Diminished humoral responses against and reduced gene expression levels of human endogenous retrovirus-K (HERV-K) in psoriasis

Antibodies against endogenous retroviruses

The human genome harbors hundreds of thousands of integrations of ancient retroviruses, amassed over millions of years of evolution. To reduce further amplification in the genome, the host prevents transcription of these now endogenous retroviruses (ERVs) through epigenetic repression and, with evolutionary time, ERVs are incapacitated by accumulating mutations and deletions. However, several members of recently endogenized ERV groups still retain the capacity to produce viral RNA, retroviral proteins, and higher order structures, including virions. The retention of viral characteristics, combined with the reversible nature of epigenetic repression, particularly as seen in cancer, allow for immunologically unanticipated ERV expression, perceived by the adaptive immune system as a genuine retroviral infection, to which it has to respond. Accordingly, antibodies reactive with ERV antigens have been detected in diverse disorders and, occasionally, in healthy individuals. Although they are part of self, the retroviral legacy of ERV antigens, and association with and, possibly, causation of disease states may set them apart from typical self-antigens. Consequently, the pathogenic or, indeed, host-protective capacity of antibodies targeting ERV antigens is likely to be context-dependent. Here, we review the immunogenicity of typical ERV proteins, with emphasis on the antibody response and its potential disease implications.

Transcriptional Landscape of Repetitive Elements in Psoriatic Skin from Large Cohort Studies: Relevance to Psoriasis Pathophysiology

While studies demonstrating the expression of repetitive elements (REs) in psoriatic skin using RNA-seq have been published before, not many studies have focused on the genome-wide expression patterns using larger cohorts. This study investigated the transcriptional landscape of differentially expressed REs in lesional and non-lesional skin from two previously published large datasets. We observed significant differential expression of REs in lesional psoriatic skin as well as the skin of healthy controls. Significant downregulation of several ERVs, HERVs (including HERV-K) and LINEs was observed in lesional psoriatic skin from both datasets. The upregulation of a small subset of HERV-Ks and Alus in lesional psoriatic skin was also reported. An interesting finding from this expression data was the significant upregulation and overlapping of tRNA repetitive elements in lesional and non-lesional psoriatic skin. The data from this study indicate the potential role of REs in the immunopathogenesis of psoriasis. The expression data from the two independent large study cohorts are powerful enough to confidently verify the differential expression of REs in relation to psoriatic skin pathology. Further studies are warranted to understand the functional impact of these repetitive elements in psoriasis pathogenesis, thereby expanding their significance as a potential targeting pathway for the disease treatment of psoriasis and other inflammatory diseases.

The Immunological Conundrum of Endogenous Retroelements

Our defenses against infection rely on the ability of the immune system to distinguish invading pathogens from self. This task is exceptionally challenging, if not seemingly impossible, in the case of retroviruses that have integrated almost seamlessly into the host. This review examines the limits of innate and adaptive immune responses elicited by endogenous retroviruses and other retroelements, the targets of immune recognition, and the consequences for host health and disease. Contrary to theoretical expectation, endogenous retroelements retain substantial immunogenicity, which manifests most profoundly when their epigenetic repression is compromised, contributing to autoinflammatory and autoimmune disease and age-related inflammation. Nevertheless, recent evidence suggests that regulated immune reactivity to endogenous retroelements is integral to immune system development and function, underpinning cancer immunosurveillance, resistance to infection, and responses to the microbiota. Elucidation of the interaction points with endogenous retroelements will therefore deepen our understanding of immune system function and contribution to disease.

Human Endogenous Retroviruses in Diseases

Human endogenous retroviruses (HERVs), which are conserved sequences of ancient retroviruses, are widely distributed in the human genome. Although most HERVs have been rendered inactive by evolution, some have continued to exhibit important cytological functions. HERVs in the human genome perform dual functions: on the one hand, they are involved in important physiological processes such as placental development and immune regulation; on the other hand, their aberrant expression is closely associated with the pathological processes of several diseases, such as cancers, autoimmune diseases, and viral infections. HERVs can also regulate a variety of host cellular functions, including the expression of protein-coding genes and regulatory elements that have evolved from HERVs. Here, we present recent research on the roles of HERVs in viral infections and cancers, including the dysregulation of HERVs in various viral infections, HERV-induced epigenetic modifications of histones (such as methylation and acetylation), and the potential mechanisms of HERV-mediated antiviral immunity. We also describe therapies to improve the efficacy of vaccines and medications either by directly or indirectly targeting HERVs, depending on the HERV.

Viral dUTPases: Modulators of Innate Immunity

Most free-living organisms encode for a deoxyuridine triphosphate nucleotidohydrolase (dUTPase; EC 3.6.1.23). dUTPases represent a family of metalloenzymes that catalyze the hydrolysis of dUTP to dUMP and pyrophosphate, preventing dUTP from being incorporated into DNA by DNA polymerases, maintaining a low dUTP/dTTP pool ratio and providing a necessary precursor for dTTP biosynthesis. Thus, dUTPases are involved in maintaining genomic integrity by preventing the uracilation of DNA. Many DNA-containing viruses, which infect mammals also encode for a dUTPase. This review will summarize studies demonstrating that, in addition to their classical enzymatic activity, some dUTPases possess novel functions that modulate the host innate immune response.

Expression of human endogenous retroviruses in pemphigus vulgaris patients

Pemphigus is a severe, potentially life-threatening autoimmune blistering mucocutaneous disease which establishes with autoreactive IgG antibodies that target cellular adhesions, precisely extracellular domains of keratinocyte proteins. Several genetic and environmental elements are believed to contribute to the pathogenesis of the disease. The extent to which the initiation and progress of this autoimmune blistering disease may be influenced by the expression of human endogenous retroviruses (HERVs) remains to be elucidated. In this study, we evaluated the expression of HERV groups (K, W, and H) in pemphigus vulgaris (PV) patients in comparison to controls. Peripheral blood samples were collected from 24 PV patients and the corresponding age- and sex-matched healthy controls to extract total RNA for evaluation of HERV-K (HML-2), HERV-W, and HERV-H, env gene expression profile by qPCR. The mRNA expression level of HERV-K, HERV-W, and HERV-H were significantly upregulated in PV patients in comparison to healthy controls (P < 0.0001). The difference in expression of studied HERVs groups between men and women was no significant (P > 0.05). Although rituximab taking patients had a decreased expression level of studied HERVs, the results were not significant (P > 0.05). According to our obtained data, HERVs expression could be measured as a possible diagnostic tool for detection of PV and monitoring of the treatment.

Comparative Expression Analysis of Human Endogenous Retrovirus Elements in Peripheral Blood of Children with Specific Language Impairment

Specific language impairment (SLI) is a psychiatric condition with a complex etiology and a substantial genetic basis that affects children's verbal communication abilities. In this study, we examined the expression of five different human endogenous retrovirus elements (HERVs) in a cohort of 25 children with SLI and 25 healthy children in the control group. Human endogenous retrovirus elements, a diverse group of repetitive DNA sequences, can potentially cause considerable genetic heterogeneity. They had been integrated in the genome of our ancestors throughout evolution and now consist of about 8.0% of the human genome. Several HERV loci are transcribed in various cell types. Their expression in peripheral blood and in the brain is altered in many neurological and psychiatric diseases. To date, HERV expression profiles have never been studied in patients with SLI. This study aimed to elucidate differentially regulated human endogenous retroelements in peripheral blood of children with SLI, in comparison with healthy controls, through quantitative reverse tran-scription-polymerase chain reaction (qRT-PCR) methodology. Our results show that two genes: HERV-K (HLM-2) gag and HERV-P env were expressed at lower levels in the blood samples from SLI children in comparison with those in the control group.

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.

HERV-E expression in peripheral mononuclear cells of patients with psoriasis

BACKGROUND

Psoriasis is a common inflammatory skin disease characterized by uncontrolled proliferation of keratinocytes and recruitment of T lymphocytes into the skin. Possible triggers for psoriasis have been attributed to drugs or pathogens such as bacteria and possibly virus. Human endogenous retroviruses (HERVs) might play a role in triggering these antiviral immune responses, since the role of HERVs in the pathogenesis of autoimmune diseases has generated considerable interest. Some studies have also reported an association of HERV-E and psoriasis. None of them investigate the HERV-E expression in peripheral blood of psoriasis. All these considerations have prompted us to perform a survey for HERV-E expression in PBMC from psoriatic patients.

METHODS

Peripheral blood mononuclear cells from 69 psoriatic patients were analyzed. Total RNA was extracted and amplified with reverse transcription polymerase chain reaction. Results were compared with those obtained in a cohort of 20 healthy donors.

RESULTS

HERV E was expressed in all samples analyzed but the level of expression was much lower in the psoriasis that in HC P<0.0001.

CONCLUSIONS

The reasons for the unexpected, low levels of HERV expression in psoriatic patients are unclear and might be in part a consequence of antiviral defense mechanisms.

HERV Envelope Proteins: Physiological Role and Pathogenic Potential in Cancer and Autoimmunity

Human endogenous retroviruses (HERVs) are relics of ancient infections accounting for about the 8% of our genome. Despite their persistence in human DNA led to the accumulation of mutations, HERVs are still contributing to the human transcriptome, and a growing number of findings suggests that their expression products may have a role in various diseases. Among HERV products, the envelope proteins (Env) are currently highly investigated for their pathogenic properties, which could likely be participating to several disorders with complex etiology, particularly in the contexts of autoimmunity and cancer. In fact, HERV Env proteins have been shown, on the one side, to trigger both innate and adaptive immunity, prompting inflammatory, cytotoxic and apoptotic reactions; and, on the other side, to prevent the immune response activation, presenting immunosuppressive properties and acting as immune downregulators. In addition, HERV Env proteins have been shown to induce abnormal cell-cell fusion, possibly contributing to tumor development and metastasizing processes. Remarkably, even highly defective HERV env genes and alternative env splicing variants can provide further mechanisms of pathogenesis. A well-known example is the HERV-K(HML2) env gene that, depending on the presence or the absence of a 292-bp deletion, can originate two proteins of different length (Np9 and Rec) proposed to have oncogenic properties. The understanding of their involvement in complex pathological disorders made HERV Env proteins potential targets for therapeutic interventions. Of note, a monoclonal antibody directed against a HERV-W Env is currently under clinical trial as therapeutic approach for multiple sclerosis, representing the first HERV-based treatment. The present review will focus on the current knowledge of the HERV Env expression, summarizing its role in human physiology and its possible pathogenic effects in various cancer and autoimmune disorders. It moreover analyzes HERV Env possible exploitation for the development of innovative therapeutic strategies.

Transcriptional landscape of human endogenous retroviruses (HERVs) and other repetitive elements in psoriatic skin

Human endogenous retrovirus (HERV) sequences make up at least 8% of the human genome. Transcripts originating from these loci as well as proteins encoded by them have been detected in various tissues. HERVs are believed to be implicated in autoimmune diseases, however the extent to which, has remained unclear. Differential expression studies have so far been limited to certain HERV subfamilies with conserved sequences. No studies have been published describing the genome-wide expression pattern of HERVs and repetitive elements in the context of psoriasis. In the present study, we analysed total RNA sequencing data from skin samples of 12 psoriasis patients and 12 healthy controls, which enabled us to describe the entire transcriptional landscape of repetitive elements. We report high levels of repetitive element expression in the skin of psoriasis patients as well as healthy controls. The majority of differentially expressed elements were downregulated in lesional and non-lesional skin, suggesting active HERV suppression in the pro-inflammatory environment of psoriatic skin. However, we also report upregulation of a small subset of HERVs previously described in the context of autoimmune diseases, such as members of the HERV-K and W families, with the potential to affect the immunopathogenesis of psoriasis.

TDP-43 regulates endogenous retrovirus-K viral protein accumulation

The concomitant expression of neuronal TAR DNA binding protein 43 (TDP-43) and human endogenous retrovirus-K (ERVK) is a hallmark of ALS. Since the involvement of TDP-43 in retrovirus replication remains controversial, we sought to evaluate whether TDP-43 exerts an effect on ERVK expression. In this study, TDP-43 bound the ERVK promoter in the context of inflammation or proteasome inhibition, with no effect on ERVK transcription. However, over-expression of ALS-associated aggregating forms of TDP-43, but not wild-type TDP-43, significantly enhanced ERVK viral protein accumulation. Human astrocytes and neurons further demonstrated cell-type specific differences in their ability to express and clear ERVK proteins during inflammation and proteasome inhibition. Astrocytes, but not neurons, were able to clear excess ERVK proteins through stress granule formation and autophagy. In vitro findings were validated in autopsy motor cortex tissue from patients with ALS and neuro-normal controls. We further confirmed marked enhancement of ERVK in cortical neurons of patients with ALS. Despite evidence of enhanced stress granule and autophagic response in ALS cortical neurons, these cells failed to clear excess ERVK protein accumulation. This highlights how multiple cellular pathways, in conjunction with disease-associated mutations, can converge to modulate the expression and clearance of viral gene products from genomic elements such as ERVK. In ALS, ERVK protein aggregation is a novel aspect of TDP-43 misregulation contributing towards the pathology of this neurodegenerative disease.

Molecular functions of human endogenous retroviruses in health and disease

Human endogenous retroviruses (HERVs) and related genetic elements form 504 distinct families and occupy ~8% of human genome. Recent success of high-throughput experimental technologies facilitated understanding functional impact of HERVs for molecular machinery of human cells. HERVs encode active retroviral proteins, which may exert important physiological functions in the body, but also may be involved in the progression of cancer and numerous human autoimmune, neurological and infectious diseases. The spectrum of related malignancies includes, but not limits to, multiple sclerosis, psoriasis, lupus, schizophrenia, multiple cancer types and HIV. In addition, HERVs regulate expression of the neighboring host genes and modify genomic regulatory landscape, e.g., by providing regulatory modules like transcription factor binding sites (TFBS). Indeed, recent bioinformatic profiling identified ~110,000 regulatory active HERV elements, which formed at least ~320,000 human TFBS. These and other peculiarities of HERVs might have played an important role in human evolution and speciation. In this paper, we focus on the current progress in understanding of normal and pathological molecular niches of HERVs, on their implications in human evolution, normal physiology and disease. We also review the available databases dealing with various aspects of HERV genetics.