SMCHD1 Promotes ATM-dependent DNA Damage Signaling and Repair of Uncapped Telomeres
SMCHD1 (structural maintenance of chromosomes flexible hinge domain containing protein 1) has been implicated in X-chromosome inactivation, imprinting and DNA damage repair. Mutations in SMCHD1 can also cause facioscapulohumoral muscular dystrophy. More recently, SMCHD1 has also been detected as component of telomeric chromatin. Here, we identify requirements of SMCHD1 for DNA damage signaling and non-homologous end joining (NHEJ) at unprotected telomeres. Co-depletion of SMCHD1 with TRF2 reduced the rate of 3 overhang removal in time course experiments and the number of telomere end fusions. In SMCHD1 deficient cells, the formation of ATM pS1981, {gamma}H2AX and 53BP1 containing telomere dysfunction induced foci (TIFs) were diminished indicating defects in checkpoint signaling. Strikingly, removal of TPP1 and subsequent activation of ATR signaling rescued telomere fusion events in TRF2-depleted SMCHD1 knockout cells. Together, these data indicate that SMCHD1 depletion reduces telomere fusions in TRF2-depleted cells due to defects in ATM-dependent DNA checkpoint signaling. SMCHD1 mediates DNA damage signaling activation upstream of ATM phosphorylation at uncapped telomeres.