BackgroundBreast cancer is the most common cancer to affect women and one of the leading causes of cancer-related deaths. Proper regulation of cell cycle checkpoints plays 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. We hypothesized that elevated Spy1 would override protective cell cycle checkpoints and support the onset of mammary tumourigenesis.MethodsWe generated a transgenic mouse model driving expression of Spy1 in the mammary epithelium. Mammary development, growth characteristics and susceptibility to tumourigenesis were studied. In vitro studies were conducted to investigate the relationship between Spy1 and p53.ResultsWe found that in the presence of wild-type p53, Spy1 protein is held ‘in check’ via protein degradation, representing a novel endogenous mechanism to ensure protected checkpoint control. Regulation of Spy1 by p53 is at the protein level and is mediated in part by Nedd4. Mutation or abrogation of p53 is sufficient to allow for accumulation of Spy1 levels resulting in mammary hyperplasia. Sustained elevation of Spy1 results in elevated proliferation of the mammary gland and susceptibility to tumourigenesis.ConclusionsThis mouse model demonstrates for the first time that degradation of the cyclin-like protein Spy1 is an essential component of p53-mediated tumour suppression. Targeting cyclin-like protein activity may therefore represent a mechanism of re-sensitizing cells to important cell cycle checkpoints in a therapeutic setting.

44Background: Breast cancer is the most common cancer to affect women and one of the 45 leading causes of cancer related deaths. Proper regulation of cell cycle checkpoints plays 46 a critical role in preventing the accumulation of deleterious mutations. Perturbations in 47 the expression or activity of mediators of cell cycle progression or checkpoint activation 48 represent important events that may increase susceptibility to the onset of carcinogenesis. 49 The atypical cyclin-like protein Spy1 was isolated in a screen for novel genes that could 50 bypass the DNA damage response. Clinical data demonstrates that protein levels of Spy1 51 are significantly elevated in ductal and lobular carcinoma of the breast. We hypothesized 52 that elevated Spy1 would override protective cell cycle checkpoints and support the onset 53 of mammary tumorigenesis. 54 Methods:We generated a transgenic mouse model driving expression of Spy1 in the 55 mammary epithelium. Mammary development, growth characteristics and susceptibility 56 to tumorigenesis was studied. In vitro studies were conducted to investigate the 57 relationship between Spy1 and p53. 58 Results:We found that in the presence of wild-type p53, Spy1 protein is held 'in check' 59 via protein degradation, representing a novel endogenous mechanism to ensure protected 60 checkpoint control. Regulation of Spy1 by p53 is at the protein level and is mediated in 61 part by Nedd4. Mutation or abrogation of p53 is sufficient to allow for accumulation of 62 Spy1 levels resulting in mammary hyperplasia. Sustained elevation of Spy1 results in 63 elevated proliferation of the mammary gland and susceptibility to tumorigenesis.64 Conclusions: This mouse model demonstrates for the first time that degradation of the 65 cyclin-like protein Spy1 is an essential component of p53-mediated tumour suppression.66 3Targeting cyclin-like protein activity may therefore represent a mechanism of re-67 sensitizing cells to important cell cycle checkpoints in a therapeutic setting. 68 Introduction 71 Breast cancer is the most prevalent form of cancer to affect women and represents the 72 second leading cause of cancer related mortality among this population. Increased 73 incidence of breast cancer in women can be attributed to the complex cellular changes the 74 female mammary gland undergoes throughout life in response to hormonal cues. A 75 delicate balance of cell cycle progression and inhibition is required at each of these 76 periods of development to ensure maintenance of genomic stability; a crucial factor in the 77 inhibition of tumourigenesis. Women with inherited mutations in genes that play 78 fundamental roles in recognition of DNA damage and activation of DNA repair pathways 79 have an elevated risk of breast cancer. Hence understanding how mammary epithelial 80 cells monitor and respond to changes in genomic instability throughout development may 81 reveal novel factors that predispose women to carcinogenesis. 82 The tumour suppressor p53 plays a critical role in DNA repair mecha...