What is CA 27-29?

Updated: Jul 11, 2019
  • Author: Nikhil G Thaker, MD; Chief Editor: Eric B Staros, MD  more...
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Answer

Answer

Cancer antigen 27-29 (or CA 27-29) is an epitope of a large transmembrane mucin glycoprotein named MUC1 that is expressed on the cell surfaces of most glandular epithelia. [1] This protein, also known as polymorphic epithelial mucin or epithelial membrane antigen, has a large extracellular region, a transmembrane sequence, and a cytosolic domain. CA 27-29 represents a sequence of mucins on the extracellular region of this glycoprotein.

Physiologically, the MUC1 protein may be involved in protection, lubrication, and cell adhesion by decreasing the degree of cell to extra-cellular matrix and cell to cell interactions. [1, 7] The MUC1 protein and its overexpression may be causally related to cancer invasion and metastasis.

The MUC1 protein is often overexpressed and aberrantly glycosylated on its extracellular surface in malignant granular cells. [7, 8] In fact, the carbohydrate epitopes on MUC1 in breast cancer cells may be antigenically different than those in normal breast. As tumor cells shed the MUC1 protein into the bloodstream, CA 27-29 is recognized by a single monoclonal antibody, which recognizes a 20 amino-acid sequence in the mucin core. [1] This is in contrast to the CA 15-3 assay, which utilizes two monoclonal antibodies. However, there is overlap, in part, with the epitopes that are recognized by both assays. [1] Early stage breast cancers have a low incidence of elevated CA 27-29 and lower absolute levels, while higher-stage breast cancers have a higher incidence of elevated levels and higher absolute levels. Since all breast cancers do not express the CA 27-29 antigen, this assay cannot detect all breast cancers.

The CA 15-3 assay can be used instead of the CA 27-29 assay. Although CA 27-29 may be more sensitive for breast cancer, its indications and limitations parallel those of CA 15-3. [5]


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