CRB1-associated diseases
The CRB1 gene was initially found to be defective in a specific clinical subtype of autosomal recessively inherited RP, RP12 (also denoted RP with preserved para-arteriolar retinal pigment epithelium - PPRPE; den Hollander et al. 1999). Patients with RP12 have a severe form of RP and can be identified by expert ophthalmologists based on a typical morphological sparing of the RPE underlying retinal blood vessels (Figure 1B). The reason for this sparing is as yet unknown.
Two years later it was also found that CRB1 mutations are often found in patients with RP and a specific vascular abnormality, named 'Coats-like exudative vasculopathy' (Figure 1C; den Hollander et al. 2001). In these patients, lipid-like substances leak from retinal vessels and infiltrate between the neural retina and the RPE, leading to retinal detachment. In severe cases, a pseudotumor is formed which sometimes can be so painful that the eye is completely removed.
In 2001 it was also found that mutations in CRB1 can cause congenital blindness (Leber congenital amaurosis - LCA; den Hollander et al. 2001; Lotery et al. 2001a) and RP without PPRPE or Coats'complication (Lotery et al. 2001b). Several studies have revealed that mutations in CRB1 underlie approximately 10% of cases with congenital blindness (Figure 1D).
Recently, in one particular family with 6 patients suffering from pigmented paravenous chorioretinal atrophy in two generations, an apparently dominantly inherited CRB1 mutation (Val162Met) was identified (McKay et al. 2005).
Figure 1: Fundus pictures of: A. Healthy individual; B. Patient with RP and preservation of the para-arteriolar retinal pigment epithelium (PPRPE); C. Patient with RP and Coats' complication, and D. Patient with congenital blindness. A. Normal fundus has an orange-yellow appearance due to the colour of the RPE and choroid. Retinal vessels can be seen in the otherwise transparent neural retina. B. Part of the fundus of a patient with RP12. Note the conspicuous sparing of the RPE underlying retinal blood vessels (arrows). C. In an RP-Coats patient, lipids can be observed near retinal vessels (white arrows) which often lead to detachment of the neural retina from the RPE. D. The patient with congenital blindness with CRB1 mutations shows a macular coloboma, which is uncommon in this kind of patients, and a papillary oedema (right side).
Mutation spectrum of CRB1
Except for one family, mutations in CRB1 have been found in families which display an autosomal recessive type of inheritance. This means that in the majority of the cases, patients are only observed in one generation of a family. Parents of a patient each carry one defective CRB1 gene and one normal gene (Figure 2). Affected children will carry two defective CRB1 genes. Not affected children will have inherited one or no defective CRB1 gene from their parents. Children with one mutation are carriers of a mutation but otherwise healthy. In a family in which a child carries two CRB1 mutations, the risk for another child to inherit the two defective CRB1 genes is 25%. The chance that a child will inherit one defective CRB1 gene is 50% and the chance that none of the defective CRB1 genes are inherited is 25%.
Figure 2: Inheritance of autosomal recessive gene defects. Both parents with normal vision carry 1 defect (green bars). The son at the left side inherited from both parents a CRB1 gene defect and has vision impairment. The daugther and son in the 2nd and 3rd positions from the left each inherited one CRB1 gene defect. They are carriers of the defect but unaffected. The daughter at the right side is unaffected and carries no gene defects.
In figure 3, the structure of the CRB1 gene is depicted. The CRB1 gene consists of 12 different building blocks (called exons) and is situated on the long arm of chromosome 1. The CRB1 protein consists of five different types of protein domains which are depicted with different colours. At the beginning of the protein, there is a signal peptide which is a recognition signal allowing the CRB1 protein to be transported through the cell membrane. The largest part of the CRB1 protein consists of 19 EGF-like motifs (in red) and 3 laminin A G-like domains (in blue). At the end of the protein there is a transmembrane domain and a cytoplasmic domain (in yellow). The cytoplasmic domain is very small (37 amino acids), but serves an important function in that it acts as an anchor point for several proteins inside the photoreceptor cells. In the past research on the cellular function of the fruitfly homologue of CRB1 protein, Crumbs, was focussed almost exclusively on the small cytoplasmic domain.
Figure 3: Schematic representation of the CRB1 gene and its protein domain structure. Some of the mutations identified in patients with an autosomal recessive retinal disease (RP + PPRPE, RP + Coats, LCA - congenital blindness; each patient two mutations) and an autosomal dominant retinal disease (PPRCA; patient with one, apparently dominant, mutation) are depicted.
Mutations in human patients with eye diseases have been found in all parts of the gene but most of them are localised in the 2nd and 3rd laminin A G-like domains. At the moment, 71 different mutations have been found in 92 patients with autosomal recessive retinal diseases (den Hollander et al. 2004). Figure 3 illustrates the mutations found in a few patients with different retinal diseases. How can different combinations of mutations result in these different diseases? We believe that different CRB1 mutations do not have an equally severe effect on CRB1 function. We hypothesize that the mutations identified in patients with Leber congenital amaurosis (e.g. Lys801Ter) result in a complete inactivation of the corresponding mutant protein. Patients with RP + PPRPE and RP with Coats' complication can also carry severe mutations but never in both CRB1 copies. Hence, there is a residual function of the resulting CRB1 protein which leads to a somewhat milder retinal dystrophy. In addition, the function of the CRB1 protein and severity of disease is likely to be influenced by the genetic background.