UCSF

Tumors acquire genetic alterations throughout their life, leading them to evolve and to accumulate heterogeneity over time. These characteristics make cancer a particularly complex and challenging disease to treat, as tumor heterogeneity has been linked to drug resistance and overall worse outcomes for patients. Using the 4-color lineage tracing Confetti mouse and the DMBA/TPA chemical carcinogenesis model of skin cancer, clonal and evolutionary dynamics were investigated at every stage of tumor development, from initiation to malignant progression and metastasis. The initiating mutagen, DMBA (dimethylbenzanthracene), leaves a characteristic fingerprint of A>T mutations in each tumor genome, and thus next generation sequencing of these tumors enabled detailed dissection of tumor clones and evolution, and relationships between primary tumors and their respective metastases. It was found that, while benign tumors can be comprised of multiple cellular populations, only one population contains an initiating A>T mutation in Hras. Thus benign tumors are clonal in origin, but surprisingly can subsequently recruit neighboring “normal” skin cells into the tumor. Like initiation, progression to malignancy was found to be a clonal event, driven by the emergence of a single dominant progressing clone. In contrast to both of these processes, however, metastasis was seen not to be clonal—multiple cell populations from the same primary tumor were observed to contribute to metastases in the lymph nodes as well as at distant sites, suggesting that metastasis is a fundamentally different process than initiation or progression. Next generation sequencing analysis further demonstrated that most metastases departed from the primary tumor synchronously rather than traveling first to a lymph node, supporting the parallel rather than the linear model of metastasis. These findings highlight the usefulness of mouse models which faithfully recapitulate all stages of human disease, from benign tumor formation to metastatic spread, in understanding the evolutionary processes which take place in cancer.