Temporal and spatial expression patterns of the CRX transcription factor and its downstream targets. Critical differences during human and mouse eye development.

Cone-rod homeobox (CRX), a paired-like homeobox transcription factor, plays a major role in photoreceptor development and maintenance of the retina. F ifteen different mutations in the CRX gene have been identified as a cause of blinding retinal dystrophy. As a step towards characterizing the underly ing pathophysiology of disease, temporal and spatial gene expression patter ns during human and mouse eye development were investigated for CRX and for downstream retinally expressed genes, postulated to be transactivated by C RX. We found that human CRX was expressed at 10.5 weeks postconception (p.c .). This was significantly later than observed in mouse development. Immuno cytochemistry in human retina showed that CRX protein was not detected unti l > 4 weeks later at 15 weeks p.c., implying that it would be unable to tra nsactivate PDEB, IRBP and arrestin, which were all expressed before 15 week s. These data therefore eliminate CRX as the major transcriptional activato r of these three genes from a wide group of retinal genes that can be trans activated by CRX in vitro. Additionally, PDEB was expressed 2 weeks before CRX whereas murine Pdeb was expressed after Crx, highlighting a potential d ifference for the role of PDEB in human eye development. Previous data had shown CRX expression in the adult human retina to be photoreceptor-specific ; however, we demonstrate that this gene is also expressed in the inner nuc lear layer (INL) of the human and mouse retina by in situ hybridization and immunocytochemistry. INL localization of murine Crx was confirmed in rd/rd ,cl mice, as in this mouse model the photoreceptors are absent. We have fou nd important differences in the temporal expression of this gene in human a nd mouse retina, although spatial expression of the CRX gene appears to be conserved. In addition, downstream targets of CRX in vitro might not repres ent in vivo function during development. These data support concerns about the extent to which we can extrapolate from rodent models regarding embryon ic development and disease pathophysiology.