ARID1B Immunohistochemistry Is an Important Test for the Diagnosis of Dedifferentiated and Undifferentiated Gynecologic Malignancies.

Simple Summary: Mounting evidence show that dedifferentiated and undifferentiated endometrial and ovarian carcinomas (DDC/UDC) are clinically distinct entities with rapid progression, poor response to adjuvant therapy, and grim outcome, even when compared to other high-grade gynecologic malignancies. Unfortunately, they are very challenging to diagnose and often receive a non-specific or ambiguous diagnosis. Immunohistochemical tests for core SWI/SNF proteins (i.e. ARID1B, SMARCA4, and SMARCB1) offer high specificity for DDC/UDC, low cost, and fast turnaround time, unlike DNA sequencing-based assays which currently have very limited use in predicting inactivation core SWI/SNF subunits. Recent reports show that among the known core SWI/SNF proteins, ARID1B inactivation are most common in these tumors, yet this test is rarely available even in tertiary centers. In this opinion I stress the importance of including ARID1B along with other core SWI/SNF proteins in the diagnostic workup of DDC/UDC. Dedifferentiated and undifferentiated endometrial and ovarian carcinomas (DDC/UDC) are aggressive malignancies defined by morphologic and molecular undifferentiation, and associated with core SWI/SNF deficiency. Their main differential diagnoses include high-grade endometrial and ovarian carcinomas that often show overlapping morphologic and molecular profiles. Loss of cell lineage markers expression by immunohistochemistry (IHC) is commonly used to assist diagnosis, but it has poor specificity, while core SWI/SNF deficiency is much more specific. Approximately half of SWI/SNF-deficient DDC/UDC are associated with loss of ARID1B expression, yet, unlike the other core SWI/SNF proteins (SMARCA4 and SMARCB1), this test is rarely available, even in tertiary centers. Mutational testing for ARID1B is increasingly common among targeted DNA sequencing panels, but it is difficult to interpret in the absence of IHC results. Overall, the importance of including ARID1B IHC as part of the routine panel for undifferentiated gynecologic malignancies should be emphasized, especially as SWI/SNF inactivation is becoming a necessary biomarker for diagnostics, clinical management, and clinical trial enrollment. [ABSTRACT FROM AUTHOR]