Phenotypical Characteristics of POC1B-Associated Retinopathy in Japanese Cohort: Cone Dystrophy With Normal Funduscopic Appearance

Abstract

Purpose: Cone/cone-rod dystrophy is a large group of retinal disorders with both phonotypic and genetic heterogeneity. The purpose of this study was to characterize the phenotype of eight patients from seven families harboring POC1B mutations in a cohort of the Japan Eye Genetics Consortium (JEGC).

Methods: Whole-exome sequencing with targeted analyses identified homozygous or compound heterozygous mutations of the POC1B gene in 7 of 548 families in the JEGC database. Ophthalmologic examinations including the best-corrected visual acuity, perimetry, fundus photography, fundus autofluorescence imaging, optical coherence tomography, and full-field and multifocal electroretinography (ERGs) were performed.

Results: There were four men and four women whose median age at the onset of symptoms was 15.6 years (range, 6–23 years) and that at the time of examination was 40.3 years (range, 22–67 years). The best-corrected visual acuity ranged from −0.08 to 1.52 logMAR units. The funduscopic appearance was normal in all the cases except in one case with faint mottling in the fovea. Optical coherence tomography revealed an absence of the interdigitation zone and blurred ellipsoid zone in the posterior pole, but the foveal structures were preserved in three cases. The full-field photopic ERGs were reduced or extinguished with normal scotopic responses. The central responses of the multifocal ERGs were preserved in two cases. The diagnosis was either generalized cone dystrophy in five cases or cone dystrophy with foveal sparing in three cases.

Conclusions: Generalized or peripheral cone dystrophy with normal funduscopic appearance is the representative phenotype of POC1B-associated retinopathy in our cohort.

Cone/cone-rod dystrophy is the name given to a large group of retinal disorders with genetically heterogeneous origin and is characterized by progressive cone dysfunction with or without rod dysfunction. The age of onset, degree of cone/rod dysfunction, and funduscopic appearance are diverse, partly because there are many genetic causes related to this disorder. Representative genotypes related to this disorder involve GUCY2D,^1–4 GUCA1A,^5–8 CRX,^9,10 RIMS1,¹¹ PROM1,^12,13 and PRPH2^14,15 as autosomal dominant; ABCA4^16–19 and KCNV2^20–22 as autosomal recessive; and RPGR^23,24 as X-linked recessive. It is notable that the clinical features of cone/cone-rod dystrophy are also diverse among the patients having mutations in the same gene or even among patients in the same family. For example, the funduscopic features vary from that of central retinal atrophy, central chorioretinal atrophy, bull's eye appearance, and normal funduscopic appearance, depending on both the genotypes and the individual.^{2,7,8,10,11,13,15,16,18,19,21–24} 

A normal funduscopic appearance is unusual but not a rare feature of cone dystrophy (COD), and it has been reported in many cases with various genotypes.^{10,21,22,24–28} Patients with normal fundus are often misdiagnosed as having optic neuropathy, amblyopia, or nonorganic visual disturbances unless they undergo detailed examinations, including optical coherence tomography (OCT) and electroretinography (ERG). However, there is no report showing that a specific genotype is strongly associated with this funduscopic feature. Thus, to determine the specific genotypes related to normal funduscopic appearance, we have searched for patients with COD that have no apparent funduscopic abnormalities from the genotype-phenotype database of Japan Eye Genetics Consortium (JEGC), and eight cases had putative biallelic mutations in the POC1B gene. 

POC1B is expressed predominantly in the ciliary region of photoreceptor cells and synapses of the outer plexiform layer of the retina,²⁹ and homozygous or compound heterozygous mutations in the POC1B gene have been reported in cases with COD or cone-rod dystrophy (CORD),^27,29,30 Leber's congenital amaurosis (LCA) with syndromic ciliopathy,³¹ and peripheral COD.²⁸ The funduscopic appearance in these cases varied from normal to peripheral abnormalities and small colobomas with small diameter vessels. However, a detailed clinical and genetic association caused by POC1B pathogenic variants has not been published. 

Thus, the purpose of this study was to characterize the phenotypical characteristics of eight patients from seven Japanese families harboring POC1B mutations in a cohort of the JEGC. 

Patients and Methods

The protocol of this study adhered to the tenets of the Declaration of Helsinki and was approved by the Ethics Committee of the participating institutions: National Institute of Sensory Organs (NISO), National Hospital Organization, Tokyo Medical Center; Nippon Medical School Chiba Hokusoh Hospital; Nagoya University Graduate School of Medicine; The Jikei University School of Medicine; Kindai University Faculty of Medicine; Aichi Medical University; and Ideta Eye Hospital. A signed informed consent was obtained from all patients. 

Participants

Eight affected patients from seven families who carried multiple POC1B variants were studied. The 8 patients were part of the 1035 cases (548 families) in the phenotype-genotype database of the JEGC. The clinical data and results of whole-exome sequencing were available for all the participants. There were 41 cases whose phenotype was “cone/cone-rod dystrophy without apparent funduscopic abnormalities” in the JEGC database, and all of the 8 cases were categorized with this phenotype. The data of two families (families 3 and 6) were partially reported by Kominami et al.²⁷ and Ito et al.³² 

Clinical Examinations

Comprehensive ophthalmologic examinations were performed on all patients and also on several unaffected family members who were examined for cosegregation analyses (Fig. 1). The clinical evaluations included measurements of the best-corrected visual acuity (BCVA), color vision tests (Ishihara color vision test and the Farnsworth Panel D-15 Color Vision tests), visual field (Goldmann kinetic perimetry and Humphrey Visual Field Analyzer; Carl Zeiss Meditec, Dublin, CA, USA), fundus photography and funduscopy, fundus autofluorescence (FAF) imaging (HRA 2; excitation light, 488 nm; barrier filter, 500 nm; Heidelberg Engineering, Heidelberg, Germany; and 200Tx; excitation light, 532 nm; barrier filter, 570–780 nm; Optos, Dunfermline, United Kingdom), spectral domain optical coherent tomographic (SD-OCT; Cirrus HDOCT, version 6.5-11.0; Carl Zeiss Meditec; and Spectralis; Heidelberg Engineering), and electrophysiologic assessments. 

Figure 1

 

Pedigrees of seven Japanese families with POC1B-associated retinopathy. The solid squares (men) and circles (women) represent the affected patients. Unaffected family members are represented by clear symbols. The slash symbol indicates deceased individuals. The generation number is shown on the left. Patients enrolled in the study are marked by an arrow, and the clinically examined individuals are indicated by a cross. Detected POC1B variants in the affected subjects are shown as follows: patient 1, c.[356C>T];[ 337G>C], p.[Thr119Ile]; [Asp113His]; patient 2, c.[356C>T]; [337G>C], p.[Thr119Ile]; [Asp113His]; patient 3, c.[356C>T]; [1355G>A], p.[Thr119Ile]; [Arg452Gln; patient 4, c.[987C>A]; [1355G>A], p.[Tyr329Ter]; [Arg452Gln]; patient 5, c.[356C>T; 356C>T], p.[Thr119Ile; Thr119Ile]; patient 6, c.[356C>T(;)356C>T]; p.[Thr119Ile(;)Thr119Ile]; patient 7, c.[356C>T]; [1355G>A], p.[Thr119Ile]; [Arg452Gln]; and patient 8, c.[356C>T(;)356C>T], p.[Thr119Ile(;)Thr119Ile].

Signal Intensity Profiling of FAF and SD-OCT Images

Signal intensity profiling of the FAF images was performed with a custom-made software (Gray scale profiling version 0.1; modified based on MATLAB 3.0; The MathWorks, Inc., Natick, MA, USA). The gray scales of the obtained FAF images were calculated, and the three-dimensional color tomographic and cross-sectional images based on the gray scales along a selected line were generated. For this study, a horizontal line across the fovea was manually selected to perform the signal intensity profiling of the autofluorescence (AF) density across the fovea. SD-OCT images were obtained, and cross-sectional images based on the gray scales along a selected line were generated according to a published method with custom-made software (Longitudinal profiling version 0.1).³³ The presence of high-intensity lines shown as peaks in the cross-sectional images were assessed for this study. 

Electrophysiologic Assessments

For the electrophysiologic assessments, full-field electroretinograms (ERGs) were recorded under both scotopic and photopic conditions in accordance with the protocol of the International Society of Clinical Electrophysiology of Vision.³⁴ ERGs were recorded with the LE4000 device (Tomey Corporation, Aichi, Japan) or the UTAS device (LKC Technologies, Gaitherburg, MD, USA). Macular function was evaluated by either multifocal ERGs (mfERGs; VERIS Science and VERIS Clinic; EDI, San Mateo, CA, USA) or focal macular ERGs (ER-80; Kowa Company, Tokyo, Japan).³⁴ All of the clinical data and images were uploaded into the JEGC databank at the NISO, and data quality was confirmed by two of the authors (KF, KT). 

Whole-Exome Sequencing and In Silico Molecular Genetic Analysis

Whole-exome sequencing and targeted sequence analyses of the 301 retinal disease-associated genes (including genes of Retnet; https://sph.uth.edu/retnet/, in the public domain) were done according to the published protocol of the NISO.³⁵ Paired-end sequence library construction and exome capturing were performed at a company (Macrogen, Kyoto, Japan/Gasan-dong, South Korea) by the Agilent Bravo automated liquid-handling platform with SureSelect XT Human All Exon kit V3-5 + UTRs kit (Agilent Technologies, Santa Clara, CA, USA). Enriched libraries were sequenced with the Illumina HiSeq 2000/HiSeq 2500 sequencer (San Diego, CA, USA; read length, 2 × 101 bp). Exome pipeline analysis was performed with a customized protocol developed for the Japanese population.³⁵ 

In silico bioinformatic analyses were performed to predict the pathogenicity of all of the identified POC1B variants. The identified variants were filtered with allele frequency of less than 1.0% of the Human Genetic Variation Database (http://www.genome.med.kyoto-u.ac.jp/SnpDB/about.html, in the public domain), and 2kJPN (https://ijgvd.megabank.tohoku.ac.jp/download_2kjpn/, in the public domain), which is specific for the Japanese population, and with a total frequency of less than 1.0% of the gnomAD Browser (http://gnomad.broadinstitute.org/, in the public domain). All identified variants were analyzed using three software prediction programs: PolyPhen2 (http://genetics.bwh.harvard.edu/pph/index.html, in the public domain), SIFT (http://sift.jcvi.org/, in the public domain), and mutation taster (http://www.mutationtaster.org/, in the public domain). Conservation in the positions of the identified variants was evaluated with primate PhyloP and phastCons scores provided by University of California-Santa Cruz based on the human genome 19 coordinates (http://genome.ucsc.edu/cgi-bin/hgTrackUi?db=hg19&g=cons46way, in the public domain). 

Direct Sequencing

The POC1B variants identified by exome sequencing and targeted analysis were further confirmed by direct sequencing of all family members. The identified regions were amplified by polymerase chain reaction (PCR) using primers synthesized by Greiner Bio-One (Tokyo, Japan). The PCR products were purified (ExoSAP-IT; USB Corp., Cleveland, OH, USA) and were used as the template for sequencing. Both strands were sequenced by an automated sequencer (Bio Matrix Research, Chiba, Japan). 

Overall, the pathogenicity prediction of all variants, confirmed by direct sequencing, were classified based on the American College of Medical Genetics (ACMG) standards and guidelines.³⁶ 

Results

Whole-exome sequencing with targeted analysis identified homozygous or compound heterozygous mutations of the POC1B gene in 8 out of 1035 cases (7 of 548 families) in the JEGC database (Fig. 1). 

Demographics, Color Vision Defect, and Visual Fields

The phenotypic findings are shown in Table 1 and 2. There were four women and four men. The median age at the initial examination was 40.3 years with a range of 22 to 67 years, and the median age at the onset of symptoms was 15.6 years with a range of 6 to 23 years. Seven out of eight patients complained of photophobia (88%) as an initial symptom. The BCVA ranged from −0.08 to 1.52 logMAR units. There were no systemic abnormalities described in the reports of all patients. 

Table 1

 

Summary of Clinical Findings 1

 

Table 2

 

Summary of Clinical Findings 2

 

The results of the Ishihara color vision tests were obtained from 10 eyes of 5 patients (Table 1, 2). Eight eyes of four patients (8/10, 80%) were deficient and the five eyes of three patients could not read any plate including the 1st plate. Therefore, these eyes could not be evaluated by Ishihara color vision test (OD of patient 1 and OU of patients 4 and 6). Three eyes of two patients could read only the 1st plate (OS of patient 1 and OU of patient 3). Two eyes of one patient were normal and identified all plates correctly (2/10, 20%). Panel D-15 was performed in seven patients (Table 1, 2; Supplementary Fig. S4). The results of six patients (6/7, 85.7%) were classified as fail. The results of patient 3, 4, 5, and 6 showed many confusion lines between the deutan and tritan axes or along the tritan axes. The results of patients 1 and 8 were fail with two crossings. Two eyes of one patient passed the test (2/14, 14%). The visual field examinations showed a central scotoma in 11 eyes in 6 patients (11/16, 69%). Patient 3 had a central scotoma by Humphrey Visual Field Analyzer. A paracentral scotoma was found in five eyes of three patients (5/16, 31%). 

Fundus and FAF Images

The findings of the funduscopic examinations were normal in all of the eyes except in patient 2-II:1 who showed small and faint retinal pigment epithelial (RPE) mottling in the fovea bilaterally (14/16, 87.5%) (Fig. 2). FAF images were obtained from 12 eyes of 6 patients and signal intensity profiling of gray scales was performed on these 12 images. (Tables 1, 2; Fig. 3; Supplementary Fig. S1). An area of high AF signal was observed in three eyes (patient 3 and 4; 3/12, 25.0%; Table 1, 2; Fig. 3), which was demonstrated as a peak of intensity at the foveola by the gray scale analysis (Supplementary Fig. S1). A band of high AF signal was found surrounding the fovea in two eyes (patient 6; 2/12, 16.7%; Fig. 3). This was seen as a band of slightly increased AF signaling between the fovea and the disc. Although this was a qualitative analysis, the FAF images were seen as hyper-AF in or around the fovea in five eyes of three cases (5/12, 41.7%; Fig. 3, Supplementary Fig. S1). No particular AF abnormalities were detected in seven eyes (7/12, 58.3%; Fig. 3, Supplementary Fig. S1). We compared the feature of cross-sectional images of gray scale profiling obtained from normal controls, CRX-retinopathy, and our patients with pathogenic POC1B variants. The profile of the eye with CRX-retinopathy showed a wider peak of hyporeflectivity as confirmed by the FAF image (Supplementary Fig. S1). However, our patients had a narrower peak of hyporeflectivity similar to that of normal controls. 

Figure 2

 

Fundus photographs of all POC1B-associated retinopathy patients. Fundus photographs of the right eyes of all patients with POC1B-associated retinopathy. No marked abnormalities are present in all of the eyes except for patient 3 (2-II:1) who showed small and faint RPE mottling in the fovea bilaterally

Figure 3

 

FAF images of six patients with POC1B-associated retinopathy (patients 3–8). FAF images of the right eyes of six patients with POC1B-associated retinopathy showing an area of high AF signal in patients 3 and 4. A band of high AF signal surrounding the fovea was found in patient 6. No particular AF abnormalities were noted in the other three patients.

SD-OCT Images

Signal intensity profiling of the two hyperreflective lines (ellipsoid zone [EZ] and interdigitation zone [IZ]) was performed for the 16 SD-OCT images (Fig. 4; Supplementary Fig. S2). The absence of the IZ and blurred EZ at the posterior polar region was observed in all the eyes (16/16) (Fig. 4), although the EZ of patient 1 and the EZ and IZ of patients 7 and 8 were relatively preserved in a narrow region of fovea as foveal sparing (6/16, 37.5%; shown as asterisks in Fig. 4; Supplementary Fig. S2). In patient 6, the EZ was completely absent bilaterally in the fovea. RPE layer was preserved in all of the eyes (16/16, 100%; Fig. 4; Supplementary Fig. S2). 

Figure 4

 

SD-OCT images of all POC1B-associated retinopathy patients. SD-OCT images of the right eyes of all patients with POC1B-associated retinopathy are shown. The absence of the IZ and blurred EZ at the wide posterior pole region in all the eyes can be seen, although the EZ of patient 1 and both the EZ and IZ of patients 7 and 8 were relatively preserved in a narrow region of the fovea as foveal sparing (shown as asterisks). In patient 6 (5-II:2), the EZ was complete absent bilaterally in the macular region. The RPE layer is preserved in all of the eyes.

Electrophysiologic Findings

The cone responses of the full-field ERGs were severely reduced or extinguished in all the eyes (16/16, 100%; Table 1, 2; Fig. 5). The rod responses were normal in all eyes (16/16, 100%; Fig. 5). The mfERGs were preserved in the central fields in four eyes of the two cases tested (4/12, 33%; Table 1, 2; Supplementary Fig. S3), and the others were extinguished in the macular region (8/12, 67%; Table 1, 2). 

Figure 5

 

Full-field ERGs of eight POC1B-associated retinopathy patients. Full-field ERGs recorded from patients and normal control are shown. The dark-adapted 0.01 (DA 0.01), dark-adapted 3.0 (DA 3.0), dark-adapted 10.0 (DA 10.0), light-adapted 3.0 (LA 3.0), and light-adapted 3.0 flicker (LA 3.0, 30 Hz Flicker) ERGs are shown. The results show extinguished or severe reduction of the cone responses in all patients, although the rod responses are well-preserved.

The clinical diagnosis was either generalized COD in 10 eyes of 5 cases or COD with foveal sparing (i.e., peripheral COD, in 6 eyes of 3 cases). 

POC1B Variants

Four possible pathogenic variants were identified: c.337G>C, p.Asp113His; c.356C>T, p.Thr119Ile; c.987C>A, p.Tyr329Ter; and c.1355G>A, p.Arg452Gln (Table 3, 4). Of these variants, Tyr329Ter is a nonsense variant and the others are missense variants. The minor allelic frequency of the four variants was less than 0.2% in two Japanese-specific databases and less than 0.03% in all ethnicities in the gnomAD database (Table 3, 4). The results of three prediction programs indicated that all missense variants were deleterious, probably damaging, and disease causing (Table 3, 4). Two missense variants, Asp113His and Thr119Ile, were located within a WD40 domain, which is critical for proper POC1B function (Fig. 6). A missense variant, Arg452Gln, was located at the same position as a reported pathogenic mutation, although the reported mutation was a nonsense mutation (Fig. 6). The conservation score of all missense variants was more than 3.0 in PhyloP, which is relatively high, and mutated amino acids in these variants were well conserved in the homologues of POC1B in other species (Table 3, 4; Fig. 7). 

Table 3

 

Results of In Silico Genetic Analysis of Four Pathogenic POC1B* Variants 1

 

Table 4

 

Results of In Silico Genetic Analysis of Four Pathogenic POC1B* Variants 2

 

Figure 6

 

Schematic representation of POC1B gene and mutations. The schematic structure of POC1B gene are shown. The encoded protein contains seven WD repetitive domains, which are located between amino acids 16 and 307 (highlighted in gray). The detailed locations of the seven WD domains are 16–55, 58–99, 101–139, 142–181, 183–223, 226–265, and 268–307 (UniProtKB - Q8TC44 [POC1B_HUMAN]; https://www.uniprot.org/uniprot/Q8TC44, in the public domain). Exon-intron structure and exon numbers are shown under the scheme. Variants in this study and previous reports are shown at the top and the variants identified in this study are shown in bold.

Figure 7

 

Alignment of POC1B family proteins. The result of Weblogo analysis derived from amino acid sequences of POC1B from seven species reported in the NCBI database are shown: Homo sapiens, Mus musculus, Xenopus tropicalis, Bos taurus, Macaca mulatta, Canis lupus familiaris, and Callithrix jacchus. Amino acid residues of D113, T119, and R452 in humans are indicated. Well-conserved residues are shown in larger letters (WebLogo; https://weblogo.berkeley.edu/logo.cgi in the public domain).

Pedigree analyses of families with POC1B variants revealed that these four variants were well cosegregated (Fig. 1). Direct sequencing of the four variants detected by whole-exome analysis was performed, and the variants were verified. According to the American College of Medical Genetics standards and guidelines, the POC1B variants were considered to be pathogenic, likely pathogenic, or of uncertain significance (Tables 3–5). 

Table 5

 

Pathogenicity Evaluation of POC1B Variants and ACMG Classification in Each Patient

 

Discussion

The proteome of the centriole 1B gene (POC1B; OMIM 614784) is one of the two POC1 homologs that function together as a highly conserved core centriole and basal body component.³⁷ The POC1B protein is localized to centrioles and appear to play roles in centriole duplication and/or maintenance and functions together with POC1A.^38,39 The WD40 repeat domain containing the cartwheel protein Poc1 is required for the structural maintenance of centrioles in Tetrahymena thermophila.³⁷ A knockdown of poc1b in zebrafish causes ciliary defects and morphologic phenotypes consistent with human ciliopathies.³⁷ A morpholino oligomer knockdown of poc1b translation in zebrafish resulted in a dose-dependent small-eye phenotype, impaired optokinetic responses, and decreased length of the photoreceptor outer segments.⁴⁰ These findings suggested that poc1b is required for the normal development and ciliogenesis of the retinal photoreceptor sensory cilia.⁴⁰ 

Homozygous or compound heterozygous mutations of the POC1B gene in three Turkish patients and a Dutch subject with COD and CORD were reported in 2014.²⁹ The three mutations were located in a highly conserved residue within the WD40 domain. This domain is associated with a wide variety of functions, including adaptor/regulatory modules in signal transduction, pre-mRNA processing, and cytoskeleton assembly. WD40 typically contains a GH dipeptide, 11 to 24 residues from its N terminus and a WD dipeptide at its C terminus of 40 residues long, hence the name WD40. Clinical and genetic findings of four affected subjects in a consanguineous Turkish family with CORD were also reported by Durlu et al.³⁰ in 2014. The recurrent variant c.317G>C, p.Arg106Pro was identified in an Iraqi patient with a severe syndromic retinal ciliopathy in a consanguineous family.³¹ 

Thus far, only 11 patients from 6 families with biallelic POC1B variants have been reported to have retinal abnormalities except in the Japanese patients (Table 6).^27–31,33 We have presented eight Japanese patients from seven families. According to previous reports, a wide variety of phenotypes were observed: one case of LCA, seven cases of CORD, and three cases of COD. The funduscopic appearance was normal in the two Turkish cases and one Chinese case, whereas the other eight cases were reported to have apparent funduscopic abnormalities either in the peripheral retina or in the macular area. On the other hand, all eight cases in the Japanese cohort were diagnosed with COD with preserved rod function (Table 6), and funduscopic examination did not reveal any abnormalities except one case with minimal RPE changes in the fovea (Fig. 2). 

Table 6

 

Previously Reported POC1B Mutations With Retinal Dystrophies

 

The FAF images showed hyper-AF in or around the fovea in five eyes of three cases (Table 1, 2; Fig. 4; Supplementary Fig. S1). These FAF abnormalities, however, were much less severe than in other macular dystrophies.^{2,7,10,11,13,15,17,21,23,24,41–43} Also, a hypo-AF, which indicates long-term RPE dysfunction, was not observed in any of the eyes. By comparing the FAF of a severe case with CRX-retinopathy that had severe photoreceptor and RPE degeneration at the macula, we would strongly suggest that patients with POC1B-associated retinopathy had never had severe hyporeflectivity, as found in patients with CRX-retinopathy. This implies that the primary lesion of our cases was the photoreceptors, and the RPE was not severely damaged even in cases with a long disease course. In fact, SD-OCT did not show any abnormalities in the RPE layer, although a loss of the IZ and blurred EZ in the posterior pole were detected in all patients. 

It is notable that foveal sparing (i.e., preserved foveal EZ and IZ) was observed in six eyes of three cases (Fig. 4; Supplementary Fig. S2). All of the eyes with foveal sparing had good BCVA between −0.08 and 0.15 logMAR units (Table 1, 2). Among them, four eyes of two cases had preserved mfERGs in the central region (Supplementary Fig. S3). 

A foveal sparing was also reported in a Chinese patient with the POC1B mutation and was diagnosed with peripheral COD.²⁸ Peripheral COD was first reported in three Japanese cases of cone dysfunction with preserved macular responses in the ERGs by Kondo et al.⁴⁴ These cases had normal funduscopic appearance in both the macula and peripheral retina. The pedigree of one family suggested an autosomal recessive inheritance; however, the causative gene has not been definitively determined. In our six eyes with preserved foveal structures in the OCT images, two eyes did not have preserved central responses in the mfERGs (patient 8; Table 1, 2; Supplementary Fig. S3), although the BVCA was 0.05 logMAR units bilaterally. This may be because the preserved foveal region was too small to evoke normal ERGs, and we suggest that the etiology of our three cases was similar to that reported by Kondo et al.⁴⁴ 

A sparing of the fovea is commonly observed in different types of macular diseases, such as in ABCA4- and PRPH2-associated retinopathies,^{17,18,43,45–48} mitochondrial retinal dystrophy,⁴⁹ macular dystrophy with CRB1 mutation,⁵⁰ and age-related macular degeneration.^51–54 The explanations for the physiologic and anatomic sparing of the fovea have been presented in many publications.^55–63 However, foveal sparing in these macular diseases is usually accompanied by RPE atrophy around the fovea, and the border between the atrophic region and normal fovea is distinct. The foveal sparing in our cases did not show any funduscopic changes because the RPE layer was normally preserved whether the photoreceptor layer is damaged or not (Fig. 2). Thus, the pathologic mechanism for the foveal sparing may differ between the cases with POC1B mutations and other macular dystrophies.^{17,18,43,45–54} Although the numbers of the eyes with foveal sparing are limited, further investigations to determine whether there is a significant association between the genotype and RPE findings (i.e., the presence or absence of RPE atrophy) in a large cohort with foveal sparing could determine the mechanism of the foveal sparing. 

The question then arises on whether the cases with foveal sparing represent an early stage that will progress to a more advanced stage with foveal abnormalities. The BCVA of the left eye of patient 1 deteriorated from 0.15 logMAR units to 0.4 logMAR units after 4 years of follow-up, and the OCT images with the reflectivity profiles also showed that the foveal sparing disappeared during the course; the EZ became blurred and the IZ disappeared at the fovea (Fig. 8, asterisks). The reflectance intensity of the IZ relative to the RPE was 0.82 at 35 years and 0.63 at 39 years. The reflectance intensity of the EZ relative to the RPE was 0.86 at 35 years and 0.77 at 39 years. These findings indicate that the reflectance of both IZ and EZ relative to the RPE were decreased during a follow-up period of 4 years. The changes in the SD-OCT images indicated that foveal sparing is observed during the natural course of the disease process in our cases with POC1B mutations and may progress to foveal dysfunction with reduced BCVA with increasing time. Because not all the patients had been followed for a long period of time, we cannot conclude whether the central foveal sparing observed in our patients could be an initial phase of the disorder or a subtype of the phenotypes in the POC1B-associated retinopathy. Long-term observations should be able to confirm the natural course of these cases. 

Figure 8

 

Horizontal and vertical OCT images of the left eye of patient 1 recorded at the age 35 years (A) and 39 years (B). Longitudinal reflectivity profiles at the fovea are shown below the OCT images. EZ and IZ are preserved in the fovea at the age 35 years but they are blurred at the age of 39 years. The reflectivity profiles show small peaks of IZ at the age 35 years but not at 39 years. The reflectance intensity of the IZ relative to the RPE was 0.82 at the age 35 years and 0.63 at the age 39 years. The reflectance intensity of the EZ relative to the RPE was 0.86 at the age 35 years and 0.77 at the age 39 years. The reflectivity was averaged in width as indicated by the yellow lines. ILM, internal limiting membrane; a.u., arbitrary unit.

The SD-OCT findings of our cases were similar to those of occult macular dystrophy with the RP1L1 mutation, Miyake's disease, in that both EZ blurring and IZ loss were observed in the affected region without RPE atrophy.^34,64,65 Because both POC1B and RP1L1 are located at the retinal photoreceptor sensory cilia, there is a possibility that the loss of IZ and blurred EZ with preserved RPE are diagnostic markers for retinal ciliopathies. There are, however, other ciliopathies affecting the retina, such as RPGR-,^66–68 RPGRIP1-,^69–71 RP1-^72–74 and CEP290-associated retinopathies,^75,76 which commonly lead to an apparent RPE degeneration. The mechanism of why RPE atrophy is less distinct in POC1B- and RP1L1-associated retinopathy is not clear. 

Normal rod function and preserved RPE structures, which are associated with normal funduscopic appearances, are characteristic features in both the Japanese and Chinese cases with POC1B-associated retinopathy. On the other hand, the reports of non-Asians identified funduscopic abnormalities in 8 of 10 cases (Table 6).^29–31,41 The phenotypic differences among the different cohorts may arise from either variations in ethnicity or the existence of recurrent R106P variants, which were not found in the Japanese or Chinese patients. However, it should be noted that the funduscopic appearances and FAF images of the POC1B-associated retinopathy in non-Asian populations were much less severe than those in COD and CORD caused by mutations of other genes.^30,41 A relatively preserved RPE function, which leads to normal funduscopic appearance, may be a common feature of POC1B-associated retinopathy in individuals of Asian ethnicities. 

Of the 41 cases in the JEGC database registered as cone/cone-rod dystrophy without apparent funduscopic abnormalities, 8 cases with POC1B-associated retinopathy were identified, while there were no cases with POC1B-associated retinopathy in 161 patients with “macular dystrophy/cone rod dystrophy with apparent funduscopic changes” in the JEGC cohort. This fact implies that POC1B-associated retinopathy is a major subset of cases with cone/cone-rod dystrophy. 

Our study has a number of limitations. Our data were obtained from the JEGC database for inherited retinal degeneration from all over Japan. The data from patients from multiple institutions were uploaded into the database. However, the examination devices used at the different institutions could have been different. Therefore, detailed quantitative analysis could not be made. More detailed quantitative analyses are needed to resolve this limitation. 

The results of Ishihara color vision test showed that five eyes of three patients (patient 1, 4, and 6) could not read any plate including the 1st plate. The results of Panel D-15 of these eyes were fail with two crossings or many confusion lines mainly between the deutan and tritan axes. It is unusual that patients with a BCVA better than 1.0 logMAR units could not identify the 1st plate of the Ishihara color vision test. To address these color vision deficiencies, another detailed color vision assessment such as 100-Hue tests would be needed. The results of Ishihara and Panel D-15 tests of patients 3 and 5 could be considered as behavior of severe red-green deficiency or achromatopsia. There are reports of congenital achromatopsia, which shows similar pattern such as cone dysfunction in the ERG, loss of IZ in the OCT, and D-15 abnormality with confusion lines between deutan and tritan.^77,78 The course of our cases was progressive and was not like that of congenital achromatopsia. However, the relationship between the results of color vision tests and OCT images could not be revealed. In addition, we could not rule out the complications of congenital red-green color deficiency based on the results of color vision tests in this study. 

We performed whole-exome sequencing with targeted analysis that could have missed the disorder-causing variants in genes outside of target (301 retinal disease-associated genes) and structural variants including large deletions in the target region. More comprehensive gene screening and analysis by methods such as whole-genome sequencing could help to determine the genetic aberrations of our cohort. We have examined patients with pathogenic POC1B variants in a relatively large Japanese cohort, but it is important to note that we have examined only eight patients. The cross-sectional nature of our study did not allow us to draw conclusions regarding the phenotype-genotype correlation of POC1B retinopathy. Although several cases had clear onset with notable visual impairment and progression and an evidence of morphologic changes, which suggests the progression of POC1B-associated retinopathy, the features and rate of progression were not determined. To address these issues, systematic longitudinal studies incorporating detailed ophthalmologic assessments in large cohort are needed, and they should help determine the mechanisms involved in the development of POC1B-associated retinopathy. 

In conclusion, the results indicate that a generalized or peripheral COD with normal funduscopic appearance is the representative phenotype of POC1B retinopathy in the Japanese. The characteristic morphologic changes in the photoreceptor layers are similar to those of occult macular dystrophy with the RP1L1 mutation, which is also one of the retinal ciliopathies and might be a distinctive phenotypic feature to differentiate POC1B-associated retinopathy from the other COD or CORD. 

Acknowledgments

The authors thank the patients and their families for participation in this study, Kazuki Yamazawa for his expert comments on genetic assessments, and Duco Hamasaki of the Bascom Palmer Eye Institute, University of Miami School of Medicine (Miami, FL, USA) for discussions and editing our manuscript. 

Supported by research grants from the Japan Agency for Medical Research and Development (AMED), the Ministry of Health, Labor and Welfare, and Japan (18ek0109282h0002 to TI); Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science (H26-26462674 to KT; 16H06269 and 16KK0193 to KF); and National Hospital Organization Network Research Fund (H30-NHO-Sensory Organs-03 to KF and KT). KF is supported by Foundation Fighting Blindness, USA and Great Britain Sasakawa Foundation Butterfield Awards, UK. The authors have no proprietary or commercial interest in any materials discussed in this article. GA is supported by a Fight for Sight (UK) Early career investigator award, NIHR-BRC at Moorfields Eye Hospital and the UCL Institute of Ophthalmology, NIHR-BRC at Great Ormond Street Hospital and UCL Institute of Child Health, and Great Britain Sasakawa Foundation Butterfield Award, UK. NP is supported by the the NIHR-BRC at Moorfields Eye Hospital and the UCL Institute of Ophthalmology. 

Disclosure: S. Kameya, None; K. Fujinami, None; S. Ueno, None; T. Hayashi, None; K. Kuniyoshi, None; R. Ideta, None; S. Kikuchi, None; D. Kubota, None; K. Yoshitake, None; S. Katagiri, None; H. Sakuramoto, None; T. Kominami, None; H. Terasaki, None; L. Yang, None; Y. Fujinami-Yokokawa, None; X. Liu, None; G. Arno, None; N. Pontikos, None; Y. Miyake, None; T. Iwata, None; K. Tsunoda, None 

Appendix

Members of the Japan Eye Genetics Consortium

Atsushi Mizota,¹ Kei Shinoda,^1,2 Natsuko Nakamura,³ Kei Mizobuchi,⁴ Toshihide Nishimura,⁵ Yoshihide Hayashizaki,⁶ Mineo Kondo,⁷ Nobuhiro Shimozawa,⁸ Masayuki Horiguchi,⁹ Shuichi Yamamoto,¹⁰ Manami Kuze,¹¹ Nobuhisa Naoi,¹² Shigeki Machida,¹³ Yoshiaki Shimada,¹⁴ Makoto Nakamura,¹⁵ Takashi Fujikado,¹⁶ Hotta Yoshihiro,¹⁷ Masayo Takahashi,¹⁸ Kiyofumi Mochizuki,¹⁹ Akira Murakami,²⁰ Hiroyuki Kondo,²¹ Susumu Ishida,²² Mitsuru Nakazawa,²³ Tetsuhisa Hatase,²⁴ Tatsuo Matsunaga,²⁵ Akiko Maeda,¹⁸ Kosuke Noda,²² Atsuhiro Tanikawa,⁹ Syuji Yamamoto,²⁶ Hiroyuki Yamamoto,²⁶ Makoto Araie,²⁷ Makoto Aihara,³ Toru Nakazawa,²⁸ Tetsuju Sekiryu,²⁹ Kenji Kashiwagi,³⁰ Kenjiro Kosaki,³¹ Carninci Piero,³² Takeo Fukuchi,³³ Atsushi Hayashi,³⁴ Katsuhiro Hosono,¹⁷ Keisuke Mori,³⁵ Kouji Tanaka,³⁶ Kouiti Furuya,³⁷ Keiichirou Suzuki,³⁷ Ryo Kohata,³ Yasuo Yanagi,³⁸ Yuriko Minegishi,³⁹ Daisuke Iejima,³⁹ Akiko Suga,³⁹ Brian P. Rossmiller,³⁹ Yang Pan,³⁹ Tomoko Oshima,³⁹ Mao Nakayama,³⁹ Megumi Yamamoto,³⁹ Naoko Minematsu,³⁹ Daisuke Mori,⁴⁰ Yusuke Kijima,⁴⁰ Go Mawatari,¹² Kentaro Kurata,¹⁷ Norihiro Yamada,⁴¹ Masayoshi Itoh,⁶ Hideya Kawaji,⁶ and Yasuhiro Murakawa⁴² 

¹Department of Ophthalmology, Teikyo University, Tokyo, Japan 

²Department of Ophthalmology, Saitama Medical University, Saitama, Japan 

³Department of Ophthalmology, The University of Tokyo, Tokyo, Japan 

⁴Department of Ophthalmology, Kindai University Faculty of Medicine, Osaka, Japan 

⁵Department of Translational Medicine Informatics, St. Marianna University School of Medicine, Kawasaki, Japan 

⁶RIKEN Preventive Medicine and Diagnosis Innovation Program, Wako, Japan 

⁷Department of Ophthalmology, Mie University Graduate School of Medicine, Mie, Japan 

⁸National Institutes of Biomedical Innovation, Health and Nutrition, Tsukuba, Japan 

⁹Department of Ophthalmology, Fujita Health University School of Medicine, Toyoake, Japan 

¹⁰Department of Ophthalmology and Visual Science, Chiba University Graduate School of Medicine, Chiba, Japan 

¹¹Department of Ophthalmology, Matsusaka Central General Hospital, Matsusaka, Japan 

¹²Department of Ophthalmology, University of Miyazaki, Miyazaki, Japan 

¹³Saitama Medical Center, Dokkyo Medical University, Koshigaya, Saitama, Japan 

¹⁴Fujita Health University, Banbuntane Hospital, Nagoya, Japan 

¹⁵Department of Ophthalmology, Kobe University Hospital, Kobe, Japan 

¹⁶Osaka University Medical School, Suita, Japan 

¹⁷Hamamatsu University School of Medicine, Hamamatsu, Japan 

¹⁸Riken Center for Developmental Biology, Kobe, Hyogo, Japan 

¹⁹Department of Ophthalmology Gifu University Graduate School of Medicine, Gifu, Japan 

²⁰Department of Ophthalmology, Juntendo University Faculty of Medicine, Tokyo, Japan. 

²¹Department of Ophthalmology, University of Occupational and Environmental Health, Kitakyuusyuu, Japan 

²²Laboratory of Ocular Cell Biology and Visual Science, Department of Ophthalmology, Faculty of Medicine and Graduate School of Medicine, Hokkaido University, Sapporo, Hokkaido, Japan 

²³Hirosaki University Graduate School of Medicine, Hirosaki-shi, Japan 

²⁴Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan 

²⁵Division of Hearing and Balance Research, National Institute of Sensory Organs, National Hospital Organization, Tokyo Medical Center, Tokyo, Japan 

²⁶Hitoshi Ophthalmology Clinic, Nishinomiya, Japan 

²⁷Kanto Central Hospital of the Mutual Aid Association of Public School Teachers, Tokyo, Japan 

²⁸Department of Ophthalmology, Graduate School of Medicine, Tohoku University, Sendai, Japan 

²⁹Department of Ophthalmology, Fukushima Medical University School of Medicine, Fukushima, Japan 

³⁰Department of Ophthalmology, University of Yamanashi, Yamanashi, Japan 

³¹Center for Medical Genetics, Keio University School of Medicine, Tokyo, Japan 

³²Division of Genomic Technologies, Laboratory for Transcriptome Technology, RIKEN Center for Integrative Medical Sciences, Yokohama, Japan 

³³Division of Ophthalmology and Visual Science, Graduate School of Medical and Dental Sciences, Niigata University, Niigata, Japan 

³⁴Department of Ophthalmology, Graduate School of Medicine and Pharmaceutical Sciences, University of Toyama, Toyama, Japan 

³⁵Department of Ophthalmology, International University of Health and Welfare, Nasu-shiobara, Japan 

³⁶Department of Ophthalmology, Nihon University Hospital, Tokyo, Japan 

³⁷Institute for Advanced Co-Creation Studies, Osaka University, Suita, Japan 

³⁸Department of Ophthalmology, Asahikawa Medical University, Asahikawa, Japan 

³⁹Division of Molecular and Cellular Biology, National Institute of Sensory Organs, National Hospital Organization Tokyo Medical Center, Tokyo, Japan 

⁴⁰Graduate School of Agricultural and Life Sciences, The University of Tokyo, Japan 

⁴¹Department of Ophthalmology, Saitama Medical University, Saitama, Japan 

⁴²RIKEN Preventive Medicine and Diagnosis Innovation Program, Yokohama, Japan