Heterogeneity of EGFR and HER2 gene status by fluorescent in situ hybridization in colorectal cancer

B80

Background: Recent studies suggest that the epidermal growth factor receptor (EGFR) gene copy number by fluorescence in situ hybridization (FISH) might predict response to anti-EGFR antibodies in advanced colorectal cancer (CRC) patients. Additionally, preclinical data indicate that HER2, a member of the EGFR family, could enhance sensitivity to tyrosine kinase inhibition. However, in CRC it is still unknown whether EGFR and HER2 gene status is stable during disease progression. Methods: EGFR and HER2 gene status is being analysed by FISH on 30 paired samples collected from patients with either: 1) a primary CRC and at least one related distant metastatic lesion; or 2) a primary CRC and a subsequent local relapse; or 3) two metachronous metastatic lesions. Evaluation of EGFR and HER2 status was based on two parameters: the specific copy numbers of both gene and chromosomecentromere, and their frequencies in tumor cells. For both genes, a FISH-positive status was retained when either a high level oftrisomy orpolysomy (≥3 copies of the gene in ≥40% of cells), or an amplification pattern were observed. EGFR protein expression by immunohistochemistry (IHC) was assessed and was further correlated to gene status. Results: 2 of 11 cases (18%) were discordant for EGFR status, but not for HER2: both cases represented paired metachronous metastatic lesions showing EGFR disomy in one sample and high level polysomy in the other one. Conversely, 3 of 11 cases (27%) were discordant for HER2 status, but not for EGFR: 2 cases showed high level polysomy in the primary tumor, being lost in the related metastasis, whilst 1 case exhibited high level polysomy in the metastasis, but not in the related primary tumor. By IHC analysis, EGFR expression was virtually absent in 2 of 9 samples bearing a FISH-positive status. The study is ongoing and final results will be presented at the meeting. Conclusions: Our preliminary results suggest for the first time that EGFR and HER2 gene status is not stable during disease progression, and they question whether the observed changes of EGFR and HER2 status are the result of any selective pressure in CRC. Moreover, our results further highlight the potential inconsistencies of EGFR assessment by IHC in a non negligible proportion of CRC. We anticipate that these findings have to be considered in future prospective studies.