PO-457 Optimised ARID1A immunohistochemistry is an accurate predictor of ARID1A mutational status in gynaecological cancers

Abstract

IntroductionARID1A is a tumour suppressor gene that is frequently mutated in clear cell and endometrioid carcinomas of the ovary and endometrium and is an important clinical biomarker for novel treatment approaches for patients with ARID1A defects. However, the accuracy of ARID1A immunohistochemistry as a surrogate for mutation status has not fully been established for patient stratification in clinical trials. Here we tested whether ARID1A immunohistochemistry (IHC) could reliably predict ARID1A mutations identified by next-generation sequencing.Material and methodsThree commercially available antibodies - EPR13501 (Abcam), D2A8U (Cell Signalling) and HPA005456 (Sigma) – were optimised for IHC using cell line models and human tissue, and screened across a cohort of 45 rare gynaecological tumours. IHC was scored independently by three pathologists using an immunoreactive score. ARID1A mutation status was assessed using 2 sequencing platforms. The concordance between ARID1A mutation and protein expression was assessed using Receiver Operator Characteristics (ROC) statistics.Results and discussionsOverall, 21 ARID1A mutations in 14/43 assessable tumours (33%) were identified, the majority of which were predicted to be deleterious: mutations were identified in 7/18 (39%) ovarian clear cell carcinomas (OCCC), 4/7 (57%) of ovarian endometrioid carcinomas, 2/5 (40%) of endometrial carcinomas and 1/6 (17%) of carcinosarcomas. ROC analysis identified greater than 95% concordance between mutation status and IHC for all three antibodies; this allowed a cut-off immunoreactive score for ARID1A mutant status to be calculated.ConclusionComprehensive assessment of concordance of ARID1A IHC and mutation status identified EPR13501, as an optimal antibody, with a 100% concordance between ARID1A mutation status and protein expression, across different gynaecological histological subtypes. It delivered the best inter-observer agreement between all pathologists, as well as a clear cost-benefit advantage. This could allow patients to be accurately stratified based on their ARID1A IHC status into early phase clinical trials.