Two novel mucin genes down-regulated in colorectal cancer identified by differential display

Epithelial mucins are large, secreted and cell surface glycoproteins involv ed in epithelial cell protection, adhesion modulation, and signaling. Using differential display, we have identified two novel mucin cDNAs (dd34 and d d29), hereafter designated MUC11 and MUC12 respectively, that are down-regu lated in colorectal cancers. Northern blots demonstrated polydisperse signa ls characteristic of mucin transcripts in RNA from normal colon that were a bsent in colorectal cancer. Both cDNAs were mapped by fluorescence in situ hybridization to chromosome band 7q22, the location of the MUC3 mucin gene, thus suggesting that there may be a cluster of mucin genes at this locus. The sequences of both differential display clones were extended by a combin ation of screening libraries and PCR, The 2.8-kb MUC11 cDNA composite encod ed 35 serine/threonine-rich, mucin-like degenerate 28 amino acid tandem rep eats. The MUC11 cDNA composite encoded a putative transmembrane mucin conta ining two extracellular cysteine-rich, EGF-like domains, a coiled-coil regi on, and a mucin-like domain consisting of 28 amino acid degenerate tandem r epeats. Distinct patterns of expression of MUC11, MUC12 and MUC3 mRNAs were observed in a range of normal human tissues. MUC12 mRNA was not expressed in any of six colorectal cancer cell Lines examined and was down-regulated or absent in 6 of 15 (40%) tumors compared with matched normal colonic tiss ue. In contrast, MUC11 showed a different pattern of mRNA expression, with four of these lines showing low levels and the other two lines showing rela tively high Levels of MUC11 transcripts. Expression of MUC11 was down-regul ated in the tumors of 12 of 15 (80%) paired samples. Structural homology of MUC12 with rat, mouse, and human MUC3 and human and rat MUC4/ASGP2 indicat e that there is a distinct subfamily of transmembrane mucins with conserved epidermal growth factor domains. The homology of MUC12 with epidermal grow th factor-like growth factors and its down-regulation in colorectal cancers , together with known interactions between rat MUC4 and c-erbB-2 growth fac tor receptors, suggests that MUC12 may be involved in epithelial cell growt h regulation.