Prevalence and fate of pro-oncogenic clones in the human colon
Normal aged tissues are thought to exist as a patchwork of mutations. The detection of cancer-driver mutations in the normal human colonic epithelium had been so far limited. Using immunohistochemistry and sequencing methods, and by screening a large number of patients and crypts, pro-oncogenic clones were detected for eight colorectal cancer driver genes; ARID1A, APC, CTNNB1, FBXW7, KRAS, PTEN, TP53 and SMAD4. Losses or gain of function in these genes were associated with biases in the processes of clone fixation and expansion, which determined their event burden in the tissue. However, by comparing the event burden in the normal tissue and the frequency of the same mutations in colorectal cancer, it was observed that a high mutational burden does not dictate the Availability of clones for Neoplastic Conversion (ANC). For example, TP53 clones had the highest event burden in the normal tissue but KRAS clones had a higher ANC. Some of the factors that could be contributing to the differential fate of pro-oncogenic clones were investigated. An increase in cell-proliferation seemed necessary to induce biases in clonal dynamics but not sufficient to explain the different fates of clones in terms of their ANC. In addition, larger KRAS patches seemed to possess different characteristics from smaller ones, often exhibiting abnormal crypt morphology and containing a reduced frequency of CD4+ immune cells. Finally, a human colonic KO organoid model was used to study the clonal behaviours in vitro and gain insight into the potential mechanisms and pathways that could be mediating mutation-associated advantages. The normal human colon appears to be more mosaic in terms of cancer-driver mutations than originally thought. This means that the status of certain cancer-associated mutations can be reclassified based on their degree of representation in the normal tissue and mutations with a higher probability of initiating cancer can be identified. Thousands of invisible pro-oncogenic clones are inferred to exist in the aged colon but not all of them may have an equal chance for neoplastic transformation. Differential fates were found to exist between clones with mutations in different genes, different amino acid changes in the same gene or even the same amino acid change. This information could in time be used for early-detection efforts by stratifying individuals based on the risk of the mutations that they might possess.