The Enigma of CRB1 and CRB1 Retinopathies
- 1.1k Downloads
Mutations in the gene Crumbs homolog 1 (CRB1) are responsible for several retinopathies that are diverse in severity and phenotype. Thus, there is considerable incentive to determine how disruption of this gene causes disease. Progress on this front will aid in developing molecular diagnostics that can predict disease severity with the ultimate goal of developing therapies for CRB1 retinopathies via gene replacement. The purpose of this review is to summarize what is known regarding CRB1 and highlights information outstanding. Doing so will provide a framework toward a thorough understanding of CRB1 at the molecular and protein level with the ultimate goal of deciphering how it contributes to the disease.
KeywordsCrb1 Leber congenital amaurosis Rd8 Retinitis pigmentosa Photoreceptor Crumbs Muller glia
41.1 Mutations in CRB1 Cause a Spectrum of Retinopathies
Retinopathies attributed to the loss of function of the gene CRB1 are diverse in both age of onset and retinal pathology. Roughly 200 disease-causing mutations in CRB1 account for various retinopathies having an autosomal recessive inheritance pattern (Talib et al. 2017). The most aggressive of these retinopathies is Leber congenital amaurosis (LCA), which is hallmarked by blindness at birth or early infancy accompanied by severe perturbations of retinal cell layering (Jacobson et al. 2003; Aleman et al. 2011). Mutations in CRB1 are the second leading cause for LCA and account for ~10–13% of cases or 10,000 people worldwide (den Hollander et al. 2008; Bujakowska et al. 2012; Alves et al. 2014). Mutations in CRB1 also cause early- and adult-onset retinitis pigmentosa (RP), accounting for ~3% of cases or 70,000 patients worldwide (Bujakowska et al. 2012; Alves et al. 2014). Other retinopathies attributed to CRB1 include RP with preserved para-arteriole retinal pigment epithelium (PPRPE) (den Hollander et al. 1999), RP with Coats-like exudative vasculopathy (den Hollander et al. 2001b; den Hollander et al. 2004), and familial foveal retinoschisis (Vincent et al. 2016). Given the multitude of diseases and phenotypes caused by CRB1 mutations, it is likely we will link more retinopathies to CRB1 as genetic testing becomes more common in the clinic.
41.2 CRB1 Has Multiple Isoforms and Is Expressed in the Photoreceptors
41.3 CRB1 Is Important for Adherens Junction Integrity
Much of what we understand about CRB1 protein function is derived from drosophila Crumbs. The most commonly studied CRB1 protein isoform was chosen due to its close homology to the drosophila protein (Izaddoost et al. 2002). Drosophila Crumbs localizes to the stalk of the photoreceptor and is critical for positioning and maintaining adherens junction (AJ) integrity (Izaddoost et al. 2002; Pellikka et al. 2002). In mammals, CRB1 serves a similar function at the subapical region of the outer limiting membrane AJs between photoreceptors and Muller glia (Pellikka et al. 2002; van de Pavert et al. 2004). The cytoplasmic domain encoded by exon 12 of CRB1 interacts with cell polarity protein Pals1, which organizes complexes to regulate apical-basal polarity (Kantardzhieva et al. 2005). Relatively little is known about the function of the extracellular portion of the protein including what the interacting partners are.
41.4 Crb1 Mouse Models Do Not Recapitulate the Human Disease
There are two prevalent mouse models that disrupt the Crb1 locus and are used to model retinopathies. A spontaneous point mutant line (rd8) was discovered that contains a nonsense mutation in exon 9 of the gene likely resulting in a truncated protein (Mehalow et al. 2003). Mice homozygous for the rd8 mutation begin to show pseudorosettes in the outer nuclear layer detectable as early as 4–6 weeks of age (Mehalow et al. 2003; Mattapallil et al. 2012). Despite these ocular lesions, degeneration occurs slowly over the course of several months (Mehalow et al. 2003). A Crb1 null model (Crb1−/−) was generated by replacing exon 1 and the upstream promoter with a targeting vector (van de Pavert et al. 2004). This essentially eliminates generation of transcripts that use exon 1. However, it is unclear what effect it may have on putative transcripts that do not use exon 1 (Fig. 41.1b, c). In the Crb1−/− model, pseudorosettes developed by 3 months of age, and at 6 months of age large rosettes were apparent along with photoreceptor degeneration (van de Pavert et al. 2004). A notable feature of both rd8 and Crb1−/− models is that they fail to recapitulate the aggressive degeneration of the LCA and early-onset RP patients, causing one to speculate whether the disconnect in phenotypes is due to modifying genes or hypomorphic alleles.
41.5 The Path Forward
The variability of phenotype and age of onset of CRB1 retinopathies highlights the necessity of taking a bottom-up approach toward understanding CRB1 function. The apparent lack of correlation between mutation location and phenotypic outcome has perplexed researchers for more than a decade. The phenotypic variability is often explained by disease-modifying genes that act in concert with CRB1. While recent work in animal models has gained traction with this hypothesis (Pellissier et al. 2014; Luhmann et al. 2015), there are still several outstanding questions regarding CRB1 at the molecular and protein level that warrant further investigation.
Knowing the temporal and cell-type expression of CRB1 is paramount for understanding the basis of CRB1 retinopathies. For instance, does early expression by retinal progenitors contribute to the disease? Is the function of CRB1 in photoreceptors versus Muller glia equally important? Further, accurate gene quantification relies on proper gene annotation (Zhao and Zhang 2015); correct annotation of the CRB1 gene is necessary for expression analyses and determining the utility of current disease models. Finally, we need a detailed understanding of CRB1 protein functions to determine which are most relevant to the disease phenotype. By improving our understanding at the gene and protein levels, we can aim to more accurately diagnose CRB1 retinopathies and lay the groundwork for developing a therapy.
- Sun Y, Vandenbriele C, Kauskot A et al (2015) A human platelet receptor protein microarray identifies the high affinity immunoglobulin E receptor subunit alpha (FcepsilonR1alpha) as an activating platelet endothelium aggregation receptor 1 (PEAR1) ligand. Mol Cell Proteomics 14:1265–1274CrossRefGoogle Scholar