HNPCC versus sporadic microsatellite-unstable colon cancers follow different routes toward loss of HLA class I expression

Background: Abnormalities in Human Leukocyte Antigen ( HLA) class I expression are common in colorectal cancer. Since HLA expression is required to activate tumor antigen-specific cytotoxic T-lymphocytes ( CTL), HLA class I abnormalities represent a mechanism by which tumors circumvent immune surveillance. Tumors with high microsatellite instability ( MSI-H) are believed to face strong selective pressure to evade CTL activity since they produce large amounts of immunogenic peptides. Previous studies identified the prevalence of HLA class I alterations in MSI-H tumors. However, those reports did not compare the frequency of alterations between hereditary and sporadic MSI-H tumors neither the mechanisms that led to HLA class I alterations in each subgroup. Methods: To characterize the HLA class I expression among sporadic MSI-H and microsatellite-stable ( MSS) tumors, and HNPCC tumors we compared immunohistochemically the expression of HLA class I, beta 2-microglobulin (beta 2m), and Antigen Processing Machinery ( APM) components in 81 right-sided sporadic and 75 HNPCC tumors. Moreover, we investigated the genetic basis for these changes. Results: HLA class I loss was seen more frequently in MSI-H tumors than in MSS tumors ( p < 0.0001). Distinct mechanisms were responsible for HLA class I loss in HNPCC and sporadic MSI-H tumors. Loss of HLA class I expression was associated with beta 2m loss in HNPCC tumors, but was correlated with APM component defects in sporadic MSI-H tumors ( p < 0.0001). In about half of the cases, loss of expression of HLA class I was concordant with the detection of one or more mutations in the beta 2m and APM components genes. Conclusion: HLA class I aberrations are found at varying frequencies in different colorectal tumor types and are caused by distinct genetic mechanisms. Chiefly, sporadic and hereditary MSI-H tumors follow different routes toward HLA class I loss of expression supporting the idea that these tumors follow different evolutionary pathways in tumorigenesis. The resulting variation in immune escape mechanisms may have repercussions in tumor progression and behavior.