Clonal biases dictate availability of colonic cancer driver mutations for transformation

Aged normal tissues harbour cancer mutations predisposing to transformation. However, previous studies of human colon have not allowed a comparison of the prevalence and behaviours of different pro-oncogenic events. Here, hotspot regions in colorectal cancer genes (APC, KRAS, TP53, FBXW7, CTNNB1) were analysed by targeted amplicon sequencing in normal tissue samples across three different sampling methods, Micro-seq, AceStrip-seq and Section-seq, to ensure both high sensitivity of detection and wide tissue coverage (Micro-seq data: 56 patients and 76,800 crypts, AceStrip-seq: 43 patients and 754,188 crypts and Section-seq: 189 patients and 4,114,532 crypts. Cancer driver mutations were detected in all genes, including APC. Inference of clone dynamics revealed that FBXW7 R465C and KRAS G12 missense mutations have strong positive biases in clone fixation and expansion, respectively. Modelling of mutational co-occurrences and temporal ordering show that selection biases associated with early biallelic loss of APC progressively favours an 'APC first' acquisition of driver events leading to cancer with age. At younger ages KRAS activation is equally likely to be first due to its associated biases.