Mucins are a diverse group of complex, highly glycosylated extracellular proteins. Mucin 2 (MUC2) protects the gastric and intestinal epithelium from chemical and mechanical injury. Anti-MUC2 reactivity is normally seen in goblet cells of the small intestine and colon and is associated with mucinous carcinomas, including those of the gastrointestinal tract and ovary. MUC2 immunohistochemistry is useful for identifying colonic, gastric, and esophageal carcinomas.
Mucins are high molecular weight glycoproteins which constitute the major component of the mucus layer that protects the gastric epithelium from chemical and mechanical aggressions. In humans, at least 14 mucin genes have been identified that code for the mucin proteins. They are designated as MUC1, MUC2, MUC3, MUC4, MUC5AC, MUC5B , MUC6 , MUC7, MUC8, MUC9, MUC11, MUC12, MUC13 and MUC16. Mucins share a common feature of a tandem-repeat domain rich in serine and threonine residues. These amino acid residues are potential O-glycosylation sites for attachment of the O-glycan chains that constitute up to 80% of the molecular weight of the final mucin glycoprotein.
The heterogeneous pattern of mucin expression, including the expression of the intestinal mucin MUC2, may provide new insights into the differentiation pathways of gastric carcinoma. The pattern of mucin expression may also be used as a clue to bring new insights into the biological behaviour of distinct clinicopathological entities related to the localisation of gastric carcinoma, namely proximal and distal gastric carcinomas. Pinto-de-Sousa et al. have shown in a comprehensive study of gastric carcinomas evaluated for expression of several mucins (MUC1, MUC2, MUC5AC and MUC6) that: (1) mucin expression is associated with tumour type (MUC5AC with diffuse and infiltrative carcinomas and MUC2 with mucinous carcinomas) but not with the clinico-biological behaviour of the tumours; and (2) mucin expression is associated with tumour location (MUC5AC with antrum carcinomas and MUC2 with cardia carcinomas), indirectly reflecting differences in tumour differentiation according to tumour location.
The following generalities apply to the patterns of Mucin expression:
MUC1 expression: apical surfaces of most epithelial cells in breast, GI, respiratory, and GU tracts.
MUC2 expression: specifically expressed in goblet cells of the small intestine & colon.
Colonic CAs – 65%, Gastric CAs – 42%, Esophageal CAs – 17%
Rare outside of GI tract – with exception of; mucinous ca of breast, clear cell-type CAs of the ovary.