Gastrointestinal stromal tumor: evidence for a smooth-muscle origin

A gastrointestinal stromal tumor, arising from the rectal ampulla of a 63-yr-old man, was investigated using conventional techniques as well as Western blot analysis of its cytoskeleton proteins. The expression of desmin, muscle-specific actins, vimentin, S-100 protein, chromogranin, neuron-specific enolase, and keratins was studied using the avidin-biotin technique. The tumor cells showed a positive reaction only to antivimentin antibody. Ultrastructural analysis failed to provide conclusive evidence for neural or muscular origin of the tumor. Western blot analysis of the tumor whole-protein extract allowed identification of the presence of gamma-smooth-muscle actin, thus suggesting an enteric smooth-muscle origin of the tumor. This result seems partially to support a parenchymal smooth-muscle origin for S-100 protein and desmin-negative gastrointestinal tumors.