SummaryInitiation of mammalian DNA replication can be reconstituted from isolated G1-phase nuclei and cell extracts, supplemented with cyclin-dependent protein kinases (CDKs). Under these conditions, cyclin E supports pre-replication complex assembly, whereas cyclin-A-associated kinase acts later to terminate assembly and activate DNA replication. The mechanism by which these events are coordinated is unknown. Here, we show that the replication factor Ciz1 interacts with cyclins E and A sequentially through distinct cyclin-binding motifs. Cyclin A displaces cyclin E from Ciz1 in a manner that is dependent on functional domains that are essential for its role in DNA replication. Furthermore, in cell-free assays, recombinant cyclin-A-CDK2 complexes and recombinant Ciz1 cooperate to promote initiation of DNA replication in late G1-phase nuclei. In addition, Ciz1 supports immobilization of cyclin A in isolated nuclei and depletion of Ciz1 by RNAi impairs immobilization, suggesting that Ciz1 promotes initiation by helping to target the kinase to a specific subnuclear compartment. We propose that Ciz1 acts to coordinate the functions of cyclins E and A in the nucleus, by delivering cyclin-A-associated kinase to sites that are specified by cyclin E, helping to ensure that they execute their functions in the same place and in the correct order.

Cip1-interacting zinc finger protein 1 (Ciz1) stimulates DNA replication in vitro and is required for mammalian cells to enter S phase. Here, we show that a significant proportion of Ciz1 is retained in nuclear foci following extraction with nuclease and high salt. This suggests that Ciz1 is normally immobilized by interaction with nonchromatin nuclear structures, consistent with the nuclear matrix. Furthermore, matrix-associated Ciz1 foci strikingly colocalize with sites of newly synthesized DNA in S phase nuclei, suggesting that Ciz1 is present in DNA replication factories. Analysis of green fluorescent proteintagged fragments indicates that nuclear immobilization of Ciz1 is mediated by sequences in its C-terminal third, encoded within amino acids 708-830. Immobilization occurs in a cell-cycle-dependent manner, most probably during late G1 or early S phase, to coincide with its reported point of action. Although C-terminal domains are sufficient for immobilization, N-terminal domains are also required to specify focal organization. Combined with previous work, which showed that the DNA replication activity of Ciz1 is encoded by N-terminal sequences, we suggest that Ciz1 is composed of two functionally distinct domains: an N-terminal replication domain and a Cterminal nuclear matrix anchor. This could contribute to the formation or function of DNA replication factories in mammalian cells.

CIZ1 is a nuclear matrix protein that cooperates with cyclin A2 (encoded by CCNA2) and CDK2 to promote mammalian DNA replication. We show here that cyclin-A-CDK2 also negatively regulates CIZ1 activity by phosphorylation at threonines 144, 192 and 293. Phosphomimetic mutants do not promote DNA replication in cell-free and cell-based assays, and also have a dominant-negative effect on replisome formation at the level of PCNA recruitment. Phosphorylation blocks direct interaction with cyclin-A-CDK2 and recruitment of endogenous cyclin A to the nuclear matrix. In contrast, phosphomimetic CIZ1 retains the ability to bind to the nuclear matrix, and its interaction with CDC6 is not affected. Phospho-T192-specific antibodies confirm that CIZ1 is phosphorylated during S phase and G2, and show that phosphorylation at this site occurs at post-initiation concentrations of cyclin-A-CDK2. Taken together, the data suggest that CIZ1 is a kinase sensor that promotes initiation of DNA replication at low kinase levels, when in a hypophosphorylated state that is permissive for cyclin-A-CDK2 interaction and delivery to licensed origins, but blocks delivery at higher kinase levels when it is phosphorylated.

The DNA replication protein Ciz1 promotes initiation of mammalian DNA replication in cooperation with cyclin A-dependent kinase, most likely by delivery cyclin A to sites where cyclin E-dependent pre-replication complex assembly has taken place. Normally, Ciz1 is anchored within nuclear matrix-associated foci that co-localize with sites of DNA replication, but in the absence of anchor domain Ciz1 retains the ability to promote initiation of DNA replication in isolated nuclei so attachment to the nuclear matrix is not essential for function. Here, we show that expression of DNA replication and nuclear matrix anchor domains of Ciz1 are uncoupled and uneven at the transcript level in a panel of common cancer cell lines, giving rise to excess DNA replication domain protein that is not attached to the nuclear matrix. Moreover Ciz1 domain expression is also uncoupled in a wide range of common solid tumours, including breast and lung, so that anchor domain transcript is elevated over control tissues, and exceeds replication domain in all primary stage I, II and III tumours tested. Notably, more than half of all stage IV tumours tested resemble established cell lines, having more replication domain than anchor domain. In cell based assays, recombinant anchor domain protein interferes with attachment of endogenous Ciz1 to the nuclear matrix, revealing a dominant negative effect that also impacts on nuclear matrix-recruitment of a key component of the pre-replication complex, Cdc6. This shows that Ciz1 normally plays a role in localizing Cdc6 to the nuclear matrix and suggests that cancer-associated uncoupled expression influences recruitment of mammalian pre-replication complexes to the nuclear matrix. These findings implicate spatially unconstrained DNA replication as a source of nuclear disorder in cancer cells. We identified a variant Ciz1 isoform with alterations in nuclear matrix attachment domain and tumour-restricted expression. Variant Ciz1 is elevated at the transcript and protein level with high frequency in small cell lung tumours, and with lower frequency in a range of other tumour types. Selective suppression of variant Ciz1 expression using inducible shRNA restricts proliferation of lung cancer cells that express it in vitro and in vivo, identifying a novel, exploitable therapeutic target with potential application in the treatment of lung cancer. Recent evidence will also be presented on the application of variant Ciz1 as a serum biomarker. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3022. doi:10.1158/1538-7445.AM2011-3022