Genetic localisation of the RP2 type of X linked retinitis pigmentosa in a large kindred

Amyloid Precursor-Like Protein 2 deletion-induced retinal synaptopathy related to congenital stationary night blindness: structural, functional and molecular characteristics

Background

Amyloid precursor protein knockout mice (APP-KO) have impaired differentiation of amacrine and horizontal cells. APP is part of a gene family and its paralogue amyloid precursor-like protein 2 (APLP2) has both shared as well as distinct expression patterns to APP, including in the retina. Given the impact of APP in the retina we investigated how APLP2 expression affected the retina using APLP2 knockout mice (APLP2-KO).

Results

Using histology, morphometric analysis with noninvasive imaging technique and electron microscopy, we showed that APLP2-KO retina displayed abnormal formation of the outer synaptic layer, accompanied with greatly impaired photoreceptor ribbon synapses in adults. Moreover, APLP2-KO displayed a significant decease in ON-bipolar, rod bipolar and type 2 OFF-cone bipolar cells (36, 21 and 63 %, respectively). Reduction of the number of bipolar cells was accompanied with disrupted dendrites, reduced expression of metabotropic glutamate receptor 6 at the dendritic tips and alteration of axon terminals in the OFF laminae of the inner plexiform layer. In contrast, the APP-KO photoreceptor ribbon synapses and bipolar cells were intact. The APLP2-KO retina displayed numerous phenotypic similarities with the congenital stationary night blindness, a non-progressive retinal degeneration disease characterized by the loss of night vision. The pathological phenotypes in the APLP2-KO mouse correlated to altered transcription of genes involved in pre- and postsynatic structure/function, including CACNA1F, GRM6, TRMP1 and Gα0, and a normal scotopic a-wave electroretinogram amplitude, markedly reduced scotopic electroretinogram b-wave and modestly reduced photopic cone response. This confirmed the impaired function of the photoreceptor ribbon synapses and retinal bipolar cells, as is also observed in congenital stationary night blindness. Since congenital stationary night blindness present at birth, we extended our analysis to retinal differentiation and showed impaired differentiation of different bipolar cell subtypes and an altered temporal sequence of development from OFF to ON laminae in the inner plexiform layer. This was associated with the altered expression patterns of bipolar cell generation and differentiation factors, including MATH3, CHX10, VSX1 and OTX2.

Conclusions

These findings demonstrate that APLP2 couples retina development and synaptic genes and present the first evidence that APLP2 expression may be linked to synaptic disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0245-z) contains supplementary material, which is available to authorized users.

RPGR-containing protein complexes in syndromic and non-syndromic retinal degeneration due to ciliary dysfunction

Dysfunction of primary cilia due to mutations in cilia-centrosomal proteins is associated with pleiotropic disorders. The primary (or sensory) cilium of photoreceptors mediates polarized trafficking of proteins for efficient phototransduction. Retinitis pigmentosa GTPase regulator (RPGR) is a cilia-centrosomal protein mutated in >70% of X-linked RP cases and 10%-20% of simplex RP males. Accumulating evidence indicates that RPGR may facilitate the orchestration of multiple ciliary protein complexes. Disruption of these complexes due to mutations in component proteins is an underlying cause of associated photoreceptor degeneration. Here, we highlight the recent developments in understanding the mechanism of cilia-dependent photoreceptor degeneration due to mutations in RPGR and PGR-interacting proteins in severe genetic diseases, including retinitis pigmentosa, Leber congenital amaurosis (LCA), Joubert syndrome, and Senior-Loken syndrome, and explore the physiological relevance of photoreceptor ciliary protein complexes.

Multiprotein complexes of Retinitis Pigmentosa GTPase regulator (RPGR), a ciliary protein mutated in X-linked Retinitis Pigmentosa (XLRP)

Mutations in Retinitis Pigmentosa GTPase Regulator (RPGR) are a frequent cause of X-linked Retinitis Pigmentosa (XLRP). The RPGR gene undergoes extensive alternative splicing and encodes for distinct protein isoforms in the retina. Extensive studies using isoform-specific antibodies and mouse mutants have revealed that RPGR predominantly localizes to the transition zone to primary cilia and associates with selected ciliary and microtubule-associated assemblies in photoreceptors. In this chapter, we have summarized recent advances on understanding the role of RPGR in photoreceptor protein trafficking. We also provide new evidence that suggests the existence of discrete RPGR multiprotein complexes in photoreceptors. Piecing together the RPGR-interactome in different subcellular compartments should provide critical insights into the role of alternative RPGR isoforms in associated orphan and syndromic retinal degenerative diseases.

Retinitis Pigmentosa GTPase Regulator (RPGR) protein isoforms in mammalian retina: insights into X-linked Retinitis Pigmentosa and associated ciliopathies

Mutations in the cilia-centrosomal protein Retinitis Pigmentosa GTPase Regulator (RPGR) are a frequent cause of retinal degeneration. The RPGR gene undergoes complex alternative splicing and encodes multiple protein isoforms. To elucidate the function of major RPGR isoforms (RPGR 1-19 and RPGR ORF15), we have generated isoform-specific antibodies and examined their expression and localization in the retina. Using sucrose-gradient centrifugation, immunofluorescence and co-immunoprecipitation methods, we show that RPGR isoforms localize to distinct sub-cellular compartments in mammalian photoreceptors and associate with a number of cilia-centrosomal proteins. The RCC1-like domain of RPGR, which is present in all major RPGR isoforms, is sufficient to target it to the cilia and centrosomes in cultured cells. Our findings indicate that multiple isotypes of RPGR may perform overlapping yet somewhat distinct transport-related functions in photoreceptors.

Summary of ocular genetic disorders and inherited systemic conditions with eye findings

Of the close to 10,000 known inherited disorders that affect humankind, a disproportionately high number affect the eye. The total number of genes responsible for the normal structure, function, and differentiation of the eye is unknown, but the list of these genes is rapidly and constantly growing. The objective of this paper is to provide a current list of mapped and/or cloned human eye genes that are responsible for inherited diseases of the eye. The ophthalmologist should be aware of recent advances in molecular technology which have resulted in significant progress towards the identification of these genes. The implications of this new knowledge will be discussed herein.

Phenotypes in three Swedish families with X-linked retinitis pigmentosa caused by different mutations in the RPGR gene

PURPOSE

To assess the clinical phenotypes in three Swedish families with X-linked retinitis pigmentosa caused by different mutations in the RPGR gene.

METHODS

Three families from different parts of Sweden, including nine patients with retinitis pigmentosa and six female carriers of X-linked retinitis pigmentosa, were examined clinically. Ophthalmologic examination included kinetic perimetry with a Goldmann perimeter using standardized objects I4e and V4e, dark adaptation final thresholds with a Goldmann-Weeker adaptometer, and full-field electroretinograms.

RESULTS

The clinical findings in the patients demonstrated a severe form of retinitis pigmentosa with visual handicap early in life. Patients with a microdeletion of exons 8 through 10 of the RPGR gene had a more severe phenotype compared to the patients with single base-pair mutations in the introns 10 and 13 of the RPGR gene, resulting in splicing defects. Furthermore, heterozygous carriers in these families displayed a wide spectrum of clinical features, from minor symptoms to severe visual disability.

CONCLUSION

These three families show a variable clinical phenotype resulting from different mutations in the RPGR gene. A microdeletion spanning at least parts of exons 8 through 10 seems to result in a severe phenotype compared to the splice defects. Heterozygous carriers of X-linked retinitis pigmentosa with these specific RPGR genotypes also show a variability of the phenotype; carriers with the microdeletion may be severely visually handicapped.

Mapping the RP2 locus for X-linked retinitis pigmentosa on proximal Xp: a genetically defined 5-cM critical region and exclusion of candidate genes by physical mapping

Genetic linkage studies have implicated at least two loci for X-linked retinitis pigmentosa (XLRP) on proximal Xp. We now report a defined genetic localization for the RP2 locus to a 5-cM interval in Xp11.3-11.23. Haplotype analysis of polymorphic markers in recombinant individuals from two XLRP families has enabled us to identify DXS8083 and DXS6616 as the new distal and proximal flanking markers for RP2. Using STS-content and YAC end-clone mapping, an approximately 1.2 Mb YAC contig has been established encompassing the proximal RP2 boundary and extending from T1MP1 to DXS1240 in Xp11.23. Several ESTs have been positioned and ordered on this contig, one of which is novel to the region, identified by sequence data-base match to a physically mapped YAC insert terminal STS. Integration of the genetic and physical data has placed four retinally expressed genes proximal to DXS6616, and thereby excluded them from a causitive role in RP2. This work now provides a much needed focus for positional cloning approaches to isolation of the defective gene.

Genetic analysis of a kindred with X-linked mental handicap and retinitis pigmentosa

A kindred is described in which X-linked nonspecific mental handicap segregates together with retinitis pigmentosa. Carrier females are mentally normal but may show signs of the X-linked retinitis pigmentosa carrier state and become symptomatic in their later years. Analysis of polymorphic DNA markers at nine loci on the short arm of the X chromosome shows that no crossing-over occurs between the disease and Xp11 markers DXS255, TIMP, DXS426, MAOA, and DXS228. The 90% confidence limits show that the locus is in the Xp21-q21 region. Haplotype analysis is consistent with the causal gene being located proximal to the Xp21 loci DXS538 and 5'-dystrophin on the short arm of the X chromosome. The posterior probability of linkage to the RP2 region of the X chromosome short arm (Xp11.4-p11.23) is .727, suggesting the possibility of a contiguous-gene-deletion syndrome. No cytogenetic abnormality has been identified.

Retinitis pigmentosa families showing apparent X linked inheritance but unlinked to the RP2 or RP3 loci

Three families with retinitis pigmentosa (RP) are described in which the disorder shows apparent X linked inheritance but does not show linkage to the RP2 and RP3 regions of the short arm of the X chromosome. The families are also inconsistent with a localisation of the disease gene between DXS164 and DXS28. In one case, reassessment of the family in the light of these results suggested that the family may have an autosomal dominant form of RP. The remaining two families are consistent with X linkage and suggest the possibility of a new X linked RP (XLRP) locus. These families highlight the difficulties in determining the mode of inheritance on the basis of pedigree structure and clinical data alone. Molecular genetics plays an important role in confirming the mode of inheritance and in detecting potential misclassifications, particularly in a group of disorders as heterogeneous as RP. They emphasise that caution is required in genetic counselling of RP families, particularly in the absence of any molecular genetic analysis.

Heterogeneity analysis in 40 X-linked retinitis pigmentosa families

Analysis of genetic heterogeneity in 40 kindreds with X-linked retinitis pigmentosa (XLRP), with 20 polymorphic markers, showed that significant heterogeneity is present (P = .001) and that 56% of kindreds are of RP3 type and that 26% are of RP2 type. The location of the RP3 locus was found to be 0.4 cM distal to OTC in the Xp21.1 region, and that of the RP2 locus was 6.5 cM proximal to DXS7 in Xp11.2-p11.3. Bayesian probabilities of linkage to RP2, RP3, or to neither locus were calculated. This showed that 20 of 40 kindreds could be assigned to one or the other locus, with a probability > .70 (14 kindreds with RP3 and 6 kindreds with RP2 disease). A further three kindreds were found to be unlinked to either locus, with a probability > .8. The remaining 17 kindreds could not be classified unambiguously. This highlights the difficulty of classifying families in the presence of genetic heterogeneity, where the two loci are separated by an estimated 16 cM.

Prenatal diagnosis of X-linked retinitis pigmentosa (RP) in five pregnancies at risk

Five pregnancies at risk for X-linked retinitis pigmentosa (RP) have been monitored by first-trimester prenatal diagnosis using DNA markers flanking the RP2 and RP3 loci. Three affected and two unaffected fetuses, including a female carrying a wild-type genotype, were predicted on the basis of marker segregation and estimation of the recombination fraction.

Retinitis pigmentosa: problems associated with genetic classification

Genetic classification of retinitis pigmentosa (RP) can be problematic, due to a large number of isolated cases, reduced penetrance, and considerable variation in expressivity. Another confounder is a high proportion of affected female carriers in X-linked RP. Based on the genetic definitions of five different authors, a reclassification experiment was conducted with 350 Danish families. Agreement existed about a small "nucleus" of familial cases. Most definitions favored autosomal dominant inheritance at the expense of X-linked. The experiment revealed that methodological differences to a large extent might explain the considerable variation among reported genetic frequencies of retinitis pigmentosa.

Attitudes towards prenatal diagnosis and selective abortion among patients with retinitis pigmentosa or choroideremia as well as among their relatives

Genetic counselling endeavours to be nondirective. However, the availability of prenatal diagnosis may direct clients towards accepting and using these methods. It is time to investigate the attitudes of clients in order to monitor the psychological and social effects of new genetic techniques. As prenatal diagnosis was possible for choroideremia (C), but not for retinitis pigmentosa (RP) in 1988-89, we used a questionnaire to compare the attitudes of C and RP patients, their relatives and C carriers to prenatal diagnosis. The response rate was low (35%) and no significant differences between RP and C groups came to light. However, C carriers accepted prenatal diagnosis and also selective abortion more easily, but, on the other hand, they showed more uncertainty than did the other groups. This indicates that the availability of prenatal diagnosis may confuse those concerned. In general, about 60% of all the respondents had a positive attitude to the prenatal diagnosis of RP or choroideremia, though only about 30% would use if for abortion. Over 80% of all the respondents wanted to know the opinion of the genetic counsellor.

Phenotype-genotype correlations in X linked retinitis pigmentosa

Retinitis pigmentosa (RP) represents a group of clinically heterogeneous retinal degenerations in which all modes of inheritance have been described. We have previously found two different clinical profiles in X linked RP as a function of age and mode of onset. The first clinical form has very early onset with severe myopia. The second form starts later with night blindness with mild myopia or none. At least two genes have been identified in X linked forms, namely RP2 (linked to DXS7, DXS255, and DXS14) and RP3 (linked to DXS84 and OTC) on the short arm of the X chromosome. In order to contribute to phenotype-genotype correlations in X linked RP, we tested the hypothesis that the two clinical profiles could be accounted for by the two different gene loci. The present study provides evidence for linkage of the clinical form with early myopia as the onset symptom with the RP2 gene (pairwise linkage to DXS255: Z = 3.13 at theta = 0), while the clinical form with later night blindness as the onset symptom is linked to the RP3 gene (pairwise linkage to OTC: Z = 4.16 at theta = 0).

A family with RP3 type of X-linked retinitis pigmentosa: an association with ciliary abnormalities

The results of linkage analysis in a family with X-linked retinitis pigmentosa (XLRP) are presented. Probe M27B (DXS255), localized to Xp11.22, was only loosely linked to XLRP, whereas pHOC3 (OTC), in the more distal Xp21.1 region, was tightly linked. In this family, the conditional probability of an RP3 locus (in Xp21.1-p11.4) was found to be 0.978 compared with 0.021 for an RP2 locus (in Xp11.4-p11.2). Risk assessment showed that 2 out of 4 "at risk" females showing no clinical abnormality have a high probability of being genetic carriers of XLRP. Some affected males have recurrent respiratory infections as a result of a condition indistinguishable from the immotile cilia syndrome; indeed, there is an association between XLRP and susceptibility to respiratory infections in the majority of affected males. The possibility that previously observed ciliary abnormalities in XLRP patients might be associated specifically with an RP3 locus abnormality is discussed.

Linkage analysis of Norrie disease with X-chromosomal ornithine aminotransferase

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