Retroviral recombination: what drives the switch?

The human microbiota is a beneficial reservoir for SARS-CoV-2 mutations

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mutations are rapidly emerging. In particular, beneficial mutations in the spike (S) protein, which can either make a person more infectious or enable immunological escape, are providing a significant obstacle to the prevention and treatment of pandemics. However, how the virus acquires a high number of beneficial mutations in a short time remains a mystery. We demonstrate here that variations of concern may be mutated due in part to the influence of the human microbiome. We searched the National Center for Biotechnology Information database for homologous fragments (HFs) after finding a mutation and the six neighboring amino acids in a viral mutation fragment. Among the approximate 8,000 HFs obtained, 61 mutations in S and other outer membrane proteins were found in bacteria, accounting for 62% of all mutation sources, which is 12-fold higher than the natural variable proportion. A significant proportion of these bacterial species-roughly 70%-come from the human microbiota, are mainly found in the lung or gut, and share a composition pattern with COVID-19 patients. Importantly, SARS-CoV-2 RNA-dependent RNA polymerase replicates corresponding bacterial mRNAs harboring mutations, producing chimeric RNAs. SARS-CoV-2 may collectively pick up mutations from the human microbiota that change the original virus's binding sites or antigenic determinants. Our study clarifies the evolving mutational mechanisms of SARS-CoV-2.

IMPORTANCE

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) mutations are rapidly emerging, in particular advantageous mutations in the spike (S) protein, which either increase transmissibility or lead to immune escape and are posing a major challenge to pandemic prevention and treatment. However, how the virus acquires a high number of advantageous mutations in a short time remains a mystery. Here, we provide evidence that the human microbiota is a reservoir of advantageous mutations and aids mutational evolution and host adaptation of SARS-CoV-2. Our findings demonstrate a conceptual breakthrough on the mutational evolution mechanisms of SARS-CoV-2 for human adaptation. SARS-CoV-2 may grab advantageous mutations from the widely existing microorganisms in the host, which is undoubtedly an "efficient" manner. Our study might open a new perspective to understand the evolution of virus mutation, which has enormous implications for comprehending the trajectory of the COVID-19 pandemic.

Genome-Wide Profiling of Transcription Initiation with STRIPE-seq

Transcription start site (TSS) usage is a critical factor in the regulation of gene expression. A number of methods for global TSS mapping have been developed, but barriers of expense, technical difficulty, time, and/or cost have limited their broader adoption. To address these issues, we developed Survey of TRanscription Initiation at Promoter Elements with high-throughput sequencing (STRIPE-seq). Requiring only three enzymatic steps with intervening bead cleanups, a STRIPE-seq library can be prepared from as little as 50 ng total RNA in ~5 h at a cost of ~$12 (US). In addition to profiling TSS usage, STRIPE-seq provides information on transcript levels that can be used for differential expression analysis. Thanks to its simplicity and low cost, we envision that STRIPE-seq could be employed by any molecular biology laboratory interested in profiling transcription initiation.

Low-bias ncRNA libraries using ordered two-template relay: Serial template jumping by a modified retroelement reverse transcriptase

Retrotransposons are noninfectious, mobile genetic elements that proliferate in host genomes via an RNA intermediate that is copied into DNA by a reverse transcriptase (RT) enzyme. RTs are important for biotechnological applications involving information capture from RNA since RNA is first converted into complementary DNA for detection or sequencing. Here, we biochemically characterized RTs from two retroelements and uncovered several activities that allowed us to design a streamlined, efficient workflow for determining the inventory of RNA sequences in processed RNA pools. The unique properties of nonretroviral RT activities obviate many technical issues associated with current methods of RNA sequence analysis, with wide applications in research, biotechnology, and diagnostics.

Selfish, non-long terminal repeat (non-LTR) retroelements and mobile group II introns encode reverse transcriptases (RTs) that can initiate DNA synthesis without substantial base pairing of primer and template. Biochemical characterization of these enzymes has been limited by recombinant expression challenges, hampering understanding of their properties and the possible exploitation of their properties for research and biotechnology. We investigated the activities of representative RTs using a modified non-LTR RT from Bombyx mori and a group II intron RT from Eubacterium rectale. Only the non-LTR RT supported robust and serial template jumping, producing one complementary DNA (cDNA) from several templates each copied end to end. We also discovered an unexpected terminal deoxynucleotidyl transferase activity of the RTs that adds nucleotide(s) of choice to 3′ ends of single- and/or double-stranded RNA or DNA. Combining these two types of activity with additional insights about nontemplated nucleotide additions to duplexed cDNA product, we developed a streamlined protocol for fusion of next-generation sequencing adaptors to both cDNA ends in a single RT reaction. When benchmarked using a reference pool of microRNAs (miRNAs), library production by Ordered Two-Template Relay (OTTR) using recombinant non-LTR retroelement RT outperformed all commercially available kits and rivaled the low bias of technically demanding home-brew protocols. We applied OTTR to inventory RNAs purified from extracellular vesicles, identifying miRNAs as well as myriad other noncoding RNAs (ncRNAs) and ncRNA fragments. Our results establish the utility of OTTR for automation-friendly, low-bias, end-to-end RNA sequence inventories of complex ncRNA samples.

Integrating Imaging, Histologic, and Genetic Features to Predict Tumor Mutation Burden of Non-Small-Cell Lung Cancer

BACKGROUND

Immune checkpoint inhibitors have dramatically changed the landscape of therapeutic management of non-small-cell lung cancer (NSCLC). Tumor mutation burden (TMB) is an important biomarker of the response to cancer immunotherapy. We investigated the relationship between TMB and the imaging, histologic, and genetic features in NSCLC.

MATERIALS AND METHODS

We evaluated the associations between the semantic imaging features (7 quantitative or semiquantitative imaging features and 13 qualitative features that reflect the tumor characteristics) and TMB and built an imaging signature for TMB using logistic regression. Finally, we integrated the imaging signature, histologic type, and TP53 genotype into a composite model.

RESULTS

Among 89 patients, 37 (41.6%) had low TMB and 52 (58.4%) had high TMB. Tumors with high TMB were more prevalent in squamous cell carcinoma (P = .017) and those with a TP53 mutation (P < .0001). The absence of concavity was significantly associated with higher TMB (P = .008). An imaging signature containing 5 features, including concavity, border definition, spiculation, thickened adjacent bronchovascular bundle and size, achieved good discrimination between tumors with low and high TMB (area under the curve [AUC], 0.79; 95% confidence interval [CI], 0.69-0.89). The composite model integrating the imaging signature, histologic type, and TP53 genotype improved the classification (AUC, 0.89; 95% CI, 0.82-0.95) compared with the imaging signature alone using the DeLong test (P = .012). The composite model achieved a high sensitivity of 95% and a specificity of 62%.

CONCLUSION

Specific computed tomography features are associated with TMB in NSCLC, and the integration of imaging, histologic, and genetic information might allow for accurate prediction of TMB.

Evolutionary modes of emergence of short interspersed nuclear element (SINE) families in grasses

Short interspersed nuclear elements (SINEs) are non-autonomous transposable elements which are propagated by retrotransposition and constitute an inherent part of the genome of most eukaryotic species. Knowledge of heterogeneous and highly abundant SINEs is crucial for de novo (or improvement of) annotation of whole genome sequences. We scanned Poaceae genome sequences of six important cereals (Oryza sativa, Triticum aestivum, Hordeum vulgare, Panicum virgatum, Sorghum bicolor, Zea mays) and Brachypodium distachyon to examine the diversity and evolution of SINE populations. We comparatively analyzed the structural features, distribution, evolutionary relation and abundance of 32 SINE families and subfamilies within grasses, comprising 11 052 individual copies. The investigation of activity profiles within the Poaceae provides insights into their species-specific diversification and amplification. We found that Poaceae SINEs (PoaS) fall into two length categories: simple SINEs of up to 180 bp and dimeric SINEs larger than 240 bp. Detailed analysis at the nucleotide level revealed that multimerization of related and unrelated SINE copies is an important evolutionary mechanism of SINE formation. We conclude that PoaS families diversify by massive reshuffling between SINE families, likely caused by insertion of truncated copies, and provide a model for this evolutionary scenario. Twenty-eight of 32 PoaS families and subfamilies show significant conservation, in particular either in the 5' or 3' regions, across Poaceae species and share large sequence stretches with one or more other PoaS families.

Analysis of 100,000 human cancer genomes reveals the landscape of tumor mutational burden

Background

High tumor mutational burden (TMB) is an emerging biomarker of sensitivity to immune checkpoint inhibitors and has been shown to be more significantly associated with response to PD-1 and PD-L1 blockade immunotherapy than PD-1 or PD-L1 expression, as measured by immunohistochemistry (IHC). The distribution of TMB and the subset of patients with high TMB has not been well characterized in the majority of cancer types.

Methods

In this study, we compare TMB measured by a targeted comprehensive genomic profiling (CGP) assay to TMB measured by exome sequencing and simulate the expected variance in TMB when sequencing less than the whole exome. We then describe the distribution of TMB across a diverse cohort of 100,000 cancer cases and test for association between somatic alterations and TMB in over 100 tumor types.

Results

We demonstrate that measurements of TMB from comprehensive genomic profiling are strongly reflective of measurements from whole exome sequencing and model that below 0.5 Mb the variance in measurement increases significantly. We find that a subset of patients exhibits high TMB across almost all types of cancer, including many rare tumor types, and characterize the relationship between high TMB and microsatellite instability status. We find that TMB increases significantly with age, showing a 2.4-fold difference between age 10 and age 90 years. Finally, we investigate the molecular basis of TMB and identify genes and mutations associated with TMB level. We identify a cluster of somatic mutations in the promoter of the gene PMS2, which occur in 10% of skin cancers and are highly associated with increased TMB.

Conclusions

These results show that a CGP assay targeting ~1.1 Mb of coding genome can accurately assess TMB compared with sequencing the whole exome. Using this method, we find that many disease types have a substantial portion of patients with high TMB who might benefit from immunotherapy. Finally, we identify novel, recurrent promoter mutations in PMS2, which may be another example of regulatory mutations contributing to tumorigenesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-017-0424-2) contains supplementary material, which is available to authorized users.

Effect of the linkers between the zinc fingers in zinc finger protein 809 on gene silencing and nuclear localization

Zinc finger protein 809 (ZFP809) belongs to the Kruppel-associated box-containing zinc finger protein (KRAB-ZFP) family and functions in repressing the expression of Moloney murine leukemia virus (MoMLV). ZFP809 binds to the primer-binding site (PBS)located downstream of the MoMLV-long terminal repeat (LTR) and induces epigenetic modifications at integration sites, such as repressive histone modifications and de novo DNA methylation. KRAB-ZFPs contain consensus TGEKP linkers between C2H2 zinc fingers. The phosphorylation of threonine residues within linkers leads to the inactivation of zinc finger binding to target sequences. ZFP809 also contains consensus linkers between zinc fingers. However, the function of ZFP809 linkers remains unknown. In the present study, we constructed ZFP809 proteins containing mutated linkers and examined their ability to silence transgene expression driven by MLV, binding ability to MLV PBS, and cellular localization. The results of the present study revealed that the linkers affected the ability of ZFP809 to silence transgene expression. Furthermore, this effect could be partly attributed to changes in the localization of ZFP809 proteins containing mutated linkers. Further characterization of ZFP809 linkers is required for understanding the functions and features of KRAB-ZFP-containing linkers.

What makes up plant genomes: The vanishing line between transposable elements and genes

The ultimate source of evolution is mutation. As the largest component in plant genomes, transposable elements (TEs) create numerous types of mutations that cannot be mimicked by other genetic mechanisms. When TEs insert into genomic sequences, they influence the expression of nearby genes as well as genes unlinked to the insertion. TEs can duplicate, mobilize, and recombine normal genes or gene fragments, with the potential to generate new genes or modify the structure of existing genes. TEs also donate their transposase coding regions for cellular functions in a process called TE domestication. Despite the host defense against TE activity, a subset of TEs survived and thrived through discreet selection of transposition activity, target site, element size, and the internal sequence. Finally, TEs have established strategies to reduce the efficacy of host defense system by increasing the cost of silencing TEs. This review discusses the recent progress in the area of plant TEs with a focus on the interaction between TEs and genes.

Long time-course monitoring of ZFP809-mediated gene silencing in transgene expression driven by promoters containing MLV-derived PBS

Expression of Moloney murine leukemia virus (MoMLV)-typed retroviral vectors is strictly suppressed in immature cells such as embryonic stem cells. The phenomenon known as gene silencing is primed by the sequence-specific binding of the zinc finger protein 809 (ZFP809) to the primer-binding site of the vectors. However, it has yet to be determined whether the ZFP809-mediated gene silencing is maintained over a long period. In this study, we established an experimental system that can monitor gene silencing during a long-term cell culture using flow cytometry technology combined with fluorescent reporters for the expression of ZFP809 and the transgene expression driven by the promoters of interest. Time-course analysis using our system revealed that ZFP809 maintains gene silencing effect even at a longtime period. Furthermore, our system was useful for the monitoring of ZFP809-mediated gene silencing regardless of the types of vectors and cell lines.

The design of RNA binders: targeting the HIV replication cycle as a case study

The human immunodeficiency virus 1 (HIV-1) replication cycle is finely tuned with many important steps involving RNA-RNA or protein-RNA interactions, all of them being potential targets for the development of new antiviral compounds. This cycle can also be considered as a good benchmark for the evaluation of early-stage strategies aiming at designing drugs that bind to RNA, with the possibility to correlate in vitro activities with antiviral properties. In this review, we highlight different approaches developed to interfere with four important steps of the HIV-1 replication cycle: the early stage of reverse transcription, the transactivation of viral transcription, the nuclear export of partially spliced transcripts and the dimerization step.

A protein ballet around the viral genome orchestrated by HIV-1 reverse transcriptase leads to an architectural switch: from nucleocapsid-condensed RNA to Vpr-bridged DNA

HIV-1 reverse transcription is achieved in the newly infected cell before viral DNA (vDNA) nuclear import. Reverse transcriptase (RT) has previously been shown to function as a molecular motor, dismantling the nucleocapsid complex that binds the viral genome as soon as plus-strand DNA synthesis initiates. We first propose a detailed model of this dismantling in close relationship with the sequential conversion from RNA to double-stranded (ds) DNA, focusing on the nucleocapsid protein (NCp7). The HIV-1 DNA-containing pre-integration complex (PIC) resulting from completion of reverse transcription is translocated through the nuclear pore. The PIC nucleoprotein architecture is poorly understood but contains at least two HIV-1 proteins initially from the virion core, namely integrase (IN) and the viral protein r (Vpr). We next present a set of electron micrographs supporting that Vpr behaves as a DNA architectural protein, initiating multiple DNA bridges over more than 500 base pairs (bp). These complexes are shown to interact with NCp7 bound to single-stranded nucleic acid regions that are thought to maintain IN binding during dsDNA synthesis, concurrently with nucleocapsid complex dismantling. This unexpected binding of Vpr conveniently leads to a compacted but filamentous folding of the vDNA that should favor its nuclear import. Finally, nucleocapsid-like aggregates engaged in dsDNA synthesis appear to efficiently bind to F-actin filaments, a property that may be involved in targeting complexes to the nuclear envelope. More generally, this article highlights unique possibilities offered by in vitro reconstitution approaches combined with macromolecular imaging to gain insights into the mechanisms that alter the nucleoprotein architecture of the HIV-1 genome, ultimately enabling its insertion into the nuclear chromatin.

HIV-1 reverse transcriptase dissociates during strand transfer

Steps in the replication of human immunodeficiency virus type 1 (HIV-1) occurring in the virus but not in the host are preferred targets of antiretroviral therapy. Strand transfer is unique; the DNA strand being made by viral reverse transcriptase (RT) is moved from one RNA template position to another. Understanding the mechanism requires knowing whether the RT directly mediates the template exchange or dissociates during the exchange, so that it occurs by polymer dynamics. Earlier work in vitro showed that the presence of an RT-trapping polymer would allow synthesis on the original or donor template but completely block transfer and subsequent synthesis on the second or acceptor template. One interpretation is that the RT must dissociate during transfer, but an alternative is that sequestration of non-polymerizing RTs prevents polymerization-independent ribonuclease H (RNase H) cleavages of the donor template necessary for strand exchange. To resolve this ambiguity, we designed a primer-template system that allows strand transfer without RNase H activity. Using an RNase H negative mutant RT, we showed that a polymer trap still prevented strand transfer. This confirms that RT dissociates during strand transfer. The presence of HIV-1 nucleocapsid protein, which promotes strand exchange, had little effect on this outcome. Additional assays showed that both the wild-type RT and a multiple nucleoside RT inhibitor-resistant HIV-1 RT containing an extended fingers domain, which is characterized by its enhanced primer-template binding affinity, were unable to transfer with the trapping polymer. This implies that common sequence variations among RTs are unlikely to alter dissociation feature.

Bicistronic lentiviruses containing a viral 2A cleavage sequence reliably co-express two proteins and restore vision to an animal model of LCA1

The disease processes underlying inherited retinal disease are complex and are not completely understood. Many of the corrective gene therapies designed to treat diseases linked to mutations in genes specifically expressed in photoreceptor cells restore function to these cells but fail to stop progression of the disease. There is growing consensus that effective treatments for these diseases will require delivery of multiple therapeutic proteins that will be selected to treat specific aspects of the disease process. The purpose of this study was to design a lentiviral transgene that reliably expresses all of the proteins it encodes and does so in a consistent manner among infected cells. We show, using both in vitro and in vivo analyses, that bicistronic lentiviral transgenes encoding two fluorescent proteins fused to a viral 2A-like cleavage peptide meet these expression criteria. To determine if this transgene design is suitable for therapeutic applications, we replaced one of the fluorescent protein genes with the gene encoding guanylate cyclase-1 (GC1) and delivered lentivirus carrying this transgene to the retinas of the GUCY1*B avian model of Leber congenital amaurosis-1 (LCA1). GUCY1*B chickens carry a null mutation in the GC1 gene that disrupts photoreceptor function and causes blindness at hatching, a phenotype that closely matches that observed in humans with LCA1. We found that treatment of these animals with the 2A lentivector encoding GC1 restored vision to these animals as evidenced by the presence of optokinetic reflexes. We conclude that 2A-like peptides, with proper optimization, can be successfully incorporated into therapeutic vectors designed to deliver multiple proteins to neural retinal. These results highlight the potential of this vector design to serve as a platform for the development of combination therapies designed to enhance or prolong the benefits of corrective gene therapies.

Single-molecule stretching studies of RNA chaperones

RNA chaperone proteins play significant roles in diverse biological contexts. The most widely studied RNA chaperones are the retroviral nucleocapsid proteins (NC), also referred to as nucleic acid (NA) chaperones. Surprisingly, the biophysical properties of the NC proteins vary significantly for different viruses, and it appears that HIV-1 NC has optimal NA chaperone activity. In this review we discuss the physical nature of the NA chaperone activity of NC. We conclude that the optimal NA chaperone must saturate NA binding, leading to strong NA aggregation and slight destabilization of all NA duplexes. Finally, rapid kinetics of the chaperone protein interaction with NA is another primary component of its NA chaperone activity. We discuss these characteristics of HIV-1 NC and compare them with those of other NA binding proteins and ligands that exhibit only some characteristics of NA chaperone activity, as studied by single molecule DNA stretching.

The remarkable frequency of human immunodeficiency virus type 1 genetic recombination

The genetic diversity of human immunodeficiency virus type 1 (HIV-1) results from a combination of point mutations and genetic recombination, and rates of both processes are unusually high. This review focuses on the mechanisms and outcomes of HIV-1 genetic recombination and on the parameters that make recombination so remarkably frequent. Experimental work has demonstrated that the process that leads to recombination--a copy choice mechanism involving the migration of reverse transcriptase between viral RNA templates--occurs several times on average during every round of HIV-1 DNA synthesis. Key biological factors that lead to high recombination rates for all retroviruses are the recombination-prone nature of their reverse transcription machinery and their pseudodiploid RNA genomes. However, HIV-1 genes recombine even more frequently than do those of many other retroviruses. This reflects the way in which HIV-1 selects genomic RNAs for coencapsidation as well as cell-to-cell transmission properties that lead to unusually frequent associations between distinct viral genotypes. HIV-1 faces strong and changeable selective conditions during replication within patients. The mode of HIV-1 persistence as integrated proviruses and strong selection for defective proviruses in vivo provide conditions for archiving alleles, which can be resuscitated years after initial provirus establishment. Recombination can facilitate drug resistance and may allow superinfecting HIV-1 strains to evade preexisting immune responses, thus adding to challenges in vaccine development. These properties converge to provide HIV-1 with the means, motive, and opportunity to recombine its genetic material at an unprecedented high rate and to allow genetic recombination to serve as one of the highest barriers to HIV-1 eradication.

Influence of vector design and host cell on the mechanism of recombination and emergence of mutant subpopulations of replicating retroviral vectors

Background

The recent advent of murine leukaemia virus (MLV)-based replication-competent retroviral (RCR) vector technology has provided exciting new tools for gene delivery, albeit the advances in vector efficiency which have been realized are also accompanied by a set of fresh challenges. The expression of additional transgene sequences, for example, increases the length of the viral genome, which can lead to reductions in replication efficiency and in turn to vector genome instability. This necessitates efforts to analyse the rate and mechanism of recombinant emergence during the replication of such vectors to provide data which should contribute to improvements in RCR vector design.

Results

In this study, we have performed detailed molecular analyses on packaged vector genomes and proviral DNA following propagation of MLV-based RCR vectors both in cell culture and in pre-formed subcutaneous tumours in vivo. The effects of strain of MLV, transgene position and host cell type on the rate of emergence of vector recombinants were quantitatively analysed by applying real-time PCR and real-time RT-PCR assays. Individual mutants were further characterized by PCR, and nucleotide sequence and structural motifs associated with these mutants were determined by sequencing. Our data indicate that virus strain, vector design and host cell influence the rate of emergence of predominating vector mutants, but not the underlying recombination mechanisms in vitro. In contrast, however, differences in the RNA secondary structural motifs associated with sequenced mutants emerging in cell culture and in solid tumours in vivo were observed.

Conclusion

Our data provide further evidence that MLV-based RCR vectors based on the Moloney strain of MLV and containing the transgene cassette in the 3' UTR region are superior to those based on Akv-MLV and/or containing the transgene cassette in the U3 region of the LTR. The observed discrepancies between the data obtained in solid tumours in vivo and our own and previously published data from infected cells in vitro demonstrates the importance of evaluating vectors designed for use in cancer gene therapy in vivo as well as in vitro.

Immune-driven recombination and loss of control after HIV superinfection

After acute HIV infection, CD8⁺ T cells are able to control viral replication to a set point. This control is often lost after superinfection, although the mechanism behind this remains unclear. In this study, we illustrate in an HLA-B27⁺ subject that loss of viral control after HIV superinfection coincides with rapid recombination events within two narrow regions of Gag and Env. Screening for CD8⁺ T cell responses revealed that each of these recombination sites (∼50 aa) encompassed distinct regions containing two immunodominant CD8 epitopes (B27-KK10 in Gag and Cw1-CL9 in Env). Viral escape and the subsequent development of variant-specific de novo CD8⁺ T cell responses against both epitopes were illustrative of the significant immune selection pressures exerted by both responses. Comprehensive analysis of the kinetics of CD8 responses and viral evolution indicated that the recombination events quickly facilitated viral escape from both dominant WT- and variant-specific responses. These data suggest that the ability of a superinfecting strain of HIV to overcome preexisting immune control may be related to its ability to rapidly recombine in critical regions under immune selection pressure. These data also support a role for cellular immune pressures in driving the selection of new recombinant forms of HIV.

Selective killing of HIV-1-positive macrophages and T cells by the Rev-dependent lentivirus carrying anthrolysin O from Bacillus anthracis

Background

The ability of Human Immunodeficiency Virus (HIV) to persist in the body has proven to be a long-standing challenge to virus eradication. Current antiretroviral therapy cannot selectively destroy infected cells; it only halts active viral replication. With therapeutic cessation or interruption, viral rebound occurs, and invariably, viral loads return to pre-treatment levels. The natural reservoirs harboring replication-competent HIV-1 include CD4 T cells and macrophages. In particular, cells from the macrophage lineage resist HIV-1-mediated killing and support sustained viral production. To develop a complementary strategy to target persistently infected cells, this proof-of-concept study explores an HIV-1 Rev-dependent lentiviral vector carrying a bacterial hemolysin, anthrolysin O (anlO) from Bacillus anthracis, to achieve selective killing of HIV-1- infected cells.

Results

We demonstrate that in the Rev-dependent lentiviral vector, anlO expression is exclusively dependent on Rev, a unique HIV-1 protein present only in infected cells. Intracellular expression and oligomerization of AnlO result in membrane pore formation and cytolysis. We have further overcome a technical hurdle in producing a Revdependent AnlO lentivirus, through the use of β-cyclodextrin derivatives to inhibit direct killing of producer cells by AnlO. Using HIV-1-infected macrophages and T cells as a model, we demonstrate that this Rev-dependent AnlO lentivirus diminishes HIV-1- positive cells.

Conclusion

The Rev-dependent lentiviral vector has demonstrated its specificity in targeting persistently infected cells. The choice of anlO as the first suicidal gene tested in this vector is based on its cytolytic activity in macrophages and T cells. We conclude that Rev-regulated expression of suicidal genes in HIV-1-positive cells is possible, although future in vivo delivery of this system needs to address numerous safety issues.

A putative RNA-interference-based immune system in prokaryotes: computational analysis of the predicted enzymatic machinery, functional analogies with eukaryotic RNAi, and hypothetical mechanisms of action

Background

All archaeal and many bacterial genomes contain Clustered Regularly Interspaced Short Palindrome Repeats (CRISPR) and variable arrays of the CRISPR-associated (cas) genes that have been previously implicated in a novel form of DNA repair on the basis of comparative analysis of their protein product sequences. However, the proximity of CRISPR and cas genes strongly suggests that they have related functions which is hard to reconcile with the repair hypothesis.

Results

The protein sequences of the numerous cas gene products were classified into ~25 distinct protein families; several new functional and structural predictions are described. Comparative-genomic analysis of CRISPR and cas genes leads to the hypothesis that the CRISPR-Cas system (CASS) is a mechanism of defense against invading phages and plasmids that functions analogously to the eukaryotic RNA interference (RNAi) systems. Specific functional analogies are drawn between several components of CASS and proteins involved in eukaryotic RNAi, including the double-stranded RNA-specific helicase-nuclease (dicer), the endonuclease cleaving target mRNAs (slicer), and the RNA-dependent RNA polymerase. However, none of the CASS components is orthologous to its apparent eukaryotic functional counterpart. It is proposed that unique inserts of CRISPR, some of which are homologous to fragments of bacteriophage and plasmid genes, function as prokaryotic siRNAs (psiRNA), by base-pairing with the target mRNAs and promoting their degradation or translation shutdown. Specific hypothetical schemes are developed for the functioning of the predicted prokaryotic siRNA system and for the formation of new CRISPR units with unique inserts encoding psiRNA conferring immunity to the respective newly encountered phages or plasmids. The unique inserts in CRISPR show virtually no similarity even between closely related bacterial strains which suggests their rapid turnover, on evolutionary scale. Corollaries of this finding are that, even among closely related prokaryotes, the most commonly encountered phages and plasmids are different and/or that the dominant phages and plasmids turn over rapidly.

Conclusion

We proposed previously that Cas proteins comprise a novel DNA repair system. The association of the cas genes with CRISPR and, especially, the presence, in CRISPR units, of unique inserts homologous to phage and plasmid genes make us abandon this hypothesis. It appears most likely that CASS is a prokaryotic system of defense against phages and plasmids that functions via the RNAi mechanism. The functioning of this system seems to involve integration of fragments of foreign genes into archaeal and bacterial chromosomes yielding heritable immunity to the respective agents. However, it appears that this inheritance is extremely unstable on the evolutionary scale such that the repertoires of unique psiRNAs are completely replaced even in closely related prokaryotes, presumably, in response to rapidly changing repertoires of dominant phages and plasmids.

This article was reviewed by: Eric Bapteste, Patrick Forterre, and Martijn Huynen.

Open peer review

Reviewed by Eric Bapteste, Patrick Forterre, and Martijn Huynen.

For the full reviews, please go to the Reviewers' comments section.

Modulating the splicing activity of Tetrahymena ribozyme via RNA self-assembly

The internal guiding sequence (IGS) is normally located at the 5' end of trans-splicing ribozymes that are derived from the Tetrahymena group I intron, and is required for the recognition of substrate RNAs and for trans-splicing reactions. Here, we separated the Tetrahymena group I intron at the L2 loop to produce two fragments: the IGS-containing substrate, and the IGS-lacking ribozyme. We show here that two fragments can complex not through the IGS interaction but under the guidance of appended interacting nucleotides, and perform trans-splicing. The splicing reactions took place both in vitro and in mammalian cells, and the spliced mRNA product from the self-assembled ribozyme complex can be translated into functional proteins in vivo. The splicing efficiency was dependent on the length of appending nucleotides.

Three new alternative splicing variants of human cytochrome P450 2D6 mRNA in human extratumoral liver tissue

AIM: To identify the new alternative splicing variants of human CYP2D6 in human extratumoral liver tissue with RT-PCR and sequencing.

METHODS: Full length of human CYP2D6 cDNAs was amplificated by reverse transcription-polymerase chain reaction (RT-PCR) from a human extratumoral liver tissue and cloned into pGEM-T vector. The cDNA was sequenced. Exons from 1 to 4 of human CYP2D6 cDNAs were also amplificated by RT-PCR from extratumoral liver tissues of 17 human hepatocellular carcinomas. Some RT-PCR products were sequenced. Exons 1 to 4 of CYP2D6 gene were amplified by PCR from extratumoral liver tissue DNA. Two PCR products from extratumoral liver tissues expressing skipped mRNA were partially sequenced.

RESULTS: One of the CYP2D6 cDNAs had 470 nucleotides from 79 to 548 (3’ portion of exons 1 to 5’portion of exon 4), and was skipped. Exons 1 to 4 of CYP2D6 cDNA were assayed with RT-PCR in 17 extratumoral liver tissues. Both wild type and skipped mRNAs were expressed in 4 samples, only wild type mRNA was expressed in 5 samples, and only skipped mRNA was expressed in 8 samples. Two more variants were identified by sequencing the RT-PCR products of exons 1 to 4 of CYP2D6 cDNA. The second variant skipped 411 nucleotides from 175 to 585. This variant was identified in 4 different liver tissues by sequencing the RT-PCR products. We sequenced partially 2 of the PCR products amplified of CYP2D6 exon 1 to exon 4 from extratumoral liver tissue genomic DNA that only expressed skipped mRNA by RT-PCR. No point mutations around exon 1, intron 1, and exon 4, and no deletion in CYP2D6 gene were detected. The third variant was the skipped exon 3 , and 153 bp was lost.

CONCLUSION: Three new alternative splicing variants of CYP2D6 mRNA have been identified. They may not be caused by gene mutation and may lose CYP2D6 activity and act as a down-regulator of CYP2D6.

Mechanistic analysis of pause site-dependent and -independent recombinogenic strand transfer from structurally diverse regions of the HIV genome

Retroviral recombinants are generated by strand transfers occurring within internal regions of the viral genome and are a major source of genetic variability. Strand transfer has been linked to "pausing" occurring at secondary structures during synthesis by reverse transcriptase. Yet, weakly structured templates lacking strong pause sites also undergo efficient transfer. In this report, transfer crossover sites on high and low structured templates from the gag-pol frameshift region (GagPol) and the env (Env) regions, respectively, were determined by using a reconstituted in vitro strand transfer assay. The assay tested transfers occurring between a donor and acceptor template over a 150-nucleotide homologous region. The majority of crossovers were in a small 23-nucleotide region near a major pause site on GagPol, clearly indicating a pause-driven mechanism. In contrast, on Env, transfers were more dispersed clustering toward the end of the homologous region. Slowing down polymerization on Env by decreasing the dNTP concentration resulted in crossovers shifting toward the beginning of the homologous region. Removal of a small 38-nucleotide region at the 3'-end of the Env acceptor had a large effect on the level of strand transfer despite very few crossovers mapping to this region. This implicated this part of the acceptor in transfers occurring at downstream positions. For Env the results support a mechanism where the acceptor rapidly binds nascent DNA, then "zippers" downstream catching up with the donor-DNA hybrid and displacing the donor. Such a mechanism may be important to recombination in low structure regions of the HIV genome.

End-to-end template jumping by the reverse transcriptase encoded by the R2 retrotransposon

The reverse transcriptase encoded by the non-long terminal repeat retrotransposon R2 has been shown to be able to jump from the 5'-end of one RNA template (the donor) to the 3'-end of a second RNA template (the acceptor) in the absence of preexisting sequence identity between the two templates. These jumps between RNA templates have similarity to the end-to-end template jumps described for the RNA-directed RNA polymerases encoded by certain RNA viruses. Here we describe for the first time the mechanism by which such end-to-end template jumps can occur. Most template jumps by the R2 reverse transcriptase are brought about by the enzyme's ability to add nontemplated (overhanging) nucleotides to the cDNA when it reaches the end of the donor RNA. The enzyme then anneals these overhanging nucleotides to sequences at the 3'-end of the acceptor RNA. The annealing is most efficient if it involves the terminal nucleotide(s) of the acceptor RNA but can occur to sites at least 5 nucleotides from the 3'-end. These end-to-end jumps are similar to steps proposed to be part of the integration reaction of non-long terminal repeat retrotransposons and can explain chimeric integration products derived from multiple RNA templates.

Role of the Reverse Transcriptase, Nucleocapsid Protein, and Template Structure in the Two-step Transfer Mechanism in Retroviral Recombination

Template switching during reverse transcription promotes recombination in retroviruses. Efficient switches have been measured in vitro on hairpin-containing RNA templates by a two-step mechanism. Pausing of the reverse transcriptase (RT) at the hairpin base allowed enhanced cleavage of the initial donor RNA template, exposing regions of the cDNA and allowing the acceptor to base pair with the cDNA. This defines the first or docking step. The primer continued synthesis on the donor, transferring or locking in a second step. Here we determine the enzyme-dependent factors that influence template switching by comparing the RTs from human immunodeficiency virus, type 1 (HIV-1), and equine infectious anemia virus (EIAV). HIV-1 RT promoted transfers with higher efficiency than EIAV RT. We found that both RTs paused strongly at the base of the hairpin. While stalled, HIV-1 RT made closely spaced cuts, whereas EIAV RT made only a single cut. Docking occurred efficiently at the multiply cut but not at the singly cut site. HIV-1 nucleocapsid (NC) protein stimulated strand transfers. It improved RNase H activity of both RTs. It allowed the EIAV RT to make a distribution of cuts, greatly stimulating docking at the base of the hairpin. Most likely, it also promoted strand exchange, allowing transfers to be initiated from sites throughout the hairpin. Minor pause sites beyond the base of the hairpin correlated with the locking sites. The strand exchange properties of NC likely promote this step. We present a model that explains the roles of RNase H specificity, template structure, and properties of NC in the two-step transfer reaction.

Evidence for the differential effects of nucleocapsid protein on strand transfer in various regions of the HIV genome

An in vitro strand transfer assay that mimicked recombinational events occurring during reverse transcription in HIV-1 was used to assess the role of nucleocapsid protein (NC) in strand transfer. Strand transfer in highly structured nucleic acid species from the U3 3' long terminal repeats, gag-pol frameshift region, and Rev response element were strongly enhanced by NC. In contrast, weakly structured templates from the env and pol-vif regions transferred well without NC and showed lower enhancement. The lack of strong polymerase pause sites in the latter regions demonstrated that non-pause driven mechanisms could also promote transfer. Assays conducted using NC zinc finger mutants supported a differential role for the two fingers in strand transfer with finger 1 (N-terminal) being more important on highly structured RNAs. Overall this report suggests a role for structural intricacies of RNA templates in determining the extent of influence of NC on recombination and illustrates that strand transfer may occur by several different mechanisms depending on the structural nature of the RNA.

Evidence for a mechanism of recombination during reverse transcription dependent on the structure of the acceptor RNA

Genetic recombination is a major force driving retroviral evolution. In retroviruses, recombination proceeds mostly through copy choice during reverse transcription. Using a reconstituted in vitro system, we have studied the mechanism of strand transfer on a major recombination hot spot we previously identified within the genome of HIV-1. We show that on this model sequence the frequency of copy choice is strongly influenced by the folding of the RNA template, namely by the presence of a stable hairpin. This structure must be specifically present on the acceptor template. We previously proposed that strand transfer follows a two-step process: docking of the nascent DNA onto the acceptor RNA and strand invasion. The frequency of recombination under copy choice conditions was not dependent on the concentration of the acceptor RNA, in contrast with strand transfer occurring at strong arrests of reverse transcription. During copy choice strand transfer, the docking step is not rate limiting. We propose that the hairpin present on the acceptor RNA could mediate strand transfer following a mechanism reminiscent of branch migration during DNA recombination.

Lymphoma- and leukemia-associated chromosomal translocations in healthy individuals

Chromosomal translocations (CTs) are hallmark mutations of hematopoietic malignancy that result in the deregulated expression of oncogenes or the generation of novel fusion genes. The polymerase chain reaction (PCR) can be used to detect illegitimate recombinations of genomic DNA sequences as a more sensitive assay than cytogenetics for determining the presence of CTs. Both direct DNA-PCR and reverse transcriptase-PCR were used to examine healthy individuals for lymphoma- and leukemia-associated CTs. Two oncogene-activating CTs [t(14;18)(q32;q21) and t(8;14)(q24;q32)] and one fusion-gene CT [t(2;5)(p23;q35)] from lymphomas and five fusion-gene CTs from leukemia [t(9;22)(q34;q11), t(4;11)(q21;q23), t(15;17)(q22;q11), t(12;21)(p13;q22), t(8;21)(q22;q22)] were detected in such studies. The biological implication is that CTs associated with malignant tumors may also be found in cells that are not neoplastic. CTs are characteristic attributes of neoplastic clones but are by themselves insufficient to cause malignant transformation. A better understanding of the special biology of non-neoplastic CT-bearing cells will provide insight into their putative role as tumor precursors. Prospective epidemiological studies are needed to determine whether such cells in healthy individuals may, in some instances, become clonogenic founders of lymphoma or leukemia.

Multiparameter single-molecule fluorescence spectroscopy reveals heterogeneity of HIV-1 reverse transcriptase:primer/template complexes

By using single-molecule multiparameter fluorescence detection, fluorescence resonance energy transfer experiments, and newly developed data analysis methods, this study demonstrates directly the existence of three structurally distinct forms of reverse transcriptase (RT):nucleic acid complexes in solution. Single-molecule multiparameter fluorescence detection also provides first information on the structure of a complex not observed by x-ray crystallography. This species did not incorporate nucleotides and is structurally distinct from the other two observed species. We determined that the nucleic acid substrate is bound at a site far removed from the nucleic acid-binding tract observed by crystallography. In contrast, the other two states are identified as being similar to the x-ray crystal structure and represent distinct enzymatically productive stages in DNA polymerization. These species differ by only a 5-A shift in the position of the nucleic acid. Addition of nucleoside triphosphate or of inorganic pyrophosphate allowed us to assign them as the educt and product state in the polymerization reaction cycle; i.e., the educt state is a complex in which the nucleic acid is positioned to allow nucleotide incorporation. The second RT:nucleic acid complex is the product state, which is formed immediately after nucleotide incorporation, but before RT translates to the next nucleotide.

Requirements for RNA heterodimerization of the human immunodeficiency virus type 1 (HIV-1) and HIV-2 genomes

Retroviruses are prone to recombination because they package two copies of the RNA genome. Whereas recombination is a frequent event within the human immunodeficiency virus type 1 (HIV-1) and HIV-2 groups, no HIV-1/HIV-2 recombinants have been reported thus far. The possibility of forming HIV-1/HIV-2 RNA heterodimers was studied in vitro. In both viruses, the dimer initiation site (DIS) hairpin is used to form dimers, but these motifs appear too dissimilar to allow RNA heterodimer formation. Multiple mutations were introduced into the HIV-2 DIS element to gradually mimic the HIV-1 hairpin. First, the loop-exposed palindrome of HIV-1 was inserted. This self-complementary sequence motif forms the base pair interactions of the kissing-loop (KL) dimer complex, but such a modification is not sufficient to permit RNA heterodimer formation. Next, the HIV-2 DIS loop size was shortened from 11 to 9 nucleotides, as in the HIV-1 DIS motif. This modification also results in the presentation of the palindromes in the same position within the hairpin loop. The change yielded a modest level of RNA heterodimers, which was not significantly improved by additional sequence changes in the loop and top base pair. No isomerization of the KL dimer to the extended duplex dimer form was observed for the heterodimers. These combined results indicate that recombination between HIV-1 and HIV-2 is severely restricted at the level of RNA dimerization.

Mechanisms of retroviral recombination

Recombination is a major source of genetic variability in retroviruses. Each viral particle contains two single-stranded genomic RNAs. Recombination mostly results from a switch in template between these two RNAs during reverse transcription. Here we emphasize the main mechanisms underlying recombination that are emerging from recent advances in biochemical and cell culture techniques. Increasing evidence supporting the involvement of RNA secondary structures now complements the predominant role classically attributed to enzyme pausing during reverse transcription. Finally, the implications of recombination on the dynamics of emergence of genomic aberrations in retroviruses are discussed.

RNase H activity is required for high-frequency repeat deletion during Moloney murine leukemia virus replication

It has been postulated that retroviral recombination, like strong stop template switching, requires the RNase H activity of reverse transcriptase. To address this hypothesis, Moloney murine leukemia virus-based vectors, which were designed to test the recombination-related property of direct repeat deletion, were encapsidated in virions engineered to contain phenotypic mixtures of wild-type and RNase H catalytic site point mutant reverse transcriptase. Integrated provirus titers per milliliter were determined for these phenotypically mixed virions, and vector proviruses were screened to determine what percentage contained repeat deletions. The results revealed a steady decline in direct repeat deletion frequency that correlated with decreases in functional RNase H, with greater than fourfold decreases in repeat deletion frequency observed when 95% of virion reverse transcriptase was RNase H defective. Parallel experiments were performed to address effects of molar excesses of RNase H relative to functional DNA polymerase. These experiments demonstrated that increasing the stoichiometry of RNase H relative to the amount of functional DNA polymerase had minimal effects on direct repeat deletion frequency. DNA synthesis was error prone when directed principally by RNase H mutant reverse transcriptase, suggesting a role for RNase H catalytic integrity in the fidelity of intracellular reverse transcription.

The kissing hairpin sequence promotes recombination within the HIV-I 5' leader region

The role of RNA-RNA template interactions in facilitating recombination during reverse transcription of minus strand DNA has been examined. The tested hypothesis is that template switching by reverse transcriptase is promoted at sites where homologous regions of two RNAs are brought in close proximity via stable intertemplate interactions. Frequency and distribution of template switching between homologous donor and acceptor RNAs were examined within the human immunodeficiency virus type I (HIV-I) 5'-untranslated region (UTR) containing the dimer initiation sequence (DIS). Results were compared with control nondimerizing templates from the pol region. The dimerizing UTR templates displayed a 4-fold higher transfer efficiency than the control. A striking 53% of transfers in the UTR mapped near the DIS, of which two-thirds occurred immediately 5' to this sequence. In the UTR template, deletion of the DIS hairpin disrupted template dimerization and caused a 4-fold drop in transfer efficiency. Insertion of the DIS within the pol template increased both dimerization and transfer efficiency. Transfer distributions revealed that in both sets of templates, DIS-induced dimerization increased the efficiency of transfers across the whole template, with the transfers peaking around the dimerization site. Overall, these results suggest that template dimerization facilitated by the unique geometry of the DIS-promoted kissing interactions effectively promotes recombination within the HIV-I 5'-UTR.

The HIV-1 repeated sequence R as a robust hot-spot for copy-choice recombination

Template switching during reverse transcription is crucial for retroviral replication. While strand transfer on the terminal repeated sequence R is essential to achieve reverse transcription, template switching from internal regions of the genome (copy choice) leads to genetic recombination. We have developed an experimental system to study copy-choice recombination in vitro along the HIV-1 genome. We identify here several genomic regions, including the R sequence, where copy choice occurred at high rates. The frequency of copy choice occurring in a given region of template was strongly influenced by the surrounding sequences, an observation that suggests a pivotal role of the folding of template RNA in the process. The sequence R, instead, constituted an exception to this rule since it was a strong hot-spot for copy choice in the different sequence contexts tested. We suggest therefore that the structure of this region has been optimised during viral evolution to ensure efficient template switching independently from the sequences that might surround it.