Note | The CEACAM5 gene encodes CEA, which is the most widely used tumor marker for the diagnosis and monitoring of various cancers (see below). CEA is composed of 642 amino acids (a molecular mass of approximately 70kDa) and has 28 potential N-linked glycosylation sites (Oikawa et al., 1987; Nicholson and Stanners, 2007). CEA contains 24-26 asparagine-linked sugar chains in one molecule (Yamashita et al., 1987) and its final molecular mass is approximately 180kDa (Thompson et al., 1991). |
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| (A) The gene structure and protein domain model of the CEA molecule. CEA encoded by the CEACAM5 gene is synthesized as a precursor with a signal peptide (S) followed by 668 amino acids of the putative molecule; the first N-terminal domain (N domain) is followed by six IgC-like domains (A1, B1, A2, B2, A3, and B3 domains) and the last C-terminal hydrophobic domain (C domain) (Oikawa et al., 1987). Exons 1 and 9 include a 5'-untranslated region (5'-UTR) and a 3'-untranslated region (3'-UTR), respectively (Schrewe et al., 1990). (B) Schematic representation of the CEA protein structure. CEA is finally anchored to the membrane by the simultaneously occurring proteolysis of the C domain and replacement with the GPI anchor immediately after synthesis (Takami et al., 1988). |
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Description | Among the main CEACAM family members (CEACAM1, CEACAM3, CEACAM4, CEACAM5, CEACAM6 CEACAM7 and CEACAM8), the genes encoding proteins with transmembrane domains were assigned the CEACAM consecutive numbers 1-4, and the genes encoding GPI-anchored proteins including CEA were assigned the CEACAM numbers 5-8 (Beauchemin et al., 1999). |
Expression | Many CEACAM family proteins, such as CEACAM1, CEACAM8, CEACAM6 and CEACAM3, are known to be expressed in hematopoietic cells (Nagel and Grunert, 1995; Baeclay et al., 1997). In contrast, CEA has not been detected in hematopoietic cells and shows a more limited tissue localization in normal tissues including the tongue, esophagus, stomach, duodenum, appendix, colon, trachea (Nap et al., 1988; Kuroki et al., 1981) and lung (Nouwen et al., 1986). The CEA expression in those organs starts during the early fetal period (9-14 weeks) and appears to continue throughout one's lifespan (Nap et al., 1988). Increased expression of CEA has also been observed in various cancers including colorectal carcinoma (Jothy et al., 1993), gastric carcinoma (Kinugasa et al., 1998), pancreatic carcinoma (Shi et al., 1994), gall bladder carcinoma (Shi et al., 1994), lung adenocarcinoma (Robbins et al., 1993), small cell lung carcinoma (Kim et al., 1992), breast carcinoma (Cournoyer et al., 1988), urinary bladder carcinoma (Shi et al., 1994), mucinous ovarian carcinoma (Thompson et al., 1994), serous ovarian carcinoma (Thompson et al., 1993) and endometrial adenocarcinoma (Thompson et al., 1993). It is usually expressed in corresponding metastatic lesions as well, due to its involvement in tumor progression and metastasis. |
Localisation | CEA is localized to columnar epithelial cells and goblet cells of the colon, mucous neck cells and pyloric mucous cells of the stomach, squamous epithelial cells of the tongue, esophagus and cervix, secretory epithelia and duct cells of sweat glands and epithelial cells of the prostate (Nap et al., 1988; Kodera et al., 1993; Prall et al., 1996). Studies using immunoelectron microscopy using a specific monoclonal antibody for CEA demonstrate that CEA is specifically localized to the apical surface of mature enterocytes (Ahnen et al., 1982; Baranov et al., 1994; Frãngsmyr et al., 1999). No staining is seen at the basolateral surfaces of the enterocytes. The structure that is specifically stained is the apical glycocalyx (= fuzzy coat)/microvillus region of the mature enterocytes. The fuzzy coat is made up of microvesicles and filaments. The microvesicles are formed by the blebbing of microvillus membrane and subsequent pinching off. The finding that CEA in normal colon is released via CEA-coated vesicles agrees with the findings which showed that more than 90% of total CEA in feces exist in a membrane-bound form and that it can be released from the membranes by phosphatidylinositol-specific phospholipase C (Kuroki et al., 1994; Kinugasa et al., 1994). |
Function | In vitro studies with tumor cell lines have demonstrated that CEA, like other CEA gene family members, can act as homophilic and heterophilic adhesion molecules when expressed on the tumor cell surface (Benchimol et al., 1989; Oikawa et al., 1992; Zhou et al., 1993). Therefore, in the tumor environment it is possible that CEA plays some role as a contact-mediating device when tumor cells are invading new sites. In normal physiology, however, it appears unlikely that CEA is involved in intercellular adhesion due to their apical localization on polarized cells. CEA in normal tissues is now considered to protect the luminal organs of the body from microbial infection by binding and trapping infectious microorganisms. Those organs are the colon and perhaps other areas, such as the upper alimentary tract, the respiratory tract, the urinary bladder, and the skin (sweat glands), where the microbial load is a routine event (Hammarström, 1999). |
Homology | The CEACAM5 gene homologue is present in primates such as chimpazees, orangutans, macaques and marmosets, but not in rodents such as mice and rats (Pavlopoulou and Scorilas, 2014). |
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