In many mouse models of skin cancer, only a few tumors typically form even though many cells competent for tumorigenesis receive the same oncogenic stimuli. These observations suggest an active selection process for tumor-initiating cells. Here, we use quantitative mRNA- and miR-Seq to determine the impact of HrasG12V on the transcriptome of keratinocytes. We discover that microRNA-203 is downregulated by HrasG12V. Using a knockout mouse model, we demonstrate that loss of microRNA-203 promotes selection and expansion of tumor-initiating cells. Conversely, restoration of microRNA-203 using an inducible model potently inhibits proliferation of these cells. We comprehensively identify microRNA-203 targets required for Hras-initiated tumorigenesis. These targets include critical regulators of the Ras pathway and essential genes required for cell division. This study establishes a role for the loss of microRNA-203 in promoting selection and expansion of Hras mutated cells and identifies a mechanism through which microRNA-203 antagonizes Hras-mediated tumorigenesis.
DNA mutations occur and accumulate during an individual's lifetime. Often these changes are harmless. But some mutations—called driver mutations—can trigger the formation of tumors. This is often because these mutations allow the cells to grow faster than normal cells. Mutations in genes in the Ras gene family are among the most common driver mutations found in human cancers. These common mutations lead to the uncontrolled activation of genes that are normally tightly controlled, which in turn allows the cells to divide more and live for longer: these are two key features of cancer cells.
So, how are Ras genes and the genes that they control regulated to prevent such dangerous over activation? One mechanism rests on binding sites in their messenger RNA sequence that are recognized by smaller RNA molecules called microRNAs. RNA molecules are created when genes are transcribed. Some RNAs, called messenger RNAs, are then decoded to create proteins. Many other RNAs, including microRNAs, do not code for proteins, but instead bind to many messenger RNA targets, and repress their ability to be decoded into proteins. Three genes, called Hras, Kras, and Nras, are regulated in this way by numerous microRNAs, which together act to dampen the normal activities of these genes.
Riemondy et al. investigate how a cancer-promoting mutation in the Hras gene affects the activities of microRNAs in mouse skin cells in culture. By measuring RNA levels, the experiments reveal that skin cells carrying this mutation produce significantly lower levels of what is normally the most highly produced microRNA in the skin. This microRNA, called microRNA-203, acts to limit the proliferation of skin cells when these cells are dividing rapidly. When the gene encoding microRNA-203 was deleted in mice, the skin cells proliferated more. These mice also developed more skin tumors than normal mice when they were exposed to cancer-causing chemicals. When the gene for microRNA-203 was added into skin cells carrying the Hras mutation and then activated, the cells both divided less and, as a results, grew less. This indicates that microRNA-203 could prevent cancerous cells from expanding in number, a key event in the initiation of tumors.
Riemondy et al. also used a variety of approaches to identify the molecules targeted by microRNA-203 in the skin, and reveal that it targets multiple signaling pathways, including components of the Ras pathway, to suppress cell proliferation. Together, these findings highlight microRNA-203 as a potential source of new treatments to prevent or slow tumor growth in humans.