Author：Enxiao LI, Ying Hu, Wenbo Han, Tianshu Liu, Fang Lv, Lili Zhao, Yang Liu, Yanhui Chen, Henghui Zhang

Background: The microsatellite instability-high (MSI-H) phenotype confers good prognosis and greater response to immunotherapy in colorectal cancer(CRC). The mutational landscape of MSI-H CRC is unclear. This study was designed to illustrate the difference mutation profile between the MSI-H and microsatellite stable (MSS) CRC. 

Methods: Tumor tissue and matched blood samples from 40 patients with colorectal cancer were collected. Microsatellite instability (MSI) status were detected by PCR-amplified for five mononucleotide repeat markers (BAT-25, BAT-26, NR-21, NR-24 and MONO-27). Mutation profiles were sequenced by a cancer gene-targeted NGS panel. 

Results: The tumor mutation burden(TMB) of the MSI-H CRC patients was significantly higher than those MSS CRC patients. Compared with the MSS CRC, MSI-H CRC involved more genes and pathways. Furthermore, we found the copy number variation (CNV) was different between the two groups. The copy number instability (CNI) score of MSI-H CRC patients was significantly lower than those MSS CRC patients. MSI-H CRC patients showed a higher frequency of TP53 gene CNV gain compared with MSS CRC (41% (7/17) in MSI-H CRC versus 13% (3/23) in MSS CRC). 

Conclusions: The mutational landscape are different between the MSI-H and MSS colorectal cancer. Compared with MSS colorectal cancer, the MSI-H colorectal cancer patients have higher tumor mutation burden(TMB) and lower copy number instability (CNI) score. Keywords: colorectal cancer, microsatellite instability, tumor mutation burden, copy number instability. Abbreviations CRC, colorectal cancer; TMB, tumor mutation burden; MSI-H, The microsatellite instability-high; MSS, microsatellite stable; CNV, copy number variation.