PAX7 immunohistochemical evaluation of Ewing sarcoma and other small round cell tumours

• Published in: Histopathology 2018-10, Vol.73 (4), p.645-652
• Main Authors: Toki, Shunichi, Wakai, Susumu, Sekimizu, Masaya, Mori, Taisuke, Ichikawa, Hitoshi, Kawai, Akira, Yoshida, Akihiko
• Format: Article
• Language: English
• Subjects: Biomarkers, Tumor - analysis; Diagnosis, Differential; DNA microarrays; Ewing sarcoma; Ewing's sarcoma; FLI-1 protein; Fusion protein; Humans; Immunohistochemistry; Monoclonal antibodies; NKX2‐2; Osteosarcoma; PAX7; PAX7 Transcription Factor - analysis; PAX7 Transcription Factor - biosynthesis; Rhabdomyosarcoma; Sarcoma, Ewing - diagnosis; small round cell tumour; Soft Tissue Neoplasms - diagnosis; Soft tissues; Synovial sarcoma; Tumors
• ISSN: 0309-0167; 1365-2559
• DOI: 10.1111/his.13689

Aims Ewing sarcoma is a small round cell tumour that affects bone and soft tissues. Although the detection of the specific fusion gene is a robust method of its diagnosis, immunohistochemistry may serve as a practical surrogate. Recent tissue microarray studies suggested that PAX7 is a novel marker, because it was expressed consistently in Ewing sarcoma, in addition to rhabdomyosarcoma and synovial sarcoma. Here, we evaluated the utility of PAX7 immunohistochemistry in whole‐tissue sections of an expanded array of round cell malignancies with adequate molecular characterisation. Methods and results We stained 30 molecularly confirmed Ewing sarcomas, one EWSR1–NFATC2 sarcoma and 141 non‐Ewing round cell tumours by a monoclonal antibody against PAX7. Staining was considered positive if at least 5% of tumour cells were stained. PAX7 was expressed in 27 of 30 Ewing sarcomas (90%), mainly in a diffuse and strong manner. Although NKX2‐2 showed similar sensitivity, PAX7 showed more extensive and strong reactivity. One EWSR1–NFATC2 sarcoma co‐expressed PAX7 and NKX2‐2. PAX7 was also expressed in 24 of 141 non‐Ewing tumours, including alveolar rhabdomyosarcomas (seven of 10), poorly differentiated synovial sarcomas (seven of 10), BCOR–CCNB3 sarcomas (eight of 10), small‐cell osteosarcoma (one of five) and desmoplastic small round cell tumour (one of 10), one‐third of which showed diffuse strong reactivity. Conclusions Although we confirmed that PAX7 is a sensitive marker for Ewing sarcoma, anti‐PAX7 antibody also stained several Ewing sarcoma mimics, whose spectrum was distinct from NKX2‐2‐positive non‐Ewing entities. Further studies are required to determine how PAX7 could be integrated into practice to classify small round cell tumours efficiently.