Inactivation of CtIP Leads to Early Embryonic Lethality Mediated by G₁ Restraint and to Tumorigenesis by Haploid Insufficiency

Abstract

CtIP interacts with a group of tumor suppressor proteins including RB (retinoblastoma protein), BRCA1, Ikaros, and CtBP, which regulate cell cycle progression through transcriptional repression as well as chromatin remodeling. However, how CtIP exerts its biological function in cell cycle progression remains elusive. To address this issue, we generated an inactivated Ctip allele in mice by inserting a neo gene into exon 5. The corresponding Ctip ؊/؊ embryos died at embryonic day 4.0 (E4.0), and the blastocysts failed to enter S phase but accumulated in G 1 , leading to a slightly elevated cell death. Mouse NIH 3T3 cells depleted of Ctip were arrested at G 1 with the concomitant increase in hypophosphorylated Rb and Cdk inhibitors, p21. However, depletion of Ctip failed to arrest Rb ؊/؊ mouse embryonic fibroblasts (MEF) or human osteosarcoma Saos-2 cells at G 1 , suggesting that this arrest is RB dependent. Importantly, the life span of Ctip ؉/؊ heterozygotes was shortened by the development of multiple types of tumors, predominantly, large lymphomas. The wild-type Ctip allele and protein remained detectable in these tumors, suggesting that haploid insufficiency of Ctip leads to tumorigenesis. Taken together, this finding uncovers a novel G 1 /S regulation in that CtIP counteracts Rbmediated G 1 restraint. Deregulation of this function leads to a defect in early embryogenesis and contributes, in part, to tumor formation.

CtIP Silencing as a Novel Mechanism of Tamoxifen Resistance in Breast Cancer

“…More importantly, it has been shown that inactivation of CtIP in mice leads to early embryonic lethality, and the life span of Ctip +/À heterozygotes, which have Ctip haploid insufficiency, was shortened due to the development of multiple types of tumors. These findings show that CtIP is a critical protein in early embryogenesis and implicates an important role of CtIP in tumorigenesis (39). In addition, CtIP interacts with the BRCT domains of BRCA1 where most mutations occur in BRCA1 breast cancer patients, and such protein-protein interaction is abolished by tumorassociated mutations in the BRCT domains (26,29,31), suggesting that interaction between CtIP and BRCA1 is of functional relevance in the breast cancer suppressor activity.…”

C-terminal-binding protein interacting protein binds directly to adenovirus early region 1A through its N-terminal region and conserved region 3

“…It is also involved in the transcriptional repression by the Rb family possibly by recruitment of CtBP and associated HDACs (reviewed by Chinnadurai, 2006b). It has recently been shown that knockout mice, which do not express CtIP die at an early stage of development, probably due to aberrant cell cycle regulation (Chen et al, 2005). It has been surmised that CtIP expression is necessary for correct phosphorylation of Rb and subsequent cell cycle progression.…”

“…The CtIP/CtBP complex plays a role in E2F/Rb-mediated repression and it has been suggested that AdE1A could alleviate this repression through dissociation of the complex (Meloni et al, 1999). Furthermore, it is possible that AdE1A can directly affect CtIP's ability to regulate Rb-dependent S-phase entry, perhaps by controlling its phosphorylation (Chen et al, 2005) (Figure 9). Moreover, CtIP's phosphorylation-dependent association with BRCA1 regulates the G2/M transition checkpoint following DNA damage (Yu and Chen, 2004).…”

“…The role of CtIP is not clear -it binds to the Rb family and it has been suggested that this is responsible for some of the transcriptional repression properties of Rb through recruitment of CtBP and associated HDACs (Meloni et al, 1999). CtIP also regulates Rb-dependent cell cycle arrest in G1 through the modulation of Rb phosphorylation (Chen et al, 2005). CtIP binds to breast cancer associated 1 (BRCA1) and is involved in the cellular response to DNA double strand breaks, being phosphorylated by ataxia telangiectasia mutated (ATM) (Wong et al, 1998;Yu et al, 1998;Li et al, 1999Li et al, , 2000Wu-Baer and Baer, 2001;Yu and Chen, 2004).…”

“…Significantly, BRCA1 catalyses CtIP ubiquitination and co-localizes with CtIP in DNA damaged-induced foci (Yu et al, 2006). Although the precise role of CtIP has yet to be determined in detail the protein must be of considerable importance to the functioning of the normal cell since CtIP knock-out mice die at an early stage in development (E4) largely through cell cycle arrest in G 1 (Chen et al, 2005).…”