Alternative titles; symbols
HGNC Approved Gene Symbol: TOPORS
Cytogenetic location: 9p21.1 Genomic coordinates (GRCh38): 9:32,540,544-32,552,586 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 9p21.1 | Retinitis pigmentosa 31 | 609923 | Autosomal dominant | 3 |
The TOPORS gene encodes a RING domain-containing E3 ubiquitin ligase that binds topoisomerase I (TOP1; 126420) and interacts with and ubiquitinates p53 (191170). It localizes in the nucleus in speckled loci that are associated with promyelocytic leukemia bodies, nuclear compartments that are implicated in transcription, DNA repair, viral defense, stress, cell cycle regulation, proteolysis, and apoptosis (summary by Chakarova et al., 2007).
Using murine p53 (191170) as bait in a yeast 2-hybrid screen of a HeLa cell cDNA library, Zhou et al. (1999) obtained a partial cDNA encoding TOPORS, which they called P53BP3. TOPORS is rich in serine and arginine and contains a leucine zipper domain, 2 PEST sequences, a central basic domain containing 2 potential bipartite nuclear localization signals, and 2 C-terminal PEST sequences. Northern blot analysis detected ubiquitous but variable expression of a 5.0-kb transcript. Highest expression was in testis, and lowest expression was in lung. Fluorescence-labeled TOPORS localized to nuclear star-like clusters in transfected HeLa cells.
Using the N-terminal 250 amino acids of topoisomerase I (TOP1; 126420) as bait in a yeast 2-hybrid screen of a HeLa cell cDNA library, followed by 5-prime RACE of leukemia cell mRNA, Haluska et al. (1999) cloned TOPORS. They extended the 5-prime coding region reported by Zhou et al. (1999) and determined that the full-length protein contains 1,045 amino acids.
Using degenerate primers to amplify cDNAs encoding RING finger proteins from human lung cDNA, followed by screening a lung cDNA library, Chu et al. (2001) cloned TOPORS, which they designated LUN. The deduced 1,045-amino acid protein has a calculated molecular mass of 119 kD. TOPORS contains an N-terminal PEST sequence, followed by a C3H4-type RING finger domain, a leucine zipper coiled-coil region, 2 PEST sequences, a nuclear localization signal, and 2 C-terminal PEST sequences. Northern blot analysis detected a 3.8-kb transcript in brain, kidney, liver, lung, spleen, pancreas, and skeletal muscle, and a 3.6-kb transcript in heart and lung. Highest expression of TOPORS was in lung. In situ hybridization of adult lung sections detected TOPORS in alveolar epithelium and in some squamous and cuboidal cells.
By immunohistochemical analysis of adult mouse, pig, and human retinas, Chakarova et al. (2011) found that TOPORS localized to the connecting cilium between the biosynthetically active inner segment and the photosensitive outer segment in retinal pigment epithelium. TOPORS was also detected in nuclei of retinal ganglion cells, where it localized to the basal body and its daughter centriole. In ciliated mammalian cell lines, including canine kidney, mouse collecting duct, and human ARPE-19 cells, TOPORS localized to the base of the primary cilium. TOPORS maintained association with the centrosome throughout the cell cycle in synchronized RPE-1 cells. Western blot analysis detected TOPORS at an apparent molecular mass of 150 kD.
The TOPORS gene contains 3 exons and spans 13 kb (Haluska et al., 1999).
By FISH, Zhou et al. (1999) mapped the TOPORS gene to chromosome 12p12.1-p11.2. However, Chu et al. (2001) mapped the TOPORS gene by FISH to chromosomes 9p21, a localization confirmed by the International Radiation Hybrid Mapping Consortium (RH98974).
By yeast 2-hybrid analysis, Zhou et al. (1999) determined that a central region of TOPORS containing the putative nuclear localization signal and 2 PEST sequences was required for interaction with p53. The leucine zipper of TOPORS was dispensable for interaction with p53.
By coimmunoprecipitation assays of transfected HeLa cells, Haluska et al. (1999) confirmed that TOPORS interacts with TOP1.
Chu et al. (2001) found that TOPORS had zinc-dependent DNA binding activity. The region from amino acids 51 to 374 was responsible for DNA binding. Chu et al. (2001) identified a palindromic DNA sequence that bound TOPORS in the upstream regulatory region of the E-cadherin gene (CDH1; 192090) and in 2 intervening regions of the talin gene (TLN1; 186745).
Rajendra et al. (2004) found that TOPORS functioned in vitro as an E3 ubiquitin ligase with the E2 enzymes UBCH5A (UBE2D1; 602961), UBCH5C (UBE2D3; 602963), and UBCH6 (UBE2E1; 602916), but not with other E2 enzymes examined. A conserved tryptophan within the RING domain of TOPORS was required for ubiquitination activity. In vitro and cellular studies implicated p53 as a ubiquitination substrate for TOPORS. Similar to MDM2 (164785), overexpression of TOPORS resulted in a proteasome-dependent decrease in p53 protein expression in a human osteosarcoma cell line.
By coimmunoprecipitation of bovine retina extracts, Chakarova et al. (2011) found that Topors coprecipitated with the centrosomal/basal body protein gamma-tubulin (TUBG; 191135) and with the dynein-dynactin protein complex (see DCTN1, 601143), which is part of the retrograde transport mechanism.
Chakarova et al. (2007) used a positional cloning approach to sequence genes in the critical region on 9p21.1 linked to a form of autosomal dominant retinitis pigmentosa with a distinct phenotype (RP31; 609923). They identified heterozygous mutations in the TOPORS gene: a 1-bp insertion in exon 3 (609507.0001) in a French Canadian family and a 2-bp deletion in exon 3 (609507.0002) in a small German family. Chakarova et al. (2007) commented that the ubiquitous nature of TOPORS expression and a lack of mutant protein in patients was highly suggestive of haploinsufficiency, rather than a dominant-negative effect, as the molecular mechanism of the disease and made the clinical phenotype amenable to rescue by somatic gene therapy.
In 3 unrelated Chinese men with RP, Xu et al. (2014) identified heterozygosity for truncating mutations in the TOPORS gene (see, e.g., 609507.0003 and 609507.0004). Sanger sequencing confirmed the mutations; none was found in 96 control individuals.
Wang et al. (2022) reviewed published RP-associated mutations in the TOPORS gene, and stated that 17 truncating variants had been reported in 47 families, including 42 families with typical RP and 5 with unclassified retinal dystrophy. All but 1 of the truncating alleles were distributed in the last exon of TOPORS and involved amino acid residues 807 to 867. The authors noted that in the gnomAD database, only 1 truncating allele was in this region, indicating that pathogenic truncating variants were significantly clustered in residues 807 to 867 (p = 1.1 x 10(-17)). The authors suggested that the underlying pathogenetic mechanism was likely a dominant-negative effect rather than haploinsufficiency.
Chakarova et al. (2011) found that zebrafish topors was expressed in all retinal cellular layers, in contrast to mammalian TOPORS. In photoreceptors, highest expression was detected in the inner segment. Morpholino-mediated knockdown of topors in zebrafish resulted in a wide range of developmental anomalies, with a high percentage of morphants showing microphthalmia, kinked tail, and edema. Some morphants also showed hydrocephaly and pericardial effusion.
In a French Canadian family with early-onset autosomal dominant retinitis pigmentosa and perivascular retinal pigment epithelium atrophy (RP31; 609923) mapping to chromosome 9p21.1 (Papaioannou et al., 2005), Chakarova et al. (2007) identified a 1-bp insertion, 2474_2475insA, in exon 3 of the TOPORS gene. The mutation was predicted to result in premature termination of the protein (Tyr825fs).
In a small German family, Chakarova et al. (2007) found that autosomal dominant retinitis pigmentosa with perivascular retinal pigment epithelium atrophy (RP31; 609923) was associated with a 2-bp deletion, which they designated c.2552_2553delGA, in exon 3 of the TOPORS gene. The mutation was predicted to result in premature termination of the protein (Arg851fs).
In a 39-year-old Chinese man (RP021) with onset of retinitis pigmentosa at age 20 years, and in his affected father, Xu et al. (2014) identified the 2-bp deletion (c.2556_2557delGA, NM_005802.4) identified by Chakarova et al. (2007).
In a 19-year-old Chinese man (RP316) with retinitis pigmentosa (RP31; 609923), Xu et al. (2014) identified heterozygosity for a 4-bp deletion (c.2550_2553delCAGA, NM_005802.4) in exon 3 of the TOPORS gene, causing a frameshift predicted to result in a premature termination codon (Asp850GlufsTer15). The mutation status of the proband's unaffected parents was not reported, but the variant was not found in 96 control individuals.
Wang et al. (2022) restudied family RP316 (as 5556) and reported that neither of the proband's unaffected parents carried the mutation.
In a 30-year-old Chinese man (RP282) with retinitis pigmentosa (RP31; 609923), Xu et al. (2014) identified heterozygosity for a 4-bp deletion (c.2554_2557delGAGA, NM_005802.4) in exon 3 of the TOPORS gene, causing a frameshift predicted to result in a premature termination codon (Glu852GlnfsTer13). The mutation status of the proband's affected mother was not reported, although the variant was not found in an unaffected maternal uncle, the proband's unaffected son, or 96 control individuals.
In the proband from a 3-generation Chinese family (14760) segregating autosomal dominant RP, Wang et al. (2022) identified heterozygosity for the previously reported 4-bp deletion (c.2554_2557del) in the TOPORS gene. The mutation status of his affected mother and maternal grandmother was not reported.
Chakarova, C. F., Khanna, H., Shah, A. Z., Patil, S. B., Sedmak, T., Murga-Zamalloa, C. A., Papaioannou, M. G., Nagel-Wolfrum, K., Lopez, I., Munro, P., Cheetham, M., Koenekoop, R. K., Rios, R. M., Matter, K., Wolfrum, U., Swaroop, A., Bhattacharya, S. S. TOPORS, implicated in retinal degeneration, is a cilia-centrosomal protein. Hum. Molec. Genet. 20: 975-987, 2011. [PubMed: 21159800] [Full Text: https://doi.org/10.1093/hmg/ddq543]
Chakarova, C. F., Papaioannou, M. G., Khanna, H., Lopez, I., Waseem, N., Shah, A., Theis, T., Friedman, J., Maubaret, C., Bujakowska, K., Veraitch, B., Abd El-Aziz, M. M., and 14 others. Mutations in TOPORS cause autosomal dominant retinitis pigmentosa with perivascular retinal pigment epithelium atrophy. Am. J. Hum. Genet. 81: 1098-1103, 2007. [PubMed: 17924349] [Full Text: https://doi.org/10.1086/521953]
Chu, D., Kakazu, N., Gorrin-Rivas, M. J., Lu, H.-P., Kawata, M., Abe, T., Ueda, K., Adachi, Y. Cloning and characterization of LUN, a novel RING finger protein that is highly expressed in lung and specifically binds to a palindromic sequence. J. Biol. Chem. 276: 14004-14013, 2001. [PubMed: 11278651] [Full Text: https://doi.org/10.1074/jbc.M010262200]
Haluska, P., Jr., Saleem, A., Rasheed, Z., Ahmed, F., Su, E. W., Liu, L. F., Rubin, E. H. Interaction between human topoisomerase I and a novel RING finger/arginine-serine protein. Nucleic Acids Res. 27: 2538-2544, 1999. [PubMed: 10352183] [Full Text: https://doi.org/10.1093/nar/27.12.2538]
Papaioannou, M., Chakarova, C. F., Prescott, D. Q. C., Waseem, N., Theis, T., Lopez, I., Gill, B., Koenekoop, R. K., Bhattacharya, S. S. A new locus (RP31) for autosomal dominant retinitis pigmentosa maps to chromosome 9p. Hum. Genet. 118: 501-503, 2005. [PubMed: 16189705] [Full Text: https://doi.org/10.1007/s00439-005-0063-3]
Rajendra, R., Malegaonkar, D., Pungaliya, P., Marshall, H., Rasheed, Z., Brownell, J., Liu, L. F., Lutzker, S., Saleem, A., Rubin, E. H. Topors functions as an E3 ubiquitin ligase with specific E2 enzymes and ubiquitinates p53. J. Biol. Chem. 279: 36440-36444, 2004. [PubMed: 15247280] [Full Text: https://doi.org/10.1074/jbc.C400300200]
Wang, J., Wang, Y., Jiang, Y., Li, X., Xiao, X., Li, S., Jia, X., Sun, W., Wang, P., Zhang, Q. Autosomal dominant retinitis pigmentosa-associated TOPORS protein truncating variants are exclusively located in the region of amino acid residues 807 to 867. Invest. Ophthal. Vis. Sci. 63: 19, 2022. [PubMed: 35579903] [Full Text: https://doi.org/10.1167/iovs.63.5.19]
Xu, Y., Guan, L., Shen, T., Zhang, J., Xiao, X., Jiang, H., Li, S., Yang, J., Jia, X., Yin, Y., Guo, X., Wang, J., Zhang, Q. Mutations of 60 known causative genes in 157 families with retinitis pigmentosa based on exome sequencing. Hum. Genet. 133: 1255-1271, 2014. [PubMed: 24938718] [Full Text: https://doi.org/10.1007/s00439-014-1460-2]
Zhou, R., Wen, H., Ao, S.-Z. Identification of a novel gene encoding a p53-associated protein. Gene 235: 93-101, 1999. [PubMed: 10415337] [Full Text: https://doi.org/10.1016/s0378-1119(99)00203-6]