SAFB1 transiently associates with chromatin and promotes γH2AX spreading after DNA damage.
Major finding: SAFB1 transiently associates with chromatin and promotes γH2AX spreading after DNA damage.
Mechanism: SAFB1 recruitment is dependent on damage-induced poly(ADP-ribosyl)ation by PARP1.
Impact: Recruitment of an architectural protein can overcome structural barriers to the DNA damage response.
Chromatin modifications must occur rapidly after DNA damage to allow DNA repair factors and other components of the DNA damage response access to damaged DNA, but excessive spreading of these chromatin modifications to undamaged loci can be harmful to the cell. Proteomic screens have revealed that scaffold attachment factor B1 (SAFB1), a nonenzymatic, architectural component of chromatin thought to mediate long-range chromatin interactions and higher-order chromatin structure, is posttranslationally modified upon DNA damage, prompting Altmeyer and colleagues to evaluate whether SAFB1 regulates damage-induced chromatin modifications. Knockdown of SAFB1 reduced histone H2AX phosphorylation (γH2AX) in response to ionizing radiation or hydroxyurea-induced DNA replication stress and reduced cell survival in association with impaired DNA damage signaling. Together with the observation that SAFB1 depletion counteracted the increase in γH2AX formation caused by histone deacetylase inhibition, these findings suggested that SAFB1 cooperates with histone acetylation to create a permissive environment at damaged chromatin for induction of the DNA damage response. SAFB1 recruitment was not dependent on canonical DNA damage response kinases but could be abolished by PARP1 knockdown or PARP inhibition, indicating that the association of SAFB1 with damaged chromatin is poly(ADP-ribosyl)ation-dependent and raising the possibility that inhibition of DNA damage-induced SAFB1 activity is part of the mechanism of action of PARP inhibitors. SAFB1 localization to sites of damage was also highly dynamic, with SAFB1 disappearing from damaged foci within minutes. Forced extension of the association between SAFB1 and damaged chromatin through fusion with the 53BP1 focus-forming domain increased H2AX phosphorylation, consistent with a model in which the dynamic exchange of SAFB1 at sites of DNA damage allows chromatin modifications and recruitment of DNA damage response factors to occur without spreading excessively.