Role of Ku in the DNA damage response activated from telomeres

Abstract

The Ku heterodimer, composed of two subunits, Ku70 and Ku80, is the main DNA binding component of the non homologous end joining (NHEJ) DNA repair pathway. Intriguingly, Ku is also present at telomeres and is required for telomere protection and regulation, but its role in telomere maintenance is not fully understood. We previously showed that Ku70 S155 is involved in regulation of the DNA damage response leading to apoptosis in response to ionizing radiation (IR). Here we show that expression of Ku70 bearing a phosphomimetic substitution (Ku70 S155D) produces a robust DNA damage response, marked by an upregulation of phospho‐H2AX and ATM activation, even in the absence of any DNA damage treatment. Telomere FISH analysis showed colocalization of γ‐H2AX foci with telomere ends. Cells expressing Ku70 S155D displayed cell cycle arrest, in particular in the G2 phase and upregulation of several factors involved in cell cycle arrest. Conversely, cells expressing Ku70 with a S155 alanine substitution (S155A) showed an increased cell growth rate and downregulation of genes involved in cell cycle checkpoints. This suggests that the phosphorylation status of Ku70 S155 regulates the DNA damage response from telomeres to control cell cycle progression, providing a novel role for Ku as a signaling factor at mammalian telomeres.