Mutations in a human homologue of Drosophila crumbs cause retinitis pigmentosa (RP12)

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Retinitis pigmentosa (RP) comprises a clinically and genetically heterogeneous group of diseases that afflicts approximately 1.5 million people worldwide. Affected individuals suffer from a progressive degeneration of the photoreceptors, eventually resulting in severe visual impairment. To isolate candidate genes for chorioretinal diseases, we cloned cDNAs specifically or preferentially expressed in the human retina and the retinal pigment epithelium (RPE) through a novel suppression subtractive hybridization (SSH) method1,2. One of these cDNAs (RET3C11) mapped to chromosome 1q31–q32.1, a region harbouring a gene involved in a severe form of autosomal recessive RP characterized by a typical preservation of the para-arteriolar RPE (RP12; ref. 3). The full-length cDNA encodes an extracellular protein with 19 EGF-like domains, 3 laminin A G-like domains and a C-type lectin domain. This protein is homologous to the Drosophila melanogaster protein crumbs (CRB), and denoted CRB1 (crumbs homologue 1). In ten unrelated RP patients with preserved para-arteriolar RPE, we identified a homozygous AluY insertion disrupting the ORF, five homozygous missense mutations and four compound heterozygous mutations in CRB1. The similarity to CRB suggests a role for CRB1 in cell-cell interaction and possibly in the maintenance of cell polarity in the retina. The distinct RPE abnormalities observed in RP12 patients suggest that CRB1 mutations trigger a novel mechanism of photoreceptor degeneration.

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Figure 1: Chromosomal map of the critical chromosomal region for RP12 and the structure of CRB1 and protein product.
Figure 2: Expression of CRB1 in human tissues and RPE cell lines.
Figure 3: Homozygous insertion of an Alu repetitive element in exon 7 of CRB1 in patient 25977.
Figure 4: Homozygous Met1041Thr missense mutation found in patient 22147.

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We thank R. Roepman and H. Yntema for their help with northern-blot analysis and cDNA library screening; M. van Schooneveld and D. Bessant for ascertaining patients with RP12; and P. de Jong for his continuous interest in this project. This work was supported by The Foundation Fighting Blindness, Inc., USA and The British Retinitis Pigmentosa Society.

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Correspondence to Frans P.M. Cremers.

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