To determine the frequency and nature of mutations in the gene ABCA4 in a cohort of patients with bull's‐eye maculopathy (BEM).
A panel of 49 subjects (comprising 40 probands/families, 7 sibling pairs and a set of three sibs) with BEM, not attributable to toxic causes, was ascertained. Blood samples from each patient were used to extract genomic DNA, with subsequent mutation screening of the entire coding sequence of ABCA4, using single‐strand conformational polymorphism (SSCP) analysis and direct sequencing.
Fourteen probands (35%) were found to have a potentially disease‐causing ABCA4 sequence variant on at least one allele. Three patients had a Gly1961Glu missense mutation, the most common variant in Stargardt disease (STGD), with 2 of these subjects having a macular dystrophy (MD) phenotype and a second ABCA4 variant previously associated with STGD. The second most common STGD mutation, Ala1038Val, was seen in one patient with cone–rod dystrophy (CORD). Five novel ABCA4 variants were detected. Two sibships were identified with a similar intra‐familial phenotype but discordant ABCA4 variants.
Variations in the ABCA4 gene are common in BEM. Two sibships showed discordant ABCA4 variants. One of these sibships illustrates that ABCA4 variants can be identified in families that have another molecular cause for their disease, due to the high prevalence of ABCA4 disease alleles in the population. The discordance evident in the second sibship may yet also be a chance finding in families with macular disease of another genetic cause, or it may represent a complex mode of inheritance determined/modified by the combination of ABCA4 alleles.
Stargardt Disease (STGD) is the commonest genetic form of juvenile or early adult onset macular degeneration, which is a genetically heterogeneous disease. Molecular diagnosis of STGD remains a challenge in a significant proportion of cases. To address this, seven patients from five putative STGD families were recruited. We performed capture next generation sequencing (CNGS) of the probands and searched for potentially disease-causing genetic variants in previously identified retinal or macular dystrophy genes. Seven disease-causing mutations in ABCA4 and two in PROM1 were identified by CNGS, which provides a confident genetic diagnosis in these five families. We also provided a genetic basis to explain the differences among putative STGD due to various mutations in different genes. Meanwhile, we show for the first time that compound heterozygous mutations in PROM1 gene could cause cone-rod dystrophy. Our findings support the enormous potential of CNGS in putative STGD molecular diagnosis.
This study aimed to identify genetic mechanisms underlying severe retinal degeneration in one large family from northern Sweden, members of which presented with early-onset autosomal recessive retinitis pigmentosa and juvenile macular dystrophy. The clinical records of affected family members were analysed retrospectively and ophthalmological and electrophysiological examinations were performed in selected cases. Mutation screening was initially performed with microarrays, interrogating known mutations in the genes associated with recessive retinitis pigmentosa, Leber congenital amaurosis and Stargardt disease. Searching for homozygous regions with putative causative disease genes was done by high-density SNP-array genotyping, followed by segregation analysis of the family members. Two distinct phenotypes of retinal dystrophy, Leber congenital amaurosis and Stargardt disease were present in the family. In the family, four patients with Leber congenital amaurosis were homozygous for a novel c.2557C>T (p.Q853X) mutation in the CRB1 gene, while of two cases with Stargardt disease, one was homozygous for c.5461-10T>C in the ABCA4 gene and another was carrier of the same mutation and a novel ABCA4 mutation c.4773+3A>G. Sequence analysis of the entire ABCA4 gene in patients with Stargardt disease revealed complex alleles with additional sequence variants, which were evaluated by bioinformatics tools. In conclusion, presence of different genetic mechanisms resulting in variable phenotype within the family is not rare and can challenge molecular geneticists, ophthalmologists and genetic counsellors.
CRB1; ABCA4; SNP-array; Stargardt disease; Leber congenital amaurosis
Stargardt disease is the most common cause of juvenile macular dystrophy. Five subjects from a two-generation Chinese family with Stargardt disease are reported in this study. All family members underwent complete ophthalmologic examinations. Patients of the family initiated the disease during childhood, developing progressively impaired central vision and bilateral atrophic macular lesions in the retinal pigmental epithelium (RPE) that resembled a “beaten-bronze” appearance. Peripheral venous blood was obtained from all patients and their family members for genetic analysis. Exome sequencing was used to analyze the exome of two patients II1, II2. A total of 50709 variations shared by the two patients were subjected to several filtering steps against existing variation databases. Identified variations were verified in all family members by PCR and Sanger sequencing. Compound heterozygous variants p.Y808X and p.G607R of the ATP-binding cassette, sub-family A (ABC1), member 4 (ABCA4) gene, which encodes the ABCA4 protein, a member of the ATP-binding cassette (ABC) transport superfamily, were identified as causative mutations for Stargardt disease of this family. Our findings provide one novel ABCA4 mutation in Chinese patients with Stargardt disease.
This study aimed to identify the underlying molecular genetic cause in four Spanish families clinically diagnosed of Retinitis Pigmentosa (RP), comprising one autosomal dominant RP (adRP), two autosomal recessive RP (arRP) and one with two possible modes of inheritance: arRP or X-Linked RP (XLRP). We performed whole exome sequencing (WES) using NimbleGen SeqCap EZ Exome V3 sample preparation kit and SOLID 5500xl platform. All variants passing filter criteria were validated by Sanger sequencing to confirm familial segregation and the absence in local control population. This strategy allowed the detection of: (i) one novel heterozygous splice-site deletion in RHO, c.937-2_944del, (ii) one rare homozygous mutation in C2orf71, c.1795T>C; p.Cys599Arg, not previously associated with the disease, (iii) two heterozygous null mutations in ABCA4, c.2041C>T; p.R681* and c.6088C>T; p.R2030*, and (iv) one mutation, c.2405-2406delAG; p.Glu802Glyfs*31 in the ORF15 of RPGR. The molecular findings for RHO and C2orf71 confirmed the initial diagnosis of adRP and arRP, respectively, while patients with the two ABCA4 mutations, both previously associated with Stargardt disease, presented symptoms of RP with early macular involvement. Finally, the X-Linked inheritance was confirmed for the family with the RPGR mutation. This latter finding allowed the inclusion of carrier sisters in our preimplantational genetic diagnosis program.
To resolve the spectrum of causative retina-specific ATP-binding cassette transporter gene (ABCA4) gene mutations in Portuguese Stargardt (STGD) patients and compare allele frequencies obtained in this cohort with those of previous population surveys.
Using a microarray technique (ABCR400 gene chip), we screened all previously reported ABCA4 gene mutations in the genomic DNA of 27 patients from 21 unrelated Stargardt families whose phenotypes had been clinically evaluated using psychophysics and electrophysiological measurements. Furthermore, we performed denaturing high performance liquid chromatography whenever one or both mutant alleles failed to be detected using the ABCR gene chip.
A total of 36 mutant alleles (out of the 54 tested) were identified in STGD patients, resulting in a detection rate of 67%. Two mutant alleles were present in 12 out of 21 STGD families (57%), whereas in four out of 21 (19%) of the families, only one mutant allele was found. We report the presence of 22 putative pathogenic alterations, including two sequence changes not found in other populations, c.2T>C (p.Met1Thr) and c.4036_4037delAC (p.Thr1346fs), and two novel disease-associated variants, c.400C>T (p.Gln134X) and c.4720G>T (p.Glu1574X). The great majority of the mutations were missense (72.7%). Seven frameshift variants (19.4%), three nonsense mutations (8.3%), and one splicing sequence change (2.7%) were also found in STGD chromosomes. The most prevalent pathologic variant was the missense mutation p.Leu11Pro. Present in 19% of the families, this mutation represents a quite high prevalence in comparison to other European populations. In addition, 23 polymorphisms were also identified, including four novel intronic sequence variants.
To our knowledge, this study represents the first report of ABCA4 mutations in Portuguese STGD patients and provides further evidence of different mutation frequency across populations. Phenotypic characterization of novel putative mutations was addressed.
To provide a comprehensive overview of all detected mutations in the ABCA4 gene in Spanish families with autosomal recessive retinal disorders, including Stargardt disease (arSTGD), cone-rod dystrophy (arCRD), and retinitis pigmentosa (arRP). Also, to assess genotype-phenotype correlation and disease progression in 10 years by considering type of variants and age of onset.
A total of 420 unrelated Spanish families: 259 arSTGD, 86 arCRD and 75 arRP.
Spanish families were analysed through a combination of ABCR400 genotyping microarray, denaturing High-Performance Liquid Chromatography (dHPLC) and High Resolution Melting (HRM) scanning. Direct sequencing was used as confirmation technique for the identified variants. Screening by Multiple Ligation Probe Analysis (MLPA) was used in order to detect possible large deletions or insertions in the ABCA4 gene. Selected families were further analysed by Next Generation Sequencing (NGS).
Main Outcome Measures
DNA sequence variants, mutation detection rates, haplotypes, age of onset, central or peripheral vision loss, night blindness.
Overall, we detected 70.5% and 36.6% of all expected ABCA4 mutations in arSTGD and arCRD patient cohorts, respectively. In the fraction of the cohort where the ABCA4 gene was completely sequenced the detection rates reached 73.6% for STGD and 66.7% for CRD. However, the frequency of possibly pathogenic ABCA4 alleles in arRP families was only slightly higher to that in the general population. Moreover, in some families mutations in other known arRP genes segregated with the disease phenotype.
An increasing understanding of causal ABCA4 alleles in arSTGD and arCRD facilitates disease diagnosis and prognosis and is also paramount in selecting patients for emerging clinical trials of therapeutic interventions. As ABCA4-associated diseases are evolving retinal dystrophies, assessment of age of onset, accurate clinical diagnosis and genetic testing are crucial. We suggest that ABCA4 mutations may be associated with an RP-like phenotype often as a consequence of severe (null) mutations and/or in cases of long-term, advanced disease. Patients with “classical” arRP phenotypes, especially from the onset of the disease, should be first screened for mutations in known arRP genes and not ABCA4.
ABCA4; Stargardt disease; cone-rod dystrophy; genotype-phenotype correlation
Stargardt disease (STGD), characterized by central visual impairment, is the most common juvenile macular dystrophy. All recessively inherited cases are thought to be due to mutations in the ABCA4 gene. Early-onset autosomal recessive retinitis pigmentosa (arRP) is a severe retinal degeneration that presents before the patient is ten years old. It has been associated with mutations in different genes, including CRB1. The aim of this study was to determine the genetic causes for two different retinal dystrophies, STGD and early-onset arRP, both segregating in one Spanish family.
Mutational analyses were performed using the ABCR400 and Leber congenital amaurosis (LCA) genotyping microarrays. Additional scanning for mutations was conducted by denaturing high performance liquid chromatography (dHPLC); results were confirmed by direct sequencing.
A patient, who exhibited a STGD phenotype, was found to be homozygous for the p.Asn1805Asp (c.5413A>G) mutation in ABCA4. However, his affected sister, who had the arRP phenotype, was found to be heterozygous for this allele; no other sequence change could be found in ABCA4. Analysis using the LCA chip revealed the p.Cys948Tyr mutation in CRB1 in heterozygous state. A second mutation (p.Trp822ter) was found in the CRB1 gene in the affected female by denaturing high performance liquid chromatography (dHPLC) and direct sequencing.
Two distinct retinal dystrophies with mutations affecting two different genes cosegregated in this family. The presence of two different phenotypes associated with mutations in two distinct genes in one single family must be considered especially when dealing with retinal dystrophies which bear high carrier frequencies in general population.
To investigate the genotype and phenotype in families with adenosine triphosphate–binding cassette, sub-family A, member 4 (ABCA4)–associated retinal degeneration.
Three families with at least one family member with known homozygous or compound heterozygote mutations in the ABCA4 gene were studied. The investigations included full field electroretinography (ff-ERG), multifocal ERG (mERG), Goldmann visual fields, optical coherence tomography (OCT), and standard ophthalmological examination. Microarray (Asper) was used for ABCA4 genotyping.
In family 1, the proband (age 23) was homozygote for the c768 G>T mutation. She was diagnosed with cone rod dystrophy (CRD) while her aunt (age 69) was compound heterozygote for the c768 G>T and c2894 A>G mutations and had autosomal recessive retinitis pigmentosa (arRP). The father (age 61) and the mother (age 60) of the proband were asymptomatic carriers of the c768 G>T mutation. In family 2, the proband (age 25) was homozygote for the c5917del. She was diagnosed with CRD. Her father and two sisters were compound heterozygote for the c5917del and c5882 G>A mutations. The eldest sister (age 23) suffered from Stargardt disease (STGD) while the youngest sister (age 12) and their father (age 48) had no visual complaints. Anyhow, their ERG measurements indicated changes corresponding to STGD. The mother (age 42), (heterozygote for the c5917 delG mutation) and the youngest child (age 9; heterozygote for the c5882 G>A mutation) had a normal phenotype. In family 3, the proband (age 43) was compound heterozygote for c768 G>T and c3113 C>T and had been diagnosed with STGD. Her son (age 12), who was homozygote for the c768 G>T mutation, had wider scotomas with earlier onset (age 6), ff-ERG cone responses in the lower range of normality, and reduced mERG. At the moment, he is classified as having STGD but may progress to CRD. The father (age 45) was asymptomatic and heterozygote for the c768 G>T mutation. The patients with progressive disorders (CRD or arRP) had prolonged implicit times for the 30 Hz flicker ff-ERG and the mERG. All patients with two mutations in the ABCA4 gene demonstrated attenuation of retinal thickness on the OCT macular map.
This study confirms that ABCA4 mutations lead to a spectrum of retinal degenerations ranging from STGD to CRD or arRP. At the time of diagnosis, it is not possible to predict the severity of the condition only from genotyping. Our results suggest that prolongation of implicit times for the ff-ERG and/or mERG seem to be associated with progressive conditions such as CRD and arRP. Since ABCA4 mutations are common in the general population, different family members can harbor various combinations of mutations resulting in diverse phenotype and prognosis in the same family, further emphasizing the importance of a combination of genetic and electrophysiological tests at the first visit and follow-up.
Stargardt disease is a common inherited macular degeneration characterized by a significant loss in central vision in the first or second decade of life, bilateral atrophic changes in the central retina associated with degeneration of photoreceptors and underlying retinal pigment epithelial cells, and the presence of yellow flecks extending from the macula. Autosomal recessive Stargardt disease, the most common macular dystrophy, is caused by mutations in the gene encoding ABCA4, a photoreceptor ATP binding cassette (ABC) transporter. Biochemical studies together with analysis of abca4 knockout mice and Stargardt patients have implicated ABCA4 as a lipid transporter that facilitates the removal of potentially toxic retinal compounds from photoreceptors following photoexcitation. An autosomal dominant form of Stargardt disease also known as Stargardt-like dystrophy is caused by mutations in a gene encoding ELOVL4, an enzyme that catalyzes the elongation of very long chain fatty acids in photoreceptors and other tissues. This review focuses on the molecular characterization of ABCA4 and ELOVL4 and their role in photoreceptor cell biology and the pathogenesis of Stargardt disease.
Stargardt Disease; ABCA4; ABC Transporters; Retinoids; ELOVL4; Elongase; Very long chain fatty acids
Stargardt disease (STGD) is the most common juvenile macular dystrophy, characterized by central visual impairment. All recessively inherited cases are thought to be due to mutations in the ABCA4 gene, mapped to 1p21-p13.
To describe a form of non-mendelian inheritance in a patient with STGD identified through the course of a conventional mutational screening performed on 77 STGD families. DNA from the patient and relatives was analyzed for variants in all 50 exons of the ABCA4 gene by screening on the ABCR400 microarray; results were confirmed by direct sequencing. Haplotype analyses, standard and high-resolution (HR) karyotypes, and multiplex ligation-dependent probe amplification (MLPA) were also performed.
A patient with STGD caused by the homozygous p.Arg1129Leu mutation in the ABCA4 gene was found to be the daughter of a noncarrier mother and a father who was heterozygous for this change. Haplotype analysis suggested that no maternal ABCA4 allele was transmitted to the patient. Microsatellite markers spanning the entire chromosome 1 identified a homozygous region of at least 4.4 Mb, involving the ABCA4 gene. The cytogenetic study revealed normal female karyotype. Further evaluation with MLPA showed the patient had a normal dosage for both copies of the ABCA4 gene, thus suggesting partial paternal isodisomy but not a maternal microdeletion.
We report that recessive STGD can rarely be inherited from only one unaffected carrier parent in a non-mendelian manner. This study also demonstrates that genomic alterations contribute to only a small fraction of disease-associated alleles for ABCA4.
Retinal dystrophies (RD) are a group of hereditary diseases that lead to debilitating visual impairment and are usually transmitted as a Mendelian trait. Pathogenic mutations can occur in any of the 100 or more disease genes identified so far, making molecular diagnosis a rather laborious process. In this work we explored the use of whole exome sequencing (WES) as a tool for identification of RD mutations, with the aim of assessing its applicability in a diagnostic context.
We ascertained 12 Spanish families with seemingly recessive RD. All of the index patients underwent mutational pre-screening by chip-based sequence hybridization and resulted to be negative for known RD mutations. With the exception of one pedigree, to simulate a standard diagnostic scenario we processed by WES only the DNA from the index patient of each family, followed by in silico data analysis. We successfully identified causative mutations in patients from 10 different families, which were later verified by Sanger sequencing and co-segregation analyses. Specifically, we detected pathogenic DNA variants (∼50% novel mutations) in the genes RP1, USH2A, CNGB3, NMNAT1, CHM, and ABCA4, responsible for retinitis pigmentosa, Usher syndrome, achromatopsia, Leber congenital amaurosis, choroideremia, or recessive Stargardt/cone-rod dystrophy cases.
Despite the absence of genetic information from other family members that could help excluding nonpathogenic DNA variants, we could detect causative mutations in a variety of genes known to represent a wide spectrum of clinical phenotypes in 83% of the patients analyzed. Considering the constant drop in costs for human exome sequencing and the relative simplicity of the analyses made, this technique could represent a valuable tool for molecular diagnostics or genetic research, even in cases for which no genotypes from family members are available.
We describe a particular form of autosomal recessive generalized choriocapillaris dystrophy phenotype associated with ABCA4 mutations.
A cohort of 30 patients with identified ABCA4 mutations and a distinct phenotype was studied. A retrospective review of history, fundus photographs, electroretinography, visual field testing, dark adaptometry, and optical coherence tomography was performed. Genetic analyses were performed by ABCA4 microarray analysis, high resolution melting, and/or next generation sequencing of all protein-coding sequences of the ABCA4 gene.
The earliest recorded manifestation of ABCA4-associated disease was a central bull's eye type of macular dystrophy that progressed to chorioretinal atrophy of the macula with coarse rounded hyperpigmentations and expanding involvement of the periphery. The mean age at first presentation was 10.3 years, the longest follow-up was 61 years. All patients had two ABCA4 mutations identified, confirming the molecular genetic diagnosis of an ABCA4-associated disease. Most patients harbored at least one mutation classified as “severe,” the most common of which was the p.N965S variant that had been found previously at a high frequency among patients with ABCA4-associated retinal dystrophies in Denmark.
Generalized choriocapillaris dystrophy is a progressive ABCA4-associated phenotype characterized by early-onset macular dystrophy that disperses and expands to widespread end-stage chorioretinal atrophy with profound visual loss. All cases in this study were confirmed as harboring two ABCA4 mutations. Most of the ABCA4 mutations were classified as “severe” explaining the early onset, panretinal degeneration, and fast progression of the disease.
This study delineates a particular type of generalized choriocapillaris dystrophy caused by mostly deleterious mutations in the ABCA4 gene. The disease presents with early-onset macular dystrophy and progresses into an end-stage of widespread choriocapillaris atrophy.
chorioretinal dystrophy; ABCA4; phenotype–genotype
Autosomal recessive Stargardt disease (STGD1) is caused by hundreds of mutations in the ABCA4 gene, which are often specific to racial and ethnic groups. Here, we investigated the ABCA4 variation and their phenotypic expression in a cohort of 44 patients of African American descent, a previously under-characterized racial group. Patients were screened for mutations in ABCA4 by next-generation sequencing and array-comparative genomic hybridization (aCGH), followed by analyses for pathogenicity by in silico programs. Thorough ophthalmic examination was performed on all patients. At least two (expected) disease-causing alleles in the ABCA4 gene were identified in 27 (61.4%) patients, one allele in 11 (25%) patients, and no ABCA4 mutations were found in six (13.6%) patients. Altogether, 39 different disease-causing ABCA4 variants, including seven new, were identified on 65 (74%) chromosomes, most of which were unique for this racial group. The most frequent ABCA4 mutation in this cohort was c.6320G>A (p.(R2107H)), representing 19.3% of all disease-associated alleles. No large copy number variants were identified in any patient. Most patients reported later onset of symptoms. In summary, the ABCA4 mutation spectrum in patients of West African descent differs significantly from that in patients of European descent, resulting in a later onset and “milder” disease.
ABCA4; Stargardt disease; next-generation sequencing; allelic heterogeneity; African American
Mutations in ABCA4 cause Stargardt disease and other blinding autosomal recessive retinal disorders. However, sequencing of the complete coding sequence in patients with clinical features of Stargardt disease sometimes fails to detect one or both mutations. For example, among 208 individuals with clear clinical evidence of ABCA4 disease ascertained at a single institution, 28 had only one disease-causing allele identified in the exons and splice junctions of the primary retinal transcript of the gene. Haplotype analysis of these 28 probands revealed 3 haplotypes shared among ten families, suggesting that 18 of the 28 missing alleles were rare enough to be present only once in the cohort. We hypothesized that mutations near rare alternate splice junctions in ABCA4 might cause disease by increasing the probability of mis-splicing at these sites. Next-generation sequencing of RNA extracted from human donor eyes revealed more than a dozen alternate exons that are occasionally incorporated into the ABCA4 transcript in normal human retina. We sequenced the genomic DNA containing 15 of these minor exons in the 28 one-allele subjects and observed five instances of two different variations in the splice signals of exon 36.1 that were not present in normal individuals (P < 10−6). Analysis of RNA obtained from the keratinocytes of patients with these mutations revealed the predicted alternate transcript. This study illustrates the utility of RNA sequence analysis of human donor tissue and patient-derived cell lines to identify mutations that would be undetectable by exome sequencing.
Stargardt-like macular degeneration (STGD3) is an early onset, autosomal dominant macular degeneration. STGD3 is characterized by a progressive pathology, the loss of central vision, atrophy of the retinal pigment epithelium, and accumulation of lipofuscin, clinical features that are also characteristic of age-related macular degeneration. The onset of clinical symptoms in STGD3, however, is typically observed within the second or third decade of life (i.e., starting in the teenage years). The clinical profile at any given age among STGD3 patients can be variable suggesting that, although STGD3 is a single gene defect, other genetic or environmental factors may play a role in moderating the final disease phenotype. Genetic studies localized the STGD3 disease locus to a small region on the short arm of human chromosome 6, and application of a positional candidate gene approach identified protein truncating mutations in the elongation of very long chain fatty acids-4 gene (ELOVL4) in patients with this disease. The ELOVL4 gene encodes a protein homologous to the ELO group of proteins that participate in fatty acid elongation in yeast. Pathogenic mutations found in the ELOVL4 gene result in altered trafficking of the protein and behave with a dominant negative effect. Mice carrying an Elovl4 mutation developed photoreceptor degeneration and depletion of very long chain fatty acids (VLCFA). ELOVL4 protein participates in the synthesis of fatty acids with chain length longer than 26 carbons. Studies on ELOVL4 indicate that VLCFA may be necessary for normal function of the retina, and the defective protein trafficking and/or altered VLCFA elongation underlies the pathology associated with STGD3. Determining the role of VLCFA in the retina and discerning the implications of abnormal trafficking of mutant ELOVL4 and depleted VLCFA content in the pathology of STGD3 will provide valuable insight in understanding the retinal structure, function, and pathology underlying STGD3 and may lead to a better understanding of the process of macular disease in general.
STGD3; ELOVL4; Fatty acids; Dominant negative effect; Protein accumulation; Animal models
Autosomal dominant Stargardt macular dystrophy caused by mutations in the Elongation of Very Long Chain fatty acids (ELOVL4) gene results in macular degeneration, leading to early childhood blindness. Transgenic mice and pigs expressing mutant ELOVL4 develop progressive photoreceptor degeneration. The mechanism by which these mutations cause macular degeneration remains unclear, but have been hypothesized to involve the loss of an ER-retention dilysine motif located in the extreme C-terminus. Dominant negative mechanisms and reduction in retinal polyunsaturated fatty acids also have been suggested. To understand the molecular mechanisms involved in disease progression in vivo, we addressed the hypothesis that the disease-linked C-terminal truncation mutant of ELOVL4 exerts a dominant negative effect on wild-type (WT) ELOVL4, altering its subcellular localization and function, which subsequently induces retinal degeneration and loss of vision.
We generated transgenic Xenopus laevis that overexpress HA-tagged murine ELOVL4 variants in rod photoreceptors.
Tagged or untagged WT ELOVL4 localized primarily to inner segments. However, the mutant protein lacking the dilysine motif was mislocalized to post-Golgi compartments and outer segment disks. Coexpression of mutant and WT ELOVL4 in rods did not result in mislocalization of the WT protein to outer segments or in the formation of aggregates. Full-length HA-tagged ELOVL4 lacking the dilysine motif (K308R/K310R) necessary for targeting the WT ELOVL4 protein to the endoplasmic reticulum was similarly mislocalized to outer segments.
We propose that expression and outer segment mislocalization of the disease-linked 5–base-pair deletion mutant ELOVL4 protein alters photoreceptor structure and function, which subsequently results in retinal degeneration, and suggest three possible mechanisms by which mutant ELOVL4 may induce retinal degeneration in STGD3.
In this study, we showed that the disease-linked 5–base-pair deletion mutant ELOVL4 is misrouted to photoreceptor outer segments. We propose that outer segment mislocalization of the mutant ELOVL4 alters photoreceptor structure and function, which subsequently results in retinal degeneration.
autosomal dominant Stargardt-like macular dystrophy (STGD3); elongation of very long chain fatty acids-4 (ELOVL4); retinal degeneration; photoreceptor outer segment
Whole-exome sequencing is a diagnostic approach for the identification of molecular defects in patients with suspected genetic disorders.
We developed technical, bioinformatic, interpretive, and validation pipelines for whole-exome sequencing in a certified clinical laboratory to identify sequence variants underlying disease phenotypes in patients.
We present data on the first 250 probands for whom referring physicians ordered whole-exome sequencing. Patients presented with a range of phenotypes suggesting potential genetic causes. Approximately 80% were children with neurologic pheno-types. Insurance coverage was similar to that for established genetic tests. We identified 86 mutated alleles that were highly likely to be causative in 62 of the 250 patients, achieving a 25% molecular diagnostic rate (95% confidence interval, 20 to 31). Among the 62 patients, 33 had autosomal dominant disease, 16 had auto-somal recessive disease, and 9 had X-linked disease. A total of 4 probands received two nonoverlapping molecular diagnoses, which potentially challenged the clinical diagnosis that had been made on the basis of history and physical examination. A total of 83% of the autosomal dominant mutant alleles and 40% of the X-linked mutant alleles occurred de novo. Recurrent clinical phenotypes occurred in patients with mutations that were highly likely to be causative in the same genes and in different genes responsible for genetically heterogeneous disorders.
Whole-exome sequencing identified the underlying genetic defect in 25% of consecutive patients referred for evaluation of a possible genetic condition. (Funded by the National Human Genome Research Institute.)
ELOVL4 was first identified as a disease-causing gene in Stargardt macular dystrophy (STGD3, MIM 600110.) To date, three ELOVL4 mutations have been identified, all of which result in truncated proteins which induce autosomal dominant juvenile macular degenerations. Based on sequence homology, ELOVL4 is thought to be another member within a family of proteins functioning in the elongation of long chain fatty acids. However, the normal function of ELOVL4 is unclear. We generated Elovl4 knockout mice to determine if Elovl4 loss affects retinal development or function. Here we show that Elovl4 knockout mice, while perinatal lethal, exhibit normal retinal development prior to death at day of birth. Further, postnatal retinal development in Elovl4 heterozygous mice appears normal. Therefore haploinsufficiency for wildtype ELOVL4 in autosomal dominant macular degeneration likely does not contribute to juvenile macular degeneration in STGD3 patients. We found, however, that Elovl4+/− mice exhibit enhanced ERG scotopic and photopic a and b waves relative to wildtype Elovl4+/+ mice suggesting that reduced Elovl4 levels may impact retinal electrophysiological responses.
STGD3; Elovl4; knockout; mouse; ERG
To determine the carrier frequency of ABCA4 mutations in order to achieve an insight into the prevalence of autosomal recessive Stargardt disease (arSTGD) in the Spanish population.
arSTGD patients (n = 133) were analysed using ABCR400 microarray and sequencing. Control subjects were analysed by two different strategies: 200 individuals were screened for the p.Arg1129Leu mutation by denaturing-HPLC and sequencing; 78 individuals were tested for variants with the microarray and sequencing.
For the first strategy in control subjects, the p.Arg1129Leu variant was found in two heterozygous individuals, which would mean a carrier frequency for any variant of ∼6.0% and a calculated arSTGD prevalence of 1:1000. For the second strategy, carrier frequency was 6.4% and therefore an estimated prevalence of the disease of 1:870.
Calculated prevalence of arSTGD based on the ABCA4 carrier frequency could be considerably higher than previous estimation. This discrepancy between observed (genotypic) and estimated (phenotypic) prevalence could be due to the existence of non-pathological or low penetrance alleles, which may result in late-onset arSTGD or may be implicated in age-related macular degeneration. This situation should be regarded with especial care when genetic counselling is given and further follow-up of these patients should be recommended.
Stargardt disease is an ABCA4-associated retinopathy, which generally follows an autosomal recessive inheritance pattern and is a frequent cause of macular degeneration in childhood. ABCA4 displays significant allelic heterogeneity whereby different mutations can cause retinal diseases with varying severity and age of onset. A genotype–phenotype model has been proposed linking ABCA4 mutations, purported ABCA4 functional protein activity and severity of disease, as measured by degree of visual loss and the age of onset. It has, however, been difficult to verify this model statistically in observational studies, as the number of individuals sharing any particular mutation combination is typically low. Seven founder mutations have been identified in a large number of Caucasian Afrikaner patients in South Africa, making it possible to test the genotype–phenotype model. A generalised linear model was developed to predict and assess the relative pathogenic contribution of the seven mutations to the age of onset of Stargardt disease. It is shown that the pathogenicity of an individual mutation can differ significantly depending on the genetic context in which it occurs. The results reported here may be used to identify suitable candidates for inclusion in clinical trials, as well as guide the genetic counselling of affected individuals and families.
ABCA4; age of onset; generalised linear model; genotype–phenotype; Stargardt disease
The authors conducted comprehensive analysis of an important and very variable eye disease gene, ABCA4, by next-generation sequencing in a large cohort of patients and follow-up analysis of identified mutations in both coding and noncoding regions of the ABCA4 locus.
To find all possible disease-associated variants in coding sequences of the ABCA4 gene in a large cohort of patients diagnosed with ABCA4-associated diseases.
One hundred sixty-eight patients who had been clinically diagnosed with Stargardt disease, cone-rod dystrophy, and other ABCA4-associated phenotypes were prescreened for mutations in ABCA4 with the ABCA4 microarray, resulting in finding 1 of 2 expected mutations in 111 patients and 0 of 2 mutations in 57 patients. The next-generation sequencing (NGS) strategy was applied to these patients to sequence the entire coding region and the splice sites of the ABCA4 gene. Identified new variants were confirmed or rejected by Sanger sequencing and analyzed for possible pathogenicity by in silico programs and, where possible, by segregation analyses.
Sequencing was successful in 159 of 168 patients and identified the second disease-associated allele in 49 of 103 (∼48%) of patients with one previously identified mutation. Among those with no mutations, both disease-associated alleles were detected in 4 of 56 patients, and one mutation was detected in 10 of 56 patients. The authors detected a total of 57 previously unknown, possibly pathogenic, variants: 29 missense, 4 nonsense, 9 small deletions and 15 splice-site-altering variants. Of these, 55 variants were deemed pathogenic by a combination of predictive methods and segregation analyses.
Many mutations in the coding sequences of the ABCA4 gene are still unknown, and many possibly reside in noncoding regions of the ABCA4 locus. Although the ABCA4 array remains a good first-pass screening option, the NGS platform is a time- and cost-efficient tool for screening large cohorts.
To identify the gene causing a severe form of progressive autosomal recessive cone-rod dystrophy presenting as Stargardt disease and to characterize clinical features in a large American family.
We characterized an American family who had an unusual retinal dystrophy with clinical features of Stargardt disease and severe progressive cone-rod dystrophy. Family members underwent complete ocular examinations with evaluation of visual acuity, visual fields, fundus examination, fluorescein angiography, and electroretinography. Genome-wide linkage analysis of the family was performed using 408 microsatellite markers spanning the entire human genome. Direct DNA sequence analysis was used for mutational analysis of the ABCA4 gene in all exons and exon-intron boundary regions and for testing cosegregation of the mutations with the disease in the family. DNA sequence analysis was used to determine the presence of the mutations in 200 unrelated controls.
The proband presented with a clinical phenotype that was initially compatible with Stargardt disease, only to progress to a severe cone-rod dystrophy over the course of a few years. The disease-causing gene in the family was linked to the ABCA4 locus on chromosomal 1p22. One novel mutation, c.655A>T, was identified in exon 6 and another novel splicing mutation, c.5312+3A>T, was identified in intron 37 of ABCA4. The mutations were not present in 200 controls. The two affected sisters in this pedigree were compound heterozygotes for the mutations. Unaffected family members either did not carry either or had only one of the two mutations.
We have identified two novel ABCA4 mutations, c.655A>T and c.5312+3A>T. When present as a compound heterozygous state, the mutations cause a phenotype of retinal dystrophy that initially manifests as Stargardt disease and slowly progresses to a severe cone-rod dystrophy. These results expand the wide range of clinical manifestations of ABCA4 mutations.
Adaptive optics scanning laser ophthalmoscopy demonstrated abnormal cone spacing in regions of abnormal fundus autofluorescence and reduced visual function in 12 patients with Stargardt disease.
To study the relationship between macular cone structure, fundus autofluorescence (AF), and visual function in patients with Stargardt disease (STGD).
High-resolution images of the macula were obtained with adaptive optics scanning laser ophthalmoscopy (AOSLO) and spectral domain optical coherence tomography in 12 patients with STGD and 27 age-matched healthy subjects. Measures of retinal structure and AF were correlated with visual function, including best-corrected visual acuity, color vision, kinetic and static perimetry, fundus-guided microperimetry, and full-field electroretinography. Mutation analysis of the ABCA4 gene was completed in all patients.
Patients were 15 to 55 years old, and visual acuity ranged from 20/25–20/320. Central scotomas were present in all patients, although the fovea was spared in three patients. The earliest cone spacing abnormalities were observed in regions of homogeneous AF, normal visual function, and normal outer retinal structure. Outer retinal structure and AF were most normal near the optic disc. Longitudinal studies showed progressive increases in AF followed by reduced AF associated with losses of visual sensitivity, outer retinal layers, and cones. At least one disease-causing mutation in the ABCA4 gene was identified in 11 of 12 patients studied; 1 of 12 patients showed no disease-causing ABCA4 mutations.
AOSLO imaging demonstrated abnormal cone spacing in regions of abnormal fundus AF and reduced visual function. These findings provide support for a model of disease progression in which lipofuscin accumulation results in homogeneously increased AF with cone spacing abnormalities, followed by heterogeneously increased AF with cone loss, then reduced AF with cone and RPE cell death. (ClinicalTrials.gov number, NCT00254605.)
We have recently characterised the genomic organisation of a novel interphotoreceptor matrix proteoglycan, IMPG1, and have mapped the gene locus to chromosome 6q13-q15 by fluorescence in situ hybridisation. As the interphotoreceptor matrix (IPM) is thought to play a critical role in retinal adhesion and the maintenance of photoreceptor cells, it is conceivable that a defect in one of the IPM components may cause degenerative lesions in retinal structures and thus may be associated with human retinopathies. By genetic linkage analysis, several retinal dystrophies including one form of autosomal dominant Stargardt-like macular dystrophy (STGD3), progressive bifocal chorioretinal atrophy (PBCRA), and North Carolina macular dystrophy (MCDR1) have previously been localised to a region on proximal 6q that overlaps the IMPG1 locus. We have therefore assessed the entire coding region of IMPG1 by exon amplification and subsequent single stranded conformational analysis in patients from 6q linked multigeneration families diagnosed with PBCRA and MCDR1, as well as a single patient from an autosomal dominant STGD pedigree unlinked to either of the two known STGD2 and STGD3 loci on chromosomes 13q and 6q, respectively. No disease associated mutations were identified. In addition, using an intragenic polymorphism, IMPG1 was excluded by genetic recombination from both the PBCRA and the MCDR1 loci. However, as the autosomal dominant Stargardt-like macular dystrophies are genetically heterogeneous, other forms of this disorder, in particular STGD3 previously linked to 6q, may be caused by mutations in IMPG1.