Background: The purpose of this study was to evaluate HER2 gene status in relation to chromosome 17 polysomy with the chromogenic in situ hybridization (CISH) technique and to compare the results with those of immunohistochemistry (IHC).
Methods and results: Sixty six cases of breast carcinoma with an immunohistochemical HER2 protein score of 1+, 2+ 3+ (HercepTest) were investigated. HER2 gene status was evaluated on paraffin sections with the CISH technique using a digoxigenin-labeled DNA probe. In HER2 positive cases with low level amplification (LLA), the copy number of chromosome 17 was determined. Thirty four tumors (51.5%) were negative and 32 (48.5%) were positive for HER2 gene amplification. Of these 10 tumors (15%) showed LLA and 22 tumors (33.5%) high level amplification (HLA). Nine of ten tumors with LLA had an equal or greater than two ratio of HER2 to chromosome 17 signals. The correlation of the results obtained by CISH and IHC showed that the concordance of the two methods was highest in the 3+ group (100%) and lower in 1+ group (89%), whereas a high degree of discordance was found in the 2+ group (69%).
Conclusions: CISH is an accurate and practical technique for the evaluation of both HER2 gene and chromosome 17 status and its application is considered necessary especially for the clarification of the 2+ results of IHC.