Mechanistically Distinct Mouse Models for CRX-Associated Retinopathy

Cone-rod homeobox (crx) protein is a paired-like homeodomain transcription factor that is essential for regulating rod and cone photoreceptor transcription. mutations in human crx are associated with the dominant retinopathies retinitis pigmentosa (rp),cone-rod dystrophy (cord) and leber congenital amaurosis (lca),with variable severity. heterozygous crx knock-out (ko) mice (+/-) have normal vision as adults and fail to model the dominant human disease. to investigate how different mutant crx proteins produce distinct disease pathologies,we generated two crx knock-in (k-in) mouse models: crxe168d2 (e168d2) and crxr90w (r90w). e168d2 mice carry a frameshift mutation in the crx activation domain,glu168del2,which is associated with severe dominant cord or lca in humans. r90w mice carry a substitution mutation in the crx homeodomain,arg90trp,which is associated with dominant mild late-onset cord and recessive lca. as seen in human patients,heterozygous e168d2 (e168d2/+) but not r90w (r90w/+) mice show severely impaired retinal function,while mice homozygous for either mutation are blind and undergo rapid photoreceptor degeneration. e168d2/+ mice also display abnormal rod/cone morphology,greater impairment of crx target gene expression than r90w/+ or +/- mice,and undergo progressive photoreceptor degeneration. surprisingly,e168d2/+ mice express more mutant crx protein than wild-type crx. e168d2neo/+,a subline of e168d2 with reduced mutant allele expression,displays a much milder retinal phenotype,demonstrating the impact of crx expression level on disease severity. both crx[e168d2] and crx[r90w] proteins fail to activate transcription in vitro,but crx[e168d2] interferes more strongly with the function of wild type (wt) crx,supporting an antimorphic mechanism. e168d2 and r90w are mechanistically distinct mouse models for crx-associated disease that will allow the elucidation of molecular mechanisms and testing of novel therapeutic approaches for different forms of crx-associated disease. © 2014 tran et al.