Abstract
β-catenin antagonizes normal epidermal growth but promotes HrasG12V-driven oncogenic growth.
Major finding: β-catenin antagonizes normal epidermal growth but promotes HrasG12V-driven oncogenic growth.
Approach: A genome-wide RNA interference screen in the mouse epidermis identified growth regulators.
Impact: Candidate genes provide insight into SCC progression and may represent therapeutic targets.
In vitro RNA interference (RNAi) screens have been used to characterize genes that are preferentially required for the growth of cancer cells but do not recapitulate the complex interactions between tumors and their microenvironment. To characterize growth regulators in a more physiologic context, Beronja and colleagues performed an in vivo genome-wide RNAi screen in rapidly proliferating murine embryonic epidermal tissue using lentiviral short hairpin RNAs (shRNA). Analysis of changes in shRNA abundance identified genes that selectively controlled either normal epidermal growth in wild-type mice or oncogene-driven hyperproliferation in mice with epidermal-specific expression of HrasG12V, which induced dose-dependent overgrowth of embryonic epidermal cells and enhanced WNT signaling. Genes essential for normal growth were enriched in those that regulate protein synthesis and RNA splicing, whereas genes specific to oncogenic growth were distinct and included those encoding RAS pathway members such as Akt3 and Raf1. Knockdown of two of the top candidate genes, Ctnnb1, which encodes β-catenin, or Mllt6 (myeloid/lymphoid or mixed-lineage leukemia translocated to, 6), which was induced by WNT signaling, specifically impaired oncogenic Hras-driven hyperplasia. However, in contrast to its role in oncogenic growth, Ctnnb1 depletion stimulated normal epidermal growth via WNT-independent regulation of cell–cell adhesion, suggesting that HrasG12V-mediated signaling switches the role of β-catenin to promote oncogenic outgrowth. Consistent with a role for these genes in skin tumorigenesis, silencing of Ctnnb1 or Mllt6 in HrasG12V-expressing epidermal cells transcriptionally suppressed protumorigenic genes and activated the expression of tumor suppressors. Moreover, depletion of either gene diminished the formation of HrasG12V-dependent squamous papillomas in mice and inhibited both the initiation and maintenance of human squamous cell carcinoma xenografts. These results demonstrate the feasibility of this in vivo screening approach and identify key regulators of oncogenic growth that may represent potential therapeutic targets.