mRNA expression of ER, PR, HER2 and Ki67 are concordant to central ihc and predict clinical outcome: A validation study from the ABCSG-6 biomarker cohort

Abstract

Background: Central immunohistochemistry (IHC) is the clinical gold standard for assessing ER, PR, HER2 and Ki67 in FFPE breast cancers, however limitations still exist concerning (pre) analytic validity. mRNA expression assays provide an alternative approach to measuring these biomarkers. XpertVR Breast Cancer STRAT4 is a CE-IVD, cartridge-based test performed on the GeneXpert platform which semi-automates sample preparation and RT-qPCR detection of ESR1, PGR, ERBB2 and MKi67 mRNA in FFPE tissues. Here we demonstrate the concordance of STRAT4 mRNA in comparison to central IHC in women treated within the phase III ABCSG-6 adjuvant endocrine therapy trial. Methods: We evaluated ESR1, PGR, ERBB2 and MKi67 mRNA expression by STRAT4 and ER, PR, HER2 and Ki67 by IHC (FISH for HER2 IHC 2+) in 525 surgical FFPE specimens from ABCSG-6. All STRAT4 and IHC/FISHtesting was performed by a central academic reference laboratory. Concordance (overall percent agreement) between STRAT4 and IHC for each marker was the predetermined primary objective. The effect of binary parameters (positive vs negative) obtained by IHC and by STRAT4 on distant recurrence free interval (DRFI) was analyzed by Cox models and described by hazard ratios (HR). Results: In this study, concordance between STRAT4 and IHC was 98.6% for ER, 92.6% for PR, 98.4% for HER2, and 88.7% for Ki67 (excluding intermediate IHC 10- 20% staining). In univariate Cox regression analyses, PR (HR 0.29, P<0.001), HER2 (HR 2.62, P=0.005), and Ki67 (HR 3.45, P=0.001) tested by central IHC and PGR (HR 0.48,P = 0.007), ERBB2 (HR 2.29, P=0.037) and MKi67 (HR 3.87, P<0.001) tested by STRAT4 were all significantly associated with DRFI whereas neither ER by IHC, (HR 0.35, P=0.077, nor ESR1 by STRAT4 (HR 0.73, P=0.597), was associated with DRFI. Conclusions: We demonstrate high concordance between centrally assessed IHC and mRNA measurements of the four main biomarkers routinely assessed in early breast cancer. This was corroborated by the similar prognostic values observed for protein versus mRNA assessments for each marker. Future investigations of the clinical utility of mRNA based measurements by STRAT4 in breast cancer cohorts are warranted.