Quantitation of polymorphic epithelial mucin: a challenge for biochemists and immunologists

Polymorphic epithelial mucin (PEM) is a heavily glycosylated protein present at the apical surface of glandular epithelial cells which is shed into the lumen of epithelial tissue. In carcinomas cell polarisation is lost, PEM is overexpressed and found on the entire cell surface. High amounts of PEM are shed into the circulation of cancer patients. CA 15.3 is the first commercial assay for the detection of PEM. After roughly one decade of use in clinical practice it is considered valuable for breast cancer therapy monitoring and, in the follow up, for early detection of metastatic disease. The extreme polymorphism of this molecule, with its varying number of multiple epitopes and tremendous variation in glycosylation which can mask catcher/tracer epitopes, impairs its precise measurement. A further impediment is complex formation with autoantibodies, as revealed by a recently developed assay.