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X-linked retinitis pigmentosa (XLRP) caused by mutations in the RPGR gene is one of the most severe forms of RP due to its early onset and intractable progression. Most cases have been associated with genetic variants within the purine-rich exon ORF15 region of this gene. RPGR retinal gene therapy is currently being investigated in several clinical trials. Therefore, it is crucial to report and functionally characterize (all novel) potentially pathogenic DNA sequence variants. Whole-exome sequencing (WES) was performed for the index patient. The splicing effects of a non-canonical splice variant were tested on cDNA from whole blood and a minigene assay. WES revealed a rare, non-canonical splice site variant predicted to disrupt the wildtype splice acceptor and create a novel acceptor site 8 nucleotides upstream of RPGR exon 12. Reverse-transcription PCR analyses confirmed the disruption of the correct splicing pattern, leading to the insertion of eight additional nucleotides in the variant transcript. Transcript analyses with minigene assays and cDNA from peripheral blood are useful tools for the characterization of splicing defects due to variants in the RPGR and may increase the diagnostic yield in RP. The functional analysis of non-canonical splice variants is required to classify those variants as pathogenic according to the ACMG's criteria.

Mutations in Norrin, the ligand of a receptor complex consisting of FZD4, LRP5 and TSPAN12, cause severe developmental blood vessel defects in the retina and progressive loss of the vascular system in the inner ear, which lead to congenital blindness and progressive hearing loss, respectively. We now examined molecular pathways involved in developmental retinal angiogenesis in a mouse model for Norrie disease. Comparison of morphometric parameters of the superficial retinal vascular plexus (SRVP), including the number of filopodia, vascular density and number of branch points together with inhibition of Notch signaling by using DAPT, suggest no direct link between Norrin and Notch signaling during formation of the SRVP. We noticed extensive vessel crossing within the SRVP, which might be a loss of Wnt- and MAP kinase-characteristic feature. In addition, endomucin was identified as a marker for central filopodia, which were aligned in a thorn-like fashion at P9 in Norrin knockout (Ndp(y/-)) mice. We also observed elevated mural cell coverage in the SRVP of Ndp(y/-) mice and explain it by an altered expression of PDGFβ and its receptor (PDGFRβ). In vivo cell proliferation assays revealed a reduced proliferation rate of isolectin B4-positive cells in the SRVP from Ndp(y/-) mice at postnatal day 6 and a decreased mitogenic activity of mutant compared with the wild-type Norrin. Our results suggest that the delayed outgrowth of the SRVP and decreased angiogenic sprouting in Ndp(y/-) mice are direct effects of the reduced proliferation of endothelial cells from the SRVP.

Purpose: To establish mouse models for RPGR-associated diseases by generating and characterizing an Rpgr mutation (in-frame deletion of exon 4) in two different genetic backgrounds (BL/6 and BALB/c)., Methods: Gene targeting in embryonic stem (ES) cells was performed to introduce a in-frame deletion of exon 4 in the Rpgr gene (Rpgr(DeltaEx4)). Subsequently, the mutation was introduced in two different inbred mouse strains by successive breeding. Mutant and wild-type mice of both strains were characterized by electroretinography (ERG) and histology at five time points (1, 3, 6, 9, and 12 months). RPGR transcript amounts were assessed by quantitative RT-PCR. A variety of photoreceptor proteins, including RPGR-ORF15, RPGRIP, PDE6delta/PrBPdelta, rhodopsin, and cone opsin, were localized on retinal sections by immunohistochemistry., Results: Mislocalization of rhodopsin and cone opsin was an early pathologic event in mutant mice of both lines. In contrast, RPGR-ORF15 as well as RPGRIP1 and PDE6delta/PrBPdelta showed similar localizations in mutant and wild-type animals. Functional and histologic studies revealed a mild rod-dominated phenotype in mutant male mice on the BL/6 background, whereas a cone-dominated phenotype was observed for the same mutation in the BALB/c background., Conclusions: Both Rpgr mutant mouse lines developed retinal disease with a striking effect of the genetic background. Cone-specific modifiers might influence the retinal phenotype in the BALB/c strain. The two lines provide models to study RPGR function in rods and cones, respectively.

Purpose: Mutations in the NDP gene impair angiogenesis in the eyes of patients diagnosed with a type of blindness belonging to the group of exudative vitreoretinopathies. This study was conducted to investigate the differential gene expression caused by the absence of Norrin (the NDP protein) in the developing mouse retina and to elucidate early pathogenic events., Methods: A comparative gene expression analysis was performed on postnatal day (p)7 retinas from a knockout mouse model for Norrie disease using gene microarrays. Subsequently, results were verified by quantitative real-time PCR analyses. Immunohistochemistry was performed for the vascular permeability marker plasmalemma vesicle associated protein (Plvap)., Results: Our study identified expression differences in Ndph(y/-) versus wild-type mice retinas at p7. Gene transcription of the neutral amino acid transporter Slc38a5, apolipoprotein D (ApoD), and angiotensin II receptor-like 1 (Agtrl1) was decreased in the knockout mouse, whereas transcript levels of adrenomedullin (Adm) and of the plasmalemma vesicle associated protein (Plvap) were increased in comparison to the wild-type. In addition, ectopic expression of Plvap was found in the developing retinal vasculature of Norrin-knockout mice on the protein level., Conclusions: These data provide molecular evidence for a role of Norrin in the development of the retinal vasculature. Expression of two genes, Plvap and Slc38a5, is considerably altered in retinal development of Norrin-knockout mice and may reflect or contribute to the pathogenesis of the disease. In particular, ectopic expression of Plvap is consistent with hallmark disease symptoms in mice and humans.

Male infertility is one possible consequence of a group of disorders arising from dysfunction of cilia. Ciliopathies include primary ciliary dyskinesia, polycystic kidney disease, Usher syndrome, nephronophthisis, Bardet-Biedl syndrome, Alstrom syndrome, and Meckel-Gruber syndrome as well as some forms of retinal degenerations. Mutations in the retinitis pigmentosa GTPase regulator gene (RPGR) are best known for leading to retinal degeneration but have also been associated with ciliary dysfunctions affecting other tissues. To further study the involvement of RPGR in ciliopathies, transgenic mouse lines overexpressing RPGR were generated. Animals carrying the transgene in varying copy numbers were investigated. We found that infertility due to aberrant spermatozoa correlated with increased copy numbers. In animals with moderately increased gene copies of Rpgr, structural disorganization in the flagellar midpiece, outer dense fibers, and fibrous sheath was apparent. In contrast, in animals with high copy numbers, condensed sperm heads were present, but the flagellum was absent in the vast majority of spermatozoa, although early steps of flagellar biogenesis were observed. This complexity of defects in flagellar assembly suggests a role of RPGR in intraflagellar transport processes.

X-linked Norrie disease, familial exudative vitreoretinopathy (FEVR), Coat's disease and retinopathy of prematurity are severe human eye diseases and can all be caused by mutations in the Norrie disease pseudoglioma gene. They all show vascular defects and characteristic features of retinal hypoxia. Only Norrie disease displays additional neurological symptoms, which are sensorineural hearing loss and mental retardation. In the present study, we analysed transcript levels of the ligand Norrin (Ndph) and its two receptors Frizzled-4 (Fzd4) and LDL-related protein receptor 5 (Lrp5) in six different brain regions (cerebellum, cortex, hippocampus, olfactory bulb, pituitary and brain stem) of 6- to 8-month-old wild-type and Ndph knockout mice by quantitative real-time PCR. No effect of the Ndph knockout allele on Fzd4 or Lrp5 receptor expression was found. Furthermore, no alterations of the transcript levels of three hypoxia-regulated angiogenic factors (Vegfa, Itgrb3 and Tie1) were observed in the absence of Norrin. Interestingly, we identified significant differences in Ndph, Fzd4 and Lrp5 transcript levels in brain regions of wild-type mice and observed highest expression of Norrin and frizzled-4 in cerebellum. Transcript analyses were correlated with morphological data obtained from cerebellum and immunohistochemical studies of blood vessels in different brain regions. Vessel density was reduced in the cerebellum of Ndph knockout mice but the number of Purkinje and granular cells was not altered. This provides the first description of a brain phenotype in Ndph knockout mice, which will help to elucidate the role of Norrin in the brain.

Purpose: To characterize developmental defects and the time course of Norrie disease in retinal and hyaloid vasculature during retinal development and to identify underlying molecular angiogenic pathways that may be affected in Norrie disease, exudative vitreoretinopathy, retinopathy of prematurity, and Coats' disease., Methods: Norrie disease pseudoglioma homologue (Ndph)-knockout mice were studied during retinal development at early postnatal (p) stages (p5, p10, p15, and p21). Histologic techniques, quantitative RT-PCR, ELISA, and Western blot analyses provided molecular data, and scanning laser ophthalmoscopy (SLO) angiography and electroretinography (ERG) were used to obtain in vivo data., Results: The data showed that regression of the hyaloid vasculature of Ndph-knockout mice occurred but was drastically delayed. The development of the superficial retinal vasculature was strongly delayed, whereas the deep retinal vasculature did not form because of the blockage of vessel outgrowth into the deep retinal layers. Subsequently, microaneurysm-like lesions formed. Several angiogenic factors were differentially transcribed during retinal development. Increased levels of hypoxia inducible factor-1alpha (HIF1alpha) and VEGFA, as well as a characteristic ERG pattern, confirmed hypoxic conditions in the inner retina of the Ndph-knockout mouse., Conclusions: These data provide evidence for a crucial role of Norrin in hyaloid vessel regression and in sprouting angiogenesis during retinal vascular development, especially in the development of the deep retinal capillary networks. They also suggest an early and a late phase of Norrie disease and may provide an explanation for similar phenotypic features of allelic retinal diseases in mice and patients as secondary consequences of pathologic hypoxia.

X-linked congenital stationary night blindness (XLCSNB) is characterized by impaired scotopic vision with associated ocular symptoms such as myopia, hyperopia, nystagmus and reduced visual acuity. Genetic mapping in families with XLCSNB revealed two different loci on the proximal short arm of the X chromosome. These two genetic subtypes can be distinguished on the basis of electroretinogram (ERG) responses and psychophysical testing as a complete (CSNB1) and an incomplete (CSNB2) form. The CSNB1 locus has been mapped to a 5-cM linkage interval in Xp11.4 (refs 2,5-7). Here we construct and analyse a contig between the markers DXS993 and DXS228, leading to the identification of a new gene mutated in CSNB1 patients. It is partially deleted in 3 families and mutation analysis in a further 21 families detected another 13 different mutations. This gene, designated NYX, encodes a protein of 481 amino acids (nyctalopin) and is expressed at low levels in tissues including retina, brain, testis and muscle. The predicted polypeptide is a glycosylphosphatidylinositol (GPI)-anchored extracellular protein with 11 typical and 2 cysteine-rich, leucine-rich repeats (LRRs). This motif is important for protein-protein interactions and members of the LRR superfamily are involved in cell adhesion and axon guidance. Future functional analysis of nyctalopin might therefore give insight into the fine-regulation of cell-cell contacts in the retina.

Purpose: To identify possible correlations between the putative mutations and the clinical characteristics in X-linked retinitis pigmentosa, RP2., Design: A retrospective, descriptive clinical study., Material: The ophthalmological files on affected persons from three Danish families with identified pathogenic mutations in the RP2 gene., Results: Mutation analysis in 14 Danish families with X-linked retinitis pigmentosa revealed disease-associated sequence alterations in eight of them. Five mutations were detected in the RP3 gene (RPGR) and three in the RP2 gene. Genotype-phenotype comparison in the three RP2 families revealed striking interfamilial phenotypic differences. Severe phenotypes were associated with a null mutation Gln26stop and a missense mutation Arg118His. These families differed mutually with respect to retinal appearance. Affected carriers had a delayed onset by three decades. Tapetal reflexes were not observed in the carriers. An in-frame deletion DeltaSer6 was associated with a milder phenotype., Conclusions: Interfamilial differences in RP2 phenotype might be related to the type and location of the mutational event. Due to a considerable overlap between RP2 and RP3 phenotypes, the genotype cannot safely be deduced from conventional clinical examination methods.

X-linked retinitis pigmentosa (XLRP) is a genetically heterogeneous group of progressive retinal degenerations. The disease process is initiated by premature apoptosis of rod photoreceptor cells in the retina, which leads to reduced visual acuity and, eventually, complete blindness. Mutations in the retinitis pigmentosa GTPase regulator ( RPGR ), a ubiquitously expressed gene at the RP3 locus in Xp21.1, account for approximately 20% of all X-linked cases. We have analysed the expression of this gene by northern blot hybridization, cDNA library screening and RT-PCR in various organs from mouse and man. These studies revealed at least 12 alternatively spliced isoforms. Some of the transcripts are tissue specific and contain novel exons, which elongate or truncate the previously reported open reading frame of the mouse and human RPGR gene. One of the newly identified exons is expressed exclusively in the human retina and mouse eye and contains a premature stop codon. The deduced polypeptide lacks 169 amino acids from the C-terminus of the ubiquitously expressed variant, including an isoprenylation site. Moreover, this exon was found to be deleted in a family with XLRP. Our results indicate tissue-dependent regulation of alternative splicing of RPGR in mouse and man. The discovery of a retina-specific transcript may explain why phenotypic abberations in RP3 are confined to the eye.