Molecular Mechanisms Leading to Null-Protein Product from Retinoschisin (RS1) Signal-Sequence Mutants in X-Linked Retinoschisis (XLRS) Disease

Section on Translational Research for Retinal and Macular Degeneration, National Institute on Deafness and Other Communication Disorders, National Institutes of Health (NIH), Bethesda, Maryland 20892, USA.
Human Mutation (Impact Factor: 5.14). 11/2010; 31(11):1251-60. DOI: 10.1002/humu.21350
Source: PubMed


Retinoschisin (RS1) is a cell-surface adhesion molecule expressed by photoreceptor and bipolar cells of the retina. The 24-kDa protein encodes two conserved sequence motifs: the initial signal sequence targets the protein for secretion while the larger discoidin domain is implicated in cell adhesion. RS1 helps to maintain the structural organization of the retinal cell layers and promotes visual signal transduction. RS1 gene mutations cause X-linked retinoschisis disease (XLRS) in males, characterized by early-onset central vision loss. We analyzed the biochemical consequences of several RS1 signal-sequence mutants (c.1A>T, c.35T>A, c.38T>C, and c.52G>A) found in our subjects. Expression analysis in COS-7 cells demonstrates that these mutations affect RS1 biosynthesis and result in an RS1 null phenotype by several different mechanisms. By comparison, discoidin-domain mutations generally lead to nonfunctional conformational variants that remain trapped inside the cell. XLRS disease has a broad heterogeneity in general, but subjects with the RS1 null-protein signal-sequence mutations are on the more severe end of the clinical phenotype. Results from the signal-sequence mutants are discussed in the context of the discoidin-domain mutations, clinical phenotypes, genotype-phenotype correlations, and implications for RS1 gene replacement therapy.

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    • "Two major biochemical signatures emerge from these point mutations: an RS1-null phenotype results from an absolute lack of RS1 protein, whereas others give a signature of functionally incompetent misfolded mutant RS1 molecules. While some misfolded proteins (Leu12His and Leu13Pro) are disposed by the large ATPdependent proteolytic machine involving the 26S proteasome (endoplasmic reticulumassociated degradation), other misfolded mutants (Glu72Lys, Arg102Trp, Arg213Trp, Asn179Asp, and Pro192S) are stabilized in the cell and form high molecular mass aggregates (Vijayasarathy et al. 2010). "
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    ABSTRACT: There is good evidence that retinoschisin (RS1) is one of the key participants in retinal cell adhesion processes controlling the formation of retinal cell layers and mosaics. Loss-of-function mutations in the X-linked retinoschisis (RS1) gene lead to splitting within the retina, a condition known as the X-linked juvenile retinoschisis (XLRS). XLRS causes impairment of visual activity in young males and frequently progresses to even more severe reduction of both central and peripheral vision with age. This perspective reviews progress in the field of RS1 biology and pathophysiology.
    Full-text · Article · Jan 2012 · Advances in Experimental Medicine and Biology
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    • "Two XLRS patients harboring " donor splice " mutation in exon 4, known to produce a null biochemical phenotype, with peripheral and central schisis, subnormal or non-detectable b-wave, and reduced a-wave, whereas another four patients, carrying missense mutations (R200C and R102W) resulted in more preserved ERG b-wave and almost normal retinal periphery with slight macular changes were reported (Bradshaw et al., 1999). We recently described (Vijayasarathy et al., 2010) a considerable difference between the severe clinical phenotype in subjects harboring RS1 null-protein signalsequence mutations and the less compromised retinal function in XLRS patients carrying missense mutation. These findings suggest that mutations, like deletions and frame shifts that putatively cause protein truncation, result in greater clinical severity of the disease, even if there is no univocal classification system for XLRS. "

    Full-text · Chapter · Aug 2011
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    ABSTRACT: Mutations in the RS1 gene that encodes the discoidin domain containing retinoschisin cause X-linked juvenile retinoschisis (XLRS), a common macular degeneration in males. Disorganization of retinal layers and electroretinogram abnormalities are hallmarks of the disease and are also found in mice deficient for the orthologous murine protein, indicating that retinoschisin is important for the maintenance of retinal cell integrity. Upon secretion, retinoschisin associates with plasma membranes of photoreceptor and bipolar cells, although the components by which the protein is linked to membranes in vivo are still unclear. Here, we show that retinoschisin fails to bind to phospholipids or unilamellar lipid vesicles. A recent proteomic approach identified the Na/K-ATPase subunits ATP1A3 and ATP1B2 as binding partners of retinoschisin. We analyzed mice deficient for retinoschisin (Rs1h(-/Y)) and ATP1B2 (Atp1b2(-/-)) to characterize the role of Na/K-ATPase interaction in the organization of retinoschisin on cellular membranes. We demonstrate that both the Na/K-ATPase and retinoschisin are significantly reduced in Atp1b2(-/-) retinas, suggesting that retinoschisin membrane association is severely impaired in the absence of ATP1A3 and ATP1B2 subunits. Conversely, the presence of ATP1A3 and ATP1B2 are obligatory for binding of exogenously applied retinoschisin to crude membranes. Also, co-expression of ATP1A3 and ATP1B2 is required for retinoschisin binding to intact Hek293 cells. Taken together, our data support a predominant role of Na/K-ATPase in anchoring retinoschisin to retinal cell surfaces. Furthermore, altered localization of ATP1A3 and ATP1B2 is a notable consequence of retinoschisin deficiency and thus may be an important downstream aspect of cellular pathology in XLRS.
    Full-text · Article · Mar 2011 · Human Molecular Genetics
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