Co-amplification of the HER2 gene and chromosome 17 centromere: a potential diagnostic pitfall in HER2 testing in breast cancer

Abstract

Co-amplification of the centromere on chromosome 17 (CEP17) and HER2 can occur in breast cancer. Such aberrant patterns (clusters) on CEP17 can be misleading to calculate the HER2/CEP17 ratio, and thus underreporting of HER2 amplification. We identified 14 breast cancers retrospectively with HER2/CEP17 co-amplification and performed FISH (fluorescence in situ hybridization) with additional chromosome 17 probes (17p11.1-q11.1, 17p11.2-p12, TP53 on 17p13.1, RARA on 17q21.1-3 and TOP2 on 17q21.3-22) to characterize the spanning of the amplicon in these cases. Furthermore, the HER2 status was analyzed by means of HER2 silver in situ hybridization (SISH) and immunohistochemistry (IHC). The co-amplification of HER2/CEP17 was compared between the three institutions. TP53 was eusomic in all cases, 17p11.2-p12 in 79% (11/14), whereas 17p11.1-q11.1 showed chromosomal gain in all cases. RARA was amplified in 10/14 cases (71%) and TOP2 in 3/14 cases (21%). HER2 was amplified with FISH/SISH in all 14 cases. 9/14 tumors were 3+ IHC positive (64%) and 3 cases were 2+ IHC positive. In our cohort the CEP17 amplicon almost always involves the HER2 but not the TOP2 locus. Overall agreement on HER2/CEP17 ratio (when applying ASCO/CAP guidelines) was only 64% (9/14 cases) between the institutions. Discrepant ratios varied from 1.1 to 14.3. The HER2/CEP17 co-amplification is not defined in the ASCO/CAP guidelines, and may result in inaccurate HER2-FISH/SISH status, particularly if only the calculated HER2/CEP17 ratio is reported. It is recommended to report separate CEP17 and HER2 signals in complex HER2/CEP17 patterns.

Assessment of HER2 status in breast cancer: overall positivity rate and accuracy by fluorescence in situ hybridization and immunohistochemistry in a single institution over 12 years: a quality control study

“…Recent papers on HER2 testing add to the recommendations mentioned above, the need for pathology institutions to participate in external national or international quality assurance and proficiency programs together with develop national/international guidelines for HER2 testing [5,26,29-35]. During the last years, pitfalls in HER2 testing as polysomy and co-amplification of HER2/CEP17 were explored and published, which pathologists must be also aware of, when reporting HER2 status in breast cancer [36-38]. …”

Evaluation of the prognostic role of centromere 17 gain and HER2/topoisomerase II alpha gene status and protein expression in patients with breast cancer treated with anthracycline-containing adjuvant chemotherapy: pooled analysis of two Hellenic Cooperative Oncology Group (HeCOG) phase III trials

“…The CEP17 FISH probe detects the alpha-satellite repeat region at the centromere of chromosome 17, at 17p11.1-17q11.1. The specificity of CEP17 remains undetermined, while this probe and a centromeric probe detecting additional neighboring regions on 17p11.2-12 yield different results concerning chromosome 17 status and, therefore, different HER2 gene amplification status, when the latter is assessed as HER2 /CEP17 ratio of >2.2 [66]. This may reflect the presence of the probed satellite repeats outside the centromeric region, which happen during evolution [67] and probably during cancer clone evolution, as well.…”

Current Challenges for HER2 Testing in Diagnostic Pathology: State of the Art and Controversial Issues

“…By coupling FISH and microarray-based comparative genomic hybridization (aCGH) analysis we have provided the first direct evidence that additional copies of CEP17 as detected by FISH are frequently caused by CEP17 gain/amplification (Figure 2) and that these phenomena are more prevalent than true chromosome 17 polysomy (38.9/55.5 versus 5.5%, respectively) (Marchio et al, 2009). This demonstration, subsequently validated by independent groups employing other techniques (Yeh et al, 2009; Moelans et al, 2010, 2011b; Varga et al, 2012), holds important clinico-therapeutic implications, as the occurrence of amplification of CEP17 can be responsible for misleading HER2 FISH results due to the HER2 /CEP17 ratio (Figure 2), precluding therefore anti-HER2 based therapy to some patients (Marchio et al, 2009). …”