Tumour heterogeneity and cancer cell plasticity

Abstract

Phenotypic and functional heterogeneity arise among cancer cells within the same tumour as a consequence of genetic change, environmental differences and reversible changes in cell properties. Some cancers also contain a hierarchy in which tumorigenic cancer stem cells differentiate into non-tumorigenic progeny. However, it remains unclear what fraction of cancers follow the stem-cell model and what clinical behaviours the model explains. Studies using lineage tracing and deep sequencing could have implications for the cancer stem-cell model and may help to determine the extent to which it accounts for therapy resistance and disease progression.

Figure 1. Cancer cell fate versus potential

a, Transplantation assays assess the potential of cancer cells to form tumors. The ability of a cell to form a tumor is context dependent: cells that can form a tumor under one set of conditions may not form a tumor in other conditions. For this reason, tumorigenesis assays must be conducted under the most permissive possible conditions so as not to underestimate the spectrum of cells with tumorigenic potential. Factors such as the site of injection, the genetic background of recipient mice, and co-injection of extracellular matrix all influence the ability of cells to form tumors. Optimization of these and other parameters can substantially increase the frequency of tumorigenic cells detected in various cancers^,,,,. b and c, Lineage tracing or fate-mapping assays assess the actual fate of tumor cells in a particular context, often the native tumor environment. Thus, while potential measures what a cell can do under permissive conditions, fate measures what a cell actually does in a particular context. Some cells with tumorigenic potential do not actually contribute to tumor growth – for example because they are in a non-permissive environment or because they are eliminated by immune effector cells. An important question is whether many (b) or few (c) cells with tumorigenic potential actually contribute to tumor growth. It will be important to integrate transplantation studies of tumorigenic potential with studies of cell fate in the native tumor environment to assess the extent to which the cancer stem cell model describes the growth and progression of individual cancers.

Figure 2. Various cancers may be hierarchically organized into subpopulations of tumorigenic and non-tumorigenic cells but some hierarchies may be steep (a), with only rare tumorigenic cells, while other hierarchies may be shallow, with common tumorigenic cells (b) or even rare non-tumorigenic cells (c)

As hierarchies become increasingly shallow, the value of distinguishing between tumorigenic and non-tumorigenic cells to understand cancer biology and improve therapy declines.

Figure 3. Potential forms of plasticity among tumorigenic and non-tumorigenic cells yield different predictions with respect to transplantability and therapy response

a–c. The differentiation of tumorigenic cells into non-tumorigenic progeny may be irreversible (a), inefficiently reversible (b), or readily reversible (c). d–f. This degree of plasticity within cancer cell hierarchies influences the outcome of transplantation assays. If differentiation is irreversible, non-tumorigenic cells should not form tumors after transplantation (d). If differentiation is inefficiently reversible, non-tumorigenic cells will inefficiently form tumors after transplantation (e). If cells efficiently and reversibly transition between tumorigenic and non-tumorigenic states then cells in the non-tumorigenic state should nonetheless form tumors after transplantation. Under these circumstances, transplantation assays may not be able to distinguish between cells in tumorigenic and non-tumorigenic states and it may not be experimentally possible to distinguish this model from tumors that are composed entirely of tumorigenic cells (f). g–i, Plasticity within cancer cell hierarchies also influences the predicted outcome of therapies that ablate tumorigenic cells. If differentiation is irreversible, therapy will convert a hierarchically organized malignancy to a benign tumor containing only non-tumorigenic cells (g). If differentiation is inefficiently reversible, a single round of therapy will deplete but not eliminate tumorigenic cells (h). If cells efficiently transition between tumorigenic and non-tumorigenic states then a single round of therapy will have little effect on tumorigenic cell frequency (i).

Figure 4. Tumorigenic cells cannot recapitulate the heterogeneity of the tumors from which they derive if those tumors have extensive genetic heterogeneity

If every tumorigenic cell carries a combination of common and unique mutations then none of these cells will recapitulate the genetic heterogeneity of the tumor from which they derive - they will all give rise to genetically distinct tumors upon transplantation. They still may give rise to hierarchically organized tumors with tumorigenic and non-tumorigenic components, as in the tumor of origin. Nonetheless, if the genetic heterogeneity involves mutations that influence cancer cell phenotype or function the genetic heterogeneity will contribute to tumor heterogeneity through mechanisms independent of cancer stem cell differentiation.

Figure 5. Clonal evolution and the differentiation of tumorigenic cells into non-tumorigenic cells can independently, or jointly, contribute to tumor heterogeneity

a, New mutations can increase the heterogeneity within tumors as long as the mutations influence cell phenotype or function. b, The differentiation of tumorigenic cells into non-tumorigenic progeny creates heterogeneity within tumors. New mutations that occur in non-tumorigenic cells would not be propagated (unless they restore tumorigenic potential). c, If mutations occur in tumorigenic cells, then both clonal evolution and the differentiation of tumorigenic cells into non-tumorigenic progeny contribute to tumor heterogeneity. This is likely what occurs in cancers that follow the stem cell model. This means that phenotypic and functional differences cannot automatically be ascribed to epigenetic differences among tumorigenic and non-tumorigenic cells as genetic heterogeneity may contribute to some of those differences.

Figure 6. Genetic changes and the inherent properties of tumorigenic cells can independently, or jointly, contribute to therapy resistance

a, Genetic alterations can confer therapy resistance (e.g. ref). b, Tumorigenic cells in certain cancers are inherently resistant to certain therapies^,,. c, Tumorigenic cells may persist despite therapy but may not be able to cause disease relapse due to an inability to regenerate significant numbers of non-tumorigenic cells in the presence of therapy. The acquisition of de novo mutations may enhance therapy resistance, enabling relapse and disease progression. CML stem cells are thought to be inherently imatinib resistant, and to persist in the presence of imatinib until they acquire an imatinib resistance mutation and progress to blast cell crisis^–.