Camiel J. F. Boon

The spectrum of ocular phenotypes caused by mutations in the BEST1 gene

Bestrophin-1 is an integral membrane protein, encoded by the BEST1 gene, which is located in the basolateral membrane of the retinal pigment epithelium. The bestrophin-1 protein forms a Ca(2+) activated Cl(-) channel and is involved in the regulation of voltage-dependent Ca(2+) channels. In addition, bestrophin-1 appears to play a role in ocular development. Over 120 different human BEST1 mutations have been described to date, associated with a broad range of ocular phenotypes. The purpose of this review is to describe this spectrum of phenotypes, which includes Best vitelliform macular dystrophy and adult-onset foveomacular vitelliform dystrophy, autosomal dominant vitreoretinochoroidopathy, the MRCS (microcornea, rod-cone dystrophy, cataract, posterior staphyloma) syndrome, and autosomal recessive bestrophinopathy. The genotype-phenotype correlations that are observed in association with BEST1 mutations are discussed. In addition, in vitro studies and animal models that clarify the pathophysiological mechanisms are reviewed.

The spectrum of retinal dystrophies caused by mutations in the peripherin/RDS gene

Peripherin/rds is an integral membrane glycoprotein, mainly located in the rod and cone outer segments. The relevance of this protein to photoreceptor outer segment morphology was first demonstrated in retinal degeneration slow (rds) mice. Thus far, over 90 human peripherin/RDS gene mutations have been identified. These mutations have been associated with a variety of retinal dystrophies, in which there is a remarkable inter- and intrafamilial variation of the retinal phenotype. In this paper, we discuss the characteristics of the peripherin/RDS gene and its protein product. An overview is presented of the broad spectrum of clinical phenotypes caused by human peripherin/RDS gene mutations, ranging from various macular dystrophies to widespread forms of retinal dystrophy such as retinitis pigmentosa. Finally, we review the proposed genotype-phenotype correlation and the pathophysiologic mechanisms underlying this group of retinal dystrophies.

A rare nonsynonymous sequence variant in C3 is associated with high risk of age-related macular degeneration

Through whole-genome sequencing of 2,230 Icelanders, we detected a rare nonsynonymous SNP (minor allele frequency = 0.55%) in the C3 gene encoding a p.Lys155Gln substitution in complement factor 3, which, following imputation into a set of Icelandic cases with age-related macular degeneration (AMD) and controls, associated with disease (odds ratio (OR) = 3.45; P = 1.1 × 10−7). This signal is independent of the previously reported common SNPs in C3 encoding p.Pro314Leu and p.Arg102Gly that associate with AMD. The association of p.Lys155Gln was replicated in AMD case-control samples of European ancestry with OR = 4.22 and P = 1.6 × 10−10, resulting in OR = 3.65 and P = 8.8 × 10−16 for all studies combined. In vitro studies have suggested that the p.Lys155Gln substitution reduces C3b binding to complement factor H, potentially creating resistance to inhibition by this factor. This resistance to inhibition in turn is predicted to result in enhanced complement activation.

Risk Alleles in CFH and ARMS2 Are Independently Associated with Systemic Complement Activation in Age-related Macular Degeneration

PURPOSE Systemic complement activation is associated with age-related macular degeneration (AMD) and has mainly been attributed to a risk allele in the complement factor H (CFH) gene. Whether other important AMD genes also influence complement activation is unclear. In the present case-control study, complement activity and concentrations of complement components and their activation products are measured in AMD patients and in unaffected controls and correlated with genetic variants in the CFH, ARMS2, C3, CFI, and CFB genes. DESIGN Case-control study. PARTICIPANTS A cohort of 197 confirmed AMD patients and 150 unaffected age-matched controls were recruited prospectively for the study. METHODS Hemolytic complement assays (AP50, CP50, and LP50), complement components (C3, CFB, CFI, and CFH), and the activation products (C3d, C5a, and SC5b-9) were analyzed in serum or plasma. The DNA samples were genotyped for 5 single nucleotide polymorphisms (SNPs) previously associated with AMD in the CFH, ARMS2, C3, CFB, and CFI genes. MAIN OUTCOME MEASURES Complement concentrations and their associations with SNPs in the CFH, ARMS2, C3, CFB, and CFI genes. RESULTS The AMD patients had increased activation of the alternative complement pathway (P = 0.003) and elevated levels of complement activation components C3d (P<0.0001) and C5a (P<0.0001), CFB (P<0.0001), and an increased C3d/C3 ratio (P<0.0001) calculated as a measure of C3 activation. While the CFH risk genotype was significantly associated with the elevated C3d/C3 ratios obtained, in the absence of CFH risk alleles the ARMS2 risk genotype also showed significantly increased levels of complement activation (P = 0.013). Furthermore, the carriers of the CFB protective allele had lower CFB concentrations. CONCLUSIONS The current study found evidence showing that in AMD risk alleles in CFH and ARMS2 are independently associated with complement activation. Especially the C3d/C3 ratio seems to be a strong marker for AMD. The findings suggest that CFH and ARMS2 share a common pathway in the pathogenesis of AMD.

Basal laminar drusen caused by compound heterozygous variants in the CFH gene.

Age-related macular degeneration (AMD) is a multifactorial disease that is strongly associated with the Tyr402His variant in the complement factor H (CFH) gene. Drusen are hallmark lesions of AMD and consist of focal-inflammatory and/or immune-mediated depositions of extracellular material at the interface of the retinal pigment epithelium (RPE) and the Bruch membrane. We evaluated the role of CFH in 30 probands with early-onset drusen and identified heterozygous nonsense, missense, and splice variants in five families. The affected individuals all carried the Tyr402His AMD risk variant on the other allele. This supports an autosomal-recessive disease model in which individuals who carry a CFH mutation on one allele and the Tyr402His variant on the other allele develop drusen. Our findings strongly suggest that monogenic inheritance of CFH variants can result in basal laminar drusen in young adults, and this can progress to maculopathy and severe vision loss later in life.

Clinical and genetic characteristics of late-onset Stargardt's disease

OBJECTIVE To describe the genotype and phenotype of patients with a late-onset Stargardts disease (STGD1). DESIGN Retrospective case series. PARTICIPANTS Twenty-one unrelated STGD1 patients with an age at onset of ≥45 years and ≥1 rare variant in the ABCA4 gene. METHODS Ophthalmologic examination, including best-corrected visual acuity (VA), Amsler grid testing, fundus photography, fluorescein angiography (FA), spectral-domain optical coherence tomography (OCT), fundus autofluorescence (FAF) imaging, full-field electroretinography (ERG), multifocal ERG, and central visual field testing. Analysis of the ABCA4 gene was performed using microarray analysis, sequencing, and multiplex ligation-dependent probe amplification. In addition, the PRPH2 and CFH genes were sequenced. MAIN OUTCOME MEASURES Age at onset, VA, fundus appearance, FA, FAF, and OCT findings; ABCA4 mutations; and genotype-phenotype correlation. RESULTS The mean age at onset was 55 years (range, 45-72 years). Seven patients were diagnosed without visual symptoms (age range, 45-83 years). The VA was ≥20/40 in 24 eyes of 14 patients (59%) owing to foveal sparing. On ophthalmoscopy, late-onset STGD1 showed flavimaculatus flecks (15 patients), small flecks surrounding mottled foveal changes (3 patients), extensive chorioretinal atrophy (2 patients), or small yellowish spots in the macula (1 patient). The fundus flecks showed increased autofluorescence on FAF. The choroidal background fluorescence on FA was obscured in 16 patients (80%). We found a single heterozygous ABCA4 variant in 11 patients (52%), 2 compound heterozygous variants in 8 patients (38%), and a homozygous variant in 2 patients (10%). No PRPH2 or CFH mutations were detected. CONCLUSIONS Late-onset STGD1 is at the mild end of the spectrum of retinal dystrophies caused by ABCA4 mutations. The VA is frequently preserved in late-onset STGD1 patients owing to foveal sparing. This phenotype may be caused by 1 or 2 ABCA4 variants. The differential diagnosis between late-onset STGD1 and age-related macular degeneration may be challenging. A thorough clinical and genetic analysis makes a distinction possible, which is important for clinical and genetic counseling. FINANCIAL DISCLOSURE(S) The authors have no proprietary or commercial interest in any of the materials discussed in this article.

The spectrum of phenotypes caused by variants in the CFH gene.

Complement factor H (CFH) is a complement inhibitor, which is present as a soluble protein and attached to cell surfaces throughout the human body. As such, CFH is a key player in complement homeostasis, inhibiting excessive activation of the complement cascade, with an emphasis on the alternative pathway. The significance of CFH is demonstrated by the broad range of phenotypes associated with specific CFH gene variants. This phenotypic spectrum includes renal phenotypes, such as membranoproliferative glomerulonephritis and atypical hemolytic uremic syndrome, as well as ocular phenotypes, such as basal laminar drusen and age-related macular degeneration. In addition, several overlapping phenotypes have been described in association with CFH gene variants. The phenotypic outcome of these CFH variants depends on their differential impact on plasma- and surface-bound CFH function. Consequently, distinct genotype-phenotype correlations may be observed.

Fundus autofluorescence imaging of retinal dystrophies

Fundus autofluorescence (FAF) is a non-invasive imaging technique that enables the visualization of lipofuscin changes in the retinal pigment epithelium. This study aims to illustrate the spectrum of FAF changes in a variety of retinal dystrophies. For this purpose, we examined patients with retinal dystrophies such as Stargardt disease, Best vitelliform macular dystrophy, and retinal dystrophies associated with mutations in the peripherin/RDS gene. All retinal dystrophies were confirmed by molecular genetic analysis. A broad range of characteristic FAF patterns was observed. Our results indicate that FAF imaging constitutes a useful additive tool in the diagnosis and follow-up of various retinal dystrophies.

Mutations in the peripherin/RDS gene are an important cause of multifocal pattern dystrophy simulating STGD1/fundus flavimaculatus.

Aim: To describe the phenotype and to analyse the peripherin/RDS gene in 10 unrelated families with multifocal pattern dystrophy simulating Stargardt disease (STGD1). Methods: The probands of 10 families and 20 affected family members underwent an ophthalmic examination including dilated fundus examination, fundus autofluorescence imaging and optical coherence tomography (OCT). In all probands and in selected family members, fluorescein angiography, electrophysiological testing and visual field analysis were performed. Blood samples were obtained from affected and unaffected family members for analysis of the peripherin/RDS gene. Results: All 10 probands carried mutations in the peripherin/RDS gene. Nine different mutations were identified, including six mutations that were not described previously. All probands showed a pattern dystrophy with yellow–white flecks in the posterior pole that strongly resembled the flecks seen in STGD1, on ophthalmoscopy as well as on autofluorescence and OCT. Clinical findings in the family members carrying the same mutation as the proband were highly variable, ranging from no visible abnormalities to retinitis pigmentosa. Conclusions: Mutations in the peripherin/RDS gene are the major cause of multifocal pattern dystrophy simulating STGD1/fundus flavimaculatus. This autosomal dominant disorder should be distinguished from autosomal recessive STGD1, in view of the different inheritance pattern and the overall better visual prognosis.

Central Areolar Choroidal Dystrophy

OBJECTIVE To describe the clinical characteristics, follow-up data and molecular genetic background in a large group of patients with central areolar choroidal dystrophy (CACD). DESIGN Retrospective case series study. PARTICIPANTS One hundred three patients with CACD from the Netherlands. METHODS Ophthalmologic examination, including color vision testing, fundus photography, fluorescein angiography, fundus autofluorescence (FAF) imaging, optical coherence tomography, full-field electroretinography (ERG), multifocal ERG, and electrooculography. Blood samples were obtained for DNA extraction and subsequent analysis of the peripherin/RDS gene, as well as haplotype analysis. MAIN OUTCOME MEASURES Clinical characteristics, phenotypic range, clinical follow-up data, and FAF findings. RESULTS The mean age at onset of visual loss was 46 years, with subsequent gradual deterioration in visual acuity. Ninety-eight patients carried a p.Arg142Trp mutation in peripherin/RDS, whereas 5 affected members of a CACD family carried a p.Arg172Gln peripherin/RDS mutation. A remarkable variation in disease severity was observed, and nonpenetrance was seen up to the age of 64 years, in up to 21% of mutation carriers. However, most macular lesions in mutation carriers displayed a typical stage of CACD. Substantial changes were seen on FAF imaging after a mean follow-up period of 11 months. Electrophysiologic data were consistent with a central cone dystrophy. The age at onset and phenotypic characteristics of CACD show considerable overlap with atrophic age-related macular degeneration (AMD). The great majority of p.Arg142Trp-carrying CACD patients originated from the southeast region of the Netherlands, and haplotype analysis strongly suggested a common founder mutation. CONCLUSIONS When caused by a p.Arg142Trp mutation in the peripherin/RDS gene, CACD causes a central cone dystrophy phenotype. This mutation, which most likely originates from a common founder in most patients, is associated with a significant degree of nonpenetrance. In the elderly patient, CACD may be confused with AMD, especially in cases with decreased penetrance.