The atypical cyclin-like protein Spy1 overrides p53-mediated tumour suppression and promotes susceptibility to breast tumourigenesis

Bre-Anne Fifield,Evie Kirou,Robert D Cardiff,Ingrid Qemo,Lisa Ann Porter

doi:10.1186/s13058-019-1211-3

Bre-Anne Fifield, Evie Kirou + Show 3 more

Open Access

https://doi.org/10.1186/s13058-019-1211-3

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Abstract

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.

Highlights

Breast cancer is the most prevalent form of cancer to affect women and represents the second leading cause of cancer-related mortality among this population
Generation of mammary tumour virus (MMTV)-Spy1 transgenic mice The flag-Spy1 coding sequence was cloned into the MMTV-SV40 plasmid (Fig. 1a) and injected into B6CBAF1/J pronuclei
Analysis of both mRNA and protein levels from 6-week-old mice revealed that mammary glands from MMTV-Spy1 mice contained significantly higher levels of Spy1 as compared to control littermates (Additional file 1: Figure S1B)

Summary

Introduction

Breast cancer is the most prevalent form of cancer to affect women and represents the second leading cause of cancer-related mortality among this population. A delicate balance of cell cycle progression and inhibition is required at each of these periods of development to ensure maintenance of genomic stability, a crucial factor in the inhibition of tumourigenesis. Women with inherited mutations in genes that play fundamental roles in recognition of DNA damage and activation of DNA repair pathways have an elevated risk of breast cancer. Understanding how mammary epithelial cells monitor and respond to changes in genomic instability throughout development may reveal novel factors that predispose women to carcinogenesis. 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. We hypothesized that elevated Spy would override protective cell cycle checkpoints and support the onset of mammary tumourigenesis

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