Original contributionThe MUC13 cell surface mucin is highly expressed by human colorectal carcinomas
Introduction
Mucins are complex glycoproteins produced by all mucosal tissues that are thought to act primarily as a physical and chemical barrier to potential pathogens and particulate matter. Some mucins are highly expressed by cancers of mucosal tissues and have diagnostic, therapeutic, and biological significance in these adenocarcinomas.
The key characteristic of mucins is the presence of large, heavily O-glycosylated domains typically composed of a variable number of tandem repeat sequences (VNTR) that are rich in serine and threonine resides. Mucins may be broadly categorized as either secreted (usually gel-forming) or transmembrane. The secreted gel-forming mucins homo-oligomerize and are the major constituent of the mucus gel that overlies all mucosal tissues. MUC2, MUC5AC, MUC5B, and MUC6 occur in a cluster on chromosome band 11p15 [1], and MUC19 is encoded for by a gene on chromosome band 12p12 [2]. The chromosome 11 mucins are produced by a variety of epithelia in organ-specific patterns: MUC2 is the major glycoprotein component of colonic mucus, synthesized and secreted by goblet cells [3]. Expression of MUC19 appears to be restricted to salivary glands [2]. The transmembrane (TM) mucins include MUC1 [4], MUC3A and MUC3B [5], [6], [7], MUC4 [8], MUC12 [9], MUC13 [10], MUC15 [11], MUC16 (also known as CA-125) [12], MUC17 [13], and MUC20 [14]. MUC3A, MUC3B, MUC12, and MUC17 occur in a cluster on chromosome band 7q22, whereas MUC4 and MUC20 are in close proximity on band 3q29. The other TM mucin genes are dispersed through the genome. A number of structural characteristics are common to TM mucins: SEA domains that form cleavage sites have been identified in MUC1, MUC3A and MUC3B, MUC12, and MUC13 [15], whereas one or more epidermal growth factor-like domains are present in MUC3A, MUC3B, MUC4, MUC12, MUC13, and MUC17. Although the true role of TM mucins remains unclear, there is emerging evidence that they are involved in host defense from infection acting as both a physical barrier and by modulating epithelial growth and apoptosis via signal transduction.
We identified the MUC13 gene and localized it to chromosome band 3q13.3 [10]. The apoprotein structure is composed of 3 epidermal growth factor-like domains, a large 151 amino acid VNTR, and a SEA domain within the extracellular component, a short transmembrane domain, and a 69 amino acid cytoplasmic domain (MUC13-CD). Within the MUC13-CD there are several potential phosphorylation sites (8 serine and 2 tyrosine residues) and a protein kinase C consensus phosphorylation motif. MUC13 mRNA is expressed most highly by the large intestine, trachea, kidney, small intestine, and gastric epithelium [10]. In malignancy, MUC13 mRNA has been detected in colorectal, esophageal, gastric, pancreatic, and lung cancers [16].
The large intestine expresses most of the known TM mucins. Immunohistochemical studies have demonstrated MUC1 and MUC4 in both colonic goblet and columnar epithelial cells where expression levels are highest in the crypt bases [17], [18]. MUC3 is expressed by both goblet and columnar cells, but expression is most pronounced in the upper crypts and surface epithelium [19], [20]. In our initial report regarding MUC13, we demonstrated expression in the small and large intestines and localized the protein to the apical membranes, goblet cell thecae, and epithelial cell cytoplasm [10]. MUC12 immunohistochemistry reveals a similar pattern (unpublished data). Studies of the remaining TM mucins have been largely restricted to mRNA to date. MUC12 is most abundantly expressed in normal colon with lesser levels in the pancreas [16]. MUC15 and MUC20 mRNAs have also been detected in the colon [11], [14]. High levels of MUC17 mRNA are seen in the transverse colon and lesser amounts in other sites within the colorectum [13]. In situ hybridization revealed MUC17 mRNA in the absorptive epithelial cells of the small intestine.
Expression of many epithelial mucins has been shown to be perturbed in colorectal neoplasia. Adenomatous polyps show MUC3 [23] and variable but increased MUC1 expression. On the other hand, MUC4 expression in hyperplastic polyps recapitulates the pattern seen in normal epithelium, whereas serrated adenomas often show reduction or loss of staining accompanied by up-regulation of MUC1 [21], [22]. Colorectal cancers frequently show altered expression of TM mucins. MUC1 mRNA levels tend to be higher in colorectal cancers (CRCs) compared with normal mucosa, whereas MUC4, MUC12, and MUC13 expression decreases with malignant transformation and MUC3 mRNA levels in CRC remain roughly comparable to those found in normal mucosa [16]. MUC1 expression detected using immunohistochemistry frequently increased with strong cytoplasmic reactivity as well as activity along the apical membranes and within luminal secretions [17]. Overexpression of MUC1 in colorectal cancers, particularly at the invasion front, has been correlated with poor prognosis [23], [24]. MUC3 is expressed in the cytoplasm and glandular lumina of CRCs [19], [20]. MUC4 expression tends to be decreased after malignant transformation, and we have previously shown an association between loss of MUC4 expression and increased tumor stage in CRC [17]. Various studies have reported expression of MUC1, MUC3, MUC4, MUC12, MUC13, and MUC17 mRNAs in panels of colorectal cancer cell lines [13], [16]. No data concerning MUC15 expression in CRCs have been published. For a comprehensive review of mucins and their expression in colorectal cancer, see Byrd and Bresalier [25].
There is little known about the expression of MUC13 protein in malignancy. Shimamura et al [26] have recently described expression of MUC13 in gastric adenocarcinomas and reported that there was an association between MUC13 expression and the intestinal phenotype with expression less common in diffuse type carcinomas. Interestingly, these authors reported no staining of normal gastric mucosa but strong apical membrane reactivity for MUC13 in normal colon and small intestinal epithelial cells. We now report that the MUC13 protein is very highly expressed by most colorectal cancers.
Section snippets
Patients
The patient cohort is composed of 99 randomly selected cases of colorectal cancer from 96 patients treated surgically at the Royal Brisbane Hospital between March 1989 and March 2001 (3 patients had 2 synchronous carcinomas each). There were 53 men and 43 women, with a mean age of 67.7 ± 11.6 years (range, 27.8-88.2 years). Ethical approval was obtained from the relevant institutional committees at all participating centers. The clinicopathological characteristics for the study series are
Expression of MUC13 in normal colon
Contiguous histologically normal colonic epithelium was present in 65 of 100 cases studied. Staining of MUC13 was most intense in the apical membrane with variable but moderate to high levels of cytoplasmic staining. Some staining of goblet cell thecae was also observed, showing that MUC13 is produced by both the columnar and goblet cells of the colon. In 4 cases, there was also evidence of intense reactivity in intracytoplasmic vesicles. There was a gradient of staining, with the most intense
Discussion
In this first comprehensive study of the expression of MUC13 in intestinal malignancy, we demonstrate that this TM mucin is expressed abundantly by virtually all colorectal cancers.
The antibody used reacts with the nonglycosylated MUC13 cytoplasmic domain, and therefore, reactivity will not be affected by variable glycosylation in cancer. As expected, staining of apical membranes was pronounced, suggesting that MUC13 is a constitutive component of the luminal membrane. However, the MUC13
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2018, Critical Reviews in Oncology/HematologyCitation Excerpt :Those patients who expressed MUC1 showed poor survival. Increased expression of MUC13 has been observed in poorly differentiated colon tumor (Walsh et al., 2007). Recently it has been identified that the major colon mucins MUC11 and MUC12 were down regulated during colorectal carcinogenesis (Williams et al., 1999) (Fig. 6).