The genes encoding mucin proteins are generally designated MUC and have homologs in many species other than humans. Thanks to the Human Genome Project and the tireless work of genetic researchers, the National Library of Medicine and NCBI have a catalog of human genes, including mucins. Here are the more than 20 catalogued human mucin genes and their normal expression. Their roles in disease are discussed at the end.
MUC1: expressed by glandular epithelium, increased in the breast during lactation
MUC2: expressed by the small intestine and other mucus membrane-containing organs
MUC3: secreted by the intestinal mucosa, occurs as closely related A and B subtypes
MUC4: transmembrane heterodimer, found on a broad range of epithelial tissue, potentially involved in cell signaling
MUC5B: expressed by the sublingual gland and submucosal tissues (mouth/throat), principle component of saliva
MUC5AC: found on the surface of respiratory epithelia, categorized as gastric mucin, two proteins (5A, 5C) are encoded by the same gene (5AC)
MUC6: associated with the gastric epithelium, categorized as a gastric mucin
MUC7: secreted in saliva
MUC8: tracheobronchial (throat, windpipe)
MUC9: also known as oviductin, estrogen-dependent oviduct protein, and oviductal glycoprotein 1 based on expression in the oviduct
MUC10: secreted by submandibular gland into saliva
MUC11/MUC12: cell surface associated with unknown function, precursor has ubiquitous tissue distribution with higher expression in the colon
MUC13: cell surface associated, expressed by ductal and glandular epithelia and intestines
MUC14: interferes with the assembly of focal adhesion complexes
MUC15: cell surface associated, appears to be expressed mainly in the placenta
MUC16: cell surface associated, expressed in the female reproductive system, respiratory tract, and eye
MUC17: cell surface associated, confers anti-adhesive properties to cancer cells that lose their polarization, offers cytoprotection and maintains luminal structure in small intestine, also potentially occurs as secreted form in respiratory tract
MUC18: integral membrane protein involved in adhesion, marker of T cell activation
MUC19: expression is restricted to the glands around the oral cavity and trachea, but not known to be a component of saliva
MUC20: cell surface associated, multiple isoforms have been observed, suppresses downstream signaling of the met proto-oncogene
MUC21: also known as epiglycanin, a novel transmembrane mucin
Some of the proteins, and thus expression of the genes, have been associated with cancer and other disorders. Both MUC1 and MUC4 have been associated with prostate cancer when down-regulated. MUC1 over-expression also appears to be involved in or associated with colorectal cancer. MUC14 is also known as endomucin 2 and gastric cancer antigen GA34, which is associated with gastric cancer. Another mucin with its aberration in its name is MUC18, which is also known as melanoma cell adhesion molecule (MCAM) or CD146, a marker of tumor progression in melanoma. Aberrant expression of MUC16 is associated with chronic dry eye, but is also known as CA-125 ovarian cancer antigen and over-expressed by ovarian cancer cells.
