Elucidation of the molecular mechanism underlying CRB1-associated blindness
04 / 2002 - 09 / 2006
Inherited forms of retinal dystrophy affect approximately 1 in 3000 individuals. The quality of life is particularly impaired in patients with Leber's congenital amaurosis (LCA), who suffer from blindness at birth, and in patients with retinitis pigmentosa (RP), experiencing progressive visual loss starting in young adulthood. We have identified mutations in the CRB1 (Crumbs homolog 1) gene in patients with LCA, in patients with RP type 12, a severe autosomal recessive form of RP, in patients with classic RP, and in RP patients with Coats-type exudative vasculopathy. The preferential expression of CRB1 in the retina and its similarity to Drosophila CRB suggest a role for the CRB1 protein in the regulation and maintenance of cell polarity or plasticity in the retina, in intracellular and extracellular protein-protein interactions, and possibly in signal transduction. There are a number of findings suggesting that human and Drosophila CRB have some evolutionary conserved functions. Both consist of a carboxyterminal 37-amino acid cytoplasmic domain, a transmembrane domain, and a large extracellular part which harbours an evolutionary conserved arrangement of laminin A G-like domains and EGF-like repeats. Drosophila CRB is located at the zonula adherens, a belt-like organiser located at the subapical membranes of ectodermally-derived epithelia. Its carboxy-terminal ERLI motif interacts with Discs lost (Dlt). We found that human homologues of Dlt, INADL and MUPP1, are highly expressed in the retina and that INADL can bind to the cytoplasmic domain of human CRB1 in vitro. Moreover, transgene expression of the transmembrane and cytoplasmic domains of human CRB1 in Drosophila embryos partially rescued the crumbs phenotype. There are as yet no clues for the function of the extracellular part of CRB proteins. In the CRB1 gene, this part contains all mutations identified in patients with inherited retinal dystrophies. Recently, apart from CRB1, several retinal dystrophy genes (e.g. USH2A, CDH23) have been identified that encode transmembrane or extracellular matrix proteins with as yet unknown functions. It can be hypothesized that a number of these proteins directly or indirectly interact and that their impaired function triggers common mechanisms of retinal dystrophy. The long-term goal of this research line is to define these common mechanisms which is a prerequisite to design studies directed towards slowing down or reversing the progression of retinal dystrophy in mouse models, and eventually, in humans. As a first step towards this goal it is the overall aim of this programme to elucidate the function of human CRB1 and to decipher its role in the pathogenesis of retinal dystrophies through the following key objectives: 1) Identification of human CRB1 interacting proteins (CIPs), 2) Determine the role of CRB1 as a regulator of cell polarisation, 3) Study the primary defects leading to retinal dystrophy in Crb1-/- mice, 4) Identify additional retinal dystrophy genes among CIPs.