Mutations in SAMD7 cause autosomal-recessive macular dystrophy with or without cone dysfunction.

Sterile alpha motif domain containing 7 (SAMD7) is a component of the Polycomb repressive complex 1, which inhibits transcription of many genes, including those activated by the transcription factor Cone-Rod Homeobox (CRX). Here we report bi-allelic mutations in SAMD7 as a cause of autosomal-recessive macular dystrophy with or without cone dysfunction. Four of these mutations affect splicing, while another mutation is a missense variant that alters the repressive effect of SAMD7 on CRX-dependent promoter activity, as shown by in vitro assays. Immunostaining of human retinal sections revealed that SAMD7 is localized in the nuclei of both rods and cones, as well as in those of cells belonging to the inner nuclear layer. These results place SAMD7 as a gene crucial for human retinal function and demonstrate a significant difference in the role of SAMD7 between the human and the mouse retina.

TULP1 and TUB Are Required for Specific Localization of PRCD to Photoreceptor Outer Segments.

Photoreceptor disc component (PRCD) is a small protein which is exclusively localized to photoreceptor outer segments, and is involved in the formation of photoreceptor outer segment discs. Mutations in PRCD are associated with retinal degeneration in humans, mice, and dogs. The purpose of this work was to identify PRCD-binding proteins in the retina. PRCD protein-protein interactions were identified when implementing the Ras recruitment system (RRS), a cytoplasmic-based yeast two-hybrid system, on a bovine retina cDNA library. An interaction between PRCD and tubby-like protein 1 (TULP1) was identified. Co-immunoprecipitation in transfected mammalian cells confirmed that PRCD interacts with TULP1, as well as with its homolog, TUB. These interactions were mediated by TULP1 and TUB highly conserved C-terminal tubby domain. PRCD localization was altered in the retinas of TULP1- and TUB-deficient mice. These results show that TULP1 and TUB, which are involved in the vesicular trafficking of several photoreceptor proteins from the inner segment to the outer segment, are also required for PRCD exclusive localization to photoreceptor outer segment discs.

PRCD is concentrated at the base of photoreceptor outer segments and is involved in outer segment disc formation.

Mutations of the photoreceptor disc component (PRCD) gene are associated with rod-cone degeneration in both dogs and humans. Prcd is expressed in the mouse eye as early as embryonic day 14. In the adult mouse retina, PRCD is expressed in the outer segments of both rod and cone photoreceptors. Immunoelectron microscopy revealed that PRCD is located at the outer segment rim and that it is highly concentrated at the base of the outer segment. Prcd-knockout mice present with progressive retinal degeneration, starting at 20 weeks of age and onwards. This process is reflected by a significant and progressive reduction of both scotopic and photopic electroretinographic responses and by thinning of the retina, and specifically of the outer nuclear layer, indicating photoreceptor loss. Electron microscopy revealed severe damage to photoreceptor outer segments, which is associated with immigration of microglia cells to the Prcd-knockout retina and accumulation of vesicles in the inter-photoreceptor space. Phagocytosis of photoreceptor outer segment discs by the retinal pigmented epithelium is severely reduced. Our data show that Prcd-knockout mice serve as a good model for retinal degeneration caused by PRCD mutations in humans. Our findings in these mice support the involvement of PRCD in outer segment disc formation of both rod and cone photoreceptors. Furthermore, they suggest a feedback mechanism which coordinates the rate of photoreceptor outer segment disc formation, shedding and phagocytosis. This study has important implications for understanding the function of PRCD in the retina, as well as for future development of treatment modalities for PRCD deficiency in humans.

Oculopharyngeal Muscular Dystrophy and Inherited Retinal Dystrophy in Bukhara Jews Due to Linked Mutations in the PABPN1 and NRL Genes.

AIM: We have previously described two unrelated Bukhara Jews (BJs) with a combination of oculopharyngeal muscular dystrophy (OPMD) and inherited retinal dystrophy (IRD), because of mutations in two linked genes: PABPN1 and NRL. Here we investigated the prevalence of the NRL mutation among BJs with OPMD. MATERIALS AND METHODS: PABPN1 and NRL mutation testing were performed by polymerase chain reaction amplification and direct sequencing on two cohorts of Bukhara Jewish patients: OPMD patients (with or without IRD) and IRD patients (without OPMD). RESULTS: Of 24 unrelated chromosomes from Bukhara Jewish OPMD patients, 19 (79%) harbored the NRL mutation. In contrast, the NRL mutation was not detected in Bukhara Jewish patients diagnosed with IRD but without OPMD. CONCLUSIONS: Our findings provide an explanation for the reoccurrence of IRD in Bukhara Jewish OPMD homozygotes. Moreover, they indicate that Bukhara Jewish OPMD patients are at high risk for carrying the NRL mutation, and should be offered appropriate genetic counseling and testing.

An intronic deletion in the PROM1 gene leads to autosomal recessive cone-rod dystrophy.

PURPOSE: To investigate the genetic basis for autosomal recessive cone-rod dystrophy (CRD) in a consanguineous Israeli Jewish family. METHODS: Patients underwent a detailed ophthalmic evaluation, including eye examination, visual field testing, optical coherence tomography (OCT), and electrophysiological tests, electroretinography (ERG) and visual evoked potential (VEP). Genome-wide homozygosity mapping using a single nucleotide polymorphism (SNP) array was performed to identify homozygous regions shared among two of the affected individuals. Mutation screening of the underlying gene was performed with direct sequencing. In silico and in vitro analyses were used to predict the effect of the identified mutation on splicing. RESULTS: The affected family members are three siblings who have various degrees of progressive visual deterioration, glare, color vision abnormalities, and night vision difficulties. Visual field tests revealed central scotomas of different extension. Cone and rod ERG responses were reduced, with cones more severely affected. Homozygosity mapping revealed several homozygous intervals shared among two of the affected individuals. One included the PROM1 gene. Sequence analysis of the 26 coding exons of PROM1 in one affected individual revealed no mutations in the coding sequence or in intronic splice sites. However, in intron 21, proximate to the intron-exon junction, we observed a homozygous 10 bp deletion between positions -26 and -17 (c.2281-26_-17del). The deletion was linked to a known SNP, c.2281-6C>G. The deletion cosegregated with the disease in the family, and was not detected in public databases or in 101 ethnically-matched control individuals. In silico analysis predicted that this deletion would lead to altered intron 21 splicing. Bioinformatic analysis predicted that a recognition site for the SRSF2 splicing factor is located within the deleted sequence. The in vitro splicing assay demonstrated that c.2281-26_-17del leads to complete exon 22 skipping. CONCLUSIONS: A novel and unique intronic mutation of PROM1, underlying autosomal recessive CRD in a consanguineous Israeli family, was found. This report expands the spectrum of pathogenic mutations of PROM1 and further demonstrates the importance of intronic mutations.

Non-syndromic retinitis pigmentosa due to mutations in the mucopolysaccharidosis type IIIC gene, heparan-alpha-glucosaminide N-acetyltransferase (HGSNAT).

Retinitis pigmentosa (RP), the most common form of inherited retinal degeneration, is clinically and genetically heterogeneous and can appear as syndromic or non-syndromic. Mucopolysaccharidosis type IIIC (MPS IIIC) is a lethal disorder, caused by mutations in the heparan-alpha-glucosaminide N-acetyltransferase (HGSNAT) gene and characterized by progressive neurological deterioration, with retinal degeneration as a prominent feature. We identified HGSNAT mutations in six patients with non-syndromic RP. Whole exome sequencing (WES) in an Ashkenazi Jewish Israeli RP patient revealed a novel homozygous HGSNAT variant, c.370A>T, which leads to partial skipping of exon 3. Screening of 66 Ashkenazi RP index cases revealed an additional family with two siblings homozygous for c.370A>T. WES in three Dutch siblings with RP revealed a complex HGSNAT variant, c.[398G>C; 1843G>A] on one allele, and c.1843G>A on the other allele. HGSNAT activity levels in blood leukocytes of patients were reduced compared with healthy controls, but usually higher than those in MPS IIIC patients. All patients were diagnosed with non-syndromic RP and did not exhibit neurological deterioration, or any phenotypic features consistent with MPS IIIC. Furthermore, four of the patients were over 60 years old, exceeding by far the life expectancy of MPS IIIC patients. HGSNAT is highly expressed in the mouse retina, and we hypothesize that the retina requires higher HGSNAT activity to maintain proper function, compared with other tissues associated with MPS IIIC, such as the brain. This report broadens the spectrum of phenotypes associated with HGSNAT mutations and highlights the critical function of HGSNAT in the human retina.

Genetic heterogeneity and consanguinity lead to a "double hit": homozygous mutations of MYO7A and PDE6B in a patient with retinitis pigmentosa.

PURPOSE: Retinitis pigmentosa (RP), the most genetically heterogeneous disorder in humans, actually represents a group of pigmentary retinopathies characterized by night blindness followed by visual-field loss. RP can appear as either syndromic or nonsyndromic. One of the most common forms of syndromic RP is Usher syndrome, characterized by the combination of RP, hearing loss, and vestibular dysfunction. METHODS: The underlying cause of the appearance of syndromic and nonsyndromic RP in three siblings from a consanguineous Israeli Muslim Arab family was studied with whole-genome homozygosity mapping followed by whole exome sequencing. RESULTS: THE FAMILY WAS FOUND TO SEGREGATE NOVEL MUTATIONS OF TWO DIFFERENT GENES: myosin VIIA (MYO7A), which causes type 1 Usher syndrome, and phosphodiesterase 6B, cyclic guanosine monophosphate-specific, rod, beta (PDE6B), which causes nonsyndromic RP. One affected child was homozygous for both mutations. Since the retinal phenotype seen in this patient results from overlapping pathologies, one might expect to find severe retinal degeneration. Indeed, he was diagnosed with RP based on an abnormal electroretinogram (ERG) at a young age (9 months). However, this early diagnosis may be biased, as two of his older siblings had already been diagnosed, leading to increased awareness. At the age of 32 months, he had relatively good vision with normal visual fields. Further testing of visual function and structure at different ages in the three siblings is needed to determine whether the two RP-causing genes mutated in this youngest sibling confer increased disease severity. CONCLUSIONS: This report further supports the genetic heterogeneity of RP, and demonstrates how consanguinity could increase intrafamilial clustering of multiple hereditary diseases. Moreover, this report provides a unique opportunity to study the clinical implications of the coexistence of pathogenic mutations in two RP-causative genes in a human patient.

Novel mutations of MYO7A and USH1G in Israeli Arab families with Usher syndrome type 1.

PURPOSE: This study investigated the genetic basis for Usher syndrome type 1 (USH1) in four consanguineous Israeli Arab families. METHODS: Haplotype analysis for all known USH1 loci was performed in each family. In families for which haplotype analysis was inconclusive, we performed genome-wide homozygosity mapping using a single nucleotide polymorphism (SNP) array. For mutation analysis, specific primers were used to PCR amplify the coding exons of the MYO7A, USH1C, and USH1G genes including intron-exon boundaries. Mutation screening was performed with direct sequencing. RESULTS: A combination of haplotype analysis and genome-wide homozygosity mapping indicated linkage to the USH1B locus in two families, USH1C in one family and USH1G in another family. Sequence analysis of the relevant genes (MYO7A, USH1C, and USH1G) led to the identification of pathogenic mutations in all families. Two of the identified mutations are novel (c.1135-1147dup in MYO7A and c.206-207insC in USH1G). CONCLUSIONS: USH1 is a genetically heterogenous condition. Of the five USH1 genes identified to date, USH1C and USH1G are the rarest contributors to USH1 etiology worldwide. It is therefore interesting that two of the four Israeli Arab families reported here have mutations in these two genes. This finding further demonstrates the unique genetic structure of the Israeli population in general, and the Israeli Arab population in particular, which due to high rates of consanguinity segregates many rare autosomal recessive genetic conditions.

Mutations in C2ORF71 cause autosomal-recessive retinitis pigmentosa.

With a worldwide prevalence of 1 in 4,000, retinitis pigmentosa (RP) is the most common form of hereditary retinal degeneration. More than 30 genes and loci have been implicated in nonsyndromic autosomal-recessive (ar) RP. Genome-wide homozygosity mapping was conducted in one Dutch and one Israeli family affected by arRP. The families were found to share a 5.9 Mb homozygous region on chromosome 2p23.1-p23.3. A missense variant in one of the genes residing in this interval, C2ORF71, has recently been reported to be associated with RP. C2ORF71, encoding a putative protein of 1,288 amino acids, was found to be specifically expressed in human retina. Furthermore, RT-PCR analysis revealed that in the mouse eye, C2orf71 is expressed as early as embryonic day 14. Mutation analysis detected a 1 bp deletion (c.946 del; p.Asn237MetfsX5) segregating with RP in the Dutch family, whereas a nonsense mutation (c.556C > T; p.Gln186X) was identified in the Israeli family. Microsatellite-marker analysis in additional Israeli families revealed cosegregation of a C2ORF71-linked haplotype in one other family, in which a 13 bp deletion (c.2756_2768 del; p.Lys919ThrfsX) was identified. Clinically, patients with mutations in C2ORF71 show signs of typical RP; these signs include poor night vision and peripheral field loss, typical retinal bone-spicule-type pigment deposits, pale appearance of the optic disk, and markedly reduced or completely extinguished electroretinograms. In conclusion, truncating mutations in C2ORF71 were identified in three unrelated families, thereby confirming the involvement of this gene in the etiology of arRP.

Genetic heterogeneity in two consanguineous families segregating early onset retinal degeneration: the pitfalls of homozygosity mapping.

Retinitis pigmentosa is the most common form of hereditary retinal degeneration, with a worldwide prevalence of 1 in 4,000. At least 28 genes and loci have been implicated in nonsyndromic autosomal recessive retinitis pigmentosa. Here we report two extended and highly consanguineous families segregating early onset retinitis pigmentosa. Despite the consanguinity in both families, we found allelic heterogeneity in one of them, in which affected individuals were compound heterozygotes for two different mutations of the CRB1 gene. In the second family we found evidence for locus heterogeneity. A novel homozygous mutation of RDH12 was found in only 14 of 17 affected individuals in this family. Our data indicate that in the other affected individuals the disease is caused by a different gene/s. These findings demonstrate that while homozygosity mapping is an efficient tool for identification of the underlying mutated genes in inbred families, both locus and allelic heterogeneity may occur even within the same consanguineous family. These observations should be taken into account, especially when studying common and heterogeneous recessive genetic conditions.

Four USH2A founder mutations underlie the majority of Usher syndrome type 2 cases among non-Ashkenazi Jews.

Type 2 Usher syndrome (USH2) is a recessively inherited disorder, characterized by the combination of early onset, moderate-to-severe, sensorineural hearing loss, and vision impairment due to retinitis pigmentosa. From 74% to 90% of USH2 cases are caused by mutations of the USH2A gene. USH2A is composed of 72 exons, encoding for usherin, an extracellular matrix protein, which plays an important role in the development and maintenance of neurosensory cells in both retina and cochlea. To date, over 70 pathogenic mutations of USH2A have been reported in individuals of various ethnicities. Many of these mutations are rare private mutations segregating in single families. The aim of the current work was to investigate the genetic basis for USH2 among Jews of various origins. We found that four USH2A mutations (c.239-240insGTAC, c.1000C>T, c.2209C>T, and c.12067-2A>G) account for 64% of mutant alleles underlying USH2 in Jewish families of non-Ashkenazi descent. Considering the very large size of the USH2A gene and the high number of mutations detected in USH2 patients worldwide, our findings have significant implications for genetic counseling and carrier screening in various Jewish populations.

Abetalipoproteinemia in Israel: evidence for a founder mutation in the Ashkenazi Jewish population and a contiguous gene deletion in an Arab patient.

Abetalipoproteinemia (ABL) is a rare autosomal recessive metabolic disorder, characterized by the absence of plasma apolipoprotein B-containing lipoproteins and very low levels of plasma triglycerides and cholesterol. ABL is caused by mutations of the MTP gene. We investigated the genetic basis for ABL in a cohort of Israeli families. In Ashkenazi Jewish patients we identified a conserved haplotype and a common MTP mutation, p.G865X, with a carrier frequency of 1:131 in this population. We also report the first case of ABL and additional abnormalities in a Muslim Arab patient, due to a homozygous contiguous gene deletion of approximately 481 kb, including MTP and eight other genes.