Leber congenital amaurosis caused by a homozygous mutation (R90W) in the homeodomain of the retinal transcription factor CRX: direct evidence for theinvolvement of CRX in the development of photoreceptor function

The CRX (cone-rod homeobox) gene is specifically expressed in developing an d mature photoreceptors and encodes an otd/Otx-like paired homeodomain prot ein. Mutant alleles of the CRX gene have recently been associated with auto somal dominant cone-rod dystrophy (CORD) as well as dominant Leber congenit al amaurosis (LCA). Since LCA is more commonly inherited in an autosomal re cessive manner, we examined a cohort of recessive LCA patients for CRX muta tions. A homozygous substitution of arginine (R) at codon 90 by tryptophan (W) was identified in the CRX homeodomain of one of the probands who was ne arly blind from birth. A group of 48 control individuals and 190 previously characterized CORD probands did not reveal this sequence change. The mutan t CRXR90W homeodomain demonstrated decreased binding to the previously iden tified cis sequence elements in the rhodopsin promoter. In transient transf ection experiments, the mutant protein showed significantly reduced ability to transactivate the rhodopsin promoter, as well as lower synergistic acti vation with the bZIP transcription factor NRL. Heterozygosity of the mutant CRX (R90W) allele was detected in both parents and in an older sibling. Op hthalmologic examination and electroretinography revealed a subtle abnormal ity of cone function in both the parents. These data suggest that the R90W mutation results in a CRX protein with reduced DNA binding and transcriptio nal regulatory activity and that the subsequent changes in photoreceptor ge ne expression lead to the very early onset severe visual impairment in LCA.