Varied clinical presentations of RP1L1 variants in Chinese patients: a study of occult macular dystrophy and vitelliform macular dystrophy

Abstract

Background

Occult Macular Dystrophy (OMD), primarily caused by retinitis pigmentosa 1-like 1 (RP1L1) variants, is a complex retinal disease characterised by progressive vision loss and a normal fundus appearance. This study aims to investigate the diverse phenotypic expressions and genotypic correlations of OMD in Chinese patients, including a rare case of Vitelliform Macular Dystrophy (VMD) associated with RP1L1.

Methods

We analysed seven OMD patients and one VMD patient, all with heterozygous pathogenic RP1L1 variants. Clinical assessments included Best Corrected Visual Acuity (BCVA), visual field testing, Spectral Domain Optical Coherence Tomography (SD-OCT), multifocal Electroretinograms (mfERGs), and microperimetry. Next-generation sequencing was utilised for genetic analysis.

Results

The OMD patients displayed a range of phenotypic variability. Most (5 out of 7) had the RP1L1 variant c.133 C > T; p.R45W, associated with central vision loss and specific patterns in SD-OCT and mfERG. Two patients exhibited different RP1L1 variants (c.3599G > T; p.G1200V and c.2880G > C; p.W960C), presenting milder phenotypes. SD-OCT revealed photoreceptor layer changes, with most patients showing decreased mfERG responses in the central rings. Interestingly, a unique case of VMD linked to the RP1L1 variant was observed, distinct from traditional OMD presentations.

Conclusions

This study highlights the phenotypic diversity within OMD and the broader spectrum of RP1L1-associated macular dystrophies, including a novel association with VMD. The findings emphasise the complexity of RP1L1 variants in determining clinical manifestations, underscoring the need for comprehensive genetic and clinical evaluations in macular dystrophies.

Background

Retinitis pigmentosa 1-like 1 gene (RP1L1, OMIM: 608,581), located on chromosome 8, encodes a component of the photoreceptor axoneme. A 1167-bp region between positions 5968 and 7135 in the RP1L1 gene encodes 25 imperfect copies of a 16-amino acid repeat module. This region has a high GC content of 62%, making it less efficiently amplifiable by polymerase chain reaction (PCR) from both RNA and genomic DNA [1]. As its name suggests, RP1L1 is highly similar to the retinitis pigmentosa 1 (encoded by RP1, OMIM: 603,937) gene. RP1 and RP1L1 are predicted to be integral to the development and maintenance of the outer segment of photoreceptors, playing a pivotal role in transporting materials between the inner and outer segments, thereby maintaining the biochemical homeostasis of the photoreceptor [2, 3]. Pathogenic variants in RP1L1 lead to progressive photoreceptor degenerative disorders like autosomal dominant occult macular dystrophy (OMD, OMIM: 613,587) and autosomal recessive retinitis pigmentosa 88 (RP88, OMIM: 618,826) [4,5,6,7]. OMD typically presents with slowly progressive visual disturbances, a normal-appearing fundus, and selective abnormalities in the photoreceptor layer observable through optical coherence tomography (OCT) [8, 9]. SD-OCT provides valuable in vivo information about the microstructure of photoreceptors in OMD. Two primary pathophysiological types of OMD have been identified [8, 10,11,12]: classical OMD, characterised by both blurred ellipsoid zone (EZ) and absence of the interdigitation zone (IZ) on SD-OCT, and subtle OMD, where at least one of these features is absent. The clinical presentation of OMD can be categorised into three stages based on the degree of IZ and EZ involvement, potentially correlating with the duration of disease onset [13, 14].

OMD was previously considered the only maculopathy caused by RP1L1 deficiency, characterised by a normal fundus. However, recent studies have identified additional phenotypes, such as adult-onset pseudo-vitelliform macular dystrophy (VMD) and conditions resembling Best disease [9, 15]. These discoveries have introduced the term “RP1L1 maculopathy” to describe the broader spectrum of maculopathies associated with RP1L1 variants.

The purpose of this study is to characterise the clinical and molecular genetic features of a cohort with OMD and a VMD caused by heterozygous pathogenic RP1L1 variants in Chinese patients.

Methods

Clinical assessment

Clinical data were collected from patients with variants in the RP1L1 gene. Examinations were conducted after obtaining written informed consent and in accordance with the principles of the Declaration of Helsinki. The procedures used in this study were approved by the local ethics committee of Southwest Hospital, Army Medical University, Chongqing, China (reference number: KY2023007). Detailed medical history and comprehensive ophthalmological examinations were performed, including decimal best-corrected visual acuity (BCVA), color fundus photography, spectral domain optical coherence tomography (SD-OCT, Heidelberg and Zeiss), fundus autofluorescence imaging (FAF; excitation: 488 nm, Spectralis HRA + OCT; Heidelberg Engineering, Dossenheim, Germany), Humphrey’s visual fields (HVFs, 30 − 2) and microperimetry (MP) (MAIA, 4 − 2, Italy and NIDEK, MP3, Japan). We recorded full-field electroretinograms (ffERGs) (Espion E2, Diagnosys LLC, Lowell, MA, USA and MonPack 3, Metrovision, France ) following the standards of the International Society for Clinical Electrophysiology of Vision (ISCEV). Multifocal ERGs (mfERGs) were recorded with a VERIS Science 6.3.2 imaging system (103-hexagon, EDI, San Mateo, CA) in accordance with the ISCEV standard protocol. The mfERGs with stable fixation were selected for further analysis.

Two subtypes of SD-OCT morphologic phenotype were applied to describe the structural changes according to a previous study [8]: (1) severe phenotype (denoted as a classical phenotype compatible with stages IIa, IIb, IIIa and IIIb), showing both blurred/flat EZ and absence of IZ; and (2) mild phenotype (denoted as subtle morphologic changes compatible to stage Ia, Ib), defined as minimal/local blurring of EZ and local absence of IZ.

The P1 amplitude decrease rate was calculated by comparing the amplitude of the P1 component in patients to a normal reference. This calculation was executed using the formula: (Normal P1 - Patient P1) / Normal P1. Values less than 10% were considered negligible and recorded as zero.

Pathogenic variant detection

Peripheral venous blood samples were collected from all subjects and unaffected family members for Genomic DNA extraction using standard procedure. Either eye gene-enriched (97 to 384 IRD genes) panel-based next-generation sequencing (NGS) or whole exome sequencing (WES) was performed. The raw reads were aligned to the human reference genome (GRCh38/hg38) using the Burrows Wheeler Aligner tool. Sequencing data processing and bioinformatics analysis followed the methods of Meng et al. [16]. Sanger sequencing (the primers are listed in Supplementary Table 1) was used to perform co-segregation analysis of candidate pathogenic variants. The pathogenicity of the candidate pathogenic variants were assessed according to the American College of Medical Genetics and Genomics (ACMG) guidelines.

Table 1 Summary of clinical characteristics and RP1L1^# variants in seven OMD patients

Results

Traditional phenotype caused by RP1L1 variant: occult macular dystrophy

We identified seven individuals from five families diagnosed with OMD, all harboring heterozygous pathogenic RP1L1 variants (Fig. 1). These patients exhibited normal fundus appearance but had central scotomas in their visual fields. They ages at the last examination ranged from 5 to 69 years, with onset ages between 0 and 57 years. The median BCVA was 0.7 (range: 0.05 to 0.82) in logMAR units. Genetic testing revealed the heterozygous variant c.133 C > T; p.R45W in five out of seven OMD patients, while two patients had the heterozygous variants c.3599G > T; p.G1200V and c.2880G > C; p.W960C, respectively (Table 1). According to the ACMG guidelines, these three variants were classified as pathogenic, likely pathogenic and pathogenic, respectively (Supplementary Table 2). Patients 6 and 7 exhibited a milder phenotype, as evident in BCVA, mfERGs and microperimetric assessments (Fig. 2).

Fig. 1

a Pedigrees of five families affected by OMD and harboring heterozygous pathogenic RP1L1 variants. Affected individuals are represented by solid squares (men) and circles (women), while unaffected members are indicated with white icons. The proband in each family is identified with an arrow. The presence of wild type is denoted by “-”. Generation numbers are indicated on the left. M1, M2 and M3 correspond to the RP1L1 variants p.R45W, p.G1200V, and p.W960C, respectively. b Illustration of the location of these three variants within the genetic and protein structures of RP1L1.

Fig. 2

Displays SD-OCT, mfERGs, and Microperimetry results for one eye of each of the seven OMD patients, who experienced a symmetrical bilateral decline in visual acuity

SD-OCT showed morphologic changes in the photoreceptor layers. Six out of seven OMD patients demonstrated blurring of the EZ and absence of the IZ in both eyes. Patient 7, however, had a preserved foveal structure with blurred and interrupted EZ and IZ in the parafoveal regions. Other retinal layers, including the outer nuclear layer (ONL) and retinal pigment epithelium (RPE), appeared normal. Patients were classified into SD-OCT stages based on spatial functional and morphological features (Fig. 2).

A decrease in mfERGs across six eccentric rings was observed in both eyes of most patients. Except for Patient 7, all showed significantly lower responses in the central rings (Fig. 3).

Fig. 3

Decline rate in mfERGs responses across six eccentric rings in both eyes of the patients. The left eye of Patient 3 was omitted due to poor fixation. All patients, except for Patient 7, exhibited significantly lower response densities in rings 1 and 2

In terms of preferred retinal loci (PRLs), seven eyes from four OMD patients with the R45W variant were assessed. Two PRLs were located within the central macula, while five were outside. The two with central PRLs exhibited similar mean microperimetry sensitivity (MS) at the fovea and parafovea. Patient 6, with the G1200V variant, showed stable fixation at the fovea with lower MS, while Patient 7, with the W960C variant, displayed normal MS at the macula (Table 2).

Table 2 Microperimetric features of the OMD patients

A special phenotype caused by RP1L1 variant: vitelliform macular dystrophy

Patient 8, a 30-year-old male, experienced progressive central visual loss in both eyes over four years. There were no reports of systemic diseases, trauma, drug abuse, smoking or consanguineous marriage in his family. His BCVA was 0.82 and 1.00 in the right and left eyes, respectively. Fundus imaging showed bright, yellow vitelliform deposits at the macula in both eyes (Fig. 4a). SD-OCT scans indicated abnormal thickening in the foveal region. MfERGs showed a marked decrease in central response, with peripheral functions relatively preserved. EOGs displayed normal Arden ratios, and ffERGs were normal (Supplementary Table 3).

Fig. 4

a The clinical presentation of Patient 8, who exhibited the phenotype of Vitelliform Macular Dystrophy (VMD). b The clinical presentation of Patient 8’s father (Patient 7), who exhibited the phenotype of Occult Macular Dystrophy (OMD).

Patient 7, a 57-year-old, the father of the aforementioned patient, had no visual complaints. His BCVA was 0.05 in both eyes. Fundus images and autofluorescence appeared normal (Fig. 4b). SD-OCT showed a preserved foveal structure with blurred and interrupted EZ and IZ in the parafoveal regions. MfERGs indicated a slight central reduction in function but preserved peripheral function. ffERGs were normal (Supplementary Table 3).

Discussion

The results of this study, contextualised within the current understanding of RP1L1-associated macular dystrophies, underscore the considerable variability and complexity in the clinical presentation and genetic underpinnings of these conditions. Our findings of seven affected individuals from five families, all harboring heterozygous pathogenic RP1L1 variants and presenting with OMD phenotypes (classical and subtle), align with established patterns of RP1L1 variants leading to progressive photoreceptor degeneration [8, 13, 14, 17].

In our cohort, the majority of OMD patients had the RP1L1 variants c.133 C > T; p.R45W. This finding is consistent with previous studies where this variant was identified as a common causative factor in OMD irrespective of ethnic or genetic backgrounds [11, 18,19,20,21,22,23]. This variant was particularly associated with worse BCVA, distinctive SD-OCT and mfERG findings, highlighting its significant clinical impact [14, 19, 23,24,25,26].

Our study further categorises OMD into clinical stages based on photoreceptor structures observed in OCT images. Most patients showed decreased amplitude in the center area observed in mfERGs, indicating disease progression with central involvement. We also analysed microperimetric features in OMD patients, revealing PRLs in most eyes of patients with the R45W variant. Despite relatively normal MS, abnormalities in microperimetry at the fovea and central area were evident, correlating with BCVA, OCT structure, and mfERG findings. Patients with G1200V and p.W960C variants exhibited good fixation, and Patient 7 showed normal MP at the macula, compatible with his good vision, central macular structure, and normal mfERGs. This comprehensive approach underscores the importance of integrating various diagnostic modalities to fully understand the spectrum of RP1L1-associated OMD.

The identification of a patient presenting with VMD due to an RP1L1 variant in our study adds another layer of complexity to the phenotypic spectrum of RP1L1 variants. This is the first case reported in the Chinese population, resonating with recent descriptions of “RP1L1 maculopathy” where visible maculopathy resembling adult-onset pseudovitelliform macular dystrophy or Best disease was observed in two reports [9, 15]. Our study extends this observation, indicating that RP1L1 variants can manifest in a broader array of phenotypes than previously recognised. Interestingly, the patient’s father (Pt. 7), who carries the same RP1L1 variant, showed an OMD phenotype, suggesting a broader spectrum of RP1L1-associated pathologies.

The variability in clinical presentations and the severity of phenotypes observed in our cohort, along with the established understanding that RP1L1 gene expression plays a crucial role in photoreceptor morphogenesis, suggest a complex interplay between genetic factors and phenotypic expression. Our observations align with the largest genetically confirmed East Asian cohort study, which identified three functional phenotypes based on mfERG abnormalities, demonstrating the wide spectrum of functional and structural damage associated with RP1L1 Variants [14, 27]. The study also emphasised the importance of assessing the EZ line in OCT for identifying functional subtypes and disease severity, a finding that resonates with our observations.

In conclusion, our study not only reaffirms the genetic and clinical diversity of RP1L1 maculopathy but also expands the known phenotype spectrum. It reinforces the need for heightened awareness among clinicians and researchers about the broad spectrum of phenotypes associated with RP1L1 variants and the importance of genetic testing for accurate diagnosis and management of these complex conditions. The findings have significant implications for patient management and future research.