CRISPR-Cas attack of HIV-1 proviral DNA can cause unintended deletion of surrounding cellular DNA.

IMPORTANCE: Although HIV replication can be effectively inhibited by antiretroviral therapy, this does not result in a cure as the available drugs do not inactivate the integrated HIV-1 DNA in infected cells. Consequently, HIV-infected individuals need lifelong therapy to prevent viral rebound. Several preclinical studies indicate that CRISPR-Cas gene-editing systems can be used to achieve permanent inactivation of the viral DNA. It was previously shown that this inactivation was due to small inactivating mutations at the targeted sites in the HIV genome and to excision or inversion of the viral DNA fragment between two target sites. We, here, demonstrate that CRISPR-Cas treatment also causes large unintended deletions, which can include surrounding chromosomal sequences. As the loss of chromosomal sequences may cause oncogenic transformation of the cell, such unintended large deletions form a potential safety risk in clinical application of this antiviral application and possibly all CRISPR-Cas gene-editing approaches.

HIV-1 3'-Polypurine Tract Mutations Confer Dolutegravir Resistance by Switching to an Integration-Independent Replication Mechanism via 1-LTR Circles.

Several recent studies indicate that mutations in the human immunodeficiency virus type 1 (HIV-1) 3'polypurine tract (3'PPT) motif can reduce sensitivity to the integrase inhibitor dolutegravir (DTG). Using an in vivo systematic evolution of ligands by exponential enrichment (SELEX) approach, we discovered that multiple different mutations in this viral RNA element can confer DTG resistance, suggesting that the inactivation of this critical reverse transcription element causes resistance. An analysis of the viral DNA products formed upon infection by these 3'PPT mutants revealed that they replicate without integration into the host cell genome, concomitant with an increased production of 1-LTR circles. As the replication of these virus variants is activated by the human T-lymphotropic virus 1 (HTLV-1) Tax protein, a factor that reverses epigenetic silencing of episomal HIV DNA, these data indicate that the 3'PPT-mutated viruses escape from the integrase inhibitor DTG by switching to an integration-independent replication mechanism. IMPORTANCE The integrase inhibitor DTG is a potent inhibitor of HIV replication and is currently recommended in drug regimens for people living with HIV. Whereas HIV normally escapes from antiviral drugs by the acquisition of specific mutations in the gene that encodes the targeted enzyme, mutational inactivation of the viral 3'PPT sequence, an RNA element that has a crucial role in the viral reverse transcription process, was found to allow HIV replication in the presence of DTG in cell culture experiments. While the integration of the viral DNA into the cellular genome is considered one of the hallmarks of retroviruses, including HIV, 3'PPT inactivation caused integration-independent replication, which can explain the reduced DTG sensitivity. Whether this exotic escape route can also contribute to viral escape in HIV-infected persons remains to be determined, but our results indicate that screening for 3'PPT mutations in patients that fail on DTG therapy should be considered.

HIV co-opts a cellular antiviral mechanism, activation of stress kinase PKR by its RNA, to enable splicing of rev/tat mRNA.

BACKGROUND: Activation of RNA-dependent stress kinase PKR, especially by viral double-stranded RNA, induces eukaryotic initiation factor 2 α-chain (eIF2α) phosphorylation, attenuating thereby translation. We report that this RNA-mediated negative control mechanism, considered a cornerstone of the cell's antiviral response, positively regulates splicing of a viral mRNA. RESULTS: Excision of the large human immunodeficiency virus (HIV) rev/tat intron depends strictly on activation of PKR by the viral RNA and on eIF2α phosphorylation. Rev/tat mRNA splicing was blocked by viral PKR antagonists Vaccinia E3L and Ebola VP35, as well as by a trans-dominant negative mutant of PKR, yet enhanced by overexpressing PKR. Expression of non-phosphorylatable mutant eIF2αS51A, but not of wild type eIF2α, abrogated efficient splicing of rev/tat mRNA. By contrast, expression of eIF2αS51D, a phosphomimetic mutant of eIF2α, left rev/tat mRNA splicing intact. Unlike eIF2αS51A, eIF2αS51D does not inhibit eIF2α phosphorylation by activated PKR. All HIV mRNA species contain terminal trans-activation response (TAR) stem-loop sequences that potentially could activate PKR, yet even upon TAR deletion, HIV mRNA production remained sensitive to inhibitors of PKR activation. Bioinformatic and mutational analyses revealed a compact RNA pseudoknot upstream of 3'-terminal TAR that promotes splicing by activating PKR. Supporting its essential role in control of splicing, this pseudoknot is conserved among diverse HIV and nonhuman primate SIVcpz isolates. The pseudoknot and 3'-terminal TAR collaborate in mediating PKR-regulated splicing of rev/tat intron, the pseudoknot being dominant. CONCLUSIONS: Our results on HIV provide the first example of a virus co-opting activation of PKR by its RNA, a cellular antiviral mechanism, to promote splicing. They raise the question whether other viruses may use local activation of host kinase PKR through RNA elements within their genome to achieve efficient splicing of their mRNA. Our experiments reveal an indispensable role for eIF2α phosphorylation in HIV rev/tat mRNA splicing that accounts for the need for PKR activation.

Transient CRISPR-Cas Treatment Can Prevent Reactivation of HIV-1 Replication in a Latently Infected T-Cell Line.

Novel therapeutic strategies aiming at the permanent inactivation of the HIV-1 reservoir in infected individuals are currently being explored, including approaches based on CRISPR-Cas gene editing. Extinction of all infectious HIV provirus in infected T-cell cultures was previously achieved when cells were transduced with lentiviral vectors for the stable expression of CRISPR-Cas9 or Cas12a systems targeting HIV DNA. Because lentiviral transduction and long-term CRISPR-Cas activity are less suitable for in vivo application of this antiviral strategy, we investigated whether HIV can also be completely inactivated by transient CRISPR-Cas activity. Latently infected SupT1 T-cells were repeatedly transfected with different Cas9 and Cas12a mRNA/protein sources in combination with dual gRNAs/crRNAs targeting highly conserved viral sequences. Upon repeated Cas9 protein treatment, viral replication could no longer be reactivated. We demonstrate that this was due to complete mutational inactivation of the proviral DNA, mostly through mutations at the target sites, but also through excision or inversion of the viral DNA fragment between the two target sites. These results demonstrate that repeated transient CRISPR-Cas treatment of a latently infected T-cell culture can lead to the permanent inactivation of HIV replication, indicating that transient CRISPR-Cas delivery methods can be considered for in vivo application.

Variations in the Abortive HIV-1 RNA Hairpin Do Not Impede Viral Sensing and Innate Immune Responses.

The highly conserved trans-acting response element (TAR) present in the RNA genome of human immunodeficiency virus 1 (HIV-1) is a stably folded hairpin structure involved in viral replication. However, TAR is also sensed by viral sensors, leading to antiviral immunity. While high variation in the TAR RNA structure renders the virus replication-incompetent, effects on viral sensing remain unclear. Here, we investigated the role of TAR RNA structure and stability on viral sensing. TAR mutants with deletions in the TAR hairpin that enhanced thermodynamic stability increased antiviral responses. Strikingly, TAR mutants with lower stability due to destabilization of the TAR hairpin also increased antiviral responses without affecting pro-inflammatory responses. Moreover, mutations that affected the TAR RNA sequence also enhanced specific antiviral responses. Our data suggest that mutations in TAR of replication-incompetent viruses can still induce immune responses via viral sensors, hereby underscoring the robustness of HIV-1 RNA sensing mechanisms.

Impact of Nuclear Export Pathway on Cytoplasmic HIV-1 RNA Transport Mechanism and Distribution.

HIV-1 full-length RNA (referred to as HIV-1 RNA here) serves as the viral genome in virions and as a template for Gag/Gag-Pol translation. We previously showed that HIV-1 RNA, which is exported via the CRM1 pathway, travels in the cytoplasm mainly through diffusion. A recent report suggested that the export pathway used by retroviral RNA could affect its cytoplasmic transport mechanism and localization. HIV-1 RNA export is directed by the viral protein Rev and the cis-acting element, Rev response element (RRE). When Rev/RRE is replaced with the constitutive transport element (CTE) from Mason-Pfizer monkey virus (MPMV), HIV-1 RNA is exported through the NXF1 pathway. To determine the effects of the export pathway on HIV-1 RNA, we tracked individual RNAs and found that the vast majority of cytoplasmic HIV-1 RNAs travel by diffusion regardless of the export pathway. However, CTE-containing HIV-1 RNA diffuses at a rate slower than that of RRE-containing HIV-1 RNA. Using in situ hybridization, we analyzed the subcellular localizations of HIV-1 RNAs in cells expressing a CTE-containing and an RRE-containing provirus. We found that these two types of HIV-1 RNAs have similar subcellular distributions. HIV-1 RNA exported through the NXF1 pathway was suggested to cluster near centrosomes. To investigate this possibility, we measured the distances between individual RNAs to the centrosomes and found that HIV-1 RNAs exported through different pathways do not exhibit significantly different distances to centrosomes. Therefore, HIV-1 RNAs exported through CRM1 and NXF1 pathways use the same RNA transport mechanism and exhibit similar cytoplasmic distributions.IMPORTANCE The unspliced HIV-1 full-length RNA (HIV-1 RNA) is packaged into virions as the genome and is translated to generate viral structural proteins and enzymes. To serve these functions, HIV-1 RNA must be exported from the nucleus to the cytoplasm. It was recently suggested that export pathways used by HIV-1 RNA could affect its cytoplasmic transport mechanisms and distribution. In the current report, we examined the HIV-1 RNA transport mechanism by following the movement of individual RNAs and identifying the distribution of RNA using in situ hybridization. Our results showed that whether exported by the CRM1 or NXF1 pathway, HIV-1 RNAs mainly use diffusion for cytoplasmic travel. Furthermore, HIV-1 RNAs exported using the CRM1 or NXF1 pathway are well mixed in the cytoplasm and do not display export pathway-specific clustering near centrosomes. Thus, the export pathways used by HIV-1 RNAs do not alter the cytoplasmic transport mechanisms or distribution.

Extinction of all infectious HIV in cell culture by the CRISPR-Cas12a system with only a single crRNA.

The CRISPR-Cas9 system has been used for genome editing of various organisms. We reported inhibition of the human immunodeficiency virus (HIV) in cell culture infections with a single guide RNA (gRNA) and subsequent viral escape, but complete inactivation of infectious HIV with certain combinations of two gRNAs. The new RNA-guided endonuclease system CRISPR-Cas12a (formerly Cpf1) may provide a more promising tool for genome engineering with increased activity and specificity. We compared Cas12a to the original Cas9 system for inactivation of the integrated HIV DNA genome. Superior antiviral activity is reported for Cas12a, which can achieve full HIV inactivation with only a single gRNA (called crRNA). We propose that the different architecture of Cas9 versus Cas12a endonuclease explains this effect. We also disclose that DNA cleavage by the Cas12a endonuclease and subsequent DNA repair causes mutations with a sequence profile that is distinct from that of Cas9. Both CRISPR systems can induce the typical small deletions around the site of DNA cleavage and subsequent repair, but Cas12a does not induce the pure DNA insertions that are routinely observed for Cas9. Although these typical signatures are apparent in many literature studies, this is the first report that documents these striking differences.

CRISPR-Cas9 Dual-gRNA Attack Causes Mutation, Excision and Inversion of the HIV-1 Proviral DNA.

Although several studies demonstrated that the HIV proviral DNA can be effectively targeted and inactivated by the CRISPR-Cas9 system, the precise inactivation mechanism has not yet been analyzed. Whereas some studies suggested efficient proviral DNA excision upon dual-gRNA/Cas9 treatment, we previously demonstrated that hypermutation of the target sites correlated with permanent virus inactivation. To better understand the mechanism underlying HIV inactivation, we analyzed the proviral DNA upon Cas9 attack with gRNA pairs. We observed that dual-gRNA targeting resulted more frequently in target site mutation than fragment excision, while fragment inversion was rarely observed. The frequencies varied for different gRNA combinations without an obvious relationship with the distance between the target sites, indicating that other gRNA and target DNA characteristics influence the DNA cleavage and repair processes.

Elimination of infectious HIV DNA by CRISPR-Cas9.

Current antiretroviral drugs can efficiently block HIV replication and prevent transmission, but do not target the HIV provirus residing in cells that constitute the viral reservoir. Because drug therapy interruption will cause viral rebound from this reservoir, HIV-infected individuals face lifelong treatment. Therefore, novel therapeutic strategies are being investigated that aim to permanently inactivate the proviral DNA, which may lead to a cure. Multiple studies showed that CRISPR-Cas9 genome editing can be used to attack HIV DNA. Here, we will focus on not only how this endonuclease attack can trigger HIV provirus inactivation, but also how virus escape occurs and this can be prevented.

On the generation of the MSD-Ñ° class of defective HIV proviruses.

Antiretroviral therapy (ART) can effectively suppress ongoing HIV replication and block disease progression, but the infection is never cured due to the persistence of a small pool of latently infected cells hosting integrated replication-competent HIV proviruses. However, the vast majority of HIV proviruses in ART-treated patients are replication-incompetent due to a variety of genetic defects. Most defective proviruses (around 90%) contain large internal deletions or are G-to-A hypermutated, resulting in destruction of most if not all viral open reading frames, which is consistent with the idea that cytotoxic T cells (CTLs) effectively remove cells that produce viral antigens. An intriguing subclass of defective proviruses (around 10%) that are consistently detected in such patients carry a small deletion or a point mutation in a relatively precise and well conserved region near the 5' end of the HIV genome, in the area that encodes the major splice donor (MSD) site and the packaging signal Ñ° in the viral RNA genome. Why this subclass of proviruses is defective has never been properly understood. We now propose a mechanistic scenario for how these MSD-Ñ° mutations can prevent viral protein expression. Based on ample results in literature, we argue that MSD inactivation triggers the activity of the 5'-polyadenylation site, resulting in the production of ultra-short non-protein-coding HIV transcripts.

IFI16 Targets the Transcription Factor Sp1 to Suppress HIV-1 Transcription and Latency Reactivation.

The interferon γ-inducible protein 16 (IFI16) is known as immune sensor of retroviral DNA intermediates. We show that IFI16 restricts HIV-1 independently of immune sensing by binding and inhibiting the host transcription factor Sp1 that drives viral gene expression. This antiretroviral activity and ability to bind Sp1 require the N-terminal pyrin domain and nuclear localization of IFI16, but not the HIN domains involved in DNA binding. Highly prevalent clade C HIV-1 strains are more resistant to IFI16 and less dependent on Sp1 than other HIV-1 subtypes. Furthermore, inhibition of Sp1 by IFI16 or pharmacologically by Mithramycin A suppresses reactivation of latent HIV-1 in CD4+ T cells. Finally, IFI16 also inhibits retrotransposition of LINE-1, known to engage Sp1, and murine IFI16 homologs restrict Friend retrovirus replication in mice. Thus, IFI16 restricts retroviruses and retrotransposons by interfering with Sp1-dependent gene expression, and evasion from this restriction may facilitate spread of HIV-1 subtype C.

Interferon-inducible TRIM22 contributes to maintenance of HIV-1 proviral latency in T cell lines.

The human immunodeficiency virus type-1 (HIV-1) establishes a state of latent infection in a small number of CD4+ T lymphocytes that, nonetheless, represent a major obstacle to viral eradication. We here show that Tripartite Motif-containing protein 22 (TRIM22), an epigenetic inhibitor of Specificity protein 1 (Sp1)-dependent HIV-1 transcription, is a relevant factor in maintaining a state of repressed HIV-1 expression at least in CD4+ T cell lines. By knocking-down (KD) TRIM22 expression, we observed an accelerated reactivation of a doxycycline (Dox)-controlled HIV-1 replication in the T lymphocytic SupT1 cell line. Furthermore, we here report for the first time that TRIM22 is a crucial factor for maintaining a state of HIV-1 quiescence in chronically infected ACH2 -T cell line while its KD potentiated HIV-1 expression in both ACH-2 and J-Lat 10.6 cell lines upon cell stimulation with either tumor necrosis factor-α (TNF-α) or histone deacetylase inhibitors (HDACi). In conclusion, TRIM22 is a novel determinant of HIV-1 latency, at least in T cell lines, thus representing a potential pharmacological target for strategies aiming at curtailing or silencing the pool of latently infected CD4+ T lymphocytes constituting the HIV-1 reservoir in individuals receiving combination antiretroviral therapy.

The Impact of HIV-1 Genetic Diversity on CRISPR-Cas9 Antiviral Activity and Viral Escape.

The clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 system is widely explored for sequence-specific attack on HIV-1 proviral DNA. We recently identified dual-guide RNA (dual-gRNA) combinations that can block HIV-1 replication permanently in infected cell cultures and prevent viral escape. Although the gRNAs were designed to target highly conserved viral sequences, their efficacy may be challenged by high genetic variation in the HIV-1 genome. We therefore evaluated the breadth of these dual-gRNA combinations against distinct HIV-1 isolates, including several subtypes. Replication of nearly all virus isolates could be prevented by at least one gRNA combination, which caused inactivation of the proviral genomes and the gradual loss of replication-competent virus over time. The dual-gRNA efficacy was not affected by most single nucleotide (nt) mismatches between gRNA and the viral target. However, 1-nt mismatches at the Cas9 cleavage site and two mismatches anywhere in the viral target sequence significantly reduced the inhibitory effect. Accordingly, sequence analysis of viruses upon breakthrough replication revealed the acquisition of escape mutations in perfectly matching and most 1-nt mismatching targets, but not in targets with a mismatch at the Cas9 cleavage site or with two mismatches. These results demonstrate that combinatorial CRISPR-Cas9 treatment can cure T cells infected by distinct HIV-1 isolates, but even minor sequence variation in conserved viral target sites can affect the efficacy of this strategy. Successful cure attempts against isolates with divergent target sequences may therefore require adaptation of the gRNAs.

The HIV-1 Tat Protein Enhances Splicing at the Major Splice Donor Site.

Transcription of the HIV-1 proviral DNA and subsequent processing of the primary transcript results in the production of a large set of unspliced and differentially spliced viral RNAs. The major splice donor site (5'ss) that is located in the untranslated leader of the HIV-1 transcript is used for the production of all spliced RNAs, and splicing at this site has to be tightly regulated to allow the balanced production of all viral RNAs and proteins. We demonstrate that the viral Tat protein, which is known to activate viral transcription, also stimulates splicing at the major 5'ss. As for the transcription effect, Tat requires the viral long terminal repeat promoter and the trans-acting responsive RNA hairpin for splicing regulation. These results indicate that HIV-1 transcription and splicing are tightly coupled processes through the coordinated action of the essential Tat protein.IMPORTANCE The HIV-1 proviral DNA encodes a single RNA transcript that is used as RNA genome and packaged into newly assembled virus particles. This full-length RNA is also used as mRNA for the production of structural and enzymatic proteins. Production of other essential viral proteins depends on alternative splicing of the primary transcript, which yields a large set of differentially spliced mRNAs. Optimal virus replication requires a balanced production of all viral RNAs, which means that the splicing process has to be strictly regulated. We show that the HIV-1 Tat protein, a factor that is well known for its transcription activating function, also stimulates splicing. Thus, Tat controls not only the level of the viral RNA but also the balance between spliced and unspliced RNAs.

Tackling HIV Persistence: Pharmacological versus CRISPR-Based Shock Strategies.

Jan Svoboda studied aspects of viral latency, in particular with respect to disease induction by avian RNA tumor viruses, which were later renamed as part of the extended retrovirus family. The course of retroviral pathogenesis is intrinsically linked to their unique property of integrating the DNA copy of the retroviral genome into that of the host cell, thus forming the provirus. Retroviral latency has recently become of major clinical interest to allow a better understanding of why we can effectively block the human immunodeficiency virus type 1 (HIV-1) in infected individuals with antiviral drugs, yet never reach a cure. We will discuss HIV-1 latency and its direct consequence-the formation of long-lasting HIV-1 reservoirs. We next focus on one of the most explored strategies in tackling HIV-1 reservoirs-the "shock and kill" strategy-which describes the broadly explored pharmacological way of kicking the latent provirus, with subsequent killing of the virus-producing cell by the immune system. We furthermore present how the clustered regularly interspaced palindromic repeats (CRISPR) and associated protein (Cas) system can be harnessed to reach the same objective by reactivating HIV-1 gene expression from latency. We will review the benefits and drawbacks of these different cure strategies.

CRISPR-Cas based antiviral strategies against HIV-1.

In bacteria and archaea, the clustered regularly interspaced short palindromic repeats (CRISPR) and associated proteins (Cas) confer adaptive immunity against exogenous DNA elements. This CRISPR-Cas system has been turned into an effective tool for editing of eukaryotic DNA genomes. Pathogenic viruses that have a double-stranded DNA (dsDNA) genome or that replicate through a dsDNA intermediate can also be targeted with this DNA editing tool. Here, we review how CRISPR-Cas was used in novel therapeutic approaches against the human immunodeficiency virus type-1 (HIV-1), focusing on approaches that aim to permanently inactivate all virus genomes or to prevent viral persistence in latent reservoirs.

Establishment of a mouse xenograft model of metastatic adrenocortical carcinoma.

Adrenocortical carcinoma is a rare neoplasm with a poor prognosis. Very important advances have been made in the identification of the genetic determinants of adrenocortical carcinoma pathogenesis but our understanding is still limited about the mechanisms that determine cancer spread and metastasis. One major problem hindering preclinical experimentation for new therapies for adrenocortical carcinoma is represented by the lack of suitable animal models for metastatic disease. With the aim to overcome these limitations, in this study we tested several protocols in order to establish a mouse xenograft model of metastatic adrenocortical carcinoma. The most efficient method, based upon intrasplenic injection followed by splenectomy, produced metastases with high efficiency, whose development could be followed over time by bioluminescence measurements. We expect that the availability of this model will greatly improve the possibilities for preclinical testing of new treatments for advanced-stage disease.