Interplay of opposing fate choices stalls oncogenic growth in skin epithelium

Abstract

Due to its exposed nature, skin is subject to continuous DNA damage and a high incidence of oncogenic mutations. To ensure normal function it repairs or expels damaged cells. However, the skin is also able to tolerate numerous long-lived oncogenic clones that can comprise a significant portion of physiologically normal adult tissue, but how it does so is unknown. In highly mutagenized skin, it is thought that oncogenic clones can compete with each other and stall one another’s growth to maintain tissue structure and function. But how such tolerance is established in the context of early, independently occurring single oncogenic events remains undiscovered. To directly address this question, we induce HrasG12V expression in single cells in the adult murine epidermis and follow them long-term. Using a novel cell fate identification (CFI) assay that can directly quantify progenitor cell fate choice in vivo, we show that HrasG12V induces an early and transient pro-growth effect driven by increased progenitor renewal. Ultimately, renewal yields to progenitor differentiation, leading to stalled clone expansion. CFI allowed us to attribute this dynamic effect on the emergence of two distinct populations within oncogenic clones, renewing progenitors along the edge and differentiating ones within the central core. As clone expansion is accompanied by progressive enlargement of the core and diminishment of the edge compartment, the interplay between the two populations works to stabilize oncogenic growth. In order to identify the molecular pathway that governs HrasG12V-driven differentiation, we employ an shRNA-mediated screen of Ras-effectors in vivo. The screen identifies Rassf5 as the top candidate, which we further validate as a novel mediator that is both necessary and sufficient for oncogene-specific progenitor differentiation. Our study provides evidence that the epidermis can tolerate an isolated oncogenic event through a novel cellular mechanism of inter-clonal competition.