Abstract
Human endogenous retroviruses (HERVs) are a remnant of repeated exogenous retroviral infections in human ancestors, which have been integrated into germline cells and proliferated through retrotransposition, recombination, and reinfection. Comprising approximately 8% of the human genome, HERV genes are capable of upregulating the expression of their encoded gene products in response to both endogenous and exogenous stimuli. Among HERV gene products, the envelope (env) proteins are currently extensively investigated for their pathogenic properties in cancer. Given that HERV was initially discovered in the germline cells and the ovary is an essential female reproductive organ, this review will focus on the current knowledge of the role of HERV env protein in ovarian cancer (OC). Our review systematically delineates the expression of HERV env protein across different histological subtypes of OC and highlights its pivotal roles in tumorigenesis and cancer progression. Elucidating the role of HERV env protein in OC offers novel perspectives for developing diagnostic approaches and therapeutic monitoring strategies in OC management.
1 Introduction
Human endogenous retroviruses (HERVs) originate from ancestral exogenous retroviral infections, where the proviral element became stably integrated into the host genome and was subsequently vertically transmitted through germline cells to progeny generations (). In the scenario of germ cell infection, the integrated retroviral element is transmitted in a Mendelian pattern and disseminated across all nucleated cells of the organism (). HERVs became stable components of the human genome, constituting roughly 8% of our DNA (; ). Under normal physiological conditions, HERVs remain transcriptionally silent; however, they can be aberrantly activated in various pathological states, comprising cancers (; ; ; ), neurodegenerative pathologies (; ), and autoimmune diseases (; ). The envelope (env) protein, a key product of HERV activation, is a transmembrane glycoprotein encoded by the env gene within the HERV genome (; ). As shown by multiple studies, aberrant level of HERV env protein has been linked to numerous types of cancer, including ovarian cancer (OC) (Wang-Johanning et al., 2007; Salavatiha et al., 2020; ), breast cancer (Wang-Johanning et al., 2008; ), melanoma (), prostate cancer (), lung cancer (Zare et al., 2018), and colorectal cancer (; ).
Germ cells act as principal vectors for the vertical propagation of HERVs within the human genome, and the ovary—a vital female reproductive organ—plays a central role in oogenesis and hormonal regulation. Nearly 70% of OC has diagnostic recognition at an advanced stage, while the pathogenesis of this disease remains inadequately characterized (). It remains imperative to understand the molecular pathogenesis of OC better, and elucidating the role of HERV in OC may provide novel perspectives into the pathogenic mechanisms underlying this disease. Numerous studies have demonstrated that HERV env protein is maintained at minimal levels in benign or normal ovarian tissues but exhibits significant upregulation in OC (Wang-Johanning et al., 2007; ; ; ). The investigation into the function of HERV env proteins in OC holds promise for uncovering novel early-stage tumor biomarkers and advancing the development of personalized therapeutic strategies (Rycaj et al., 2015; ).
Based on the tissue origin and pathological features, OC is mainly classified into three main categories: epithelial ovarian cancer (EOC), malignant ovarian germ cell tumor (MOGCT), and sex cord-stromal tumors (SCST) (; ). EOC constitutes the predominant histological subtype of OC (∼90%), MOGCT comprises the least OC (3–4%), and SCST is approximately 6% of OC (; ; ). EOC is a heterogeneous disease commonly classified into five major histotypes of invasive disease: high-grade serous carcinoma (HGSOC), low-grade serous carcinoma (LGSOC), mucinous carcinoma (MOC), endometrioid carcinoma (ENOC), and clear cell carcinoma (CCOC) (; Sambasivan, 2022). MOGCT is predominantly categorized into five types: dysgerminoma, yolk sac tumor, immature teratoma, embryonal carcinoma, and mixed germ cell tumors (). With a low prevalence of SCST and the three histologic subtypes of MOGCT—immature teratoma, embryonal carcinoma, and mixed germ cell tumors, the function of HERV env protein in them has been less investigated. Therefore, this review will mainly address the role of HERV env protein in EOC and two main types of MOGCT—dysgerminoma and yolk sac tumor (Table 1).
TABLE 1
| Tumor type | HERV type | Gene | Detection | Sample | Description | References |
|---|---|---|---|---|---|---|
| Epithelial ovarian cancer (EOC) | HEMO | env | Protein | Tissue | The transcriptional activity of HEMO is upregulated in CCOC tissues | |
| HERV-K | env | Protein | Tissue | HERV-K env expression is inversely correlated with OC malignancy potential and histologic grade | Wang-Johanning et al. (2007) | |
| HERV-R | env | Protein | Tissue | HERV-R env protein levels were markedly higher in stage I in contrast to stages II-IV | ||
| HERV-W | Promoter regions | Protein | Tissue | Hypomethylation of HERV-W promoter CpG sites drives env transcriptional activation | ||
| HERV-E, HERV-K, and ERV3 | env | mRNA | Tissue | Higher levels of HERV-E, HERV-K, and ERV3 relative to those in normal ovarian epithelial tissues | Wang-Johanning et al. (2007), | |
| HERV-K | env | Transcript | Tissue | Both HERV-K env spliced Rec and Np9 transcripts were detected in ovarian serous carcinoma tissues | Wang-Johanning et al. (2007) | |
| HERV-K | env | Protein | Cell | High expression levels of HERV-K env protein were observed on the cell surface and in the cytoplasm of EOC cells | Wang-Johanning et al. (2007) | |
| HERV-K | env | Transcript | Cell | Both HERV-K env spliced Rec and Np9 transcripts were detected in EOC cells (DOV13 and SKOV3) | Wang-Johanning et al. (2007) | |
| HERV-E, HERV-K, and ERV3 | env | Antibody | Plasma from patients | Anti-HERV-K env titers were significantly higher than anti-HERV-E and anti-ERV3 | Wang-Johanning et al. (2007),Rycaj et al. (2015) | |
| HERV-K | env | Protein | Ascites-derived cells from patients | Both primary and metastatic ascites-derived cells and ascites samples exhibited elevated HERV-K env protein on their surface | Rycaj et al. (2015) | |
| Dysgerminoma | HERV-K | gag and env | RNA | Tissue | Dysgerminoma tissues shared HERV-K expression of gag and env RNA | |
| HERV-K | gag, pol, and env | RNA transcript | Tissue | High levels of the corresponding RNA transcripts were observed through non-overlapping probes | ||
| Yolk sac tumor | HERV-K | gag and env | RNA | Tissue | HERV-K gag and env RNA are detected in yolk sac tumor tissues | |
| HERV-K | env | mRNA | Cell | Yolk sac tumor cells show an intermediate level of HERV-K env mRNA | ||
| HERV-K | gag and env | Antibody | Plasma from patients | Serum from yolk sac tumor patients exhibited significantly higher anti-HERV-K gag and env antibodies |
Comprehensive overview of HERV env in OC.
2 The structure, classification, and activation of HERV
HERVs are a distinct class of retrotransposons embedded in the human genome and are pivotal agents of genome evolution (; ). The canonical genomic organization of HERVs comprises four core genes—gag, pro, pol, and env—encoding structural and enzymatic proteins, flanked by two LTRs (Xue et al., 2020; ). LTR harbors core promoter and enhancer elements that orchestrate transcriptional regulation of both HERV-derived sequences and adjacent host genes through epigenetic modifications and transcription factors recruitment (; ). The specific functions of the HERV components are shown in (Figure 1a). The primer binding site (PBS) is positioned between 5′LTR and gag, and the polypurine tract (PPT) is located between env and 3′LTR. The gag gene encodes the structural components including capsid, nucleocapsid, and matrix protein. The pro gene encodes a viral protease called dUTPse. The pol gene generates viral enzymes including reverse transcriptase (RT), Ribonuclease H (RNase H), and integrase (; ; ). The env gene encodes env protein consisting of a 55 kDa surface glycoprotein (SU), which determines the specificity of host cell receptor recognition, and a 39 kDa transmembrane (TM) subunit, which is vital in anchoring the viral receptor to the host cell membrane and facilitating the fusion process between viral particles and the host cell, ensuring successful viral entry. TM subunit contains the immunosuppressive domain (ISD) involved in host immune regulation (; ; ) (Figure 1c). The HERV-K env gene is capable of producing not only the env protein but also two proteins of distinct lengths, namely Np9 and Rec, depending on the presence or absence of a 292-bp deletion, proposed to have oncogenic properties (; ; Soleimani-Jelodar et al., 2024) (Figure 1b). Rec is a 14 kDa accessory protein that functionally serves as the Rev and Rex proteins for HIV and HTLV respectively, and Np9 is a 9 kDa protein identical to its first 14 amino acids with HERV-K Rec (; ; ).
FIGURE 1
Due to the lack of proper nomenclature and the continuous increase in the knowledge of HERV, the classification of HERVs has been incomplete for a long time and is constantly being revised. The conventional nomenclature of HERV subtypes refers to the first-letter amino acid code of the tRNA of the primary binding site during the reverse transcription process (e.g., HERV-K for lysine, HERV-H for histidine, and HERV-W for tryptophan) (Tristem, 2000; Vargiu et al., 2016). Nevertheless, this naming approach has notable limitations, particularly when distinct HERV families utilize the same tRNA species (
In most human tissues under physiological conditions, HERV expression remains epigenetically silenced and is generally undetectable at baseline levels (She et al., 2022;
3 The role of HERV env protein in EOC
EOC is the predominant histological subtype of OC, arising from malignant transformation of ovarian surface epithelial cells or fallopian tube epithelium (Siegel et al., 2022). This aggressive malignancy is characterized by frequent late-stage diagnoses and accounts for the majority of gynecological cancer mortality (
Emerging evidence has characterized a novel HERV env protein, designated as HEMO [human endogenous MER34 (medium-reiteration-frequency-family-34) ORF], which illustrates marked upregulation of transcriptional activity in CCOC tissues compared to normal ovarian epithelium and other histologic subtypes of EOC (
Congruent with the elevated expression of HERV env proteins in EOC cells and tissues described above, RT-qPCR analysis of HERV env mRNA in EOC tissues indicated higher levels of HERV-E, HERV-K, and endogenous retroviruses type 3 (ERV3) in contrast to those in normal ovarian epithelial tissues (Wang-Johanning et al., 2007;
To explore the impact of the HERV-K env on the genesis of EOC cell lines, the HERV-K env gene was knocked out in EOC cell lines SKOV3 and OVCAR3 through the CRISPR-Cas9 system, which indicated dramatic attenuation of tumor cell proliferation, migration, and invasion (
A potentially useful source of tumor-associated antigens (TAA) for therapeutic vaccination targeting may be found in the HERV-K env protein. The development of a cancer vaccine for EOC based on HERV-K env protein functioned as TAA revealed that HERV-K env-specific cytotoxic T lymphocytes (CTLs) can elicit robust cytotoxicity towards autologous EOC cells. Compared to benign ovarian disease cells that hardly exhibit expression of HERV-K env, HERV-K env-specific CTLs demonstrated preferential cytotoxicity against autologous EOC cells with HERV-K env expression. Furthermore, PBMCs isolated from EOC patients were found to harbor HERV-K env-specific CTLs, which were capable of mounting cytolytic responses against HERV-K env-positive EOC cells upon in vitro reactivation (Rycaj et al., 2015). A thorough exploration of HERV in EOC was conducted by combining data from The Cancer Genome Atlas (TCGA) with an independent dataset obtained from Hammersmith Hospital. This study uncovered that a distinct HERV expression signature not only serves as a prognostic indicator for HGSOC but also indicates a profound correlation with enhanced effector T cell infiltration within the tumor microenvironment. Furthermore, experiments in vitro suggested that upregulated baseline HERV expression may contribute to increased tumor immunogenicity and potentially influence therapeutic responsiveness to DNA methyltransferase inhibitors (DNMTi). This implies that manipulation of the expression of HERV by DNMTi resulted in improved EOC cell killing by cytotoxic immune cells, and the serum HERV expression scores of EOC patients can predict the level of immune infiltration is anticipated to be one of the most crucial markers for both diagnosis and disease progression tracking (
4 The role of HERV in two main classes of MOGCT
4.1 Dysgerminoma
Dysgerminoma is one of the most common MOGCTs, occurring mainly in women under 30 years of age, with similarities in pathologic features to the classic testicular spermatogonia seminoma (
4.2 Yolk sac tumor
Yolk sac tumor, representing the second most prevalent MOGCTs, are characterized by their complex histopathological composition, including endoderm-like differentiated extra-embryonic tissues, immature embryonic endodermal derivatives, and mesenchymal components (
5 Regulation of HERV env expression-HERV LTR
HERV LTR precisely regulates HERV env expression through its promoters, enhancers, transcription factor binding sites, and epigenetic regulatory mechanisms (
P53 may modulate the transcription of the HERV LTR through direct binding, with its regulatory effects—either activation or repression, depending on the cellular environment and the sequence context of the LTRs.
6 Conclusion and perspectives
Although there are numerous subgroups of HERV-K that have been extensively studied in tumors, international uniform standards of HERV classification have not yet been determined. This makes the advancement of HERV-related research relatively challenging on a global scale. Besides, most studies report significantly upregulated expression of HERV-K env in OC, but few studies indicate no significant difference in HERV-K env expression between OC tissues and normal ovarian tissues (
The pathognomonic overexpression of HERV env protein in cancer cells has emerged as a promising source of biomarkers for cancer diagnosis and therapeutic monitoring (
Statements
Author contributions
JZ: Writing – original draft, Writing – review and editing. DS: Writing – review and editing. YZ: Writing – review and editing. QG: Writing – review and editing. CB: Writing – review and editing. HX: Writing – review and editing. YS: Writing – review and editing. QQG: Writing – review and editing. MZ: Writing – review and editing. JW: Writing – review and editing. LM: Writing – review and editing.
Funding
The author(s) declare that financial support was received for the research and/or publication of this article. This research was funded by the Suzhou Municipal Science and Technology Development Plan (No. SKYD2023002), the open project of Jiangsu Key Laboratory of Medical Laboratory, Jiangsu University (No. JSKLM-Z-2024-008), the Talent Research Project of Suzhou Health Talent Plan (No. GSWS2023005) and the Kunshan First People’s Hospital medical health technology innovation project (No. KET DCX202401).
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
human endogenous retroviruses (HERVs), ovarian cancer (OC), epithelial ovarian cancer (EOC), envelope (env), herv-k
Citation
Zhang J, Sun D, Zhan Y, Gao Q, Bao C, Xiang H, Shen Y, Gao Q, Zhang M, Wang J and Mao L (2025) The role of HERV envelope protein in ovarian cancer. Front. Cell Dev. Biol. 13:1618542. doi: 10.3389/fcell.2025.1618542
Received
26 April 2025
Accepted
22 July 2025
Published
31 July 2025
Volume
13 - 2025
Edited by
Claudia Matteucci, University of Rome Tor Vergata, Italy
Reviewed by
Tara Patricia Hurst, University of Oxford, United Kingdom
Vivek Kumar, University of South Florida, United States
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© 2025 Zhang, Sun, Zhan, Gao, Bao, Xiang, Shen, Gao, Zhang, Wang and Mao.
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*Correspondence: Lingxiang Mao, maolingxiang@aliyun.com
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