Previous researchers investigated HERVs as genomic parasites because many HERVs have lost the capacity to form functional viral particles owing to mutations and deletions of coding genes. Nevertheless, more recent studies have shown that HERVs participate in various biological processes in humans and are associated with dysregulation in numerous diseases, particularly malignant tumors [15, 16]. Nonetheless, the study of HERV expression and function in urologic neoplasms remains an area requiring further study. Thus, this paper aims to review the role of HERVs in urologic neoplasms and analyze the feasibility of utilizing HERVs in immunotherapies for urologic neoplasm patients.

Similar to exogenous retroviruses, HERVs possess four core structural genes (env, gag, pol, and pro), flanked at both ends by long terminal repeats (LTRs) harboring regulatory functions [1]. The gag gene encodes the major structural polyprotein Gag, which is processed by host proteases into capsid and nucleocapsid proteins. The env gene encodes envelope (Env) glycoproteins responsible for receptor recognition, binding, and membrane fusion, comprising surface (SU) and transmembrane subunits. The pol gene encodes enzymes crucial for viral replication, including reverse transcriptase (RT) and integrase, facilitating DNA synthesis and integration into the host genome. LTRs contain essential cis-acting regulatory elements, such as promoters and enhancers [17,18,19]. HERVs are phylogenetically classified into three major classes (I, II, III) based on pol gene sequence similarity. Further subdivision into distinct groups relies on the complementarity of their primer binding site sequences to specific transfer RNAs [1, 20, 21].

Although most HERVs have lost transcriptional activity over millions of years of evolution, consensus indicates they can be reactivated by external stimuli and epigenetic dysregulation [4, 20, 22, 23] (Fig. 2). Substantial evidence shows HERVs expression can be upregulated by environmental factors, including chemical compounds, physical agents, and exogenous viral infections.

For instance, exogenous brain injury activates HERVs expression [24], γ-irradiation upregulates HERV-R [25], and ultraviolet C irradiation induces HERV-K expression [26]. Copper ions exhibit a dual role in melanoma, potentially inducing or inhibiting HERV activation depending on concentration [27]. Interferon-γ stimulation promotes HERV expression in tumor cells [28], while silver nanoparticles induce HERV-W expression in a size-dependent manner [29]. However, the precise molecular mechanisms underlying HERV activation by these physical and chemical factors remain poorly defined.

Exogenous viral infections are potent inducers of HERV expression. In multiple sclerosis (MS), Epstein-Barr virus triggers HERV-K transactivation in lymphoblastoid cell lines [30] and may induce early HERV-W expression via activation of human herpesvirus-6A in the central nervous system, contributing to MS pathogenesis [31, 32]. Elevated HERV expression is observed in HIV-1-infected individuals [33, 34]. Specifically, the HIV-1 Tat protein activates HERV-K transcription through the NF-κB and NF-AT pathways [35,36,37]. Latent proteins LANA and vFLIP encoded by Kaposi's sarcoma-associated herpesvirus modulate MAPK/NF-κB signaling and cytokine production, mediating HERV-K activation implicated in Kaposi's sarcoma (KS) development [34, 38, 39]. Human cytomegalovirus (HCMV) infection increases HERV expression in various cells [40]; clinically, HCMV infection associates with elevated HERV-K and HERV-W expression in renal transplant recipients, although the specific mechanisms are undefined [41, 42].

Epigenetic regulation constitutes the primary host mechanism controlling HERV expression levels [43], primarily through DNA methylation and histone modifications [44]. Regulatory proteins and small RNAs also contribute to HERV activation [45, 46].

Accumulating evidence suggests that DNA hypomethylation (demethylation) derepresses HERVs, leading to aberrant expression. Genome-wide studies reveal lower HERV DNA methylation levels in human embryonic stem cells compared to differentiated cells, like fetal lung fibroblasts, correlating with increased transcriptional activity downstream of HERV LTRs [47]. Correspondingly, DNA methylation is recognized as a key mechanism for silencing HERVs and other repetitive elements [48,49,50]. Specifically, in melanoma, LTR hypomethylation correlates with elevated HERV-K expression and a more malignant phenotype [51]. In placental and choriocarcinoma cells, HERV-W promoter methylation inversely correlates with its mRNA and protein expression [52]. Hypomethylation contributes to activated HERV-H expression in head and neck tumors compared to normal tissues [53]. Furthermore, varying degrees of HERV hypomethylation are reported in urothelial, testicular, ovarian, and other malignancies [51, 54, 55], collectively implicating DNA hypomethylation as a major driver of HERV derepression.

Histone acetylation and methylation also significantly impact HERV expression, exhibiting a more complex relationship than DNA methylation [46]. Histone acetylation generally promotes open chromatin and transcriptional activation of associated HERVs [19, 56]. For example, several investigators have induced HERV-H expression in cancer tissue using histone acetylation inhibitors [57]. Furthermore, inhibition of histone deacetylases can interfere with HERV-Fc1 silencing in a variety of human cells, which means that HERV activation and expression are promoted by histone acetylation [58]. The impact of histone methylation on HERVs is more complex than that of histone acetylation, and different methylation types have different effects on gene activity [44, 46, 59]. In B lymphocytes deficient in the histone methyltransferase SETDB1, ERVs are activated in a lineage-specific manner due to the difference in transcription factors that target proviral regulatory elements [60]. However, the silencing mechanism of LTRs could be related to the evolutionary age of LTRs. Unlike early evolutionary LTRs that are mainly silenced by DNA methylation, intermediate-age LTRs may be more sensitive to histone H3 lysine 9 (H3K9) methylation silencing; researchers have shown that the histone methyltransferases EHMT2, SETDB1, and SUV39H1 are involved in LTRs silencing in humans [61]. Of concern, there may be reciprocal links between histone acetylation and histone methylation, and one lysine acetyltransferase, Tip60, can promote histone H3K9 trimethylation by positively regulating the histone methyltransferases SUV39H1 and SETDB1, thereby inhibiting HERV expression in colorectal cancer cells to suppress malignancy development [62]. In summary, studies on the regulatory role of histone modifications in HERVs are still quite limited.

Collectively, epigenetic derepression initiates oncogenic cascades through direct transcriptional reprogramming and innate immune activation (Fig. 3). Direct transcriptional reprogramming: Genome-wide DNA hypomethylation reactivates LTR elements that function as alternative promoters or enhancers for proto-oncogenes. In Hodgkin's lymphoma, THE1B LTR hypomethylation drives lineage-inappropriate expression of the colony-stimulating factor 1 receptor (CSF1R), which is essential for tumor survival [63]. Acute myeloid leukemia exploits derepressed ERV-derived enhancers -- normally silenced by H3K9me3 modifications -- to establish oncogenic transcriptional circuits governing apoptosis resistance [64]. This transcriptional hijacking extends to solid tumors, where LTR hypomethylation activates HERV-K expression in melanoma [51] and reactivates placenta-restricted HERV-W loci in testicular carcinomas [54], with parallel hypomethylation patterns documented in head/neck and ovarian malignancies [53, 55].

Innate immune activation through viral mimicry: Hypomethylation-induced HERV transcripts [53,54,55] or SETDB1/H3K9me3-mediated derepression [59] generate double-stranded RNA (dsRNA) species that engage cytosolic pattern recognition receptors. This activates the mitochondrial antiviral-signaling protein (MAVS) cascade, triggering IRF3-STAT1 phosphorylation and subsequent type I interferon production [28]. While this response recruits dendritic cells and CD8⁺ T lymphocytes-creating an immunogenic microenvironment [28]-it concurrently induces compensatory immunosuppressive checkpoints, including PD-L1 [59]. Therapeutically, WEE1 inhibition exploits this pathway by inducing endogenous retroviral dsRNA to potentiate anti-PD-L1 efficacy [59].

The generation of malignancies is an extremely complex process that results from the activation of oncogenes or the inactivation of tumor suppressor genes due to a series of genetic, epigenetic, or environmental factors [65,66,67]. Mechanistically, these processes originate from epigenetic HERV derepression (Section "Regulatory mechanism of HERVs"), which drives transcriptional hijacking of oncogenes and dsRNA-dependent immune remodeling. Available evidence suggests that HERVs also play a role in malignancy generation through various mechanisms (Fig. 4).

HERVs can activate the expression of oncogenes as alternative promoters [15, 68,69,70]. For example, Hodgkin's lymphoma cells can aberrantly express CSF-1R and are sensitive to CSF-1R inhibitor therapy; thus, the CSF-1R pathway could be a potential target for the treatment of Hodgkin's lymphoma. However, CSF-1R transcription in Hodgkin's lymphoma does not originate from its normal promoter but from an LTR element of the MaLR THE1B family upstream of its normal promoter [63], and the Hodgkin-Reed-Sternberg gene transcribed from activated LTRs plays a role in determining the phenotype of classical Hodgkin lymphoma [71]. HERVs can also enhance the expression of relevant oncogenes. In acute myeloid leukemia, the chromatin profiles of ERVs are thought to include enhancers, and ERVs are thought to be associated with disease phenotypes and cancer progression [64]. The transcribed and translated HERV protein products influence tumor formation by affecting signaling pathways and inducing tumor immune escape [20, 72, 73]. Noncoding RNAs (ncRNAs) from HERVs have the ability to affect genome function [46]. Studies have shown that ncRNAs from HERVs are highly expressed in human triple-negative breast cancer (TNBC), and these ncRNAs bind to the metastasis suppressor ZMYND8, thereby promoting the proliferation and invasion of TNBC and leading to a poor prognosis [74]. In addition, many highly conserved endogenous retroviral-associated adenocarcinoma RNAs are specifically activated in adenocarcinomas and are associated with poor survival rates [75]. Phylogenetic and sequence analyses of HERV-K have shown that at least 16% of the elements undergo significant rearrangements [76], which may contribute to copy number variations generated by nonallelic homologous recombination mediated by HERVs [77]. HERV-mediated insertions may lead to the activation of oncogenes or the disruption of tumor suppressor genes [46, 78], but this mechanism still needs to be further explored and validated. One of the important functions of HERVs is trophoblast cell fusion mediated by synctin-1, which is encoded by HERV-W, and the syncyin-2 protein, which is encoded by HERV-FDR [79, 80] and is essential for normal placental development [81]. In melanoma, the HERV-K-encoded protein can mediate the fusion of melanoma cells, which may give rise to multinucleated cells and cause genetic evolution [82]. HERVs paradoxically shape immunogenicity through epigenetic dsRNA release. While MAVS-IRF3-STAT1 signaling recruits T cells [28], compensatory PD-L1 induction facilitates evasion [59]. Notably, certain HERV proteins (e.g., syncytin-2) directly suppress immunity [83]. In addition, there are many other HERV ENV proteins with immunosuppressive effects [19] whose immunosuppressive mechanisms are unknown, and some studies suggest that they may indirectly reduce the activation and proliferation of T cells by regulating the activity of dendritic cells [84].