The Maf-family transcription factor Nrl is a key regulator of photoreceptor differentiation in mammals. Ablation of the Nrl gene in mice leads to functional cones at the expense of rods. We show that a 2.5-kb Nrl promoter segment directs the expression of enhanced GFP specifically to rod photoreceptors and the pineal gland of transgenic mice. GFP is detected shortly after terminal cell division, corresponding to the timing of rod genesis revealed by birthdating studies. In Nrl ؊/؊ retinas, the GFP؉ photoreceptors express S-opsin, consistent with the transformation of rod precursors into cones. We report the gene profiles of freshly isolated flow-sorted GFP؉ photoreceptors from wild-type and Nrl ؊/؊ retinas at five distinct developmental stages. Our results provide a framework for establishing gene regulatory networks that lead to mature functional photoreceptors from postmitotic precursors. Differentially expressed rod and cone genes are excellent candidates for retinopathies.gene profiling ͉ gene regulation ͉ neuronal differentiation ͉ retina ͉ transcription factor E volution of higher-order sensory and behavioral functions in mammals is accompanied by increasingly complex regulation of gene expression (1). As much as 10% of the human genome is presumably dedicated to the control of transcription. Exquisitely timed expression of cell-type-specific genes, together with spatial and quantitative precision, depends on the interaction between transcriptional control machinery and extracellular signals (2, 3). Neuronal heterogeneity and functional diversity result from combinatorial and cooperative actions of regulatory proteins that form complicated yet precise transcriptional networks to generate unique gene expression profiles. A key transcription factor, combined with its cognate regulatory cis-sequence codes, specifies a particular node in the gene regulatory networks that guide differentiation and development (4).The retina offers an ideal paradigm for investigating regulatory networks underlying neuronal differentiation. The genesis of six types of neurons and Müller glia in the vertebrate retina proceeds in a predictable sequence during development (5). Subsets of multipotent retinal neuroepithelial progenitors exit the cell cycle at specific time points and acquire a particular cell fate under the influence of intrinsic genetic program and extrinsic factors (5-7). Pioneering studies using thymidine labeling and retroviral vectors established the order and birthdates of neurons in developing retina (5,(8)(9)(10)). The current model of retinal differentiation proposes that a heterogeneous pool of progenitors passes through states of competence, where it can generate a distinct subset of neurons (5). One can predict that, at the molecular level, this competence is acquired by combinatorial action of specific transcriptional regulatory proteins. Genetic ablation studies of transcription factors involved in early murine eye specification are consistent with combinatorial regulation (11-13).Rod and cone photorecep...

X-linked retinitis pigmentosa (XLRP) is a clinically and genetically heterogeneous degenerative disease of the retina. At least five loci have been mapped for XLRP; of these, RP2 and RP3 account for 10%-20% and 70%-90% of genetically identifiable disease, respectively. However, mutations in the respective genes, RP2 and RPGR, were detected in only 10% and 20% of families with XLRP. Mutations in an alternatively spliced RPGR exon, ORF15, have recently been shown to account for 60% of XLRP in a European cohort of 47 families. We have performed, in a North American cohort of 234 families with RP, a comprehensive screen of the RP2 and RPGR (including ORF15) genes and their 5' upstream regions. Of these families, 91 (39%) show definitive X-linked inheritance, an additional 88 (38%) reveal a pattern consistent with X-linked disease, and the remaining 55 (23%) are simplex male patients with RP who had an early onset and/or severe disease. In agreement with the previous studies, we show that mutations in the RP2 gene and in the original 19 RPGR exons are detected in <10% and approximately 20% of XLRP probands, respectively. Our studies have revealed RPGR-ORF15 mutations in an additional 30% of 91 well-documented families with X-linked recessive inheritance and in 22% of the total 234 probands analyzed. We suggest that mutations in an as-yet-uncharacterized RPGR exon(s), intronic changes, or another gene in the region might be responsible for the disease in the remainder of this North American cohort. We also discuss the implications of our studies for genetic diagnosis, genotype-phenotype correlations, and gene-based therapy.

The rd3 mouse is one of the oldest identified models of early-onset retinal degeneration. Using the positional candidate approach, we have identified a C-->T substitution in a novel gene, Rd3, that encodes an evolutionarily conserved protein of 195 amino acids. The rd3 mutation results in a predicted stop codon after residue 106. This change is observed in four rd3 lines derived from the original collected mice but not in the nine wild-type mouse strains that were examined. Rd3 is preferentially expressed in the retina and exhibits increasing expression through early postnatal development. In transiently transfected COS-1 cells, the RD3-fusion protein shows subnuclear localization adjacent to promyelocytic leukemia-gene-product bodies. The truncated mutant RD3 protein is detectable in COS-1 cells but appears to get degraded rapidly. To explore potential association of the human RD3 gene at chromosome 1q32 with retinopathies, we performed a mutation screen of 881 probands from North America, India, and Europe. In addition to several alterations of uncertain significance, we identified a homozygous alteration in the invariant G nucleotide of the RD3 exon 2 donor splice site in two siblings with Leber congenital amaurosis. This mutation is predicted to result in premature truncation of the RD3 protein, segregates with the disease, and is not detected in 121 ethnically matched control individuals. We suggest that the retinopathy-associated RD3 protein is part of subnuclear protein complexes involved in diverse processes, such as transcription and splicing.