Injury-associated induction of two novel and replication-defective murine retroviral RNAs in the liver of mice

The prevalence of human endogenous retroviruses in the plasma of major burn patients

BACKGROUND

Approximately 8% of the human genome is composed of retroviral sequences, which are known as human endogenous retroviruses (HERVs) and, have been implicated in both health status and disease. Recently, indirect evidence for a possible role of retroviral elements in the systemic response to stress signals has been provided by several studies. In the present study, we sought to evaluate the relationship between HERVs and major burn in humans.

METHOD

We investigated the prevalence of HERV families by reverse transcriptase PCR (RT-PCR) in cell-free plasma samples from patients with burns and from normal individuals.

RESULTS

Different prevalences of HERV families were observed in the plasma samples from the burn patient group and normal group. Compared with the prevalences of HERV-W and HERV-K in the normal group, in the burn patient group, the prevalence of HERV-W was significantly lower (P<0.001), but the prevalence of HERV-K was higher (P=0.059).

CONCLUSIONS

Our study of the prevalences of HERVs revealed that the activation of certain HERV families may be influenced not only by burns but also by the initial treatments that were used to address these injuries.

Tropism, cytotoxicity, and inflammatory properties of two envelope genes of murine leukemia virus type-endogenous retroviruses of C57BL/6J mice

Envelope (env) proteins of certain endogenous retroviruses (ERVs) participate in various pathophysiological processes. In this study, we characterized pathophysiologic properties of two murine leukemia virus-type ERV (MuLV-ERV) env genes cloned from the ovary of C57BL/6J mice. The two env genes (named ENV_OV1 and ENV_OV2), with 1,926 bp coding region, originated from two MuLV-ERV loci on chromosomes 8 and 18, respectively. ENV_OV1 and ENV_OV2 were ~75 kDa and predominantly expressed on the cell membrane. They were capable of producing pseudotype murine leukemia virus virions. Tropism trait and infectivity of ENV_OV2 were similar to the polytropic env; however, ENV_OV1 had very low level of infectivity. Overexpression of ENV_OV2, but not ENV_OV1, exerted cytotoxic effects and induced expression of COX-2, IL-1β, IL-6, and iNOS. These findings suggest that the ENV_OV1 and ENV_OV2 are capable of serving as an env protein for virion assembly, and they exert differential cytotoxicity and modulation of inflammatory mediators.

Endogenous retroviruses in systemic response to stress signals

Infection of germline cells with retroviruses initiates permanent proviral colonization of the germline genome. The germline-integrated proviruses, called endogenous retroviruses (ERVs), are inherited to offspring in a Mendelian order and belong to the transposable element family. Endogenous retroviruses and other long terminal repeat retroelements constitute ~8% and ~10% of the human and mouse genomes, respectively. It is likely that each individual has a distinct genomic ERV profile. Recent studies have revealed that a substantial fraction of ERVs retains the coding potentials necessary for virion assembly and replication. There are several layers of potential mechanisms controlling ERV expression: intracellular transcription environment (e.g., transcription factor pool, splicing machinery, hormones), epigenetic status of the genome (e.g., proviral methylation, histone acetylation), profile of transcription regulatory elements on each ERV's promoter, and a range of stress signals (e.g., injury, infection, environment). Endogenous retroviruses may exert pathophysiologic effects by infection followed by random reintegration into the genome, by their gene products (e.g., envelope, superantigen), and by altering the expression of neighboring genes. Several studies have provided evidence that ERVs are associated with a range of pathogenic processes involving multiple sclerosis, systemic lupus erythematosus, breast cancer, and the response to burn injury. For instance, the proinflammatory properties of the human ERV-W envelope protein play a central role in demyelination of oligodendrocytes. As reviewed in this article, recent advances in ERV biology and mammalian genomics suggest that ERVs may have a profound influence on various pathogenic processes including the response to injury and infection. Understanding the roles of ERVs in the pathogenesis of injury and infection will broaden insights into the underlying mechanisms of systemic immune disorder and organ failure in these patients.

Identification of putative endogenous retroviruses actively transcribed in the brain

Remnant proviral sequences in the genome resulting from the ancient germline infection of exogenous retroviruses are called endogenous retroviruses (ERVs). The transcriptional activation of human ERVs (HERVs) in the brain of patients with some neurologic diseases suggests that ERVs may participate in certain disease processes in the central nervous system. In this study, we identified putative murine ERVs (MuERVs) which are transcriptionally active in the brain and characterized their biological properties to better understand the ERVs' roles in the brain pathophysiology. The brain and selective non-nervous tissues (heart, muscle, adrenal gland, and salivary gland) of female C57BL/6J mice were subjected to RT-PCR analyses of MuERV expression by amplifying the 3'-end U3 regions and full-length/subgenomic transcripts. The expression patterns of the U3 regions and subgenomic transcripts in the brain were unique compared to the other tissues as well as the genomic MuERV profile. Two putative MuERVs (8,027 and 5,668 bp) were mapped on the mouse genome (chromosome 10, and chromosomes 4 and 8, respectively) using the MuERV U3 sequences, which were evidently expressed in the brain, as probes. Biological properties of these putative MuERVs, such as transcription potential, primer binding site, coding potential, integration age, recombination, and flanking host genes, were characterized. In particular, one of the two putative MuERV isolates had coding potentials for intact group specific antigen (gag), and truncated polymerase (pol) and envelope (env) polypeptides, while the other was defective for all three polypeptides. The findings from this study suggest that a specific group of MuERVs are constitutively expressed in the brain and they may participate in normal and pathogenic events pertaining to the brain through their replication gene products (e.g., gag and env polypeptides) as well as interactions with flanking host genes.

macroH2A1-dependent silencing of endogenous murine leukemia viruses

We show that macroH2A1 histone variants are important for repressing the expression of endogenous murine leukemia viruses (MLVs) in mouse liver. Intact MLV proviruses and proviruses with deletions in env were nearly silent in normal mouse liver and showed substantial derepression in macroH2A1 knockout liver. In contrast, MLV proviruses with a deletion in the 5' end of pro-pol were expressed in normal liver and showed relatively low levels of derepression in knockout liver. macroH2A1 nucleosomes were enriched on endogenous MLVs, with the highest enrichment occurring on the 5' end of pro-pol. The absence of macroH2A1 also led to a localized loss of DNA methylation on the 5' ends of MLV proviruses. These results demonstrate that macroH2A1 histones have a significant role in silencing endogenous MLVs in vivo and suggest that specific internal MLV sequences are targeted by a macroH2A1-dependent silencing mechanism.

Genome-wide expression profiles of endogenous retroviruses in lymphoid tissues and their biological properties

Endogenous retroviruses (ERVs) constitute approximately 8-10% of the human and mouse genome. Some autoimmune diseases are attributed to the altered expression of ERVs. In this study, we examined the ERV expression profiles in lymphoid tissues and analyzed their biological properties. Tissues (spleen, thymus, and lymph nodes [axillary, inguinal, and mesenteric]) from C57BL/6J mice were analyzed for differential murine ERV (MuERV) expression by RT-PCR examination of polymorphic U3 sequences. Each tissue had a unique profile of MuERV expression. A genomic map identifying 60 putative MuERVs was established using 22 unique U3s as probes and their biological properties (primer binding site, coding potential, transcription regulatory element, tropism, recombination event, and integration age) were characterized. Interestingly, 12 putative MuERVs retained intact coding potentials for all three polypeptides essential for virus assembly and replication. We suggest that MuERV expression is differentially regulated in conjunction with the transcriptional environment of individual lymphoid tissues.

Genome-wide changes in expression profile of murine endogenous retroviruses (MuERVs) in distant organs after burn injury

Background

Previous studies have shown that burn-elicited stress signals alter expression of certain murine endogenous retroviruses (MuERVs) in distant organs of mice. These findings suggest that MuERVs may participate in a network of pathophysiologic events during post-burn systemic response. To gain a better understanding of the biological roles of MuERVs in post-burn systemic response, we examined the genome-wide changes in the MuERV expression profiles in distant organs and the biological properties of the putative-burn related MuERVs were characterized.

Results

Female C57BL/6J mice were subjected to an approximately 18 % total body surface area flame burn and tissues (liver, lung, and kidney) were harvested at 3 hours and 24 hours after injury. The changes in the MuERV expression profiles in these tissues were examined by RT-PCR using a primer set flanking the non-ecotropic MuERV U3 promoter region within the 3' long terminal repeat. There were differential changes in the expression profiles of MuERV U3 regions after injury in all three tissues examined. Subsequently, a total of 31 unique U3 promoter sequences were identified from the tissues of both burn and no burn mice. An analysis of viral tropisms revealed that putative MuERVs harboring these U3 promoter sequences were presumed to be either xenotropic or polytropic. Some putative transcription regulatory elements were present predominantly in U3 promoter sequences isolated from burn and no burn mice, respectively. In addition, in silico mapping using these U3 sequences as a probe against the mouse genome database identified 59 putative MuERVs. The biological properties (coding potentials for retroviral polypeptides, primer binding sites, tropisms, branching ages, recombination events, and neighboring host genes) of each putative MuERV were characterized. In particular, 16 putative MuERVs identified in this study retained intact coding potentials for all three retroviral polypeptides (gag, pol, and env). None of the putative MuERVs identified in this study were mapped to the coding sequences of host genes.

Conclusion

In this study, we identified and characterized putative MuERVs whose expression might be altered in response to burn-elicited systemic stress signals. Further investigation is needed to understand the role of these MuERVs in post-burn systemic pathogenesis, in particular, via characterization of their interaction with host genes, MuERV gene products, and viral activities.

Murine leukemia following irradiation conditioning for transplantation of lentivirally-modified hematopoietic stem cells

Emerging reports are conclusively demonstrating the mutagenic risks involved in using retroviral vectors for gene therapy. Animal studies, as well as cases from a human clinical trial, have proven the potential of insertional leukemogenesis caused by a retroviral vector. Here, we report the observation of six T-lymphoblastic leukemia cases arising during the course of a gene therapy study for hemophilia B after transplantation of ex vivo transduced hematopoietic stem cells (HSCs) by a lentivirus vector. Three of these animals comprised secondary recipients of the same donor and LAM-PCR was performed to identify the vector integration loci. We located integrations in repeat elements of known genes, including a candidate brain-tumor locus, but none of these clones could be tracked in the leukemic blasts. Although transduced clones with an intact proviral cassette were detected in the spleen of the leukemic animals, they comprised a very small proportion, not correlating to the levels of leukemic blasts. After propagation of the latter in NOD/SCID mice, we could no longer detect the proviral cassette suggesting that the leukemic blasts were untransduced. We did, however, detect increased levels of reverse transcriptase activity in the leukemic blasts which may suggest activation of endogenous retroviruses. This study demonstrates that tumors arising in these type of gene therapy protocols are not necessarily due to vector insertional mutagenesis and highlights the importance of careful functional studies to delineate the nature of tumorigenesis.

CD14-dependent Modulation of Transcriptional Activities of Endogenous Retroviruses in the Lung after Injury

Lipopolysaccharide (LPS) plays a central role in the pathogenesis of distant organs after burn. Recent studies demonstrated the regulation of mouse endogenous retroviruses (MuERVs) in several organs after burn. In this study, the role of CD14, a LPS receptor, in burn-mediated regulation of MuERV expression in the lung was investigated. CD14 knockout (KO) and wild type (WT) mice were subjected to burn followed by RT-PCR analysis of alterations in the MuERV expression in the lung 1 day after injury. Even without injury, CD14 KO mice had a unique profile of MuERV expression compared to WT. Three bands (Lung-1, Lung-2, and Lung-3) in CD14 KO were downregulated after injury. Lung-2 and Lung-3 transcripts were almost identical to 2 previously described defective env transcripts of MuERVs, respectively. The Lung-1-1 transcript was a double spliced message generated by the env and a set of novel splicing signals, whereas the Lung-1-2 transcript was a defective env transcript. Only the Lung-1-1 transcript had a significant ORF capable of encoding a gag-pol fusion polypeptide. Putative proviral sequences of Lung-1-1 and Lung-1-2 transcripts were mapped to chromosomes 4 and 11, respectively. The results from this study suggest that the absence of CD14 expression in CD14 KO mice contributes to the transcriptional regulation of MuERVs in the lung after injury.