BRCA1 in the DNA damage response and at telomeres

Abstract

Mutations of the breast and ovarian cancer susceptibility gene 1 (BRCA1) account for about 40-45% of hereditary breast cancer cases. Moreover, a significant fraction of sporadic (non-hereditary) breast and ovarian cancers exhibit reduced or absent expression of the BRCA1 protein, suggesting an additional role for BRCA1 in sporadic cancers. BRCA1 follows the classic pattern of a highly penetrant Knudsen-type tumor suppressor gene in which one allele is inactivated through a germ-line mutation and the other is mutated or deleted within the tumor. BRCA1 is a multi-functional protein but it is not fully understood which function(s) is (are) most important for tumor suppression, nor is it clear why BRCA1-mutations confer a high risk for breast and ovarian cancers and not a broad spectrum of tumor types. Here, we will review BRCA1 functions in the DNA damage response (DDR), which are likely to contribute to tumor suppression. In the process, we will highlight some of the controversies and unresolved issues in the field. We will also describe a recently identified and under-investigated role for BRCA1 in the regulation of telomeres and the implications of this role in the DDR and cancer suppression.

Keywords: DNA damage response; DNA damage signaling; ataxia-telangiectasia mutated; base excision repair; breast cancer susceptibility gene 1; homology-directed repair; telomeres.

Figure 1

Breast and ovarian cancer susceptibility gene 1 protein interactions that contribute to its role in the DNA damage response. In response to DNA damage, BRCA1 is phosphorylated at various sites by several kinases (e.g., ATM, CHK2, and/or ATR) and forms several different types of complexes that are recruited to the sites of DNA damage through various mechanisms. The roles of these complexes in DNA damage signaling and repair are only partially understood. The formation of these BRCA1 complexes is dependent upon the mutually exclusive interactions of its BRCT domains with phosphorylated motifs within Abraxis, BACH1, or CtIP. BRCA1 functions to recruit BRCA2 to DNA damage sites through an intermediary protein, PALB2 (partner and localizer of BRCA2). The interaction of the BRCA1 N-terminal RING domain with its binding partner BARD1 is required for tumor suppression, since BRCA1-mutations that disrupt this interaction lead to cancer.

Figure 2

ATM activation by ionizing radiation (IR) vs. oxidative stress. (A) In IR-induced activation, the MRN complex, a DNA damage sensor, is recruited to DSBs; and MRN then recruits ATM. In undamaged cells, ATM is a dimer held in the inactive state by PP2A. After IR, PP2A dissociates from ATM, allowing autophosphorylation on S1981 and conversion to a monomer at the MRN/DSB site. The protein BAAT1 binds to activated ATM and prevents dephosphorylation by PP2A. Another step in ATM activation involves binding of TIP60 to chromatin near the DSB and acetylation of ATM, which is required for its full activation. (B) In response to oxidative stress, ATM is directly oxidized, forming a disulfide-linked dimer, which is phosphorylated on S1981 and activated.

Figure 3

Recruitment of BRCA1 to sites of double-strand DNA breaks. Two possible mechanisms by which the BRCA1 complex A can be recruited to ionizing radiation-induced foci (IRIF) are illustrated in (A,B). Both involve post-translational modifications of DDR proteins, including phosphorylation and ubiquitination. In (A), the RNF8/Ubc13 complex ubiquitinates MDC1, and the ubiquitin-interacting motif (UIM) of RAP80 interacts with ubiquitinated MDC1. In (B), RNF8/Ubc13 polyubiquitinates a nearby histone H2AX and the UIM of RAP80 interacts with the ubiquitinated H2AX protein. In each case, phosphorylated Abraxis interacts with the BRCT domain of BRCA1 and the RAP80 protein, thus recruiting BRCA1 to the site of the DSB.

Figure 4

Double-strand break repair by homology-directed repair (HDR) or non-homologous end joining (NHEJ). In response to IR, the MRN complex recognizes and binds to the broken ends of DNA at DSBs. DSB repair can occur by NHEJ or HDR, depending upon the phase of the cell cycle and the relative levels of BRCA1 vs. 53BP1 which has been phosphorylated by ATM and is complexed with RIF1. Some of the proteins involved in NHEJ and HDR are shown in the boxes. In addition to HDR, BRCA1 may participate in one subtype of NHEJ, but the role of BRCA1 in NHEJ is still not certain.

Figure 5

Model for role of BRCA1 in telomere maintenance. (A) Shows a linear representation of a normal functional telomere. For simplicity, not all of the telomere-associated proteins are shown. BRCA1 is recruited to the telomere by RAD50, a component of the MRN complex, which is also present at the telomere. When BRCA1 is over-expressed, more BRCA1 is present at the telomere. BRCA1 causes overall telomere shortening, but the 3′ G-strand overhang is lengthened, as illustrated in (B). (C) Shows a critically short and dysfunctional telomere with little or no 3′ G-strand overhang in cells with no functional BRCA1. A DDR is activated with resultant chromosomal aberrations due to end-end fusions and translocations (a dicentric chromosome is illustrated). The G-strand overhang is represented by a thick black line. The thick red lines represent double-stranded telomeric DNA, while the sub-telomeric DNA is shown as blue lines.