DNA double-strand breaks are considered the principal lethal damage resulting from irradiation and/or exposure to cross-linking drugs (e.g., cisplatin, mitomycin, doxorubicin, bleomycin, etc.). Typically, double-strand breaks lead to arrest in the cell cycle progression and to activation of the cellular DNA repair machinery, and failure of DNA repair generally leads to genome abnormality, and eventually cell death.
Among other mechanisms, homologous recombination significantly contributes to the repair of DNA damage, and particularly double strand breaks. Several genes are involved in homologous recombination and include BRCA1, BRCA2, RAD51, RAD54, XRCC2, and XRCC3. Where these genes are mutated, cells will frequently exhibit high levels of genomic instability and hypersensitivity toward irradiation and/or crosslinking agents. On the other hand, in many cancer cells, and especially those that are genomically instable, elevated rates of homologous recombination have been observed. In these cancer cells, elevated expression levels of wild-type RAD51 have been observed, which appears to suggest that RAD51 (which is also involved in maintaining genomic stability) may also be responsible for the resistance of cancer cells to DNA-damaging radio- or chemotherapy.
RAD51 is a eukaryotic recombinase and is homologous to the E. coli RecA protein. RAD51 has ATP-dependent DNA binding activity, multimerizes to form a nucleoprotein filament on single-stranded DNA, and is reported to catalyze homologous DNA pairing and strand exchange reactions in vitro. After treating cells with irradiation and/or DNA damaging agents, dramatic amounts of RAD51-foci can be observed at the site of DNA damage, which may further include other proteins related to homologous recombination (e.g., BRCA1, Rad54, BLM, and RPA), and especially BRCA2.
BRCA2 was identified as a breast tumor suppressor based on various studies of familial breast cancer, and deficiency in BRCA2 is often characterized by cumulative chromosome abnormalities, including chromosomal breaks, aberrant mitotic exchanges, and aneuploidy. More recently it has been demonstrated that BRCA2 is also required for homology-directed repair of chromosomal breaks. Consistent with its involvement in DNA repair, mouse embryos lacking BRCA2 exhibit radiation hypersensitivity, which is also characteristic of mouse embryos lacking RAD51.
The potential role of BRCA2 in DNA repair was first revealed by identification of its interaction with RAD51. It has been shown that the six highly conserved BRC repeats are involved in the interaction between RAD51 and BRCA2, and that the interaction between the BRC repeats of BRCA2 and RAD51 is critical for cellular response to DNA damage caused by methyl methanesulfonate. Among other support, such criticality is reflected in the findings that radiation-induced RAD51 foci formation is diminished in BRCA2-deficient cells, and in cells in which the interaction between BRCA2 and RAD51 is disrupted using BRC peptides. Also, the RAD51-DNA binding ability is specifically abolished in vitro in the presence of excess BRC peptide, as well as the RAD51 nucleoprotein filament formation is disrupted.
Thus, while numerous data appear to highlight the significance of BRCA2/RAD51 interaction in DNA repair, effective strategies to selectively interfere with such interaction as a treatment modality for BRCA2-associated cancer are elusive. Therefore, there is still a need for compositions and methods to interfere with BRCA2/RAD51 interaction in DNA repair, and especially in the context of treatment and chemoprevention of neoplastic diseases.