横纹肌肉瘤目前根据其形态学、免疫组织化学和分子遗传学特征的不同被归为四种亚型（泡泡状、胚胎状、梭形细胞/硬化状、多形状），其中泡泡状亚型的特征是包括PAX3或PAX7和FOXO1的经常性易位。鉴别这种易位对正确分类和预后很重要。本研究旨在探讨FOXO1免疫组织化学在横纹肌肉瘤分类中的诊断效用。使用一种针对FOXO1表位的单克隆抗体研究了105例横纹肌肉瘤。所有25例泡泡状横纹肌肉瘤中均表达FOXO1（一种融合癌基因蛋白）；其中84％的肿瘤细胞均弥漫性表达，其余泡泡状横纹肌肉瘤至少有60％的病变细胞表现为中度染色。除三例梭形细胞状横纹肌肉瘤外，其他80例胚胎状、多形状、梭形细胞/硬化状横纹肌肉瘤均未表达FOXO1（96.3％特异性），使用核染色阈值确定20％的肿瘤细胞为阳性。所有横纹肌肉瘤亚型中都出现不同程度的胞浆染色。非肿瘤淋巴细胞、内皮细胞和Schwann细胞也显示出不同程度的核反FOXO1免疫反应性。综上，我们的发现表明，FOXO1免疫组织化学是横纹肌肉瘤中PAX3/7::FOXO1融合癌基因蛋白的高度敏感且相对特异的代用标志物。胞浆反应性、非肿瘤组织中的表达以及非泡泡状横纹肌肉瘤的核染色限制可能是解释中的潜在难题。本文受版权保护。版权所有。

Rhabdomyosarcomas currently are classified into one of four subtypes (alveolar, embryonal, spindle cell/sclerosing, or pleomorphic) according to their morphological, immunohistochemical, and molecular genetic features. The alveolar subtype is characterized by a recurrent translocation involving PAX3 or PAX7 and FOXO1; identification of this translocation is important for appropriate classification and prognostication. In this study, we aimed to explore the diagnostic utility of FOXO1 immunohistochemistry for rhabdomyosarcoma classification.A monoclonal antibody targeting a FOXO1 epitope retained in the fusion oncoprotein was used to study 105 rhabdomyosarcomas. FOXO1 was positive for expression by immunohistochemistry in all 25 alveolar rhabdomyosarcomas with 84% showing diffuse expression in greater than 90% of neoplastic cells; the remainder of alveolar rhabdomyosarcomas displayed at least moderate staining in a minimum of 60% of lesional cells. Apart from three spindle cell rhabdomyosarcomas showing heterogeneous nuclear immunoreactivity in 40-80% of tumor cells, the 80 cases of embryonal, pleomorphic, and spindle cell/sclerosing rhabdomyosarcoma were negative for FOXO1 expression (96.3% specific) when using a threshold of nuclear staining in 20% of neoplastic cells to determine positivity. Variable cytoplasmic staining was present in a fraction of all rhabdomyosarcoma subtypes. Non-neoplastic lymphocytes, endothelial cells, and Schwann cells also showed variably intense nuclear anti-FOXO1 immunoreactivity.Taken together, our findings suggest that FOXO1 immunohistochemistry is a highly sensitive and relatively specific surrogate marker of the PAX3/7::FOXO1 fusion oncoprotein in rhabdomyosarcoma. Cytoplasmic immunoreactivity, expression in non-neoplastic tissues, and limited nuclear staining of non-alveolar rhabdomyosarcomas represent potential pitfalls in interpretation.This article is protected by copyright. All rights reserved.