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Degradation of BRCA2 in Alkyltransferase-Mediated DNA Repair and Its Clinical Implications

¹ Mouse Cancer Genetics Program, Center for Cancer Research, and ² Pathology Histotechnology Laboratory, Science Applications International Corporation-Frederick, Inc., National Cancer Institute at Frederick, Frederick, Maryland; and ³ Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania

Requests for reprints: Shyam K. Sharan, National Cancer Institute-Frederick, Building 560, Room 32-31C, 1050 Boyles Street, Frederick, MD 21702. Phone: 301-846-5140; Fax: 301-846-7017; E-mail: ssharan{at}mail.ncifcrf.gov.

Key Words: Mouse Models • BRCA2 • O⁶-Alkylguanine-DNA alkyltransferase (AGT) • O⁶-Methylguanine DNA methyl transferase • O⁶-Benzylguanine

Germ-line mutations in BRCA2 have been linked to early-onset familial breast cancer. BRCA2 is known to play a key role in repairing double-strand breaks. Here, we describe the involvement of BRCA2 in O⁶-alkylguanine DNA alkyltransferase (AGT)–mediated repair of O⁶-methylguanine adducts. We show that BRCA2 physically associates and undergoes repair-mediated degradation with AGT. In contrast, BRCA2 with a 29-amino-acid deletion in an evolutionarily conserved domain does not bind to alkylated AGT; the two proteins are not degraded; and mouse embryonic fibroblasts are specifically sensitive to alkylating agents that result in O⁶-methylguanine adducts. We show that O⁶-benzylguanine (O⁶BG), a nontoxic inhibitor of AGT, can also induce BRCA2 degradation. BRCA2 is a viable target for cancer therapy because BRCA2-deficient cells are hypersensitive to chemotherapeutic DNA-damaging agents. We show a marked effect of O⁶BG pretreatment on cell sensitivity to cisplatin. We also show the efficacy of this approach on a wide range of human tumor cell lines, which suggests that chemosensitization of tumors by targeted degradation of BRCA2 may be an important consideration when devising cancer therapeutics. [Cancer Res 2008;68(23):9973–81]