Genetic mechanisms of immune evasion in colorectal cancer [Research Articles]

To understand the genetic drivers of immune recognition and evasion in colorectal cancer (CRC), we analyzed 1,211 CRC primary tumor samples, including 179 classified as microsatellite instability-high (MSI-high). This set includes The Cancer Genome Atlas CRC cohort of 592 samples, completed and analyzed here. MSI-high, a hypermutated, immunogenic subtype of CRC, had a high rate of significantly mutated genes in important immune modulating pathways and in the antigen presentation machinery, including biallelic losses of B2M and HLA genes due to copy number alterations and copy-neutral loss of heterozygosity (CN-LOH). WNT/β-catenin signaling genes were significantly mutated in all CRC subtypes, and activated WNT/β-catenin signaling was correlated with the absence of T-cell infiltration. This large-scale genomic analysis of CRC demonstrates that MSI-high cases frequently undergo an immunoediting process that provides them with genetic events allowing immune escape despite high mutational load and frequent lymphocytic infiltration, and furthermore, that CRC tumors have genetic and methylation events associated with activated WNT signaling and T-cell exclusion.

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