Molecular analysis and phenotypic study in 14 Chinese families with Bietti crystalline dystrophy

Natural history of progressive vision loss in Bietti crystalline dystrophy: a model-based meta-analysis

PURPOSE

Bietti crystalline corneoretinal dystrophy (BCD) is an autosomal recessive progressive retinal degenerative disease due to mutations in the CYP4V2 gene. Best-corrected visual acuity (BCVA) is a common primary endpoint in clinical trials for retinal diseases, but the natural history of BCVA loss remains unclear because of the heterogeneity of manifestations in BCD patients.

METHODS

Based on the individual data of untreated BCD patients, a disease progression model was established using the change in BCVA from baseline as an index, and covariates including age of onset, age, duration of disease, baseline BCVA, gender, race (East Asian/non-East Asian), genotype, and family history. Then, based on the final model, the natural disease progression characteristics of BCD were simulated.

RESULT

A total of 14 studies met the inclusion criteria, with a total sample size of 117 cases, including 6 studies (N=80) with East Asian populations and 9 studies (N=37) with non-East Asian populations. The change of BCVA from baseline increased linearly with time, and the disease progression model of BCD was successfully established. BCVA increased by 0.06 logarithm of the minimum angle of resolution (LogMAR) per year in BCD patients. BCVA increased by 0.09 LogMAR per year in patients with BCVA≥0.5LogMAR and disease duration more than 10 years.

CONCLUSIONS

For the first time, we successfully established a BCD disease progression model based on the change in BCVA from baseline. The mean visual acuity loss increased linearly with the progression of the disease. A sharper loss of BCVA may be expected in patients with BCVA≥0.5LogMAR and disease duration ≥10 years.

Crystalline Hepatopathy Associated With Bietti Crystalline Dystrophy: A Striking Manifestation of Disordered Fatty Acid Metabolism

Bietti crystalline dystrophy (BCD) is a rare heritable retinal disease characterized by crystal deposition primarily in the retina. It is associated with atrophy of the retinal pigment epithelium (RPE) and is caused by variants in CYP4V2 , which encodes a cytochrome P450 hemethiolate protein superfamily member. CYP4V2 is involved in the selective hydrolysis of saturated medium chain fatty acids, and patients with BCD demonstrate abnormalities in fatty acid metabolism, including abnormal lipid profiles and the accumulation of the pathogenic crystals within the RPE, which leads to the visual pathologies characteristic of BCD. However, the precise identity of the crystals is currently unknown, and BCD has no established extraocular manifestations. Here, we report granulomatous hepatitis associated with abundant diffuse crystalline clefts in the hepatic parenchyma in 3 patients with retinal dystrophy and dyslipidemia: 2 with pathogenic CYP4V2 variants and 1 patient with clinical ophthalmologic findings suggestive of BCD but without available genetic testing. The unique and striking histologic features unifying the liver biopsies in all 3 patients strongly support a process related to abnormal fatty acid metabolism underlying the genetic disease of BCD, expanding the spectrum of BCD and shedding light on the importance of CYP4V2 in systemic fatty acid metabolism.

Clinical and genetic characterization of a large cohort of Chinese patients with Bietti crystalline retinopathy

PURPOSE

To investigate the clinical and genetic characteristics for a large cohort of Chinese patients with Bietti crystalline retinopathy (BCR).

METHODS

A total of 208 Chinese BCR patients from 175 families were recruited. Comprehensive clinical evaluations and genetic analysis were performed. Genotype-phenotype correlations were evaluated through statistical analysis.

RESULTS

The patients' median age was 37 years (range, 20-76 years). The median best corrected visual acuity (BCVA) was 0.8 LogMAR unit (range, 2.8 to -0.12). A significant decline of BCVA was revealed in patients over 40 years old (P<0.001). Two clinical types were observed: peripheral type (type P) and central type (type C). Significantly more type C patients had a worse central visual acuity, but a more preserved retinal function (P<0.05). Molecular screening detected biallelic CYP4V2 pathogenic variants in 98.3% (172/175) of the families, including 19 novel ones. The most frequent pathogenic variant was c.802-8_810del17insGC, with the allele frequency of 55.7% (195/350), followed by c.992A>C (28/350, 8%) and c.1091-2A>G (23/350, 6.6%). BCR patients with one c.802-8_810del17insGC and one truncating variant (IVS6-8/Tru) had BCVA>1.3 LogMAR unit (Snellen equivalent<20/400) at a younger age than those with homozygous c.802-8_810del17insGC variants (homo IVS6-8) (P=0.031).

CONCLUSIONS

BCR patients preserved relatively good vision before 40 years old. Two distinct clinical types of BCR were observed. BCR patients with IVS6-8/Tru had an earlier decline in visual acuity than those with homo IVS6-8. Our findings enhance the knowledge of BCR and will be helpful in patient selection for gene therapy.

CHARACTERIZATION OF MACULAR NEOVASCULARIZATION IN BIETTI CRYSTALLINE DYSTROPHY USING MULTIMODAL IMAGING MODALITIES

PURPOSE

To characterize the clinical features of macular neovascularization (MNV) secondary to Bietti crystalline dystrophy.

METHODS

The imaging data of 157 eyes in 79 patients with Bietti crystalline dystrophy were retrospectively reviewed. 12 individuals (19 eyes) were found to have MNVs. Multimodal retinal imaging was used to evaluate the features of MNVs and the primary chorioretinopathy.

RESULTS

The MNV lesions were shown as typical type 2 MNVs with subretinal hyperreflective material (SHRM), and usually detected along the borders of the retinal pigment epithelium/choriocapillaris dropout. The active MNVs were noted in earlier stages of Bietti crystalline dystrophy, while the activity was observed to be reduced in advanced cases. On spectral domain optical coherence tomography, the outer retinal structures were demonstrated to be partially preserved above the SHRMs compared with the extensive atrophy contiguously. Fibrotic scaring of the MNVs was commonly observed and arteriolarization was usually shown within the scars.

CONCLUSION

MNV was demonstrated to be a common complication secondary to Bietti crystalline dystrophy. The lesions were typical type 2 MNV of varied activities possibly associated with the degrees of the primary degeneration. Choriocapillaris hypoperfusion may participate in MNV development.

Diagnostic and Management Strategies of Bietti Crystalline Dystrophy: Current Perspectives

Bietti crystalline dystrophy (BCD) is a rare, genetically determined chorioretinal dystrophy presenting with intraretinal crystalline deposits and varying degrees of progressive chorioretinal atrophy commencing at the posterior pole. In some cases, there can be concomitant corneal crystals noted first in the superior or inferior limbus. CYP4V2 gene, a member of the cytochrome P450 family is responsible for the disease and more than 100 mutations have been defined thus far. However, a genotype-phenotype correlation has not been established yet. Visual impairment commonly occurs between the second and third decades of life. By the fifth or sixth decade of life, vision loss can become so severe that the patient may potentially become legally blind. Multitudes of multimodal imaging modalities can be utilized to demonstrate the clinical features, course, and complications of the disease. This present review aims to reiterate the clinical features of BCD, update the clinical perspectives with the help of multimodal imaging techniques, and overview its genetic background with future therapeutic approaches.

Corneal deposits and nerve alterations in Bietti Corneoretinal Crystalline Dystrophy imaged using in vivo confocal microscopy

PURPOSE

To evaluate the imaging features of corneal deposits and nerve alterations in Chinese patients with Bietti Corneoretinal Crystalline Dystrophy (BCD) using in vivo confocal microscopy (IVCM).

METHODS

Twenty patients with BCD and 20 age- and sex-matched healthy controls were enrolled in this retrospective, observational study. Corneal deposits and sub-basal nerve plexus (SNP) were observed by IVCM. Parameters of SNP including total nerve density/number, main nerve trunk density/number, and branch nerve density/number were analyzed by Neuron J.

RESULTS

Corneal deposits were observed in both eyes of all patients by IVCM. These crystals appeared as dot-shaped, needle-shaped, and rod-shaped hyperreflective bodies and were located not only in the sub-epithelium and stroma of cornea, but in endothelium which were not reported before. There was a decrease of total nerve density (P < 0.001), main nerve trunk density (P = 0.007), and branch nerve density (P = 0.001), in BCD compared to controls. The number of total nerves/frame (P = 0.001), main nerve trunks/frame (P = 0.005), and branch nerves/frame (P = 0.006) in BCD were lower than controls.

CONCLUSION

New findings in locations of corneal crystals by IVCM expand the phenotype spectrum of BCD. Corneal deposits may be useful for diagnosis of BCD, especially ones without retinal deposits. Corneal nerve parameters were reduced in BCD, which may provide new insights to be further explored to contribute to our understanding of BCD. IVCM is a promising tool to evaluate corneal deposits and nerve alterations in BCD.

AAV2-mediated gene therapy for Bietti crystalline dystrophy provides functional CYP4V2 in multiple relevant cell models

Bietti crystalline dystrophy (BCD) is an inherited retinal disease (IRD) caused by mutations in the CYP4V2 gene. It is a relatively common cause of IRD in east Asia. A number of features of this disease make it highly amenable to gene supplementation therapy. This study aims to validate a series of essential precursor in vitro experiments prior to developing a clinical gene therapy for BCD. We demonstrated that HEK293, ARPE19, and patient induced pluripotent stem cell (iPSC)-derived RPE cells transduced with AAV2 vectors encoding codon optimization of CYP4V2 (AAV2.coCYP4V2) resulted in elevated protein expression levels of CYP4V2 compared to those transduced with AAV2 vectors encoding wild type CYP4V2 (AAV2.wtCYP4V2), as assessed by immunocytochemistry and western blot. Similarly, we observed significantly increased CYP4V2 enzyme activity in cells transduced with AAV2.coCYP4V2 compared to those transduced with AAV2.wtCYP4V2. We also showed CYP4V2 expression in human RPE/choroid explants transduced with AAV2.coCYP4V2 compared to those transduced with AAV2.wtCYP4V2. These preclinical data support the further development of a gene supplementation therapy for a currently untreatable blinding condition—BCD. Codon-optimized CYP4V2 transgene was superior to wild type in terms of protein expression and enzyme activity. Ex vivo culture of human RPE cells provided an effective approach to test AAV-mediated transgene delivery.

Whole-exome sequencing identified genes known to be responsible for retinitis pigmentosa in 28 Chinese families

PURPOSE

Retinitis pigmentosa (RP) is a group of highly heterogenetic inherited retinal degeneration diseases. Molecular genetic diagnosis of RP is quite challenging because of the complicated disease-causing mutation spectrum. The aim of this study was to explore the mutation spectrum in Chinese RP patients using next-generation sequencing technology and to explore the genotype-phenotype relationship.

METHOD

In this study, a cost-effective strategy using whole-exome sequencing (WES) was employed to address the genetic diagnosis of 28 RP families in China. One to two patients and zero to two healthy relatives were sequenced in each family. All mutations in WES data that passed through the filtering procedure were searched in relation to 662 gene defects that can cause vision-associated phenotypes (including 89 RP genes in the RetNet Database). All patients visiting the outpatient department received comprehensive ophthalmic examinations.

RESULT

Twenty-five putative pathogenic mutations of 12 genes were detected by WES and were all confirmed by Sanger sequencing in 20 (20/28, 71.4%) families, including the 12 following genes: USH2A, CYP4V2, PRPF31, RHO, RP1, CNGA1, CNGB1, EYS, PRPF3, RP2, RPGR, and TOPORS. Three families were rediagnosed as having Bietti crystalline dystrophy (BCD). USH2A (4/20, 20%) and CYP4V2 (3/20, 15%) were found to be the most frequent mutated genes. Seven novel mutations were identified in this research, including mutations in USH2A1, USH2A2, PRPF31, RP2, TOPORS, CNGB1, and RPGR. Phenotype and genotype relationships in the 12 RP genes were analyzed, which revealed later disease onset and more severe visual function defects in CYP4V2.

CONCLUSION

Twenty-five putative pathogenic mutations of 12 genes were detected by WES, and these were all confirmed by Sanger sequencing in 20 (20/28, 71.4%) families, including seven novel mutations. USH2A and CYP4V2 were found to be the most frequent genes in this research. Phenotype and genotype relationships were revealed, and the mutation spectrum of RP in Chinese populations was expanded in this research, which may benefit future cutting-edge therapies.

PREDICTED PROTEIN STRUCTURE VARIATIONS INDICATE THE CLINICAL PRESENTATION OF CYP4V2-RELATED BIETTI CRYSTALLINE DYSTROPHY

PURPOSE

To investigate the relationship between different CYP4V2 disease-causing variants and disease severity in Bietti crystalline dystrophy (BCD).

METHODS

Twenty-one subjects from 19 unrelated families with a clinical diagnosis of BCD were enrolled. A novel severity prediction score for BCD based on the predicted molecular impact of CYP4V2 variants was applied for grouping and subsequent analyses. The more severe variants led to less CYP4V2 protein function preservation and a higher severity prediction score.

RESULTS

All subjects harbored two alleles of CYP4V2 disease-causing variants, of which c.802-8_810del17insGC was the most prevalent (14/21, 66.67%) and c.1507G>C was novel. According to the severity score, the subjects were categorized into severe, moderate, and mild groups with different preservation of central vision (mean logMAR visual acuity 0.95 ± 0.82, 0.89 ± 1.22, and 0.56 ± 0.64, respectively). The patients with a lower severity score had slower disease progression.

CONCLUSION

This is the first cohort study of BCD in Taiwan, and we established a novel BCD severity index based on the molecular impact of different CYP4V2 variants. More severe impairment of CYP4V2 protein led to a more severe disease course with earlier progression. Our results could be helpful in identifying a therapeutic window for patients with BCD.

Identification of a novel compound heterozygous CYP4V2 variant in a patient with autosomal recessive retinitis pigmentosa

Retinitis pigmentosa (RP) belongs to a family of retinal disorders that is characterized by the progressive degeneration of rod and cone photoreceptors. The aim of the present study was to screen for possible disease-causing genetic variants in a non-consanguineous Chinese family with non-syndromic autosomal recessive RP. Whole-exome sequencing (WES) was performed in samples from the affected individual (the proband) and those from the two children of the proband. A novel compound heterozygous variant of c.C958T (p.R320X) and c.G1355A (p.R452H) in the Cytochrome P450 family 4 subfamily V member 2 (CYP4V2) gene was identified through WES. Subsequently, this variant was validated by direct Sanger sequencing. This compound heterozygous variant was found to be absent from other unaffected family members and 400 ethnically-matched healthy control individuals. In addition, this compound variant was co-segregated with the RP phenotype in an autosomal recessive manner. In silico analysis revealed that both c.C958T (p.R320X) and c.G1355A (p.R452H) could compromise the protein function of CYP4V2. These results strongly suggest this compound variant to be a disease-causing variant, which expands upon the spectrum of currently known CYP4V2 genetic variants associated with retinal diseases.

Intereye Symmetry in Bietti Crystalline Dystrophy

PURPOSE

To evaluate anatomic and functional intereye symmetry among individuals with Bietti crystalline dystrophy (BCD) using clinical and multimodal imaging methods, with a focus on the number, area, and distribution of the characteristic retinal crystalline deposits.

DESIGN

Observational case series with prospective and retrospective data.

METHODS

Setting: Multicenter.

STUDY POPULATION

Thirteen Australian and New Zealand participants (26 eyes) with confirmed biallelic CYP4V2 mutations and a characteristic BCD fundus appearance. Procedures and main outcome measures: Crystals visible on color fundus photography were manually counted. Crystals were superimposed on aligned multimodal fundus images. Spearman's correlation coefficients (ρ), intraclass correlation coefficients (ICCs), and Bland-Altman plots were used to quantify symmetry between eyes.

MAIN OUTCOME MEASURES

Fundus crystal area and count, and absent-autofluorescence (absent-AF) area.

RESULTS

Median participant age was 48 years (interquartile range: 40-60 years). Intereye symmetry was high for fundus crystal area (ρ = 1.00, 95% confidence interval [CI]: 1.00-1.00; ICC = 0.97, 95% CI: 0.88-0.99), fundus crystal count (ρ = 0.98, 95% CI: 0.92-1.00; ICC = 0.97, 95% CI: 0.89-0.99), and absent-AF area (ρ = 0.88, 95% CI: 0.53-0.98; ICC = 0.98, 95% CI: 0.90-0.99). Average foveal volume, foveal crystal count and area, average and central foveal thickness, best corrected visual acuity, and average macular and central foveal sensitivity were not highly correlated between eyes.

CONCLUSIONS

This study demonstrated strong intereye symmetry measured by fundus crystal area, fundus crystal number, and absent-AF area. This may influence the choice of outcome measures for future therapeutic trials for BCD and provides valuable clinical information for ophthalmologists involved in the care and counseling of patients with BCD.

[Generation and characterization of Cyp4v3 gene knockout mice]

OBJECTIVE

Bietti crystalline dystrophy (BCD) is a rare degenerative eye disease caused by mutations in the CYP4V2 gene, and Cyp4v3 is the murine ortholog to CYP4V2. To better understand the molecular pathogenesis of this disease and to explore the potential treatment we have established a Cyp4v3 knock-out mouse model.

METHODS

Cyp4v3^-/- mice were generated by clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 in embryonic stem cells of C57BL/6J mice. Ocular morphologic characteristics were evaluated via fundus imaging, histologic analysis of rods and cones via immunofluorescence, and phalloidin stain to observe retinal pigment epithelium (RPE) in whole-mounts, electroretinogram (ERG) was also conducted to examine the retinal function.

RESULTS

The characteristic features of BCD recurred in the Cyp4v3^-/- mice, including retinal crystalline deposits, atrophy and degeneration of RPE cells, and ERG amplitude decline of dark and light adapted a- and b- wave; however, the immunofluorescence stain of rod and cone cells did not show obvious differences when compared with the wild type (WT) mice. In the early stage of the disease, no crystal-like deposits were found in the fundus, ERG detection of the retinal function did not find a significant decline, and the morphological structure and quantity of the neural retina and RPE did not change significantly. Crystalline deposits occurred and converged when the Cyp4v3^-/- mice at the end of 6 months, and the deposits disappeared when the Cyp4v3^-/- mice at the end of 12 months. The ERG amplitude started to decline when the Cyp4v3^-/- mice at the end of 6 months and deteriorated at the end of 12 months. The RPE cells of the 12-month old Cyp4v3^-/- mice showed irregular shape by phalloidin staining of F-actin. The Cyp4v3^-/- mice behaved normally and were viable and fertile when maintained under specific pathogen-free (SPF) housing conditions.

CONCLUSION

Just like BCD patients, the disease progress of Cyp4v3^-/- mouse is correlated with the age, which provides a good model for pathogenesis and gene therapy study in the future. The atrophy and degeneration of RPE take the lead in progressing of the disease, but the mechanism is not clear yet.

CYP4V2 fatty acid omega hydroxylase, a druggable target for the treatment of metabolic associated fatty liver disease (MAFLD)

Fatty acids are essential in maintaining cellular homeostasis by providing lipids for energy production, cell membrane integrity, protein modification, and the structural demands of proliferating cells. Fatty acids and their derivatives are critical bioactive signaling molecules that influence many cellular processes, including metabolism, cell survival, proliferation, migration, angiogenesis, and cell barrier function. The CYP4 Omega hydroxylase gene family hydroxylate various short, medium, long, and very-long-chain saturated, unsaturated and polyunsaturated fatty acids. Selective members of the CYP4 family metabolize vitamins and biochemicals with long alkyl side chains and bioactive prostaglandins, leukotrienes, and arachidonic acids. It is uncertain of the physiological role of different members of the CYP4 omega hydroxylase gene family in the metabolic control of physiological and pathological processes in the liver. CYP4V2 is a unique member of the CYP4 family. CYP4V2 inactivation in retinal pigment epithelial cells leads to cholesterol accumulation and Bietti's Crystalline Dystrophy (BCD) pathogenesis. This commentary provides information on the role CYP4V2 has in metabolic syndrome and nonalcoholic fatty liver disease progression. This is accomplished by identifying its role in BCD, its control of cholesterol synthesis and lipid droplet formation in C. elegans, and the putative function in cardiovascular disease and gastrointestinal/hepatic pathologies.

Genotype and Ocular Phenotype in Sixteen Chinese Patients with Bietti Corneoretinal Crystalline Dystrophy

OBJECTIVE

To investigate CYP4V2 gene variants and ocular clinical characteristics of Bietti corneoretinal crystalline dystrophy in China so as to provide more references for genotype and phenotype of BCD.

METHODS

Sixteen Chinese probands were recruited in Beijing Tongren Hospital in a retrospective study. All patients underwent CYP4V2 gene detection and ophthalmic clinical examinations.

RESULTS

CYP4V2 gene variants were detected in all patients. Eight variants were identified, and the most common one was c.802-8_810del17bpinsGC. Onset age of BCD was from 12 to 44 years, and the first symptoms mostly were decreased visual acuity or night blindness. Corneal crystalline depositions were observed in all patients and were found not only in epithelium and superficial stroma near the limbus but also in corneal endothelium. OCT showed atrophy of RPE in all patients, outer retinal tubulation in ten patients, macular edema in four patients, macular hole in three patients with one accompanied with retinal detachment, and choroidal neovascularization in one patient.

CONCLUSION

CYP4V2 gene variants were detected in all patients consistent with the genetic locus homogeneity of BCD, and c.802-8_810del17bpinsGC was the most common mutation. Corneal crystalline depositions were observed in all patients, which may be features of BCD and helpful for the diagnosis of BCD patients, especially those in the advanced stage without typical fundus crystalline depositions or without gene detection. However, considerable phenotypic variability was detected. Corneal crystalline deposits were observed not only in epithelium and superficial stroma but also in endothelium, which has not been reported before. This may provide further evidence for the variable phenotypic expression between affected individuals.

Multimodal imaging features and genetic findings in Bietti crystalline dystrophy

Background

Bietti crystalline dystrophy (BCD) is a distinct entity of retinitis pigmentosa with a wide range of genotypic and phenotypic variabilities. The goal of the present study was to investigate the morphological, functional and genetic features of BCD.

Methods

A full series of multimodal imaging was performed in four Chinese patients with BCD, including fundus photography, fundus autofluorescence, fundus fluorescein angiography (FFA), indocyanine green (ICG) angiography, optical coherence tomography (OCT) and microperimetry. Electrophysiological tests including full-field electroretinography (ERG) and multifocal ERG were employed. CYP4V2 gene sequencing was performed.

Results

Intraretinal crystalline deposits were observed in fundus photographs in all patients. The crystals were better appreciated in infrared images. Autofluorescence imaging demonstrated multifocal patchy hypofluorescence, suggesting massive RPE atrophy. FFA and ICG angiography further confirmed atrophy of the RPE and the underlying choroidal vessels. OCT revealed disruption of the photoreceptors, RPE and the choroid. Outer retinal tubulations (ORTs) confining to the outer nuclear layer were detected in three out of four patients. Full-field ERG showed markedly diminished responses. Multifocal ERG displayed reduced central and peripheral responses in a patient with normal vision. Gene sequencing identified two deletion mutations in CYP4V2, c.802_807del and c.810delT. BCD complicated by choroidal neovascularization (CNV) was diagnosed in one patient, and intravitreal anti-vascular endothelial growth factor (VEGF) injection was given with favorable response.

Conclusions

Multimodal imaging features and electrophysiological findings of BCD patients were comprehensively discussed. A novel deletion mutation, c.802_807del, in the CYP4V2 gene was reported. ORTs are important changes in the outer retina of BCD patients, further investigation of this structure may provide insights into pathology of BCD. Intravitreal anti-VEGF therapy was effective for treatment of BCD complicated by CNV.

Novel mutations in CYP4V2 in Bietti corneoretinal crystalline dystrophy: Next-generation sequencing technology and genotype-phenotype correlations

Purpose

To identify any novel mutations in CYP4V2 in 85 Chinese families with Bietti corneoretinal crystalline dystrophy (BCD) by using next-generation sequencing, and to summarize the mutation spectrum in this population, along with any genotype-phenotype correlations.

Methods

A total of 90 patients with BCD from 85 unrelated Chinese families were recruited. All probands were analyzed by using gene chip-based next-generation sequencing, to capture and sequence all the exons of 57 known hereditary retinal degeneration-associated genes. The candidate variants were validated with PCR and Sanger sequencing.

Results

Twenty-eight mutations were detected in all patients, including thirteen novel mutations (five missense, six deletions, one splicing and one frame-shift mutations) and 15 previously reported mutations. Mutations in 64 patients were inherited from their parents, while three patients had de novo mutations. c.802-8_810del17insGC was the most common mutation, accounting for 78% of the mutations. Although 16 patients were homozygous at this site, the clinical features of all 16 patients were highly heterogeneous.

Conclusions

These results expand the spectrum of mutations in CYP4V2, and suggest that mutations in CYP4V2 may be common in the Chinese population. The phenotype of patients with the homozygous mutation (hom.c.802-8_810del17insGC) is highly heterogeneous.

Molecular Functionality of Cytochrome P450 4 (CYP4) Genetic Polymorphisms and Their Clinical Implications

Enzymes in the cytochrome P450 4 (CYP4) family are involved in the metabolism of fatty acids, xenobiotics, therapeutic drugs, and signaling molecules, including eicosanoids, leukotrienes, and prostanoids. As CYP4 enzymes play a role in the maintenance of fatty acids and fatty-acid-derived bioactive molecules within a normal range, they have been implicated in various biological functions, including inflammation, skin barrier, eye function, cardiovascular health, and cancer. Numerous studies have indicated that genetic variants of CYP4 genes cause inter-individual variations in metabolism and disease susceptibility. Genetic variants of CYP4A11, 4F2 genes are associated with cardiovascular diseases. Mutations of CYP4B1, CYP4Z1, and other CYP4 genes that generate 20-HETE are a potential risk for cancer. CYP4V2 gene variants are associated with ocular disease, while those of CYP4F22 are linked to skin disease and CYP4F3B is associated with the inflammatory response. The present study comprehensively collected research to provide an updated view of the molecular functionality of CYP4 genes and their associations with human diseases. Functional analysis of CYP4 genes with clinical implications is necessary to understand inter-individual variations in disease susceptibility and for the development of alternative treatment strategies.

Current perspectives in Bietti crystalline dystrophy

Bietti crystalline dystrophy (BCD) is a rare-inherited disease caused by mutations in the CYP4V2 gene and characterized by the presence of multiple shimmering yellow-white deposits in the posterior pole of the retina in association with atrophy of the retinal pigment epithelium (RPE) and chorioretinal atrophy. The additional presence of glittering dots located at the corneal limbus is also a frequent finding. The CYP4V2 protein belongs to the cytochrome P450 subfamily 4 and is mainly expressed in the retina and the RPE and less expressed in the cornea. The disease has its metabolic origin in the diminished transformation of fatty acid substrates into n-3 polyunsaturated fatty acids due to a dysregulation of the lipid metabolism. In this review, we provide updated insights on clinical and molecular characteristics of BCD including underlying mechanisms of BCD, genetic diagnosis, progress in the identification of causative genetic and epigenetic factors, available techniques of exploration and development of novel therapies. This information will help clinicians to improve accuracy of BCD diagnosis, providing the patient reliable information regarding prognosis and clinical prediction of the disease course.

CYP4V2 mutation screening in an Iranian Bietti crystalline dystrophy pedigree and evidence for clustering of CYP4V2 mutations

Purpose

To report the genetic analysis of an Iranian Bietti crystalline dystrophy (BCD)-affected family, and to review previously reported mutations in the gene and assess the distribution of affected amino acids in the encoded protein.

Methods

The eleven exons of CYP4V2 were sequenced in the DNA of the proband of the Iranian BCD family. The putative disease-causing variation was screened in all affected and non-affected members. BCD causing CYP4V2 mutations previously reported in the literature were compiled, and positions of amino acids affected by nonsense and missense mutations were mapped onto the primary structure of the CYP4V2 protein.

Results

C.1219G > T in CYP4V2 that causes p.Glu407* was identified as cause of BCD in the Iranian family. The mutation segregated with disease status. Clinical presentations were similar among affected members, except that one patient presented with retinal macular hole. Twelve nonsense and 47 missense mutations in CYP4V2 were compiled. Inspection of distribution of amino acids affected by the mutations suggested non-random distribution and clustering of affected amino acids in nine regions of the protein, including regions that contain the heme binding site, the metal binding site, and a region between these binding sites. The most C-terminus proximal nonsense mutation affected position 482.

Conclusions

This study presents results of the genetic analysis of an Iranian BCD family. Protein regions affected by mutations within the nine mutation clusters include regions well conserved among orthologous proteins and human CYP4 proteins, some of which are associated with known functions. The findings may serve to identify reasonable candidate gene region targets for gene editing therapy approaches.

Identification of novel CYP4V2 genotypes associated with Bietti crystalline dystrophy and atypical anterior segment phenotypes in Spanish patients

PURPOSE

To identify the spectrum of disease-causing CYP4V2 variants in Spanish patients with clinically diagnosed Bietti crystalline dystrophy (BCD) over an 8-year period and to analyse the phenotype-genotype correlation of the identified variants.

METHODS

Four unrelated Spanish probands with a clinical diagnosis of BCD were recruited. Ophthalmological examination included visual acuity (VA), slit lamp examination, in vivo corneal confocal microscopy, funduscopy and fluoresceinic angiography. Genomic DNA was obtained from blood samples, and the exons and flanking intron sequences of the CYP4V2 gene were screened by Sanger sequencing. Family members of the patients with mutations in CYP4V2 gene were subsequently studied.

RESULTS

Clinical examination revealed retinal and corneal patterns compatible with BCD in all the participants. We identified a total of six CYP4V2 variants among the four carriers. As far as we know, the variant p.(Trp244Cysfs*33) has not previously been reported. This variant along with p.(Ala204Thr) and p.(Arg443Trp) were combined in three novel pathogenic phenotypes that share the presence of bilateral limbic glistening deposits, severe retinal damage and visual impairment and a fast rate of progression of the disease.

CONCLUSION

To the best of our knowledge, this study represents the largest effort to determine the genetic alterations underlying BCD in Spain to date. Our results show that analysis of CYP4V2 variants is required for a reliable diagnosis of BCD. We report a high prevalence of anterior segment changes in this Spanish BCD cohort, which we consider representative of the Spanish patients.

Comprehensive screening of CYP4V2 in a cohort of Chinese patients with Bietti crystalline dystrophy

PURPOSE

Bietti crystalline dystrophy (BCD) is an autosomal recessive retinal degeneration disorder caused by mutations in CYP4V2. The aim of this study is to describe the genetic and clinical findings in 128 unrelated Chinese patients diagnosed with BCD.

METHODS

Ophthalmological evaluations were performed in all patients. All coding regions of CYP4V2 were amplified and sequenced directly. Real-time quantitative PCR was performed to detect copy number variations. Haplotype analysis was performed in 70 patients with c.802-8_810del17insGC and in 93 normal controls.

RESULTS

A total of 28 mutations in CYP4V2, including eight novel mutations, were identified in 125 patients. The most common mutation was c.802-8_810del17insGC, with an allele frequency of 62.6%, followed by p.H331P (8.7%) and c.1091-2A>G (7.5%). A novel large deletion encompassing exon 8 of CYP4V2 was detected. Haplotype analysis revealed four common haplotypes in patients with c.802-8_810del17insGC. A 17.6 kb haplotype CT(delCT)TA(Indel)A was the most common and was observed in 34.5% of the c.802-8_810del17insGC mutant alleles. The patients with mutations in CYP4V2 showed wide intra- and interfamilial variability in clinical severity.

CONCLUSIONS

The findings expand the mutational spectrum of CYP4V2 and further confirm the c.802-8_810del17insGC mutation was due to a founder effect in a large cohort of Chinese patients.

Identification and population history of CYP4V2 mutations in patients with Bietti crystalline corneoretinal dystrophy

To identify known and novel CYP4V2 mutations in patients with Bietti crystalline cornea (BCD), expand the spectrum of CYP4V2 mutations, and characterize the population history of the c.802-8_810del17insGC mutation common in Asian populations, genomic DNA was isolated from peripheral blood samples from 58 unrelated patients with clinical diagnoses of BCD. Exons and flanking intronic regions of the CYP4V2 gene were dideoxy DNA sequenced. Nonpathogenic polymorphisms were excluded and known mutations were identified by sequencing 192 unaffected individuals from similar ethnic backgrounds and examination of online databases. The age of the c.802-8_810del17insGC mutation was estimated using three independent approaches. A total of 28 CYP4V2 mutations, 9 of which were novel, were detected in the 58 patients with BCD. These included 19 missense, 4 nonsense, 2 deletion, 2 splice site, and 1 insertion-deletion mutations. Two missense variants of uncertain significance were also detected. The age of the c.802-8_810del17insGC mutation was estimated to be 1040-8200 generations in the Chinese and 300-1100 generations in the Japanese populations. These results expand the mutation spectrum of CYP4V2, and provide insight into the origin of the c.802-8_810del17insGC mutation in the Chinese population and its transmission to the Japanese population.

Genetics of Bietti Crystalline Dystrophy

Bietti crystalline dystrophy (BCD) is an inherited retinal degenerative disease characterized by crystalline deposits in the retina, followed by progressive atrophy of the retinal pigment epithelium (RPE), choriocapillaris, and photoreceptors. CYP4V2 has been identified as the causative gene for BCD. The CYP4V2 gene belongs to the cytochrome P450 superfamily and encodes for fatty acid ω-hydroxylase of both saturated and unsaturated fatty acids. The CYP4V2 protein is localized most abundantly within the endoplasmic reticulum in the RPE and is postulated to play a role in the physiological lipid recycling system between the RPE and photoreceptors to maintain visual function. Electroretinographic assessments have revealed progressive dysfunction of rod and cone photoreceptors in patients with BCD. Several genotypes have been associated with more severe phenotypes based on clinical and electrophysiological findings. With the advent of multimodal imaging with spectral domain optical coherence tomography, fundus autofluorescence, and adaptive optics scanning laser ophthalmoscopy, more precise delineation of BCD severity and progression is now possible, allowing for the potential future development of targets for gene therapy.

Detailed functional and structural phenotype of Bietti crystalline dystrophy associated with mutations in CYP4V2 complicated by choroidal neovascularization

PURPOSE

To describe in detail the phenotype of a patient with Bietti crystalline dystrophy (BCD) complicated by choroidal neovascularization (CNV) and the response to intravitreal Bevacizumab (Avastin^®; Genentech/Roche).

METHODS

A 34-year-old woman with BCD and mutations in CYP4V2 (c.802-8_806del13/p.H331P:c992A>C) underwent a complete ophthalmic examination, full-field flash electroretinography (ERG), kinetic and two-color dark-adapted perimetry, and dark-adaptometry. Imaging was performed with spectral domain optical coherence tomography (SD-OCT), near infrared (NIR) and short wavelength (SW) fundus autofluorescence (FAF), and fluorescein angiography (FA).

RESULTS

Best-corrected visual acuity (BCVA) was 20/20 and 20/60 for the right and left eye, respectively. There were corneal paralimbal crystal-like deposits. Kinetic fields were normal in the peripheral extent. Retinal crystals were most obvious on NIR-reflectance and corresponded with hyperreflectivities within the RPE on SD-OCT. There was parafoveal/perifoveal hypofluorescence on SW-FAF and NIR-FAF. Rod > cone sensitivity loss surrounded fixation and extended to ~10° of eccentricity corresponding to regions of photoreceptor outer segment-retinal pigmented epithelium (RPE) interdigitation abnormalities. The outer nuclear layer was normal in thickness. Recovery of sensitivity following a ~76% rhodopsin bleach was normal. ERGs were normal. A subretinal hemorrhage in the left eye co-localized with elevation of the RPE on SD-OCT and leakage on FA, suggestive of CNV. Three monthly intravitreal injections of Bevacizumab led to restoration of BCVA to baseline (20/25).

CONCLUSION

crystals in BCD were predominantly located within the RPE. Photoreceptor outer segment and apical RPE abnormalities underlie the relatively extensive retinal dysfunction observed in relatively early-stage BCD. Intravitreal Bevacizumab was effective in treating CNV in this setting.

Identification of CYP4V2 mutation in 36 Chinese families with Bietti crystalline corneoretinal dystrophy

Bietti crystalline corneoretinal dystrophy (BCD) is an inherited eye disease that is most common in the Chinese. It is caused by a mutation in the CYP4V2 gene. In this study, 43 Chinese BCD families were recruited; most patients manifested the characteristic phenotype of BCD, with 2 families initially misdiagnosed with retinitis pigmentosa. Five patients in our cohort presented with BCD and choroidal neovascularization (CNV), and 1 patient presented with typical BCD and abnormality in the terminals of both fingers and toes. A total of 17 pathogenic mutations involving 68 alleles were identified from 36 families using targeted exon sequencing and Sanger sequencing; we achieved a diagnostic rate of approximately 84%. Fifteen families were found to carry homozygous mutations, 17 families carried compound heterozygous mutations, and 4 families carried a single heterozygous mutation. Of the mutations identified, four variants c.802-8_810del17bpinsGC, c.802-8_810del17bpinsGT, c.992A > C (p.H331P), and c.1091-2A > G accounted for 71% of the mutations identified in CYP4V2. These mutations were hotspots in Chinese populations for BCD. Five among them were novel and predicted to be disease-causing, including c.65T > A (p.L22H), c.681_4delTGAG (p.S227Rfs*1), c.802-8_810del17bpinsGT, c.965_7delAAG (p.321delE), and c.994G > A (p.D332N). No apparent correlation between genotype and phenotype was identified. Our findings broaden the spectrum of CYP4V2 mutations that cause BCD and the phenotypic spectrum of the disease in Chinese families. These results will be useful for the genetic diagnosis of BCD, genetic consultation, and gene therapy in the future.

Novel CYP4V2 mutations associated with Bietti crystalline corneoretinal dystrophy in Chinese patients

AIM

To analyze the CYP4V2 mutations in five unrelated Chinese patients with Bietti crystalline corneoretinal dystrophy (BCD) and to provide clinical features of these patients. BCD is a rare monogenic autosomal recessively inherited disorder characterized by the presence of crystals in the retina and retinal pigment epithelium atrophy. Mutations in the CYP4V2 gene have been found to be causative for BCD.

METHODS

Ophthalmic examinations were carried out in the affected individuals. Peripheral blood samples were collected and genomic DNA was extracted. All exons and flanking intronic regions of the CYP4V2 gene were amplified with polymerase chain reaction and screened for mutations by direct DNA sequencing. One hundred control chromosomes were also screened to exclude nonpathogenic polymorphisms.

RESULTS

Fundus examination revealed the presence of tiny yellowish-sparkling crystals at the posterior pole of the fundus and atrophy of the retinal pigment epithelium in all patients. Choroid neovascularization was noted in one patient. Five different CYP4V2 mutations were identified, including two missense mutations (p.F73L, p.R400H), two splice site mutations (c.802-8_810del17insGC, c.1091-2A>G), and one single base-pair deletion (p.T479TfsX7 or c.1437delC). The two splice site mutations were identified in three of the patients with BCD. Mutation p.T479TfsX7 was a novel mutation not observed in any of 100 ethnically matched control chromosomes.

CONCLUSION

Mutation c.802-8_810del17insGC and c.1091-2A>G are common mutations in Chinese patients with BCD. Our results expand the allelic heterogeneity of BCD.

Identification of novel CYP4V2 gene mutations in 92 Chinese families with Bietti's crystalline corneoretinal dystrophy

PURPOSE

To characterize the spectrum of CYP4V2 gene mutations in 92 unrelated Chinese probands with Bietti's crystalline dystrophy (BCD) and to describe the molecular and clinical characteristics of four novel CYP4V2 mutations associated with BCD.

METHODS

All study participants underwent a complete ophthalmological examination. Mutational screening of CYP4V2 coding regions and flanking intron sequences was examined via directional Sanger sequencing, with allele separation confirmed by screening other family members. Subsequent in silico analysis of the mutational consequence on protein function was undertaken, with the impact of the novel mutation on pre-mRNA splicing examined via RT-PCR.

RESULTS

Fifteen disease-causing variants were identified in 92 probands with BCD, including four novel mutations and eleven previously reported mutations. The most prevalent mutation was c.802_810del17insGC, which was detected in 69 unrelated families, with an allele frequency of 52.7% (97/184). Homozygosity was revealed in 35 unrelated families, and compound heterozygosity was observed in 43 subjects. Four patients harbored four novel variants, with these mutations cosegregated within all affected individuals and were not found in unaffected family members and 100 unrelated controls. Transcriptional analysis of a novel splice mutation revealed altered RNA splicing. In silico analysis predicted that the missense variant, p.Tyr343Asp, disrupted the CYP4V2 surface electrostatic potential distribution and spatial conformation. Among the patients with four novel mutations, genotype did not always correlate with age at onset, disease course, or electroretinogram (ERG) changes, with phenotypic variations even noted within the same genotype.

CONCLUSIONS

The c.802_810del17insCG mutation was the most common mutation in the 92 Chinese probands with BCD examined. Four novel mutations were identified, contributing to the spectrum of CYP4V2 mutations associated with BCD, with no clear link established between disease phenotype and genotype.

Spectral domain optical coherence tomographic findings of bietti crystalline dystrophy

We analyzed the OCT features of 24 eyes of 12 patients with Bietti crystalline dystrophy (BCD) with the Heidelberg HRA2-OCT. Seventeen of 24 eyes were in intermediate stage of the disease and seven in advanced stage of the disease at the time of latest OCT examination performed in 2014. Outer retinal tubulations and retinal hyperreflective dots were present in 20 of 24 eyes. The remaining four eyes had advanced disease with very thin retina. Appearance of bright plaque on top of RPE-Bruch membrane was present in all eyes. Choroidal hyperreflective spots were noted in 19 of 24 eyes. The remaining five eyes had advanced disease stage with very thin choroid. Mean central macular thickness was 163.08 μm ± 62.52 for all eyes (170.35 μm ± 56.46 in eyes with intermediate disease and 145.42 μm ± 77.2 in eyes with advanced disease). Mean subfoveal choroidal thickness was 95.37 μm ± 55.93 for the study eyes (116.47 ± 46.92 μm in eyes with intermediate disease and 44.14 μm ± 42.43 in eyes with advanced disease). Choroidal hyperreflective spots were noted in 21 of 24 eyes (87.5%). SD-OCT shows the disease progression in retinal and choroidal layers delicately in eyes with BCD and expands our knowledge about the ongoing disease process.

Novel insights into the molecular pathogenesis of CYP4V2-associated Bietti's retinal dystrophy

Bietti's crystalline dystrophy (BCD) is a rare, autosomal recessive retinal degenerative disease associated with mutations in CYP4V2. In this study, we describe the genetic and clinical findings in 19 unrelated BCD patients recruited from five international retinal dystrophy clinics. Patients underwent ophthalmic examinations and were screened for CYP4V2 mutations by Sanger sequencing and quantitative polymerase chain reaction (qPCR) copy number variation screening. Eight CYP4V2 mutations were found in 10/19 patients, including three patients in whom only monoallelic mutations were detected. Four novel mutations were identified: c.604G>A; p.(Glu202Lys), c.242C>G; p.(Thr81Arg), c.604+4A>G; p.(?), and c.1249dup; p.(Thr417Asnfs*2). In addition, we identified a heterozygous paternally inherited genomic deletion of at least 3.8 Mb, encompassing the complete CYP4V2 gene and several other genes, which is novel. Clinically, patients demonstrated phenotypic variability, predominantly showing choroidal sclerosis, attenuated vessels, and crystalline deposits of varying degrees of severity. To our knowledge, our study reports the first heterozygous CYP4V2 deletion and hence a novel mutational mechanism underlying BCD. Our results emphasize the importance of copy number screening in BCD. Finally, the identification of CYP4V2-negative patients with indistinguishable phenotypes from CYP4V2-positive patients might suggest the presence of mutations outside the coding regions of CYP4V2, or locus heterogeneity, which is unreported so far.

Generation and characterization of a murine model of Bietti crystalline dystrophy

PURPOSE

Bietti crystalline dystrophy (BCD) is a rare, autosomal recessive, progressive, degenerative eye disease caused by mutations in the CYP4V2 gene, for which no treatments are currently available. Cyp4v3 is the murine ortholog to CYP4V2, and to better understand the molecular pathogenesis of this disease we have established a Cyp4v3-null mouse line.

METHODS

Cyp4v3(-/-) mice were generated by homologous recombination in embryonic stem cells. Ocular morphologic characteristics were evaluated via fundus imaging, plasma lipid profiling, and histologic analysis via Oil Red O reactivity, hematoxylin and eosin staining, and transmission electron microscopy.

RESULTS

The Cyp4v3(-/-) mouse recapitulates the characteristic features of corneoretinal crystal accumulation and systemic dyslipidemia seen in BCD. The Cyp4v3(-/-) mice behave normally and are viable and fertile when maintained under specific pathogen-free (SPF) housing conditions.

CONCLUSIONS

Cyp4v3(-/-) mice represent a promising preclinical model that may be used to better understand the disease etiology and to evaluate pharmacotherapies for this devastating condition.

An Overview of Rare and Unusual Clinical Features of Bietti's Crystalline Dystrophy

Bietti's crystalline dystrophy (BCD) is a rare disease presenting with the appearance of intraretinal crystalline deposits and varying degrees of chorioretinal atrophy commencing at the posterior pole. Within time, intraretinal crystals gradually disappear and chorioretinal atrophy extends beyond the macula even resulting in complete chorioretinal atrophy. Concomitant corneal crystals can be noted in 1/2 - 1/3 of the patients, and the presence of corneal crystals is not a must for establishing the diagnosis. For the past decade, genetic evaluations and newer imaging modalities expand our knowledge about the disease. CYP4V2 gene is found to be the gene responsible for the disease process and new mutations are still being described. Modern imaging modalities, such as a spectral domain optical coherence tomography (SD-OCT) shed light on the anatomic features of the disease. By this, we reiterate the rare and unusual clinical features of BCD.