Stem cell quiescence acts as a tumour suppressor in squamous tumours

In some organs, adult stem cells are uniquely poised to serve as cancer cells of origin. It is unclear, however, whether tumorigenesis is influenced by the activation state of the adult stem cell. Hair follicle stem cells (HFSCs) act as cancer cells of origin for cutaneous squamous cell carcinoma and undergo defined cycles of quiescence and activation. The data presented here show that HFSCs are unable to initiate tumours during the quiescent phase of the hair cycle, indicating that the mechanisms that keep HFSCs dormant are dominant over the gain of oncogenes (such as Ras) or the loss of tumour suppressors (such as p53). Furthermore, Pten activity is necessary for quiescence-based tumour suppression, as its deletion alleviates tumour suppression without affecting proliferation. These data demonstrate that stem cell quiescence is a form of tumour suppression in HFSCs, and that Pten plays a role in maintaining quiescence in the presence of tumorigenic stimuli. Lowry and colleagues report the potential role of stem cell quiescence as a tumour suppressive mechanism. They show that although hair follicle stem cell activation allows tumour formation in response to oncogenic stimuli, tumours are not initiated during the quiescent phase of the hair cycle. They further find that the presence of Pten is important in maintaining hair follicle stem cell quiescence in this setting.