The Atypical Cyclin-Like Protein Spy1 Overrides p53-Mediated Tumour Suppression and Promotes Susceptibility to Breast Tumorigenesis

Breast cancer is the most common cancer to affect women and one of the leading causes of cancer related deaths. Maintenance of genomic stability and proper regulation of cell cycle checkpoints play a critical role in preventing the accumulation of deleterious mutations. Perturbations in the expression or activity of mediators of cell cycle progression or checkpoint activation represent important events that may increase susceptibility to the onset of carcinogenesis. The atypical cyclin-like protein Spy1 was isolated in a screen for novel genes that could bypass the DNA damage response. Clinical data demonstrates that protein levels of Spy1 are significantly elevated in ductal and lobular carcinoma of the breast. Using a transgenic mouse driving expression of Spy1 in the mammary epithelium we demonstrate that sustained elevation of Spy1 leads to enhanced proliferation and an increased susceptibility to mammary tumour formation. We find that Spy1 is targeted for degradation by the tumour suppressor p53 to protect checkpoint control. When crossed with p53 deficient mice, elevation of Spy1 leads to an increase in hyperplastic alveolar nodules. Targeting cyclin-like protein activity may therefore represent a mechanism of re-sensitizing cells to important cell cycle checkpoints in a therapeutic setting.