A complete, homozygous CRX deletion causing nullizygosity is a new genetic mechanism for Leber congenital amaurosis

Abstract

CRX is a transcription factor required for activating the expression of many photoreceptor-neuron genes. CRX may be mutated in three forms of human blindness; Leber congenital amaurosis (LCA), cone-rod degeneration (CRD) and retinitis pigmentosa (RP). The pathogenic mechanism in most cases is likely dominant negative, with gain of function. We report a novel, complete homozygous CRX deletion in LCA. We identified a Lebanese family with 3 affected LCA cases. The proband was sequenced by NGS. Quantitative PCR, array comparative genomic hybridization, and long range PCR were performed. Full eye examinations, OCT and photography were performed. We identified a homozygous 56,000 bp deletion of CRX, which co-segregates and is heterozygous in four parents, who report normal vision. The blind children with LCA manifest severe retinal degeneration, a phenotype typical for CRX and LCA. We hypothesized that a single copy of CRX (haplo-insufficiency) in the causes mild abnormal foveal development, but not LCA. Two parents had significant inner and outer foveal and photoreceptor abnormalities. This is the first reported case of a homozygous, complete CRX deletion. Nullizygosity of CRX thus causes LCA while haplo-insufficiency of CRX causes abnormal foveal development, but not LCA. Our data suggest a new disease mechanism for CRX.

Gene Therapy of Dominant CRX-Leber Congenital Amaurosis using Patient Stem Cell-Derived Retinal Organoids

“…CRX controls expression of most rod and cone photoreceptor genes through its interaction with bZIP transcription factor NRL ( Mitton et al., 2000 ) and/or an intricate network of regulatory proteins ( Hennig et al., 2008 ). Heterozygous mutations in CRX cause early-onset retinal dystrophies with extensive phenotypic heterogeneity, with rare reports of biallelic variants in LCA ( Huang et al., 2012 ; Hull et al., 2014 ; Ibrahim et al., 2018 ; Rivolta et al., 2001 ; Swaroop et al., 1999 ). Notably, a majority of reported dominant LCA can be attributed to CRX .…”

Clinical and Genetic Characteristics of 18 Patients from 13 Japanese Families with CRX-associated retinal disorder: Identification of Genotype-phenotype Association

“…A locus and gene for ADCORD (CORD2) was first mapped and identified as CRX in 1994 18,23 . Since then, over 90 variants in the CRX gene have been associated with a wide range of different phenotypes of IRDs, including CORD, LCA, MD, and RP (The Human Gene Mutation Database; http://www.hgmd.cf.ac.uk/ac/index.php; accessed on 1 August 2018) [24][25][26][27][28][29][30][31][32][33][34][35] . The predominant mode of inheritance in reported families is AD, and in a few patients, including Japanese with RP or LCA caused by homozygous CRX variants was reported 29,36 .…”

Allele-specific gene editing to rescue dominant CRX-associated LCA7 phenotypes in a retinal organoid model

“…Allele-specific editing of mutant CRX allele in patientderived hiPSCs rescues photoreceptor defects in LCA7 retinal organoids Based on data published in Crx +/À mice (Furukawa et al, 1999), and a case study examining a human cohort with a CRX deletion mutation (Ibrahim et al, 2018), we know that one copy of CRX is sufficient to allow for proper maturation (although delayed) and function in photoreceptor cells. Therefore, we wanted to explore a therapeutic approach that would eliminate the mutant allele and allow the wild-type protein to be solely expressed.…”