BRCA1: cell cycle checkpoint, genetic instability, DNA damage response and cancer evolution

Abstract

Germline mutations of the breast cancer associated gene 1 (BRCA1) predispose women to breast and ovarian cancers. BRCA1 is a large protein with multiple functional domains and interacts with numerous proteins that are involved in many important biological processes/pathways. Mounting evidence indicates that BRCA1 is involved in all phases of the cell cycle and regulates orderly events during cell cycle progression. BRCA1 deficiency, consequently causes abnormalities in the S-phase checkpoint, the G(2)/M checkpoint, the spindle checkpoint and centrosome duplication. The genetic instability caused by BRCA1 deficiency, however, also triggers cellular responses to DNA damage that blocks cell proliferation and induces apoptosis. Thus BRCA1 mutant cells cannot develop further into full-grown tumors unless this cellular defense is broken. Functional analysis of BRCA1 in cell cycle checkpoints, genome integrity, DNA damage response (DDR) and tumor evolution should benefit our understanding of the mechanisms underlying BRCA1 associated tumorigenesis, as well as the development of therapeutic approaches for this lethal disease.

Figure 1

Current views of BRCA1 functions G₁ and G₁/S cell cycle checkpoint. (A) A model illustrating a negative role for BRCA1 in G₁ arrest. BRCA1 binds to hypophosphorylated RB, which interacts with E2F to form an active complex that blocks transcription. BRCA1 and RB also recruit HDC, which deacetylates histones associated with the promoter, thereby promoting formation of nucleosomes that inhibit transcription. (B and C) Expression of BRCA1 may cause G₁/S arrest through a p53-independent (B), and p53 dependent (C) mechanisms. (D) UV radiation may also cause BRCA1-mediated G₁/S arrest, although it is not clear whether p21 is involved.

Figure 2

A model illustrating a role of BRCA1 in the C₂/M cell cycle checkpoint. BRCA1 can be phosphorylated by ATR, ATM and Chk2. BRCA1 regulates expression and cellular localization of Chk1, although it is not clear whether Chk1 can phosphorylates BRCA1. Both Chk1 and Chk2 inactivate Cdc25 by phosphorylate it at Ser-216. The phosphorylation of Cdc25 at Ser-126 not only inactivates this protein but also allows it to bind to 14-3-3α, leading to its exporting from the nucleus. This results in the decrease of the active form of Cdc25, which is a phosphatase involved in dephosphorylation of Cdc2. The dephosphorylated form of Cdc2 is the active form that promotes cell progression from G₂ to M phase. Therefore, the reduced amount of Cdc25 results in the decrease of active form of Cdc2, which prevents G₂ to M transition. Another two factors are also involved in this pathway. One is Cyclin B1, which forms an active complex with Cdc2 to promote cell progression from G₂ to M phase, and the other is Wee1, a kinase that puts phosphate on an inhibitory site of Cdc2, thereby inhibiting function of Cdc2. Thus, expression changes of these genes could theoretically affect G₂/M cell cycle checkpoint.

Figure 3

The spindle checkpoint, activation and deficiency. (A) A bipolar spindle showing an unattached kinetochore (arrow). (B) The activation of the spindle checkpoint at the kinetochore leads to the formation of the BubR1-Bub3-Mad2-Cdc20 complex (MCC). The MCC binds and inhibits activity of PPC, preventing sister chromatids from separation. It is shown that Brca1 interacts with Mad2 promoter and positively regulates its expression. (C and D) Images of Brca1^Δ11/Δ11 (C) and wild-type (D) MEF cells stained with Dapi and an anti-α-tubulin antibody. Arrows pointed to lagging chromosomes. (E and F) Morphology difference between wild-type (E) and Brca1^Δ11/Δ11 MEF (F) cells in responding to nocodazole treatment. Brca1^Δ11/Δ11 MEF cells at mitotic phase (as revealed by both round up and positive for phosphorylated histone H3 antibody staining) exhibited significantly more fragmented cells. Phosphorylated histone H3 antibody staining also indicated that many mutant cells contained fragmented chromosomes.

Figure 4

A model illustrating connections among cell cycle checkpoints, centrosome duplication, DNA damage repair, genetic instability, DNA damage response, developmental abnormalities and tumorigenesis caused by BRCA1 deficiency.