Calponin and h-caldesmon in soft tissue tumors: Consistent h-caldesmon immunoreactivity in gastrointestinal stromal tumors indicates traits of smoothmuscle differentiation

Currently, the immunohistochemical evaluation of smooth muscle differentiat ion is usually based on desmin, which also reacts with skeletal muscle and is not present in all smooth muscle tumors, and a-smooth muscle actin, whic h reacts with myoepithelial cells. Neither marker typically reacts with gas trointestinal stromal tumors (GISTs), previously classified as smooth muscl e tumors or presently often classified as smooth muscle/stromal tumors. Two cytoskeleton-associated actin-binding proteins, calponin (CALP) and h-cald esmon (HCD), are putative smooth muscle markers that also react with myoepi thelia These markers are of particular interest in the immunohistochemical analysis of tumors; neither of them has been extensively documented in soft tissue tumors. In this study, we evaluated selected normal and reactive ti ssues and more than 250 mesenchymal tumors for CALF and HCD. Both markers w ere expressed in parenchymal and vascular smooth muscle cells in various or gans and in myoepithelial cells. CALF also reacted with myofibroblasts of d esmoplastic stroma. All of our 25 benign smooth muscle tumors from various locations were positive for CALP and HCD, as were most of the retroperitone al and uterine leiomyosarcomas. HCD was more specific, because CALF also re acted with myofibroblastic lesions. The common reactivity of malignant fibr ous histiocytomas with CALP and HCD suggests a combination of myofibroblast ic and smooth muscle differentiation in these tumors. The GISTs (c-kit posi tive, usually actin negative) showed nearly consistent HCD reactivity, sugg esting traits of smooth muscle differentiation. GISTs were usually CALP neg ative and showed a CALF expression pattern similar to that of a-smooth musc le actin. Although nonmuscle, non-myofibroblastic tumors were negative for CALF and HCD, synovial sarcomas showed streaks of CALP-positive cells of un known significance. CALF and HCD should be explored as markers to identify myofibroblastic and smooth muscle cell differentiation in mesenchymal tumor s.