Abstract
The shieldin complex, comprised of SHLD1, SHLD2, SHLD3, and REV7, is a 53BP1 effector complex.
Major finding: The shieldin complex, comprised of SHLD1, SHLD2, SHLD3, and REV7, is a 53BP1 effector complex.
Concept: Shieldin interacts with the CST complex to suppress DNA double-strand break resection.
Impact: Mutations in shieldin may confer resistance to PARP inhibitors in patients with BRCA1-deficient tumors.
Repair of DNA double-strand breaks (DSB) by homologous recombination (HR) requires end resection to generate a 3′ overhang, whereas nonhomologous end joining (NHEJ) can join unresected ends. In cancer, mutations in BRCA1 suppress DSB resection, impairing HR and conferring sensitivity to PARP inhibitors. In BRCA1-deficient cells, the chromatin-binding protein 53BP1 is involved in shielding DNA ends, preventing the resection of DSBs. However, the mechanism by which this occurs has not been fully elucidated. To identify proteins involved in the 53BP1 pathway, Noordermeer, Adam, Setiaputra, and colleagues performed a CRISPR/Cas9 screen find genes that, when mutated, restore HR in BRCA1-deficient cells. This screen identified SHLD1 as a 53BP1 effector, and immunoprecipitation coupled to mass spectrometry identified the other interaction partners, SHLD2, SHLD3, and REV7, altogether comprising a complex termed shieldin. The shieldin complex localized to sites of DNA DSBs in a 53BP1- and RIF1-dependent manner and bound to single-stranded DNA via the SHLD2 subunit. Loss of the shieldin complex impaired NHEJ and conferred PARP inhibitor resistance in BRCA1-deficient tumor cells in vitro and in vitro. Further, mutations in shieldin subunits restored HR, rendering BRCA1-deficient cells resistant to PARP inhibitors. Altogether, these findings suggest a model by which shieldin protects DNA ends to facilitate 53BP1-dependent DNA repair. In a related study, Mirman, Lottersberger, and colleagues sought to determine the mechanism by which 53BP1–RIF1–shieldin suppresses the generation of recombinogenic 3′ overhangs. Newly replicated telomeres are resected to generate long 3′ overhangs that are filled in by the Polα primase. A CTC1–STN1–TEN1 (CST) complex interacted with shieldin and localized with Polα at sites of DNA damage in a 53BP1- and shieldin-dependent manner. Consistent with these findings, CST and Polα enhanced the efficacy of PARP1 inhibitors in BRCA-deficient cells. These findings suggest that CST–Polα-mediated fill-in regulates 53BP1-mediated repair of DSBs. Taken together, these studies provide insight into the role of 53BP1 in DNA DSB repair.
Note: Research Watch is written by Cancer Discovery editorial staff. Readers are encouraged to consult the original articles for full details. For more Research Watch, visit Cancer Discovery online at http://cancerdiscovery.aacrjournals.org/CDNews.
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