CRISPR/Cas9 and HIV1
Larry H. Bernstein, MD, FCAP, Curator
Harnessing the CRISPR/Cas9 system to disrupt latent HIV-1 provirus
Scientific Reports 2013; 2510(3) http://dx.doi.org:/10.1038/srep02510
Even though highly active anti-retroviral therapy is able to keep HIV-1 replication under control, the virus can lie in a dormant state within the host genome, known as a latent reservoir, and poses a threat to re-emerge at any time. However, novel technologies aimed at disrupting HIV-1 provirus may be capable of eradicating viral genomes from infected individuals. In this study, we showed the potential of the CRISPR/Cas9 system to edit the HIV-1 genome and block its expression. When LTR-targeting CRISPR/Cas9 components were transfected into HIV-1 LTR expression-dormant and -inducible T cells, a significant loss of LTR-driven expression was observed after stimulation. Sequence analysis confirmed that this CRISPR/Cas9 system efficiently cleaved and mutated LTR target sites. More importantly, this system was also able to remove internal viral genes from the host cell chromosome. Our results suggest that the CRISPR/Cas9 system may be a useful tool for curing HIV-1 infection.
Integration of reverse transcribed viral DNA into the host cell genome is an essential step during the HIV-1 life cycle1. The integrated retroviral DNA is termed a provirus, which serves as the fundamental source of viral protein production. HIV-1 gene expression is regulated by LTR promoter and enhancer activities, where cellular transcription factors such as NF-κB, SP-1 and TBP bind to promote RNA polymerase II processivity. Subsequently, Tat protein is expressed from early double-spliced transcripts and binds to the trans activation response (TAR) region of HIV-1 RNA for its efficient elongation2.
Latent infection occurs when the HIV-1 provirus becomes transcriptionally inactive, resulting in a latent reservoir that has become the main obstacle in preventing viral eradication from HIV-1 infected individuals. However, the mechanisms of viral silencing and reactivation remain incompletely understood3. Previous studies have suggested that the position of the integration site strongly influences viral gene expression and may be one of the determinants of HIV-1 latency4. While highly active anti-retroviral therapy (HARRT) has dramatically decreased mortality from HIV-1 infection, there is currently no effective strategy to target the latent form of HIV-1 proviruses5.
Over the last decade, novel genome-editing methods that utilize artificial nucleases such as zinc finger nucleases (ZFNs)6 and transcription activator like-effector nucleases (TALENs)7 have been developed. These molecularly engineered nucleases recognize and cleave specific nucleotide sequences in target genomes for digestion, resulting in various mutations such as substitutions, deletions and insertions induced by host DNA repair machinery. These technologies have enabled the production of genome-manipulated animals in a wide range of species such as Drosophila8, Zebrafish9 and Rat10. However, ZFNs or TALENs remain somewhat difficult and time-consuming to design, develop, and empirically test in a cellular context11. Recently, a third genome-editing method was developed based on clustered regularly interspaced short palindromic repeat (CRISPR) systems. CRISPR systems were originally identified in bacteria and archaea12 as part of an adaptive immune system, dependent on a complex consisting of CRISPR RNAs (crRNAs) and CRISPR-associated (Cas) proteins to degrade complimentary sequences of invading viral and plasmid DNA. Mali et al. created a novel version of the genome-editing tool applicable to mammalian cells, termed the CRISPR/Cas9 system, which is based on modifications of the Streptococcus pyogenes type II CRISPR system in crRNA fused to trans-encoded tracrRNA13. This CRISPR/Cas9 system is composed of guide RNA (gRNA) and a human codon-optimized Cas9 nuclease that forms an RNA-protein complex to digest unique target sequences matching those of gRNA. The CRISPR/Cas9 system can be utilized by simple transfection of designed gRNA and a humanized Cas9 (hCas9) expression plasmid into target mammalian cells, making it a promising tool for various applications.
In this study, we tested the ability of the CRISPR/Cas9 system to suppress HIV-1 expression by editing HIV-1 integrated proviral DNA. Cas9 and gRNA, designed to target HIV-1 LTR, were transfected and significantly inhibited LTR-driven expression under the control of Tat. This LTR-targeted CRISPR/Cas9 system can also excise provirus from the cellular genome.
CRISPR/Cas9 system can target the latent form of HIV-1 provirus in Jurkat cell
Because the putative latently infected cells are CD4+ T cells, we next tested the genome editing potential of the CRISPR/Cas9 system in these cells.
In this study, we successfully disrupted the expression of HIV-1 provirus utilizing the CRISPR/Cas9 system (Fig. 1). Importantly, this disruption not only restricted transcriptionally active provirus, it also blocked the expression of latently integrated provirus (Fig. 3). Cas9 proteins are predicted to contain RuvC and HNH motifs15, which possess autonomous ssDNA cleavage activity. Interestingly, mutants lacking one of the motifs become nicking endonucleases16. It is plausible that the independent nicking activity of each domain may enhance efficient access to the heterochromatin state of latently integrated provirus. Another possibility is that Cas9 has a highly efficient target surveillance system similar to what has been previously reported for the Cas3 system17.
T6 gRNA that targeted the NF-κB binding site, also strongly suppressed the LTR promoter activity (Fig. 1). However, the effect was weaker than that of T5 gRNA. In this study we used an LTIG vector modified from the LTR of HIV-1 strain NL4-3 that possesses two adjacent NF-κB binding sites18. The T6 target site is at the end of the 5′ NF-κB binding site, meaning that mutations may not completely render transcription inactive since the 3′ NF-κB binding site may remain functional. On the other hand, T5 gRNA that targeted TAR, is profoundly effective in disrupting HIV-1 gene expression. The putative cleavage site was positioned at the neck of the stem loop region of TAR, which is critical for Cyclin T1-Tat-TAR ternary complex formation19. Therefore, the TAR sequence may be one of the best targets for blocking HIV-1 provirus expression. Target specificity of the CRISPR/Cas system is very high and a single mutation can disrupt targeting20, meaning that some provirus may escape from this genome-editing machinery if mutations arise in target sequences. However, given that the TAR region is relatively conserved and there is little variation among HIV-1 subtypes21, it could still be an appropriate target for the elimination of latently infected provirus.
Perhaps the most important finding in this study is that we could excise provirus from the host genome of HIV-1 infected cells, which may provide a ray of hope to eradicate HIV-1 from infected individuals. However, there are numerous hurdles that must be cleared before utilizing genome editing for HIV-1 eradication therapies such as gene therapy. First, the efficiency of genome-editing and/or proviral excision should be quantified in HIV infected primary cells, including latently infected CD4+ quiescent T cells. Second, an efficient delivery system must be developed. Fortunately, the CRISPR/Cas9 system has the advantage in size compared with TALENs22. Thus, the CRISPR system has the potential to be delivered by lentivirus vectors, whereas TALENs do not because of their large size and repeat sequences23. The final hurdle concerns possible off-target effects, which are pertinent concerns for all genome-editing strategies that may lead to nonspecific gene modification events. If Cas9 has off-target effects, then removal of the off-target activity may be the best approach before utilizing CRISPR/Cas system for anti-HIV treatment.
Elimination of HIV-1 Genomes from Human T-lymphoid Cells by CRISPR/Cas9 Gene Editing
Scientific Reports 6, Article number: 22555 (2016) http://dx.doi.org:/10.1038/srep22555
We employed an RNA-guided CRISPR/Cas9 DNA editing system to precisely remove the entire HIV-1 genome spanning between 5′ and 3′ LTRs of integrated HIV-1 proviral DNA copies from latently infected human CD4+ T-cells. Comprehensive assessment of whole-genome sequencing of HIV-1 eradicated cells ruled out any off-target effects by our CRISPR/Cas9 technology that might compromise the integrity of the host genome and further showed no effect on several cell health indices including viability, cell cycle and apoptosis. Persistent co-expression of Cas9 and the specific targeting guide RNAs in HIV-1-eradicated T-cells protected them against new infection by HIV-1. Lentivirus-delivered CRISPR/Cas9 significantly diminished HIV-1 replication in infected primary CD4+ T-cell cultures and drastically reduced viral load in ex vivo culture of CD4+ T-cells obtained from HIV-1 infected patients. Thus, gene editing using CRISPR/Cas9 may provide a new therapeutic path for eliminating HIV-1 DNA from CD4+ T-cells and potentially serve as a novel and effective platform toward curing AIDS.
AIDS remains a major public health problem, as over 35 million people worldwide are HIV-1-infected and new infections continue at steady rate of greater than two million per year. Antiretroviral therapy (ART) effectively controls viremia in virtually all HIV-1 patients and partially restores the primary host cell (CD4+ T-cells), but fails to eliminate HIV-1 from latently-infected T-cells1,2. In latently-infected CD4+ T cells, integrated proviral DNA copies persist in a dormant state, but can be reactivated to produce replication-competent virus when T-cells are activated, resulting in rapid viral rebound upon interruption of antiretroviral treatment3,4,5,6,7,8. Therefore, most HIV-1-infected individuals, even those who respond very well to ART, must maintain life-long ART due to persistence of HIV-1-infected reservoir cells. During latency HIV infected cells produce little or no viral protein, thereby avoiding viral cytopathic effects and evading clearance by the host immune system. Because the resting CD4+ memory T-cell compartment9is thought to be the most prominent latently-infected cell pool, it is a key focus of research aimed at eradicating latent HIV-1 infection.
Recent efforts to eradicate HIV-1 from this cell population have used primarily a “shock and kill” approach, with the rationale that inducing HIV reactivation in CD4+ memory T-cells may trigger elimination of virus-producing cells by cytolysis or host immune responses. For example, epigenetic modification of chromatin structure is critical for establishing viral reactivation. Consequently, inhibition of histone deacetylase (HDAC) by Trichostatin A (TSA) and vorinostat (SAHA) led to reactivation of latent virus in cell lines10,11,12. Accordingly, other HDACi, including vorinostat, valproic acid, panobinostat and rombidepsin have been tested ex vivo and have led, in the best cases, to transient increases in viremia13,14. Similarly, protein kinase C agonists, can potently reactivate HIV either singly or in combination with HDACi15,16. However, there are multiple limitations of this approach: (i) since a large fraction of HIV genomes in this reservoir are non-functional, not all integrated provirus can produce replication-competent virus17; (ii) total numbers of CD4+ T cells reactivated from resting CD4+ T cell HIV-1 reservoirs, has been found by viral outgrowth assays to be much smaller than the numbers of cells infected, as detected by PCR-based assays, suggesting that not all cells within this reservoir are reactivated18; (iii) the cytotoxic T lymphocyte (CTL) immune response is not sufficiently robust to eliminate the reactivated infected cells19 and (iv) uninfected T-cells are not protected from HIV infection and can therefore sustain viral rebound.
These observations suggest that a cure strategy for HIV-1 infection should include methods that directly eliminate the proviral genome from the majority of HIV-1-positive cells, including CD4+ T-cells, and protect cells from future infection, with little or no harm to the host. The clustered, regularly-interspaced, short palindromic repeats (CRISPR)/CRISPR-associated 9 (Cas9) nuclease has wide utility for genome editing in a broad range of organisms including yeast, Drosophila, zebrafish, C. elegans, and mice, and has been applied in a broad range of in vivo and in vitro studies toward human diseases20,21,22,23,24. Recently we modified the CRISPR/Cas9 system to enable recognition of specific DNA sequences positioned within the HIV-1 promoter spanning the 5′ long terminal sequence (LTR)25,26. Using this modified system, we now demonstrate excision of integrated copies of the proviral DNA fragment from a latently HIV-1-infected human T-lymphoid cell line, completely eliminating HDAC inhibition-elicited viral production. Results of whole-genome sequencing and comprehensive bioinformatic analysis ruled out any genotoxicity to host cell DNA. Further, we found that lentivirally-delivered CRISPR/Cas9 reduces viral replication upon HIV-1 infection of primary cultured CD4+ T-cells. The results point toward this approach as a promising potential therapeutic avenue to eradicating HIV-1 from T reservoir cells of host patients, to prevent AIDS re-emergence.
Despite its remarkable therapeutic success and efficacy, ART treatment is unable to eradicate HIV-1 from infected patients who must therefore undergo life-long treatment. A new therapeutic strategy is thus needed in order to achieve permanent remission allowing patients to stop ART and reduce it’s attendant costs and potential long-term side effects. Our findings address key barriers to this goal, as we developed CRISPR/Cas9 techniques that eradicated integrated copies of HIV-1 from human CD4+ T-cells, inhibited HIV-1 infection in primary cultured human CD4+ T-cells, and suppressed viral replication ex vivo in peripheral blood mononuclear cells (PBMCs) and CD4+ T-cells of HIV-1+ patients. They also address a further key issue, providing evidence that such gene editing effectively impedes viral replication without causing genotoxicity to host DNA or eliciting destructive effects via host cell pathways. Prior studies using gene editing based on zinc finger nuclease (ZFN), transcription activator-like effector nuclease (TALEN), and CRISPR/Cas9 systems prompted much interest in their potential abilities to suppress viral infection, either by altering virus receptors or introducing mutations in the viral genome (for review see26,30). All these studies suggest that gene editing strategies can be engineered for targeting specific regions of the viral genome and once efficiently delivered to infected cells, their robust antiviral activity effectively suppresses viral replication. However, there are several important issues that require close attention including the careful design of the targeting strategy that achieves the highest levels of specificity and safety with optimum efficiency of editing.
In this study, due to the complexity associated with determination of the sites and numbers of randomly integrated proviral HIV-1 DNA in in vitro infected primary cell culture and the difficulty in full scale characterization of the InDel/Excision by Cas9/gRNAs in these cells, as a first step, we chose to use the clonal 2D10 cell line as a human T-cell latency model to establish: (i) the ability of Cas9/gRNA in removing the entire coding sequence of the integrated copies of the HIV-1 DNA using ultradeep whole genome sequencing and (ii) investigate its safely related to off-target effects and cell viability. Once these goals were accomplished, we shifted our attention to primary cell culture as well as patient samples to examine the efficiency of the CRISPR/Cas9 in affecting viral DNA load in a laboratory setting.
We found that CRISPR/Cas9 edited multiple copies of viral DNA scattered among the chromosomes. Combined treatment of latently-infected T cells with Cas9 plus gRNAs A and B that recognize specific DNA motifs within the LTR U3 region efficiently eliminated the entire viral DNA fragment spanning between the two LTRs. The remaining 5′ LTR and 3′ LTR cleavage sites by Cas9 and gRNA B in chromosome 1, and by Cas9 and gRNAs A and B in chromosome 16, were joined by host DNA repair at sites located precisely three nucleotides upstream of the PAM. Genome-wide assessment of CRISPR/Cas9-treated HIV-1-infected 2D10 cells clearly verified complete excision of the integrated copies of viral DNA from the second intron of RSBN1 and exon 2 of MSRB1 genes. To address the critical issue related to its specificity and potential off-target and adverse effects, we analyzed this comprehensively and at an unprecedented level of detail, by whole-genome sequencing and bioinformatic analyses. These revealed many naturally-occurring mutations in the genomes of control cells and gRNAs A- and B-mediated HIV-1 DNA eradication. The mutations discovered included naturally-occurring InDels, base excisions, and base substitutions, all of which are, more or less, expected in rapidly growing cells in culture, including Jurkat 2D10 cells. The critical issue is our discovery that none of these mutations resulted from our gene-editing system, as we identified no sequence identities with either gRNA A or B within 1200 nucleotides of any such mutation site. Further, our method of HIV-1 DNA excision had no adverse effects on proximal or distal cellular genes and showed no impact on cell viability, cell cycle progression or proliferation, and did not induce apoptosis, thus strongly supporting its safety at this translational phase, by all in vitromeasures assessed in cultured cells. We found that the expression levels of Cas9 and the gRNAs diminished after several passages and eventually disappeared, but as long as Cas9 and single or multiplex gRNAs were present, cells remained protected against new HIV-1infection.
Another key translational feasibility question we addressed is whether CRISPR/Cas9-mediated HIV-1 eradication can prevent or suppress HIV-1 infection in the most relevant human and patient target cell populations. We provide a critical new advance, by observing in PBMCs and CD4+ T-cells from HIV-1 infected patients that lentivirally-delivered Cas9/gRNAs A/B significantly decreased viral copy numbers and protein levels. Using primer sets directed within the LTR, we amplified and detected residual viral DNA fragments that were not completely deleted in these cells, yet were affected by Cas9/gRNAs and contained InDel mutants near the PAM sequence. These findings verified that CRISPR/Cas9 exerted efficacious antiviral activity in the PBMCs of HIV-1 patients. We also found that introducing Cas9/gRNAs A/B via lentiviral delivery into primary cultured human CD4+ HIV-1JRFL– or HIV-1NL4-3-infected T-cells significantly reduced viral copy numbers, corroborating earlier findings by us and others that stably-integrated HIV-1-directed Cas9 and gRNAs (distinct from our gRNAs A and B used presently) conferred resistance to HIV-1 infection in cell lines31,32. With the notion that CRISPR/Cas9 can target both integrated, as well as episomal DNA sequences, as evidenced by its editing ability of various human viruses as well as plasmid DNAs in either configuration31,32,33,34,35,36, it is likely that both the integrated as well as pre-integrated, free-floating intracellular HIV-1 DNA are edited by Cas9/gRNA.
As noted, during the course of our studies no ART was included prior to the treatment with CRISPR/Cas9 as our goal in this study was to determine the extent of viral suppression during the productive stage of viral infection. We observed a significant level of suppression suggesting that CRISPR/Cas9 may effectively disable expression of the functionally active integrated copies of HIV-1 DNA in the host chromosome. This notion is supported by our observations using 2D10 CD4+ T-cells where the latent copies of HIV-1 that are integrated in chromosomes 1 and 16 were effectively eliminated by CRISPR/Cas9. Our future studies are aimed to address the impact of CRISPR/Cas9 in in vitro infected CD4+ T-cells where the virus is controlled by ART and a cohort of naïve and ART-treated patient CD4+ T-cells. Results from these studies should determine whether or not, in the context of ART, the virus enters into the latent stage and remains responsive to CRISPR/Cas9. Of note, results from these ex vivo studies using ART treated patient PBMCs and CD4+ T-cells show that CRISPR/Cas9 effectively suppresses viral replication by introducing InDel mutations.
Our findings show comprehensively and conclusively that the entire coding sequence of host-integrated HIV-1 was eradicated in human 2D10 T cells, providing a strong first step of support for potential translatability of such a system to T-cell-directed HIV-1 therapies in patients. The complete absence of genomic and off-target functional effects in all assays also provides critical support for the promise of developing this approach for future therapeutic applications.
When evaluating a therapeutic strategy based on CRISPR/Cas9, it is critical to understand that not only will HIV-1 be eliminated from latently infected cells, but the majority of uninfected cells will become resistant to HIV infection. Thus, there is a high likelihood that rebounding viral infections will be contained by the resistant cells. Still, some formidable challenges remain before this type of strategy can be implemented. First, it will be important to maximize elimination of viral sequences from patients. This will require analysis of the HIV-1 quasi-species harbored by patients’ CD4+ T-cells and design of suitable, i.e. personalized CRISPRs. Second, improved delivery of CRISPR/Cas9 will be required to target the majority of circulating T-cells. In summary, our novel ex vivo findings that our lentiviral delivery-based approach reduced HIV-1 DNA copy numbers and protein levels in PBMCs of HIV-1 infected patients provides strong proof-of-concept evidence that CRISPR/Cas9 can be effectively utilized as part of HIV Cure strategies.
Introduction: The use of antiretroviral therapy has led to a significant decrease in morbidity and mortality in HIV-infected individuals. Nevertheless, gene-based therapies represent a promising therapeutic paradigm for HIV-1, as they have the potential for sustained viral inhibition and reduced treatment interventions. One new method amendable to a gene-based therapy is the clustered regularly interspaced short palindromic repeats (CRISPR)-associated protein-9 nuclease (Cas9) gene editing system.
Areas covered: CRISPR/Cas9 can be engineered to successfully modulate an array of disease-causing genetic elements. We discuss the diverse roles that CRISPR/Cas9 may play in targeting HIV and eradicating infection. The Cas9 nuclease coupled with one or more small guide RNAs can target the provirus to mediate excision of the integrated viral genome. Moreover, a modified nuclease-deficient Cas9 fused to transcription activation domains may induce targeted activation of proviral gene expression allowing for the purging of the latent reservoirs. These technologies can also be exploited to target host dependency factors such as the co-receptor CCR5, thus preventing cellular entry of the virus.
Expert opinion: The diversity of the CRISPR/Cas9 technologies offers great promise for targeting different stages of the viral life cycle, and have the capacity for mediating an effective and sustained genetic therapy against HIV.
Expert Opinion on Biological Therapy Sept 2003; 3(6):951-963
Expert Opinion on Biological Therapy Feb 2009; 9(2): 161-170