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

A novel homozygous missense variant in POC1B causes cone dystrophy in a consanguineous Pakistani family

BACKGROUND

Cone dystrophy is a heterogeneous hereditary retinal disorder with disease symptoms appearing in the late first or early second decades of life.

METHODS

A consanguineous Pakistani family with three affected individuals underwent detailed clinical and genetic investigation.

RESULTS

The proband, a 63-years old male, showed severely reduced day vision, a visual acuity of counting fingers (CF), color vision deficiency, high myopia and photophobia. Fundus images showed bilateral peripapillary atrophy, bilateral dull foveal reflex, tilted disc, tessellated fundus, and hyperfluorescence at the peripheral superior temporal arcade and leak at an early stage. OCT macula and angiography findings suggested traction near the disc in the right eye and sub-retinal fluid at the fovea in the left eye. Retinal layers were normal toward the periphery but disorganized near the disc. Full visual field tests showed bilateral central scotoma, while single visual field tests indicated bilateral generalized depression of the visual field. The proband showed normal bilateral intraocular pressure, normal choroidal vessels, and unremarkable anterior segment. Exome sequencing identified a novel homozygous missense variant (POC1B:NM_172240.3:c.1391T>C;p.L464P) in the proband. Existing evidence supported pathogenicity of the identified variant in the family.

CONCLUSION

In conclusion, we document a first-ever Pakistani family with POC1B-related cone 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.