Dominant mutations in RP1L1 are responsible for occult macular dystrophy

Abstract

Occult macular dystrophy (OMD) is an inherited macular dystrophy characterized by progressive loss of macular function but normal ophthalmoscopic appearance. Typical OMD is characterized by a central cone dysfunction leading to a loss of vision despite normal ophthalmoscopic appearance, normal fluorescein angiography, and normal full-field electroretinogram (ERGs), but the amplitudes of the focal macular ERGs and multifocal ERGs are significantly reduced at the central retina. Linkage analysis of two OMD families was performed by the SNP High Throughput Linkage analysis system (SNP HiTLink), localizing the disease locus to chromosome 8p22-p23. Among the 128 genes in the linkage region, 22 genes were expressed in the retina, and four candidate genes were selected. No mutations were found in the first three candidate genes, methionine sulfoxide reductase A (MSRA), GATA binding 4 (GATA4), and pericentriolar material 1 (PCM1). However, amino acid substitution of p.Arg45Trp in retinitis pigmentosa 1-like 1 (RP1L1) was found in three OMD families and p.Trp960Arg in a remaining OMD family. These two mutations were detected in all affected individuals but in none of the 876 controls. Immunohistochemistry of RP1L1 in the retina section of cynomolgus monkey revealed expression in the rod and cone photoreceptor, supporting a role of RP1L1 in the photoreceptors that, when disrupted by mutation, leads to OMD. Identification of RP1L1 mutations as causative for OMD has potentially broader implications for understanding the differential cone photoreceptor functions in the fovea and the peripheral retina.

Figure 1

Autosomal OMD Families and DNA Sequencing of RP1L1 (A) The four families shown demonstrate dominant inheritance of the OMD phenotype. In all presented families, none of the patients had ocular diseases other than OMD, except senile cataract and diabetic retinopathy. Control family members were confirmed to be normal via a complete ophthalmic examination including focal macular ERGs or multifocal ERGs. (B) DNA sequencing of both p.Arg45Trp and p.Trp960Arg mutations found in four independent families.

Figure 2

Linkage Analysis and Haplotype Analysis of Family 1 (A) Parametric multipoint linkage analysis of family 1. Horizontal axis indicates cumulative position (cM) from the short arm of chromosome 1. As a result of computational capacity, family 1 was divided into two branches for calculation of LOD scores. No other chromosomes except chromosome 8 yielded a positive LOD score. (B) Parametric multipoint linkage analysis of family 1 and mutations in RP1L1. A maximum LOD score of 3.77 was obtained at 8p32.1-8p22. A haplotype bounded by rs365309 (physical position: 9,064,350 in the hg18 assembly of the UCSC Genome Browser) and rs2632841 (18,716,063) was shared by all affected individuals. Horizontal axis indicates the position (cM) on the short arm of chromosome 8. Vertical axis indicates the parametric multipoint LOD score. Mutations (p.Arg45Trp and p.Trp960Arg) are demonstrated.

Figure 3

Immunohistochemistry of RP1L1 in the Cynomolgus Monkey Retina Localization of RP1L1 in the rod and cone photoreceptors in the Cynomolgus monkey (Macaca fascicularis). Retina labeled with anti-human RP1L1 (red, top); same section labeled with retinal cone specific marker, peanut agglutinin lectin (PNA, green, middle); merged image (bottom). Yellow signal present in cone photoreceptor resulted from combination of the red signal of RP1L1 and the green signal of PNA. Cell nuclei were stained with DAPI (blue). PL, photoreceptor layer; ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; IPL, inner plexiform layer; GCL, ganglion cell layer, Scale bars represent 20 μm.