Mucin Families in Gastrointestinal Tract

Boland, Clement

Abstract

Mucins are the principal glycoprotein constituents of the mucus coat that lines the gastrointestinal tract. Histochemists have traditionally recognized that sulfomucins and sialomucins are differentially distributed through normal colonic epithelium, and that different forms are expressed during diseases processes, including colorectal neoplasia. We have purified mucins from the colon and divided them into biochemically distinct families that correspond to the sialomucin and sulfomucin families. Our preliminary data suggest that sialomucins carry all of the tumor antigens, and other investigating groups have suggested that this family of mucins may mediate tumor metastasis and play a role in tumor immunology. Six human mucin genes have been cloned, and all share a common structural motif in which there are multiple tandem repeat units in the middle of the apomucin core protein that is densely glycosylated in the fully processed mucin. To date, no glycoprotein product has been identified for any of the mucin genes, and it is not known how the epithelial cells processes each apomucin core protein faithfully. In this proposal, we will identify which genes encode for each of the mucin families. We will identify the genetic source of the mucins that carry tumor antigens in the colon. We will determine the distribution of mucin gene expression in normal and neoplastic tissues using a histochemical technique that will destroy carbohydrate portion of mucins in situ to expose the apomucins for recognition by antibodies. We will perform in situ hybridization and use reverse transcriptase PCR to determine mucin gene expression in colorectal tissues. It is unknown how an epithelial cell can differentially process glycoproteins that give rise to different families of mucin. We propose to use mucin """"""""minigenes"""""""" in which we will splice together different portions of mucin genes to find which portion of the mucin apoprotein determines the nature of its glycosylation. Finally, we have developed a technique whereby genomic lesions may be assayed by extracting DNA from tissues removed from single histological sections. By using mucin histochemistry and genomic analysis, we have proposed to begin to explore the genetic basis of tumor antigen expression. In summary, we plan to better understand the relationship between recently cloned mucin genes and the fully processed gene products. This will provide insight into the mechanism by which carbohydrate tumor antigens are expressed in neoplastic colorectal epithelium. The experiments will also provide insight into the process by which the cell accomplishes selective protein glycosylation.