BRCA2 and Cell Cycling
Shortly after the identification of BRCA2, the gene was shown to be regulated in a cell cycle–dependent manner and to be associated with cellular proliferation. Levels of BRCA2 are low in the G1 phase of the cell cycle but increase as the cell approaches the G1-S boundary. The highest levels of BRCA2 are detected in S phase, as with BRCA1. The cell cycle kinetics of BRCA2 appear to be very similar to the kinetics of BRCA1; this suggests that these proteins may be coordinately regulated, consistent with the finding that they interact with each other.
BRCA2 and Response to DNA Damage
Several experiments have supported the hypothesis that BRCA2 plays a role in double-strand break repair. First, Morimatsu et al. successfully identified RAD51 as a binding partner of mouse Brca2. Subsequently, Hasty and other researchers demonstrated that Rad51 binds to human BRCA2 (residues 3196-3232) Rad51 is known to be critical for mitotic and meiotic recombination and double-strand DNA break repair. These amino acids are 95% conserved between mouse and human BRCA2, suggesting that the domain is important for BRCA2 function.
Further evidence supporting a role for BRCA2 in DNA damage repair has been provided by studies of Brca2-mutant embryos, which display hypersensitivity to ionizing radiation. After mutant embryos were exposed to gamma radiation, the inner cell mass outgrowth of these embryos was completely ablated, an effect not seen in heterozygous or normal embryos. Mouse embryo fibroblasts from viable Brca2 homozygous mutant animals with mutations toward the 3’ portion of the BRC repeats also are hypersensitive to agents that create double-strand DNA breaks.
Studies in a pancreatic cancer cell line (CAPAN-1) with both copies of BRCA2 deleted have shown that the lack of these repeats leads to hypersensitivity to methyl methanesulfonate treatment. This sensitivity persists with the introduction of BRCA2 containing mutated BRC repeats but is reversed by the introduction of wild-type BRCA2. In addition, CAPAN-1 cells were demonstrated to be highly sensitive to drugs inducing double-strand DNA breaks, such as etoposide, and to ionizing radiation.
Tumors in mice derived from CAPAN-1 cell implantation also demonstrated hypersensitivity to radiation and mitoxantrone; this finding suggests that irradiation or treatment with DNA-damaging agents may be a useful method for elimination of BRCA2-null cells-a suggestion with clear potential clinical significance for women with BRCA2-related breast cancer.
Angela DeMichele and Barbara L. Weber
A. DeMichele: Departments of Medicine and Epidemiology, The University of Pennsylvania Cancer Center, Philadelphia, Pennsylvania
B. L. Weber: Departments of Medicine and Genetics, Breast Cancer Program, Population Science and Cancer Control, The University of Pennsylvania Cancer Center, Philadelphia, Pennsylvania