BCOR (BSB-128), MMab

Summary and Explanation

Using next-generation sequencing, an increasing number of novel gene fusions and other abnormalities have emerged recently in the spectrum of EWSR1-negative Small Blue Round Cell Tumors (SBRCTs). A subset of SBRCTs harboring either BCOR gene fusions (BCOR-CCNB3, BCOR-MAML3), BCOR internal tandem duplications (ITD), or YWHAENUTM2B share a transcriptional signature including high BCOR mRNA expression, as well as similar histologic features. Furthermore, other tumors such as Clear Cell Sarcoma of Kidney (CCSK) and Primitive Myxoid Mesenchymal Tumor of Infancy (PMMTI) also demonstrate BCOR ITDs and high BCOR gene expression. Recent studies have found the IHC of BCOR to be a highly sensitive marker for SBRCTs and CCSKs with BCOR abnormalities and YWHAE rearrangements and can be used as a useful diagnostic marker in these various molecular subsets.

In another study, Strong diffuse nuclear BCOR staining (defined as >95% of tumor cells) was seen in the round cell component of 20 (100%) classic YWHAE-NUTM2 high-grade Endometrial Stromal Sarcomas and the 3 unusual high-grade Endometrial Stromal Sarcomas. It was concluded that BCOR immunohistochemical staining is a highly sensitive marker for YWHAE-NUTM2 high-grade Endometrial Stromal Sarcomas with both classic and unusual morphology and identifies a subset of high-grade Endometrial Stromal Sarcomas with BCOR alterations, including BCOR rearrangement and internal tandem duplication.