Darwinian Dynamics of Intratumoral Heterogeneity: Not Solely Random Mutations but Also Variable Environmental Selection Forces

Abstract

Spatial heterogeneity in tumors is generally thought to result from branching clonal evolution driven by random mutations that accumulate during tumor development. However, this concept rests on the implicit assumption that cancer cells never evolve to a fitness maximum because they can always acquire mutations that increase proliferative capacity. In this study, we investigated the validity of this assumption. Using evolutionary game theory, we demonstrate that local cancer cell populations will rapidly converge to the fittest phenotype given a stable environment. In such settings, cellular spatial heterogeneity in a tumor will be largely governed by regional variations in environmental conditions, e.g. alterations in blood flow. Model simulations specifically predict a common spatial pattern in which cancer cells at the tumor-host interface exhibit invasion-promoting, rapidly-proliferating phenotypic properties, while cells in the tumor core maximize their population density by promoting supportive tissue infrastructures e.g. to promote angiogenesis. We tested model predictions through detailed quantitative image analysis of phenotypic spatial distribution in histological sections of 10 patients with stage 2 invasive breast cancers. CAIX, GLUT1 and Ki67 were upregulated in the tumor edge consistent with an acid-producing invasive, proliferative phenotype. Cells in the tumor core were 20% denser than the edge, exhibiting upregulation of CAXII, HIF-1α and cleaved caspase-3, consistent with a more static and less proliferative phenotype. Similarly, vascularity was consistently lower in the tumor center compared to the tumor edges. Lymphocytic immune responses to tumor antigens also trended to higher level in the tumor edge, although this effect did not reach statistical significance. Like invasive species in nature, cancer cells at the leading edge of the tumor possess a different phenotype from cells in the tumor core. Our results suggest that at least some of the molecular heterogeneity in cancer cells in tumors is governed by predictable regional variations in environmental selection forces, arguing against the assumption that cancer cells can evolve toward a local fitness maximum by random accumulation of mutations. Major Findings Like invasive species in nature, cancer cells at the leading edge of the tumor possess a different phenotype from cells in the tumor core. We conclude that at least some intratumoral heterogeneity in the molecular properties of cancer cells is governed by predictable regional variations in environmental selection forces.

Natural Selection in Cancer Biology: From Molecular Snowflakes to Trait Hallmarks

“…For example, regions of low blood flow, which are commonly observed in tumor imaging, would select for tumors that are adapted to such conditions as reduced availability of substrate and blood-derived growth factors. Detailed analysis of spatial molecular heterogeneity in 10 clinical breast cancers showed a consistent regional distribution meeting this prediction, which supported the adaptive hypothesis (Lloyd et al 2016). …”

“…Recent molecular evidence for cancer cell adaptation comes from examination of spatial heterogeneity in breast cancers (Lloyd et al 2016). Evolutionary theory predicted that spatial variation of phenotypes could result from local variations in environmental factors that select for different phenotypic properties.…”

“…The problem is not the specific mutations, which are highly variable and heterogeneous. Rather, the problem is adaptive evolution, and recognizing this has led some clinicians to refocus on “the possibility to track and treat evolution” (Willyard 2016), by applying standard tools of evolutionary biology, such as phylogenetic reconstruction of ancestral lineages and evolutionary origins (Kostadinov et al 2013; Jamal-Hanjani et al 2014; Willyard 2016) or mathematical models of fitness optimization through evolutionary adaptation (Misale et al 2015; Enriquez-Navas et al 2016; Lloyd et al 2016). …”

Phenotypic changes of acid-adapted cancer cells push them toward aggressiveness in their evolution in the tumor microenvironment

“…Notably, these appear to emerge at the leading edge of invading tumors. 18 We showed this phenomenon in 3D cell culture above (Fig. 2C).…”

“…17 As a result, the early malignant cancers are a dynamically evolving school of cells living in their distinct acidic microhabitats that ensure the emergence of more aggressive phenotype as "pioneers." 18 To investigate this, we adapted cells at to growth at acidic pH (6.5) until their growth rates were equivalent to their parental cells grown at high pH (7.4). As shown in Figure 1A, acid adapted MCF-7 cells have different shape and size than their parental cells that they are evolved from.…”