Intracellular retention of mutant retinoschisin is the pathological mechanism underlying X-linked retinoschisis

X-linked retinoschisis results in visual loss in early life with splitting within the inner retinal layers. Many missense and protein truncating mutations of the causative gene RS1 (encoding retinoschisin) have been identified but disease severity is not mutation-dependent. Retinoschisin is a soluble secretory protein predicted to have a globular conformation. Missense mutations would be expected to interfere with protein folding leading to an abnormal conformation and intracellular retention and elimination. To test this hypothesis we have expressed seven pathological RS1 mutations (L12H, C59S, G70S, R102W, G109R, R141G and R213W) in COS-7 cells and investigated their intracellular processing and transport. Using immunoblotting and confocal fluorescent immunocytochemistry we show normal secretion of WT RS1, but either reduced (C59S and R141G) or absent (L12H, G70S, R102W, G109R and R213W) secretion of mutant RS1 and intracellular retention. In addition, we show that L12H RS1 is degraded by proteasomes and in vitro transcription/translation revealed the defects in both cleavage of its signal peptide and translocation into the endoplasmic reticulum. Our results indicate the pathological basis of RS1 is intracellular retention of the majority of mutant proteins, which may explain why disease severity is not mutation-specific. Furthermore, we have shown that in vitro expression of RS1 may be a useful functional assay to investigate the pathogenicity of sequence changes within the RS1 gene.