Null Retinoschisin-Protein Expression from anRS1c354del1-ins18 Mutation Causing Progressive and Severe XLRS in a Cross-Sectional Family Study

Abstract

Purpose Explore the retinoschisin (RS1) protein biochemical phenotype from an RS1 exon-5 deletion/insertion frame-shift mutation in an X-linked retinoschisis (XLRS) family and describe the clinical and electrophysiological features. Methods Six XLRS males underwent ophthalmologic examination and electroretinogram (ERG) recording. The RS1 gene was sequenced. Mutant RS1-RNA and protein expression were assessed by transfecting COS-7 cells with minigene constructs. Results All six males carried the RS1 c354del1-ins18 mutation in which an 18-bp insertion replaced nucleotide 354, duplicating the adjacent upstream intron-4-to-exon-5 junction and causing a premature termination codon downstream. Analysis indicated normal pre-mRNA splicing producing mRNA transcripts. Truncated RS1 protein was expressed transiently but was degraded rapidly by a proteasomal pathway rather than by nonsense mediated mRNA decay. Two boys, 1.5 and 5 years old (y/o), had foveal cysts and minimal peripheral schisis, and retained near-normal scotopic b-wave amplitude and normal ERG waveforms. The 5 y/o's ERG was reduced when repeated three years later. Four older XLRS relatives 32-45 y/o had substantial b-wave loss and strongly “electronegative” ERGs; three had overt macular atrophy. Cross-sectional family analysis showed the b/a-wave amplitude ratio as inversely related to age in the six males. Conclusions The c354del1-ins18 mutation causes an RS1 null biochemical phenotype and a progressive clinical phenotype in a 5-y/o male, while the older XLRS relatives had macular atrophy and marked ERG changes. The phenotypic heterogeneity with age by cross-sectional study of this family mutation argues that XLRS disease is not stationary and raises questions regarding factors involved in progression.

Synaptic pathology and therapeutic repair in adult retinoschisis mouse by AAV-RS1 transfer

“…These conditions are collectively termed CSNB and have a characteristic "electronegative" ERG waveform that retains the initial negative-going a-wave (from rod photoreceptor signaling), but lacks the following positive-going b-wave (involving postsynaptic DBC activity). However, we also note that none of the CSNB gene mutants and KO models of synaptic pathology in Table 1 exhibit a pattern identical to that found in Rs1-KO, which may reflect the difference between a primary in Rs1-KO mice (42) and in XLRS patients (43). Our results support the notion that RS1 plays an important role in maintaining the integrity of the photoreceptor-bipolar synapse.…”

“…This corresponds well with progressive failure of ERG b-wave generation in XLRS disease (42,43). RGS7, RGS11, and Gβ5 serve to accelerate GTP hydrolysis by Gαo and thereby set the time course of synaptic signaling.…”

Molecular Modeling of Protein Structure, Biology of Disease and Clinical Electroretinography in Human X-Linked Retinoschisis (XLRS)

“…A substantial change from the "cystic" (< 25 years old) to the "atrophic" (> 25 years old) pattern of the retina with age was recently described (Lesch et al, 2008). We also showed (Vijayasarathy et al, 2009) worsening of retinal function by age in a family harboring a null mutation in exon 5 of the RS1 gene indicating disease progression with age. In this study, the two younger males, ages 1.5 and 5 years, had well preserved b-waves extending above the pre-flash baseline and nearly normal ERG a-wave and b-wave configuration with a classical XLRS fundus appearance.…”

“…The identification of Ltype voltage-gated calcium channels (Shi et al, 2009), Na/K ATPase (alpha3, beta2 isoforms) (Friedrich et al), the sterile alpha and TIR motif-containing protein SARM1 (Molday, 2007), and phospholipids/Ca 2+ (Vijayasarathy et al, 2007) as potential RS1 interacting ligands corroborate a role for RS1 in cell signaling events as well. However, ERG show considerable variation in this disease and ophthalmologists have to be aware about the possibility of relative ERG b-wave preservation especially in presence of atypical or mild manifestation of the disease (Eksandh et al, 2005;Sieving et al, 1999a;Vijayasarathy et al, 2009). These observations may imply that the degree of ERG dysfunctions may not correlate with the extent of fundus involvement and that synaptic integrity might be impaired secondarily over a greater extent of the fundus.…”

“…Several reports based on visual acuity loss (Apushkin et al, 2005), fundus changes with schisis cavities modification (Lesch et al, 2008), and retinal function deterioration Vijayasarathy et al, 2009) have focused on the "progressive" nature of XLRS disease. In detail, visual acuity is mostly impaired starting in grade school (Apushkin et al, 2005) due to macular schisis.…”