Subclonal diversity arises early even in small colorectal tumours and contributes to differential growth fates

Abstract

Objective and designThe goal of the study was to determine whether the mutational profile of early colorectal polyps correlated with growth behaviour. The growth of small polyps (6–9 mm) that were first identified during routine screening of patients was monitored over time by interval imaging with CT colonography. Mutations in these lesions with known growth rates were identified by targeted next-generation sequencing. The timing of mutational events was estimated using computer modelling and statistical inference considering several parameters including allele frequency and fitness.ResultsThe mutational landscape of small polyps is varied both within individual polyps and among the group as a whole but no single alteration was correlated with growth behaviour. Polyps carried 0–3 pathogenic mutations with the most frequent being in APC, KRAS/NRAS, BRAF, FBXW7 and TP53. In polyps with two or more pathogenic mutations, allele frequencies were often variable, indicating the presence of multiple populations within a single tumour. Based on computer modelling, detectable mutations occurred at a mean polyp size of 30±35 crypts, well before the tumour is of a clinically detectable size.ConclusionsThese data indicate that small colon polyps can have multiple pathogenic mutations in crucial driver genes that arise early in the existence of a tumour. Understanding the molecular pathway of tumourigenesis and clonal evolution in polyps that are at risk for progressing to invasive cancers will allow us to begin to better predict which polyps are more likely to progress into adenocarcinomas and which patients are at greater risk of developing advanced disease.

A population genetics perspective on the determinants of intra-tumor heterogeneity

“…Thus, tumor growth may accompany rapid genetic diversification and a small tumor can already harbor extensive diversity. This prediction is consistent with recent findings that early tumors display high-levels of genomic diversity [33, 46, 178]. …”

“…Subsequent studies have corroborated aspects of ‘Big Bang’ dynamics [46, 47, 247], suggesting that they may be relatively common in colorectal tumors. Others have considered more strict models of neutrality.…”

“…In theory, the earlier the initial clonal expansion is detected, the less diverse and less fit the cell population (since natural selection operates on diversity), and the better the prognosis. Even relatively early tumors exhibit high genomic diversity [33, 46], which may reflect processes that were already operative for some time and likely will continue in the absence of intervention. The term “early” is indeed relative as the first clonal expansions will be invisible and the lesion must be large enough to be detected and sampled.…”

New insights into the earliest stages of colorectal tumorigenesis

“…Sottoriva and colleagues demonstrated the presence of both sub-clonal mutations and copy number events in colon adenomas [23, 28]. Similarly, Sievers and colleagues recently reported sub-clonal driver mutations, including the KRAS G12D variant, in small (6–9mm) colon polyps [29]. Utilizing a unique cohort of patients with small polyps for which volumetric growth rate was available, they were able to determine that detectable, sub-clonal mutations must have arisen before the polyp was of a detectable size.…”

Cancer evolution and heterogeneity

“…1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35 This multiregional analysis (MRA) sequencing approach enabled us not only to observe spatial heterogeneity, but also to calculate temporal alterations and eventually disclose the evolution of tumors. There are two types of somatic aberration in a tumor: ubiquitous aberrations (founder mutations, trunk mutations, or clonal mutations) and scattered aberrations (progressor mutations, branch/leaf mutations, or subclonal mutations).…”