Newfoundland rod-cone dystrophy, an early-onset retinal dystrophy, is caused by splice-junction mutations in RLBP1

Abstract

Some isolated populations exhibit an increased prevalence of rare recessive diseases. The island of Newfoundland is a characteristic geographic isolate, settled by a small number of families primarily during the late 1700s and early 1800s. During our studies of this population, we identified a group of families exhibiting a retinal dystrophy reminiscent of retinitis punctata albescens but with a substantially lower age at onset and more-rapid and distinctive progression, a disorder that we termed "Newfoundland rod-cone dystrophy" (NFRCD). The size of one of these families was sufficient to allow us to perform a genomewide screen to map the NFRCD locus. We detected significant linkage to markers on the long arm of chromosome 15, in a region encompassing RLBP1, the gene encoding the cellular retinaldehyde-binding protein. Previously, mutations in RLBP1 have been associated with other retinal dystrophies, leading us to hypothesize that RLBP1 mutations might also cause NFRCD. To test this hypothesis, we sequenced all coding exons and splice junctions of RLBP1. We detected two sequence alterations, each of which is likely to be pathogenic, since each segregates with the disease and is predicted to interfere with mRNA splicing. In contrast to some previously reported RLBP1 mutations, which yield a protein that may retain some residual activity, each NFRCD mutation is likely to give rise to a null allele. This difference may account for the severe phenotype in these families and exemplifies the molecular continuum that underlies clinically distinct but genetically related entities.

Figure 1

Fundus appearance at early, middle, and late stages of NFRCD. The left eyes of NF-001-p09, at age 54 years, exhibiting RPA and scallop-bordered atrophy inferonasally (A and B), of NF-001-p24 at age 34, with RPA (C), of NF-004-p02, at age 66 years, with peripheral lacunar atrophy (D), and of NF-001-p18, at ages 27 years (E) and 46 years (F), showing progression from RPA to advanced atrophy. Also shown is a composite of the fundus of NF-001-p18, at age 27 years (G).

Figure 2

Visual-field defects in NFRCD with Goldmann perimeter: NF-001-p16, at age 26 years, with visual acuity 6/6, showing ring scotoma at 6–25 degrees (a), NF-001-p24, at age 34 years, with visual acuity 6/15, ring scotoma at 1–30 degrees (b), NF-001-p09, at age 54 years, with visual acuity 6/60, showing the central 10 degrees and peripheral islands only (c), and NF-004-p01, at age 57 years, with visual acuity restricted to detection of hand motion, peripheral islands only (d).

Figure 3

Mutation and haplotype data for six Newfoundland pedigrees diagnosed with NFRCD. All individuals were genotyped for D15S205 and D15S127 and were sequenced for RLBP1. Haplotypes were then constructed for D15S205-SpliceMut1-SpliceMut2-D15S127, with polymorphic alleles expressed in terms of base pairs. SpliceMut1 = 324G→A; SpliceMut2 = IVS3+2 T→C. Each colored bar represents a unique haplotype: yellow = 135-324G→A-wt-142; blue = 124-324G→A-wt-134; red = 159-wt-IVS3+2 T→C-136; green = 135-wt- IVS3+2 T→C-140; gray = non–disease-associated haplotypes.

Figure 4

Location of mutations in RLBP1, and pedigrees harboring the mutations. A, Two splice mutations observed at the exon 3/intron 3 junction and predicted to activate a cryptic splice site 6 bp into intron 3. This splice site will create a transcript that, if translated, will produce a protein that terminates early in exon 4. SpliceMut1 = 324G→A; SpliceMut2 = IVS3+2 T→C. B, Subsets of the large pedigree NF-001 and of pedigrees NF-005 and NF-008. Patients NF-001-p01 and NF-001-p02 are homozygous for the 324G→A mutation. NF-001-p38 is homozygous for the IVS3+2 T→C mutation, whereas NF-001-p45 is heterozygous for each mutation. Patients in NF-005 and NF-008 are heterozygous for both alterations.