Autosomal Recessive Rod-Cone Dystrophy Associated With Compound Heterozygous Variants in ARL3 Gene

Abstract

Purpose:ARL3 (ADP-ribosylation factor-like 3) variants cause autosomal dominant retinitis pigmentosa (RP) or autosomal recessive Joubert syndrome. We found a family with rod-cone dystrophy (RCD) and verified it was associated with compound heterozygous variants in ARL3 gene.Methods: Ophthalmic examinations including optical coherence tomography and electroretinogram (ERG) were performed. Targeted next generation sequencing (NGS) was performed for the proband using a custom designed panel. Sanger sequencing and co-segregation were conducted in the family members. Changes of protein structure mediated by the variants were studied in vitro. ARL3 protein stability and its interaction with RP2 protein were assessed by cycloheximide chase assay and co-immunoprecipitation (Co-IP) assay.Results: Visual acuity of the 18-year-old male proband was 0.25 in the right and 0.20 in the left eye, while his non-consanguineous parents and sister was normal. The proband showed signs of RCD, including nyctalopia, peripheral field loss, bone-spicule deposits in the retina, and reduced ERG responses. The father, aged 50 years old, showed visual acuity of 1.0 in both eyes. Unlike the proband, he presented late onset and mild cone-rod dystrophy (CRD), including macular atrophy, central scotomata, moderate reduction in photopic ERG responses. None of all the family members had hearing abnormality, mental dysplasia or gait instability. We identified two novel compound heterozygous variants (c.91A>G, p.T31A; c.353G>T, p.C118F) in ARL3 in the proband, while his father only had variant c.91A>G. Bioinformatics analysis indicated amino acid positions of the two variants are highly conserved among species. The in silico tools predicted the variants to be harmful. Protein structure analysis showed the two variants had potential to alter the protein structure. Based on the ACMG guidelines, the two variants were likely pathogenic. In addition, the ARL3 mutations destabilized ARL3 protein, and the mutation c.353G>T disrupted the interaction between ARL3 and RP2 in HEK293T cells.Conclusions: We showed novel compound heterozygous variants in ARL3 were associated with early onset of autosomal recessive RCD, while c.91A>G along may be associated with a late onset of dominant CRD. The two variants in ARL3 could be causative by destabilizing ARL3 protein and impairing its interaction with RP2 protein.

A Novel ARL3 Gene Mutation Associated With Autosomal Dominant Retinal Degeneration

“…Homozygous mutation in ARL3 was identified as a cause of autosomal recessive Joubert syndrome, a neurodevelopmental disorder that may include retinal dystrophy, in two unrelated families (Alkanderi et al, 2018). A role for ARL3 in non-syndromic autosomal recessive IRDs has also been reported (Sheikh et al, 2019;Fu et al, 2021). Two consanguineous Pakistani families with common ancestry both had affected members with a homozygous c.296 > T (p.Arg99Ile) variant (Sheikh et al, 2019).…”

“…Two consanguineous Pakistani families with common ancestry both had affected members with a homozygous c.296 > T (p.Arg99Ile) variant (Sheikh et al, 2019). Novel compound heterozygous ARL3 variants (c.91A > G, p.Thr31Ala; c.353G > T, p.Cys118Phe) were associated with IRD in a Chinese family (Fu et al, 2021). The role of ARL3 in dominant IRDs was first suggested based on the identification of a rare, coding heterozygous p.Tyr90Cys variant in a small family (four available members) of European Caucasian descent (Strom et al, 2016).…”

“…Autosomal recessive IRDs caused by ARL3 mutations have also been described by phenotype (Sheikh et al, 2019;Fu et al, 2021). In two Pakistani families that shared the same homozygous pathogenic ARL3 variant (p.Arg99Ile; Sheikh et al, 2019), there was imaging evidence of maculopathy (whether atrophy or bull's eye changes) in the second to the fourth decades of life and reduced acuities.…”

“…OCTs have abnormalities in outer retinal layers, but there appears to be a well-preserved outer nuclear layer in the scans. The heterozygous father (p.Thr31Ala), the only other family member with a phenotype, is described as having normal visual acuity, a central scotoma, color vision abnormalities, OCT with foveal area preservation but para-and peri-central outer retinal layer abnormalities, and possibly a bull's eye maculopathy ("circular degeneration around macular fovea"; Fu et al, 2021). Both family members seem to show a widespread rod and cone degeneration with different severities.…”

“…Specifically, the c.269A > G (p.Tyr90Cys) variant was determined to be a de novo mutation in two unrelated families, each with two generations of affected members (Strom et al, 2016;Holtan et al, 2019). ARL3 has also been implicated as an autosomal recessive cause of Joubert syndrome and non-syndromic retinal degeneration (Alkanderi et al, 2018;Sheikh et al, 2019;Fu et al, 2021).…”