The CIZ1 protein is required for timely entry to S phase following exit from quiescence (1). We show here that CIZ1 is also required for accurate entry into quiescence. Analysis of quiescence entry following contact inhibition in a CIZ1-deletion model has revealed a role in dampening a previously unreported quiescence checkpoint, evidenced by ubiquitous post-replicative phosphorylation of histone H2AX and activation of checkpoint kinases ATM and CHK1. Subsequent emergence of proliferating colonies indicates escape from checkpoint-mediated arrest, and begins to implicate inappropriate response to contact induced signaling as a novel endogenous driver of oncogenesis. Unscheduled checkpoint activation was also observed in bone marrow cells from juvenile CIZ1 null mice, and high frequency B-cell lymphoma in young adults in the absence of any additional oncogenic stimulation, supporting the idea that CIZ1 plays a very early role in tumorigenesis.

Analysis of cell cycle progression using immunofluorescence and flow cytometry of primary fibroblasts from post-natal CIZ1 null mice revealed cell cycle elongation, with extended expression of cyclins D2 and E1, increased cyclin A2, and altered p21, p27 and CDK2, but no checkpoint activation. Thus, CIZ1 null cells can tolerate altered expression, targeting and recruitment of multiple cell cycle regulators. Checkpoint activation occurs only after exit from the cell cycle, reflecting aberrant response to contact induced signaling and failure to accurately transition into a stable quiescent state.

Inappropriate expression of CIZ1 splice variants has been previously linked with both adult and pediatric cancers (2-4), and shown to play a proliferation-promoting role in lung (4) and breast (5) tumor models. Furthermore, CIZ1 b-variant has biomarker capability for the early detection of lung cancer (4). Despite these advances, the precise role of CIZ1 in the cell is not yet understood. The CIZ1 knock out mouse model described here, and a related model (6), indicate a tumor suppressor function, which is apparently at odds with the DNA replication- and proliferation-promoting roles reported previously. The current work helps to reconcile these observations by showing that CIZ1s established activity in the promotion of cyclin A-mediated initiation of DNA replication (1) is as a coordinator. This in fact, influences multiple cyclins and CDKis, however, impairment of this capability during cell cycle progression may not underlie CIZ1s role in tumorigenesis. Our central finding is that altered cell cycle homeostasis in CIZ1 null cells does not lead to checkpoint activation, until they are driven to contact induced quiescence.

1 Copeland et al (2010) J Cell Sci 123, 1108-15

2 Warder & Keherly (2003) J Biomed Sci.10, 406-17

3 Rahman et al (2007) Human Mutation 28, 993-1004

4 Higgins et al (2012) Proc Natl Acad Sci U S A 109, E3128-35

5 den Hollander et al (2006) Cancer Research 66, 11021-30

6 Nishibe et al (2013) FEBS Lett 587, 1529-35

Citation Format: Rosemary H.C Wilson, Jo-An Roulson, Sumia Bageghni, Ghadeer Albadrani, Justin F-X Ainscough, Dawn Coverley. Quiescence induced checkpoint activation in the absence of CIZ1. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5096. doi:10.1158/1538-7445.AM2014-5096