Analysis of HER2 gene status in breast cancer with HER2 protein overexpression

Abstract

To study the HER2 gene status (by fluorescence in situ hybridization (FISH) in breast cancer with HER2 protein overexpression, the correlation between gene amplification and protein overexpression, as well as the rate and significance of chromosome 17 aneusomy. One hundred and twenty archival cases of breast cancer with formalin-fixed and paraffin-embedded tumor tissues with 2+ (42 cases) and 3+ (78 cases) HER2 protein overexpression by immunohistochemistry (IHC, HercepTest, Dako) were tested by FISH (PathVysion, Vysis) for HER2 gene status. The rate of chromosome 17 aneusomy was also analyzed. Amongst the 42 samples with IHC 2+, HER2 gene amplification was identified in 32 cases (76.19%), which included 11 cases with low amplification (ratio 2 approximately 4), 20 cases with moderate amplification (ratio 4 approximately 10) and 1 case with high amplification (ratio>10). Amongst the 78 samples with IHC 3+, HER2 gene amplification was identified in 71 cases (91.03%), which included 9 cases with low amplification, 48 cases with moderate amplification and 14 cases with high amplification. Chromosome 17 aneusomy was found in 83 cases (83/120, 69.17%), in which 14 cases (11.67%) showed hypodisomy (chromosome 17 copy number per cell<or=1.75), 52 cases (43.33%) showed low polysomy (chromosome 17 copy number per cell=2.26 approximately 3.75) and 17 cases (14.17%) showed high polysomy (chromosome 17 copy number per cell>or=3.76). Amongst the 42 cases with IHC 2+, 25 samples (59.52%) had chromosome 17 aneusomy, including 3 cases with hypodisomy, 18 cases with low polysomy and 4 cases with high polysomy. Amongst the 78 cases with IHC 3+, 58 samples (74.36%) had aneusomy 17, including 11 cases with hypodisomy, 34 cases with low polysomy and 13 cases with high polysomy. Most of IHC 2+ and FISH-positive cases had low or moderate HER2 gene amplification, while most of the IHC 3+ and FISH-positive cases had moderate or high gene amplification (P=0.0003). Six of the 7 samples with IHC 3+ and FISH-negativity had chromosome 17 aneusomy and 5 of the 10 samples with IHC 2+ and FISH-negativity had such aneusomy. A high concordance rate is noted between IHC 3+ and FISH positive results. The rate of FISH positive in IHC 2+ patients was higher than reported in other studies. Low or moderate HER2 gene amplification in IHC 2+ and moderate or high gene amplification in IHC 3+ occurs quite frequently. Chromosome 17 aneusomy (including hypodisomy, low polysomy and high polysomy) is also a relatively common phenomenon in our cohort with HER2 overexpression, with predominance of low polysomy.