O-18 PARP1 expression in hormone estrogen receptor negative breast cancer: Preferential expression in basal-like and HER2-positive tumours

breast cancer series prepared as tissue microarray (n = 1858) using IHC (HercepTest, DakoCytomation) and chromogenic ISH (CISH; DuoCISH, DakoCytomation) in order to identify relationships with clinico-pathological variables and patient outcome. None of these cases have received anti-HER2 therapy. There was excellent overall concordance between HercepTest negative (scores 0/1+) and positive (3+) with CISH positive/negative (defined as HER2/Chr17 copy number ratio of P2; p < 0.001). Twelve percent of cases were identified as HER2 positive (those with 3+ HercepTest scores or 2+ with gene amplification). Of the 74 borderline HercepTest 2+ cases, 44 cases (59%) showed HER2 gene amplification. We identified that HercepTest 2+ non-amplified cases were not significantly different from those amplified 2+ or 3+ cases with respect to their clinical outcome (BCSS and DFS). The overall concordance between HercepTest and CISH analysis for HER2 status was excellent. All HercepTest 2+ cases identified were observed to have poor outcomes similar to those HercepTest 3+ cases regardless of gene amplification status. In the current clinical environment, cases exhibiting IHC 2+ with non-amplified HER2 gene status are not offered targeted HER2 therapy but do exhibit aggressive clinical behavioural characteristics and therefore optimal treatment strategies for these patients need to be determined.