Tumor Heterogeneity in Colorectal Cancer: What Do We Know So Far?

Colorectal cancer is not one disease but rather a collection of neoplastic diseases. Due to heterogeneity in the disease biology, therapy response, and prognosis, extensive disease stratification is required. Therefore, TNM stage, microsatellite status, tumor grade, lymphovascular invasion, and other parameters are assessed in the pathology report to indicate the extent and prognosis of the disease. The mutation status of KRAS, BRAF, and NRAS is also investigated in a metastatic context to predict the response to anti-EGFR therapy. Recently, 4 distinct molecular subtypes of colorectal cancer have been described that have both prognostic and therapeutic relevance. In addition, characterization of the inflammatory infiltrate revealed major differences in the amount and location of inflammatory cells in distinct colorectal tumor types. Together, all of these parameters help to stratify patients into different therapeutic and prognostic subgroups. However, this stratification is not unambiguous since tumors often display intratumoral heterogeneity, whereby several subpopulations within one tumor show differences in morphology, inflammatory infiltrate, mutational status, or gene expression profile. This article gives an overview of all of the current known data with regard to tumor heterogeneity at both inter- and intratumoral levels.