Potential Involvement of Myostatin in Smooth Muscle Differentiation in Pleomorphic Leiomyosarcoma

High-grade sarcomas often lack typical morphological features and exhibit no clear differentiation, often leading to a diagnosis of undifferentiated sarcoma (US). Pleomorphic leiomyosarcoma (PLMS) is a high-grade sarcoma consisting of a typical leiomyosarcoma (LMS) component alongside dedifferentiated high-grade areas. A few decades ago, PLMS was regarded as a subtype of high-grade sarcoma previously referred to as malignant fibrous histiocytoma; it is now classified as a variant of LMS. The mechanisms underlying myogenic differentiation and their relevance to the pathological diagnosis of high-grade sarcomas remain poorly understood. To investigate the gene expression networks associated with myogenic differentiation, we employed Cap Analysis of Gene Expression (CAGE) to distinguish PLMS from other high-grade sarcoma subtypes. We analyzed 27 frozen high-grade sarcoma samples, comprising 10 PLMSs, 11 high-grade myxofibrosarcomas, 3 dedifferentiated liposarcomas, 2 USs, and 1 high-grade sarcoma not otherwise specified, using CAGE profiling. Hierarchical clustering based on differentially expressed genes identified by CAGE separated 7 of the 10 PLMSs from other high-grade sarcomas, while the remaining 3 PLMSs clustered with a single US case. CAGE analysis also revealed that the myostatin (MSTN) promoter (false discovery rate [FDR] < 0.05) was more strongly activated in the high-grade sarcoma group lacking morphological and immunohistochemical smooth muscle differentiation than in the PLMS group, whereas the alpha smooth muscle actin (ACTA2) promoter (FDR < 0.05) was more prominently activated in the PLMS group. Immunohistochemical analysis showed reduced or absent myostatin expression in PLMSs, in contrast to diffuse myostatin expression in other high-grade sarcomas. Smooth muscle actin, encoded by ACTA2, was expressed in all 10 PLMS cases but only in 11 of 17 other high-grade sarcomas. Furthermore, both conventional immunohistochemistry and double immunostaining revealed that myostatin and myogenic markers exhibited largely mutually exclusive expression patterns within these tumors. A validation study was performed using 59 soft tissue sarcoma cases, including 27 PLMSs and 16 LMSs. Loss or reduction in myostatin expression was confirmed in both LMS and PLMS, and the ratio of myostatin loss was comparable (62.5% in LMS vs. 63% in PLMS). Collectively, these findings suggest that myostatin contributes to smooth muscle differentiation in high-grade sarcomas and has potential utility as a diagnostic marker.