Using tumour phylogenetics to identify the roots of metastasis in humans

In cancer, much uncertainty remains regarding the origins of metastatic disease. Models of metastatic progression offer competing views on when dissemination occurs (at an early or late stage of tumour development), whether metastases at different sites arise independently and directly from the primary tumour or give rise to each other, and whether dynamic cell exchange occurs between synchronously growing lesions. Although it is probable that many routes can lead to the establishment of systemic disease, clinical observations suggest that distinct modes of metastasis might prevail in different tumour types. Gaining a more-comprehensive understanding of the evolutionary processes that underlie metastasis is not only relevant from a basic biological perspective, but also has profound clinical implications. The 'tree of life' of metastatic cancer contains answers to many outstanding questions about the development of systemic disease, but has only been reconstructed in a limited number of patients. Here we review available data on the phylogenetic relationships between primary solid tumours and their metastases, and examine to what degree they support different models of metastatic progression. We provide a description of experimental methods for lineage tracing in human cancer, ranging from broad DNA-sequencing approaches to more-targeted techniques, and discuss their respective benefits and caveats. Finally, we propose future research questions in the area of cancer phylogenetics.