Coamplification with colocalization of the human epidermal growth factor receptor 2 and centromeric-17 signals on fluorescence in situ hybridization in invasive breast carcinoma: An exceedingly rare findingT Pai1, O Shetty1, A Patil2, T Shet2, SB Desai1
1 Division of Molecular Pathology, Tata Memorial Centre; Department of Pathology, Tata Memorial Centre, Mumbai, Maharashtra, India
2 Department of Pathology, Tata Memorial Centre, Mumbai, Maharashtra, India
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0022-3859.186394
Source of Support: None, Conflict of Interest: None
Human epidermal growth factor receptor 2 (HER2) gene amplification is a well-established prognostic and predictive marker in breast cancer. Introduction of the HER2 targeted treatment, especially trastuzumab (Herceptin, Genentech, South San Francisco, CA, USA), created a revolutionary change in the management of early and metastatic HER2-positive breast cancer. Hence, it is of utmost importance to assess the HER2 status of the tumor accurately to determine the eligibility for HER2-targeted therapy. Fluorescence in situ hybridization (FISH) technique is considered as the gold standard for HER2 status determination, and most of the cases are easily interpreted by this technique.  However, a small subset of cases with unusual FISH patterns exists with interpretative difficulties.
With this preamble, we herewith present two cases of invasive breast carcinoma, no special type, Grade III; one case (Case 1) with positive steroid hormone receptor status (estrogen [ER] and progesterone [PR]) and other case (Case 2) with negative ER and PR status. In both cases, immunohistochemistry for cerbB2 (CerbB2/HER2 monoclonal antibody, Biocare Medical, CA, USA) was positive (Score 3+). Interphase FISH test was performed using Vysis HER2 DNA FISH probe kit (Abbott Molecular Inc., Des Plaines, IL, USA) to confirm the HER2 status.
The cases were reviewed by two pathologists independently for FISH. Case 1 had multiple HER2 (with an average of 9.0 signals/nuclei) and centromeric-17 (CEP17) signals (with an average of 5.0 signals/nuclei) and HER2/CEP17 of 1.80 and Case 2 with an average of 14.8 HER2 signals and 5.1 CEP17 signals had HER2/CEP17 of 2.90. An interesting and common finding in both was that the majority of the signals were colocalized, resulting in a multiple-merged yellow signals ranging from 5 to 20 signals/nuclei [Figure 1]a, which is a rarely observed pattern. The possibility of these being artifacts was ruled out as the signals were regular in size, confined within the tumor nuclei, not seen across other bandwidths, and the adjacent stromal cells and lymphocytes displayed regular 2 HER2 and CEP17 signals [Figure 1]b. Although the mean HER2/CEP17 ratio was 1.8, Case 1 was reported as HER2 amplified taking into consideration mean HER2 copy number ≥6 as per the 2013 ASCO/CAP guidelines.  The report included a comment on coamplification with colocalization of HER2 and CEP17 signals in both cases.
The colocalization pattern is an exceedingly unusual pattern and creates diagnostic difficulty. Ours is a tertiary referral cancer center in India, and till date since 2006, we have performed FISH assessment for HER2 status in approximately 1788 cases of breast cancer, of which only two cases (0.1%) have demonstrated such a pattern which is concordant to that reported in literature.  This signal pattern is explained by the mechanism of extension and incorporation of the HER2 amplicon into the pericentromeric region, resulting in the coamplification with colocalization; however, additional technique, namely, array comparative genomic hybridization is recommended for confirmation of these results.  Cases with extended HER2 amplicons have been associated with decreased objective response to trastuzumab, , and thus, it would be prudent to mention this pattern in the FISH report.
Although rare, such FISH patterns are encountered in routine diagnostics, and taking into consideration their possible therapeutic relevance, such findings should be mentioned in the reports.
The authors would like to thank Mr. Sandeep Dhanavade, Ms. Sonali Tambe and Mr. Vinayak Kadam for their excellent technical assistance in performing the FISH test.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.