Synovial sarcomas are histologically classified into bipha-sic and monophasic types according to the presence or absence of glandular epithelial differentiation. Synovial sarcomas are characterized by a t(X;18)(p11;q11). The SYT gene at 18q11 is fused with either SSX1 or SSX2 at Xp11. Although there is only a 13-amino-acid difference between the two types of fusion genes, the SYT-SSX fusion type correlates with overall survival and histologic features of the tumor.3334 In patients with localized disease at presentation, SYT-SSX fusion type appears to be the only significant prognostic factor in a multivariate analysis. Of 202 patients with localized synovial sarcoma at diagnosis, the median and 5-year survival for the SYT-SSX2 group were 13.7 years and 77%, respectively, compared to 9.2 years and 61%, respectively, for the SYT-SSX1 group.34
Studies in ES and ARMS have also established that alternative forms of the specific aberrant fusion transcripts may have an important impact on one or more clinical features or prognosis. The PAX3-FKHR fusion is an adverse prognostic factor in ARMS.35 Therefore, not only is PAX-FKHR fusion transcript the defining feature of ARMS, but also the specific type of gene fusion present in the tumor identifies, among patients with metastatic disease, a high-risk subgroup (with PAX3-FKHR) and a favorable-outcome subgroup (with PAX7-FKHR), as shown by a recent study of the Children's Oncology Group.36
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