Alternative titles; symbols
HGNC Approved Gene Symbol: STN1
Cytogenetic location: 10q24.33 Genomic coordinates (GRCh38) : 10:103,877,569-103,918,184 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 10q24.33 | Cerebroretinal microangiopathy with calcifications and cysts 2 | 617341 | Autosomal recessive | 3 |
The STN1 gene encodes a member of the human CTC1 (613129)-STN1- TEN1 (613130) (CST) complex, which plays a role in telomere C-strand synthesis as well as in genomewide replication and the recovery from replication stress (Miyake et al., 2009; summary by Simon et al., 2016).
Casteel et al. (2009) cloned mouse Obfc1, which they called Aaf44. The deduced 378-amino acid protein contains an oligonucleotide/oligosaccharide-binding (OB) fold domain that is predicted to bind single-stranded DNA (ssDNA). Database analysis revealed Aaf44 orthologs in several species, including human. The human AAF44 protein shares 71% identity with mouse Aaf44, with highest conservation in the OB fold domain. Northern blot analysis detected low and variable Aaf44 expression in several mouse tissues. Endogenous Aaf44 protein had an apparent molecular mass of 44 kD by SDS-PAGE. Aaf44 and Aaf132 (Ctc1; 613129) colocalized in nuclei of transfected synchronized HeLa cells, with exclusion from nucleoli. The 2 proteins colocalized with PCNA (176740), presumably within DNA replication foci.
By searching databases for sequences similar to S. pombe Stn1, followed by RT-PCR of HeLa cell mRNA, Miyake et al. (2009) cloned OBFC1, which they called STN1. The deduced 368-amino acid protein contains an N-terminal OB fold. Immunohistochemical analysis and FISH revealed that STN1 colocalized with telomeric DNA. It did not appear to localize with replication foci.
Hartz (2009) mapped the STN1 gene to chromosome 10q24.33 based on an alignment of the STN1 sequence (GenBank AK026212) with the genomic sequence (GRCh37).
Casteel et al. (2009) mapped the mouse Stn1 gene to chromosome 19.
Cryoelectron Microscopy
The CTC1 (613129)-STN1-TEN1 (613130) complex, also known as the CST complex, is essential for telomere maintenance and resolution of stalled replication forks genomewide. Lim et al. (2020) reported the 3.0-angstrom cryoelectron microscopy structure of human CST bound to telomeric single-stranded DNA, which assembles as a decameric supercomplex. The atomic model of the 134-kD CTC1 subunit, built almost entirely de novo, revealed the overall architecture of CST and the DNA-binding anchor site. The carboxyl-terminal domain of STN1 interacts with CTC1 at 2 separate docking sites, allowing allosteric mediation of CST decamer assembly. Furthermore, ssDNA appears to staple 2 monomers to nucleate decamer assembly. CTC1 has stronger structural similarity to replication protein A (RPA; see 179835) than the expected similarity to yeast Cdc13. The decameric structure suggested that CST can organize ssDNA analogously to the nucleosome's organization of double-stranded DNA.
Using immunoprecipitation analysis, Casteel et al. (2009) showed that epitope-tagged mouse Aaf44 and Aaf132 interacted in transfected HEK293 cells. Expression of Aaf44 appeared to stabilize the Aaf132 protein. When expressed individually, Aaf44 and Aaf132 weakly bound single-stranded oligo(dC), but when expressed together, they showed strong ssDNA binding. When coexpressed, Aaf44 and Aaf132 formed an AAF complex that stimulated DNA primase activity by calf or human polymerase-alpha-primase (see 176636) on a poly(dT) template. Small interfering RNA directed against AAF44 in human cell lines inhibited DNA synthesis, but did not affect cell viability.
Using Western blot analysis of coimmunoprecipitated proteins and yeast 2-hybrid assays, Miyake et al. (2009) showed that STN1 interacted with CTC1 and TEN1 in a complex, which the authors called the CST complex. STN1 interacted with both CTC1 and TEN1, but CTC1 and TEN1 did not show significant interaction. The CST complex immunoprecipitated telomeric DNA, but not Alu repeat DNA, from HeLa cells. Only a fraction of telomeres associated with the CST complex, and association of the CST complex with telomeres did not vary during the cell cycle. The CST complex, but not its individual components, bound ssDNA with high affinity and in a sequence-independent manner. The CST complex appeared to protect telomeres independently of POT1 (606478). Knockdown studies suggested that the CST complex and POT1 play redundant roles in telomere protection.
Chen et al. (2012) demonstrated that the human CST complex, implicated in telomere protection and DNA metabolism, inhibits telomerase (see 602322) activity through primer sequestration and physical interaction with the protection of telomeres 1 (POT1)-TPP1 (609377) telomerase processivity factor. CST competes with POT1-TPP1 for telomeric DNA, and CST-telomeric-DNA binding increases during late S/G2 phase only on telomerase action, coinciding with telomerase shut-off. Depletion of CST allows excessive telomerase activity, promoting telomere elongation. Chen et al. (2012) proposed that through binding of the telomerase-extended telomere, CST limits telomerase action at individual telomeres to approximately one binding and extension event per cell cycle. The authors suggested that their findings defined the sequence of events that occur to first enable and then terminate telomerase-mediated telomere elongation.
Gu and Chang (2013) found that CTC1 mutations associated with Coats plus syndrome (CRMCC1; 612199) promoted telomere dysfunction by decreasing the stability of STN1 and reducing the ability of STN1 to interact with DNA polymerase-alpha (POLA; see 312040). They proposed that failure of STN1 to interact with POLA would make inactivating STN1 mutations incompatible with survival.
Using purified recombinant proteins, Ganduri and Lue (2017) showed that the STN1 component of the CST complex alone stimulated the synthesis of RNA-DNA chimeras by human primase-polymerase-alpha (PP; see 620063), primarily by affecting the coupling between primase and polymerase. Mutation analysis revealed that the PP-stimulatory activity of STN1 resided predominantly in its N-terminal OB fold domain. The OB fold directly interacted with POLA2 and also bound to single-stranded DNA. These binding activities of STN1 were, however, separable, and the PP stimulatory activity of STN1 was related to its POLA2-binding activity rather than its DNA-binding activity. The main interaction between human POLA2 and STN1 occurred through their OB fold domains.
In 2 unrelated patients, both born of consanguineous Pakistani parents, with cerebroretinal microangiopathy with calcifications and cysts-2 (CRMCC2; 617341), Simon et al. (2016) identified different homozygous missense mutations in the STN1 gene (R135T, 613128.0001 and D157Y, 613128.0002). The mutations, which were found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the families. Patient fibroblasts were abnormally large, contained cytoplasmic vacuoles and extended podia, and grew poorly due to replication defects. The cells showed premature cellular senescence and increased apoptotic nuclei compared to controls, as well as micronuclei and torn DNA nuclear bridges. Cells showed several replication defects, including a decreased ability to release cells from S-phase and attenuation in the recovery from replication fork stalling after stress. These defects could be rescued with wildtype STN1. Telomere lengths from peripheral blood leukocytes were normal in the first patient, but shortened in the second patient; telomere lengths from fibroblasts were normal in the second patient. However, fibroblasts from both patients showed telomeric defects, such as abnormal telomere C-strand synthesis, fused chromosome ends, and telomere dysfunction-induced foci. Simon et al. (2016) suggested that the long single-stranded G-rich telomere sequences could abrogate telomere protection and activate DNA damage response and repair. Neither mutation could rescue telangiectatic defects observed in zebrafish embryos with morpholino knockdown of the stn1 gene, suggesting that both mutations resulted in a loss of function.
Simon et al. (2016) found that morpholino knockdown of the stn1 gene in zebrafish embryos resulted in decreased red blood cells and an arrest in T-cell progenitors, as well as increased vascularity. The vascular defects could be rescued by wildtype stn1 and by thalidomide treatment.
In a 12-year-old girl, born of consanguineous Pakistani parents, with cerebroretinal microangiopathy with calcifications and cysts-2 (CRMCC2; 617341), Simon et al. (2016) identified a homozygous c.404G-C transversion (c.404G-C, NM_024928) in exon 5 of the STN1 gene, resulting in an arg135-to-thr (R135T) substitution at a highly conserved residue. The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. It was not found in the dbSNP (build 135), 1000 Genomes Project, or Exome Sequencing Project databases, or in 160 ethnically matched controls.
In a 19-year-old man, born of consanguineous Pakistani parents, with cerebroretinal microangiopathy with calcifications and cysts-2 (CRMCC2; 617341), Simon et al. (2016) identified a homozygous c.469G-T transversion (c.469G-T, NM_024928) in exon 6 of the STN1 gene, resulting in an asp157-to-tyr (D157Y) substitution at a highly conserved residue. The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. It was not found in the dbSNP (build 135), 1000 Genomes Project, or Exome Sequencing Project databases, or in 160 ethnically matched controls.
Casteel, D. E., Zhuang, S., Zeng, Y., Perrino, F. W., Boss, G. R., Goulian, M., Pilz, R. B. A DNA polymerase-alpha-primase cofactor with homology to replication protein A-32 regulates DNA replication in mammalian cells. J. Biol. Chem. 284: 5807-5818, 2009. [PubMed: 19119139] [Full Text: https://doi.org/10.1074/jbc.M807593200]
Chen, L.-Y., Redon, S., Lingner, J. The human CST complex is a terminator of telomerase activity. Nature 488: 540-544, 2012. [PubMed: 22763445] [Full Text: https://doi.org/10.1038/nature11269]
Ganduri, S., Lue, N. F. STN1-POLA2 interaction provides a basis for primase-pol alpha stimulation by human STN1. Nucleic Acids Res. 45: 9455-9466, 2017. [PubMed: 28934486] [Full Text: https://doi.org/10.1093/nar/gkx621]
Gu, P., Chang, S. Functional characterization of human CTC1 mutations reveals novel mechanisms responsible for the pathogenesis of the telomere disease Coats plus. Aging Cell 12: 1100-1109, 2013. [PubMed: 23869908] [Full Text: https://doi.org/10.1111/acel.12139]
Hartz, P. A. Personal Communication. Baltimore, Md. 11/20/2009.
Lim, C. J., Barbour, A. T., Zaug, A. J., Goodrich, K. J., McKay, A. E., Wuttke, D. S., Cech, T. R. The structure of human CST reveals a decameric assembly bound to telomeric DNA. Science 368: 1081-1085, 2020. [PubMed: 32499435] [Full Text: https://doi.org/10.1126/science.aaz9649]
Miyake, Y., Nakamura, M., Nabetani, A., Shimamura, S., Tamura, M., Yonehara, S., Saito, M., Ishikawa, F. RPA-like mammalian Ctc1-Stn1-Ten1 complex binds to single-stranded DNA and protects telomeres independently of the Pot1 pathway. Molec. Cell 36: 193-206, 2009. [PubMed: 19854130] [Full Text: https://doi.org/10.1016/j.molcel.2009.08.009]
Simon, A. J., Lev, A., Zhang, Y., Weiss, B., Rylova, A., Eyal, E., Kol, N., Barel, O., Cesarkas, K., Soudack, M., Greenberg-Kushnir, N., Rhodes, M., and 21 others. Mutations in STN1 cause Coats plus syndrome and are associated with genomic and telomere defects. J. Exp. Med. 213: 1429-1440, 2016. [PubMed: 27432940] [Full Text: https://doi.org/10.1084/jem.20151618]