Smoothelin is a specific marker for smooth muscle neoplasms of the gastrointestinal tract.

Smoothelin is a smooth muscle-specific cytoskeletal protein exclusively found in differentiated smooth muscle cells. This contrasts with other smooth muscle proteins (eg, h-caldesmon, alpha-smooth muscle actin, desmin, smooth muscle myosin), which are expressed in proliferative (early) stages of smooth muscle development and occasionally in other cell types (striated muscle, myofibroblasts, myoepithelial cells, pericytes). Smoothelin has been shown to be expressed predominantly in visceral smooth muscle and to a lesser extent in vascular smooth muscle. Smoothelin expression in mesenchymal tumors of the gastrointestinal (GI) tract has not been evaluated earlier. The purpose of this study was to determine whether immunostaining for smoothelin could help distinguish smooth muscle neoplasms from their morphologic mimics, particularly KIT-negative gastrointestinal stromal tumors (GISTs), desmin-positive GISTs, and desmoid fibromatosis. A total of 150 mesenchymal neoplasms of the GI tract, abdominal cavity, and retroperitoneum were retrieved from consult and surgical pathology archives, including 54 GISTs (8 KIT-negative; 13 desmin-positive), 17 GI leiomyosarcomas (LMS), 11 GI mural leiomyomas, 13 leiomyomas of the muscularis mucosae, 12 gastric schwannomas, 15 inflammatory myofibroblastic tumors, 9 cases of mesenteric desmoid fibromatosis, 10 dedifferentiated liposarcomas, and 9 malignant peripheral nerve sheath tumors. Immunostaining for smoothelin was performed on all cases. Cytoplasmic and nuclear staining was recorded. Cytoplasmic expression of smoothelin was present in all 24 (100%) benign smooth muscle tumors (mural leiomyomas and leiomyomas of the muscularis mucosae). In contrast, only 4 (24%) GI LMS showed cytoplasmic staining for smoothelin. None of the GISTs, desmoid tumors, inflammatory myofibroblastic tumors, schwannomas, dedifferentiated liposarcomas, or malignant peripheral nerve sheath tumors showed cytoplasmic reactivity for smoothelin. Interestingly, 7 (41%) GI LMS and 12 (22%) GISTs (all except 2 with an epithelioid component) showed multifocal, exclusively nuclear staining for smoothelin. Nuclear expression of smoothelin was not detected in any of the other tumor types examined. In summary, diffuse cytoplasmic staining for smoothelin is highly sensitive and specific for benign leiomyomas of the GI tract. Aberrant nuclear expression is common in GI LMS and may also be seen in GISTs, especially epithelioid and mixed-type tumors. These findings suggest that the extent and pattern of smoothelin expression may help differentiate between benign and malignant mesenchymal tumors of the GI tract, and may be useful in distinguishing leiomyomas from KIT-negative and/or desmin-positive GISTs.