Regulation of telomere structure and function in A. thaliana

Miguel A. Vega-Palas

Our group focuses on the study of Arabidopsis thaliana telomeres. We are studying the influence of the epigenetic characteristics of telomeric regions and of different telomeric proteins on telomeres biology.

Telomeres localize at the ends of eukaryotic chromosomes and associate with multiple proteins giving rise to nucleoprotein complexes that solve two fundamental problems. On the one hand, they solve the replication problem that arises as a consequence of the limitation that regular DNA polymerases have to synthesize the lagging strand at the end of linear DNA molecules. On the other hand, they solve the protection problem that arises at DNA ends and Double Strand Breaks. The end-replication problem is usually solved by the enzyme telomerase, which is a retrotranscriptase that adds telomeric repeats to the 3’ chromosome ends by using a RNA template, and by additional proteins that also contribute to fill-in the 3’ end extended by telomerase. With regard to the end-protection problem, two conserved protein complexes contribute to bypass it by preventing the end of the chromosomes from being recognized as DNA breaks. These capping complexes, which are known as shelterin and CST, also contribute to solve the end-replication problem. Whereas functional telomerase and CST complexes exist in Arabidopsis, the existence of a functional shelterin complex remains to be clearly stablished.

Regulación de la estructura y de la función de los telómeros en A. thaliana

The epigenetic features of defined chromosomal domains condition their biochemical and functional properties. Hence the interest in studying the epigenetic marks present at relevant chromosomal loci. Telomeric regions, which include telomeres and subtelomeres, have been traditionally considered heterochromatic. However, whereas the heterochromatic nature of subtelomeres has been widely accepted, the epigenetic status of telomeres has remained controversial. This has been largely due to the difficulty to study telomere epigenetics. The epigenetic modifications of telomeres have been usually analysed by microscopy or by chromatin immunoprecipitation (ChIP). However, these analyses could be challenged by subtelomeres and/or interstitial telomeric sequences (ITSs). Whereas telomeres and subtelomeres cannot be differentiated by standard microscopy techniques, telomeres and ITSs might not be differentiated in ChIP analyses if these analyses are not properly controlled. Hence, studies focusing on the epigenetic features of telomeres have to be carefully designed and interpreted.

Our group has designed and set up multiple techniques that allow the analysis of the chromatin organization of Arabidopsis telomeres independently of ITSs. These techniques are based on the study of ChIP experiments followed by hybridization with a telomeric probe, PCR or sequencing, or on the analysis of Whole Genome Bisulfite Sequencing studies. In addition, we have set up PCR techniques to study the length of Arabidopsis telomeres and how this length or environmental factors influence telomeres capping and cell proliferation. By using these techniques, we have found that Arabidopsis telomeres have low levels of heterochromatic marks whereas subtelomeric regions and ITSs associate with heterochromatin. Thus, telomeric regions in Arabidopsis have a bimodal chromatin organization with telomeres lacking of heterochromatic marks and heterochromatic subtelomeres. Since we have also found that Arabidopsis heterochromatin control telomere length, we have proposed that subtelomeric heterochromatin could influence telomere biology. In addition, we have verified that the CST complex exerts a widespread end-capping protection of Arabidopsis telomeres.

Our group has also found that the epigenetic features of human telomeres are not heterochromatic. Thus, since human subtelomeres are known to organize as heterochromatin, telomeric regions in humans also have a bimodal chromatin organization. This organization is quite novel because both, telomeres and subtelomeres, have been found to organize as heterochromatin in other model systems. We have proposed that the bimodal organization of human telomeres should be considered when epigenetic drugs are used in clinical trials for cancer treatment.

  • A Nested PCR Telomere Fusion Assay Highlights the Widespread End-Capping Protection of Arabidopsis CTC1. Vaquero-Sedas MI, Vega-Palas MA. Int J Mol Sci. 2024 Jan 4;25(1):672. doi: 10.3390/ijms25010672.
  • Epigenetic nature of Arabidopsis thaliana telomeres. Vaquero-Sedas MI, Vega-Palas MA. Plant Physiol. 2023 Jan 2;191(1):47-55. doi: 10.1093/plphys/kiac471.
  • A complex network of interactions governs DNA methylation at telomeric regions. Farrell C, Vaquero-Sedas MI, Cubiles MD, Thompson M, Vega-Vaquero A, Pellegrini M, Vega-Palas MA. Nucleic Acids Res. 2022 Feb 22;50(3):1449-1464. doi: 10.1093/nar/gkac012.
  • Targeting Cancer through the Epigenetic Features of Telomeric Regions. Vaquero-Sedas MI, Vega-Palas MA. Trends Cell Biol. 2019 Apr;29(4):281-290. doi: 10.1016/j.tcb.2018.12.006.
  • Epigenetic features of human telomeres. Cubiles MD, Barroso S, Vaquero-Sedas MI, Enguix A, Aguilera A, Vega-Palas MA. Nucleic Acids Res. 2018 Mar 16;46(5):2347-2355. doi: 10.1093/nar/gky006.
  • Novel features of telomere biology revealed by the absence of telomeric DNA methylation. Vega-Vaquero A, Bonora G, Morselli M, Vaquero-Sedas MI, Rubbi L, Pellegrini M, Vega-Palas MA. Genome Res. 2016 Aug;26(8):1047-56. doi: 10.1101/gr.202465.115.
  • Determination of Arabidopsis thaliana telomere length by PCR. Vaquero-Sedas MI, Vega-Palas MA. Sci Rep. 2014 Jul 2;4:5540. doi: 10.1038/srep05540.

     

  • On the chromatin structure of eukaryotic telomeres. Vaquero-Sedas MI, Vega-Palas MA. Epigenetics. 2011 Sep 1;6(9):1055-8. doi: 10.4161/epi.6.9.16845.
  • Telomeric transcriptional silencing in a natural context. Vega-Palas MA, Venditti S, Di Mauro E. Nat Genet. 1997 Mar;15(3):232-3. doi: 10.1038/ng0397-232.
  • NtcA, a global nitrogen regulator from the cyanobacterium Synechococcus that belongs to the Crp family of bacterial regulators. Vega-Palas MA, Flores E, Herrero A. Mol Microbiol. 1992 Jul;6(13):1853-9. doi: 10.1111/j.1365-2958.1992.tb01357.x.
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