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BRCA1 tumour suppression occurs via heterochromatin-mediated silencing

Abstract

Mutations in the tumour suppressor gene BRCA1 lead to breast and/or ovarian cancer. Here we show that loss of Brca1 in mice results in transcriptional de-repression of the tandemly repeated satellite DNA. Brca1 deficiency is accompanied by a reduction of condensed DNA regions in the genome and loss of ubiquitylation of histone H2A at satellite repeats. BRCA1 binds to satellite DNA regions and ubiquitylates H2A in vivo. Ectopic expression of H2A fused to ubiquitin reverses the effects of BRCA1 loss, indicating that BRCA1 maintains heterochromatin structure via ubiquitylation of histone H2A. Satellite DNA de-repression was also observed in mouse and human BRCA1-deficient breast cancers. Ectopic expression of satellite DNA can phenocopy BRCA1 loss in centrosome amplification, cell-cycle checkpoint defects, DNA damage and genomic instability. We propose that the role of BRCA1 in maintaining global heterochromatin integrity accounts for many of its tumour suppressor functions.

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Figure 1: Brca1 deficiency impairs heterochromatin structure.
Figure 2: BRCA1 and its ubiquitin E3 ligase activity are required for gene silencing in constitutive heterochromatin.
Figure 3: Ubiquitylated histone H2A is directly involved in BRCA1-mediated heterochromatic silencing.
Figure 4: De-repression of satellite DNA transcription occurs in Brca1 -deficient breast cancers.
Figure 5: Ectopic expression of satellite DNA transcripts leads to genomic instability in human mammary epithelial cells.

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Gene Expression Omnibus

Data deposits

Microarray data have been deposited in the GEO database under the accession number GSE6310 (brain BRCA1 conditional knockout).

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Acknowledgements

We thank B. Miller for assistance in culturing NPCs; E. Ke for discussion and analysis of Affymetrix data; A. Yanai for the BRCA1 targeting shRNA construct; C. Lilley for assistance with western blotting; and Z. You for assistance with the LI-COR Odyssey Infrared Imaging System. We thank A. Berns for his sustained interest in this work and providing mutant mice and materials, and M. Vidal for providing mouse embryo fibroblasts containing a conditional deletion allele of Ring1B. Q.Z. was supported by the California Breast Cancer Research Program and Ruth L. Kirschtein National Research Service Award. G.M.P. was supported by a fellowship of the California Institute of Regenerative Medicine. H.S. is a recipient of ASPET-Merck fellowship. I.M.V. is an American Cancer Society Professor of Molecular Biology, and holds the Irwin and Joan Jacobs Chair in Exemplary Life Science. This work was supported in part by grants from the NIH, Ipsen/Biomeasure, Sanofi Aventis, and the H.N. and Frances C. Berger Foundation. F.H.G. is supported by NIH NS52842, NS50217 and the Lookout Fund.

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Q.Z. generated and Q.Z., N.T. and G.M.P. maintained all the knockout mice. G.M.P. and Q.Z. made the initial heterochromatin observation. Q.Z. and A.M.H. performed confocal microscopy experiments. G.M.P., Q.Z. and N.T. performed ChIP experiments. RNA isolation and microarray experiments were performed by Q.Z., G.M.P. and N.T. Microdissection of murine brains were performed by G.M.P. and A.M.H. under the guidance of F.H.G. G.M.P. designed the H2A–ubiquitin fusion experiments that were performed by Q.Z., G.M.P. and N.T. Satellite RNA experiments were designed by G.M.P. and Q.Z. and performed by Q.Z., G.M.P. and N.T. H.S. established the embryonic neural stem cell isolation and culture. P.M.N. obtained, isolated and curated the clinical patient samples, which were analysed by Q.Z., G.M.P. and N.T. All other experiments were performed by Q.Z., G.M.P. and N.T. All experiments and experimental design was performed under the supervision of I.M.V. G.M.P., Q.Z. and I.M.V. wrote the manuscript.

Corresponding authors

Correspondence to Gerald M. Pao or Inder M. Verma.

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The authors declare no competing financial interests.

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Zhu, Q., Pao, G., Huynh, A. et al. BRCA1 tumour suppression occurs via heterochromatin-mediated silencing. Nature 477, 179–184 (2011). https://doi.org/10.1038/nature10371

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