HER2 expression and HER2 gene amplification in thymic epithelial tumors: a narrative review

Introduction

Thymic epithelial tumors (TETs) are a rare group of neoplasms situated in the anterior mediastinum (1). Although infrequent, they are the most common primary tumors of the thymus and the prevascular mediastinum. According to the World Health Organization (WHO) histological classification, TETs include thymomas, thymic carcinomas and neuroendocrine thymic tumors (2). Thymomas tend to grow slowly, and early-stage tumors have high survival rates after surgical removal. In contrast, thymic carcinomas are highly aggressive, with a greater potential to spread and lower survival rates compared to thymomas (1-4). Surgery remains the main treatment for resectable TET. Surgical options may vary according to tumor size and local invasiveness, expertise and equipment; minimally invasive access (intercostal, subcostal and subxiphoid) using thoracoscopic or robot-assisted platforms are privileged for smaller lesions without local invasion, while open surgery (sternotomy, thoracotomy or combined access) is reserved for more complex cases (1-4). For advanced cases, multimodal treatment protocols are implemented. Due to their rarity and histological diversity, molecular profile of TET is not well-defined and understanding their development is quite challenging. For the same reason, elaboration of large-scale studies is difficult. Although advances in molecular profiling have improved our understanding of many solid cancers, TET remains poorly characterized. Many targets have been investigated, but none have been consistently identified that could lead the way for the development of targeted therapies (5). The human epidermal growth factor receptor 2 (HER2/ERBB2) has gained interest as a treatment target due to its role in oncogenesis and its therapeutic significance in various malignancies (6-9); however, its potential role in treating TET remains unclear. Ongoing research tries to identify its expression and possible clinical value (10-18). This review summarizes the expression patterns, gene amplification status, and clinical implications of HER2 in thymomas and thymic carcinomas. We present this article in accordance with the Narrative Review reporting checklist (available at https://med.amegroups.com/article/view/10.21037/med-2025-1-48/rc).

Methods

The current recommendations for assessing the quality of narrative review articles were followed in creating this review (19). Literature research was conducted via PubMed, Embase and Web of Science databases using the terms [HER2] OR [ERBB2] OR [ErbB2] OR [HER2/neu] OR [c-erB2] AND [thymic epithelial tumors], [HER2] OR [ERBB2] OR [ErbB2] OR [HER2/neu] OR [c-erB2] AND [thymoma], and [HER2] OR [ERBB2] OR [ErbB2] OR [HER2/neu] OR [c-erB2] AND [thymic carcinoma]. There were no restrictions on publication date. Papers related to pediatric cases were excluded. Individual case reports were also excluded, even though none was detected. Only articles in English were included. After analysis, eight articles met the research criteria. Upon reviewing the references of these articles, one additional was identified, leading to nine articles in total. The search strategy is shown in Table 1.

The role of HER2 in tumor biology

HER2 is a transmembrane receptor tyrosine kinase and a member of the ERBB family, which includes epidermal growth factor receptor (EGFR) (ERBB1), HER3 (ERBB3), and HER4 (ERBB4) (6). Unlike other ERBB receptors, HER2 does not have a known ligand and stays in a constantly active conformation, making it a powerful signaling partner in heterodimerization. HER2 overexpression and gene amplification are well-known factors driving subgroups of breast, gastric, and lung cancers, and are linked to aggressive behavior and poor prognosis (6-9).

HER2 activation triggers downstream signaling through the PI3K/AKT and MAPK pathways, thereby promoting cell proliferation, survival, and angiogenesis. Its role in modulating the tumor microenvironment includes immune suppression, activation of cancer-associated fibroblasts (CAFs), and increased vascularization (6). These mechanisms contribute to tumor progression and resistance to therapy. The transmission of mitogenic signals results in an increased proliferation and prolonged survival of tumor cells when this system is aberrantly stimulated by activating mutations, gene amplification, or an increased production of ligands by tumor cells. Particular medications such as monoclonal antibodies and tyrosine kinase inhibitors have been created to target these molecules (6-9). These substances bind specifically to the receptor, stop downstream signaling, and prevent abnormal cell division.

HER2 in TETs

In 2003, Pan et al. analyzed tissue samples from 63 cases of thymomas and 17 cases of thymic carcinomas collected between 1981 and 1997 (10). The series included 47 men and 33 women, with a mean age of 59 years (range, 25–77 years). Immunohistochemical analyses were performed, and immunopositivity was scored as:

• 0, undetectable;
• 1+, focal and weak positivity in less than 10% of tumor cells;
• 2+, heterogeneous positivity in 10–50% of tumor cells;
• 3+, strong positivity in more than 50% of tumor cells.

To evaluate HER2 gene amplification, dual-color fluorescence in situ hybridization (FISH) analysis was performed. Samples were deemed gene amplification positive if tight clusters of HER2 signals or a HER2-to-CEP17 signal ratio per cell of more than 2 was observed (10).

Thymomas generally did not express HER2 protein, except for four cases: two type B2 thymomas with focal membranous positivity (1+), and one type A and one type B3 thymoma with heterogeneous positivity (2+). Thymic carcinomas more frequently expressed HER2 protein, demonstrating focal to strong membranous staining in 8 of the 17 cases. The difference was statistically significant. Despite the increased expression of HER2, no gene amplification was detected by FISH analysis in any of the tumors. Therefore, the authors concluded that it is unlikely that patients with advanced TETs would benefit from trastuzumab treatment (10).

Kojika et al. analyzed surgically removed thymic tumor samples from 7 cases of type B3 thymoma, 68 cases of other thymomas, and 12 cases of thymic carcinoma, using tissue microarray (11). To phenotypically distinguish between B3 thymomas and thymic carcinomas, the researchers employed a comprehensive panel of immunohistochemical markers. HER2 immunostaining was deemed positive when more than 20% of the neoplastic cells presented intermediate or strong staining. There was no immunohistochemical expression (staining score 0) of HER2 in any of the tumors (11).

The Eastern Cooperative Oncology Group examined the expression of three markers (EGFR, C-kit, and HER2) in patients with advanced or recurrent non-resectable TET treated in the E1C97 trial (12). This phase II study aimed to evaluate the effectiveness of octreotide plus prednisone in patients with thymoma who had positive pretreatment octreotide scans. Histological samples from 34 patients (31 with thymomas and 3 with thymic carcinomas) were available and suitable for immunohistochemical analysis. Unlike the other markers that showed immunopositivity, HER2 immunoreactivity was consistently negative across all evaluated tumors (12).

In 2012, Weissferdt et al. studied the expression of HER family receptors and their ligands [epidermal growth factor (EGF), transforming growth factor-α (TGF-α), amphiregulin, and epiregulin] in 24 primary thymic squamous cell carcinomas (13). They also examined EGFR and HER2 gene amplification by FISH. The immunohistochemical analysis revealed that the receptors epidermal growth factor receptor (EGFR) (33.3%), phosphorylated epidermal growth factor receptor (pEGFR) (33.3%), HER2 (58.3%), and HER3 (45.8%), along with the ligands TGF-α (54.2%), amphiregulin (25.0%), and epiregulin (91.7%), were expressed in the tumor cells (13). Conversely, while one case showed HER2 gene amplification, none of the cases exhibited EGFR gene amplification. The authors concluded that although amplification of EGFR and HER2 is rare in thymic carcinoma, the results indicate that HER receptor and ligand protein expression is common, suggesting that targeted therapy against these molecules might be a potential treatment option (13).

Omatsu et al. conducted an immunohistochemical analysis in 44 patients with TET (14). There were 22 patients with thymoma and 22 patients with thymic carcinoma. HER2 expression was positive in only one case of thymic carcinoma out of 22, whereas there was no expression observed in any of the thymoma cases (0/22).

Mimae et al. evaluated 140 TET samples (103 thymomas and 37 thymic carcinomas) for HER2 expression and gene amplification using bright-field in situ hybridization (BISH). No HER2 expression was found in any of the tumors. Similarly, no cases were BISH-amplified for HER2 (15).

The researchers of the CUSTOM trial aimed to determine molecular biomarkers and assess their prevalence and clinical significance in patients with advanced non-small cell lung cancer, small cell lung cancer, and thymic neoplasms (16). More specifically, 98 patients with thymic malignancies (41 thymomas, 48 thymic carcinomas, and 9 other histologies) were enrolled in this group. Gene amplification of HER2 was assessed. HER2 mutations were not demonstrated in any of the patients with thymic tumors. HER2 amplification was detected in one patient (16).

Tiseo et al. retrospectively analyzed the mutational status of several targetable oncogenes, including HER2, in 112 TET (87 thymomas, 20 thymic carcinomas, and 5 thymic neuroendocrine tumors). All tumor samples tested for EGFR, KRAS, BRAF, PDGFR-alpha and -beta, HER2, and c-MET were wild-type (17).

In the series of Enkner et al., a molecular analysis was performed in 37 thymomas (18 type A and 19 type B3) and 37 thymic carcinomas. Immunohistochemical analysis of the tumor cells showed no HER2 expression (18).

Diagnostic and therapeutic implications

Diagnostic tools

HER2 status in TET has been evaluated by next-generation sequencing (NGS), immunohistochemistry and FISH (10-18). Given the different biological significance of protein expression and gene amplification or mutation, applying the appropriate diagnostic methods is crucial for accurate assessment.

Protein expression is the process through which a gene’s information is used to produce its corresponding protein. Its mechanism involves DNA being transcribed into mRNA, which is then translated into protein. It determines the amount of functional protein in the cell, regardless of gene copy number (6). The discrepancies demonstrated in the studies taken into account in this review paper could be due to the differences in immunostaining scores (membrane staining vs overall staining) and in the different antibodies used, even though the clones 4B5 and CB11 were the antibodies that were used in the studies reported herein (when mentioned).

Gene amplification can cause protein overexpression, but protein levels can also rise without gene amplification due to regulatory changes or environmental factors (6-8). Recognizing the difference is essential because both processes can drive disease progression but require different treatments. Gene amplification is an increase in the number of copies of a specific gene within a cell’s genome. It occurs through DNA replication errors, gene duplications, or chromosomal rearrangements (6). As a result, having more copies of the gene can lead to a higher potential for transcription, which may increase protein production—though not always. For example, amplification of the ERBB2 gene in breast cancer can cause aggressive tumor growth due to elevated receptor levels (7-9,20,21).

There are studies using NGS to unravel the genomic profile of TET (22-24). Szpechcinski et al. detected two single nucleotide variants in the ERBB2 gene, one considered pathogenic and the other with uncertain clinical significance. On the other hand, Girard et al. demonstrated clinically relevant genomic alteration in the ERBB2/3 genes.

Targeted therapies

Although HER2-targeted therapies such as trastuzumab and pertuzumab have revolutionized the treatment of HER2-amplified breast and gastric cancers, their role in TET remains experimental (6-18,20,21). The presence of HER2 protein expression without gene amplification raises questions about the effectiveness of these agents in TET. Nevertheless, emerging therapies, including antibody-drug conjugates [e.g., T-DM1 (trastuzumab emtansine), T-DXd (trastuzumab deruxtecan)] and pan-HER inhibitors, may provide new treatment options (25,26), and for that reason immunohistochemical analysis still needs to be performed More specifically, T-DM1 targets HER2-positive breast cancer, combining trastuzumab (HER2-targeted antibody) with DM1 (a microtubule inhibitor). Its mechanism of action consists in the internalization of T-DM1, after binding to HER2, and the release of DM1 inside the cell, disrupting cell division. Its administration is approved for metastatic HER2-positive breast cancer and as adjuvant therapy in early-stage disease.

On the other hand, T-DXd targets HER2-positive (amplified) and HER2-low breast cancers. This agent consists of trastuzumab linked to deruxtecan (a topoisomerase I inhibitor). It administers a powerful active compound with an elevated drug-to-antibody ratio and a detachable linker, facilitating a collateral cytotoxicity, eliminating thus adjacent tumor cells despite low HER2 expression (25,27). Its effectiveness in breast cancer with low HER2 expression (no gene amplification and no +3 immunopositivity needed) opens the door to its use in other cancer types. In many countries, T-Dxd is currently approved as a treatment option in patients with HER2-mutant NSCLC and studies including HER2 protein expression as a patient selection biomarker are still ongoing (28). Research about clinical implications of HER2 in TET is thus justified. There are already trials (NCT06248515) designed to explore the efficacy of other antibody-drug conjugates (the Trop-2-directed sacituzumab govitecan-hziy) in TETs (advanced thymomas and thymic carcinomas), thus leading the way for the development treatments based on HER2 (29).

Challenges and future directions

The rarity and histological heterogeneity of TET render the conduction of large-scale research and clinical trials difficult. As this review demonstrates, there are only a limited number of studies reporting on HER2 expression and amplification in TET. These studies are generally conducted in small patient series, including heterogeneous histological types of TET and applying different scoring systems which complicate drawing pertinent conclusions. It is also possible that like other cancers, tumors can develop resistance to HER2-targeted therapies (25). Future research in larger series of TET is needed to further explore HER2 protein expression in order to elucidate its potential as a therapeutic target for antibody-drug conjugates in patients with unresectable/recurrent TET. The role of HER2 in the interactions with the tumor microenvironment should also be explored (6).

Conclusions

For patients with unresectable TET, currently no targeted treatment options are available. Studies on the prevalence of HER2 alterations, a common treatment target in other types of cancer, are sparse. HER2 plays a still undefined role in TETs, especially in thymic carcinoma. Although gene amplification is rare, protein expression is more frequent, even though not consistent among the available studies, suggesting potential for targeted therapy. Further studies are needed to determine the therapeutic potential of HER2 in these rare cancers. International collaboration, database enrollment, and sharing could positively contribute to the research of treatment targets in TET.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://med.amegroups.com/article/view/10.21037/med-2025-1-48/rc

Peer Review File: Available at https://med.amegroups.com/article/view/10.21037/med-2025-1-48/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://med.amegroups.com/article/view/10.21037/med-2025-1-48/coif). J.M.H.H. serves as an unpaid editorial board member of Mediastinum from August 2024 to July 2026. J.M.H.H. received 2 KOTK grants for the study of lung cancer and malignant mesothelioma, and funding from BOF for research in lung transplantation. P.E.V.S. received consulting fees, honoraria for lectures, and participated in advisory boards for AstraZeneca, BMS, MSD, Roche, and Janssen. He was also past-president of IASLC. The other authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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