Demystified. Human endogenous retroviruses

The role of infections in the emergence of non-communicable diseases (NCDs): Compelling needs for novel strategies in the developing world

The emergence of non-communicable diseases (NCDs) follows multiple aetiological pathways requiring recognition for effective control and prevention. Infections are proving to be conventional, emerging and re-emerging aetiological factors for many NCDs. This review explores the possible mechanisms by which infections induce NCDs citing examples of studies in Africa and elsewhere where NCDs and infections are proposed or confirmed to be causally linked and also discusses the implications and challenges of these observations for science and medicine. The need to re-evaluate and expand early community and individual preventive and control strategies that will lead to reduction and even elimination of NCDs especially in Africa and other developing countries where infections are prevalent is highlighted.

Cloning and transcriptional activity of a novel ovarian-specific promoter from rat retrovirus-like elements

The long terminal repeats (LTRs) are the control centers for retrovirus gene expression, which possess all of the requisite signals. It has been proved that the LTRs of Moloney murine leukemia virus (MoMLV) could constitutively activate genes in diverse cell types. Recently, a retrovirus-like element, OSP-1 (ovarian-specific promoter 1), was extracted from rat ovary according to the LTRs of MoMLV, whose name was derived from the fact of ovarian-specific transcription. It was reasonable to speculate that the tissue-specificity was acquired through mutations and that there should be abound other mutants, active or inactive. In the present study, we isolated several homologous sequences to OSP-1 and detected their function. Consequently, one of them could also drive target gene expression specifically to ovarian cell lines and was named OSP-2 which shared 98% similarity to OSP-1. On the other hand, we picked up other two closest sequences and proved them inactive, which was 97% and 95% similar to OSP-1, respectively. Sequence analysis revealed the different mutations around/within the binding sites of transcriptional factors that might play important roles in tissue-specificity. In summary, we extracted a novel ovarian-specific promoter as well as other nonfunctional mutants, which in part shed light on the study of ovarian-specific transcription. In addition, it also provided a new tool in cancer gene therapy and to create transgenic animals.

EBV LMP-2A employs a novel mechanism to transactivate the HERV-K18 superantigen through its ITAM

EBV encodes latent membrane protein (LMP)-2A that functions as a homologue of the activated BCR. We have previously shown that LMP-2A transactivates a human endogenous retrovirus, HERV-K18, in infected B-lymphocytes. The Env protein of HERV-K18 encodes a superantigen that strongly stimulates a large number of T cells. To delineate the mechanism through which LMP-2A transactivates HERV-K18 env, we tested a panel of tyrosine mutants of LMP-2A in a murine B lymphoma that stably harbors HERV-K18. Our analysis revealed that the immunoreceptor tyrosine-based activation motif (ITAM) of LMP-2A is important for HERV-K18 env transactivation. ITAM contains 2 tyrosines that initiate signaling cascades when both residues are phosphorylated. However, in our study, single-tyrosine mutants of ITAM still retained full induction of HERV-K18 env. After truncating 25 kb of genomic sequence downstream of HERV-K18, LMP-2A failed to transactivate HERV-K18 env. Thus, an LMP-2A-inducible element is located downstream of HERV-K18.

Trex1 prevents cell-intrinsic initiation of autoimmunity

Detection of nucleic acids and induction of type I interferons (IFNs) are principal elements of antiviral defense but can cause autoimmunity if misregulated. Cytosolic DNA detection activates a potent, cell-intrinsic antiviral response through a poorly defined pathway. In a screen for proteins relevant to this IFN-stimulatory DNA (ISD) response, we identify 3' repair exonuclease 1 (Trex1). Mutations in the human trex1 gene cause Aicardi-Goutieres syndrome (AGS) and chilblain lupus, but the molecular basis of these diseases is unknown. We define Trex1 as an essential negative regulator of the ISD response and delineate the genetic pathway linking Trex1 deficiency to lethal autoimmunity. We show that single-stranded DNA derived from endogenous retroelements accumulates in Trex1-deficient cells, and that Trex1 can metabolize reverse-transcribed DNA. These findings reveal a cell-intrinsic mechanism for initiation of autoimmunity, implicate the ISD pathway as the cause of AGS, and suggest an unanticipated contribution of endogenous retroelements to autoimmunity.

Human endogenous retrovirus K (HML-2) elements in the plasma of people with lymphoma and breast cancer

Actively replicating endogenous retroviruses entered the human genome millions of years ago and became a stable part of the inherited genetic material. They subsequently acquired multiple mutations, leading to the assumption that these viruses no longer replicate. However, certain human tumor cell lines have been shown to release endogenous retroviral particles. Here we show that RNA from human endogenous retrovirus K (HERV-K) (HML-2), a relatively recent entrant into the human genome, can be found in very high titers in the plasma of patients with lymphomas and breast cancer as measured by either reverse transcriptase PCR or nucleic acid sequence-based amplification. Further, these titers drop dramatically with cancer treatment. We also demonstrate the presence of reverse transcriptase and viral RNA in plasma fractions that contain both immature and correctly processed HERV-K (HML-2) Gag and envelope proteins. Finally, using immunoelectron microscopy, we show the presence of HERV-K (HML-2) virus-like particles in the plasma of lymphoma patients. Taken together, these findings demonstrate that elements of the endogenous retrovirus HERV-K (HML-2) can be found in the blood of modern-day humans with certain cancers.

Variable transcriptional activity of endogenous retroviruses in human breast cancer

Human endogenous retroviruses (HERVs) account for up to 9% of the human genome and include more than 800 elements related to betaretroviruses. While mouse mammary tumor virus (MMTV) is the accepted etiological agent of mammary tumors in mice, the role of retroviral elements in human breast cancer remains elusive. Here, we performed a comprehensive microarray-based analysis of overall retroviral transcriptional activities in 46 mammary gland tissue specimens representing pairs of nonmalignant and tumor samples from 23 patients. An analysis of nonmalignant tissue samples revealed a distinct, mammary gland-specific HERV expression profile that consists of 18 constitutively active HERV taxa. For corresponding tumor samples, a general trend toward lower levels of HERV transcription was observed, suggesting common regulatory mechanisms. In various subsets of patients, however, increased transcript levels of single class I HERV families (HERV-T, HERV-E, and HERV-F) and several class II families, including HML-6, were detected. An analysis of transcribed HML-6 sequences revealed either the activation of some or the increased activity of several proviral loci. No evidence for MMTV or human MMTV-like virus transcripts was found, indicating that transcriptionally active, MMTV analogous, exogenous viruses were not present in the breast cancer samples analyzed.

Induced prion protein controls immune-activated retroviruses in the mouse spleen

The prion protein (PrP) is crucially involved in transmissible spongiform encephalopathies (TSE), but neither its exact role in disease nor its physiological function are known. Here we show for mice, using histological, immunochemical and PCR-based methods, that stimulation of innate resistance was followed by appearance of numerous endogenous retroviruses and ensuing PrP up-regulation in germinal centers of the spleen. Subsequently, the activated retroviruses disappeared in a PrP-dependent manner. Our results reveal the regular involvement of endogenous retroviruses in murine immune responses and provide evidence for an essential function of PrP in the control of the retroviral activity. The interaction between PrP and ubiquitous endogenous retroviruses may allow new interpretations of TSE pathophysiology and explain the evolutionary conservation of PrP.

Comparative longitudinal studies of HERV-K and HIV-1 RNA titers in HIV-1-infected patients receiving successful versus unsuccessful highly active antiretroviral therapy

The viral kinetics of HERV-K in HIV-1-infected patients receiving highly active antiretroviral therapy (HAART) is not unknown. HERV-K kinetic modeling may provide insight into factors altering the effectiveness of HAART in suppressing HIV-1 burden. We conducted a longitudinal study measuring the HERV-K RNA titers in four patients with successful HIV-1-suppressive HAART and in six patients undergoing HAART failure. HERV-K titers were usually undetectable in patients with successful HAART, and when detected, HERV-K titers remained below 5000 copies/ml. On the other hand, HERV-K RNA was consistently detected in patients who failed to respond to HAART before and after HIV-1 rebounds (p < 0.001). Elevated HERV-K RNA titers frequently preceded HIV-1 rebounds. These results suggest that HERV-K viral load may predict HIV-1 reactivation. HERV-K RNA testing might be clinically useful in predicting the onset of HIV-1 resistance due to suboptimal antiretroviral drug levels and/or poor adherence to treatment.

Phytoestrogens: perpetrators or protectors?

Phytoestrogens are estrogen-like substances produced by plants that account for some of the constituents present in vegetation that may be responsible for the health benefits of a diet rich in fruit and vegetables. Phytoestrogens have a plethora of different actions that they are capable of exerting on cellular metabolism. This review will focus on some of the major non-estrogen receptor-mediated cellular effects used by phytoestrogens and will draw attention to the fact that while they may have a number of beneficial effects, particularly in offering a protective effect against some hormone-dependent cancers, such as breast and prostate cancer, they may also have possible unfavorable effects by interfering with the functioning of normal cellular activities such as receptor-mediated signal transduction and DNA replication, as well as being genotoxic, mutagenic and promoting the proliferation of some cancer cells.

Insertional polymorphisms: a new lease of life for endogenous retroviruses in human disease

Human endogenous retroviruses (HERVs) result from ancestral infection by infectious viruses over millions of years of primate evolution. Some are transcriptionally active, express proteins and therefore have the potential to cause disease. Here we review the controversial attempts to link them with cancer and autoimmunity. The main difficulty is that most HERVs investigated to date are present at the same locus in 100% of the population. However, a new class of insertionally polymorphic HERV-K family members, present in a minority of individuals, has recently been described. We propose that insertionally polymorphic HERVs could be novel genetic risk factors and hence provide a new lease of life for research into HERVs and disease.

Endogenous retrovirus expression in testis and epididymis

ERVs (endogenous retroviruses), which comprise 8–10% of mouse and human genomes, are present in thousands of copies, ranging in size from complete 9 kb virus to truncated partial sequences. Despite well-documented differential expression of ERVs in normal and diseased tissues, their biological significance remains controversial. Work in this laboratory revealed remarkably high ERV expression in mouse epididymis, but not in testis. Similar early studies revealed expression of human ERV-E4.1 in both testis and epididymis, but expression of other HERVs (human ERVs) was not examined. Using degenerate primers to conserved regions of reverse transcriptase specific for each of nine HERV families, we have detected expression of six HERV families in epididymis and three in testis. Differential HERV expression may reflect the fully differentiated state of epididymal epithelium in contrast with the immature germ cell population in the testis. These two tissues may therefore lay the groundwork not only for understanding the influence of cellular differentiation on HERV expression, but also to reveal HERVs that are routinely exposed to sperm.

Does an apple a day keep the doctor away because a phytoestrogen a day keeps the virus at bay? A review of the anti-viral properties of phytoestrogens

From dengue to herpes and influenza to AIDS, the phytoestrogens that are present in many fruits and vegetables have been shown to exert anti-viral properties. Here we review the various different anti-viral mechanisms employed by phytoestrogens.

Role of endogenous retroviruses in autoimmune diseases

Molecular epidemiologic proof that HERVs and other retroelements are involved in autoimmunity or other disorders is complicated by their large numbers in the human genome. As discussed, most HERVs are no longer functional or active because of the accumulation of mutations, frameshifts, and deletions. Detection or quantification of HERV transcripts that may be pathologically involved in a particular autoimmune disease thus is often compromised by the presence in great excess of related, but nonfunctional, RNA. This phenomenon should not deter active work in the field, although it will require development of improved methods to discriminate accurately between closely related RNA transcripts. Development of improved immunologic methods to precisely identify epitopes on autoantigens or rare self-reactive T-cell clones may further implicate HERVs and the other repetitive elements in regulation of the immune system in health and disease.

HIV-1 infection increases the expression of human endogenous retroviruses type K (HERV-K) in vitro

Antibodies to HERV-K antigens have been linked to HIV-1 infection and expression of HERV-K proteins generates T-cell cytotoxic responses in many cancers. HERV-K RNA and protein abundance was measured in HIV-1-infected and control cells. In vitro exposure of HIV-1 laboratory-adapted and primary isolates on U87MG cells increased the expression of HERV-K RNA in a dose-dependent manner. HERV-K RNA and protein burdens were significantly increased in HIV-1-producing H9 cell lines compared to H9 cells. The expression of HERV-K was synergistically increased in HIV-1-infected PBMCs after stimulation with PMA/ionomycin. Furthermore, the expression of HERV-K in PBMCs, and particularly in CD4(+) T cells, was higher in HIV-1 patients compared to control subjects. The expression of HERV-K might be related to HIV-1 pathogenesis and AIDS-associated cancers.

Detection of HERV-K(HML-2) viral RNA in plasma of HIV type 1-infected individuals

Approximately 8% of the human genome sequence is composed by human endogenous retroviruses (HERVs), most of which are defective. HERV-K(HML-2) is the youngest and most active family and has maintained some proviruses with intact open reading frames (ORFs) that code for viral proteins that may assemble into viral particles. Many HERV-K(HML-2) sequences are polymorphic in humans (present in some individuals but not in others) and probably many others may be unfixed (not inserted permanently in a specific chromosomal location of the human genome). In the present study HIV-1 and HCV-1-positive plasma samples were screened for the presence of HERV-K(HML-2) RNA in an RT-PCR using HERV-K pol specific primers. HERV-K(HML-2) viral RNA sequences were found almost universally in HIV-1(+) plasma samples (95.33%) but were rarely detected in HCV-1 patients (5.2%) or control subjects (7.69%). Other HERV-K(HML-2) viral segments of the RNA genome including gag, prt, and both env regions, surface (su), and transmembrane (tm) were amplified from HERV-K pol-positive plasma of HIV-1 patients. Type 1 and type 2 HERV-K(HML- 2) viral RNA genomes were found to coexist in the same plasma of HIV-1 patients. These results suggest the HERV-K(HML-2) viral particles are induced in HIV-1-infected individuals.

Methylation of endogenous human retroelements in health and disease

Retroelements constitute approximately 45% of the human genome. Long interspersed nuclear element (LINE) autonomous retrotransposons are predominantly represented by LINE-1, nonautonomous small interspersed nuclear elements (SINEs) are primarily represented by ALUs, and LTR retrotransposons by several families of human endogenous retroviruses (HERVs). The vast majority of LINE and HERV elements are densely methylated in normal somatic cells and contained in inactive chromatin. Methylation and chromatin structure together ensure a stable equilibrium between retroelements and their host. Hypomethylation and expression in developing germ cells opens a "window of opportunity" for retrotransposition and recombination that contribute to human evolution, but also inherited disease. In somatic cells, the presence of retroelements may be exploited to organize the genome into active and inactive regions, to separate domains and functional regions within one chromatin domain, to suppress transcriptional noise, and to regulate transcript stability. Retroelements, particularly ALUs, may also fulfill physiological roles during responses to stress and infections. Reactivation and hypomethylation of LINEs and HERVs may be important in the pathophysiology of cancer and various autoimmune diseases, contributing to chromosomal instability and chronically aberrant immune responses. The emerging insights into the pathophysiological importance of endogenous retroelements accentuate the gaps in our knowledge of how these elements are controlled in normal developing and mature cells.

Cutting Edge: Epstein-Barr Virus Transactivates the HERV-K18 Superantigen by Docking to the Human Complement Receptor 2 (CD21) on Primary B Cells

EBV, a ubiquitous human herpesvirus, is the causative agent of infectious mononucleosis and is associated with many carcinomas. We have previously shown that the EBV latent genes LMP-1 and LMP-2A (for latent membrane proteins 1 and 2A), transactivate a human endogenous retrovirus (HERV), HERV-K18, in infected B lymphocytes. The envelope (Env) protein of HERV-K18 encodes a superantigen that strongly stimulates a large number of T cells. In this study we report that HERV-K18 env is transactivated even earlier in the infection process, before the establishment of latency; namely, we found that EBV, through its interaction with its cellular receptor CD21, induces the HERV-K18 env gene in resting B lymphocytes. This transactivation is direct and immediate, as up-regulation of transcripts can be detected within 30 min after EBV exposure. Thus, EBV binding to human CD21 on resting B cells triggers the expression of an endogenous superantigen. The biological significance of this superantigen expression for the EBV life cycle is discussed.

A new Real-Time-RT-PCR for quantitation of human endogenous retroviruses type K (HERV-K) RNA load in plasma samples: increased HERV-K RNA titers in HIV-1 patients with HAART non-suppressive regimens

Viral components of the human endogenous retroviruses type K (HERV-K) have been largely detected in plasma from HIV-1 infected individuals. A Sybr Green Real-Time RT-PCR approach was optimized for detection and quantitation of HERV-K RNA titers in plasma samples using the iCycler technology. The method detected 1000 HERV-K RNA copies/mL of plasma sample. The Intra- and Inter-assay performance revealed a coefficient of variations that ranged from 0.2 to 2.46%, demonstrating accuracy and reproducibility. We quantified the HERV-K RNA load in 20 HIV-1 patients receiving highly active antiretroviral therapy (HAART). We found increased HERV-K RNA titers in patients with non-suppressive HAART (patients who may develop drug-resistance and/or received suboptimal therapeutic doses), compared to suppressive regimens (p < 0.001). HERV-K RNA was not detected in HCV-1 positive or seronegative controls. Sequencing of Real-Time RT-PCR products revealed particular HERV-K subtypes activated in the HIV-1 infection. The application of this assay could expand the understanding of the role of HERV-K in the HIV-1 infection and others pathological conditions.

The potential role of human endogenous retrovirus K10 in the pathogenesis of rheumatoid arthritis: a preliminary study

OBJECTIVE

To examine whether human endogenous retrovirus K10 is associated with autoimmune rheumatic disease.

DESIGN

A novel multiplex reverse transcription polymerase chain reaction (RT-PCR) system was developed to investigate HERV-K10 mRNA expression in patients with rheumatoid arthritis.

METHODS

40 patients with rheumatoid arthritis, 17 with osteoarthritis, and 27 healthy individuals were recruited and total RNA was extracted from peripheral blood mononuclear cells (PBMCs) and analysed using multiplex RT-PCR for the level of HERV-K10 gag mRNA expression. Southern blot and DNA sequencing confirmed the authenticity of the PCR products.

RESULTS

Using the histidyl tRNA synthetase (HtRNAS) gene as a housekeeping gene in the optimised multiplex RT-PCR, a significantly higher level of HERV-K10 gag mRNA expression was found in rheumatoid arthritis than in osteoarthritis (p = 0.01) or in the healthy controls (p = 0.02).

CONCLUSION

There is enhanced mRNA expression of the HERV-K10 gag region in rheumatoid arthritis compared with osteoarthritis or healthy controls. This could contribute to the pathogenesis of rheumatoid arthritis.

Aberrant transcription from an unrelated promoter can result in MDR-1 expression following drug selection in vitro and in relapsed lymphoma samples

The development of drug resistance in the treatment of cancer remains a major problem. The hallmark of multidrug resistance is cross-resistance to multiple structurally unrelated compounds. The MDR-1 gene encoding P-glycoprotein mediates one of the most extensively studied mechanisms of drug resistance. Previous studies led to the proposal that two promoters control expression of the MDR-1 gene, and these were designated the upstream and downstream promoters. In the present article, we provide evidence that transcripts originating from the putative upstream promoter of MDR-1 are in fact aberrant transcripts whose expression is regulated by nearby genomic sequences that include a human endogenous retroviral long terminal repeat (LTR). Expression of this LTR occurs in all cells. We show that following drug selection, especially in cases where gene amplification has occurred, MDR-1 transcripts can begin near this retroviral LTR with transcription proceeding in the direction opposite of the usual LTR transcription. Because expression of these aberrant MDR-1 transcripts (AMT) is found primarily in drug-resistant cell lines, we conclude that the development of drug resistance or the attendant drug exposure might have a role in the activation of this phenomenon or the selection of cells expressing AMTs. Demonstration of similar aberrant transcripts in tumor samples obtained from patients with relapsed lymphoma suggests that this phenomenon may also occur clinically.

T for two: when helpers need help

This article discusses forms of cell cooperation involving CD4 T cells and their impact on adaptive immunity. In particular, evidence will be presented in favor of cooperation among CD4 T cells of different specificity, its molecular basis and functional significance. Th-Th cooperation, or help for helpers, will be discussed in relation to the induction of anti-tumor responses and, in converse, the generation of autoimmunity. A model is proposed where Th-Th cooperation may represent the structure/function substrate for the control of self/non-self discrimination and peripheral tolerance in vivo.

Characterization of Anti-Myosin Monoclonal Antibodies

The characterization of monoclonal antibodies (MAbs) with regard to reactivity and specificity is important for the successful application as a molecular probe and/or diagnostic reagent. Furthermore, it is recognized that some monoclonal reagents perform well in some assay systems but not others. In this study, the reactivity profiles of two anti-myosin MAbs (H1 and DH2, raised against human cardiac myosin) were evaluated in enzyme-linked immunosorbent assay (ELISA), slot-blotting, and immunocytochemistry. Both antibodies performed well in slot-blotting against myosin heavy chain preparations from cardiac and skeletal muscle and from non-human sources. In general, MAb H1 demonstrated strong to moderate reactivity in all assay systems, whilst MAb DH2 faired poorly in ELISA. MAb H1 also showed reactivity to synthetic peptides of myosin, one of which possessed a motif (ERRDA, single amino acid code) that was found in other human and nonhuman myosin protein sequences that could explain its cross-reactive profile. Intriguingly, this motif was found on viral and other pathogenic agents associated with myocarditis. Hence, it is speculated that this region could give some credence to the mechanism of molecular mimicry associated with some cardiac diseases. Overall, MAb H1 may serve as a useful probe of myosin structure.

A molecular cytogenetic study of chromosome evolution in chimpanzee

We applied multitude multicolor banding (mMCB) in combination with a novel FISH DNA probe set including subcentromeric, subtelomeric and whole chromosome painting probes (subCTM) to characterize a Pan paniscus (PPA) cell line. These powerful techniques allowed us to refine the breakpoints of a pericentric inversion on chimpanzee chromosome 4, and discovered a novel cryptic pericentric inversion in chimpanzee chromosome 11. mMCB provided a starting point for mapping and high resolution analysis of breakpoints on PPA chromosome 4, which are within a long terminal repeat (LTR) and surrounded by segmental duplications, as well as the integration/expansion sites of the interstitial heterochromatin on chimpanzee chromosomes 6 and 14. Moreover, we found evidence at hand for different types of heterochromatin in the chimpanzee genome. Finally, shedding new light on the human/chimpanzee speciation, karyotypes of three members of the genus Pan were studied by mMCB and no cytogenetic differences were found although the phylogenetic distance between these subspecies is suggested to be 2.5 million years.

Causes and consequences of DNA hypomethylation in human cancer

While specific genes are hypermethylated in the genome of cancer cells, overall methylcytosine content is often decreased as a consequence of hypomethylation affecting many repetitive sequences. Hypomethylation is also observed at a number of single-copy genes. While global hypomethylation is highly prevalent across all cancer types, it often displays considerable specificity with regard to tumor type, tumor stage, and sequences affected. Following an overview of hypomethylation alterations in various cancers, this review focuses on 3 hypotheses. First, hypomethylation at a single-copy gene may occur as a 2-step process, in which selection for gene function follows upon random hypo methylation. In this fashion, hypomethylation facilitates the adaptation of cancer cells to the ever-changing tumor tissue microenvironment, particularly during metastasis. Second, the development of global hypomethylation is intimately linked to chromatin restructuring and nuclear disorganization in cancer cells, reflected in a large number of changes in histone-modifying enzymes and other chromatin regulators. Third, DNA hypomethylation may occur at least partly as a consequence of cell cycle deregulation disturbing the coordination between DNA replication and activity of DNA methyltransferases. Finally, because of their relation to tumor progression and metastasis, DNA hypomethylation markers may be particularly useful to classify cancer and predict their clinical course.

How retroviruses select their genomes

As retroviruses assemble in infected cells, two copies of their full-length, unspliced RNA genomes are selected for packaging from a cellular milieu that contains a substantial excess of non-viral and spliced viral RNAs. Understanding the molecular details of genome packaging is important for the development of new antiviral strategies and to enhance the efficacy of retroviral vectors used in human gene therapy. Recent studies of viral RNA structure in vitro and in vivo and high-resolution studies of RNA fragments and protein-RNA complexes are helping to unravel the mechanism of genome packaging and providing the first glimpses of the initial stages of retrovirus assembly.

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.

Human endogenous retroviruses: transposable elements with potential?

Human endogenous retroviruses (HERVs) are a significant component of a wider family of retroelements that constitute part of the human genome. These viruses, perhaps representative of previous exogenous retroviral infection, have been integrated and passed through successive generations within the germ line. The retention of HERVs and isolated elements, such as long-terminal repeats, could have the potential to harm. In this review we describe HERVs within the context of the family of known transposable elements and survey these viruses in terms of superantigens and molecular mimics. It is entirely possible that these mechanisms provide the potential for undesired immune responses.

Demystified...recombinant antibodies

Recombinant antibodies are important tools for biomedical research and are increasingly being used as clinical diagnostic/therapeutic reagents. In this article, a background to humanized antibodies is given, together with details of the generation of antibody fragments--for example, single chain Fv fragments. Phage antibody fragments are fast becoming popular and can be generated by simple established methods of affinity enrichment from libraries derived from immune cells. Phage display methodology can also be used for the affinity enrichment of existing antibody fragments to provide a reagent with a higher affinity. Here, phage antibodies are demystified to provide a greater understanding of the potential of these reagents and to engage clinicians and biomedical scientists alike to think about potential applications in pathology and clinical settings.

Continued colonization of the human genome by mitochondrial DNA

Integration of mitochondrial DNA fragments into nuclear chromosomes (giving rise to nuclear DNA sequences of mitochondrial origin, or NUMTs) is an ongoing process that shapes nuclear genomes. In yeast this process depends on double-strand-break repair. Since NUMTs lack amplification and specific integration mechanisms, they represent the prototype of exogenous insertions in the nucleus. From sequence analysis of the genome of Homo sapiens, followed by sampling humans from different ethnic backgrounds, and chimpanzees, we have identified 27 NUMTs that are specific to humans and must have colonized human chromosomes in the last 4–6 million years. Thus, we measured the fixation rate of NUMTs in the human genome. Six such NUMTs show insertion polymorphism and provide a useful set of DNA markers for human population genetics. We also found that during recent human evolution, Chromosomes 18 and Y have been more susceptible to colonization by NUMTs. Surprisingly, 23 out of 27 human-specific NUMTs are inserted in known or predicted genes, mainly in introns. Some individuals carry a NUMT insertion in a tumor-suppressor gene and in a putative angiogenesis inhibitor. Therefore in humans, but not in yeast, NUMT integrations preferentially target coding or regulatory sequences. This is indeed the case for novel insertions associated with human diseases and those driven by environmental insults. We thus propose a mutagenic phenomenon that may be responsible for a variety of genetic diseases in humans and suggest that genetic or environmental factors that increase the frequency of chromosome breaks provide the impetus for the continued colonization of the human genome by mitochondrial DNA.

DNA from mitochondria has regularly inserted into the human nuclear genome. Some insertions are polymorphic, revealing that the invasion of the human genome is an ongoing process

Retrotransposon-derived elements in the mammalian genome: a potential source of disease

The plethora of genomic information gathered by the sequencing of the human and mouse genomes has paved the way for a new era of genetics. While in the past we focused mainly on the small percentage of DNA that codes for proteins, we can now concentrate on the remainder, i.e. the noncoding sequences that interrupt and separate genes. This portion of the genome is made up, in most part, of repetitive DNA sequences including DNA transposons, long terminal repeat (LTR) retrotransposons, LINEs (long interspersed nuclear elements) and SINEs (short interspersed nuclear elements). Some of these elements are transcriptionally active and can transpose or retrotranspose around the genome, resulting in insertional mutagenesis that can cause disease. In these cases, insertions have occurred in the coding sequence. However, recent evidence suggests that the main effect of these elements is their ability to influence transcription of neighbouring genes. The elements themselves contain promoters that can initiate transcription of flanking genomic DNA. Furthermore, they are susceptible to epigenetic silencing, which is often stochastic and incomplete, resulting in complex patterns of transcription. This review discusses some diseases in both human and mouse that are caused by these repetitive elements.