The tumor suppressor BRCA1 contains multiple functional domains that interact with many proteins. After DNA damage, BRCA1 is phosphorylated by CHK2 at serine 988, followed by a change in its intracellular location. To study the functions of CHK2-dependent phosphorylation of BRCA1, we generated a mouse model carrying the mutation S971A (S971 in mouse Brca1 corresponds to S988 in human BRCA1) by gene targeting.
Brca1S971A/S971A mice were born at the expected ratio without a developmental defect, unlike previously reported Brca1 mutant mice. However, Brca1 S971A/S971A mice suffered a moderately increased risk of spontaneous tumor formation, with a majority of females developing uterus hyperplasia and ovarian abnormalities by 2 years of age. After treatment with DNA-damaging agents, Brca1 S971A/S971A mice exhibited several abnormalities, including increased body weight, abnormal hair growth pattern, lymphoma, mammary tumors, and endometrial tumors. In addition, the onset of tumor formation became accelerated, and 80% of the mutant mice had developed tumors by 1 year of age. We demonstrated that the Brca1 S971A/S971A cells displayed reduced ability to activate the G 2 /M cell cycle checkpoint upon ␥-irradiation and to stabilize p53 following N-methyl-N-nitro-N-nitrosoguanidine treatment. These observations suggest that Chk2 phosphorylation of S971 is involved in Brca1 function in modulating the DNA damage response and repressing tumor formation.Germ line mutations of BRCA1 predispose women to breast cancer and ovarian cancer (1, 7). Although the specific molecular events leading to tumorigenesis remain elusive, several lines of evidence indicate that BRCA1 is involved in genetic stability control, DNA damage repair, centrosome duplication, apoptosis, and cell cycle control (reviewed in references 13, 16, 48, and 56). BRCA1 functions are mediated by several different mechanisms, including phosphorylation. It has been shown that BRCA1 undergoes hyperphosphorylation during late G 1 and S phases and is transiently dephosphorylated soon after M phase (42,47).During the DNA damage response, BRCA1 is phosphorylated by several protein kinases, such as ATM, ATR, CHK1, CHK2, and MDC1 (10, 31, 58). ATM controls cell cycle arrest in G 1 and G 2 . The control mechanism of G 2 arrest by ATM is unclear, but recently CHK2, the mammalian homolog of the budding yeast Rad53 and fission yeast Cds1 checkpoint kinases, was cloned and found to be linked to ATM: in response to ionizing radiation (IR), CHK2 is rapidly phosphorylated and activated in an ATM-dependent manner (18,21,33). In Chk2 mutant mouse embryonic stem (ES) cells, maintenance of G 2 arrest and reduced Cdc2 kinase activity in response to IR are defective (24).The first evidence that CHK2 acts as a tumor suppressor came from a finding that a subset of patients with Li-Fraumeni syndrome, which is characterized by multiple tumors at an early age, contained germ line mutations in CHK2 (5). Somatic mutations of CHK2 have also been found in diverse types of sporadic cancer, includ...