Abstract
The pro-NHEJ and anti-HR functions of 53BP1 are dependent on RIF1 and PTIP, respectively.
Major finding: The pro-NHEJ and anti-HR functions of 53BP1 are dependent on RIF1 and PTIP, respectively.
Mechanism: PTIP and RIF1 interact with distinct phosphorylation sites on 53BP1 at double-strand breaks.
Impact: Inhibition of PTIP may selectively promote restored HR in heterozygous BRCA1 mutation carriers.
Productive repair of DNA double-strand breaks (DSB) during immunoglobulin class switch recombination (CSR) and telomere uncapping is mediated by nonhomologous end-joining (NHEJ) driven by the DNA damage response protein, tumor protein p53 binding protein 1 (53BP1). In the absence of BRCA1, which opposes 53BP1 activity in S phase, 53BP1 suppresses homologous recombination (HR), leading to aberrant chromosomes and genomic instability. RAP1 interacting factor homolog (RIF1) has been implicated in the regulation of 53BP1 function at DSBs, but loss of RIF1 does not fully restore HR in BRCA1-deficient cells, suggesting that other proteins may also contribute to 53BP1 activity. Callen and colleagues found that expression of a phosphomutant 53BP1 (53BP18A) was sufficient to induce CSR but did not block HR or promote genomic instability in BRCA1-deficient B cells treated with PARP inhibitor (PARPi), indicating that these functions are separable. Mutant 53BP18A interacted with RIF1 but was unable to recruit PAX-interacting (with transcription-activation domain) protein 1 (PAXIP1, also known as PTIP) to DSBs following irradiation. Similar to the 53BP18A mutant, PTIP depletion did not impair NHEJ during CSR; however, PTIP was necessary for NHEJ-mediated repair of uncapped telomeres. Moreover, PTIP deficiency enhanced RAD51 foci formation and rescued HR in BRCA1-null cells, resulting in insensitivity to PARPi and increased genome stability and cell survival. The ability of PTIP to suppress HR required recruitment of PTIP to DSBs via interaction with 53BP1 at 8 N-terminal ataxia telangiectasia mutated (ATM)-dependent phosphorylation sites, whereas RIF1 was independently recruited to DNA damage foci via binding to 53BP1 at 7 more C-terminal phosphorylation sites. These findings identify distinct phosphoprotein interactions that modulate the pro-NHEJ and anti-HR functions of 53BP1 and suggest that inhibition of PTIP may selectively restore HR in carriers with heterozygous BRCA1 mutations without affecting B-cell recombination.