Abstract P2-08-04: <i>HER2</i> gene status change after taxanes based chemotherapy: be a ware of miss-interpretation of polyploidization! Impact for patient management.

Abstract Background: The status of the HER2 gene is not static and may change between the primary tumor, lymph node metastases and distant metastases. This status change can be a consequence of the natural evolution of the tumor or can be induced by therapy. The HER2 gene status is, in the majority of cases, established at the moment of diagnosis. After chemotherapy, monitoring this marker can be a real challenge because of ploidy changes induced by drugs. We made a HER2 genetic examination by Fluorescent in situ Hybridization (FISH) of invasive breast cancers before and after taxane treatment. After treatment we found supernumerary HER2 gene copies up to 15 copies in the cases non-amplified at diagnosis. Material and methods: All 410 invasive breast carcinoma samples were collected in a single laboratory (Service de Pathologie, Centre Jean Perrin). Tumor samples were collected by biopsy at diagnosis or by surgical excision after taxanes chemotherapy. For all cases we had matched samples, one before and one after therapy. Tumor samples were formalin-fixed paraffin-embedded. IHC staining was done to detect the expression of HER2 protein. Only cases which were IHC 2+ or IHC 1+ (with scattered HER2 heterogeneous staining) were checked by FISH. Results: After taxane-based neoadjuvant chemotherapy (4–8 cycles) thirty patients (HER2 negative at Dg) out of 344 showed supernumerary HER2 gene copies detected in giant cells with large polyploid nuclei. The proportion of polyploid post-therapy cells among regular-size cancer cells (diploid) was from 15 to 30%. The FISH examination of polyploid cells revealed an increase of HER2 copy number (and chromosome 17centromere) up to 15. The same copy number raise was observed with EGFR and ALK FISH probes. The ratio of all 30 cases with two distinct tumor cell populations (near diploid cells and giant cells) was up to 1.65 (not amplified). In one third of cases the average number of HER2 gene copies was higher than 6. Conclusions and recommendations: As a result of taxane administration, polyploid cells are formed and that most (if not all) chromosomes have been replicated simultaneously because of absent chromosomal segregation secondary to microtubule inhibition by taxanes. This HER2 copy increase is distinct from the mechanisms accounting for a more restricted gene amplification during carcinogenesis. The viability of giant polyploid cells is particularly low and most if not all of these cells succumb to mitotic catastrophe, an onco-suppressive mechanism causing cell cycle arrest and cell death. Irrespective of this consideration, it would be a critical error to classify cases as HER2 amplified solely only the bases of HER2 copy number &amp;gt;6. We recommend that the detection of more than 6 HER2 copies after taxane-based chemotherapy should be prompt a validation step involving the exclusion of polyploid cells from the analysis, as well as hybridization with a second and third probe specific for a chromosome other than 17. This validation step appears crucial to avoid unnecessary, costly treatments with HER2-targeted therapies after the failure of taxane-based chemotherapies. The clinico-pathological parameters of tumors with giant cells will be discussed. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P2-08-04.