A homozygous frameshift mutation in LRAT causes retinitis punctata albescens.

PURPOSE: To determine the genetic defect and to describe the clinical characteristics in patients with retinitis punctata albescens (RPA) and fundus albipunctatus (FAP). DESIGN: Case series/observational study. PARTICIPANTS: We included 13 patients affected by RPA or FAP. METHODS: Thirteen patients were collected from 8 families with a retinal dystrophy characterized by tiny, yellow-white dots on funduscopy, typical for FAP or RPA. All patients underwent full ophthalmologic examinations, including visual field assessment. Fundus photography, and electroretinography were performed in 12 patients, and optical coherence tomography and fundus autofluorescence were performed in 4 patients. DNA samples of all patients were screened for mutations in RLBP1 and for mutations in RDH5 in patients who did not carry mutations in RLBP1. DNA samples of 2 sibling pairs of nonconsanguineous families who carried mutations neither in RLBP1 nor in RDH5 were analyzed by genome-wide homozygosity mapping. Sequence analysis was performed of LRAT, a candidate gene in a shared homozygous region. MAIN OUTCOME MEASURES: We assessed DNA sequence variants, best-corrected visual acuity, fundus appearance, visual field measurements, electroretinogram responses, optical coherence tomography, and fundus autofluorescence. RESULTS: A homozygous frameshift mutation was identified in LRAT in 4 patients with RPA. Mutations in RLBP1 were identified in 7 patients with RPA and in 1 patient with FAP and cone dystrophy. One patient had compound heterozygous mutations in RDH5 and suffered from FAP with mild maculopathy. CONCLUSIONS: A genetic defect was identified in LRAT as a novel cause of RPA. LRAT is therefore the fourth gene involved in the visual cycle that may cause a white-dot retinopathy. We also revealed that mutations in RLBP1 may lead to FAP with cone dystrophy.

Next-generation genetic testing for retinitis pigmentosa.

Molecular diagnostics for patients with retinitis pigmentosa (RP) has been hampered by extreme genetic and clinical heterogeneity, with 52 causative genes known to date. Here, we developed a comprehensive next-generation sequencing (NGS) approach for the clinical molecular diagnostics of RP. All known inherited retinal disease genes (n = 111) were captured and simultaneously analyzed using NGS in 100 RP patients without a molecular diagnosis. A systematic data analysis pipeline was developed and validated to prioritize and predict the pathogenicity of all genetic variants identified in each patient, which enabled us to reduce the number of potential pathogenic variants from approximately 1,200 to zero to nine per patient. Subsequent segregation analysis and in silico predictions of pathogenicity resulted in a molecular diagnosis in 36 RP patients, comprising 27 recessive, six dominant, and three X-linked cases. Intriguingly, De novo mutations were present in at least three out of 28 isolated cases with causative mutations. This study demonstrates the enormous potential and clinical utility of NGS in molecular diagnosis of genetically heterogeneous diseases such as RP. De novo dominant mutations appear to play a significant role in patients with isolated RP, having major implications for genetic counselling.

Molecular genetic analysis of retinitis pigmentosa in Indonesia using genome-wide homozygosity mapping.

PURPOSE: Retinitis pigmentosa (RP) is a clinically and genetically heterogeneous retinal disorder. Despite tremendous knowledge about the genes involved in RP, little is known about the genetic causes of RP in Indonesia. Here, we aim to identify the molecular genetic causes underlying RP in a small cohort of Indonesian patients, using genome-wide homozygosity mapping. METHODS: DNA samples from affected and healthy individuals from 14 Indonesian families segregating autosomal recessive, X-linked, or isolated RP were collected. Homozygosity mapping was conducted using Illumina 6k or Affymetrix 5.0 single nucleotide polymorphism (SNP) arrays. Known autosomal recessive RP (arRP) genes residing in homozygous regions and X-linked RP genes were sequenced for mutations. RESULTS: In ten out of the 14 families, homozygous regions were identified that contained genes known to be involved in the pathogenesis of RP. Sequence analysis of these genes revealed seven novel homozygous mutations in ATP-binding cassette, sub-family A, member 4 (ABCA4), crumbs homolog 1 (CRB1), eyes shut homolog (Drosophila) (EYS), c-mer proto-oncogene tyrosine kinase (MERTK), nuclear receptor subfamily 2, group E, member 3 (NR2E3) and phosphodiesterase 6A, cGMP-specific, rod, alpha (PDE6A), all segregating in the respective families. No mutations were identified in the X-linked genes retinitis pigmentosa GTPase regulator (RPGR) and retinitis pigmentosa 2 (X-linked recessive; RP2). CONCLUSIONS: Homozygosity mapping is a powerful tool to identify the genetic defects underlying RP in the Indonesian population. Compared to studies involving patients from other populations, the same genes appear to be implicated in the etiology of recessive RP in Indonesia, although all mutations that were discovered are novel and as such may be unique for this population.

Exome sequencing and cis-regulatory mapping identify mutations in MAK, a gene encoding a regulator of ciliary length, as a cause of retinitis pigmentosa.

A fundamental challenge in analyzing exome-sequence data is distinguishing pathogenic mutations from background polymorphisms. To address this problem in the context of a genetically heterogeneous disease, retinitis pigmentosa (RP), we devised a candidate-gene prioritization strategy called cis-regulatory mapping that utilizes ChIP-seq data for the photoreceptor transcription factor CRX to rank candidate genes. Exome sequencing combined with this approach identified a homozygous nonsense mutation in male germ cell-associated kinase (MAK) in the single affected member of a consanguineous Turkish family with RP. MAK encodes a cilium-associated mitogen-activated protein kinase whose function is conserved from the ciliated alga, Chlamydomonas reinhardtii, to humans. Mutations in MAK orthologs in mice and other model organisms result in abnormally long cilia and, in mice, rapid photoreceptor degeneration. Subsequent sequence analyses of additional individuals with RP identified five probands with missense mutations in MAK. Two of these mutations alter amino acids that are conserved in all known kinases, and an in vitro kinase assay indicates that these mutations result in a loss of kinase activity. Thus, kinase activity appears to be critical for MAK function in humans. This study highlights a previously underappreciated role for CRX as a direct transcriptional regulator of ciliary genes in photoreceptors. In addition, it demonstrates the effectiveness of CRX-based cis-regulatory mapping in prioritizing candidate genes from exome data and suggests that this strategy should be generally applicable to a range of retinal diseases.

High-resolution homozygosity mapping is a powerful tool to detect novel mutations causative of autosomal recessive RP in the Dutch population.

PURPOSE: To determine the genetic defects underlying autosomal recessive retinitis pigmentosa (arRP) in the Dutch population and in a subset of patients originating from other countries. The hypothesis was that, because there has been little migration over the past centuries in certain areas of The Netherlands, a significant fraction of Dutch arRP patients carry their genetic defect in the homozygous state. METHODS: High-resolution genome-wide SNP genotyping on SNP arrays and subsequent homozygosity mapping were performed in a large cohort of 186 mainly nonconsanguineous arRP families living in The Netherlands. Candidate genes residing in homozygous regions were sequenced. RESULTS: In ~94% of the affected individuals, large homozygous sequences were identified in their genome. In 42 probands, at least one of these homozygous regions contained one of the 26 known arRP genes. Sequence analysis of the corresponding genes in each of these patients revealed 21 mutations and two possible pathogenic changes, 14 of which were novel. All mutations were identified in only a single family, illustrating the genetic diversity within the Dutch population. CONCLUSIONS: This report demonstrates that homozygosity mapping is a powerful tool for identifying the genetic defect underlying genetically heterogeneous recessive disorders like RP, even in populations with little consanguinity.

Mutations in the EYS gene account for approximately 5% of autosomal recessive retinitis pigmentosa and cause a fairly homogeneous phenotype.

OBJECTIVE: To determine the prevalence of mutations in the EYS gene in a cohort of patients affected by autosomal recessive retinitis pigmentosa (RP) and to describe the associated phenotype. DESIGN: Case series. PARTICIPANTS: Two hundred forty-five patients affected by autosomal recessive RP. METHODS: All coding exons of EYS were screened for mutations by polymerase chain reaction amplification and sequence analysis. All 12 patients carrying mutations in EYS were re-examined, which included Goldmann kinetic perimetry, electroretinography, and high-resolution spectral-domain optical coherence tomography (OCT). MAIN OUTCOME MEASURES: DNA sequence variants, best-corrected visual acuity, fundus appearance, visual field assessments using Goldmann kinetic perimetry, electroretinogram responses, and OCT images. RESULTS: Nine novel truncating mutations and one previously described mutation in EYS were identified in 11 families. In addition, 18 missense changes of uncertain pathogenicity were found. Patients carrying mutations in EYS demonstrated classic RP with night blindness as the initial symptom, followed by gradual constriction of the visual field and a decline of visual acuity later in life. The onset of symptoms typically occurred between the second and fourth decade of life. The fundus displayed bone spicules increasing in density with age and generalized atrophy of the retinal pigment epithelium and choriocapillaris with relative sparing of the posterior pole until later in the disease process, when atrophic macular changes occurred. CONCLUSIONS: Mutations in EYS account for approximately 5% of autosomal recessive RP patients in a cohort of patients consisting predominantly of patients of western European ancestry. The EYS-associated RP phenotype is typical and fairly homogeneous in most patients.

Homozygosity mapping in patients with cone-rod dystrophy: novel mutations and clinical characterizations.

PURPOSE: To determine the genetic defect and to describe the clinical characteristics in a cohort of mainly nonconsanguineous cone-rod dystrophy (CRD) patients. METHODS: One hundred thirty-nine patients with diagnosed CRD were recruited. Ninety of them were screened for known mutations in ABCA4, and those carrying one or two mutations were excluded from further research. Genome-wide homozygosity mapping was performed in the remaining 108. Known genes associated with autosomal recessive retinal dystrophies located within a homozygous region were screened for mutations. Patients in whom a mutation was detected underwent further ophthalmic examination. RESULTS: Homozygous sequence variants were identified in eight CRD families, six of which were nonconsanguineous. The variants were detected in the following six genes: ABCA4, CABP4, CERKL, EYS, KCNV2, and PROM1. Patients carrying mutations in ABCA4, CERKL, and PROM1 had typical CRD symptoms, but a variety of retinal appearances on funduscopy, optical coherence tomography, and autofluorescence imaging. CONCLUSIONS: Homozygosity mapping led to the identification of new mutations in consanguineous and nonconsanguineous patients with retinal dystrophy. Detailed clinical characterization revealed a variety of retinal appearances, ranging from nearly normal to extensive retinal remodeling, retinal thinning, and debris accumulation. Although CRD was initially diagnosed in all patients, the molecular findings led to a reappraisal of the diagnosis in patients carrying mutations in EYS, CABP4, and KCNV2.

Novel null mutations in the EYS gene are a frequent cause of autosomal recessive retinitis pigmentosa in the Israeli population.

PURPOSE: To characterize the role of EYS, a recently identified retinal disease gene, in families with inherited retinal degenerations in the Israeli and Palestinian populations. METHODS: Clinical and molecular analyses included family history, ocular examination, full-field electroretinography (ERG), perimetry, autozygosity mapping, mutation detection, and estimation of mutation age. RESULTS: Autozygosity mapping was performed in 171 consanguineous Israeli and Palestinian families with inherited retinal degenerations. Large homozygous regions, harboring the EYS gene, were identified in 15 of the families. EYS mutation analysis in the 15 index cases, followed by genotyping of specific mutations in an additional 121 cases of inherited retinal degenerations, revealed five novel null mutations, two of which are founder mutations, in 10 Israeli and Palestinian families with autosomal recessive retinitis pigmentosa (arRP). The most common mutation identified was a founder mutation in the Moroccan Jewish subpopulation. The ESTIAGE program produced an estimate that the age of the most recent common ancestor was 26 generations. The retinal phenotype in most patients was typical yet relatively severe RP, with an early age of onset and nonrecordable ERGs on presentation. CONCLUSIONS: The results demonstrate that EYS is currently the most commonly mutated arRP gene in the Israeli population, mainly due to founder mutations. EYS mutations were associated with an RP phenotype in all patients. The authors concluded that the gene plays only a minor role in causing other retinal phenotypes.

A common allele in RPGRIP1L is a modifier of retinal degeneration in ciliopathies.

Despite rapid advances in the identification of genes involved in disease, the predictive power of the genotype remains limited, in part owing to poorly understood effects of second-site modifiers. Here we demonstrate that a polymorphic coding variant of RPGRIP1L (retinitis pigmentosa GTPase regulator-interacting protein-1 like), a ciliary gene mutated in Meckel-Gruber (MKS) and Joubert (JBTS) syndromes, is associated with the development of retinal degeneration in individuals with ciliopathies caused by mutations in other genes. As part of our resequencing efforts of the ciliary proteome, we identified several putative loss-of-function RPGRIP1L mutations, including one common variant, A229T. Multiple genetic lines of evidence showed this allele to be associated with photoreceptor loss in ciliopathies. Moreover, we show that RPGRIP1L interacts biochemically with RPGR, loss of which causes retinal degeneration, and that the Thr229-encoded protein significantly compromises this interaction. Our data represent an example of modification of a discrete phenotype of syndromic disease and highlight the importance of a multifaceted approach for the discovery of modifier alleles of intermediate frequency and effect.

Identification of a 2 Mb human ortholog of Drosophila eyes shut/spacemaker that is mutated in patients with retinitis pigmentosa.

In patients with autosomal-recessive retinitis pigmentosa (arRP), homozygosity mapping was performed for detection of regions harboring genes that might be causative for RP. In one affected sib pair, a shared homozygous region of 5.0 Mb was identified on chromosome 6, within the RP25 locus. One of the genes residing in this interval was the retina-expressed gene EGFL11. Several genes resembling EGFL11 were predicted just centromeric of EGFL11. Extensive long-range RT-PCR, combined with 5'- and 3'- RACE analysis, resulted in the identification of a 10-kb transcript, starting with the annotated exons of EGFL11 and spanning 44 exons and 2 Mb of genomic DNA. The transcript is predicted to encode a 3165-aa extracellular protein containing 28 EGF-like and five laminin A G-like domains. Interestingly, the second part of the protein was found to be the human ortholog of Drosophila eyes shut (eys), also known as spacemaker, a protein essential for photoreceptor morphology. Mutation analysis in the sib pair homozygous at RP25 revealed a nonsense mutation (p.Tyr3156X) segregating with RP. The same mutation was identified homozygously in three arRP siblings of an unrelated family. A frame-shift mutation (pPro2238ProfsX16) was found in an isolated RP patient. In conclusion, we identified a gene, coined eyes shut homolog (EYS), consisting of EGFL11 and the human ortholog of Drosophila eys, which is mutated in patients with arRP. With a size of 2 Mb, it is one of the largest human genes, and it is by far the largest retinal dystrophy gene. The discovery of EYS might shed light on a critical component of photoreceptor morphogenesis.

Mutations of the CEP290 gene encoding a centrosomal protein cause Meckel-Gruber syndrome.

Meckel-Gruber syndrome (MKS) is an autosomal recessive, lethal multisystemic disorder characterized by meningooccipital encephalocele, cystic kidney dysplasia, hepatobiliary ductal plate malformation, and postaxial polydactyly. Recently, genes for MKS1 and MKS3 were identified, putting MKS on the list of ciliary disorders (ciliopathies). By positional cloning in a distantly related multiplex family, we mapped a novel locus for MKS to a 3-Mb interval on 12q21. Sequencing of the CEP290 gene located in the minimal critical region showed a homozygous 1-bp deletion supposed to lead to loss of function of the encoded centrosomal protein CEP290/nephrocystin-6. CEP290 is thought to be involved in chromosome segregation and localizes to cilia, centrosomes, and the nucleus. Subsequent analysis of another consanguineous multiplex family revealed homozygous haplotypes and the same frameshift mutation. Our findings add to the increasing body of evidence that ciliopathies can cause a broad spectrum of disease phenotypes, and pleiotropic effects of CEP290 mutations range from single organ involvement with isolated Leber congenital amaurosis to Joubert syndrome and lethal early embryonic multisystemic malformations in Meckel-Gruber syndrome. We compiled clinical and genetic data of all patients with CEP290 mutations described so far. No clear-cut genotype-phenotype correlations were apparent as almost all mutations are nonsense, frameshift, or splice-site changes and scattered throughout the gene irrespective of the patients' phenotypes. Conclusively, other factors than the type and location of CEP290 mutations may underlie phenotypic variability.

Identification of novel mutations in patients with Leber congenital amaurosis and juvenile RP by genome-wide homozygosity mapping with SNP microarrays.

PURPOSE: Leber congenital amaurosis (LCA) and juvenile retinitis pigmentosa (RP) cause severe visual impairment early in life. Thus far, mutations in 13 genes have been associated with autosomal recessive LCA and juvenile RP. The purpose of this study was to use homozygosity mapping to identify mutations in known LCA and juvenile RP genes. METHODS: The genomes of 93 consanguineous and nonconsanguineous patients with LCA and juvenile RP were analyzed for homozygous chromosomal regions by using SNP microarrays. This patient cohort was highly selected, as mutations in the known genes had been excluded with the LCA mutation chip, or a significant number of LCA genes had been excluded by comprehensive mutation analysis. Known LCA and juvenile RP genes residing in the identified homozygous regions were analyzed by sequencing. Detailed ophthalmic examinations were performed on the genotyped patients. RESULTS: Ten homozygous mutations, including seven novel mutations, were identified in the CRB1, LRAT, RPE65, and TULP1 genes in 12 patients. Ten patients were from consanguineous marriages, but in two patients no consanguinity was reported. In 10 of the 12 patients, the causative mutation was present in the largest or second largest homozygous segment of the patient's genome. CONCLUSIONS: Homozygosity mapping using SNP microarrays identified mutations in a significant proportion (30%) of consanguineous patients with LCA and juvenile RP and in a small number (3%) of nonconsanguineous patients. Significant homozygous regions which did not map to known LCA or juvenile RP genes and may be instrumental in identifying novel disease genes were detected in 33 patients.

Molecular and phenotypic analysis of a family with autosomal recessive cone-rod dystrophy and Stargardt disease.

PURPOSE: To identify the causative gene mutations in three siblings with severe progressive autosomal recessive cone-rod dystrophy (arCRD) and their fifth paternal cousin with Stargardt disease (STGD1) and to specify the phenotypes. METHODS: We evaluated eight sibs of one family, three family members displayed arCRD, and one STGD1. All of them were screened for mutations using a new microarray for autosomal recessive retinitis pigmentosa. RESULTS: We found a new pathologic ATP-binding cassette transporter (ABCA4) splice-site mutation, c.3523-2A>T and the previously reported c.5327C>T (p.P1776L) missense mutation in the arCRD patients. The three siblings shared these two ABCA4 mutations and showed similar phenotypes. An unusual aspect was nystagmus which presented in one of the arCRD patients. In the STGD1 patient we found the c.5327C>T (p.P1776L) missense mutation and a novel c.868C>T (p.R290W) missense mutation. CONCLUSIONS: Two new ABCA4 mutations were identified in a family with arCRD and STGD1. A new finding was nystagmus associated with arCRD in one of the patients.

Clinical and genetic heterogeneity in multifocal vitelliform dystrophy.

OBJECTIVE: To describe the clinical and genetic findings in 15 patients with multifocal vitelliform lesions. METHODS: All patients and, if possible, affected family members underwent an ophthalmic examination and their genomic DNA was analyzed for mutations in the vitelliform macular dystrophy 2 (VMD2) gene. Patients who did not have a mutation in the VMD2 gene were screened for mutations in the peripherin/RDS gene. RESULTS: Patient age at onset of the disease was highly variable, ranging from 5 to 59 years. The peripheral lesions varied in number, size, and overall appearance but showed similar characteristics at autofluorescence imaging and optical coherence tomography compared with the central vitelliform lesion. Mutations in the VMD2 gene were identified in 9 of 15 patients. One patient without a VMD2 mutation carried a sequence variant in the 5' untranslated region of the peripherin/RDS gene. CONCLUSIONS: Multifocal vitelliform dystrophy is a clinically and genetically heterogeneous retinal disease that can be caused by mutations in the VMD2 gene. Other genes associated with this phenotype remain to be identified. CLINICAL RELEVANCE: Clinical and molecular genetic characterization of multifocal vitelliform dystrophy may lead to better understanding of the pathophysiological mechanisms underlying this phenotype and may enable a more accurate prognosis in individual patients.

Mutations in LCA5, encoding the ciliary protein lebercilin, cause Leber congenital amaurosis.

Leber congenital amaurosis (LCA) causes blindness or severe visual impairment at or within a few months of birth. Here we show, using homozygosity mapping, that the LCA5 gene on chromosome 6q14, which encodes the previously unknown ciliary protein lebercilin, is associated with this disease. We detected homozygous nonsense and frameshift mutations in LCA5 in five families affected with LCA. In a sixth family, the LCA5 transcript was completely absent. LCA5 is expressed widely throughout development, although the phenotype in affected individuals is limited to the eye. Lebercilin localizes to the connecting cilia of photoreceptors and to the microtubules, centrioles and primary cilia of cultured mammalian cells. Using tandem affinity purification, we identified 24 proteins that link lebercilin to centrosomal and ciliary functions. Members of this interactome represent candidate genes for LCA and other ciliopathies. Our findings emphasize the emerging role of disrupted ciliary processes in the molecular pathogenesis of LCA.

Mutations in the CEP290 (NPHP6) gene are a frequent cause of Leber congenital amaurosis.

Leber congenital amaurosis (LCA) is one of the main causes of childhood blindness. To date, mutations in eight genes have been described, which together account for approximately 45% of LCA cases. We localized the genetic defect in a consanguineous LCA-affected family from Quebec and identified a splice defect in a gene encoding a centrosomal protein (CEP290). The defect is caused by an intronic mutation (c.2991+1655A-->G) that creates a strong splice-donor site and inserts a cryptic exon in the CEP290 messenger RNA. This mutation was detected in 16 (21%) of 76 unrelated patients with LCA, either homozygously or in combination with a second deleterious mutation on the other allele. CEP290 mutations therefore represent one of the most frequent causes of LCA identified so far.

Microarray-based mutation detection and phenotypic characterization of patients with Leber congenital amaurosis.

PURPOSE: To test the efficiency of a microarray chip as a diagnostic tool in a cohort of northwestern European patients with Leber congenital amaurosis (LCA) and to perform a genotype-phenotype analysis in patients in whom pathologic mutations were identified. METHODS: DNAs from 58 patients with LCA were analyzed using a microarray chip containing previously identified disease-associated sequence variants in six LCA genes. Mutations identified by chip analysis were confirmed by sequence analysis. On identification of one mutation, all protein coding exons of the relevant genes were sequenced. In addition, sequence analysis of the RDH12 gene was performed in 22 patients. Patients with mutations were phenotyped. RESULTS: Pathogenic mutations were identified in 19 of the 58 patients with LCA (32.8%). Four novel sequence variants were identified. Mutations were most frequently found in CRB1 (15.5%), followed by GUCY2D (10.3%). The p.R768W mutation was found in 8 of 10 GUCY2D alleles, suggesting that it is a founder mutation in the northwest of Europe. In early childhood, patients with AIPL1 or GUCY2D mutations show normal fundi. Those with AIPL1-associated LCA progress to an RP-like fundus before the age of 8, whereas patients with GUCY2D-associated LCA still have relatively normal fundi in their mid-20s. Patients with CRB1 mutations present with distinct fundus abnormalities at birth and consistently show characteristics of RP12. Pathogenic GUCY2D mutations result in the most severe form of LCA. CONCLUSIONS: Microarray-based mutation detection allowed the identification of 32% of LCA sequence variants and represents an efficient first-pass screening tool. Mutations in CRB1, and to a lesser extent, in GUCY2D, underlie most LCA cases in this cohort. The present study establishes a genotype-phenotype correlation for AIPL1, CRB1, and GUCY2D.

Characterization of the Crumbs homolog 2 (CRB2) gene and analysis of its role in retinitis pigmentosa and Leber congenital amaurosis.

PURPOSE: Mutations in the Crumbs homolog 1 (CRB1) gene cause autosomal recessive retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA). Database searches reveal two other Crumbs homologs on chromosomes 9q33.3 and 19p13.3. The purpose of this study was to characterize the Crumbs homolog 2 (CRB2) gene on 9q33.3, to analyze its expression pattern, and to determine whether mutations in CRB2 are associated with RP and LCA. METHODS: The CRB2 mRNA and its expression pattern in human tissues were analyzed by reverse transcription-polymerase chain reaction (RT-PCR). The cellular expression of Crb2 in the mouse eye was determined by mRNA in situ hybridizations. The open reading frame and splice junctions of CRB2 were analyzed for mutations by single-strand conformation analysis and direct nucleotide sequencing in 85 RP patients and 79 LCA patients. RESULTS: The CRB2 gene consists of 13 exons and encodes a 1285 amino acid transmembrane protein. CRB2 is mainly expressed in retina, brain, and kidney. In mouse retina Crb2 expression was detected in all cell layers. Mutation analysis of the CRB2 gene revealed 11 sequence variants leading to an amino acid substitution. Three of them were not identified in control individuals and affect conserved amino acid residues. However, the patients that carry these sequence variants do not have a second sequence variant on the other allele, excluding autosomal recessive inheritance of CRB2 sequence variants as a cause of their disease. CONCLUSIONS: This study shows that CRB2 sequence variants are not a common cause of autosomal recessive RP and LCA. It is possible that a more complex clinical phenotype is associated with the loss or altered function of CRB2 in humans due to its expression in tissues other than the retina.

CRB1 mutation spectrum in inherited retinal dystrophies.

Mutations in the Crumbs homologue 1 (CRB1) gene have been reported in patients with a variety of autosomal recessive retinal dystrophies, including retinitis pigmentosa (RP) with preserved paraarteriolar retinal pigment epithelium (PPRPE), RP with Coats-like exudative vasculopathy, early onset RP without PPRPE, and Leber congenital amaurosis (LCA). We extended our investigations of CRB1 in these retinal dystrophies, and identified nine novel CRB1 sequence variants. In addition, we screened patients with "classic" RP and classic Coats disease (without RP), but no pathologic sequence variants were found in the CRB1 gene. In total, 71 different sequence variants have been identified on 184 CRB1 alleles of patients with retinal dystrophies, including amino acid substitutions, frameshift, nonsense, and splice site mutations, in-frame deletions, and large insertions. Recent studies in two animal models, mouse and Drosophila, and in vivo high-resolution microscopy in patients with LCA, have shed light on the role of CRB1 in the pathogenesis of retinal dystrophies and its function in the photoreceptors. In this article, we provide an overview of the currently known CRB1 sequence variants, predict their effect, and propose a genotype-phenotype correlation model for CRB1 mutations.