Comparison Between CRB1-Associated Retinal Degeneration in Human Patients and Corresponding Mouse Models

Abstract

Purpose: : To assess similarities and differences between the human retinal degeneration caused by mutations in the CRB1 gene and mouse retinopathies caused by the lack of one or more Crumbs complex proteins Crb1 and Crb2.

Methods: : Patients with CRB1-associated retinal degeneration (CRB1-RD) were evaluated clinically and with en face and cross-sectional imaging. Crb1-Crb2 double knock-out mice (‘DKOs’) were generated by cross-breeding Crb1 knock-out mice with a Crb2-Chx10Cre conditional knock-out line. Crb2-Crb1 DKOs were examined at one, three, and six months of age with confocal scanning-laser ophthalmoscopy (cSLO) to assess retinal morphology and vasculature, spectral-domain optical coherence tomography (SD-OCT) to determine the retinal layer structure, and Ganzfeld electroretinography (ERG) to measure retinal function.

Results: : Patients with CRB1-RD showed early onset retina-wide severe photoreceptor dysfunction by ERG and retinal disorganization and thickening by OCT (Jacobson et al., HMG 2003;12:1073 and Aleman et al. IOVS 2011;52:6898). We found that Crb1 knock-out mice crossed with a Crb2-Chx10Cre knock-out line might have a similar retinal phenotype, whereas Crb1 knock-outs alone do not.It turned out that the fundus in native SLO imaging had initially a regular appearance and only changed when signs of degeneration were present, particularly visible in autofluorescence mode. In contrast, the corresponding SD-OCT analysis revealed from early on that retinal lamination appeared to be limited to a ‘generic’ inner and outer layer with hardly discernible sublayers. Histological workup suggests extensive cellular dislocation as a possible cause for this lack of organisation. With increasing age, a progressive retinal degeneration became visible. At this point, cSLO imaging revealed an increasing number of lesions that gave the fundus a spotty and patchy appearance, as well as several vascular alterations. In OCT, a thinning of the disorganized retina with age was observed. In line with these results, the functional analysis showed initially reduced, but recordable ERG responses at one month of age, and no discernible responses at later time points.

Conclusions: : The impairment of the Crumbs complex in human CRB1 deficiency may lead to a unique phenotype that demonstrates an abnormally thick retina and a lack of proper lamination. Here, we show that Crb2-Crb1 DKO mice have a similarly disorganized retina, accompanied by a progressive reduction of the ERG response that is also typical for the human patients. Our results suggest that these mice are a valuable model for CRB1-RD and may help to increase our understanding of the underlying disease mechanisms.

Keywords: retinal degenerations: cell biology • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • electroretinography: non-clinical