ONCOFETAL MUCIN M1 EPITOPE FAMILY - CHARACTERIZATION AND EXPRESSION DURING COLONIC CARCINOGENESIS

The gastric mucin MI antigens, markers associated with colonic carcinogenesis, have been characterized by new antimucin monoclonal antibodies (MAbs). These MAbs, obtained against mucins isolated from a human ovarian mucinous cyst (MAbs 19MI, 21MI and 45MI) and from a pancreatic adenocarcinoma (MAb 96RA), were compared with 5 other anti-MI mucin MAbs described previously, which characterized the a, b, c, d and e mucin MI epitopes. Using immunoperoxidase, these new MAbs exclusively stained the surface gastric epithelium of normal human gastro-intestinal tract and reacted with fetal, precancerous and cancerous colonic mucosa, but not with normal colon. Immunoradiofixation studies showed that these new MAbs are directed against 3 epitopes (f, g and h) which are different from the a, b, c, d and e mucin MI epitopes, though present on the same a immunoreactive high-molecular-weight components (> 1,000 kDa) with a density of 1.4 by CsCl-density-gradient ultracentrifugation. MI antigenicity is characterized by a family of 8 different MI epitopes which were destroyed with beta-mercaptoethanol (except for the f epitope), sensitive to a 5 hr trypsin treatment and resistant to 5 mM periodate (except for the h epitope). Some epitopes (b, c and d) showed increasing immunoreactivity after 20 mM periodate treatment, suggesting cryptic location. In rat-colon adenocarcinomas, MI mucin epitopes were masked but could by decrypted using high periodate treatment, similar to normal rat gastric mucosa, thus suggesting the absence of drastic changes in the saccharide coat of the peptide mucin portion bearing MI epitopes. Cryptic location, periodate resistance, sensitivity to protease and conformational behavior strongly suggest that the peptidic core of gastric (or fetal colonic) mucin plays a role in MI immunoreactivity. Indeed, the resurgence of MI antigens during colonic carcinogenesis is due to re-expression of the peptide core of gastric (or fetal colonic) mucins.