Originally published as JCO Early Release 10.1200/JCO.2007.13.4296 on September 15 2008

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Polysomy 17 in Breast Cancer: Clinicopathologic Significance and Impact on HER-2 Testing

Isabelle Vanden Bempt, Peter Van Loo, Maria Drijkoningen, Patrick Neven, Ann Smeets, Marie-Rose Christiaens, Robert Paridaens, Christiane De Wolf-Peeters

From the Departments of Pathology, Obstetrics and Gynecology, Surgery-Senology, and General Medical Oncology and Multidisciplinary Breast Centre, University Hospital Gasthuisberg, and Departments of Human Genetics and Electrical Engineering, Katholieke Universiteit Leuven; and Department of Molecular and Developmental Genetics, Flanders Institute for Biotechnology, Leuven, Belgium

Corresponding author: Isabelle Vanden Bempt, PhD, Department of Pathology, Minderbroedersstraat 12, 3000 Leuven, Belgium; e-mail: isabelle.vandenbempt{at}uz.kuleuven.ac.be

	ABSTRACT

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Purpose Polysomy 17 is frequently found in breast cancer and may complicate the interpretation of HER-2 testing results. We investigated the impact of polysomy 17 on HER-2 testing and studied its clinicopathologic significance in relation to HER2 gene amplification.

Patients and Methods In 226 patients with primary invasive breast carcinoma, HER2 gene and chromosome 17 copy numbers were determined by dual-color fluorescent in situ hybridization (FISH). The interpretation of FISH results was based on either absolute HER2 gene copy number or the ratio HER2/chromosome 17. Results were correlated with HER-2 protein expression on immunohistochemistry (IHC), HER2 mRNA expression by reverse transcriptase polymerase chain reaction (RT-PCR), and with various clinicopathologic parameters.

Results All cases with an equivocal HER-2 result by FISH, either by absolute HER2 copy number (44 of 226 patients; 19.5%) or by the ratio HER2/chromosome 17 (three of 226 patients; 1.3%), displayed polysomy 17. On its own, polysomy 17 was not associated with HER-2 overexpression on IHC or increased HER2 mRNA levels by RT-PCR. Moreover, and in contrast with HER2 gene amplification, polysomy 17 was not associated with high tumor grade, hormone receptor negativity, or reduced disease-free survival.

Conclusion Polysomy 17 affects HER-2 testing in breast cancer and is a major cause of equivocal results by FISH. We show that tumors displaying polysomy 17 in the absence of HER2 gene amplification resemble more HER-2–negative than HER-2–positive tumors. These findings highlight the need for clinical trials to investigative whether polysomy 17 tumors benefit from HER-2–targeted therapy.

	INTRODUCTION

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The search for prognostic markers and therapeutic targets in human breast cancer has revealed a major role for the HER-2 oncoprotein. Overexpression of HER-2 has been reported in 15% to 25% of invasive breast carcinomas.^1,2 In most cases, this can be attributed to amplification of the HER2 oncogene located on the long arm of chromosome 17 (17q12).³ Both HER-2 overexpression and HER2 gene amplification have been correlated with poor clinical outcome.^4-6 Apart from its prognostic value, the HER-2 status has major therapeutic implications. Not only does HER-2 overexpression predict response to certain chemotherapeutic agents, such as anthracyclines or paclitaxel, it is also considered to be a strong predictive marker for clinical benefit from HER-2–targeted therapy (trastuzumab) in the metastatic setting and, more recently, also in the adjuvant setting.^7-12 Although tumors not expressing HER-2 have virtually no chance of responding to trastuzumab, moderate or even high levels of expression are not always associated with a therapeutic success. Moreover, treating patients with breast cancer with trastuzumab is expensive and not without risk, because serious cardiac toxicity has been observed in approximately 1% to 4% of patients.¹³ Therefore, correct identification of patients who will benefit from trastuzumab therapy is of utmost importance.

A wide variety of techniques can be applied to determine the HER-2 status in breast cancer tissue. Of these, immunohistochemistry (IHC) and fluorescent in situ hybridization (FISH) are most frequently used. Although both methods have shown high concordance in some studies, reproducibility remains poor in others.^14,15 Recently, an expert team assembled by the American Society of Clinical Oncology (ASCO) and the College of American Pathologists (CAP) has developed guidelines for HER-2 testing in breast cancer.¹⁶ Accordingly, HER-2 testing results should be reported as either positive, negative, or equivocal. The latter group represents a gray area of breast tumors scoring 2+ on IHC or having a modest increase in HER2 gene copy number by FISH. Interestingly, equivocal HER-2 testing results have been related to chromosome 17 polysomy.^17-19 Indeed, tumors featuring an increased chromosome 17 copy number will contain more copies of the HER2 gene, which could result in elevated HER-2 expression. At present, it remains unknown to what extent polysomy 17 obscures the interpretation of HER-2 testing results and whether polysomy 17 tumors share biologic characteristics with true HER-2–positive breast cancers. In the present study, we aimed to clarify this issue.

	PATIENTS AND METHODS

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Patients
Since 2002, routine HER-2 FISH analysis (PathVysion; Vysis, Downers Grove, IL) has been performed at the Pathology Department of the University Hospital Gasthuisberg in Leuven, Belgium, on patients with breast cancer showing an equivocal or positive HER-2 result on IHC (score 2+/3+). From this series of 751 patients, patients with noninvasive breast carcinomas or metastatic lesions and patients lacking clinical or pathologic data, as well as referral patients, were excluded. The remaining 171 patients as well as a control series of 55 consecutive patients with a negative HER-2 result on IHC were recruited into the present study. Table 1 lists the clinicopathologic characteristics of the 226 patients included in the study. Patients underwent mastectomy or local wide excision of their primary breast tumor with an axillary lymph node dissection at least at level I and II. Histopathologic examination was performed on hematoxylin and eosin–stained sections, and tumors were classified and graded according to the WHO Classification and the Elston and Ellis grading system, respectively.^20,21 Disease-free survival was defined as the time period (in months) between the date of surgery and the date of recurrence or distant metastasis. All patients had the best standard of care for local and systemic treatment; trastuzumab was not yet standard of care in the adjuvant or neoadjuvant setting. This study was approved by the Local Commission for Medical Ethics and Clinical Studies.

FISH
FISH analysis (PathVysion, Vysis) was performed manually according to the manufacturer's recommendations. Using a Zeiss Axioplan 2 epifluorescence microscope (Carl Zeiss, Göttingen, Germany), we counted signals in at least 100 tumor nuclei in two or more separate regions of the tissue section. Averages of HER2 gene and chromosome 17 copy number counts were rounded off to the nearest whole number; in case HER2 gene amplification appeared as clusters of uncountable HER2 signals, we estimated the average HER2 gene copy number. FISH results were interpreted according to two different scoring methods: (1) based on absolute HER2 gene copy number (HER2 absolute) or (2) based on the ratio HER2 gene/chromosome 17 copy number (HER2/Chr17 ratio). Actually, these scoring methods were developed to be used in combination with, respectively, the Oncor INFORM HER-2/neu test kit (Ventana Medical Systems Inc, Tucson, AZ) or the PathVysion test kit. We have previously compared both FISH test kits in a series of 20 patients with breast cancer and found that the count of absolute HER2 gene copy number was nearly identical for both kits.²² Therefore, we applied both scoring methods in combination with the PathVysion test kit. As proposed by the ASCO/CAP guidelines,¹⁶ an absolute HER2 gene copy number lower than four or an HER2/Chr17 ratio of less than 1.8 was considered HER-2 negative; an absolute HER2 copy number between four and six or an HER2/Chr17 ratio between 1.8 and 2.2 was considered HER-2 equivocal, and an absolute HER2 gene copy number greater than six or an HER2/Chr17 ratio higher than 2.2 was considered HER-2 positive. Polysomy 17 was defined as an average chromosome 17 copy number 3.^19,23 Lymphocytes, (myo)fibroblasts, and normal epithelial cells served as internal control.

Quantitative Reverse Transcriptase Polymerase Chain Reaction
In 157 of 226 patients, representative frozen tumor tissue was available for quantitative reverse transcriptase polymerase chain reaction (RT-PCR) analysis. For each patient, total RNA was extracted from 20-µm sections using the RNeasy mini kit (Qiagen, Hilden, Germany). RNA purity and concentration were checked spectrophotometrically (Nanodrop Technologies, Wilmington, DE). One µg of total RNA was reverse transcribed and PCR reactions on the resulting cDNA were performed in the ABI-Prism 7900 HT Sequence Detector (Applied Biosystems, Lennik, Belgium). PCR primers and probes for HER2 and housekeeping gene GAPDH were obtained from Applied Biosystems (TaqMan Gene Expression Assays). Each sample was analyzed in triplicate in a MicroAmp optical 96-well reaction plate (Applied Biosystems). A sample of normal breast tissue was used as a calibrator and the Ct-method was applied to determine relative gene expression levels.²⁴

Statistical Analysis
Differences in HER2 mRNA expression levels between different subgroups were assayed by a Wilcoxon rank sum test. Differences in clinicopathologic variables between subgroups were checked using ² tests. The Bonferroni method was used for multiple testing correction. Survival analysis was performed using the Kaplan-Meier method. Survival differences between subgroups were assayed by log-rank tests. A P value of less than .05 was considered statistically significant.

	RESULTS

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Comparison Between IHC and FISH for HER-2 Testing
Comparison between IHC and FISH for HER-2 testing is outlined in Table 2. An equivocal HER-2 status by FISH was found in 44 (19.5%) of 226 patients based on absolute HER2 gene copy number and in three (1.3%) of 226 patients based on the ratio HER2/Chr17. Note that none of these patients showed overexpression on IHC (score 3+). Remarkably, all patients with an equivocal HER-2 status by FISH as well as those cases showing discordant HER-2 testing results displayed polysomy 17.

HER2 mRNA Expression by Quantitative RT-PCR
As illustrated in Figure 1, polysomy 17 tumors had low relative HER2 mRNA expression values comparable to those found in the HER-2–negative group (mean expression, 0.914 v 0.912; P = .1865). By contrast, HER-2–positive tumors generally had increased relative expression values, with most cases showing at least a five-fold increase in HER2 mRNA expression compared with normal breast tissue. In HER-2–positive cases, HER2 mRNA expression levels were significantly higher than those in HER-2–negative (mean, 7.831 v 0.912; P < 10^–15) and polysomy 17 tumors (mean, 7.831 v 0.914; P < 10^–16).

View larger version (16K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 1. HER2 mRNA expression by quantitative reverse transcriptase polymerase chain reaction.

View larger version (12K): [in this window] [in a new window] [PowerPoint Slide for Teaching]   Fig 2. Kaplan-Meier survival curves illustrate shorter disease-free survival in patients with HER-2–positive tumors as compared with patients with HER-2–negative tumors (P < .001).

	DISCUSSION

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In the present study, we provide evidence that polysomy 17 and HER2 gene amplification are two distinct genetic aberrations with a different clinicopathologic significance in breast cancer. First, we show that polysomy 17 on its own does not result in HER-2 overexpression, neither at the protein nor at the mRNA level. Indeed, we did not encounter any breast tumor showing HER-2 overexpression on IHC (score 3+) and polysomy 17 in the absence of HER2 gene amplification. Moreover, and in accordance with Dal Lago et al,²⁶ we did not find increased HER2 mRNA expression by quantitative RT-PCR in polysomy 17 tumors, not even in those tumors showing up to 10 HER2 gene copies. Second, we could demonstrate that HER2 gene amplification and polysomy 17 have a different clinicopathologic impact in breast cancer. Whereas HER2 gene amplification was clearly associated with high tumor grade, hormone receptor negativity, and reduced disease-free survival, polysomy 17 did not show any significant association with adverse clinicopathologic parameters. Nevertheless, a trend toward shorter disease-free survival was observed in polysomy 17 tumors. Because polysomy 17 may reflect aneuploidy and increased chromosomal instability, it can be expected that tumors harboring this anomaly will behave more aggressively than those without it.^30-32 Still, our data suggest that the clinicopathologic impact of polysomy 17 is not as strong as that of HER2 gene amplification in breast cancer and that tumors displaying polysomy 17 in the absence of HER2 gene amplification behave more similar to HER-2–negative than to HER-2–positive tumors.

Our current findings could have important clinical implications. Because polysomy 17 on its own is not associated with HER-2 overexpression and because it does not have the same clinicopathologic significance as true HER2 gene amplification, one may wonder whether polysomy 17 tumors benefit from HER-2–targeted therapy such as trastuzumab, which targets the HER-2 protein at the tumor cell membrane. Indeed, the best therapeutic response rates have been observed in breast cancers showing HER-2 overexpression by IHC.^28,33 Recently, trastuzumab response has been reported in two patients showing polysomy 17 in the absence of HER2 gene amplification and in one patient showing neither polysomy 17 nor HER2 gene amplification. Of interest, all three patients showed HER-2 overexpression on IHC.²⁸ We speculate that in such rare cases, HER-2 overexpression might result from deregulated gene transcription. Further phase III trials are needed to elucidate whether polysomy 17 tumors benefit from HER-2–targeted therapy.

It is important to realize that polysomy 17 has a substantial impact on the interpretation of HER-2 testing results, especially in those patients with an equivocal HER-2 status on IHC (score 2+). Indeed, in patients in whom polysomy 17 results in a moderate increase in HER2 gene copy number (four to six), HER-2 FISH results could be interpreted as equivocal if only absolute HER2 copies are counted. As such, we found that 37 (37.4%) of 99 tumors with an IHC score of 2+ were still considered equivocal after FISH analysis based on absolute HER2 copy number, whereas only one tumor (1.0%) remained equivocal based on the HER2/chromosome 17 ratio. On the basis of these data, and given that approximately 15% of newly diagnosed patients with breast cancer show an IHC score 2+,¹⁶ we estimate that 5.6% and 0.15% of breast carcinomas remain equivocal after FISH testing, depending on whether a control probe for chromosome 17 is used. Remarkably, the one tumor (0.15%) showing an equivocal HER-2 status on both IHC and FISH, even after correction for chromosome 17 copy number, also displayed polysomy 17, with a mean ratio of 10/5 or 2.0. In this particular case, quantitative RT-PCR indicated no increase in HER2 mRNA expression, suggesting a negative HER-2 status after all. In the end, one may wonder whether quantitative RT-PCR could be a valuable alternative for HER-2 testing in routine clinical practice. Still, the need for representative fresh or frozen breast cancer tissue for optimal RT-PCR testing results, as well as inevitable dilution of invasive tumor cells with normal and stromal cell populations or noninvasive breast lesions, limits the use of RT-PCR for routine HER-2 testing.

In conclusion, polysomy 17 is a major cause of equivocal HER-2 testing results by FISH. We provide evidence that polysomy 17 and HER2 gene amplification have a distinct impact on the clinicopathologic parameters in breast cancer and that polysomy 17 tumors should be regarded HER-2 negative. Indeed, HER2 gene amplification usually results in excessive HER-2 expression levels and defines a distinct clinicopathologic breast cancer entity characterized by high tumor grade, reduced hormone receptor expression, and poor prognosis. By contrast, polysomy 17 is not related to HER-2 overexpression or adverse clinicopathologic features but may rather reflect increased chromosomal instability in breast cancer. These findings underscore the importance of using dual-color systems for HER-2 FISH testing and urge the need for clinical trials to investigate whether polysomy 17 tumors benefit from HER-2–targeted therapy.

	AUTHORS’ DISCLOSURES OF POTENTIAL CONFLICTS OF INTEREST

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The author(s) indicated no potential conflicts of interest.

	AUTHOR CONTRIBUTIONS

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Conception and design: Isabelle Vanden Bempt, Peter Van Loo, Christiane De Wolf-Peeters

Financial support: Patrick Neven, Christiane De Wolf-Peeters

Provision of study materials or patients: Maria Drijkoningen, Patrick Neven, Ann Smeets, Marie-Rose Christiaens, Robert Paridaens

Collection and assembly of data: Isabelle Vanden Bempt, Maria Drijkoningen, Patrick Neven, Ann Smeets, Marie-Rose Christiaens, Robert Paridaens

Data analysis and interpretation: Isabelle Vanden Bempt, Peter Van Loo, Maria Drijkoningen, Patrick Neven, Marie-Rose Christiaens, Robert Paridaens, Christiane De Wolf-Peeters

Manuscript writing: Isabelle Vanden Bempt, Peter Van Loo, Christiane De Wolf-Peeters

Final approval of manuscript: Isabelle Vanden Bempt, Peter Van Loo, Maria Drijkoningen, Patrick Neven, Ann Smeets, Marie-Rose Christiaens, Robert Paridaens, Christiane De Wolf-Peeters

	ACKNOWLEDGMENTS

 
We thank Peter Vandenberghe, MD, PhD, and Iwona Wlodarska, PhD, from the Centre of Human Genetics for helpful collaboration and Miet Vanherck, Johan Van Even, Helga Van Den Bosch, and Lieve Ophalvens for excellent technical assistance.

	NOTES

 
published online ahead of print at www.jco.org on September 15, 2008.

I.V.B. and P.V.L. contributed equally to this work.

Authors’ disclosures of potential conflicts of interest and author contributions are found at the end of this article.

	REFERENCES

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Submitted July 9, 2007; accepted May 14, 2008.

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This Article Abstract Full Text (PDF) Purchase Article View Shopping Cart Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Save to my personal folders Download to citation manager Rights & Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Vanden Bempt, I. Articles by De Wolf-Peeters, C. Search for Related Content PubMed PubMed Citation Articles by Vanden Bempt, I. Articles by De Wolf-Peeters, C. Related Articles Related Editorial Social Bookmarking                            What's this?