A novel homozygous R764H mutation in crumbs homolog 1 causes autosomal recessive retinitis pigmentosa

Abstract

Purpose: Retinitis pigmentosa (RP; MIM 268000) is a hereditary disease characterized by poor night vision and progressive loss of photoreceptors, eventually leading to blindness. This degenerative process primarily affects peripheral vision due to the loss of rods. Autosomal recessive RP (arRP) is clinically and genetically heterogeneous. It has been associated with mutations in different genes, including CRB1 (crumbs homolog 1). The aim of this study was to determine the causative gene in a Tunisian patient with arRP born to non-consanguineous parents.

Methods: Four accessible family members were included. They underwent full ophthalmic examination with best-corrected Snellen visual acuity, fundus photography and fluorescein angiography. Haplotype analysis was used to evaluate homozygosity in the family to 20 arRP loci. All exons and intron-exon junctions of candidate genes not excluded by haplotype analysis were PCR amplified and directly sequenced.

Results: The proband was a 43-year-old female patient. Best-corrected visual acuity was 20/63 (right eye) and 20/80 (left eye). Visual loss began during the third decade. Funduscopic examination and fluorescein angiography revealed typical advanced RP changes with bone spicule-like pigment deposits in the posterior pole and the midperiphery along with retinal atrophy, narrowing of the vessels, and waxy optic discs. Haplotype analysis revealed homozygosity with microsatellite markers D1S412 and D1S413 on chromosome 1q31.3. These markers flanked CRB1. Our results excluded linkage of all the other arRP loci/genes tested. Sequencing of the 12 coding exons and splice sites of CRB1 disclosed a homozygous missense mutation in exon 7 at nucleotide c. 2291G>A, resulting in an arginine to histidine substitution (p.R764H).

Conclusions: R764H is a novel mutation associated with CRB1-related arRP. Previously, an R764C mutation was reported. Extending the mutation spectrum of CRB1 with additional families is important for genotype-phenotype correlations and characterization of the scope of mutation.

Figure 1

Pedigree structure of the investigated family. Squares and circles represent men and women, respectively. The arrow points to the proband (II:1) affected with retinitis pigmentosa.

Figure 2

Color fundus photographs of the proband at 43 years of age. Left (A) and right (B) eyes showing typical advanced retinitis pigmentosa changes with bone spicule-like pigment deposits in the posterior pole and midperiphery along with retinal atrophy, narrowing of the vessels, and waxy optic discs.

Figure 3

Partial electropherogram of exon 7 of CRB1. A: Electropherogram of the proband showing a c. 2291 homozygous G to A transition mutation in exon 7 of the crumbs homolog 1 gene leading to a p.Arg764His amino acid change. B: Normal sequence. Arrows indicate the position of the mutated nucleotide.

Figure 4

Alignment of various CRB1 orthologs. Human R764 is in bold. Conserved amino acid are replaced by a dot.

Figure 5

Schematic representation of the crumbs homolog 1 gene with retinitis pigmentosa–causing mutations. The reported mutation is in bold.