Abstract P5-02-05: Assessment of HER2 status in invasive breast cancer with increased centromere 17 copy number by fluorescence in situ hybridization
Authors: Jang Min Hye , Kim Eun Joo , Kim Hyun Jeong , Park So Yeon 
Published: May 2015

Background: A subset of breast cancers can show increased copy numbers of chromosome 17 centromere (CEP17) by in situ hybridization (ISH). However, recent studies have revealed that true polysomy 17 is a rare event in breast cancer, and increased copy number of CEP17 represents amplification or coly number gain in the centeromeric region. In this situation, the utility of CEP17 in ISH is very limited and thus, alternative methods for precise assessment of HER2 status are necessary. ISH using probes of other genes on chromosome 17 as additional reference genes has been proposed by 2013 ASCO/CAP guideline and several previous studies. In this study, we applied this method to breast cancers with increased CEP17 copy number (≥2.6), and compared it with conventional method based on ASCO/CAP guideline 2007.

Methods: After reviewing all HER2 fluorescence in situ hybridization (FISH) reports based on HER2/CEP17 ratio, documented from June 2004 to December 2011 at our institution, we identified 300 cases (29.6%) with ≥2.6 CEP17 copy number from 1013 breast cancers. We performed FISH with probes for RARA, SMS, TP53 genes on 243 breast cancers with available tissue blacks, using tissue microarrays. If one or more genes showed &lt; 2.6 copy number, the largest number of them was chosen for alternative to CEP17 and re-graded the HER2 status based on HER2: alternative gene ratio.

Results: Of 243 breast cancers with ≥ 2.6 CEP17 copy number, 2 cases showed ≥ 2.6 copy numbers in all RARA, SMS and TP53 genes. Of 151 breast cancers classified as non-amplified based on HER2:CEP17 ratio, 42 (27.8%) were re-graded as amplified after additional FISH studies, and 7 of 8 cases which had been classified as equivocal were re-graded as amplified. Of the 47 cases with mean HER2 copy number of 4 to 6, 30 (63.8%) were upgraded from non-amplified to amplified, and 5 (10.6%) were upgraded from equivocal to amplified. Two (1.9%) of 75 cases with &lt; 4 mean HER2 copy number were upgraded from non-amplified to amplified, and 24 (23.8%) were changed from non-amplified to equivocal. After re-grading, equivocal cases were significantly increased, compared to the original reports (14.1% vs. 3.3%). And the concordance between FISH and immunohistochemistry became poorer (kappa value: 0.390 vs. 0.624).

Conclusion: Using additional reference genes can be an effective way for accurate HER2 evaluation in breast cancer with increased CEP17 copy number. However, it still has some limitations. It can lead to over-grading of HER2 status and increase of the equivocal cases, when tumor has loss of reference genes. Additional study for searching most stable genes that barely show copy number alteration is needed to simplify the procedure and to increase accuracy.

Citation Format: Min Hye Jang, Eun Joo Kim, Hyun Jeong Kim, So Yeon Park. Assessment of HER2 status in invasive breast cancer with increased centromere 17 copy number by fluorescence in situ hybridization [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P5-02-05.