Whole-exome sequencing identifies mutations in GPR179 leading to autosomal-recessive complete congenital stationary night blindness

Abstract

Congenital stationary night blindness (CSNB) is a heterogeneous retinal disorder characterized by visual impairment under low light conditions. This disorder is due to a signal transmission defect from rod photoreceptors to adjacent bipolar cells in the retina. Two forms can be distinguished clinically, complete CSNB (cCSNB) or incomplete CSNB; the two forms are distinguished on the basis of the affected signaling pathway. Mutations in NYX, GRM6, and TRPM1, expressed in the outer plexiform layer (OPL) lead to disruption of the ON-bipolar cell response and have been seen in patients with cCSNB. Whole-exome sequencing in cCSNB patients lacking mutations in the known genes led to the identification of a homozygous missense mutation (c.1807C>T [p.His603Tyr]) in one consanguineous autosomal-recessive cCSNB family and a homozygous frameshift mutation in GPR179 (c.278delC [p.Pro93Glnfs(∗)57]) in a simplex male cCSNB patient. Additional screening with Sanger sequencing of 40 patients identified three other cCSNB patients harboring additional allelic mutations in GPR179. Although, immunhistological studies revealed Gpr179 in the OPL in wild-type mouse retina, Gpr179 did not colocalize with specific ON-bipolar markers. Interestingly, Gpr179 was highly concentrated in horizontal cells and Müller cell endfeet. The involvement of these cells in cCSNB and the specific function of GPR179 remain to be elucidated.

Figure 1

GPR179 mutations in CSNB. (A) GPR179 structure containing 11 coding exons (NM_001004334.2). Different mutations identified in cCSNB patients are depicted. (B) The specific domains for GPR179 were estimated by a prediction program (UniProtKB/Swiss-Prot).

Figure 2

3D Model of the Transmembrane Region of GPR179 (A) 3D homology model based on the 3D model of the wild-type squid rhodopsin (2ZIY). Wild-type Gly455 and His603 residues are indicated in green and the mutated Asp455 and Tyr603 in orange. (B) Superimposition of 3D models of the wild-type residues and the Gly455Asp alteration (aspartate in orange).

Figure 3

Indirect Expression Analysis of Gpr179 in rd1 and Wild-Type Mice Expression of Gpr179 (1459268_at) compared to the expression of Nyx (1446344_at, a known molecule expressed in the inner nuclear layer, also implicated in cCSNB) during rod degeneration in the rd1 mouse. Neural retinas from rd1 and wild-type mice on identical genetic backgrounds were hybridized to the mouse genome 430 2.0 array (Affymetrix, High Wycombe, UK). The expression profiles are similar from postnatal day (PND) 5 to PND12. Thereafter, the expression of Gpr179, as well as Nyx, increases in the rd1 retina. This phenomenon correlates temporally with the loss of rod photoreceptor cells and is likely due to the unaffected inner retinal cells in the rd1 specimen at this age.

Figure 4

Gpr179 Immunohistochemistry on Retinal Sections of Wild-Type Mice GPR179 signal (green) in the OPL and ILM double labeled with other retinal markers (red) were detected in wild-type mice by confocal microscopy. The scale bar represents 20 μm. The following abbreviations are used: ONL, outer nuclear layer; OPL, outer plexiform layer; INL, inner nuclear layer; GCL, ganglion cell layer; and ILM, inner limiting membrane. Gpr179 did not colocalize with the presynaptic marker Basoon (A), the ON-bipolar cell markers Goα (B) and PKCα (C), or with the ganglion cell labeled with calretinin (D). Some cells in the upper part of INL were surrounded by specific Gpr179 staining (arrow in A). The shape and the localization might indicate that these cells represent horizontal cells. Bipolar cell dendrites, stained with PKCα seem to surround Gpr179 (arrow in C). Calretinin antibody labeled specifically the ganglion cells, and their dendrites did not colocalize with Gpr179 (arrow in D). Instead, it seems that Gpr179 is highly concentrated in Müller cell endfeet (D); similar results have been previously shown for a potassium channel Kir4.1.

Figure 5

Genes Underlying CSNB Different forms of CSNB in human are classified according to their electroretinographic feature, mode of inheritance, clinical phenotype, and mutated genes. Patients discussed herein show a complete Schubert-Bornschein type of ERG. The following abbreviations are used: cCSNB, complete CSNB; icCSNB, incomplete CSNB; ar, autosomal recessive; ad, autosomal dominant. Genes are indicated in italics and underlined. Chromosomal location is given between brackets. The phenotype of patients with mutations in icCSNB is more variable and can even lead to progressive cone or cone-rod dystrophy.