SUMMARY Defective DNA repair by homologous recombination (HR) is thought to be a major contributor to tumorigenesis in individuals carrying Brca1 mutations. Here we show that DNA breaks in Brca1-deficient cells are aberrantly joined into complex chromosome rearrangements by a process dependent on the non-homologous end joining (NHEJ) factors, 53BP1 and DNA Ligase 4. Loss of 53BP1 alleviates hypersensitivity of Brca1 mutant cells to PARP inhibition and restores error-free repair by HR. Mechanistically, 53BP1 deletion promotes ATM-dependent processing of broken DNA ends to produce recombinogenic single-stranded DNA competent for HR. In contrast, Lig4 deficiency does not rescue the HR defect in Brca1 mutant cells, but prevents the joining of chromatid breaks into chromosome rearrangements. Our results illustrate that HR and NHEJ compete to process DNA breaks that arise during DNA replication, and that shifting the balance between these pathways can be exploited to selectively protect or kill cells harboring Brca1 mutations.

SUMMARY The DNA damage response (DDR) protein 53BP1 protects DNA ends from excessive resection in G1, and thereby favors repair by non-homologous end joining (NHEJ) as opposed to homologous recombination (HR). During S phase, BRCA1 antagonizes 53BP1 to promote HR. The pro-NHEJ and anti-recombinase functions of 53BP1 are mediated in part by RIF1, the only known factor that requires 53BP1 phosphorylation for its recruitment to double strand breaks (DSBs). Here we show that a 53BP1 phospho-mutant 53BP18A, comprising alanine substitutions of the 8 most N-terminal S/TQ phosphorylation sites, mimics 53BP1 deficiency by restoring genome stability in BRCA1 deficient cells yet behaves like wild-type 53BP1 with respect to immunoglobulin class switch recombination (CSR). 53BP18A recruits RIF1 but fails to recruit the DDR protein PTIP to DSBs, and disruption of PTIP phenocopies 53BP18A. We conclude that 53BP1 promotes productive CSR and suppresses mutagenic DNA repair through distinct phospho-dependent interactions with RIF1 and PTIP.

Purpose:The PRL-3 phosphatase has been found expressed at higher levels in metastasis than in primary tumors of patients with colorectal cancer. In the present study, we evaluated the expression of PRL-3 in ovarian cancer tissue and its role in ovarian cancer cell growth. Experimental Design: PRL-3 phosphatase expression was evaluated in 84 ovarian tumor samples. PRL-3 expression has been knocked down using specific small interfering RNAs to determine its role in ovarian cancer cell growth in vitro. Results: In ovarian cancers, PRL-3 expression correlates with disease progression, being higher in advanced (stage III) than in early (stage I) tumors. In situ measurements of PRL-3 expression showed that it was confined to the epithelial neoplastic cells.The molecular mechanism underlying PRL-3 overexpression in ovarian cancers is independent from amplification of the corresponding genomic locus. Ovarian cancer cells growing in culture have high levels of expression of this phosphatase. PRL-3^specific knockdown using small interfering RNA severely impaired the growth of cells without affecting the expression of the closely related homologue PRL-1. Intriguingly, the growth of human colon carcinoma cells expressing lower levels of the PRL-3 was not affected by the PRL-3 knockdown. Conclusions: Altogether, these results show that PRL-3 expression is associated with ovarian cancer progression and point to a key role for this phosphatase in the control of ovarian cancer cells growth.This strongly suggests that PRL-3 should be considered as a target for the discovery of new anticancer agents to be tested against this malignancy.PRL-3 phosphatase, also known as PTP4A3, belongs to a small class of tyrosine phosphatases, which includes PRL-1 and PRL-2 (1). These proteins are prenylated in vitro and in vivo and this posttranslational modification is important for their intracellular distribution (1, 2). Increasing importance in the cancer field has been gained by PRL-3 phosphatase, which has been reported to be overexpressed in colorectal metastasis in respect to nonmetastatic tumor or normal colorectal epithelium (3, 4). Liver metastasis from other cancers such as pancreas, esophagus, or stomach did not express high levels of PRL-3, suggesting a degree of specificity for colorectal cancer (4). The overexpression of PRL-3 in colorectal cancer metastasis was associated with gene amplification in a small subset of cases (3). Normal Chinese hamster ovary cells transfected with PRL-3 showed increased mobility and invasiveness and these effects were markedly reduced when a catalytically inactive mutant was used (5). Furthermore, cells ectopically expressing PRL-3 are able to form experimental metastasis in mice and this effect is dependent on the catalytic activity of the phosphatase (6, 7). These results indicate that PRL-3 could be an important factor contributing to the invasive-metastatic properties of cancer cells. In the present study, we have investigated the involvement of the PRL-3 phosphatase in primary ovarian ...

In contrast to BRCA1, CtIP has indispensable roles in promoting resection and embryonic development.