Mechanisms of blindness: Animal models provide insight into distinct CRX‐associated retinopathies

Abstract

The homeodomain transcription factor CRX is a crucial regulator of mammalian photoreceptor gene expression. Mutations in the human CRX gene are associated with dominant inherited retinopathies Retinitis Pigmentosa (RP), Cone-Rod Dystrophy (CoRD) and Leber Congenital Amaurosis (LCA), of varying severity. In vitro and in vivo assessment of mutant CRX proteins have revealed pathogenic mechanisms for several mutations, but no comprehensive mutation-disease correlation has yet been reported. Here we describe four different classes of disease-causing CRX mutations, characterized by mutation type, pathogenetic mechanism, and the molecular activity of the mutant protein: I) hypomorphic missense mutations with reduced DNA binding, II) antimorphic missense mutations with variable DNA binding, III) antimorphic frameshift/nonsense mutations with intact DNA binding and IV) antimorphic frameshift mutations with reduced DNA binding. Mammalian models representing three of these classes have been characterized. Models carrying Class I mutations display a mild dominant retinal phenotype and recessive LCA, while models carrying Class III and IV mutations display characteristically distinct dominant LCA phenotypes. These animal models also reveal unexpected pathogenic mechanisms underlying CRX-associated retinopathies. The complexity of genotype-phenotype correlation for CRX-associated diseases highlights the value of developing comprehensive 'true-to-disease' animal models for understanding pathologic mechanisms and testing novel therapeutic approaches.

Photoreceptor cell fate specification in vertebrates

“…A comparison of Crx and Otx2 ChIP-seq datasets shows that their transcriptional targets largely overlap in the retina, although there are also some unique targets (Samuel et al, 2014). Crx is expressed in both rods and cones and, as a result, mutations in this gene can lead to diseases that affect one or both photoreceptor types, such as Leber's congenital amaurosis and cone-rod dystrophy (Box 2) (Freund et al, 1997;Swaroop et al, 1999;Tran and Chen, 2014 The distinction between rod and cone gene expression profiles occurs downstream (or independently) of Otx2 and Crx, and two key factors that control this specification event are Rorβ and Nrl (Swaroop et al, 2010). Otx2 and Rorβ are needed to initiate the expression of the transcription factor Nrl in a subpopulation of newly postmitotic precursors: these Nrl-expressing cells will develop into rod photoreceptors (Fig.…”

Graded gene expression changes determine phenotype severity in mouse models of CRX-associated retinopathies

“…The homeodomain transcription factor CRX (Cone-rod homeobox protein) is expressed in both rods and cones and plays a central role in mediating photoreceptor transcription. CRX works with rod-specific, cone-specific and general transcription factors to control photoreceptor gene expression [ 9 – 14 ]. In particular, CRX and the rod-specific transcription factor NRL (Neural retina leucine zipper protein) cooperatively regulate rod gene transcription and have highly overlapping DNA-binding patterns [ 15 – 17 ].…”

Multimodal Imaging in Autosomal Dominant Cone-Rod Dystrophy Caused by Novel <b><i>CRX</i></b> Variant

“…Therefore, patients' phenotypic differences led by these 2 proteins, which are predicted to be prematurely stopped with the same length (AA 186) and with the only difference of 2 amino acids, could be explained by the mechanism of the allele-specific overexpression of the mutant CRX protein; that is, a different level of expression of the mutant CRX protein that interferes with the function of wild type CRX, impacts the disease severity, as shown in mouse models with at least 2 different class III variations [14]. It is not clear though if this phenomenon is conserved for all Class III variations and which are the underlying cellular mechanisms involved [13]. Alternatively, we can speculate that these frameshift variations are likely to stop protein translation due to nonsense-mediated mRNA decay [15] and, in this case, other genetic factors are therefore probably involved to explain the profound effect on the resultant phenotype.…”

“…Both variations fall in the class III (antimorphic frameshift/nonsense variations with intact DNA-binding) of a 4-degree system of classification, as reviewed by Tran and Chen [13]. Therefore, patients' phenotypic differences led by these 2 proteins, which are predicted to be prematurely stopped with the same length (AA 186) and with the only difference of 2 amino acids, could be explained by the mechanism of the allele-specific overexpression of the mutant CRX protein; that is, a different level of expression of the mutant CRX protein that interferes with the function of wild type CRX, impacts the disease severity, as shown in mouse models with at least 2 different class III variations [14].…”