Intratumor Heterogeneity and Evolution of Colorectal Cancer

Identification of novel targets, and the development of target-based precision medicine for personalized cancer therapy is the biggest challenge in cancer research. Gradual and progressive genetic alterations drive the tumor from its benign stage to malignancies. Study of intra-tumor heterogeneity in terms of genetic alterations enable us to understand the clonal evolution which plays a key role in progression and metastases of tumors, and can lead to the development of personalized cancer therapeutic strategies. However, unlike other types of cancer, colorectal cancer (CRC) has been studied less, despite being a major threat world-wide, especially in the Chinese population. Here, in order to understand the intratumor heterogeneity and clonal evolution, we performed high depth whole exome sequencing followed by bioinformatic and statistical analysis for multi-region colorectal tumor samples. In this prospective study, we recruited different stages of 68 CRC patients with primary tumor at right-sided colon (18), left-sided colon (20) and rectum (30). In total, we sequenced 206 tumor regions including 176 primary tumors, 19 lymph node metastasis (LN) and 11 extranodal tumor deposits (ENTD) samples. Our result showed extreme intratumor heterogeneity with a Darwinian pattern of clonal evolution. We also identified that clonal evolution pattern of left-sided CRC was more complex and divergent than right-sided CRC, suggesting the evolutionary diversity in the initiation and progression of left-sided CRC. Genetic and evolutionary evidence found that both LN and ENTD were of polyclonal origin, and ENTD was an entity distinct from LN and evolves later. In addition, extensive heterogeneity was found in driver mutations in KRAS and PIK3CA genes, suggesting major limitations of single biopsies in clinical diagnosis and for developing precision medicine for the patients with CRC. In conclusion, our study showed a Darwinian pattern of evolution in CRC with spatio-temporal differences in evolution between left-sided and right-sided CRC patients.