BASC, a super complex of BRCA1-associated proteins involved in the recognition and repair of aberrant DNA structures.
Journal: 2000/May - Genes and Development
ISSN: 0890-9369
PUBMED: 10783165
Abstract:
We report the identities of the members of a group of proteins that associate with BRCA1 to form a large complex that we have named BASC (BRCA1-associated genome surveillance complex). This complex includes tumor suppressors and DNA damage repair proteins MSH2, MSH6, MLH1, ATM, BLM, and the RAD50-MRE11-NBS1 protein complex. In addition, DNA replication factor C (RFC), a protein complex that facilitates the loading of PCNA onto DNA, is also part of BASC. We find that BRCA1, the BLM helicase, and the RAD50-MRE11-NBS1 complex colocalize to large nuclear foci that contain PCNA when cells are treated with agents that interfere with DNA synthesis. The association of BRCA1 with MSH2 and MSH6, which are required for transcription-coupled repair, provides a possible explanation for the role of BRCA1 in this pathway. Strikingly, all members of this complex have roles in recognition of abnormal DNA structures or damaged DNA, suggesting that BASC may serve as a sensor for DNA damage. Several of these proteins also have roles in DNA replication-associated repair. Collectively, these results suggest that BRCA1 may function as a coordinator of multiple activities required for maintenance of genomic integrity during the process of DNA replication and point to a central role for BRCA1 in DNA repair.
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Genes Dev 14(8): 927-939

BASC, a super complex of BRCA1-associated proteins involved in the recognition and repair of aberrant DNA structures

Verna and Mars McLean Department of Biochemistry and Molecular Biology, Department of Cellular and Molecular Biology, Howard Hughes Medical Institute, and Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, Texas 77030 USA; Laboratory of Molecular Genetics, New York Blood Center, New York, New York 10021 USA
These authors contributed equally.
Corresponding author.
Received 2000 Jan 21; Accepted 2000 Mar 2.

Abstract

We report the identities of the members of a group of proteins that associate with BRCA1 to form a large complex that we have named BASC (BRCA1-associated genome surveillance complex). This complex includes tumor suppressors and DNA damage repair proteins MSH2, MSH6, MLH1, ATM, BLM, and the RAD50–MRE11–NBS1 protein complex. In addition, DNA replication factor C (RFC), a protein complex that facilitates the loading of PCNA onto DNA, is also part of BASC. We find that BRCA1, the BLM helicase, and the RAD50–MRE11–NBS1 complex colocalize to large nuclear foci that contain PCNA when cells are treated with agents that interfere with DNA synthesis. The association of BRCA1 with MSH2 and MSH6, which are required for transcription-coupled repair, provides a possible explanation for the role of BRCA1 in this pathway. Strikingly, all members of this complex have roles in recognition of abnormal DNA structures or damaged DNA, suggesting that BASC may serve as a sensor for DNA damage. Several of these proteins also have roles in DNA replication-associated repair. Collectively, these results suggest that BRCA1 may function as a coordinator of multiple activities required for maintenance of genomic integrity during the process of DNA replication and point to a central role for BRCA1 in DNA repair.

Keywords: BASC, BRCA1, DNA repair, DNA structure, cancer
Abstract

Two general classes of cancer genes have been identified (Kinzler and Vogelstein 1997). The first class consists of genes that control cell proliferation and tumor growth such as growth factors, cyclin-dependent kinase (Cdk) regulators such as cyclins, Cdk inhibitors (CKIs) and the retinoblastoma protein, apoptotic factors, and angiogenesis factors. These genes, when mutated or overproduced, promote the inappropriate accumulation of cells. The second class consists of genes that control the stability of the genome and prevent the accumulation of mutations in the first class of genes. These genes are called antimutators or caretaker genes and include DNA repair proteins, cell cycle checkpoint regulators, and genes that maintain the fidelity of chromosome segregation. Many genes of the second class have been identified, including the mismatch-repair genes, MSH2 and MLH1, which are linked to hereditary nonpolyposis colorectal cancer (Kinzler and Vogelstein 1996); the breast cancer susceptibility genes 1 and 2 (BRCA1 and BRCA2; Futreal et al. 1994; Miki et al. 1994); the ATM gene, which is mutated in the cancer predisposition syndrome ataxia telangiectasia (AT; Savitsky et al. 1995); and the XP excision repair genes that are responsible for xeroderma pigmentosum. Other genetic disease genes that function in genome maintenance include NBS1, the gene mutated in Nijmegen breakage syndrome (NBS; Carney et al. 1998), BLM, which encodes a RecQ type DNA helicase and is mutated in Blooms' syndrome (Ellis et al. 1995), and MRE11, which is mutated in a variant of AT (Stewart et al. 1999). These proteins all function in DNA metabolism and repair. In addition, there is evidence that several of these proteins also participate in cell cycle checkpoint functions that halt cell cycle progression in the presence of damaged DNA (Shiloh and Rotman 1996; Jongmans et al. 1997).

BRCA1 contains an amino-terminal RING finger domain, a carboxy-terminal BRCT domain, and a SQ cluster domain (SCD) (Bork et al. 1997; Cortez et al. 1999). Disruption of the BRCA1 gene in mice causes embryonic lethality (Hakem et al. 1996; Gowen et al. 1996). Targeted deletion of exon 11 of BRCA1 in mouse mammary epithelial cells results in mammary tumor formation after long latency and genetic instability characterized by aneuploidy, chromosomal rearrangements, or alteration of p53 transcription (Xu et al. 1999a). The BRCA1 protein abundance is cell cycle regulated with low levels in G0 and G1 cells that increase as cells enter S phase (Chen et al. 1996; Ruffner and Verma 1997). BRCA1 localizes to nuclear foci during S phase that rapidly disperse when cells are treated with DNA damaging agents (Scully et al. 1997b). The BRCA1 protein is hyperphosphorylated in response to DNA damage and DNA replication blocks. Genetic evidence indicates that BRCA1 is required for transcription-coupled repair of oxidative DNA damage (Gowen et al. 1998) and homologous recombination in response to double-strand breaks (Moynahan et al. 1999). In addition, BRCA1 has been implicated in G2/M checkpoint control (Xu et al. 1999).

Biochemical evidence also supports a role for BRCA1 in DNA damage repair. BRCA1 is associated and colocalized with the DNA repair protein hRad51 (Scully et al. 1997c). BRCA1 associates with and is phosphorylated by the ATM protein kinase, a global regulator of the DNA damage response (Cortez et al. 1999). In addition, BRCA1 associates with the RAD50–MRE11–NBS1 complex, which functions in homologous recombination, nonhomologous end joining, meiotic recombination, and telomere maintenance (Zhong et al. 1999).

To further understand the function of BRCA1, we used immunoprecipitation and mass spectrometry to identify BRCA1-associated proteins. We found that BRCA1 resides in a large multisubunit protein complex of tumor suppressors, DNA damage sensors, and signal transducers that we have named BASC for BRCA1-associated genome surveillance complex.

Acknowledgments

We are grateful to Drs. Robert Roeder and Yoshihiro Nakatani for HeLa nuclear extracts, Dr. John Petrini for sharing Nbs1 antibodies, Dr. Bruce Stillman for providing anti-RFC140 antibodies, and Dr. David Livingston for sharing unpublished results. We also thank Tim H. Lee for technical assistance. D.C. is a Fellow of the Jane Coffin Childs Memorial Fund for Medical Research. This work was supported by grants GM44664 and Q1187 (Welch) to S.J.E. S.J.E. is an investigator with the Howard Hughes Medical Institute.

The publication costs of this article were defrayed in part by payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 USC section 1734 solely to indicate this fact.

Acknowledgments

Footnotes

E-MAIL ude.cmt.mcb@niqj; FAX (713) 798-1625.

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