Mutational Heterogeneity in APC and KRAS Arises at the Crypt level and Leads to Polyclonality in Early Colorectal Tumorigenesis

Purpose:  The majority of genomic alterations causing intratumoral heterogeneity (ITH) in colorectal cancer (CRC) are thought to arise during early stages of carcinogenesis as a burst but only after truncal mutations in APC have expanded a single founder clone. We have investigated if the initial source of ITH is consequent to multiple independent lineages derived from different crypts harboring distinct truncal APC and driver KRAS mutations, thus challenging the prevailing monoclonal monocryptal model. <p>Experimental design: High-depth next-generation sequencing and SNP arrays were performed in whole lesion extracts of 37 FAP colorectal adenomas. Also, ultra-sensitive genotyping of hotspot mutations of APC and KRAS was performed using nanofluidic PCRs in matched bulk biopsies (n=59) and crypts (n=591) from 18 adenomas and 7 carcinomas and adjacent normal tissues.</p> <p>Results: Multiple co-occurring truncal APC and driver KRAS alterations were uncovered in whole lesion extracts from adenomas and subsequently confirmed to belong to multiple clones. Ultra-sensitive genotyping of bulk biopsies and crypts revealed novel undetected APC mutations that were prominent among carcinomas, whereas abundant wild-type APC crypts were detected in adenomas. KRAS mutational heterogeneity within crypts was evident in both adenomas and carcinomas with a higher degree of concordance between biopsy and crypt genotyping in carcinomas. Non-random heterogeneity among crypts was also observed.</p> Conclusions: The striking degree of non-random intercrypt heterogeneity in truncal and driver gene mutations observed in adenomas and carcinomas is consistent with a polycryptal model derived from multiple independent initiation linages as the source of early ITH in colorectal carcinogenesis.

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