members in the family were found to carry this heterozygous missense mutation. Our results emphasize the importance of c.437T<A (p.V146D) substitution in RDH12 and provide further support for the causative role of this mutation in the pathogenesis and clinical diagnosis of RP.

Genome-wide homozygosity mapping in families with leber congenital amaurosis identifies mutations in AIPL1 and RDH12 genes

Leber congenital amaurosis (LCA) causes severe visual impairment and blindness very early in life. Mutant alleles of several genes acting in different pathways, of which all have critical roles for normal retinal function, were involved in LCA development. The purpose of this study was to use genome-wide genotyping to identify LCA-causing loci in two Turkish families. Genome-wide genotyping and haplotype analysis were performed for prioritization of candidate genes for mutation screening in families with LCA. Identified informative critical choromosomal regions obtained by homozygosity mapping from the families were searched for overlapping of any LCA causative genes. Corresponding clinical phenotypes of the patients with identified mutations were evaluated. In this study, two families were shown to be linked to two different LCA loci covering retinol dehydrogenase 12 (RDH12) and aryl-hydrocarbon-interacting protein-like1 (AIPL1) genes. Mutation screening revealed a novel p.Gln141* mutation in the AIPL1 gene and a previously described p.Thr49Met mutation in the RDH12 gene in a homozygous state. Our patients with the RDH12 mutation had the distinct macular coloboma sign, and the patient with the AIPL1 mutation developed microphthalmia and severe widespread retinal pigment epithelial atrophy, in contrast to previously reported cases. It is currently evident that mutation screening needs to be done in at least 18 genes known to be associated with LCA. Thus, homozygosity mapping is an alternative technique to improve the molecular diagnosis in LCA, which is a group of genetically and clinically heterogeneous diseases causing retinal degeneration. The patients without mutation in known genes may further be analyzed by using next-generation sequencing.

Mutation screening of retinal dystrophy patients by targeted capture from tagged pooled DNAs and next generation sequencing

Purpose

Retinal dystrophies are genetically heterogeneous, resulting from mutations in over 200 genes. Prior to the development of massively parallel sequencing, comprehensive genetic screening was unobtainable for most patients. Identifying the causative genetic mutation facilitates genetic counselling, carrier testing and prenatal/pre-implantation diagnosis, and often leads to a clearer prognosis. In addition, in a proportion of cases, when the mutation is known treatment can be optimised and patients are eligible for enrolment into clinical trials for gene-specific therapies.

Methods

Patient genomic DNA was sheared, tagged and pooled in batches of four samples, prior to targeted capture and next generation sequencing. The enrichment reagent was designed against genes listed on the RetNet database (July 2010). Sequence data were aligned to the human genome and variants were filtered to identify potential pathogenic mutations. These were confirmed by Sanger sequencing.

Results

Molecular analysis of 20 DNAs from retinal dystrophy patients identified likely pathogenic mutations in 12 cases, many of them known and/or confirmed by segregation. These included previously described mutations in ABCA4 (c.6088C>T,p.R2030*; c.5882G>A,p.G1961E), BBS2 (c.1895G>C,p.R632P), GUCY2D (c.2512C>T,p.R838C), PROM1 (c.1117C>T,p.R373C), RDH12 (c.601T>C,p.C201R; c.506G>A,p.R169Q), RPGRIP1 (c.3565C>T,p.R1189*) and SPATA7 (c.253C>T,p.R85*) and new mutations in ABCA4 (c.3328+1G>C), CRB1 (c.2832_2842+23del), RP2 (c.884-1G>T) and USH2A (c.12874A>G,p.N4292D).

Conclusions

Tagging and pooling DNA prior to targeted capture of known retinal dystrophy genes identified mutations in 60% of cases. This relatively high success rate may reflect enrichment for consanguineous cases in the local Yorkshire population, and the use of multiplex families. Nevertheless this is a promising high throughput approach to retinal dystrophy diagnostics.

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.

Exome sequencing identifies RDH12 compound heterozygous mutations in a family with severe retinitis pigmentosa

OBJECTIVE

Retinitis pigmentosa (RP) is the most prevalent type of inherited retinal degeneration and one of the commonest causes of genetically determined visual dysfunction worldwide. To date, approximately 35 genes have been associated with nonsyndromic autosomal recessive RP (arRP), however the small contribution of each gene to the total prevalence of arRP and the lack of a clear genotype-phenotype correlation complicate the genetic analysis in affected patients. Next generation sequencing technologies are powerful and cost-effective methods for detecting causative mutations in both sporadic and familial RP cases.

METHODS

A Mexican family with 5 members affected from arRP was studied. All patients underwent a complete ophthalmologic examination. Molecular methods included genome-wide SNP homozygosity mapping, exome sequencing analysis, and Sanger-sequencing confirmation of causal mutations.

RESULTS

No regions of shared homozygosity among affected subjects were identified. Exome sequencing in a single patient allowed the detection of two missense mutations in the RDH12 gene: a c.446T>C transition predicting a novel p.L149P substitution, and a c.295C>A transversion predicting a previously reported p.L99I replacement. Sanger sequencing confirmed that all affected subjects carried both RDH12 mutations.

CONCLUSIONS

This study adds to the molecular spectrum of RDH12-related retinopathy and offers an additional example of the power of exome sequencing in the diagnosis of recessively inherited retinal degenerations.

Dynamic changes in the secondary structure of ECE-1 and XCE account for their different substrate specificities

Background

X-converting enzyme (XCE) involved in nervous control of respiration, is a member of the M13 family of zinc peptidases, for which no natural substrate has been identified yet. In contrast, it’s well characterized homologue endothelin-converting enzyme-1 (ECE-1) showed broad substrate specificity and acts as endopeptidase as well as dipeptidase. To explore the structural differences between XCE and ECE-1, homology model of XCE was built using the complex structure of ECE-1 with phosphoramidon (pdb-id: 3DWB) as template. Phosphoramidon was docked into the binding site of XCE whereas phosphate oxygen of the inhibitor was used as water molecule to design the apo forms of both enzymes. Molecular dynamics simulation of both enzymes was performed to analyze the dynamic nature of their active site residues in the absence and presence of the inhibitor.

Results

Homology model of XCE explained the role of non-conserved residues of its S2’ subsite. Molecular dynamics (MD) simulations identified the flexible transitions of F149/I150, N566/N571, W714/W719, and R145/R723 residues of ECE-1/XCE for the strong binding of the inhibitor. Secondary structure calculations using DSSP method reveals the folding of R145/R723 residue of ECE-1/XCE into β-sheet structure while unfolding of the S2’ subsite residues in aECE-1 and sustained compact folding of that of aXCE. The results evaluated are in good agreement with available experimental data, thus providing detailed molecular models which can explain the structural and specificities differences between both zinc peptidases.

Conclusions

Secondary structure changes of both enzymes during the simulation time revealed the importance of β-sheet structure of R145/R723 for its binding with the terminal carboxylate group of the inhibitor. Unfolding of the α-helix comprising the S2’ subsite residues in aECE-1 correlate well with its endopeptidase activity while their compact folding in aXCE may account for the inactivity of the enzyme towards large C-terminal containing substrates.

RDH12 retinopathy: novel mutations and phenotypic description

PURPOSE

To identify patients with autosomal recessive retinal dystrophy caused by mutations in the gene, retinal dehydrogenase 12 (RDH12), and to report the associated phenotype.

METHODS

After giving informed consent, all patients underwent full clinical evaluation. Patients were selected for mutation analysis based upon positive results from the Asper Ophthalmics Leber congenital amaurosis arrayed primer extansion (APEX) microarray screening, linkage analysis, or their clinical phenotype. All coding exons of RDH12 were screened by direct Sanger sequencing. Potential variants were checked for segregation in the respective families and screened in controls, and their pathogenicity analyzed using in silico prediction programs.

RESULTS

Screening of 389 probands by the APEX microarray and/or direct sequencing identified bi-allelic mutations in 29 families. Seventeen novel mutations were identified. The phenotype in these patients presented with a severe early-onset rod-cone dystrophy. Funduscopy showed severe generalized retinal pigment epithelial and retinal atrophy, which progressed to dense, widespread intraretinal pigment migration by adulthood. The macula showed severe atrophy, with pigmentation and yellowing, and corresponding loss of fundus autofluorescence. Optical coherence tomography revealed marked retinal thinning and excavation at the macula.

CONCLUSIONS

RDH12 mutations account for approximately 7% of disease in our cohort of patients diagnosed with Leber congenital amaurosis and early-onset retinal dystrophy. The clinical features of this disorder are highly characteristic and facilitate candidate gene screening. The term RDH12 retinopathy is proposed as a more accurate description.

Evidence that proteosome inhibitors and chemical chaperones can rescue the activity of retinol dehydrogenase 12 mutant T49M

Retinol dehydrogenase 12 (RDH12) is a microsomal enzyme that catalyzes the reduction of all-trans-retinaldehyde to all-trans-retinol when expressed in cells. Mutations in RDH12 cause severe retinal degeneration; however, some of the disease-associated RDH12 mutants retain significant catalytic activity. Our previous study (Lee et al., 2010 [9]) demonstrated that the catalytically active T49M and I51N variants of RDH12 undergo accelerated degradation through the ubiquitin-proteosome system, which results in reduced levels of these proteins in the cells. Here, we investigated whether the stabilization of T49M or I51N RDH12 protein levels through the inhibition of proteosome activity or improved folding could rescue their retinaldehyde reductase activity. For the T49M variant, the inhibition of proteosome activity resulted in an increased level of T49M protein in the microsomal fraction. The higher level of the T49M variant in microsomes correlated with the higher microsomal retinaldehyde reductase activity. T49M-expressing living cells treated with the inhibitors of proteosome activity or with dimethyl sulfoxide exhibited an increase in the conversion of retinaldehyde to retinol, consistent with the recovery of functional RDH12 protein. On the other hand, accumulation of the I51N variant in the microsomes did not result in higher retinaldehyde reductase activity of the microsomes or cells. These results provide a proof of concept that, at least in the case of the T49M variant, the prevention of accelerated degradation could lead to restoration of its function in the cells. This finding justifies further search for more efficient and clinically relevant compounds for stabilizing the T49M variant activity.

Key enzymes of the retinoid (visual) cycle in vertebrate retina

A major goal in vision research over the past few decades has been to understand the molecular details of retinoid processing within the retinoid (visual) cycle. This includes the consequences of side reactions that result from delayed all-trans-retinal clearance and condensation with phospholipids that characterize a variety of serious retinal diseases. Knowledge of the basic retinoid biochemistry involved in these diseases is essential for development of effective therapeutics. Photoisomerization of the 11-cis-retinal chromophore of rhodopsin triggers a complex set of metabolic transformations collectively termed phototransduction that ultimately lead to light perception. Continuity of vision depends on continuous conversion of all-trans-retinal back to the 11-cis-retinal isomer. This process takes place in a series of reactions known as the retinoid cycle, which occur in photoreceptor and RPE cells. All-trans-retinal, the initial substrate of this cycle, is a chemically reactive aldehyde that can form toxic conjugates with proteins and lipids. Therefore, much experimental effort has been devoted to elucidate molecular mechanisms of the retinoid cycle and all-trans-retinal-mediated retinal degeneration, resulting in delineation of many key steps involved in regenerating 11-cis-retinal. Three particularly important reactions are catalyzed by enzymes broadly classified as acyltransferases, short-chain dehydrogenases/reductases and carotenoid/retinoid isomerases/oxygenases. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Retinol dehydrogenase 12 detoxifies 4-hydroxynonenal in photoreceptor cells

Mutations of the photoreceptor retinol dehydrogenase 12 (RDH12) gene cause the early onset retinal dystrophy Leber congenital amaurosis (LCA) by mechanisms not completely resolved. Determining the physiological role of RDH12 in photoreceptors is the focus of this study. Previous studies showed that RDH12, and the closely related retinol dehydrogenase RDH11, can enzymatically reduce toxic lipid peroxidation products such as 4-hydroxynonenal (4-HNE), in vitro. To explore the significance of this activity, we investigated the ability of RDH11 and RDH12 to protect stably transfected HEK-293 cells against the toxicity of 4-HNE. Both enzymes protected against 4-HNE modification of proteins and 4-HNE-induced apoptosis in HEK-293 cells. In the retina, exposure to bright light induced lipid peroxidation, 4-HNE production, and 4-HNE modification of proteins in photoreceptor inner segments, where RDH11 and RDH12 are located. In mouse retina, RDH12-but not RDH11-protected against adduct formation, suggesting that 4-HNE is a physiological substrate of RDH12. RDH12-but not RDH11-also protected against light-induced apoptosis of photoreceptors. We conclude that in mouse retina RDH12 reduces 4-HNE to a nontoxic alcohol, protecting cellular macromolecules against oxidative modification and protecting photoreceptors from light-induced apoptosis. This activity is of particular significance to the understanding of the molecular mechanisms of RDH12-induced LCA.

Disease-associated variants of microsomal retinol dehydrogenase 12 (RDH12) are degraded at mutant-specific rates

Mutations in retinol dehydrogenase 12 (RDH12) cause severe retinal degeneration. However, some of the disease-associated RDH12 mutants retain significant catalytic activity, indicating the existence of additional pathophysiological mechanisms. This study demonstrates that the catalytically active T49M and I51N mutants undergo accelerated degradation, which results in their reduced cellular levels. Inhibition of proteasome leads to significant accumulation of ubiquitylated T49M and I51N. Furthermore, the degree of ubiquitylation strongly correlates with the half-lives of the proteins. These results suggest that the accelerated degradation of RDH12 mutants by the ubiquitin-proteasome system contributes to the pathophysiology and phenotypic variability associated with mutations in the RDH12 gene.

Rdh12 activity and effects on retinoid processing in the murine retina

RDH12 mutations are responsible for early-onset autosomal recessive retinal dystrophy, which results in profound retinal pathology and severe visual handicap in patients. To investigate the function of RDH12 within the network of retinoid dehydrogenases/reductases (RDHs) present in retina, we studied the retinal phenotype of Rdh12-deficient mice. In vivo rates of all-trans-retinal reduction and 11-cis-retinal formation during recovery from bleaching were similar in Rdh12-deficient and wild-type mice matched for an Rpe65 polymorphism that impacts visual cycle efficiency. However, retinal homogenates from Rdh12-deficient mice exhibited markedly decreased capacity to reduce exogenous retinaldehydes in vitro. Furthermore, in vivo levels of the bisretinoid compound diretinoid-pyridinium-ethanolamine (A2E) were increased in Rdh12-deficient mice of various genetic backgrounds. Conversely, in vivo levels of retinoic acid and total retinol were significantly decreased. Rdh12 transcript levels in wild-type mice homozygous for the Rpe65-Leu(450) polymorphism were greater than in Rpe65-Met(450) mice and increased during postnatal development in wild-type mice and Nrl-deficient mice having an all-cone retina. Rdh12-deficient mice did not exhibit increased retinal degeneration relative to wild-type mice at advanced ages, when bred on the light-sensitive BALB/c background, or when heterozygous for a null allele of superoxide dismutase 2 (Sod2(+/-)). Our findings suggest that a critical function of RDH12 is the reduction of all-trans-retinal that exceeds the reductive capacity of the photoreceptor outer segments.

Limited roles of Rdh8, Rdh12, and Abca4 in all-trans-retinal clearance in mouse retina

PURPOSE

Although the retinoid cycle is essential for vision, all-trans-retinal and the side products of this cycle are toxic. Delayed clearance of all-trans-retinal causes accumulation of its condensation products, A2E, and all-trans-retinal dimer (RALdi), both associated with human macular degeneration. The protective roles were examined of the all-trans-RDHs, Rdh8 and Rdh12, and the ATP-binding cassette transporter Abca4, retinoid cycle enzymes involved in all-trans-retinal clearance.

METHODS

Mice genetically engineered to lack Rdh8, Rdh12, and Abca4, either singly or in various combinations, were investigated because all-trans-retinal clearance is achieved by all-trans-RDHs and Abca4. Knockout mice were evaluated by spectral-domain optical coherence tomography (SD-OCT), electroretinography, retinal morphology, and visual retinoid profiling with HPLC and MS. ARPE19 cells were examined to evaluate A2E and RALdi oxidation and toxicity induced by exposure to UV and blue light.

RESULTS

Rdh8(-/-)Abca4(-/-) and Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice displayed slowly progressive, severe retinal degeneration under room light conditions. Intense light-induced acute retinal degeneration was detected by SD-OCT in Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice. Amounts of A2E in the RPE correlated with diminished all-trans-retinal clearance, and the highest A2E amounts were found in Rdh8(-/-)Rdh12(-/-)Abca4(-/-) mice. However oxidized A2E was not found in any of these mice, and A2E oxidation was not induced by blue light and UV illumination of A2E-loaded ARPE19 cells. Of interest, addition of all-trans-retinal did activate retinoic acid receptors in cultured cells.

CONCLUSIONS

Rdh8, Rdh12, and Abca4 all protect the retina and reduce A2E production by facilitating all-trans-retinal clearance. Delayed all-trans-retinal clearance contributes more than A2E oxidation to light-induced cellular toxicity.

Homology modeling in drug discovery: current trends and applications

As structural genomics (SG) projects continue to deposit representative 3D structures of proteins, homology modeling methods will play an increasing role in structure-based drug discovery. Although computational structure prediction methods provide a cost-effective alternative in the absence of experimental structures, developing accurate enough models still remains a big challenge. In this contribution, we report the current developments in this field, discuss in silico modeling limitations, and review the successful application of this technique to different stages of the drug discovery process.

Hereditary retinal disease

PURPOSE OF REVIEW

This article reports recent advances in the diagnosis, genetic analysis, and treatment of hereditary retinal disease.

RECENT FINDINGS

Clinicians and scientists continue to reveal the relationship between phenotype and genotype in hereditary retinal diseases. Persistent investigation and progressive technology are advancing the efficiency of mutation discovery. This technology is also leading to readily available genetic testing that aids clinicians in the diagnosis of these diseases. Functional genetic studies, and laboratory and human clinical trials are occurring that may lead to future treatment of these disorders.

SUMMARY

A literature review of the recent discoveries and potential treatments for retinitis pigmentosa, Leber's congenital amaurosis, X-linked retinoschisis, Best's disease, Stargardt's disease, and congenital stationary night blindness is presented, along with a guide for clinicians seeking genetic testing of patients.

Leber congenital amaurosis: genes, proteins and disease mechanisms

Leber congenital amaurosis (LCA) is the most severe retinal dystrophy causing blindness or severe visual impairment before the age of 1 year. Linkage analysis, homozygosity mapping and candidate gene analysis facilitated the identification of 14 genes mutated in patients with LCA and juvenile retinal degeneration, which together explain approximately 70% of the cases. Several of these genes have also been implicated in other non-syndromic or syndromic retinal diseases, such as retinitis pigmentosa and Joubert syndrome, respectively. CEP290 (15%), GUCY2D (12%), and CRB1 (10%) are the most frequently mutated LCA genes; one intronic CEP290 mutation (p.Cys998X) is found in approximately 20% of all LCA patients from north-western Europe, although this frequency is lower in other populations. Despite the large degree of genetic and allelic heterogeneity, it is possible to identify the causative mutations in approximately 55% of LCA patients by employing a microarray-based, allele-specific primer extension analysis of all known DNA variants. The LCA genes encode proteins with a wide variety of retinal functions, such as photoreceptor morphogenesis (CRB1, CRX), phototransduction (AIPL1, GUCY2D), vitamin A cycling (LRAT, RDH12, RPE65), guanine synthesis (IMPDH1), and outer segment phagocytosis (MERTK). Recently, several defects were identified that are likely to affect intra-photoreceptor ciliary transport processes (CEP290, LCA5, RPGRIP1, TULP1). As the eye represents an accessible and immune-privileged organ, it appears to be uniquely suitable for human gene replacement therapy. Rodent (Crb1, Lrat, Mertk, Rpe65, Rpgrip1), avian (Gucy2D) and canine (Rpe65) models for LCA and profound visual impairment have been successfully corrected employing adeno-associated virus or lentivirus-based gene therapy. Moreover, phase 1 clinical trials have been carried out in humans with RPE65 deficiencies. Apart from ethical considerations inherently linked to treating children, major obstacles for the treatment of LCA could be the putative developmental deficiencies in the visual cortex in persons blind from birth (amblyopia), the absence of sufficient numbers of viable photoreceptor or RPE cells in LCA patients, and the unknown and possibly toxic effects of overexpression of transduced genes. Future LCA research will focus on the identification of the remaining causal genes, the elucidation of the molecular mechanisms of disease in the retina, and the development of gene therapy approaches for different genetic subtypes of LCA.

Retinol dehydrogenases RDH11 and RDH12 in the mouse retina: expression levels during development and regulation by oxidative stress

PURPOSE

RDH11 and RDH12 are closely related retinol dehydrogenases expressed in the retina. RDH12 has been linked to the early-onset retinal dystrophy Leber congenital amaurosis, whereas RDH11 has not been associated with human disease. To understand their physiological roles, the authors investigated their expression during development and their regulation by light-induced oxidative stress in mouse retina.

METHODS

Quantitative RT-PCR and immunoblot analysis were used for quantification of RDH11 and RDH12 during development and oxidative stress. Expression during development was measured between embryonic day (E) 12 and postnatal day (P) 210 (7 months) in C57BL/6 mouse eyes. Expression during light-induced oxidative stress was measured between 2 and 24 hours of exposure to light in BALB/c mouse retina.

RESULTS

The RDH11 level was low and remarkably constant during development and oxidative stress. RDH12 expression started at P7 and increased until P30 to approximately sevenfold higher than RDH11. Oxidative stress induced by exposure to constant bright light led to a rapid and significant decrease of RDH12 protein.

CONCLUSIONS

The low and constant expression of RDH11 suggested a housekeeping function for this enzyme. The onset of RDH12 expression during the maturation of photoreceptor cells suggested a function related to the visual process. The light-induced rapid decrease of RDH12 protein, preceding the decrease of the mRNA, suggested a specific degradation of the protein rather than a regulation of gene expression.

Effect of lipid peroxidation products on the activity of human retinol dehydrogenase 12 (RDH12) and retinoid metabolism

Mutations in human Retinol Dehydrogenase 12 (RDH12) are known to cause photoreceptor cell death but the physiological function of RDH12 in photoreceptors remains poorly understood. In vitro, RDH12 recognizes both retinoids and medium-chain aldehydes as substrates. Our previous study suggested that RDH12 protects cells against toxic levels of retinaldehyde and retinoic acid [S.A. Lee, O.V. Belyaeva, I.K. Popov, N.Y. Kedishvili, Overproduction of bioactive retinoic acid in cells expressing disease-associated mutants of retinol dehydrogenase 12, J. Biol. Chem. 282 (2007) 35621-35628]. Here, we investigated whether RDH12 can also protect cells against highly reactive medium-chain aldehydes. Analysis of cell survival demonstrated that RDH12 was protective against nonanal but not against 4-hydroxynonenal. At high concentrations, nonanal inhibited the activity of RDH12 towards retinaldehyde, suggesting that nonanal was metabolized by RDH12. 4-Hydroxynonenal did not inhibit the RDH12 retinaldehyde reductase activity, but it strongly inhibited the activities of lecithin:retinol acyl transferase and aldehyde dehydrogenase, resulting in decreased levels of retinyl esters and retinoic acid and accumulation of unesterified retinol. Thus, the results of this study showed that RDH12 is more effective in protection against retinaldehyde than against medium-chain aldehydes, and that medium-chain aldehydes, especially 4-hydroxynonenal, severely disrupt cellular retinoid homeostasis. Together, these findings provide a new insight into the effects of lipid peroxidation products and the impact of oxidative stress on retinoid metabolism.

Retinal morphology in patients with BBS1 and BBS10 related Bardet-Biedl Syndrome evaluated by Fourier-domain optical coherence tomography

Retinal dystrophy in Bardet-Biedl Syndrome (BBS) is caused by defective genes that are expressed within ciliated cells such as photoreceptors. The purpose of this study was to characterize and compare the retinal structure and lamination of two groups of patients, carrying mutations in BBS1 or BBS10. Eight patients with BBS (ages 11.9-28.5 years) and mutations in BBS1 (4/8) or BBS10 (4/8) were tested. A high-resolution hand-held probe Fourier-domain optical coherence tomography system (Fd-OCT) was used for retinal image acquisition. Macular scans were evaluated with respect to structure, retinal layering and photoreceptor integrity. Micro-structural in-vivo analysis showed abnormalities within retinal layers but preserved retinal lamination. Photoreceptor integrity was disrupted in all patients. Macular scans from patients with BBS10 mutations most often showed 'deposits' adjacent and anterior to Bruch's membrane. Age, genotype and presence of macular changes did not correlate with the structural changes observed. Retinal dystrophy in BBS is reflected by major changes in the outer retinal layers. This is the first report of in-vivo micro-structural analysis of retinal layers in patients with BBS. Mutations in different BBS genes seem to be associated with similar micro-structural changes in retinal layers.

Overproduction of bioactive retinoic acid in cells expressing disease-associated mutants of retinol dehydrogenase 12

Retinol dehydrogenase 12 (RDH12) is an NADP(+)-dependent oxidoreductase that in vitro catalyzes the reduction of all-trans-retinaldehyde to all-trans-retinol or the oxidation of retinol to retinaldehyde depending on substrate and cofactor availability. Recent studies have linked the mutations in RDH12 to severe early-onset autosomal recessive retinal dystrophy. The biochemical basis of photoreceptor cell death caused by mutations in RDH12 is not clear because the physiological role of RDH12 is not yet fully understood. Here we demonstrate that, although bi-directional in vitro, in living cells, RDH12 acts exclusively as a retinaldehyde reductase, shifting the retinoid homeostasis toward the increased levels of retinol and decreased levels of bioactive retinoic acid. The retinaldehyde reductase activity of RDH12 protects the cells from retinaldehyde-induced cell death, especially at high retinaldehyde concentrations, and this protective effect correlates with the lower levels of retinoic acid in RDH12-expressing cells. Disease-associated mutants of RDH12, T49M and I51N, exhibit significant residual activity in vitro, but are unable to control retinoic acid levels in the cells because of their dramatically reduced affinity for NADPH and much lower protein expression levels. These results suggest that RDH12 acts as a regulator of retinoic acid biosynthesis and protects photoreceptors against overproduction of retinoic acid from all-trans-retinaldehyde, which diffuses into the inner segments of photoreceptors from illuminated rhodopsin. These results provide a novel insight into the mechanism of retinal degeneration associated with mutations in RDH12 and are consistent with the observation that RDH12-null mice are highly susceptible to light-induced retinal apoptosis in cone and rod photoreceptors.