Mining modern genomics for cancer therapies is predicated on weeding out “bystander” alterations (nonconsequential mutations) and identifying “driver” mutations responsible for tumorigenesis and/or metastasis. We used a direct in vivo RNA interference (RNAi) strategy to screen for genes that upon repression predispose mice to squamous cell carcinomas (SCCs). Seven of our top hits—including Myh9, which encodes nonmuscle myosin IIa—have not been linked to tumor development, yet tissue-specific Myh9 RNAi and Myh9 knockout trigger invasive SCC formation on tumor-susceptible backgrounds. In human and mouse keratinocytes, myosin IIa's function is manifested not only in conventional actin-related processes but also in regulating posttranscriptional p53 stabilization. Myosin IIa is diminished in human SCCs with poor survival, which suggests that in vivo RNAi technology might be useful for identifying potent but low-penetrance tumor suppressors.

Hair production is fueled by stem cells (SCs), which transition between cyclical bouts of rest and activity. Here, we explore why hair growth wanes with age. We show that aged hair follicle SCs (HFSCs) in mice exhibit enhanced resting and abbreviated growth phases and are delayed in response to tissue-regenerating cues. Aged HFSCs are poor at initiating proliferation and show diminished self-renewing capacity upon extensive use. Only modestly restored by parabiosis, these features are rooted in elevated cell-intrinsic sensitivity and local elevation in bone morphogenic protein (BMP) signaling. Transcriptional profiling presents differences consistent with defects in aged HFSC activation. Notably, BMP-/calcium-regulated, nuclear factor of activated T-cell c1 (NFATc1) in HFSCs becomes recalcitrant to its normal down-regulating cues, and NFATc1 ChIP-sequencing analyses reveal a marked enrichment of NFATc1 target genes within the age-related signature. Moreover, aged HFSCs display more youthful levels of hair regeneration when BMP and/or NFATc1 are inhibited. These results provide unique insights into how skin SCs age.BMP signaling | hair cycle | quiescence I n adult tissues, stem cells (SCs) must replace cells lost to acute injury and normal biological activity (homeostasis). Aging can be viewed as a failure to maintain proper tissue homeostasis, resulting in a decline in tissue function and delayed response to tissue damage (1). Age-related extrinsic changes in external, systemic, and/or local tissue environment, coupled with intrinsic changes from repetitive use, are all potential underlying causes for SC malfunction. However, the relative contributions of these factors on SC aging vary among SC populations. Studies on hematopoietic and melanocyte SCs show that age-related intrinsic perturbations can impair SC function (2-4). Mesenchymal SCs, cardiac SCs, and liver progenitor cells also show age-related declines in performance (5-7). The impact of extrinsic perturbations is evident from studies on muscle and neural SCs, where exposure to a youthful systemic environment can restore SC functional capabilities (7-10). Most recently, it was shown that cardiomyocytes rely upon systemic growth and differentiation factor 11 (GDF11), a member of the transforming growth factor β (TGF-β) superfamily, which declines with age (11).The skin has some of the most recognizable age-associated changes. In humans and other mammals, skin shows an agerelated decline in homeostasis, with both dermal and epidermal thinning, reductions in epidermal proliferation and injury repair, loss of dermal elasticity, wrinkling, and notably, hair thinning and eventual loss (12). Periods of rest in hair follicles (HFs) also become longer as animals age, and in humans, hair density declines with age. It has been suggested that the progressive dormancy of HFs during aging is a reflection of a declining capacity of SCs to initiate a new hair cycle, but this has not been formally tested and the underlying mechanisms remain largely unexplored.HFs underg...

Summary Tissue growth is the multifaceted outcome of a cell’s intrinsic capabilities and its interactions with the surrounding environment. Decoding these complexities is essential for understanding human development and tumorigenesis. Here, we tackle this problem by carrying out the first genome-wide RNAi-mediated screens in mice. Focusing on skin development and oncogenic (HrasG12V-induced) hyperplasia, our screens uncover novel as well as anticipated regulators of embryonic epidermal growth. Among top oncogenic screen hits are Mllt6 and the Wnt effector β-catenin; they maintain HrasG12V-dependent hyperproliferation. We also expose β-catenin as an unanticipated antagonist of normal epidermal growth, functioning through Wnt-independent intercellular adhesion. Finally, we document physiological relevance to mouse and human cancers, thereby establishing the feasibility of in vivo mammalian genome-wide investigations to dissect tissue development and tumorigenesis. By documenting some oncogenic growth regulators, we pave the way for future investigations of other hits and raise promise for unearthing new targets for cancer therapies.