Human endogenous retrovirus (HERV)-K transcripts in gonadoblastomas and gonadoblastoma-derived germ cell tumours

Human endogenous retroviruses role in cancer cell stemness

Cancer incidence and mortality, metastasis, drug resistance and recurrence are still the critical issues of oncological diseases. In this scenario, increasing scientific evidences demonstrate that the activation of human endogenous retroviruses (HERVs) is involved in the aggressiveness of tumors such as melanoma, breast, germ cell, renal, ovarian, liver and haematological cancers. In their dynamic regulation, HERVs have also proved to be important determinants of pluripotency in human embryonic stem cells (ESC) and of the reprogramming process of induced pluripotent stem cells (iPSCs). In many types of tumors, essential characteristics of aggressiveness have been associated with the achievement of stemness features, often accompanied with the identification of defined subpopulations, termed cancer stem cells (CSCs), which possess stem cell-like properties and sustain tumorigenesis. Indeed, CSCs show high self-renewal capacity with a peculiar potential in tumor initiation, progression, metastasis, heterogeneity, recurrence, radiotherapy and drug resistance. However, HERVs role in CSCs biology is still not fully elucidated. In this regard, CD133 is a widely recognized marker of CSCs, and our group demonstrated, for the first time, the requirement of HERV-K activation to expand and maintain a CD133+ melanoma cell subpopulation with stemness features in response to microenvironmental modifications. The review will discuss HERVs expression as cancer hallmark, with particular focus on their role in the regulation of cancer stemness features and the potential involvement as targets for therapy.

Vaccination directed against the human endogenous retrovirus-K (HERV-K) gag protein slows HERV-K gag expressing cell growth in a murine model system

Background

Human endogenous retroviruses (HERVs) are remnants of ancestral infections and chromosomally integrated in all cells of an individual, are transmitted only vertically and are defective in viral replication. However enhanced expression of HERV-K accompanied by the emergence of anti-HERV-K-directed immune responses has been observed inter-alia in HIV-infected individuals and tumor patients. Therefore HERV-K might serve as a tumor-specific antigen or even as a constant target for the development of an HIV vaccine.

Results

To verify our hypothesis, we tested the immunogenicity of HERV-K Gag by using a recombinant vaccinia virus (MVA-HK_con) expressing the HERV-K Gag protein and established an animal model to test its vaccination efficacy. Murine renal carcinoma cells (Renca) were genetically altered to express E. coli beta-galactosidase (RLZ cells) and the HERV-K Gag protein (RLZ-HKGag cells). Subcutaneous application of RLZ-HKGag cells into syngenic BALB/c mice resulted in the formation of local tumors in MVA vaccinated mice. MVA-HK_con vaccination reduced the tumor growth. Furthermore, intravenous injection of RLZ-HKGag cells led to the formation of pulmonary metastases. Vaccination of tumor-bearing mice with MVA-HK_con drastically reduced the number of pulmonary RLZ-HKGag tumor nodules compared to vaccination with wild-type MVA.

Conclusion

The data demonstrate that HERV-K Gag is a useful target for vaccine development and might offer new treatment opportunities for cancer patients.

Vaccination directed against the human endogenous retrovirus-K envelope protein inhibits tumor growth in a murine model system

Human endogenous retrovirus (HERV) genomes are chromosomally integrated in all cells of an individual. They are normally transcriptionally silenced and transmitted only vertically. Enhanced expression of HERV-K accompanied by the emergence of anti-HERV-K-directed immune responses has been observed in tumor patients and HIV-infected individuals. As HERV-K is usually not expressed and immunological tolerance development is unlikely, it is an appropriate target for the development of immunotherapies. We generated a recombinant vaccinia virus (MVA-HKenv) expressing the HERV-K envelope glycoprotein (ENV), based on the modified vaccinia virus Ankara (MVA), and established an animal model to test its vaccination efficacy. Murine renal carcinoma cells (Renca) were genetically altered to express E. coli beta-galactosidase (RLZ cells) or the HERV-K ENV gene (RLZ-HKenv cells). Intravenous injection of RLZ-HKenv cells into syngenic BALB/c mice led to the formation of pulmonary metastases, which were detectable by X-gal staining. A single vaccination of tumor-bearing mice with MVA-HKenv drastically reduced the number of pulmonary RLZ-HKenv tumor nodules compared to vaccination with wild-type MVA. Prophylactic vaccination of mice with MVA-HKenv precluded the formation of RLZ-HKenv tumor nodules, whereas wild-type MVA-vaccinated animals succumbed to metastasis. Protection from tumor formation correlated with enhanced HERV-K ENV-specific killing activity of splenocytes. These data demonstrate for the first time that HERV-K ENV is a useful target for vaccine development and might offer new treatment opportunities for diverse types of cancer.

Staufen-1 interacts with the human endogenous retrovirus family HERV-K(HML-2) rec and gag proteins and increases virion production

The human endogenous retrovirus family HERV-K(HML-2) Rec protein is an RNA transport factor that enhances nuclear export of intron-containing retroviral transcripts. Using the yeast two-hybrid approach, we have newly identified human Staufen-1 as a Rec-interacting protein. The interaction was confirmed by coimmunoprecipitation experiments, and the relevant site in Staufen-1 has been mapped to double-stranded RNA binding domain 4 (RBD4). Staufen-1 is in several aspects functionally related to retroviral RNA transport proteins. It binds mRNAs and targets its ribonuclear cargo to polysomes for efficient translation. We observed an accumulation of Staufen-1 in the nucleus of Rec-expressing cells and colocalization in the nucleoli as well as in the cytoplasm. Overexpression of Staufen-1 resulted in a 5-fold enhancement in nuclear export and/or translation of unspliced HERV-K(HML-2) viral RNAs in the presence of Rec and its Rec-responsive element (RcRE) binding site together with a clear increase in virus production. Staufen-1 was previously shown to interact with the Gag protein of HIV-1, promoting Gag oligomerization and RNA encapsidation. We demonstrate here that Staufen-1 also binds to the Gag protein of HERV-K(HML-2). Under stress conditions, Rec colocalizes with Staufen-1 in stress granules in cells that express viral RNA but not in mRNA-decay-related processing bodies. Our results suggest a new role for Staufen-1 as a cellular Rec and HERV-K(HML-2) Gag cofactor.

Reactivation of codogenic endogenous retroviral (ERV) envelope genes in human endometrial carcinoma and prestages: Emergence of new molecular targets

Endometrial carcinoma (EnCa) is the most common invasive gynaecologic carcinoma. Over 85% of EnCa are classified as endometrioid, expressing steroid hormone receptors and mostly involving pathological prestages. Human endogenous retroviruses (ERV) are chromosomally integrated genes, account for about 8% of the human genome and are implicated in the etiology of carcinomas. The majority of ERV envelope (env) coding genes are either not present or not consistently represented between common gene expression microarrays. The aim of this study was to analyse the absolute gene expression of all known 21 ERV env genes including 19 codogenic and two env genes with premature stop codons in EnCa, endometrium as well as in hyperplasia and polyps. For EnCa seven env genes had high expression with >200 mol/ng cDNA (e.g. envH1-3, Syncytin-1, envT), two middle >50 mol/ng cDNA (envFc2, erv-3) and 12 low <50 mol/ng cDNA (e.g. Syncytin-2, envV2). Regarding tumor parameters, Syncytin-1 and Syncytin-2 were significantly over-expressed in advanced stage pT2 compared to pT1b. In less differentiated EnCa Syncytin-1, erv-3, envT and envFc2 were significantly over-expressed. Syncytin-1, Syncytin-2 and erv-3 were specific to glandular epithelial cells of polyps, hyperplasia and EnCa using immunohistochemistry. An analysis of 10 patient-matched EnCa with endometrium revealed that the ERV-W 5' long terminal repeat regulating Syncytin-1 was hypomethylated, including the ERE and CRE overlapping MeCP2 sites. Functional analyses showed that 10 env genes were regulated by methylation in EnCa using the RL95-2 cell line. In conclusion, over-expressed env genes could serve as indicators for pathological pre-stages and EnCa.

The Rec protein of HERV-K(HML-2) upregulates androgen receptor activity by binding to the human small glutamine-rich tetratricopeptide repeat protein (hSGT)

The expression of endogenous retroviruses of the HERV-K(HML-2) family is strongly upregulated in germ cell tumors and several other cancers. Although the accessory Rec protein of HERV-K(HML-2) has been shown to induce carcinoma in situ in transgenic mice, to increase the activity of c-myc and to interact with the androgen receptor (AR), whether or not Rec expression is indeed implicated causally in the initiation or progression of any human malignancies remains unclear. We used the yeast two-hybrid system involving the Rec protein of a recently integrated HERV-K(HML-2) element in an effort to identify potential Rec-related oncogenic mechanisms. This revealed the human small glutamine-rich tetratricopeptide repeat (TPR)-containing protein (hSGT) to be a cellular binding partner. The interaction of Rec with this known negative regulator of the AR was confirmed by coimmunoprecipitation, pull-down assays and colocalization studies. The interaction involves the TPR motif of hSGT and takes place in the cytoplasm and in the nucleoli. Using an AR-responsive promoter and gene we could demonstrate that Rec interference with hSGT resulted in an up to five-fold increase in the activity of AR. Furthermore, in AR positive cells, Rec was shown to act as transactivator by enhancing AR-mediated activation of the HERV-K(HML-2) LTR promoter. This is in line with previous observations of elevated HERV-K(HML-2) expression in steroid-regulated tissues. On the basis of our findings we propose a "vicious cycle" model of Rec-driven hyperactivation of the AR leading to increased cell proliferation, inhibition of apoptosis and eventually to tumor induction or promotion.

Characterization of the human endogenous retrovirus K Gag protein: identification of protease cleavage sites

Background

Viral genomes of the human endogenous retrovirus K (HERV-K) family are integrated into the human chromosome and are transmitted vertically as Mendelian genes. Although viral particles are released by some transformed cells, they have never been shown to be infectious. In general, gammaretroviruses are produced as immature viral particles by accumulation of the Gag polyproteins at the plasma membrane, which subsequently bud from the cell surface. After release from the cell, Gag is further processed by proteolytic cleavage by the viral protease (PR), which results in morphologically mature particles with condensed cores. The HERV-K Gag polyprotein processing and function has not yet been precisely determined.

Results

We generated a recombinant poxvirus, encoding the human endogenous retrovirus K consensus gag-pro-pol genes (MVA-HERV-K_con) and obtained high levels of HERV-K Gag expression. The resulting retroviral particle assembled at the plasma membrane, as is typical for gammaretroviruses; and immature as well as mature retrovirus-like particles (VLPs) were observed around the infected cells. VLPs were purified, concentrated and separated by two-dimensional gel electrophoresis. The HERV-K Gag fragments were identified by mass spectroscopy and N-terminal sequencing which revealed that HERV-K Gag is processed into MA, a short spacer peptide, p15, CA and NC.

Conclusion

The cleavage sites of HERV-K Gag were mapped and found to be highly conserved among HERV-K genomes. The consensus HERV-K gag gene used in this study is known to support viral, infectivity [1], and thus the cleavage sites that were mapped in this study for all the Gag components are relevant for HERV-K infectivity.

Potential mechanisms of endogenous retroviral-mediated genomic instability in human cancer

Malignancy results from a complex combination of genetic and epigenetic changes, the full effects of which are still largely unknown. Here we summarize current knowledge of the origin, retrotranspositional activity, epigenetic state, and transcription of human endogenous retroviruses (HERVs), and then discuss the potential effects of their deregulation in cancer. Evidence suggests that cancer-associated epigenetic changes most likely underlie potential HERV-mediated effects on genome and transcriptome instability and may play a role in malignancy. Despite our currently limited understanding of the importance of HERVs or other transposable elements in cancer development, we believe that the emerging era of high-throughput sequencing of cancer genomes, epigenomes, and transcriptomes will provide unprecedented opportunities to investigate these roles in the future.

Detection of T lymphocytes specific for human endogenous retrovirus K (HERV-K) in patients with seminoma

Human endogenous retrovirus K (HERV-K) is distinctive among the retroviruses that comprise about 8% of the human genome in that multiple HERV-K proviruses encode full-length viral proteins, and many HERV-K proviruses formed during recent human evolution. HERV-K gag proteins are found in the cytoplasm of primary tumor cells of patients with seminoma. We identified HERV-K-specific T cells in patients with a past history of seminoma using the interferon-gamma ELISPOT assay and an MHC-HERV-K peptide-specific tetramer. A minority of apparently healthy subjects without evident germ cell tumors also made HERV-K-specific T cell responses. In summary, we detected T cell reactivity to HERV-K peptides in both past seminoma patients and a minority of apparently healthy controls.

Endogenous retroviruses and animal reproduction

Endogenous retroviruses (ERV), as part of the host genetic heritage, are transmissible to the next generation in a Mendelian way. Their abundance in animal genomes and their expression primarily detected in germ cells, embryonic tissues and cancer cell lines, raised the question of their biological significance. This article reviews the possible role of ERVs in the physiology and diseases of animal reproduction, from Drosophila to human. In males, there is no trivial involvement of ERVs in a physiological process. Conversely, a spermatogenesis defect was associated in the human male with HERV-K expression and HERV15-induced chromosomal alteration, leading to cancer and infertility, respectively. In females, the study of insect ERVs (IERV) pointed out the overlap between genetics and virology with the genetic-dependent regulation of ZAM and the non-infectious and infectious life cycles of gypsy. The pattern of ERVs expression in rodent, ovine and human females suggest a hormone-dependent mechanism consistent with the mammalian oestrus cycle regulation. The differentiation of the mammary epithelium and breast tumorigenesis involving the mouse mammary tumour viruses (MMTV) illustrate the intimate connection between endogenous and exogenous retroviruses. Last, as a major site of ERVs transcription, placenta contributed to our understanding of ERVs modulation of neighbouring gene expression. As an interface, i.e. a site of conflicts and exchanges, placenta should resist infection and protect the foetus against the maternal immune system. Retroviral envelopes could theoretically provide such features due to receptor interference, immunosuppression and fusion properties, as shown by the HERV-W envelope involved in the syncytiotrophoblast formation. We conclude with an insight on the evolutionary and epigenetic consequences of the relationships of ERV guests with their animal hosts.

Of mice, cats, and men: Is human breast cancer a Zoonosis?

Mouse mammary tumor virus (MMTV), a member of the betaretroviridae, is the most common cause of breast cancer (BC) in mice. MMTV is transmitted in mice both in the germline as endogenous proviruses and exogenously as infectious virions. Here, we review a variety of evidence accumulated for six decades that has suggested that a human homologue of MMTV may exist. The findings include recent studies from several independent laboratories that have detected sequences very closely related to MMTV in DNA isolated from human BC tumors. Other laboratories, however, have failed to detect the MMTV-related sequences in human DNA samples, and conclusive evidence for a human mammary tumor virus has been elusive. We also reviewed additional studies, suggesting that betaretroviruses are present in a much wider range of species than previously known, including rodents, felines, and primates. The observation that a subset of cats may be infected with a close homologue of MMTV may be of epidemiological significance for human BC. Cats may become infected by MMTV from mice, and in turn may transmit the virus to humans, possibly after selection for variants with an expanded host range.

Expression and functions of human endogenous retroviruses in the placenta: an update

The placenta is unique amongst normal tissues in transcribing many different human endogenous retrovirus (HERV) families at high levels and this has led to the suggestion that HERVs may fulfil important functions in reproduction. This review discusses our current knowledge of the placental expression of HERVs, in particular the envelope proteins of ERV3 and HERV-W which may have critical roles in placental function.

Expression of the human endogenous retrovirus HERV-W family in various human tissues and cancer cells

We examined the structural genes (gag, pol and env) of the human endogenous retrovirus (HERV-W) family from 12 normal human tissues and 18 human cancer cell lines using RT-PCR. For the gag and pol genes, their expression patterns showed tissue or cell specificity, depending on the samples, whereas the env gene was expressed in all tissues and cancer cells examined. To identify active HERV-W elements in tissues and cancer cells, the RT-PCR products were cloned and sequenced. Ninety-five clones for the env gene, 55 clones for the pol gene and 17 clones for the gag gene of the HERV-W family were isolated from human tissues and sequenced, while 85 clones for the env gene, 61 clones for the pol gene and no clones for the gag gene of the HERV-W family were isolated and sequenced from cancer cells. Among these clones, 50 clones from tissues and 44 from cancer cells showed putative amino acids of the HERV-W env gene, while only two clones from cancer cells showed putative amino acids of the HERV-W pol gene. Phylogenetic analysis indicated that several clones identified previously from human monochromosomes had sister relationships with the clones from the different tissues and cancer cells. These data suggest that HERV-W elements are actively expressed in human tissues and cancer cells. These active HERV-W elements deserve further investigation as potential causative agents of various human diseases including cancers.

Detecting the expression of human endogenous retrovirus E envelope transcripts in human prostate adenocarcinoma

BACKGROUND

The expression of human endogenous retrovirus (HERV) mRNA and proteins was associated recently with diseases that include human malignancies. The authors report that, in the current study, transcripts encoding the envelope region of an HERV family, HERV-E, were expressed in human prostate carcinoma.

METHODS

RNA was isolated from various prostate tissues and was tested for the expression of various HERV envelope (env) genes by reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, RNA in situ hybridization (ISH), and Northern blot analysis. Variants of HERV that appeared in prostate carcinoma tissues were sequenced, and HERV-E was expressed in prokaryotic and eukaryotic systems.

RESULTS

In the current study, the authors found that the mRNA of the env gene of one particular family of HERVs, HERV-E, was expressed in some prostate carcinoma tissues (38.8% positive; n = 49 specimens) but not in normal prostate tissues using RT-PCR, RNA ISH, and Northern blot assays. The expression of HERV-E transcripts in prostate tumor epithelial cells was confirmed further by ISH using an HERV-E specific antisense probe. Approximately 50% of the cDNA of HERV-E obtained from prostate carcinoma specimens contained no stop codon and expressed proteins in prokaryotic or eukaryotic expression systems. Furthermore, the expression of both HERV-E and ERV3 (another class of HERV) was detected in the same prostate carcinoma tissues.

CONCLUSIONS

The expression and distribution of multiple HERV-E endogenous retroviral elements in prostate carcinoma, but not in normal control specimens, suggests that they may serve as novel tumor markers for the early diagnosis and immunotherapy of patients with prostate carcinoma.

Microsatellite instability in gonadal tumors of XY pure gonadal dysgenesis patients

To investigate genetic alternation accompanied by malignant transformation in gonadal tumors of XY pure gonadal dysgenesis patients, we investigated microsatellite instability in the hMSH1, hMSH2, TP53, and DCC loci, and ras mutations in two patients. The gonadal tumors from the patients were combined gonadoblastoma and dysgerminoma. Microsatellite instability and/or loss of heterozygotes (LOH) at hMSH1, hMSH2, and TP53 were detected in the dysgerminoma lesions of the both patients, but were not observed in any normal tissues. In the analyses of the H-, K-, or N-ras genes, where specific mutations have been frequently reported, no mutations were observed in the tumors. It is suggested therefore that microsatellite instability plays an important role in malignant transformation of gonadal tumors in patients with XY pure gonadal dysgenesis.

Reactivity of germ cell maturation stage-specific markers in spermatocytic seminoma: diagnostic and etiological implications

It is generally accepted that testicular seminomas and spermatocytic seminomas have separate pathogeneses, although the origin of these two types of germ cell tumors of the adult testis remains a matter of debate. Although an embryonic germ cell origin seems to be most likely for seminomas, a spermatogonia-spermatocyte origin has been suggested for spermatocytic seminoma. To shed more light on the etiology of spermatocytic seminomas, we undertook an immunohistochemical and molecular approach using SCP1 (synaptonemal complex protein 1), SSX (synovial sarcoma on X chromosome), and XPA (xeroderma pigmentosum type A) as targets. Although a stage-specific expression pattern has been reported for SCP1 and SSX in normal spermatogenesis, we demonstrate here that it also exists for XPA. In fact, immunohistochemistry shows that the proteins of SCP1 and XPA are specifically present in the stage of primary and pachytene spermatocytes. In contrast, SSX was found in spermatogonia and primary spermatocytes, as well as in germ cells, from at least the 17th week of intrauterine development onward. Although no protein encoded by any of these genes was detected in tumor cells of a series of testicular seminomas, all tested spermatocytic seminomas were positive, in agreement with expression analysis. These data support the model that seminomas originate from an embryonic germ cell, and they imply that the cell of origin of spermatocytic seminomas is at least capable of maturing to the stage of spermatogonia-pachytene spermatocyte.