Spatial mutation patterns as markers of early colorectal tumor cell mobility

Abstract

A growing body of evidence suggests that a subset of human cancers grows as single clonal expansions. In such a nearly neutral evolution scenario, it is possible to infer the early ancestral tree of a full-grown tumor. We hypothesized that early tree reconstruction can provide insights into the mobility phenotypes of tumor cells during their first few cell divisions. We explored this hypothesis by means of a computational multiscale model of tumor expansion incorporating the glandular structure of colorectal tumors. After calibrating the model to multiregional and single gland data from 19 human colorectal tumors using approximate Bayesian computation, we examined the role of early tumor cell mobility in shaping the private mutation patterns of the final tumor. The simulations showed that early cell mixing in the first tumor gland can result in side-variegated patterns where the same private mutations could be detected on opposite tumor sides. In contrast, absence of early mixing led to nonvariegated, sectional mutation patterns. These results suggest that the patterns of detectable private mutations in colorectal tumors may be a marker of early cell movement and hence the invasive and metastatic potential of the tumor at the start of the growth. In alignment with our hypothesis, we found evidence of early abnormal cell movement in 9 of 15 invasive colorectal carcinomas ("born to be bad"), but in none of 4 benign adenomas. If validated with a larger dataset, the private mutation patterns may be used for outcome prediction among screen-detected lesions with unknown invasive potential.

Minimal Barriers to Invasion During Human Colorectal Tumor Growth

“…The observed dissociation of phenotypic and genotypic progression during tumor growth is consistent with near neutral evolution or Big Bang tumorigenesis 11 where the founder cell already starts at a high fitness level and has the driver mutations sufficient for rapid growth (Fig 1B). Aligned with this scenario, most driver mutations in this and other studies 9,18 were clonal, that is present in all sampled areas. The subclones were defined largely by passenger mutations that show little evidence of selection in cancers [19][20][21] , and unsurprisingly, the subclones mapped by saturation microdissections were present in both the invasive and non-invasive components without distinct boundaries between those components.…”

“…In particular, the observed phylogeographies are consistent with the neutral Big Bang model of tumorigenesis, a type of branching evolution that has been extensively documented in these tumors. [9][10][11] In summary, and similarly to a recent study in early stage breast cancers 12 , our results indicate that multiclonal invasion is common in colorectal tumors and indicate that there are only minimal barriers to invasion after the start of tumor growth.…”

“…Given a founder cell with the driver mutations and an epigenome sufficient for rapid growth and adaptation, invasion can occur very early and by multiple, essentially identical subclones. The ancestral trees inferred in this study likely reflect very early events in the first tumor gland because subclonal mutations that occur later would not be detectable by exome sequencing 9,15 . Consequently, the final locations of subclones within a tumor depend primarily on cell movement or mixing during early growth 9 , which in turn leads to the observed jigsaw-arranged subclone patterns that exhibit little correlation between physical and ancestral distances.…”

“…To leverage this gland level clonality, we microdissected individual spots (small regions consisting of 2-5 adjacent glands each) from multiple regions of microscope slides (Fig 2a) obtained from 12 human colorectal tumors (Table 1). From each primary tumor, two (one for tumor R) arbitrarily located sections were collected and a median of 13 spots (range: [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] per slide were microdissected. The average minimum distance between neighboring spots was 2.9 mm, with an average pairwise distance of 8.4 mm between dots on the same slide.…”

“…Although a lack of clear morphologic boundaries or size differences between the jigsaw arrayed subclones is consistent with tumorigenesis by near-neutral subclones, the presence of clonal selection is difficult to rule out 14,15 . Indeed, a key challenge in recognizing selection during tumor growth is that the relative fitness of a subclone needs to be very high in order to "outrun" its neighboring, neutrally expanding subclones 9,16,17 . Focusing instead on established canonical driver mutations, we found that 11 of 12 tumors had at least one public driver mutation (tumor H had none).…”