A homozygous POC1B variant causes recessive cone-rod dystrophy

Phenotypic and genotypic features of POC1B-associated cone dystrophy

PURPOSE

Patients with cone dystrophy (CD) can present with virtually normal retinal appearance, which may delay diagnosis. This study describes the inconspicuous clinical features of POC1B-associated CD in two Saudi families.

METHODS

This is a retrospective case study. Clinical data analyzed included multimodal retinal imaging and electroretinography of the affected individuals. Genetic analysis was done for all probands.

RESULTS

Three affected males from two Saudi families with POC1B-associated CD were included. The ages at presentation ranged from 18 to 34 years. Ophthalmic examination showed decreased Snellen visual acuities (range: 20/100-20/300) and color vision bilaterally. Fundus examination showed only mild vascular attenuation. Macular optical coherence tomography showed reduced reflectivity of the external limiting membrane, ellipsoid, and interdigitation zones. Full-field electroretinography demonstrated undetectable light-adapted responses and normal dark-adapted responses in all patients. Next-generation sequencing showed one proband to be homozygous for a previously unpublished nonsense variant in POC1B (NM_172240):c.672C>G; p(Tyr224*). Whole exome sequencing for the second proband showed a novel homozygous frameshifting variant in POC1B: c.991del; p(Arg331Glufs*13).

CONCLUSION

We described two novel variants in POC1B and the associated subtle, yet significant retinal features. POC1B-associated CD is a rare cause of visual loss in patients with relatively normal fundus appearance. Deep phenotyping is necessary in formulating appropriate differential diagnosis.

A mild form of POC1B-associated retinal dystrophy with relatively preserved cone system function

PURPOSE

Biallelic variants in POC1B are rare causes of autosomal recessive cone dystrophy associated with generalized cone system dysfunction. In this report, we describe the clinical characteristics of a Japanese male patient with POC1B-associated retinopathy with relatively preserved cone system function.

METHODS

We performed whole-exome sequencing (WES) to identify the disease-causing variants and a comprehensive ophthalmic examination, including full-field and multifocal electroretinography (ffERG and mfERG).

RESULTS

Our WES analysis identified novel compound heterozygous POC1B variants (p.Arg106Gln and p.Arg452Ter) in the patient. His unaffected mother carried the p.Arg452Ter variant heterozygously. The patient experienced decreased visual acuity in his 50s. At the age of 63, his corrected visual acuity was 20/22 in the right and 20/20 in the left eye. Fundus and fundus autofluorescence images for each eye showed no remarkable finding, except for a subtle hyperautofluorescent spot in the fovea of the left eye. Cross-sectional optical coherence tomography demonstrated blurred but a relatively preserved ellipsoid zone. The ffERG showed that amplitudes of rod and standard-flash responses were within the reference range, whereas the cone and light-adapted 30-Hz flicker amplitudes were close to, or slightly below, the reference range. The mfERG revealed substantially reduced responses with relative preservation of central function.

CONCLUSIONS

We reported the case of an older patient with POC1B-associated retinopathy, demonstrating late-onset visual decrease, good visual acuity, and relatively preserved cone system function. The disease condition was much milder than previously reported in patients with POC1B-associated retinopathy.

Seroreactivity against retinal proteins in a case of POC1B gene associated cone dystrophy with normal funduscopic appearance: a systematic approach to diagnosis

PURPOSE

To report a case of cone dystrophy, associated with autosomal recessive homozygote POC1B gene variant, mimicking autoimmune retinopathy.

CASE

A 45-year-old female presented with a complaint of decreased vision in both eyes. Her best corrected visual acuity was 20/32 in the right eye and 20/50 in the left eye. Anterior segment and dilated fundus examinations were unremarkable. Spectral domain optical coherence tomography showed a subfoveal blurred dome-shaped ellipsoid zone and an extinguished interdigitation zone affecting the entire macula. Full field electroretinography revealed reduced cone responses. The differential diagnosis included inflammatory chorioretinopathies, autoimmune retinopathies (paraneoplastic or nonparaneoplastic), and hereditary retinal dystrophies. No remarkable finding was observed on combined fluorescein and indocyanine green angiographies. Paraneoplastic autoimmune antibody panel revealed nothing; however, aldolase, enolase, pyruvate kinase M2, and glyceraldehyde-3-phosphate dehydrogenase antibodies were positive on autoimmune retinopathy panel. To exclude hereditary retinal dystrophies, whole-exome sequencing (WES) was applied. WES identified an autosomal recessive homozygote POC1B gene variant (c.680A>G, p.His227Arg). Cone dystrophy diagnosis was given.

CONCLUSION

Cone dystrophy associated with POC1B gene variant may present without visible fundus abnormalities. It should be kept in mind that retinal autoantibodies may be positive in such a hereditary dystrophy case due to long-term exposure of the immune system to self-antigens. Therefore, autoimmune retinopathy is a diagnosis of exclusion and should not be diagnosed until all other causes, including hereditary dystrophies, have been ruled out.

Clinical Characteristics of POC1B-Associated Retinopathy and Assignment of Pathogenicity to Novel Deep Intronic and Non-Canonical Splice Site Variants

Mutations in POC1B are a rare cause of inherited retinal degeneration. In this study, we present a thorough phenotypic and genotypic characterization of three individuals harboring putatively pathogenic variants in the POC1B gene. All patients displayed a similar, slowly progressive retinopathy (cone dystrophy or cone-rod dystrophy) with normal funduscopy but disrupted outer retinal layers on optical coherence tomography and variable age of onset. Other symptoms were decreased visual acuity and photophobia. Whole genome sequencing revealed a novel homozygous frameshift variant in one patient. Another patient was shown to harbor a novel deep intronic variant in compound heterozygous state with a previously reported canonical splice site variant. The third patient showed a novel nonsense variant and a novel non-canonical splice site variant. We aimed to validate the effect of the deep intronic variant and the non-canonical splice site variant by means of in vitro splice assays. In addition, direct RNA analysis was performed in one patient. Splicing analysis revealed that the non-canonical splice site variant c.561-3T>C leads to exon skipping while the novel deep intronic variant c.1033-327T>A causes pseudoexon activation. Our data expand the genetic landscape of POC1B mutations and confirm the benefit of genome sequencing in combination with downstream functional validation using minigene assays for the analysis of putative splice variants. In addition, we provide clinical multimodal phenotyping of the affected individuals.