Assessments of HER2 Gene Status in Breast Carcinomas with Polysomy of Chromosome 17
Z Gatalica, S Repertinger, L Hatcher, P Ulmer. Creighton University School of Medicine, Omaha, NE
Background: Fluorescent in-situ hybridization (FISH) analysis is used to assess HER2 gene copy number and detect gene amplification as a part of clinical algorithm for treatment of breast cancer patients. Given the significant clinical benefits of trastuzumab therapy available to patients with HER2-positive breast cancer, it is of paramount importance to accurately identify all patients eligible for this therapy. The HER2 gene is located on chromosome 17q11.2-q12. Interpretation of FISH assays depends on scoring the precise number of HER2 hybridization signals per tumor cell nucleus. Current guidelines define HER2-positive tumors by FISH as having an average HER2 gene copy number of more than 6 copies per nucleus or as a HER2 gene to chromosome 17 ratio of more than 2.2.
Design: Commercially available Locus Specific Identifier (LSI) HER2 DNA probe (190 Kb Spectrum Orange directly labeled fluorescent DNA probe) and the Chromosome Enumeration Probe (CEP) 17 DNA probe (5.4 Kb Spectrum Green directly labeled fluorescent DNA) were used for FISH (PathVysion, Abbot Molecular) on formalin fixed paraffin embedded breast cancer cases. Polysomy 17 was defined as three or more CEP17 signals per nucleus (average for 30 cells). HER2 protein expression was investigated using standard immunohistochemical methods.
Results: Polysomy 17 was detected in 73 cases (12% of all tested patients). Average CEP17 copy number was 4.54 (range 3.0 to 10.4). 19 cases (26%) qualified as unequivocally HER2 gene amplified using HER2/CEP17 ratio (>2.2) guidelines, and 33 cases (45%) had on average 6 or more HER2 gene signals per nucleus. Ten cases with 6 or more CEP17 had concomitant HER2 gene amplification (HER2/CEP17 > 2.2) in 4 cases. Chromosome 17 polysomy showed no correlation with HER2 protein expression, but a positive trend was observed between HER protein expression and HER2 gene amplification (HER2/CEP17 ratio). A positive association between polysomy 17 and higher cancer stage was observed.
Conclusions: A substantial proportion of breast carcinomas harbor multiple copies of chromosome 17. Potentially clinically significant polysomy 17 (>6) is seen in 45% of cases, which could result in 42% discordant interpretation between absolute HER2 copy number and HER2/CEP17 gene amplification ratio. A minor fraction of carcinomas with polysomy 17 overexpresses HER2 protein, usually associated with HER2 gene amplification (HER2/CEP17 ratio>2.2), indicating that ratio is probably predictive of a therapeutic response in this subgroup of patients.
Tuesday, March 10, 2009 9:30 AM
Poster Session III # 54, Tuesday Morning