Division of Anatomic Pathology, Mayo Clinic, Rochester, MN, USA.
Am J Clin Pathol. 2022 Jan 6;157(1):33-39. doi: 10.1093/ajcp/aqab091.
Differentiating malignant pleural mesothelioma from benign reactive mesothelial processes can be quite challenging. Ancillary tests such as BRCA1-associated protein 1 (BAP1) immunohistochemistry and p16 fluorescence in situ hybridization (FISH) are helpful tools to aid in this distinction. Immunohistochemistry for methylthioadenosine phosphorylase (MTAP) has recently been proposed as an effective surrogate marker for p16 FISH and is an attractive alternative test due to shorter turnaround time. There are little data regarding the specificity of MTAP loss for mesothelioma or whether it may be useful to distinguish mesothelioma from the most common entity in the differential diagnosis, sarcomatoid carcinoma.
We studied well-characterized cases of sarcomatoid carcinoma (n = 34) and sarcomatoid mesothelioma (n = 62), which were stained for MTAP (clone 2G4) and BAP1 (clone C-4).
Loss of MTAP expression was observed in 17 (50%) of 34 pulmonary sarcomatoid carcinomas; BAP1 expression was retained in all of the cases in which it was performed (n = 31). MTAP expression was lost in 38 (61%) of 62 sarcomatoid mesotheliomas; BAP1 was lost in 6 (10%) of 62. In the six cases with BAP1 loss, five also had loss of MTAP, while MTAP expression was retained in one.
Loss of MTAP expression by immunohistochemistry is common in pulmonary sarcomatoid carcinoma, as it is present in half of cases. This rate is similar to what is observed in sarcomatoid mesothelioma (61%). Therefore, this stain is not useful to distinguish between these two malignancies. MTAP loss is more common than BAP1 loss in the setting of sarcomatoid mesothelioma (61% vs 10%, respectively).
区分恶性胸膜间皮瘤和良性反应性间皮过程可能具有挑战性。辅助测试，如 BRCA1 相关蛋白 1（BAP1）免疫组化和 p16 荧光原位杂交（FISH），是有助于区分两者的有用工具。甲基硫腺苷磷酸化酶（MTAP）的免疫组化最近被提议作为 p16 FISH 的有效替代标志物，由于周转时间较短，因此是一种有吸引力的替代测试。关于 MTAP 缺失对间皮瘤的特异性或它是否有助于区分间皮瘤与鉴别诊断中最常见的实体肉瘤样癌的数据很少。
我们研究了特征明确的肉瘤样癌（n=34）和肉瘤样间皮瘤（n=62），这些肿瘤均进行了 MTAP（克隆 2G4）和 BAP1（克隆 C-4）染色。
17 例（50%）肺肉瘤样癌中观察到 MTAP 表达缺失；进行 BAP1 表达检测的所有病例（n=31）中均保留 BAP1 表达。62 例肉瘤样间皮瘤中有 38 例（61%）MTAP 表达缺失；62 例中有 6 例（10%）BAP1 缺失。在 BAP1 缺失的 6 例中，5 例也有 MTAP 缺失，而 1 例保留 MTAP 表达。
肺肉瘤样癌中免疫组化 MTAP 表达缺失很常见，半数病例存在这种情况。这一发生率与肉瘤样间皮瘤（61%）相似。因此，这种染色不能用于区分这两种恶性肿瘤。在肉瘤样间皮瘤中，MTAP 缺失比 BAP1 缺失更常见（分别为 61%和 10%）。
