*6.1. CRX*

The *CRX* gene encodes an OTX-like homeodomain transcription factor which is essential for photoreceptor development, maturation and survival [121–124]. Transcription factors are essential for control of the maturation of progenitor cells [123–126]. Only one spontaneously occurring large animal model of a retinal transcription factor homeobox mutation has been described, the *CrxRdy* cat [127,128]. The heterozygous animal has a severe phenotype of cone-led retinal dystrophy and is a model for one form of severe early childhood onset blindness (LCA7). While most mutations in *CRX* result in LCA7, other phenotypes including cone–rod dystrophy, RP and macular degeneration have been described [129–131]. *CRX* mutations have been reported to account for between 0.6% and 2.35% of LCA [131–134].

As with other transcription factors, CRX has a characteristic structure with a DNA binding domain (homeodomain) near its N-terminal and a transactivation domain at the C-terminal. The *CrxRdy*/+ mutant cat models Class III *CRX* mutations, which are antimorphic frameshift/nonsense mutations with intact DNA binding, but a lack of target gene transactivation [135]. The *CrxRdy*/+ cat mutation is caused by a 1 bp deletion (c.546delC; p.Pro185LysfsTer2) in the final exon of the *Crx* gene [127]. This mutant mRNA avoids nonsense mediated decay and produces a truncated Crx protein with an intact DNA-binding domain but disrupted transactivation domain [128]. Both the mutant transcript and protein accumulate at higher levels than the wildtype versions, possibly due to increased stability of the mutant mRNA compared to the wildtype. In vitro studies showed that the mutant mRNA fails to activate its own promoter [128]. This suggests that the mutant protein has a dominant negative effect by binding promoter recognition sites, but fails in its transactivation function. The result is

misregulated gene expression; for example, *Rho* and cone arrestin (*Arr3*) mRNA levels are severely decreased. Truncated photoreceptor outer segments are produced, but the photoreceptors fail to fully mature [128]. A decreased rod ERG is detectable, which shows evidence of partial maturation before this is halted and the ERG amplitudes decline, paralleling rod degeneration. Cone function is more severely affected, with no cone ERG responses being detectable [128].
