Related Experiment Video

Updated: Feb 7, 2026

Identifying the Effects of BRCA1 Mutations on Homologous Recombination using Cells that Express Endogenous Wild-type BRCA1
08:53

Identifying the Effects of BRCA1 Mutations on Homologous Recombination using Cells that Express Endogenous Wild-type BRCA1

Published on: February 17, 2011

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Shieldin complex promotes DNA end-joining and counters homologous recombination in BRCA1-null cells

Harveer Dev1,2, Ting-Wei Will Chiang1, Chloe Lescale3

  • 1The Wellcome Trust/Cancer Research UK Gurdon Institute and Department of Biochemistry, University of Cambridge, Cambridge, UK.

Nature Cell Biology
|July 20, 2018
PubMed

Insights

Scientists discovered a protein complex called Shieldin (SHLD1/2) that helps cancer cells resist PARP inhibitors. Inactivating Shieldin could make PARP inhibitor treatments more effective for BRCA1-deficient cancers.

Area of Science:

  • Molecular Biology
  • Cancer Biology
  • Genetics

Background:

  • BRCA1 deficiencies are linked to various cancers, including breast, ovarian, and prostate.
  • Tumors with BRCA1 deficiencies exhibit heightened sensitivity to poly(ADP-ribose) polymerase (PARP) inhibitors.
  • Understanding resistance mechanisms to PARP inhibitors is crucial for improving cancer therapy.

Purpose of the Study:

  • To investigate the mechanisms of resistance to PARP inhibitors in BRCA1-deficient cancer cells.
  • To identify novel proteins involved in PARP inhibitor resistance.
  • To elucidate the functional role of identified proteins in DNA repair pathways.

Main Methods:

  • Whole-genome CRISPR-Cas9 synthetic-viability/resistance screens were performed on BRCA1-deficient breast cancer cells.
  • Cells were treated with PARP inhibitors to identify genes conferring resistance.
  • Biochemical and cellular assays were used to characterize protein interactions and functions.

Main Results:

  • Two uncharacterized proteins, C20orf196 and FAM35A, were identified as key mediators of PARP inhibitor resistance.
  • These proteins form a complex named 'Shieldin' (SHLD1/2), with FAM35A interacting with single-stranded DNA.
  • Shieldin functions downstream of 53BP1/RIF1/MAD2L2 to inhibit DNA double-strand break (DSB) resection and counteract homologous recombination in BRCA1-deficient cells.
  • Inactivation of Shieldin sensitizes BRCA1-deficient cells to PARP inhibitors and cisplatin.
  • Reduced SHLD1/2 expression was observed in human breast cancers resistant to PARP inhibitors.

Conclusions:

  • The Shieldin complex (SHLD1/2) is a critical mediator of resistance to PARP inhibitors in BRCA1-deficient cancers.
  • Shieldin antagonizes DNA double-strand break repair pathways, specifically by restricting resection and homologous recombination.
  • Targeting Shieldin or understanding its expression status may offer new strategies for patient stratification and overcoming PARP inhibitor resistance.

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