Novel mutations in RDH5 cause fundus albipunctatus in two consanguineous Pakistani families

Abstract

Purpose: To identify the underlying genetic causes of fundus albipunctatus (FA), a rare form of congenital stationary night blindness that is characterized by the presence of white dots in the midperiphery of the retina and delayed dark adaptation, in Pakistan.

Methods: Two families with FA were identified by fundus examination, and genome-wide single nucleotide polymorphism genotyping was performed for two individuals from family A and six individuals from family B. Genotyping data were subsequently used to identify the identical homozygous regions present in the affected individuals of both families using the online homozygosity mapping tool Homozygosity Mapper. Candidate genes selected from the homozygous regions were sequenced.

Results: Three identical homozygous regions were identified in affected persons of family A (on chromosomes 8, 10, and 12), whereas a single shared homozygous region on chromosome 12 was found in family B. In both families, the homozygous region on chromosome 12 harbored the retinol dehydrogenase 5 (RDH5) gene, in which mutations are known to be causative of FA. RDH5 sequence analysis revealed a novel five base pair deletion, c.913_917delGTGCT (p.Val305Hisfs*29), in family A, and a novel missense mutation, c.758T>G (p.Met253Arg), in family B.

Conclusions: We identified two novel disease-causing RDH5 mutations in Pakistani families with FA, which will improve diagnosis and genetic counseling, and may even lead to treatment of this disease in these families.

Figure 1

Pedigrees and sequencing results. A: Segregation of the mutation in family A. B: Segregation of the mutation in family B. C and D: Sequence electropherograms of affected individuals carrying homozygous variants (upper panels) and unaffected heterozygous carriers (middle panels) of families A (C) and B (D), along with the results of a control individual (wild-type [wt], lower panels). Arrows point to the probands; individuals tested with single nucleotide polymorphism (SNP) microarrays are indicated with asterisks.

Figure 2

Fundus photographs of affected individuals from both families. A, B: Right and left eye, respectively, of affected individual IV-1 of family A (see arrow, Figure 1A). C, D: Right and left eye, respectively, of affected individual IV-7 of family B (see arrow, Figure 1B). E, F: Right and left eye, respectively, of affected individual VI-2 of family B. G, H: Right and left eye, respectively, of affected individual VI-3 of family B.

Figure 3

Homozygosity mapping results. A: Plot of homozygous regions identified in affected individuals in family A using Homozygosity Mapper analysis. B: Plot of homozygous regions identified in affected individuals in family B using Homozygosity Mapper analysis. The red lines indicate homozygous regions shared by affected individuals in each family. The arrows indicate the homozygous regions that harbor RDH5.

Figure 4

Amino acid conservation of amino acids 245–260 of RDH5 in different species. Gray shading indicates amino acids that are identical to human RDH5 amino acids.

Figure 5

Schematic representation of RDH5 and all mutations thus far published. The membrane-embedded N-terminus consists of 18 amino acids and the ectodomain, present in the lumen of the smooth endoplasmic reticulum (SER), spans amino acids 19–288. A C-terminal membrane-spanning domain encompasses amino acids 289–310, and a small cytosolic tail of eight amino acids resides in the cytosol of retinal pigment epithelium (RPE) cells. Both missense and protein-truncating mutations are distributed across the entire protein. Mutations identified in this study are indicated in red.