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Kristof Van Schil, Marcus Karlstetter, Alexander Aslanidis, Bart Peter Leroy, Frauke Coppieters, Fanny Depasse, Thomas Langmann, Elfride De Baere; Recessive RHO mutation E150K and SAMD7 regulatory variants in a consanguineous family with retinitis pigmentosa . Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):2876.
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© ARVO (1962-2015); The Authors (2016-present)
To identify the genetic cause of a retinitis pigmentosa phenotype observed in a Turkish consanguineous family.
The family consists of five sibs, two of which are affected. All family members underwent homozygosity mapping using the HumanCytoSNP-12 BeadChips (Illumina). Subsequent candidate gene screening was performed by Sanger sequencing. Functional analysis of non-coding SAMD7 variants was performed by luciferase assays in HEK293 cells and electroporation assays in mouse retinal explants with SAMD7 CBR-reported constructs (Hlawatsch et al. 2013).
Homozygosity mapping revealed several large homozygous regions, in which two interesting candidate genes were located, namely RHO and SAMD7. A homozygous RHO mutation (c.448G>A, p.E150K) was found in the two affected sibs, while all other sibs were heterozygous carriers.<br /> No coding SAMD7 mutations were found. Interestingly, sequencing of the SAMD7 promoter and an enhancer region in the two affected sibs revealed four homozygous variants located in the binding regions of the cone-rod homeobox (CRX) transcription factor. The variants are known SNPs, with a low minor allele frequency of 1,6 %. The first three SNPs are located in a CRX-binding region called CBR1, while the fourth SNP is located in CBR2. A potential regulatory effect of these SNPs on SAMD7 expression was assessed by in vitro luciferase assays in HEK293 cells and electroporation assays in mouse retinal explants, using constructs with either CBR1 or CBR2 apart, or in tandem. The combined CBR1/CBR2 mutated construct showed a significantly decreased SAMD7 reporter activity compared with the wild type (WT) CBR1/CBR2 construct, both in the cellular luciferase and mouse electroporation assays respectively.
A rare recessive RHO mutation (E150K) was found in RP patients from consanguineous origin, consistent with previous reports (Kumaramanickavel et al., 1994; Azam et al., 2009 and Zhang et al., 2013). Moreover, functional analysis of four variants located in non-coding, CRX-binding regions of SAMD7, suggested a regulatory and synergistic effect of these upstream SNPs on SAMD7 expression. As Samd7 has recently been identified as a novel Crx-regulated transcriptional repressor in retina (Hlawatsch et al., 2013), we hypothesize that these SAMD7 variants might have a modifying effect on the retinal phenotype observed in this family.
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