Super-Resolution Imaging of Telomeres Reveals that Compaction of Telomeric DNA by Shelterin Protects Chromosome Terminii

Abstract

Telomeres, which are tandem TTAGGG repeats at the ends of mammalian chromosomes, are susceptible to degradation by nucleases and misrecognition as breaks in DNA. Telomeres are protected against DNA damage response (DDR) pathways by the shelterin complex. It has been proposed that higher order remodeling of telomeric chromatin by shelterin plays a role in this protection, but telomeres cannot be studied by crosslinking and chromosome capture methods due to failure of sequence discrimination between repetitive TTAGGG tracts. Using photoactivated localization microscopy (PALM) we imaged telomeres in human cells at ∼15 nm of resolution. We found that shelterin mediates formation of compact telomeric structures (∼150 nm in diameter). Knockdown of TRF1, TRF2 and TIN2 subunits of shelterin results in decompaction and up to 30-fold increase in volume of these telomeric structures. Mutations that abrogate TRF1 or TRF2 dimerization also lead to different levels of telomere decompaction, which positively correlates with DDR signal accumulation at telomeres in these cells. The changes in telomere structure are not due to DDR accumulation in TRF2 mutant cells, because similar levels of decompaction are observed after inactivating the ataxia telangiectasia mutated (ATM) pathway using RNAi. Our results demonstrate that shelterin remodels telomeric chromatin into compact structures by inter-repeat crosslinking of telomeric tracts.