Retinal expression of the X-linked juvenile retinoschisis (RS1) gene is controlled by an upstream CpG island and two opposing CRX-bound regions

X-linked juvenile retinoschisis (XLRS) is an orphan retinal disease in males caused by mutations in the RS1 gene. Previously we have characterized cone-rod homeobox (CRX)-responsive elements in the promoter region of RS1 driving selective gene expression in the retina. Here, we expanded our identification and functional analysis of cis-regulatory elements controlling quantitative expression of RS1 in vitro and in vivo. Sequence analysis identified a CpG island 3 kb upstream of the transcription start site (TSS). In addition, chromatin immunoprecipitation coupled to microarrays (ChIP-Chip) targeting the retinal transcription factor CRX was performed. Thereby, we identified a second CRX-bound region (CBR2) in the first intron of RS1 which contains six evolutionarily conserved CRX binding motifs. In vitro luciferase reporter gene assays and dsRed reporter electroporation of mouse retinal organ cultures demonstrated a strong constitutive and orientation-independent enhancing effect of the upstream CpG island. The intronic CBR2 potently suppressed CBR1-driven RS1 promoter activity in vitro but failed to regulate a CBR1-reporter in short-term cultured mouse retinae. We conclude that a CpG island enhancer and two CBRs may act in a combinatorial fashion to fine-tune RS1 transcript levels in the retina.

Research highlights► We identified an upstream CpG island in the human retinoschisin (RS1) gene known to be associated with X-linked juvenile retinoschisis. ► We further identified two highly conserved CRX-bound regions (CBRs) at the RS1 gene locus. ► Electroporations of reporters into ex vivo mouse retinal organ cultures were performed and confirmed our in vitro data. ► CBR1 in the proximal promoter drives basic RS1 expression. ► RS1 expression was enhanced by the CpG island and modulated by the intronic CBR2.