Chromosome 17 polysomy has been identified in 5% to 50% of invasive breast cancers; even though a relationship with human epidermal growth factor receptor 2 (HER2/ neu ) status has been reported, other studies have shown that coincident centromere 17 (Cep17) amplification may be the cause of an overestimation of chromosome 17 polysomy in fluorescence in situ hybridization (FISH) testing.
To evaluate polysomy/amplification of Cep17 in invasive breast cancer with relation to proliferative activity (Ki-67), estrogen receptor, progesterone receptor, and HER2/ neu status, in an attempt to identify a subgroup of patients with a worse prognosis.
A total of 647 cases of invasive ductal breast cancer were collected and subjected to FISH analysis for HER2/neu gene and centromere 17 alteration, HercepTest for HER2/ neu protein expression, and routine immunohistochemistry for Ki-67 and hormone receptor status.
Copy number gain of Cep17 was observed in 27.3% of cases. Within this group, HER2/neu gene amplification was detected in 14.1% of cases, whereas HER2/ neu expression was scored 3+ in 20.1% of cases; about half of the HER2/ neu overexpressing cases (9.8%) did not show amplification by FISH. Moreover, 69% of polysomic cases showed high Ki-67 index.
(1) Centromere 17-altered cases are frequently HER2/ neu overexpressing but not amplified, resulting in HercepTest/FISH disagreement; (2) HER2/neu amplification is seen at a higher incidence in cases without Cep17 copy number alterations, which are therefore not necessarily due to chromosome 17 disorder; (3) proliferation index is significantly higher in aneusomic tumors. These data suggest that the presence of Cep17 alterations could identify a subset of breast cancers with more aggressive biological and clinical behavior, which may show nonresponsiveness to conventional therapy independently of HER2/neu amplification status.