Validation of the Xpert breast cancer STRAT 4 essay on GeneXpert instruments to assess hormone receptor status, KI-67 and HER-2 mutational status in breast cancer tissue samples

Abstract

Breast cancer is the commonest cause of cancer-related mortality in African females where patients often present later and with advanced disease. Causes for delayed diagnosis include restricted diagnostic access and international controversy on interpretation of ancillary tests like immunohistochemistry (IHC). Fine needle aspirates (FNAC) are an attractive alternative although may have reduced sensitivity. The STRAT-4 assay (Cepheid, Sunnyvale) is a semi-quantitative reverse-transcription RNA assay which detects mRNA expression in breast samples for Oestrogen Receptor (ESR1), Progesterone receptor (PGR), human epidermal growth factor receptor (HER2/ERBB2) and the proliferation marker, Ki67. We assessed the performance of this assay on both formalin-fixed paraffin-embedded (FFPE, n=31) and matched FNAC (n=20) samples from patients presenting with breast cancer to the Johannesburg academic hospitals, as a pilot study. IHC and Fluorescent in-situ hybridisation analysis (performed on HER2-indeterminate samples) was compared to the expression of the target genes for FFPE and mRNA expression on FNAC samples were compared to the FFPE results for both mRNA expression and IHC. Concordance was highest for hormone receptor (HR) expression with an overall percent agreement (OPA) for ERBB2 of 96.7% and of 81.3%, for ESR1 expression of 93.3% and 93.3% and for PGR expression at 86.7% and 94.4% and lowest for Ki67 expression at 83.9% and 66.7% for FFPE and FNAC samples respectively. This suggests that the STRAT-4 assay may be a useful ancillary test in determining HR and Ki67 status in FFPE samples and that use on FNAC samples is feasible. Future studies should expand the sample numbers and establish locally relevant cut-offs.