Human POT1 Prevents Severe Telomere Instability Induced by Homology Directed DNA Repair

Abstract

The evolutionarily conserved POT1 protein binds single stranded G-rich telomeric DNA and has been implicated in contributing to telomeric DNA maintenance and the suppression of DNA damage checkpoint signaling. Here, we explore human POT1 function through genetics and proteomics discovering that the complete absence of POT1 leads to severe telomere maintenance defects that had not been anticipated from previous depletion studies. Conditional deletion of POT1 in HEK293E cells gives rise to rapid telomere elongation and length heterogeneity, branched telomeric DNA structures, telomeric R-loops and telomere fragility. We determine the telomeric proteome upon POT1-loss implementing an improved telomeric chromatin isolation protocol. We identify a large set of proteins involved in nucleic acid metabolism that engage with telomeres upon POT1-loss. Inactivation of the homology directed repair machinery suppresses POT1-loss mediated telomeric DNA defects. Our results unravel as major function of human POT1 the suppression of telomere instability induced by homology directed repair.