Over time skin cells accumulate mutations with the potential to give rise to cancer, yet often mutant clones remain contained in clusters and do not spread. Researchers in the Beronja lab (Human Biology Division) study the molecular and cellular mechanisms that regulate epithelial growth in development and cancer. New research from the lab, now published in the journal eLife, addresses a fundamental question in the field: What happens when cells acquire an oncogene that will eventually progress to cancer? In the study, Sandoval and colleagues describe a mechanism in which competition between two different populations within a malignant cell cluster results in stabilized oncogenic growth.
To follow a single mutant clone’s fate, the researchers created a genetically engineered mouse model in which oncogenic Hras (HrasG12V) can be activated in epidermal cells. This allows the investigators to follow Hras-activated cells as they grow into stable clones and then evaluate intra-clone dynamics. The investigators first noticed that activation of the oncogene in skin cells promotes the creation of more daughter cells that maintain the potential to divide (progenitor cell renewal) rather than terminally differentiate. As a result, over a long period of time HrasG12V clones are larger than WT, but surprisingly, these clones are smaller than expected given their high renewal rate.
This finding led the group to investigate additional growth suppression mechanisms and develop a novel assay to quantify progenitor cell fates in vivo. The cell fate identification (CFI) assay designed for this study determines cell fate considering location in the epidermis. Dr. Sandoval, the leading scientist in the study, provided background on the novel assay: “Recent work shows that division and differentiation in the epidermis are dynamic. An individual cell divides in repose to the fate choices of its neighbor. In normal tissue, if one keratinocyte differentiates, a neighboring cell will divide. This interplay ensures that the epidermis remains at homeostasis. A strength of the CFI assay is that it allowed us to quantify division and differentiation in the epidermis while also recording cell position within the tissue.”
By administering two different markers (EdU and BrdU) two hours apart and following cells that have incorporated the first marker versus those that are positive for both, the researchers can determine cell cycle intervals in the epidermis. Using the CFI assay, the investigators discovered that proliferation and cell fate choice are dynamic and fluctuate over time. For instance, although a high rate of renewal is observed in the first 8-10 weeks after oncogenic activation, the renewal rate then reverts to WT levels, potentially contributing to the restricted growth observed over longer time intervals.
To understand the mechanism regulating the growth of oncogene-expressing cells, the investigators studied the composition of HrasG12V clones by considering two distinct populations: ‘edge’ cells, which are in contact with WT neighbors, and ‘inner’ cells, which are found in the center of the clone and are in contact with other HrasG12V-expressing progenitors. They found that the proportion of edge to inner cells significantly decreases over time. Using the CFI assay, the researchers determined that the edge and inner cells lean toward different fates; edge cells favored renewal, whereas inner cells often chose differentiation, which eventually eliminates them from the tissue. This finding suggested that two opposing cell fates within a clone might ultimately restrict its growth.
Dr. Slobodan Beronja, the principal investigator in the study, explained the implications of this finding: “The mechanism behind suppression of oncogenic clone growth (i.e., suppression of cell transformation) is based on the general organization of the tissue, rather than some molecular mechanism activated by the oncogenic signal. In other words, the critical suppressor of growth here is the epidermal clone shape (governed by tissue topology), which by being round, ensures that at some point inner cell population will outgrow the outer cell population (as area expands faster than circumference).”
In summary, this study proposes a model in which the biological outcome of an oncogene-expressing clone is modulated by two progenitor populations with different fates that are ultimately governed by the round clone shape characteristic of epidermal organization. This model provides an explanation for the observation that oncogene-driven clones residing in aged epidermis grow to a relatively uniform size. “Without the CFI assay, we would not have been able to discover the heterogeneity in differentiation within oncogenic clones. It would be very interesting to study how this heterogeneity, which caused the inhibition of oncogenic growth, breaks down, leading to hyperplasia”, said Dr. Sandoval.
This work was supported by grants from the National Cancer Institute, the National Institute of Arthritis and Skin Diseases, and a graduate student fellowship from the Cellular and Molecular Biology Training Grant.
Fred Hutch/UW Cancer Consortium member Slobodan Beronja contributed to this study.