Quantitative HER2 measurement and PI3K mutation profile in matched primary and metastatic breast cancer tissues.

Abstract

614 Background: HER2 status of primary breast cancer (PBC) is routinely used to determine systemic treatment for metastatic breast cancer (MBC) patients. Discordance rates of HER2 status between PBC and MBC range from 5.5% to 29% based on published meta-analyses. The clinical benefit of re-assessing HER2 in MBC tissues remains controversial. In this study, we measured quantitative HER2 expression in matched PBC and MBC tissues and correlated changes of HER2 with mutations in the catalytic domain of PI3 kinase(PIK3CA). Methods: Total HER2 protein expression (H2T) was quantified by the HERmark assay in 41 matched PBC and MBC formalin-fixed, paraffin-embedded specimens. PIK3CA mutation status in exons 9 (E545K and E542K) and 20 (H1047R) was determined using a validated pyrosequencing assay. Results: MBC samples included 5 lymph node, 13 viscera, 6 brain, and 17 soft tissue lesions (N=41). 27 (66%) cases showed higher H2T in MBC than in matched PBC; and 14 (34%) cases had higher H2T in PBC than in matched MBC, indicating an overall increase of H2T in matched MBC lesions (fold change 0.25-17.57; p=0.005, paired Wilcoxon rank sum test). HER2 positive conversion (HERmark negative/equivocal in PBC, but positive in matched MBC) was found in 6 (15%) cases, while HER2 negative conversion (HERmark positive in PBC, but negative/equivocal in matched MBC) was seen in 2 (5%) cases. HER2 status was unchanged in 33 (80%) cases. PIK3CA mutations were detected in 13 (32%) of PBC and 19 (46%) of MBC samples. Among the HER2 positive conversion cases, PIK3CA mutation was identified in 50% (3/6) PBC and 67% (4/6) MBC, compared to 0% (0/2, PBC or MBC) in the HER2 negative conversion cases. Among cases with unchanged HER2 status, PIK3CA mutation was observed in 30% (10/33) PBC and 42% (14/33) MBC. Conclusions: Quantitative HER2 assessment revealed a 20% discordance in HER2 status between matched PBC and MBC tissues, with more frequent conversion from low HER2 in PBC to high HER2 in MBC. PIK3CA mutation was observed more frequently in patients who converted from HER2 negative PBC to HER2 positive MBC. These results suggest that re-assessment of biomarkers in MBC tissues may better inform the selection of therapeutic options for patients with MBC.