Elevated levels of the endogenous retrovirus ERV3 in human sebaceous glands

A spatial portrait of the human sebaceous gland transcriptional program

Sebaceous glands (SG) and their oily secretion (sebum) are indispensable for maintaining skin structure and function, and their deregulation causes skin disorders including but not limited to acne. Recent studies also indicate that sebum may have important immunomodulatory activities and may influence whole-body energy metabolism. However, the progressive transcriptional changes of sebocytes that lead to sebum production have never been characterized in detail. Here, we exploited the high cellular resolution provided by sebaceous hyperplasia and integrated spatial transcriptomics, pseudo time analysis, RNA velocity, and functional enrichment to map the landscape of sebaceous differentiation. Our results were validated by comparison with published SG transcriptome data and further corroborated by assessing the protein expression pattern of a subset of the transcripts in the public repository Human Protein Atlas. Departing from four sebocyte differentiation stages generated by unsupervised clustering, we demonstrate consecutive modulation of cellular functions associable with specific gene sets, from cell proliferation and oxidative phosphorylation via lipid synthesis to cell death. Both validation methods confirmed the biological significance of our results. Our report is complemented by a freely available and browsable online tool. Our data provide the first high-resolution spatial portrait of the SG transcriptional landscape and deliver starting points for experimentally assessing novel candidate molecules for regulating SG homeostasis in health and disease.

Oncogenic Transformation Drives DNA Methylation Loss and Transcriptional Activation at Transposable Element Loci

Transposable elements (TE) are typically silenced by DNA methylation and repressive histone modifications in differentiated healthy human tissues. However, TE expression increases in a wide range of cancers and is correlated with global hypomethylation of cancer genomes. We assessed expression and DNA methylation of TEs in fibroblast cells that were serially transduced with hTERT, SV40, and HRASR24C to immortalize and then transform them, modeling the different steps of the tumorigenesis process. RNA sequencing and whole-genome bisulfite sequencing were performed at each stage of transformation. TE expression significantly increased as cells progressed through transformation, with the largest increase in expression after the final stage of transformation, consistent with data from human tumors. The upregulated TEs were dominated by endogenous retroviruses [long terminal repeats (LTR)]. Most differentially methylated regions (DMR) in all stages were hypomethylated, with the greatest hypomethylation in the final stage of transformation. A majority of the DMRs overlapped TEs from the RepeatMasker database, indicating that TEs are preferentially demethylated. Many hypomethylated TEs displayed a concordant increase in expression. Demethylation began during immortalization and continued into transformation, while upregulation of TE transcription occurred in transformation. Numerous LTR elements upregulated in the model were also identified in The Cancer Genome Atlas datasets of breast, colon, and prostate cancer. Overall, these findings indicate that TEs, specifically endogenous retroviruses, are demethylated and transcribed during transformation.

SIGNIFICANCE

Analysis of epigenetic and transcriptional changes in a transformation model reveals that transposable element expression and methylation are dysregulated during oncogenic transformation.

CancerHERVdb: Human Endogenous Retrovirus (HERV) Expression Database for Human Cancer Accelerates Studies of the Retrovirome and Predictions for HERV-Based Therapies

In this study, we sought to create a database summarizing the expression of human endogenous retroviruses (HERVs) in various human cancers. HERVs are suitable therapeutic targets due to their abundance in the human genome, overexpression in various malignancies, and involvement in various cancer pathways. We identified articles on HERVs from PubMed and then prescreened and automatically categorized them using the portable document format (PDF) data extractor (PDE) R package. We discovered 196 primary research articles with HERV expression data from cancer tissues or cancer cell lines. HERV RNA and protein expression was reported in brain, breast, cervical, colorectal, endocrine, gastrointestinal, kidney/renal/pelvis, liver, lung, genital, oral cavity, pharynx, ovary, pancreas, prostate, skin, testicular, urinary/bladder, and uterus cancers, leukemias, lymphomas, and myelomas. Additionally, we discovered reports of HERV RNA-only overexpression in soft tissue cancers including heart, thyroid, bone, and joint cancers. The CancerHERVdb database is hosted in the form of interactive visualizations of the expression data and a summary data table at https://erikstricker.shinyapps.io/cancerHERVdb/. The user can filter the findings according to cancer type, HERV family, HERV gene, or a combination thereof and easily export the results with the corresponding reference list. In our report, we provide examples of potential uses of the CancerHERVdb, such as identification of cancers suitable for off-target treatment with the multiple sclerosis-associated retrovirus (MSRV)-Env-targeting antibody GNbAC1 (now named temelimab) currently in phase 2b clinical trials for multiple sclerosis or the discovery of cancers overexpressing HERV-H long terminal repeat-associating 2 (HHLA2), a newly emerging immune checkpoint. In summary, the CancerHERVdb allows cross-study comparisons, encourages data exploration, and informs about potential off-target effects of HERV-targeting treatments. IMPORTANCE Human endogenous retroviruses (HERVs), which in the past have inserted themselves in various regions of the human genome, are to various degrees activated in virtually every cancer type. While a centralized naming system and resources summarizing HERV levels in cancers are lacking, the CancerHERVdb database provides a consolidated resource for cross-study comparisons, data exploration, and targeted searches of HERV activation. The user can access data extracted from hundreds of articles spanning 25 human cancer categories. Therefore, the CancerHERVdb database can aid in the identification of prognostic and risk markers, drivers of cancer, tumor-specific targets, multicancer spanning signals, and targets for immune therapies. Consequently, the CancerHERVdb database is of direct relevance for clinical as well as basic research.

Retroviruses of the Human Virobiota: The Recycling of Viral Genes and the Resulting Advantages for Human Hosts During Evolution

All humans are colonized by a vast diversity of microbes (bacteria, archaea, protozoa, yeast, and fungi; collectively referred to as the microbiota) and viruses (the virobiota). This latter group includes viruses infecting prokaryotic cells (bacteriophages), viruses infecting eukaryotic-host cells, and virus-derived genetic elements present in host chromosomes. Although these eukaryotic viruses are mostly known to be pathogens, they are also able to establish mutualistic relationships with humans. Little is known about the mutualistic aspects of viral infection. Nevertheless, it is clear that evolution of some animal virus-host interactions has led to benefits in the health of the hosts, as is the case with symbiogenesis and endogenization of retroviruses that has exerted a neuroprotective effect on the human brain, and an important role in the fetal development, thus on the evolution of host species. In this review, we summarize how retroviruses provide amazing examples of cooperative-evolution, i.e., successful exchange between viruses and host, and how, in some cases, the benefits have become essential for the hosts’ survival.

Endogenous Retrovirus 3 - History, Physiology, and Pathology

Endogenous viral elements (EVE) seem to be present in all eukaryotic genomes. The composition of EVE varies between different species. The endogenous retrovirus 3 (ERV3) is one of these elements that is present only in humans and other Catarrhini. Conservation of ERV3 in most of the investigated Catarrhini and the expression pattern in normal tissues suggest a putative physiological role of ERV3. On the other hand, ERV3 has been implicated in the pathogenesis of auto-immunity and cancer. In the present review we summarize knowledge about this interesting EVE. We propose the model that expression of ERV3 (and probably other EVE loci) under pathological conditions might be part of a metazoan SOS response.

Human endogenous retrovirus W activity in cartilage of osteoarthritis patients

The etiology of viruses in osteoarthritis remains controversial because the prevalence of viral nucleic acid sequences in peripheral blood or synovial fluid from osteoarthritis patients and that in healthy control subjects are similar. Until now the presence of virus has not been analyzed in cartilage. We screened cartilage and chondrocytes from advanced and non-/early osteoarthritis patients for parvovirus B19, herpes simplex virus-1, Epstein Barr virus, cytomegalovirus, human herpes virus-6, hepatitis C virus, and human endogenous retroviruses transcripts. Endogenous retroviruses transcripts, but none of the other viruses, were detected in 15 out the 17 patients. Sequencing identified the virus as HERV-WE1 and E2. HERV-W activity was confirmed by high expression levels of syncytin, dsRNA, virus budding, and the presence of virus-like particles in all advanced osteoarthritis cartilages examined. Low levels of HERV-WE1, but not E2 envelope RNA, were observed in 3 out of 8 non-/early osteoarthritis patients, while only 3 out of 7 chondrocytes cultures displayed low levels of syncytin, and just one was positive for virus-like particles. This study demonstrates for the first time activation of HERV-W in cartilage of osteoarthritis patients; however, a causative role for HERV-W in development or deterioration of the disease remains to be proven.

Expression of selected human endogenous retroviral sequences in skin and peripheral blood mononuclear cells in morphea

INTRODUCTION

Morphea or localized scleroderma is a relatively rare disease whose main symptom is excessive skin fibrosis. Here we focus on the involvement of human endogenous retroviruses (HERVs) in morphea. The HERVs are a vast and intensely growing field in genomics. HERVs are of special interest as far as autoimmune disorders are concerned, yet little effort has been made until now to assess the possible changes of their expression in morphea.

MATERIAL AND METHODS

Six sequences of particular interest were chosen for this study. Real-time polymerase chain reaction was performed on samples derived from peripheral blood mononuclear cells (PBMCs) and skin biopsies. The results were normalized to the level of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) transcription.

RESULTS

In PBMCs we found a statistically significant decrease of transcription of HERV-E pol, while HERV-K env, HERV-R pol-env, and HERV-W env were found to be up-regulated. In skin biopsies HERV-K env was strongly up-regulated. On the other hand, we noted a decrease of transcription of HERV-H env 62, HERV-K10 gag, HERV-R pol-env, and HERV-W env. In PBMCs we found a statistically significant decrease of transcription of HERV-E pol (-81.8%, p < 0.001), while HERV-K env (+94.1%, p = 0.010), HERV-R pol-env (+140.0%, p < 0.001), and HERV-W env (+97.7%, p < 0.001) were found to be up-regulated. In skin biopsies HERV-K env was strongly up-regulated (+713.0%, p = 0.003). On the other hand, we noted a decrease of transcription of HERV-H env 62 (-83.5%, p < 0.001, HERV-K10 gag (-33.7%, p = 0.044), HERV-R pol-env (-71.3%, p < 0.001), and HERV-W env (-59.3%, p = 0.029).

CONCLUSIONS

The studied HERV sequences generally show an increase of transcription in PBMCs of morphea patients, while being down-regulated in their skin, with some exceptions for both types of tissue.

Human endogenous retrovirus-R Env glycoprotein as possible autoantigen in autoimmune disease

It has long been discussed whether endogenous retroviruses (ERVs) are involved in the pathogenesis of autoimmune diseases. Among various human endogenous retroviruses (HERVs), we have focused on HERV-R. To investigate the biological roles of HERV-R, we earlier established transgenic rats carrying the full sequence of the viral genome. In these HERV-R rats, however, no disease occurred. Another trigger that induces autoimmunity may be essential for the recognition of HERV-R products by the immune system. Thus, in this study, we mated HERV-R rats with env-pX rats (transgenic rats carrying the env-pX gene of human T cell leukemia virus type I) that develop autoimmune diseases, and generated double transgenic (DTG) rats. In DTG rats, autoimmune diseases occurred similarly in env-pX rats. Interestingly, deposition of rat IgM but not IgG was observed on the glomerular endothelial cells. Such IgM deposition was never seen in the parental HERV-R or env-pX rats. We considered that in situ formation of immune complexes consisted of the HERV-R env glycoprotein and anti-HERV-R env IgM antibodies (Abs) in DTG rats, according to the following evidence: (1) No dense deposit, representing deposition of circulating immune complexes, was seen on glomerular endothelial cells. (2) IgM Abs reactive with HERV-R env glycoprotein were generated in the serum. (3) HERV-R env glycoprotein was expressed in the kidney, specifically on glomerular endothelial cells. (4) IgM deposition was partly colocalized with the HERV-R env glycoprotein on the glomeruli. These findings strongly suggest that the HERV-R env glycoprotein is recognized as an autoantigen in the host with autoimmune diseases.

Triggering psoriasis: the role of infections and medications

Psoriasis can be provoked or exacerbated by a variety of different environmental factors, particularly infections and drugs. Strong evidence exists for the induction of guttate psoriasis by a preceding tonsillar Streptococcus pyogenes infection, whereas disease exacerbation has been linked with skin and/or gut colonization by Staphylococcus aureus, Malassezia, and Candida albicans. The role, if any, of viruses (papillomaviruses, HIV, and endogenous retroviruses) present in lesional skin is at present unknown. The use of various drugs, such as lithium, beta-blockers, antimalarial agents, nonsteroidal anti-inflammatory drugs, and angiotensin-converting enzyme inhibitors, has also been associated with induction or worsening of disease in psoriatic patients.

Expression of multiple human endogenous retrovirus surface envelope proteins in ovarian cancer

Individual classes of human endogenous retrovirus (HERV) genes and proteins are expressed in cancer, but expression of more than one type of HERV is rare. We report here the expression of multiple HERV genes and proteins in ovarian cell lines and tissues. Expression of HERV-K env mRNA was greater in ovarian epithelial tumors than in normal ovarian tissues (N = 254). The expression of this protein on the surface and in the cytoplasm of ovarian cancer cells was confirmed using anti-HERV-K specific antibody by flow cytometric analysis. The frequency of expression of HERV-K env protein in multitissue microarrays (N = 641) was determined by immunohistochemistry and a significant correlation with tumor histotype was found. A significantly increased expression of HERV-K was observed in tumors with low malignant potential and low grade, relative to expression in normal ovarian tissues. The increase in expression of HERV-K env protein took place in a stepwise fashion in serous papillary adenocarcinoma. Interestingly, we found that other classes of HERV env mRNAs, including ERV3 and HERV-E, are expressed in the same ovarian cancer tissues that expressed HERV-K. Furthermore, anti-HERV antibodies including anti-ERV3 (30%), anti-HERV-E (40%) and anti-HERV-K (55%) were detected in patients with ovarian cancer, but not in normal female controls. HERV env proteins are frequently transcribed and translated in ovarian epithelial tumors, and multiple HERV families are detectable in ovarian cancer. HERV env proteins, and especially those expressed on the cell surface, may serve as novel tumor targets for detection, diagnosis and immunotherapy of ovarian cancer.

Short communication: expression of human endogenous retrovirus-R gene links to differentiation of squamous cells

To investigate the biological roles of human endogenous retrovirus-R (HERV-R) in vivo, we established transgenic rats carrying the full sequence of the viral genome with control of its own long terminal repeat promoter. The Env protein was expressed on the surface of the epidermis of fetal HERV-R transgenic rats on day 10 of gestation. The epidermal Env expression disappeared by day 18 of gestation. After day 18 of gestation, the Env protein was detected in the prickle layer of the esophageal epithelium of transgenic rats. Interestingly, it was not detected in the basal layer of the epithelium, and the expression in the granular layer was weaker than in the prickle layer. These findings suggest that expression of HERV-R is linked not only to the development but also to the differentiation of squamous cells. Next, we examined alterations in the expression of the HERV-R env gene in cultured human squamous cells after exposure to all-trans retinoic acids (ATRA). The env expression was increased by ATRA in a dose-dependent manner, while the expression of transglutaminase 1 (TGM1), a terminal marker for squamous differentiation, was decreased. TGM1 is expressed in the granular layer of the squamous epithelium, and ATRA suppresses the differentiation of cultured squamous cells. Thus, these in vitro data also suggest that HERV-R expression is regulated by a mechanism closely related to the differentiation of squamous cells. This study is the first to demonstrate the association of HERV-R expression and differentiation of squamous cells.

ERV3 and related sequences in humans: structure and RNA expression

The ERV3 locus at chromosome 7q11 is a much studied human endogenous retroviral (HERV) sequence, owing to an env open reading frame (ORF) and placental RNA and protein expression. An analysis of the human genome demonstrated that ERV3 is one of a group of 41 highly related elements (ERV3-like HERVs) which use proline, isoleucine, or arginine tRNA in their primer binding sites. In addition to elements closely related to ERV3, the group included the previously known retinoic acid-inducible element, RRHERVI, also referred to as HERV15, but was separate from the related HERV-E elements. The ERV3-like elements are defective. The only element with an ORF among gag, pro, pol, and env genes was the env ORF of the original ERV3 locus. A search in dbEST revealed ERV3 RNA expression in placenta, skin, carcinoid tumor, and adrenal glands. Expression was also studied with newly developed real-time quantitative PCRs (QPCR) of ERV3 and HERV-E(4-1) env sequences. Results from a novel histone 3.3 RNA QPCR result served as the expression control. QPCR results for ERV3 were compatible with previously published results, with a stronger expression in adrenal gland and placenta than in 15 other human tissues. The expression of the envelope (env) of ERV3 at chromosome 7q11 was also studied by using stringent in situ hybridization. Expression was found in corpus luteum, testis, adrenal gland, Hassal's bodies in thymus, brown fat, pituitary gland, and epithelium of the lung. We conclude that ERV3 env is most strongly expressed in adrenal and sebaceous glands as well as in placenta.

The role of human endogenous retroviruses in trophoblast differentiation and placental development

A major portion of the human genome appears to be of retroviral origin. These endogenous retroviral elements are expressed in a variety of normal tissues and during disease states, such as autoimmune and malignant conditions. Recently, potential roles have been described for endogenous retroviral envelope proteins in normal differentiation of human villous cytotrophoblast into syncytiotrophoblast. This article provides a brief critical review of the current state of knowledge concerning the expression of the env regions of three endogenous retroviral elements: ERV-3, HERV-W, and HERV-FRD. A testable model of villous cytotrophoblast differentiation is constructed, in which a complementary expression of endogenous retroviral envelope proteins initiates hCG production, decreased cell proliferation, and intercellular fusion.

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.

Differential expression of a human endogenous retrovirus E transmembrane envelope glycoprotein in normal, psoriatic and atopic dermatitis human skin

BACKGROUND

Psoriasis is a common inflammatory skin disease characterized by uncontrolled proliferation of keratinocytes and recruitment of T lymphocytes into the skin. The possible role of human endogenous retroviruses (HERVs) in the induction of psoriasis has been suggested, based upon the previous observations of retrovirus-like particles in psoriasis from skin lesional plaques, urine and stimulated lymphocytes.

OBJECTIVES

To investigate the expression of HERV-E transmembrane envelope glycoprotein (HERV-E env) in normal, psoriatic and atopic human skin, and to examine the influence of ultraviolet (UV) B irradiation on HERV-E env expression in normal human epidermal keratinocytes.

METHODS

The analysis was performed on both skin biopsies and organotypic skin cultures using immunofluorescence and Western immunoblotting. UVB irradiation (312 nm) of cultured normal human keratinocytes was performed using a dose of 30 mJ cm(-2).

RESULTS

Positive staining was observed in most of the psoriatic and atopic skin samples, whereas only 15% of the normal skin samples were faintly positive. In addition, the pattern of expression of HERV-E env differed markedly in psoriasis vs. atopy. By Western blotting analysis, two main proteins of 54 and 57 kDa were detected in extracts of normal skin, normal keratinocyte cultures and reconstructed epidermis from psoriatic and normal punch biopsies. An increased level of expression of these proteins was noted in extracts from psoriatic vs. normal reconstructed epidermis. The overexpression of the 57-kDa protein in normal human cultured keratinocytes was dramatically reduced by UVB irradiation.

CONCLUSIONS

These data suggest for the first time that HERV-E env is expressed in normal and pathological human skin. Further studies are now required to elucidate the role of such viral proteins in the pathogenesis of psoriasis.

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.

Tissue specific high level expression of a full length human endogenous retrovirus genome transgene, HERV-R, under control of its own promoter in rats

Human endogenous retrovirus-R (HERV-R) is one of a full length HERV with a long open reading frame in the env region. The env transcripts are expressed in various human tissues. To investigate the biological role of HERV-R in vivo, we established two lines of transgenic rats carrying a full sequence of HERV-R under control of its own long terminal repeat (LTR) promoter. One line with tandem integration of multiple copies of the transgene expressed HERV-R mRNA in various organs with different expression levels and relatively higher in Harderian and submandibular salivary glands. In another line, the transgene was integrated as a single copy in a haploid and the expression was detected only in Harderian and submandibular salivary glands. In the placenta, one of the tissues with high levels of the HERV-R expression in humans, the transcription was evident starting the 12th day after gestation. A rabbit antiserum against synthetic peptides corresponding with the HERV-R env gene sequence led to detection of an 85 kDa product as a glycoprotein in the Harderian glands. While no pathological significance was observed in either line, the transgenic rat may prove to be a suitable model for analyzing the role of HERV-R function in vivo.

Developmental expression of HERV-R (ERV3) and HERV-K in human tissue

The human endogenous retroviruses (HERVs), ERV3 (HERV-R) and HERV-K, are both known to be transcriptionally active in human placenta. In the case of ERV3 there is also indirect evidence for its participation in cellular differentiation. In this study we examined the expression of ERV3 (HERV-R) and HERV-K in human normal fetal tissues by in situ hybridization. The highest level of ERV3 env expression was detected in primitive adrenal cortex. Elevated levels of expression were also found in the following developing tissues: kidneys (tubules), tongue, heart, liver, and central nervous system. Tissue-specific expression was found for HERV-K rec (former cORF) but not for pol/int transcripts. The highest rec expression was found in placenta and levels slightly higher than sense control were found in the rest of the tissues examined. Pol/Int was not possible to quantitate. It appears that ERV3 is expressed in an organ-specific way during embryogenesis and might suggest a possible role in the development and differentiation of human tissues.

Cytokine regulation of env gene expression of human endogenous retrovirus-R in human vascular endothelial cells

To determine whether human endogenous retroviruses are implicated in the pathogenesis of inflammatory vascular diseases of unknown etiology, we examined mRNA expression of a human endogenous retrovirus, HERV-R, which has a long open reading frame in the env region, in cultured human vascular endothelial and smooth muscle cells stimulated in the presence of various cytokines. mRNA of HERV-R was always evident in these cells but not in fibroblastic cells. Levels of expression in vascular endothelial cells were significantly regulated by treatment with tumor necrosis factor-alpha, interleukin (IL)-1alpha, and IL-1beta as up-regulators and interferon-gamma as a down-regulator. These observations are interpreted to mean that HERV-R expression may be up- or down-regulated at sites of inflammation in vessels in vivo and hence may play a pathogenetic role in inflammatory vascular diseases in humans, perhaps similar to endogenous retroviruses in mouse models of polyarteritis nodosa in humans.

Beneficial role of human endogenous retroviruses: facts and hypotheses

Human endogenous retroviruses (HERVs) have recently been suggested as mediators of normal biological processes such as cellular differentiation and regulation of gene expression. Moreover, a direct role for HERVs in pathogenesis and the development of disease is now better appreciated. Elucidation of the mechanisms regulating HERV biology should provide information about fundamental cellular activities and the pathogenesis of multifactorial diseases such as cancer and autoimmune disease. The importance of understanding the roles of HERVs is underscored by the recently obtained insight that activation of endogenous retroviruses poses potential risks following xenotransplantation and in gene therapy using retroviral vectors. Furthermore, HERV-encoded superantigens have recently been implicated as causes of autoimmune disease. This review discusses the established and possible biological roles of HERVs, and proposes hypotheses concerning their involvement as mediators of fundamental cellular responses. We propose that the evolutionary persistence of endogenous retroviruses in the genomes of eukaryotic cells reflects their indispensability in important normal functions in specialized cellular environments. HERVs can also be potentially hazardous through their involvement in the development of disease. In addition, the creation of new retroviruses can occur through recombination, between different HERVs and between HERVs and exogenous retroviruses.

Physiological knockout of the envelope gene of the single-copy ERV-3 human endogenous retrovirus in a fraction of the Caucasian population

ERV-3 is an evolutionarily conserved single-copy human endogenous retrovirus with a coding envelope gene potentially involved in important placental functions. We have investigated the sequence variability of this gene among 150 unrelated Caucasian individuals and found eight polymorphic sites. One of them corresponds to the introduction of a stop codon resulting in the production of a severely truncated ERV-3 envelope protein lacking both the fusion peptide and the immunosuppressive domain of the protein. The stop codon is observed in a homozygous state in approximately 1% of Caucasian individuals without evidence for counterselection, thus precluding the involvement of any essential function of the gene in placental implantation and development. This natural knockout provides a mean to investigate other potential roles for this otherwise highly conserved gene.

Demethylation of ERV3, an endogenous retrovirus regulating the Krüppel-related zinc finger gene H-plk, in several human cell lines arrested during early monocyte development

The activation of H-plk (Human-proviral linked Krüppel), a human Krüppel-related zinc finger gene in organs such as placenta, adrenal cortex, and testis, is probably due to insertion of an endogenous retrovirus, ERV3, upstream of the gene. Several differently spliced transcripts originate from this locus, e.g., a transcript encoding the retroviral envelope protein and a few differentially spliced transcripts encoding both the env and the zinc finger protein. During a screening for zinc finger proteins expressed during monocyte differentiation, two H-plk encoding cDNA clones were isolated from the human monoblast cell line U-937. Northern blot analysis of a panel of human hematopoietic cell lines showed high levels of constitutive expression of this zinc finger transcript in two monocytic cell lines (U-937 and THP-1) but not in any of the other cell lines or tissues tested. In addition, the H-plk transcript was upregulated by the phorbolester PMA in U-937 and in an additional monocytic cell line, MonoMac 6. Genomic Southern blot analysis of a panel of hematopoietic cell lines, after cleavage with the methylation sensitive enzyme Xho I, led to the detection of tissue specific demethylation in all three monocytic cell lines. The Xho I site was mapped to a position just downstream of the regulatory region of the endogenous retrovirus. By analysis of the U-937 cell line with two additional restriction enzymes, Nar I and Sma I, the demethylation was shown to affect at least three independent CpG dinucleotides in this region of the gene. In summary, the present data provide evidence for specific demethylation of this genomic region, in cells of monocytic origin, resulting in enhanced transcription of the genetic regions derived from both the env region of the retrovirus and the Krüppel-related zinc finger gene.

Evidence against a role for human T-cell lymphotrophic virus type I (HTLV-I) in the pathogenesis of American cutaneous T-cell lymphoma

We used a standard polymerase chain reaction (PCR)/Southern blot assay (sensitivity > 10(-5)) to detect human T-cell lymphotrophic virus type I (HTLV-I) proviral pX, pol, and env genes in the lesional skin of 42 American patients with cutaneous T-cell lymphoma (CTCL). As in some prior reports using similar methods, a variable proportion of PCR tests were positive (seven of 42 for pX, three of 42 for pol, and two of 37 for env), resulting in an overall positive test rate of 12 of 121 (10%). To determine the significance of these positive test results, we performed several additional studies. D1S80 polymorphism analysis of CTCL cases and HTLV-I PCR analysis of non-CTCL dermatosis controls showed no evidence that positive PCR tests resulted from sample mislabeling, gross HTLV-I contamination, or human endogenous retroviruses. We then modified the standard PCR assay to incorporate ultraviolet (UV) light to destroy low-level PCR contamination. With this modified assay (sensitivity > 10(-5)), only three of 12 previously positive cases were still positive, suggesting that the earlier positives were due to trace contamination of PCR reagents or trace contamination of sample DNA. This interpretation was also supported by: (i) a match between pX and pol sequences cloned from one PCR-positive specimen and the MT4-positive control, (ii) our inability to confirm HTLV-I in any PCR-positive case using genomic dot blotting (sensitivity > 10(-2)), and (iii) negative PCR results when new samples from two of the remaining positive cases were analyzed. Finally, we used our modified UV/ PCR/Southern blot assay to test an additional 28 cases of American CTCL for pX. All of them were negative. Although these studies of 70 cases of American CTCL do not exclude the possibility that another virus is involved in the pathogenesis of this disease, they provide strong evidence against a role for HTLV-I. Furthermore, they emphasize the need for special strategies to control for false-positive PCR tests that can result from even trace levels of contamination with viral DNA. As a consequence, associations between diseases and viruses should be viewed skeptically if they are based primarily on conventional PCR data.

Expression of the endogenous retrovirus ERV3 (HERV-R) during induced monocytic differentiation in the U-937 cell line

ERV3 (HERV-R) is a complete human endogenous retrovirus located on the long arm of chromosome 7. LTR-env-gene-spliced mRNA of 9 and 3.5 Kb is widely expressed in human tissues and cells, but gag-pol mRNA has not been found. Further, the env gp70 gene contains an open reading frame throughout its length and its expression has recently been detected as a full-length protein. The highest expression of ERV3 detected so far is in placenta and the lowest in cytotrophoblasts and choriocarcinoma cell lines. In this report we have studied ERV3 mRNA and protein expression in the human monoblastic cell line U-937 during differentiation into monocytes/macrophages. Differentiation of U-937 cells was induced by 1,25a-dihydroxyvitamin D3 (vitD3), retinoic acid (RA), gamma interferon (IFN-gamma) and phorbol-myristate-acetate (PMA-TPA). The expression of ERV3 env mRNA was found to be differentiation-associated, with high expression detected in the late stages of monocytic development. Using TPA, the expression of ERV3 env was detected as 9- and 3.5-kb transcripts by Northern blotting, as mRNA by in situ hybridization and as a cytoplasmic 65-kDa protein by immunofluorescence and Western blots. Low levels of basal expression were found, with up-regulation of both message and protein at 24 to 48 hr after addition of TPA. Induction with vitD3, IFN-gamma and RA produced higher levels of mRNA at earlier time points. It is concluded that the U-937 cell line represents an excellent model system for further studies to study the relationship between ERV3 expression and cellular differentiation.