P3-05-07: Genetic Heterogeneity of Amplification Status in Breast Invasive Carcinoma with 2+ HER2 Immunostaining: What Can We Learn?

Abstract

Abstract Background The genetic heterogeneity of HER2 gene amplification (GA) in breast cancer has previously been described, but the clinical significance of this phenomenon remains unknown. We studied the genetic categories of a series of consecutive 2+ IHC cases over 5 years, with a focus on cases with HER2 GA detected in minor clone(s). We compared the HER2 status in primary tumors and positive axillary lymph nodes (ALN) when available to test the hypothesis that HER2 amplified cells are more aggressive and metastase quicker than non amplified cells. Material and methods: From January 1st. 2006 to May 30. 2011, 4491 invasive breast carcinomas had HER2 immunohistochemical (IHC) and/or HER2 gene status evaluation on their tumor sample in Institut Gustave Roussy. The distribution according to their IHC was as follows: 0 in 2915 cases (65%), 1+ in 569 cases (12.6%), 2+ in 536 cases (11.8%) and 3+ in 471 cases (10.6%). All 2+ samples were checked by Fluorescence in situ hybridization (FISH). For each case we analysed 100 invasive cells in 10 microscopic fields. Heterogeneous amplification was defined as presence of 5–50% of amplified cells (ratio>2,2) within the tumors otherwise classified as not amplified or borderline (ratio for 100 cells: <2,2 according to the ASCO 2007 and CAP criteria). The IHC 2+ tumors show five main genetic categories. Results: FISH with HER2/cen17 probes can classify IHC 2+ tumors into 5 genetic categories: normal gene status (two chromosomes (chr) 17 and two HER2 genes): 90 cases; chr 17 “ polysomy “/17q gain: 145 cases; HER2 gain: 122 cases; HER2 amplification (major clone): 135 cases and chr 17 monosomy: 44 cases. We focused our interest to IHC 2+ cases where the major clone showed no HER2 amplification, but minority clone (cut off 5%) showed the HER2 amplification (ratio >2,2 or >6 HER2 copies per cell). We found 48 such cases (10% of 2+ cases). These cases had been reported as not amplified (42/48) or borderline (6/48). To find out if the amplified cells are those which metastase, we checked 10 involved ALN: they all showed the similar genetic heterogeneity as a primary tumor with a global ratio < 2,2. Other cases with involved ALN are in ongoing study by FISH. Conclusions: The genetic heterogeneity of IHC 2+ tumors is a common event and five different genetic categories can be detected. A part of the HER2 negative cases contained one or several HER2 amplified clones. Preliminary results show a similar genetic heterogeneity of metastatic population. Our results on the small series of cases do not confirm the hypothesis that the only amplified clones metastase, but all clones (amplified and not amplified) has a metastatic capacity. A multi-center collaboration is needed to collect the high number of cases with heterogeneous amplification to: 1) Find out a proportion of such patients in HER2 IHC 2+ group (10% in our study). 2) Study a status of the gene in relapse cases. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P3-05-07.