Hypomorphic mutations in RCBTB1 cause autosomal recessive isolated and syndromic inherited retinal dystrophy

Abstract

Purpose : We identified a homozygous missense variant in RCBTB1 c.973C>T p.(His325Tyr) in individuals with a syndromic inherited retinal dystrophy (IRD), consisting of retinitis pigmentosa (RP), hypothyroidism, primary ovarian insufficiency, and mild intellectual disability. Here, we aimed to investigate the contribution of RCBTB1 mutations in IRD and to assess the underlying pathogenetic mechanism.

Methods : Whole exome sequencing data of over 1000 IRD patients were inspected for RCBTB1 mutations, and targeted next-generation sequencing of the coding region of RCBTB1 was performed in 280 RD patients. Segregration analysis was performed using Sanger sequencing. Flanking SNPs and microsatellite markers were genotyped for haplotyping. RCBTB1 expression analysis was performed in cDNA from human retina and retinal pigment epithelium (RPE) using qPCR. RCBTB1 immunostaining was performed on human and murine sections. As RCBTB1 has previously been identified as a Cullin3 substrate adaptor, different components of the Cullin3 and Nrf2 pathway were quantified using qPCR.

Results : In six families with isolated and syndromic IRD five distinct homozygous missense variants were found in RCBTB1. All changes segregate with disease, affect highly conserved amino acids and in silico predictions are suggestive for a deleterious effect. Three variants are located in the 6th repeat of the RCC1 domain, while two variants localize in the first BTB domain. A founder haplotype was identified for variant c.919G>A, p.(Val307Met), occurring in two families of Italian and Greek origin, respectively. Ocular phenotypes range from typical RP starting in the second decade to chorioretinal dystrophy with a later age of onset. RCBTB1 mRNA expression was demonstrated in human retina and RPE and protein immunostaining was observed mainly in the inner retina. Different components of the Cullin3 and Nrf2 pathway displayed decreased mRNA expression in patients’ lymphocytes.

Conclusions : Hypomorphic RCBTB1 missense mutations clustering in two protein domains were identified in families with non-syndromic and syndromic IRD respectively, putting forward RCBTB1 as a new IRD disease gene. A founder effect was demonstrated for one RCBTB1 mutation in Mediterranean families. Finally, our data suggest a potential role of the ubiquitination pathway in the pathogenetic mechanism underlying RCBTB1-associated IRD.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.