Mismatch repair (MMR) proteins is a group of nuclear enzymes, which in all proliferating cells participate in repair of base-base mismatch, that occur during DNA replication. The proteins form complexes (heterodimers) that bind to areas of abnormal DNA and initiates its removal. Loss of MMR proteins leads to an accumulation of DNA replication errors in the proliferating cells, particularly in areas of the genome with short repetitive nucleotide sequences, a phenomenon known as microsatellite instability (MSI). Hence, MMR protein deficiency in cells is closely related to a high degree of MSI (MSI-H), in contrast to cells with a low degree of MSI (MSI-L) and cells that are MSI stable (MSS). In human, nine genes with MMR function have been identified, of which five have particular clinical interest because they may be mutated in families with hereditary non-polyposis colorectal cancer (HNPCC) (the relative frequency in parenthesis): MLH1 (49%), PMS1 (0.3%), PMS2 (2%), MSH2 (38%), and MSH6 (9%). More than 300 different mutations have been identified. Persons carrying a mutation have normal MMR proteins, but in case of a DNA damage involving the nonmutated allel, the protein production stops (loss of heterozygosity). When forming complexes MLH1 heterodimerizes with PMS2, PMS1 or MLH3 (another MMR protein, for which a mutation has not been detected), while MSH2 heterodimerizes with MSH6. In case of MSH2 deficiency, MSH6 protein is also lost, possibly because of protein instability.
Carriers of an MLH1 or MSH2 mutation has a more than 70% lifetime risk of developing a colorectal carcinoma and also markedly increased risk of developing endometrial carcinomas (50%), as well as an less increased risk of developing carcinomas of stomach, biliary tract, ovary and urinary tract, as well as brain tumours and sebaceous skin tumours. The distribution of extraintestinal tumours is somewhat dependent on the type of mutation. About 5% of colorectal carcinomas are associated with a germline mutation. In about 15% of colorectal carcinomas loss of MLH1 protein occurs without a mutation but because of a hypermethylation of the MLH1 promoter. MSI-H (MMR protein deficient) colorectal carcinomas are associated with significant survival advantage independently of grade and stage, compared to MSS colorectal carcinomas (which are characterized by chromosomal instability). MSI-H carcinomas usually display high prevalence of activated intraepithelial T-lymphocytes (which is suspected to increase tumour cell apoptosis and, hence, to impede the growth potential). They are more often localized in the right hemicolon, and show a slightly deviant morphologic spectrum, as there is an overrepresentation of mucinous adenocarcinomas and medullary carcinomas (inappropriately designated low differentiated [adeno-]carcinomas). It appears that among colorectal carcinomas the response to chemotherapy is different in MSI-H and MSS cases, but the data are conflicting.
Compared to molecular biological techniques, immunohistochemical analysis of MMR protein expression is much simpler and cheaper. Immunohistochemical analysis helps to pinpoint the affected gene and should be readily accessible in a pathology laboratory. Staining for MLH1, MSH2 and MSH6 in colorectal carcinomas should be carried out in patients < 55 years-of-age or with a family history of these tumours. Staining may also be carried out prior to planned chemotherapy. At some hospitals, all colorectal carcinomas are analyzed because of the prognostic implications.
Tonsil is found to be a recommendable positive tissue control for MSH6. Virtually all mantle zone B-cells must show an at least weak to moderate nuclear staining reaction, while a moderate to strong nuclear staining reaction must be seen in the proliferating germinal centre B-cells.
Colon adenocarcinoma with loss of MSH6 expression is recommended as negative tissue control. No nuclear staining reaction should be seen in the neoplastic cells, whereas a nuclear staining reaction must be seen in stromal cells serving as internal positive tissue control.
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