The nuclear receptor NR2E3 plays a role in human retinal photoreceptor differentiation and degeneration

Enhanced S-cone syndrome in a Japanese family with a nonsense NR2E3 mutation (Q350X)

PURPOSE

To describe the clinical characteristics of a Japanese male patient with enhanced S-cone syndrome (ESCS) and investigate the existence of mutations in the NR2E3 gene, which encodes a photoreceptor cell-specific nuclear receptor.

METHODS

Fundus examinations, fluorescein angiography, colour vision tests, spectral sensitivity, and full-field and spectral electroretinography were performed. Mutation screening of the NR2E3 gene was performed with polymerase chain reaction (PCR) amplification and direct sequencing.

RESULTS

We identified a novel homozygous mutation (c.1048C > T), that converts glutamine (CAA) to a termination codon (TAA) at amino acid position 350. The subject's unaffected parents were heterozygous for the mutation, consistent with autosomal recessive transmission. The electroretinographic findings revealed that the patient had neither rod nor 30-Hz flicker responses but did have cone responses with large a-wave and low b-wave amplitudes, similar to the rod-plus-cone responses, and also substantial short wavelength-sensitive (S) cone and extremely diminished long/middle wavelength-sensitive (L/M) cone responses. In the right eye, spectral sensitivity in the fovea revealed both functional S-cone and remarkably reduced L- and M-cone sensitivities, which was compatible with the decreased visual acuity (VA) and red/green colour vision defects noted in this eye. In contrast, the patient had good VA and normal red/green colour vision in the left eye.

CONCLUSION

The nonsense mutation results in a truncated NR2E3 protein lacking 61 amino acids within the ligand-binding domain (LBD) that consists of 190 amino acids of the C-terminus end. Therefore, null function of the LBD is likely to cause ESCS in the patient. The clinical findings for this patient suggest that his left eye, with its functional L/M- and S-cones, was at an earlier stage of the syndrome than his right eye.

Mouse genetic approaches to access pathways important in retinal function

The laboratory mouse is a premier research tool for understanding human biology and disease. With the availability of the first complete draft sequences of the mouse genome and ongoing large scale screens for new mouse mutations to serve as novel models, the usefulness of the mouse will only increase. The vision research field, in particular, which has identified many human diseases for which no models are yet available, will benefit from the newly developed tools in mouse molecular genetics.

Chromatin immunoprecipitation assay on the rainbow trout opsin proximal promoters illustrates binding of NF-kappaB and c-jun to the SWS1 promoter in the retina

Misexpression of opsins has been linked to apoptosis of photoreceptor cells in the vertebrate retina. Salmonid fish lose their ultraviolet-sensitive (UVS) cones through post-natal developmental apoptosis mediated by thyroid hormone (TH). In order to identify genetic mechanisms that may play a role in the loss of UVS cones, the transcriptional regulation of the SWS1 opsin in the rainbow trout (Oncorhynchus mykiss) was investigated. The Transfac database was interrogated with promoter sequence acquired by genome-walking PCR using MatInspector V2.2 to identify putative transcription factor (TF) binding sites. Putative binding sites for AP-1 (c-jun) and NF-kappaB were found in the SWS1 opsin promoter and were chosen for further investigation due to their high MatInspector scores, their established role in photoreceptor apoptosis, and their relative exclusion from other opsin promoters. NF-kappaB and c-jun proteins were visualized in rainbow trout retinal tissue with immunohistochemistry and c-jun was identified in rainbow trout retinal protein homogenate by immunoblot. A chromatin immunoprecipitation-polymerase chain reaction technique was employed to examine the in vivo interaction of c-jun and NF-kappaB proteins with their proposed binding sites in the opsin promoters. This analysis demonstrated that NF-kappaB and c-jun bind to the SWS1 opsin promoter, but not to the other rod and cone opsin promoters tested. Given the role of NF-kappaB and c-jun during photoreceptor apoptosis, the influence of their activity through TH and their selective binding to the SWS1 opsin promoter in rainbow trout, these TFs represent good candidates of mechanisms underlying UVS cone degeneration in salmonids.

The role of opsin expression and apoptosis in determination of cone types in human retina

In primates, short wavelength sensitive cones (S cones) and medium- or long-wavelength-sensitive cones (L/M cones) are two separate populations. Each cone type has a different developmental timecourse, contributes to different intra-retinal circuits, and transmits different types of information to the brain. However, in fetal human retina a significant population of cones express both S and L/M opsin (S+L/M cones), raising questions about whether S+L/M cones die or change opsin expression during development. We have utilized fetal, postnatal and adult human retinae to study the immunohistochemical distribution and morphology of S+L/M cones during development. Because S cones appear to be at higher density in fetal compared to adult retinae, we used antibodies to S opsin and alpha-transducin to estimate the proportion of S-cones, and TUNEL labelling to detect apoptotic death in the L/M, S or S+L/M population during development. S cones were present in central retina from fetal week (Fwk)11 and covered the retina by Fwk20. L/M cones appeared in the foveal cone mosaic 3-4 weeks after S-opsin was first detected, and covered the retina by birth. S+L/M cones were detected in all retinae older than Fwk14. They were most numerous at the retinal eccentricity where L/M opsin was just appearing; i.e. at the 'front' of L/M opsin expression. In this region, five morphological types of cones were present. (1) Heavily labelled S cones had thick cell bodies, a thick basal axon and pedicle, and a nucleus at any level of the outer nuclear layer (ONL). (2) Heavily labelled L/M cones were wine goblet shaped with a small round cell body, a large nucleus at the outer ONL edge, and a thin axon with a prominent synaptic pedicle. (3) Goblet-shaped S+L/M cones. (4) Goblet-shaped cones lightly labelled for S-opsin. (5) Cones that were not immunoreactive to either opsin. Only type 1 S cones were present peripheral to the L/M expression front, and their labelling intensity, morphology and distribution indicates that these are the 'true blue' cones of the adult mosaic. Only type 2 L/M cones were present in the foveal cone mosaic. Types 3 and 4 were most numerous within 500-750 microm of the L/M expression front, but type 3 S+L/M cones were also scattered throughout more central regions in fetal, infant and adult retinae. S+L/M cones comprised 5-10% of opsin immunoreactive cones at the L/M front in fetal and early postnatal retinas but 0.01-0.03% throughout P8mo and adult retinae. We found no evidence of significant levels of apoptosis in L/M cones at the expression front, suggesting that this decrease was not due to cell death. The findings suggest that goblet-shaped cones destined to express L or M opsin may initially and transiently express S opsin. Near the optic disc, at Fwk17 S cone density was around 2000 cells mm(-2), which dropped 50% by Fwk20 and stabilized at around 500 cells mm(-2) by birth. Double labelling with alpha-transducin showed that throughout this period 8-10% of all cones expressed S opsin. TUNEL labelling found no significant apoptosis in the S cone population. The decrease in S cone density near the optic disc occurs in the absence of apoptosis, and is likely due to other developmental events acting on the photoreceptor layer, including displacement of cones towards the fovea.

X-linked retinoschisis: A clinical and molecular genetic review

X-linked retinoschisis is a leading cause of macular degeneration in male children. It is characterized by a high degree of clinical variability. Clinical features include a stellate foveal retinoschisis, with or without peripheral retinoschisis. The schisis occurs within the inner retina, primarily at the level of the nerve fiber layer. The disease-causing gene, X-linked retinoschisis 1, has recently been identified, and is expressed in photoreceptor and bipolar cells. This gene codes for retinoschisin, a secreted protein containing a discoidin domain which may be involved in cellular adhesion or cell-cell interactions. The identification of this gene allows for improved diagnosis and contributes to the understanding of this condition. Visual prognosis is variable, as X-linked retinoschisis exhibits a high degree of phenotypic variability. Although there is no treatment to halt the progressive maculopathy, clinical management is directed toward treatment of amblyopia and surgical correction of certain complications. X-linked retinoschisis is an important condition to study, both to improve the clinical management of this disorder, and to better understand retinal function and development. Herein, we review the clinical, histopathologic, and molecular genetic and treatment options of X-linked retinoschisis.

PROGRESSTOWARDUNDERSTANDING THEGENETIC ANDBIOCHEMICALMECHANISMS OFINHERITEDPHOTORECEPTORDEGENERATIONS

More than 80 genes associated with human photoreceptor degenerations have been identified. Attention must now turn toward defining the mechanisms that lead to photoreceptor death, which occurs years to decades after the birth of the cells. Consequently, this review focuses on topics that offer insights into such mechanisms, including the one-hit or constant risk model of photoreceptor death; topological patterns of photoreceptor degeneration; mutations in ubiquitously expressed splicing factor genes associated only with photoreceptor degeneration; disorders of the retinal pigment epithelium; modifier genes; and global gene expression analysis of the retina, which will greatly increase our understanding of the downstream events that occur in response to a mutation.

Molecular aspects of vertebrate retinal development

The formation of retina from neural plate has been mapped extensively by anatomical and molecular methods. The major cascades of transcription factor expression have been identified, and deficits resulting from transcription factor knockouts are well characterized. There is extensive cross-regulation, both positive and negative, at the transcriptional level between transcription factors and this is vital in the formation of neural compartments. Many transcription factors are important at both early stages of optic cup formation and later stages of terminal differentiation of retinal cell types. The transcription factor cascades can be regulated by extrinsic factors, and some of the intracellular signaling pathways whereby this is achieved have been identified. Defining the quantitative interactions between regulatory molecules will be the next step in understanding this excellent model of vertebrate central nervous system (CNS) development.