The ectopic expression of gastric mucin in extramammary and mammary Paget's disease

The immunohistochemical analysis of the ectopic expression of gastric mucin has been used as a valuable tool for the identification of several neoplasms. We performed an immunohistochemical examination of the expression of gastric, intestinal, and mammary-type mucins in 25 lesions from patients with extramammary Paget's disease (ExMPD) and in 10 lesions from patients with mammary Paget's disease (MPD). All ExMPD lesions were immunopositive for HGM-45, a marker of gastric surface mucous cells, whereas only 40% of MPD lesions were weakly immunopositive. No markers for gastric-gland mucous cells were found in almost all ExMPD and MPD lesions, and markers for intestinal mucin were recognized in only a few lesions. Antibodies specific for MUC1, a marker of mammary-type mucin, gave positive results in both ExMPD and MPD lesions at a higher frequency than antibodies specific for gross cystic disease fluid protein-15. Our analysis indicates that Paget's cells contain gastric surface-type mucin in ExMPD and they contain mammary-type mucin in both ExMPD and MPD. The ectopic localization of mucosubstances may be a further step in the understanding of these lesions. Moreover, antibodies against MUC1 seem to be a useful tool for the diagnosis of both ExMPD and MPD.