Alternative titles; symbols
HGNC Approved Gene Symbol: NRL
Cytogenetic location: 14q11.2-q12 Genomic coordinates (GRCh38): 14:24,078,662-24,114,949 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 14q11.2-q12 | Retinal degeneration, autosomal recessive, clumped pigment type | 3 | ||
| Retinitis pigmentosa 27 | 613750 | Autosomal dominant | 3 |
The neural retina leucine zipper (NRL), a basic motif-leucine zipper (bZIP) transcription factor of the Maf- subfamily, is a phosphorylated protein that is specifically expressed in rod photoreceptors and pineal gland, but not in cones or other cell types. NRL is required for rod photoreceptor differentiation during retinal development (summary by Kanda et al., 2007).
Using subtraction cloning, Swaroop et al. (1991, 1992) identified a gene, designated NRL, that is expressed specifically in neuronal cells of retina. The NRL gene encodes a putative DNA-binding protein of the 'leucine zipper' family with strong similarity to the DNA-binding domain of the MAF oncogene product. The authors suggested that the NRL gene product might play a role in the regulation of retinal development and/or differentiation.
By Southern blot analysis of genomic DNA from a human/rodent somatic cell hybrid panel, Yang-Feng and Swaroop (1992) mapped the NRL gene to human chromosome 14 and sublocalized the gene to 14q11.1-q11.2 by in situ hybridization. Because of its specific pattern of expression, NRL was considered a candidate gene for retinal diseases.
Dahl et al. (1992) synthesized oligonucleotide primer sequences of 8 short tandem repeat polymorphism (STRP) markers that span the area 14q11.2-q32. Also synthesized were primer pair sequences for NRL that bracketed a polymorphic CA repeat fragment approximately 300 bp long. Genetic linkage studies in 17 families demonstrated the most likely position for NRL to be between D14S54 proximally and D14S50 distally. Bespalova et al. (1993) demonstrated that the homologous gene is located on mouse chromosome 14 and Farjo et al. (1993) provided molecular characterization of the murine gene.
NRL regulates the expression of several rod-specific genes, and missense mutations in the human NRL gene are associated with autosomal dominant retinitis pigmentosa. Mitton et al. (2003) used yeast 2-hybrid screening to identify FIZ1 (609133) as an NRL-interacting protein in retina. FIZ1 suppressed NRL- but not CRX (602225)-mediated transactivation of rhodopsin (180380) promoter activity in a transiently transfected monkey kidney cell line.
Using mouse microarrays, Yoshida et al. (2004) generated expression profiles of the wildtype and Nrl -/- retina at 3 distinct stages of photoreceptor differentiation. Comparative data analysis revealed 161 differentially expressed genes, of which 78 exhibited significantly lower and 83 higher expression in the Nrl -/- retina. Hierarchical clustering was used to predict the function of these genes in a temporal context. The differentially expressed genes primarily encoded proteins associated with signal transduction, transcriptional regulation, intracellular transport, and other processes, which could correspond to differences between rods and cones and/or retinal remodeling in the absence of rods. Chromatin immunoprecipitation assay showed that in addition to the rod phototransduction genes, Nrl might modulate the promoters of many functionally diverse genes in vivo.
Preceding the study of MacLaren et al. (2006), brain- and retina-derived stem cells transplanted into adult retina had shown little evidence of being able to integrate into the outer nuclear layer and differentiate into new photoreceptors. Furthermore, there had been no demonstration that transplanted cells form functional synaptic connections with other neurons in the recipient retina or restore visual function. MacLaren et al. (2006) hypothesized that committed progenitor or precursor cells at later ontogenetic stages might have a higher probability of success upon transplantation. In studies in mice, MacLaren et al. (2006) showed that adult wildtype and degenerating mammalian retinas can effectively incorporate rod photoreceptor precursor cells into the outer nuclear layer (ONL) of the retina. These cells differentiated, formed functional synaptic connections with downstream targets in the recipient retina, and contributed to visual function. Rather than the environment of the mature retina inhibiting photoreceptor maturation, they showed that transplantation of precursor cells at a specific ontogenetic stage, defined by activation of the transcription factor Nrl, results in their integration and subsequent differentiation into rod photoreceptors, even in retinal degeneration. Conversely, progenitor or stem cells that had not yet begun to express Nrl did not show this property and failed to integrate. Identification of the optimal ontogenetic stage for donor cells might facilitate the generation of appropriate cells for transplantation into humans from either embryonic or adult-derived stem cells.
Hao et al. (2014) found that developing and adult Nrl -/- mice lacked expression of a retina-specific Reep6 (609346) splice variant, Reep6.1, that includes exon 5. In contrast, expression of the Reep6.2 variant, which lacks exon 5, was intact in both early retina and liver of Nrl -/- mice. Hao et al. (2014) showed that Nrl bound an enhancer element in Reep6 intron 1 and, along with Nono (300084), promoted inclusion of exon 5 in Reep6.1 transcripts. Nrl had no effect on expression of the Reep6.2 transcript.
Retinitis Pigmentosa 27
Farjo et al. (1997) determined the complete sequence of the human NRL gene, identified a polymorphic (CA)n repeat (identical to D14S64) within an NRL-containing cosmid, and refined the location of the NRL gene by linkage analysis. Since a locus for autosomal recessive retinitis pigmentosa was thought to map to 14q11 in Sardinian families (Wright et al., 1995), and because mutations in rhodopsin (180380), a gene regulated by the NRL protein, cause RP, NRL was considered a valid candidate gene for retinopathies. In a panel of patients representing independent families with inherited retinal degeneration, Farjo et al. (1997) sequenced genomic PCR products of the NRL gene and of the rhodopsin-NRL response element. No causative mutations were identified.
In all affected members of a large autosomal dominant retinitis pigmentosa family (RP251) showing linkage to D14S64 (RP27; 613750), Bessant et al. (1999) identified a mutation in the NRL gene (S50T; 162080.0001).
Hernan et al. (2012) screened the NRL gene in 50 Spanish autosomal dominant RP probands and identified a heterozygous missense mutation in 1 (M96T; 162080.0004). The 3 affected individuals in the proband's family had less severe RP with later onset of symptoms than previously reported with mutations in the NRL gene; in vitro functional analysis showed that the M96T mutant increased transactivation to a lesser degree than the S50T or P51L (see Martinez-Gimeno et al., 2001) mutant proteins.
Clumped Pigmentary Retinal Degeneration
Mice lacking the Nrl gene have no rod photoreceptors and an increased number of short wavelength-sensitive cones. Nishiguchi et al. (2004) identified the phenotype associated with the loss of NRL function in humans (see 613750). They identified 2 sibs who carried 2 allelic mutations of the NRL gene: a 1-bp insertion (224insC; 162080.0002), resulting in a frameshift and a predicted null allele, and a leu160-to-pro substitution (L160P; 162080.0003), which altered a highly conserved residue in the domain involved in DNA binding site recognition. In vitro luciferase reporter assays demonstrated that the L160P mutant had severely reduced transcriptional activity compared with the wildtype protein, consistent with a severe loss of function. The affected patients had night blindness since early childhood, consistent with a severe reduction in rod function. Color vision was normal, suggesting the presence of all cone color types; nevertheless, a comparison of central visual fields evaluated with white-on-white and blue-on-yellow light stimuli was consistent with a relatively enhanced function of short wavelength-sensitive cones in the macula. The fundi had signs of retinal degeneration (such as vascular attenuation) and clusters of large, clumped, pigment deposits in the peripheral fundus at the level of the retinal pigment epithelium. Nishiguchi et al. (2004) noted that no humans with an NRL -/- genotype had previously been reported; only dominant NRL mutations that were unlikely to be null alleles had been reported. All of the published dominant NRL mutations were missense changes affecting 1 of 3 residues: ser50, pro51, or gly122. Patients with recessive NRL mutations had features resembling those caused by mutation in the NR2E3 gene (604485), the only previously known cause of enhanced S-cone syndrome (ESCS; 268100) in humans. In addition to the preservation of S-cone function, patients with recessive NR2E3 or NRL mutations have a similar pattern of intraretinal pigmentation of the fundus. This so-called clumped pigmentary retinal degeneration is found in approximately 0.5% of RP cases. Approximately half of all patients with clumped pigmentary retinal degeneration have mutations in the NR2E3 gene and are considered to have enhanced S-cone syndrome. Nishiguchi et al. (2004) concluded that mutations in NRL are a much less common cause of clumped pigmentary retinal degeneration than mutations in NR2E3.
Mutant NRL Function
Kanda et al. (2007) reported functional analyses of 17 amino acid variations and/or mutations in the NRL gene using transfection studies of HEK293 and COS1 cells. Six mutations at residues 50 and 51, including S50T, identified in patients with autosomal dominant retinitis pigmentosa resulted in a major NRL isoform that exhibited reduced phosphorylation but enhanced transcriptional activation of the rhodopsin promoter. Truncated NRL products, including 224insC, did not localize to the nucleus because of absence of the bZIP domain. The L160P mutation did not bind to the NRL-response element and showed decreased transcriptional activity. Other sequence variations were of uncertain significance. Kanda et al. (2007) concluded that gain-of-function mutations result in autosomal dominant disease, while loss-of-function mutations result in autosomal recessive disease. The findings also suggested that differential phosphorylation of NRL fine tunes its transcriptional regulatory activity, leading to a more precise control of gene expression.
Mears et al. (2001) generated mice with deletion of the NRL gene. Nrl -/- mice had complete loss of rod function and supernormal cone function, mediated by S cones. The photoreceptors in the Nrl -/- retina had cone-like nuclear morphology and short, sparse outer segments with abnormal disks. Analysis of retinal gene expression confirmed the apparent functional transformation of rods into S cones in the Nrl -/- retina. Mears et al. (2001) suggested that NRL acts as a molecular switch during rod-cell development by directly modulating rod-specific genes while simultaneously inhibiting the S-cone pathway through the activation of NR2E3 (604485).
In all affected members of a family (RP251) with a form of autosomal dominant retinitis pigmentosa showing linkage to D14S64 (RP27; 613750), Bessant et al. (1999) identified a T-to-A transversion at nucleotide 1942 of the NRL gene, resulting in a ser50-to-thr (S50T) substitution of the NRL protein. The mutation was not seen in unaffected family members or in 250 normal controls. Ser50 is located in 1 of 2 highly conserved regions of the transactivation (TA) domain, and is present in other members of the Maf family of proteins (see 602020) that contain a TA domain. Transient expression of NRL-S50T protein in CV-1 and 293 cells resulted in increased transactivation of the RHO promoter compared with wildtype NRL. The mutation abolished an HphI site.
By screening a panel of 200 autosomal dominant retinitis pigmentosa families, Bessant et al. (2000) found the S50T mutation in 3 additional families. Comparison of marker haplotypes in affected individuals from these families revealed a common disease haplotype. The exclusion of this locus and 9 other RP loci in several families indicated the existence of at least 1 other autosomal dominant RP locus.
Bessant et al. (2003) reviewed the clinical records of 21 patients with autosomal dominant RP due to the S50T mutation in the NRL gene. This mutation is associated with selective loss of scotopic function before age 20 years. With time, the photopic system becomes affected as well, leading to loss of the photopic visual field and of visual acuity.
By functional studies in cell culture, Kanda et al. (2007) determined that the S50T mutation resulted in a protein with reduced phosphorylation and enhanced transcriptional activation.
Using a mammalian 2-hybrid system, Perveen et al. (2007) demonstrated that the transactivation domain of the NRL gene interacts with p300 (602700) and that the S50T mutation enhances that interaction.
In a brother and sister with a clinical diagnosis of retinal degeneration of the clumped pigment type (see 613750), Nishiguchi et al. (2004) identified compound heterozygosity for NRL mutations: a 1-bp insertion (224insC), resulting in a frameshift at codon 75 and a premature stop 19 codons downstream, and the L160P mutation (162080.0003). The 1-bp insertion was interpreted as a null allele because the stop codon early in the reading frame would likely result in nonsense-mediated decay of the mutant RNA transcript and, even if the RNA were translated, the resulting protein would have no basic leucine zipper domain. An unaffected daughter of the brother was heterozygous for the 1-bp insertion.
By functional studies in cell culture, Kanda et al. (2007) determined that the 224insC mutant NRL protein did not localize to the nucleus because of the absence of the bZIP domain.
In a brother and sister with a clinical diagnosis of retinal degeneration of the clumped pigment type (see 613750), Nishiguchi et al. (2004) identified compound heterozygosity for 2 mutations in the NRL gene: a 479T-C transition resulting in a leu160-to-pro (L160P) and the 224insC mutation (162080.0002).
By functional studies in cell culture, Kanda et al. (2007) determined that the L160P mutant NRL protein did not bind to the NRL-response element and showed decreased transcriptional activity.
In a Spanish patient with retinitis pigmentosa (RP27; 613750), Hernan et al. (2012) identified heterozygosity for a 287T-C transition in the NRL gene, resulting in a met96-to-thr (M96T) substitution at a conserved residue. The proband's mother and a maternal aunt were also heterozygous for the mutation, which was not found in 127 controls. The 3 affected individuals had onset of night blindness in the second or third decade of life. The mutation was also present in the proband's sister and a cousin, who remained asymptomatic at ages 37 and 45 years, respectively. Hernan et al. (2012) noted that the RP phenotype in this family was less severe and had later onset of symptoms than previously reported with other NRL mutations; in vitro functional analysis demonstrated that the M96T mutant increased transactivation to a lesser degree than the S50T (162080.0001) or P51L (see Martinez-Gimeno et al., 2001) mutant proteins.
Bespalova, I. N., Farjo, Q., Mortlock, D. P., Jackson, A. U., Meisler, M. H., Swaroop, A., Burmeister, M. Mapping of the neural retina leucine zipper gene, Nrl, to mouse chromosome 14. Mammalian Genome 4: 618-620, 1993. [PubMed: 8268663] [Full Text: https://doi.org/10.1007/BF00361397]
Bessant, D. A. R., Holder, G. E., Fitzke, F. W., Payne, A. M., Bhattacharya, S. S., Bird, A. C. Phenotype of retinitis pigmentosa associated with the ser50thr mutation in the NRL gene. Arch. Ophthal. 121: 793-802, 2003. [PubMed: 12796249] [Full Text: https://doi.org/10.1001/archopht.121.6.793]
Bessant, D. A. R., Payne, A. M., Mitton, K. P., Wang, Q.-L., Swain, P. K., Plant, C., Bird, A. C., Zack, D. J., Swaroop, A., Bhattacharya, S. S. A mutation in NRL is associated with autosomal dominant retinitis pigmentosa. Nature Genet. 21: 355-356, 1999. [PubMed: 10192380] [Full Text: https://doi.org/10.1038/7678]
Bessant, D. A. R., Payne, A. M., Plant, C., Bird, A. C., Swaroop, A., Bhattacharya, S. S. NRL S50T mutation and the importance of 'founder effects' in inherited retinal dystrophies. Europ. J. Hum. Genet. 8: 783-787, 2000. [PubMed: 11039579] [Full Text: https://doi.org/10.1038/sj.ejhg.5200538]
Dahl, S. P., Jackson, A., Kimberling, W. J., Blackwood, D., Swaroop, A. Genetic mapping of NRL, a human retina-specific gene located on chromosome 14. (Abstract) Am. J. Hum. Genet. 51 (suppl.): A185 only, 1992.
Farjo, Q., Jackson, A., Pieke-Dahl, S., Scott, K., Kimberling, W. J., Sieving, P. A., Richards, J. E., Swaroop, A. Human bZIP transcription factor gene NRL: structure, genomic sequence, and fine linkage mapping at 14q11.2 and negative mutation analysis in patients with retinal degeneration. Genomics 45: 395-401, 1997. [PubMed: 9344665] [Full Text: https://doi.org/10.1006/geno.1997.4964]
Farjo, Q., Jackson, A. U., Xu, J., Gryzenia, M., Skolnick, C., Agarwal, N., Swaroop, A. Molecular characterization of the murine neural retina leucine zipper gene, Nrl. Genomics 18: 216-222, 1993. [PubMed: 8288222] [Full Text: https://doi.org/10.1006/geno.1993.1458]
Hao, H., Veleri, S., Sun, B., Kim, D. S., Keeley, P. W., Kim, J.-W., Yang, H.-J., Yadav, S. P., Manjunath, S. H., Sood, R., Liu, P., Reese, B. E., Swaroop, A. Regulation of a novel isoform of receptor expression enhancing protein REEP6 in rod photoreceptors by bZIP transcription factor NRL. Hum. Molec. Genet. 23: 4260-4271, 2014. [PubMed: 24691551] [Full Text: https://doi.org/10.1093/hmg/ddu143]
Hernan, I., Gamundi, M. J., Borras, E., Maseras, M., Garcia-Sandoval, B., Blanco-Kelly, F., Ayuso, C., Carballo, M. Novel p.M96T variant of NRL and shRNA-based suppression and replacement of NRL mutants associated with autosomal dominant retinitis pigmentosa. Clin. Genet. 82: 446-452, 2012. [PubMed: 21981118] [Full Text: https://doi.org/10.1111/j.1399-0004.2011.01796.x]
Kanda, A., Friedman, J. S., Nishiguchi, K. M., Swaroop, A. Retinopathy mutations in the bZIP protein NRL alter phosphorylation and transcriptional activity. Hum. Mutat. 28: 589-598, 2007. [PubMed: 17335001] [Full Text: https://doi.org/10.1002/humu.20488]
MacLaren, R. E., Pearson, R. A., MacNeil, A., Douglas, R. H., Salt, T. E., Akimoto, M., Swaroop, A., Sowden, J. C., Ali, R. R. Retinal repair by transplantation of photoreceptor precursors. Nature 444: 203-207, 2006. [PubMed: 17093405] [Full Text: https://doi.org/10.1038/nature05161]
Martinez-Gimeno, M., Maseras, M., Baiget, M., Beneito, M., Antinolo, G., Ayuso, C., Carballo, M. Mutations P51L and G122E in retinal transcription factor NRL associated with autosomal dominant and sporadic retinitis pigmentosa. (Abstract) Hum. Mutat. 17: 520 only, 2001. Note: Full Article Online. [PubMed: 11385710] [Full Text: https://doi.org/10.1002/humu.1135]
Mears, A. J., Kondo, M., Swain, P. K., Takada, Y., Bush, R. A., Saunders, T. L., Sieving, P. A., Swaroop, A. Nrl is required for rod photoreceptor development. Nature Genet. 29: 447-452, 2001. [PubMed: 11694879] [Full Text: https://doi.org/10.1038/ng774]
Mitton, K. P., Swain, P. K., Khanna, H., Dowd, M., Apel, I. J., Swaroop, A. Interaction of retinal bZIP transcription factor NRL with Flt3-interacting zinc-finger protein Fiz1: possible role of Fiz1 as a transcriptional repressor. Hum. Molec. Genet. 12: 365-373, 2003. [PubMed: 12566383] [Full Text: https://doi.org/10.1093/hmg/ddg035]
Nishiguchi, K. M., Friedman, J. S., Sandberg, M. A., Swaroop, A., Berson, E. L., Dryja, T. P. Recessive NRL mutations in patients with clumped pigmentary retinal degeneration and relative preservation of blue cone function. Proc. Nat. Acad. Sci. 101: 17819-17824, 2004. [PubMed: 15591106] [Full Text: https://doi.org/10.1073/pnas.0408183101]
Perveen, R., Favor, J., Jamieson, R. V., Ray, D. W., Black, G. C. M. A heterozygous c-Maf transactivation domain mutation causes congenital cataract and enhances target gene activation. Hum. Molec. Genet. 16: 1030-1038, 2007. [PubMed: 17374726] [Full Text: https://doi.org/10.1093/hmg/ddm048]
Swaroop, A., Xu, J., Agarwal, N., Weissman, S. M. A simple and efficient cDNA library subtraction procedure: isolation of human retina-specific cDNA clones. Nucleic Acids Res. 19: 1954 only, 1991. [PubMed: 2030979] [Full Text: https://doi.org/10.1093/nar/19.8.1954]
Swaroop, A., Xu, J., Pawar, H., Jackson, A., Skolnick, C., Agarwal, N. A conserved retina-specific gene encodes a basic motif/leucine zipper domain. Proc. Nat. Acad. Sci. 89: 266-270, 1992. [PubMed: 1729696] [Full Text: https://doi.org/10.1073/pnas.89.1.266]
Wright, A. F., Mansfield, D. C., Bruford, E. A., Teague, P. W., Thomson, K. L., Riise, R., Jay, M., Patton, M. A., Jeffery, S., Schinzel, A., Tommerup, N., Fossarello, M. Genetic studies in autosomal recessive forms of retinitis pigmentosa. In: Anderson, R. E.; LaVail, M. M.; Hollyfield, J. G. (eds.): Degenerative diseases of the retina. New York: Plenum Press (pub.) 1995. Pp. 293-302.
Yang-Feng, T. L., Swaroop, A. Neural retina-specific leucine zipper gene NRL (D14S46E) maps to human chromosome 14q11.1-q11.2. Genomics 14: 491-492, 1992. [PubMed: 1427865] [Full Text: https://doi.org/10.1016/s0888-7543(05)80248-4]
Yoshida, S., Mears, A. J., Friedman, J. S., Carter, T., He, S., Oh, E., Jing, Y., Farjo, R., Fleury, G., Barlow, C., Hero, A. O., Swaroop, A. Expression profiling of the developing and mature Nrl -/- mouse retina: identification of retinal disease candidates and transcriptional regulatory targets of Nrl. Hum. Molec. Genet. 13: 1487-1503, 2004. [PubMed: 15163632] [Full Text: https://doi.org/10.1093/hmg/ddh160]