Decreased gene dosage of Cdc25A inhibits cellular transformation and mammary tumorigenesis in mice

Proc Amer Assoc Cancer Res, Volume 46, 2005 2584 The Cdk-activating Cdc25A phosphatase is overexpressed in various cancers including breast cancer. Cdc25A downregulation is critical for S-phase checkpoint, and Cdc25A overexpression could promote not only aberrant proliferation but also survival with chromosomal instability. While Cdc25A has been suggested to be an oncogene, it has not been examined how genetic manipulations to suppress Cdc25A expression affect carcinogenesis. To determine the impact of Cdc25A downregulation, we generated Cdc25A-null mice by gene targeting. Cdc25A−/− mice died in utero during the peri-implantation period (E4.5-7.5). Cdc25A+/− mice showed no appreciable developmental defect, with normal fertility and nursing. However, Cdc25A+/− mice displayed marked resistance to tumorigenesis induced by MMTV -driven oncogenes. After crossbreeding, all MMTV-ras transgenic mice in the Cdc25A +/+ background developed mammary tumors with latency of 20 weeks. In contrast, 50% of MMTV-ras mice in the Cdc25A +/− background did not display detectable tumors during 24-month observation. The neu oncogene functions upstream of ras in mammary carcinogenesis. MMTV-neu mice in the Cdc25A +/+ background developed tumors with latency of 25 weeks, while tumor latency in MMTV-neu; Cdc25A +/− mice was prolonged to 34 weeks. The tumor suppressive effect of Cdc25A heterozygosity was consistent with decreased potential of Cdc25A +/− mouse embryonic fibroblasts (MEFs) to undergo transformation. Cdc25A +/− MEFs proliferated normally, but exhibited significantly decreased colony formation in soft agar following retroviral expression of H-rasV12 plus c-myc or a dominant negative (DN) p53 mutant. Consistently, suppression of Cdc25A expression was inhibitory on transformation of MCF-10A human non-transformed mammary epithelial cells. Lentiviruses encoding two distinct anti-Cdc25A short hairpin RNAs decreased Cdc25A levels in MCF-10A cells by 30% and 60%, respectively, which was correlated with inhibition of colony formation after transduction with H-rasV12 plus c-myc or DNp53. However, lowering Cdc25A levels after expression of the oncogenes was not inhibitory on colony formation, suggesting the importance of Cdc25A at initial phases of transformation. We are investigating impacts of Cdc25A downregulation on pathways controlling checkpoint and carcinogenesis, using genomics/proteomics. These studies provide evidence that Cdc25A is a rate-limiting oncogene for mammary carcinogenesis, and recapitulate that Cdc25 is a promising therapeutic target. This work is supported in part by NIH grants, CA100204 and HD38085, and a DOD grant, DAMD17-02-1-0413.