Phenotype characterization and transcript analysis in RCBTB1-associated retinopathy

Abstract

Purpose : Variants in the RCC1 and BTB domain-containing protein 1 (RCBTB1) gene have been implicated in a spectrum of inherited retinal diseases (IRDs), whereas haploinsufficiency was reported in Coats’ disease. Herein, we report the phenotype of a 44-year-old Singaporean-Chinese female and her pre-symptomatic sibling, who each possess compound heterozygous mutations in RCBTB1. Variant pathogenicity, with respect to RCBTB1 transcript expression in patient-derived induced pluripotent stem cells (iPSCs) and fibroblasts, was evaluated.

Methods : The natural history was documented by a series of ophthalmic examinations including electroretinography (ERG), fundus autofluorescence (AF) imaging, spectral-domain optical coherence tomography, visual field and microperimetry. Proband DNA was genetically analysed using a 537-gene NGS panel (MVL laboratories, Oregon, USA) independently of whole genome sequencing (WGS; GenomeOne, NSW). Targeted Sanger sequencing was used for confirmation of detected variants and cascade family testing. Expression of RCBTB1 transcripts in iPSCs and fibroblasts, derived from the proband and healthy controls was quantified by qRT-PCR.

Results : Visual acuity decreased from 83 to 72 (OD) and 81 to 73 (OS) over 4 years. Retinal pigment epithelial (RPE) atrophy enlarged by 1.2 (OD) and 1.0 (OS) mm²/year. Total macular volume declined by 0.09 (OD) and 0.12 (OS) mm³/year. Microperimetry demonstrated enlarging scotoma (OS). Speckled AF signals were seen in the posterior pole in both patients. Generalized mild cone dysfunction and maculopathy were noted on full-field and multifocal ERG. Panel testing and WGS independently revealed biallelic frameshifting mutations, c.170del (p. Gly57Glufs*12) and c.707del (p. Asn236Thrfs*11) in RCBTB1, which segregated with disease. Additionally, qRT-PCR of RCBTB1 RNA from patient-derived iPSCs demonstrated an absence of transcript from the c.707del allele, suggesting its degradation in the patient-derived iPSCs. RCBTB1 mRNA expression was significantly decreased in patient-derived iPSCs compared to that in healthy controls, indicating the alternate transcript is most likely subject to nonsense-mediated mRNA decay.

Conclusions : Macular volume change and RPE atrophy expansion rates are feasible endpoints for monitoring RCBTB1-associated retinopathy. We provided further functional evidence of pathogenicity for two disease causing variants using patient-derived iPSCs.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.