Cadherins are a family of calcium-dependent transmembrane cell adhesion glycoproteins. In connecting cells they comprise a part of the zonula adherens and desmosomes. Each type of cadherin has a specific extracellular binding site and the proteins preferentially interact with themselves in a homophilic manner. Besides cell adhesion, the cadherins are involved in shape, motility and proliferation of cells. ECAD (syn. CD325), 120kDa, chromosome 16q22.1 (CDH1 gene), is a critical regulator of epithelial junction formation. It interacts with the cytoskeleton through a number of associated proteins: The ECAD internal domain binds with alpha, beta, gamma, and p120 catenins to anchor the ECAD complex to the actin cytoskeleton of the cell. ECAD is expressed in virtually all epithelial cells with the exception of adrenocortical cells. The expression in liver cells is weaker than in most other epithelia. ECAD is also expressed in melanocytes (adhering to squamous epithelial cells). Mesothelial cells and granulosa cells are ECAD negative. Mesenchymal cells and neural crest cells are mostly negative but some types may show a weak reaction. Testicular germinative cells are weakly positive.
Decreased expression or function of ECAD appears to be an important step in tumour progression. Compared to normal tissues, ECAD expression may be down-regulated in malignant tumours, and in highly invasive, poorly differentiated carcinomas ECAD may be entirely lost, as a consequence of biallelic inactivation of CDH1 (by promoter methylation, mutation or allelic loss in any combination). Somatic mutation of ECAD is associated with increased activation of EGFR that may explain enhanced motility of tumor cells. Hereditary autosomal dominant diffuse-type gastric carcinoma usually results from germline truncating mutations in the ECAD gene. These patients also have an increased risk of lobular breast carcinoma. Overall, 50 % - 90 % of carcinomas are positive with a strong membranous reactivity. The following tumours are almost always positive: adenocarcinoma of colorectum, stomach, pancreas, prostate, endometrium, uterine cervix, and thyroid. Among ovarian carcinomas, the mucinous type is almost always positive, while varying positivity is seen in the other types. Among breast carcinomas the ductal type (including the tubulolobular subtype) is almost always positive (at least in part of the tumour) while the lobular type is negative in 85 – 90 %. Decreased expression in breast ductal carcinoma is associated with higher frequency of lymph node metastases and shorter survival. In some of these cases the down regulation of ECAD is transient, as the metastases show a higher expression of the protein. In urothelial carcinoma and squamous cell carcinoma ECAD is found in most cases but the expressioni s often reduced compared to normal cells. Hepatocellular carcinoma and renal cell carcinoma are positive in about half of the cases. Carcinoids are usually positive. Among malignant mesotheliomas, about 30% are positive. Most malignant melanomas stain for ECAD. However, the melanoma metastases are frequently negative. Among sarcomas, ECAD positivity is associated with an epithelioid growth pattern. No positivity in seen in gliomas and virtually none in malignant lymphomas. Seminomas are varying positive.
Staining for ECAD may be useful for the differentiation between ductal and lobular breast carcinomas. ECAD positive malignant mesothelioma may be differentiated from reactive mesothelial cells. However, ECAD staining cannot be recommended in the differentiation between malignant mesothelioma and adenocarcinoma. Identification of ECAD may be useful for prognostic studies in carcinomas but standards have not been established.
Liver and colon are recommended as positive and negative tissue controls for ECAD. In liver, the protocol must be calibrated to provide an at least moderate, distinct membranous staining reaction of the hepatocytes. Epithelium of the bile ducts should display a strong and distinct membranous staining intensity.
In the colon, only epithelium should display a strong membranous staining reaction. No staining reaction must be seen in stromal cells such as lymphocytes, plasma cells, smooth muscle cells or endothelial cells.