No abstract

Defects of DNA mismatch repair (MMR) cause the high level microsatellite instability (MSI-H) phenotype. MSI-H cancers may develop either sporadically or in the context of the hereditary nonpolyposis colorectal cancer (HNPCC) syndrome that is caused by germline mutations of MMR genes. In colorectal cancer (CRC), MSI-H is characterized by a dense lymphocytic infiltration, reflecting a high immunogenicity of these cancers. As a consequence of immunoselection, MSI-H CRCs frequently display a loss of human leukocyte antigen (HLA) class I antigen presentation caused by mutations of the b2-microglobulin (b2m) gene. To examine the implications of b2m mutations during MSI-H colorectal tumor development, we analyzed the prevalence of b2m mutations in MSI-H colorectal adenomas (n 5 38) and carcinomas (n 5 104) of different stages. Mutations were observed in 6/38 (15.8%) MSI-H adenomas and 29/104 (27.9%) MSI-H CRCs. A higher frequency of b2m mutations was observed in MSI-H CRC patients with germline mutations of MMR genes MLH1 or MSH2 (36.4%) compared with patients without germline mutations (15.4%). The high frequency of b2m mutations in HNPCC-associated MSI-H CRCs is in line with the hypothesis that immunoselection may be particularly pronounced in HNPCC patients with inherited predisposition to develop MSI-H cancers. b2m mutations were positively related to stage in tumors without distant metastases (UICC I-III), suggesting that loss of b2m expression may promote local progression of colorectal MSI-H tumors. However, no b2m mutations were observed in metastasized CRCs (UICC stage IV, p 5 0.04). These results suggest that functional b2m may be necessary for distant metastasis formation in CRC patients. ' 2007 Wiley-Liss, Inc.

Mismatch repair (MMR) deficiency is a major mechanism of colorectal tumorigenesis that is observed in 10-15% of sporadic colorectal cancers and those associated with the hereditary nonpolyposis colorectal cancer (HNPCC) syndrome. MMR deficiency leads to the accumulation of mutations mainly at short repetitive sequences termed microsatellites, constituting the high level microsatellite instability (MSI-H) phenotype. In recent years, several genes have been described that harbor microsatellites in their coding region (coding microsatellites, cMS) and are frequently affected by mutations in MMR-deficient cancers. However, evidence for a functional role of most of the known cMS-containing genes is missing, and further analyses are needed for a better understanding of MSI tumorigenesis. Here, we examined in detail alterations of the absent in melanoma 2 (AIM2) gene that shows a high frequency of cMS frameshift mutations in MSI-H colorectal, gastric, and endometrial tumors. AIM2 belongs to the HIN-200 family of interferon (IFN)-inducible proteins, its role in colon carcinogenesis, however, is unknown. Sequencing of the entire coding region of AIM2 revealed a high frequency of frameshift and missense mutations in primary MSI-H colon cancers (9/20) and cell lines (9/15). Biallelic AIM2 alterations were detected in 8 MSI-H colon tumors and cell lines. In addition, AIM2 promoter hypermethylation conferred insensitivity to IFN-gamma-induced AIM2 expression of three MSI-H colon cancer cell lines. These results demonstrate that inactivation of AIM2 by genetic and epigenetic mechanisms is frequent in MMR-deficient colorectal cancers, thus suggesting that AIM2 is a mutational target relevant for the progression of MSI-H colorectal cancers.