Identification of two novel mutations in CDHR1 in consanguineous Spanish families with autosomal recessive retinal dystrophy

Rescue of cone and rod photoreceptor function in a CDHR1-model of age-related retinal degeneration

Age-related macular degeneration (AMD) is the most common cause of untreatable blindness in the developed world. Recently, CDHR1 has been identified as the cause of a subset of AMD that has the appearance of the "dry" form, or geographic atrophy. Biallelic variants in CDHR1-a specialized protocadherin highly expressed in cone and rod photoreceptors-result in blindness from shortened photoreceptor outer segments and progressive photoreceptor cell death. Here we demonstrate long-term morphological, ultrastructural, functional, and behavioral rescue following CDHR1 gene therapy in a relevant murine model, sustained to 23-months after injection. This represents the first demonstration of rescue of a monogenic cadherinopathy in vivo. Moreover, the durability of CDHR1 gene therapy seems to be near complete-with morphological findings of the rescued retina not obviously different from wildtype throughout the lifespan of the mouse model. A follow-on clinical trial in patients with CDHR1-associated retinal degeneration is warranted. Hypomorphic CDHR1 variants may mimic advanced dry AMD. Accurate clinical classification is now critical, as their pathogenesis and treatment are distinct.

Clinical Phenotypes of CDHR1-Associated Retinal Dystrophies

The retinal dystrophy phenotype associated with CDHR1 retinopathy is clinically heterogenous. In this study, we describe the clinical and molecular findings of a retinal dystrophy cohort (10 patients) attributed to autosomal recessive CDHR1 and report novel variants in populations not previously identified with CDHR1-related retinopathy. Seven patients had evaluations covering at least a three-year period. The mean age of individuals at first symptoms was 36 ± 8.5 years (range 5–45 years). Visual acuity at the last visit ranged from 20/20 to 20/2000 (mean LogMAR 0.8 or 20/125). Three clinical subgroups were identified: rod–cone dystrophy (RCD), cone–rod dystrophy (CRD), and maculopathy. Extinguished scotopic electroretinography responses were noted in the RCD patients. Macular involvement was noted in all patients and documented on color fundus photography, fundus autofluorescence, and optical coherence tomography. Notable asymmetry of the degree of macular atrophy was present in two patients. The possible association between CDHR1 variants and clinical findings was predicted using molecular modeling.

Novel mutations in PDE6A and CDHR1 cause retinitis pigmentosa in Pakistani families

AIM

To investigate the genetic basis of autosomal recessive retinitis pigmentosa (arRP) in two consanguineous/ endogamous Pakistani families.

METHODS

Whole exome sequencing (WES) was performed on genomic DNA samples of patients with arRP to identify disease causing mutations. Sanger sequencing was performed to confirm familial segregation of identified mutations, and potential pathogenicity was determined by predictions of the mutations' functions.

RESULTS

A novel homozygous frameshift mutation [NM_000440.2:c.1054delG, p. (Gln352Argfs*4); Chr5:g.149286886del (GRCh37)] in the PDE6A gene in an endogamous family and a novel homozygous splice site mutation [NM_033100.3:c.1168-1G>A, Chr10:g.85968484G>A (GRCh37)] in the CDHR1 gene in a consanguineous family were identified. The PDE6A variant p. (Gln352Argfs*4) was predicted to be deleterious or pathogenic, whilst the CDHR1 variant c.1168-1G>A was predicted to result in potential alteration of splicing.

CONCLUSION

This study expands the spectrum of genetic variants for arRP in Pakistani families.

A rare case of RGR/CDHR1 haplotype identified in Bulgarian patient with cone-rod dystrophy

AIM

To present a rare clinical case of CDHR1-related retinopathy with cone and rod involvementconfirmed clinically, electrophysiologically and genetically as a cone-rod dystrophy.

MATERIAL AND METHODS

A 26-year-old woman underwent detailed ophthalmic examinationincluding fundus photography, full-field and multifocal electroretinography, visual field testing, optical coherence tomography and fluorescein angiography, which established the clinical diagnosis. Next-generation sequencing of a custom panel including 140 of the most common genes for inherited retinal degenerations was used for mutation screening.

RESULTS

The symptoms onset was two years ago included gradual loss of vision and photophobia. The clinical findings were reduced visual acuity, central and peripheral scotomas, sporadic pigmentary cells localized mainly in the peripheral retina, a thinner retina in the macula and peripherally, moderate retinal vessels attenuation and reduced cone and rod ERG responses. The genetic analysisfound that the patient was homozygous for two already reported mutations: RGR-c.196A>C (p.Ser66Arg) variant and a co-segregating frame-shift deletion in CDHR1-c.2522_2528delTCTCTGA (p.Ile841Serfs119*). Segregation analysis showed that the two mutations were transmitted by the asymptomatic heterozygous parents.

CONCLUSION

The rare haplotype of RGR mutation co-segregating incis- with CDHR1 mutation in our patient has been previously described in Albanian patients with recessive retinal dystrophy. Our findings add further support to the hypothesis of a common ancestral haplotype spread in the Balkan population. The comprehensive clinical, electrophysiological and genetic testing of patients with rare hereditary retinal dystrophies is essential for the correct diagnosis and the choice of potential novel therapies.

Deep phenotyping of the Cdhr1-/- mouse validates its use in pre-clinical studies for human CDHR1-associated retinal degeneration

PURPOSE

To validate the Cdhr1^-/- mouse as a model for human CDHR1-associated retinal degeneration, which may present as cone-rod dystrophy or geographic atrophy.

METHODS

Deep phenotyping of Cdhr1^-/-(n = 56) and C57BL6J wildtype control mice (n = 45) was undertaken using in vivo multimodal retinal imaging and dark- and light-adapted electroretinography (ERG) over 15 months to evaluate rod- and cone-photoreceptor responses and retinal morphology.

RESULTS

Cdhr1^-/- retinas exhibited outer retinal thinning on optical coherence tomography (OCT) at 1-month versus C57BL6J (mean 14.6% reduction; P < 0.0001), with progressive degeneration to 15 months. The OCT layer representing photoreceptor outer segments was more significantly shortened in Cdhr1^-/- eyes at 1 month (mean 33.7% reduction; P < 0.0001), remained stable to 3 months and was not identifiable at later timepoints. Outer retinal thinning was more pronounced at inferior versus superior retinal locations in Cdhr1^-/- eyes (P < 0.002 at 3-9 months). Dark-adapted ERG identified severe functional deficits in Cdhr1^-/- mice at 1 month of age versus C57BL6J (mean 62% reduction) that continued to decline to 15 months (P < 0.0001). Light-adapted flicker identified severe deficits in cone function at 1 month (mean 70% reduction), with improved function to 3 months followed by progressive decline (P < 0.0001).

CONCLUSIONS

The Cdhr1^-/- mouse exhibits structural and functional evidence of progressive outer retinal degeneration at a slow rate. Early functional deficits affecting both rod and cone photoreceptors in the context of relatively mild structural changes reflect the human phenotype. This study validates the use of the Cdhr1^-/- mouse for the pre-clinical evaluation of therapeutics for human CDHR1-associated retinal degeneration.

Aqueous Humor Proteomic Alterations Associated with Visual Field Index Parameters in Glaucoma Patients: A Pilot Study

PURPOSE

The purpose of this study was to discover the aqueous humor proteomic changes associated with visual field indices in glaucoma patients.

METHODS

Aqueous humor samples were analyzed using Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS). The visual fields were analyzed with the Humphrey Visual Field analyzer. Statistical analyses were performed to discover the relationship between the aqueous humor proteins and visual field parameters including Pattern Standard Deviation (PSD), Visual Field Index (VFI), Mean Deviation (MD) and Glaucoma Hemifield Test (GHT).

RESULTS

In total, 222 proteins were identified in 49 aqueous humor samples. A total of 11, 9, 7, and 6 proteins were significantly correlated with PSD, VFI, MD, and GHT respectively. These proteins include apolipoprotein D, members of complement pathway (C1S, C4A, C4B, C8B, and CD14), and immunoglobulin family (IKHV3-9, IGKV2-28).

CONCLUSION

Several proteins involved in immune responses (immunoglobulins and complement factors) and neurodegeneration (apolipoprotein D) were identified to be associated with abnormal visual field parameters. These findings provide targets for future studies investigating precise molecular mechanisms and new therapies for glaucomatous optic neuropathy.

Retinoids in the visual cycle: role of the retinal G protein-coupled receptor

Driven by the energy of a photon, the visual pigments in rod and cone photoreceptor cells isomerize 11-cis-retinal to the all-trans configuration. This photochemical reaction initiates the signal transduction pathway that eventually leads to the transmission of a visual signal to the brain and leaves the opsins insensitive to further light stimulation. For the eye to restore light sensitivity, opsins require recharging with 11-cis-retinal. This trans-cis back conversion is achieved through a series of enzymatic reactions composing the retinoid (visual) cycle. Although it is evident that the classical retinoid cycle is critical for vision, the existence of an adjunct pathway for 11-cis-retinal regeneration has been debated for many years. Retinal pigment epithelium (RPE)-retinal G protein-coupled receptor (RGR) has been identified previously as a mammalian retinaldehyde photoisomerase homologous to retinochrome found in invertebrates. Using pharmacological, genetic, and biochemical approaches, researchers have now established the physiological relevance of the RGR in 11-cis-retinal regeneration. The photoisomerase activity of RGR in the RPE and Müller glia explains how the eye can remain responsive in daylight. In this review, we will focus on retinoid metabolism in the eye and visual chromophore regeneration mediated by RGR.

Proteasome-Mediated Regulation of Cdhr1a by Siah1 Modulates Photoreceptor Development and Survival in Zebrafish

Congenital retinal dystrophies are a major cause of unpreventable and incurable blindness worldwide. Mutations in CDHR1, a retina specific cadherin, are associated with cone-rod dystrophy. The ubiquitin proteasome system (UPS) is responsible for mediating orderly and precise targeting of protein degradation to maintain biological homeostasis and coordinate proper development, including retinal development. Recently, our lab uncovered that the seven in absentia (Siah) family of E3 ubiquitin ligases play a role in optic fissure fusion and identified Cdhr1a as a potential target of Siah. Using two-color whole mount in situ hybridization and immunohistochemistry, we detected siah1 and cdhr1a co-expression as well as protein localization in the retinal outer nuclear layer (ONL), and more precisely in the connecting cilium of rods and cones between 3-5 days post fertilization (dpf). We confirmed that Siah1 targets Cdhr1a for proteasomal degradation by co-transfection and co-immunoprecipitation in cell culture. To analyze the functional importance of this interaction, we created two transgenic zebrafish lines that express siah1 or an inactive siah1 (siah1ΔRING) under the control of the heat shock promoter to modulate Siah activity during photoreceptor development. Overexpression of siah1, but not siah1ΔRING, resulted in a decrease in the number of rods and cones at 72 h post fertilization (hpf). The number of retinal ganglion cells, amacrine and bipolar cells was not affected by Siah1 overexpression, and there was no significant reduction of proliferating cells in the Siah1 overexpressing retina. We did, however, detect increased cell death, confirmed by an increase in the number of TUNEL + cells in the ONL, which was proteasome-dependent, as proteasome inhibition rescued the cell death phenotype. Furthermore, reduction in rods and cones resulting from increased Siah1 expression was rescued by injection of cdhr1a mRNA, and to an even greater extent by injection of a Siah1-insensitive cdhr1a variant mRNA. Lastly, CRISPR induced loss of Cdhr1a function phenocopied Siah1 overexpression resulting in a significant reduction of rods and cones. Taken together, our work provides the first evidence that Cdhr1a plays a role during early photoreceptor development and that Cdhr1a is regulated by Siah1 via the UPS.

Identification of a novel homozygous nonsense mutation in the CDHR1 gene in a Chinese family with autosomal recessive retinitis pigmentosa

BACKGROUND

Retinitis pigmentosa (RP) is a group of hereditary retinal diseases that often lead to blindness. Although 80 genes associated with RP have been observed, the genetic mechanism of approximately 40% RP cases remains unknown. This study was to investigate the disease-causing gene in a Han Chinese family with autosomal recessive RP (arRP).

METHODS

A Chinese arRP family (RP-2373), consisting of three affected siblings and eight unaffected family members, was recruited in this study. All participants underwent complete ophthalmic examinations, including visual field testing, best-corrected visual acuity, fundus photography and electroretinography. Whole exome sequencing was performed on the three patients and Sanger sequencing was utilized to confirm the mutations identified in all family members and 2010 unrelated controls.

RESULTS

A novel homozygous nonsense mutation, c.1231C > T (p.Q411X) in the Cadherin-Related Family Member 1 (CDHR1) gene was identified in the RP-2373 family. The proband and her two affected sisters were found to carry a homozygous mutation that led to a substitution of Glutamine to a stop codon. Other unaffected members and 2010 ethnic-matched controls lacked this mutation. These data showed a complete co-segregation of the CDHR1 mutation with arRP in this family. The p.Q411X mutation was observed to affect highly conserved amino acid residue of CHDR1.

CONCLUSION

Our study expanded the CDHR1 mutation spectrum of RP in the Chinese population, which might help to better understand RP molecular pathogenesis.

Genetic spectrum of retinal dystrophies in Tunisia

We report the molecular basis of the largest Tunisian cohort with inherited retinal dystrophies (IRD) reported to date, identify disease-causing pathogenic variants and describe genotype-phenotype correlations. A subset of 26 families from a cohort of 73 families with clinical diagnosis of autosomal recessive IRD (AR-IRD) excluding Usher syndrome was analyzed by whole exome sequencing and autozygosity mapping. Causative pathogenic variants were identified in 50 families (68.4%), 42% of which were novel. The most prevalent pathogenic variants were observed in ABCA4 (14%) and RPE65, CRB1 and CERKL (8% each). 26 variants (8 novel and 18 known) in 19 genes were identified in 26 families (14 missense substitutions, 5 deletions, 4 nonsense pathogenic variants and 3 splice site variants), with further allelic heterogeneity arising from different pathogenic variants in the same gene. The most common phenotype in our cohort is retinitis pigmentosa (23%) and cone rod dystrophy (23%) followed by Leber congenital amaurosis (19.2%). We report the association of new disease phenotypes. This research was carried out in Tunisian patients with IRD in order to delineate the genetic population architecture.

Expanding the Clinical and Molecular Heterogeneity of Nonsyndromic Inherited Retinal Dystrophies

A cohort of 172 patients diagnosed clinically with nonsyndromic retinal dystrophies, from 110 families underwent full ophthalmologic examination, including retinal imaging, electrophysiology, and optical coherence tomography, when feasible. Molecular analysis was performed using targeted next-generation sequencing (NGS). Variants were filtered and prioritized according to the minimum allele frequency, and finally classified according to the American College of Medical Genetics and Genomics guidelines. Multiplex ligation-dependent probe amplification and array comparative genomic hybridization were performed to validate copy number variations identified by NGS. The diagnostic yield of this study was 62% of studied families. Thirty novel mutations were identified. The study found phenotypic intra- and interfamilial variability in families with mutations in C1QTNF5, CERKL, and PROM1; biallelic mutations in PDE6B in a unilateral retinitis pigmentosa patient; interocular asymmetry RP in 50% of the symptomatic RPGR-mutated females; the first case with possible digenism between CNGA1 and CNGB1; and a ROM1 duplication in two unrelated retinitis pigmentosa families. Ten unrelated cases were reclassified. This study highlights the clinical utility of targeted NGS for nonsyndromic inherited retinal dystrophy cases and the importance of full ophthalmologic examination, which allows new genotype-phenotype associations and expands the knowledge of this group of disorders. Identifying the cause of disease is essential to improve patient management, provide accurate genetic counseling, and take advantage of gene therapy-based treatments.

Characteristic Ocular Features in Cases of Autosomal Recessive PROM1 Cone-Rod Dystrophy

Purpose

To define characteristic ocular features in a group of patients with autosomal recessive (AR) PROM1 cone-rod dystrophy (CRD).

Methods

Three males and one female from three unrelated families were first seen at the ages of 15 to 22 years and diagnosed with CRD. Clinical testing available for review included full-field electroretinogram (ERG) in three patients, as well as near-infrared autofluorescence (NIR-AF), spectral-domain optical coherence tomography (SD-OCT), and color fundus photography in all four patients. Whole exome sequencing (WES) was performed on all cases, and whole genome sequencing (WGS) was performed in two families.

Results

WES found compound heterozygous PROM1 variants in one isolated male, plus heterozygous variants in the remaining patients. WGS uncovered deleterious PROM1 variants in these two families. ERG showed markedly reduced cone-isolated amplitudes and variably reduced rod-isolated amplitudes. The dark-adapted combined rod and cone responses demonstrated notably reduced a-wave amplitudes and moderately reduced b-waves, and the resultant waveform resembled the normal rod-isolated response. On fundus examination, oval-shaped macular lesions were observed, as were several small, circular hypoautofluorescent lesions within the posterior pole on NIR-AF. Three patients showed extramacular circular atrophic lesions.

Conclusions

The autofluorescence changes, peripheral retinal abnormalities, and ERG findings have not been emphasized in previous reports of AR PROM1, but they became a recognizable phenotype in this cohort of patients. A similar constellation of findings may be observed in CRD due to CDHR1, a functionally related gene. The pattern of abnormalities reported herein may help to focus genetic screening in patients with these findings.

A novel, homozygous nonsense variant of the CDHR1 gene in a Chinese family causes autosomal recessive retinal dystrophy by NGS-based genetic diagnosis

Retinal dystrophy is an inherited, heterogeneous, chronic and progressive disorder of visual functions. The mutations of patients with autosomal recessive retinal retinopathy cone‐and‐rod dysfunction and macular dystrophy have not been well described in the Chinese population. In this study, a three‐generation Chinese retinal dystrophy family was recruited. Ophthalmic examinations were performed. Targeted next generation sequencing (TGS) was used to identify causative genes, and Sanger sequencing was conducted to verify candidate mutations and co‐segregation. Reverse transcription (RT)‐PCR was applied to investigate the spatial and temporal expression patterns of cdhr1 gene in mouse. A novel, homozygous, deleterious and nonsense variant (c.T1641A; p.Y547*) in the CDHR1 gene was identified in the family with autosomal recessive retinal dystrophy, which was co‐segregated with the clinical phenotypes in this family. RT‐PCR analysis revealed that cdhr1 is ubiquitously expressed in eye, particularly very high expression in retina; high expression in lens, sclera, and cornea; and high expression in brain. In conclusion, our study is the first to indicate that the novel homozygous variant c.T1641A (p.Y547*) in the CHDR1 gene might be the disease‐causing mutation for retinal dystrophy in our patient, extending its mutation spectrums. These findings further the understanding of the molecular pathogenesis of this disease and provide new insights for diagnosis as well as new implications for genetic counselling.

Dry age-related macular degeneration like pathology in aged 5XFAD mice: Ultrastructure and microarray analysis

Age-related macular degeneration (AMD) is a leading cause of blindness in the elderly. The two types of AMD are: dry and wet AMD. While laser-induced choroidal neovascularization has been used extensively in the studies of wet AMD, there is no established mouse model that fully recapitulates the cardinal features of dry AMD. A lack of appropriate mouse model for dry AMD has hampered the translational research on the pathogenesis of the disease and the development of therapeutic agents. We hypothesized that 5XFAD mice, an animal model for the study of Alzheimer’s disease, can be used as a mouse model for dry AMD with regard to the amyloid beta (Aβ) related pathology. In this study, the ultrastructure of the retinal pigment epithelium (RPE) of 5XFAD mice was analyzed using transmission electron microscopy. Of importance, the aged 5XFAD mice show ultrastructural changes in the RPE and Bruch’s membrane (BM) that are compatible with the cardinal features of human dry AMD, including a loss of apical microvilli and basal infolding of the RPE, increased BM thickness, basal laminar and linear deposits, and accumulation of lipofuscin granules and undigested photoreceptor outer segment-laden phagosomes. In microarray-based analysis, the RPE complex of the aged 5XFAD mice shows differential gene expression profiles consistent with dry AMD in the inflammation response, immune reaction pathway, and decreased retinol metabolism. Taken together, we suggest that aged 5XFAD mice can be used as a mouse model of dry AMD to study Aβ related pathology and develop a new therapeutic approaches.

CDHR1 mutations in retinal dystrophies

We report ophthalmic and genetic findings in patients with autosomal recessive retinitis pigmentosa (RP), cone-rod dystrophy (CRD) or cone dystrophy (CD) harboring potential pathogenic variants in the CDHR1 gene. Detailed ophthalmic examination was performed in seven sporadic and six familial subjects. Mutation screening was done using a customized next generation sequencing panel targeting 105 genes implicated in inherited retinal disorders. In one family, homozygosity mapping with subsequent candidate gene analysis was performed. Stringent filtering for rare and potentially disease causing variants following a model of autosomal recessive inheritance led to the identification of eleven different CDHR1 variants in nine index cases. All variants were novel at the time of their identification. In silico analyses confirmed their pathogenic potential. Minigene assays were performed for two non-canonical splice site variants and revealed missplicing for the mutant alleles. Mutations in CDHR1 are a rare cause of retinal dystrophy. Our study further expands the mutational spectrum of this gene and the associated clinical presentation.

Whole exome sequencing using Ion Proton system enables reliable genetic diagnosis of inherited retinal dystrophies

Inherited retinal dystrophies (IRD) comprise a wide group of clinically and genetically complex diseases that progressively affect the retina. Over recent years, the development of next-generation sequencing (NGS) methods has transformed our ability to diagnose heterogeneous diseases. In this work, we have evaluated the implementation of whole exome sequencing (WES) for the molecular diagnosis of IRD. Using Ion Proton^TM system, we simultaneously analyzed 212 genes that are responsible for more than 25 syndromic and non-syndromic IRD. This approach was used to evaluate 59 unrelated families, with the pathogenic variant(s) successfully identified in 71.18% of cases. Interestingly, the mutation detection rate varied substantially depending on the IRD subtype. Overall, we found 63 different mutations (21 novel) in 29 distinct genes, and performed in vivo functional studies to determine the deleterious impact of variants identified in MERTK, CDH23, and RPGRIP1. In addition, we provide evidences that support CDHR1 as a gene responsible for autosomal recessive retinitis pigmentosa with early macular affectation, and present data regarding the disease mechanism of this gene. Altogether, these results demonstrate that targeted WES of all IRD genes is a reliable, hypothesis-free approach, and a cost- and time-effective strategy for the routine genetic diagnosis of retinal dystrophies.

Identification of Novel and Recurrent Disease-Causing Mutations in Retinal Dystrophies Using Whole Exome Sequencing (WES): Benefits and Limitations

Inherited retinal dystrophies (IRDs) are Mendelian diseases with tremendous genetic and phenotypic heterogeneity. Identification of the underlying genetic basis of these dystrophies is therefore challenging. In this study we employed whole exome sequencing (WES) in 11 families with IRDs and identified disease-causing variants in 8 of them. Sequence analysis of about 250 IRD-associated genes revealed 3 previously reported disease-associated variants in RHO, BEST1 and RP1. We further identified 5 novel pathogenic variants in RPGRIP1 (p.Ser964Profs*37), PRPF8 (p.Tyr2334Leufs*51), CDHR1 (p.Pro133Arg and c.439-17G>A) and PRPF31 (p.Glu183_Met193dup). In addition to confirming the power of WES in genetic diagnosis of IRDs, we document challenges in data analysis and show cases where the underlying genetic causes of IRDs were missed by WES and required additional techniques. For example, the mutation c.439-17G>A in CDHR1 would be rated unlikely applying the standard WES analysis. Only transcript analysis in patient fibroblasts confirmed the pathogenic nature of this variant that affected splicing of CDHR1 by activating a cryptic splice-acceptor site. In another example, a 33-base pair duplication in PRPF31 missed by WES could be identified only via targeted analysis by Sanger sequencing. We discuss the advantages and challenges of using WES to identify mutations in heterogeneous diseases like IRDs.

Molecular basis for photoreceptor outer segment architecture

To serve vision, vertebrate rod and cone photoreceptors must detect photons, convert the light stimuli into cellular signals, and then convey the encoded information to downstream neurons. Rods and cones are sensory neurons that each rely on specialized ciliary organelles to detect light. These organelles, called outer segments, possess elaborate architectures that include many hundreds of light-sensitive membranous disks arrayed one atop another in precise register. These stacked disks capture light and initiate the chain of molecular and cellular events that underlie normal vision. Outer segment organization is challenged by an inherently dynamic nature; these organelles are subject to a renewal process that replaces a significant fraction of their disks (up to ∼10%) on a daily basis. In addition, a broad range of environmental and genetic insults can disrupt outer segment morphology to impair photoreceptor function and viability. In this chapter, we survey the major progress that has been made for understanding the molecular basis of outer segment architecture. We also discuss key aspects of organelle lipid and protein composition, and highlight distributions, interactions, and potential structural functions of key OS-resident molecules, including: kinesin-2, actin, RP1, prominin-1, protocadherin 21, peripherin-2/rds, rom-1, glutamic acid-rich proteins, and rhodopsin. Finally, we identify key knowledge gaps and challenges that remain for understanding how normal outer segment architecture is established and maintained.