In vitro Inactivation of Latent HSV by Targeted Mutagenesis Using an HSV-specific Homing Endonuclease. Mol Ther Nucleic Acids. 2014;3:e146. [PMID: 24496438 PMCID: PMC3951911 DOI: 10.1038/mtna.2013.75]

Suppression of HSV-1 infection and viral reactivation by CRISPR-Cas9 gene editing in 2D and 3D culture models

Although our understanding of herpes simplex virus type 1 (HSV-1) biology has been considerably enhanced, developing therapeutic strategies to eliminate HSV-1 in latently infected individuals remains a public health concern. Current antiviral drugs used for the treatment of HSV-1 complications are not specific and do not address latent infection. We recently developed a CRISPR-Cas9-based gene editing platform to specifically target the HSV-1 genome. In this study, we further used 2D Vero cell culture and 3D human induced pluripotent stem cell-derived cerebral organoid (CO) models to assess the effectiveness of our editing constructs targeting viral ICP0 or ICP27 genes. We found that targeting the ICP0 or ICP27 genes with AAV2-CRISPR-Cas9 vectors in Vero cells drastically suppressed HSV-1 replication. In addition, we productively infected COs with HSV-1, characterized the viral replication kinetics, and established a viral latency model. Finally, we discovered that ICP0- or ICP27-targeting AAV2-CRISPR-Cas9 vector significantly reduced viral rebound in the COs that were latently infected with HSV-1. In summary, our results suggest that CRISPR-Cas9 gene editing of HSV-1 is an efficient therapeutic approach to eliminate the latent viral reservoir and treat HSV-1-associated complications.

Summary of the Centers for Disease Control and Prevention/National Institute of Allergy and Infectious Diseases Joint Workshop on Genital Herpes: 3-4 November 2022

Genital herpes is caused by infection with herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) and currently has no cure. The disease is the second-most common sexually transmitted infection in the United States, with an estimated 18.6 million prevalent genital infections caused by HSV-2 alone. Genital herpes diagnostics and treatments are not optimal, and no vaccine is currently available. The Centers for Disease Control and Prevention and the National Institute of Allergy and Infectious Diseases convened a workshop entitled "CDC/NIAID Joint Workshop on Genital Herpes." This report summarizes 8 sessions on the epidemiology of genital herpes, neonatal HSV, HSV diagnostics, vaccines, treatments, cures, prevention, and patient advocacy perspective intended to identify opportunities in herpes research and foster the development of strategies to diagnose, treat, cure, and prevent genital herpes.

HSV-1 exploits host heterochromatin for nuclear egress

Herpes simplex virus (HSV-1) progeny form in the nucleus and exit to successfully infect other cells. Newly formed capsids navigate complex chromatin architecture to reach the inner nuclear membrane (INM) and egress. Here, we demonstrate by transmission electron microscopy (TEM) that HSV-1 capsids traverse heterochromatin associated with trimethylation on histone H3 lysine 27 (H3K27me3) and the histone variant macroH2A1. Through chromatin profiling during infection, we revealed global redistribution of these marks whereby massive host genomic regions bound by macroH2A1 and H3K27me3 correlate with decreased host transcription in active compartments. We found that the loss of these markers resulted in significantly lower viral titers but did not impact viral genome or protein accumulation. Strikingly, we discovered that loss of macroH2A1 or H3K27me3 resulted in nuclear trapping of capsids. Finally, by live-capsid tracking, we quantified this decreased capsid movement. Thus, our work demonstrates that HSV-1 takes advantage of the dynamic nature of host heterochromatin formation during infection for efficient nuclear egress.

Longitudinal Detection of Twenty DNA and RNA Viruses in Allogeneic Hematopoietic Stem Cell Transplant Recipients Plasma

Metagenomics revealed novel and routinely overlooked viruses, representing sources of unrecognized infections after allogeneic hematopoietic stem cell transplantation (allo-HSCT). We aim to describe DNA and RNA virus prevalence and kinetics in allo-HSCT recipients' plasma for one year post HSCT. We included 109 adult patients with first allo-HSCT from 1 March 2017 to 31 January 2019 in this observational cohort study. Seventeen DNA and three RNA viral species were screened with qualitative and/or quantitative r(RT)-PCR assays using plasma samples collected at 0, 1, 3, 6, and 12 months post HSCT. TTV infected 97% of patients, followed by HPgV-1 (prevalence: 26-36%). TTV (median 3.29 × 10⁵ copies/mL) and HPgV-1 (median 1.18 × 10⁶ copies/mL) viral loads peaked at month 3. At least one Polyomaviridae virus (BKPyV, JCPyV, MCPyV, HPyV6/7) was detected in >10% of patients. HPyV6 and HPyV7 prevalence reached 27% and 12% at month 3; CMV prevalence reached 27%. HSV, VZV, EBV, HHV-7, HAdV and B19V prevalence remained <5%. HPyV9, TSPyV, HBoV, EV and HPg-V2 were never detected. At month 3, 72% of patients had co-infections. TTV and HPgV-1 infections were highly prevalent. BKPyV, MCPyV and HPyV6/7 were frequently detected relative to classical culprits. Further investigation is needed into associations between these viral infections and immune reconstitution or clinical outcomes.

The Impact of Antiviral Resistance on Herpetic Keratitis

ABSTRACT

Herpes simplex keratitis resistance to antiviral treatment presents a growing concern. The herpes simplex virus has many different mechanisms of resistance to antiviral treatment, which have been well described. Resistance to acyclovir occurs because of mutations in the viral thymidylate kinase and DNA polymerase that decrease this enzyme's affinity for its substrate. This article discusses factors that explain the prevalence of this resistance, the ability for recurrences in immunocompromised populations, current treatments for acyclovir-resistant herpes simplex keratitis, and novel therapies for this growing concern.

Current landscape in antiviral drug development against herpes simplex virus infections

Herpes simplex viruses (HSV) are common human pathogens with a combined global seroprevalence of 90% in the adult population. HSV-1 causes orofacial herpes but can cause severe diseases, such as the potentially fatal herpes encephalitis and herpes keratitis, a prevalent cause of infectious blindness. The hallmark of HSV is lifelong latent infections and viral reactivations, leading to recurrent lesions or asymptomatic shedding. HSV-1 and HSV-2 can cause recurrent, painful, and socially limiting genital lesions, which predispose to human immunodeficiency virus infections, and can lead to neonatal herpes infections, a life-threatening condition for the newborn. Despite massive efforts, there is no vaccine against HSV, as both viruses share the capability to evade the antiviral defenses of human and to establish lifelong latency. Recurrent and primary HSV infections are treated with nucleoside analogs, but the treatments do not completely eliminate viral shedding and transmission. Drug-resistant HSV strains can emerge in relation to long-term prophylactic treatment. Such strains are likely to be resistant to other chemotherapies, justifying the development of novel antiviral treatments. The importance of developing new therapies against HSV has been recognized by the World Health Organization. In this review, we discuss the current approaches for developing novel antiviral therapies against HSV, such as small molecule inhibitors, biopharmaceuticals, natural products, gene editing, and oligonucleotide-based therapies. These approaches may have potential in the future to answer the unmet medical need. Furthermore, novel approaches are presented for potential eradication of latent HSV.

Antiviral Targeting of Varicella Zoster Virus Replication and Neuronal Reactivation Using CRISPR/Cas9 Cleavage of the Duplicated Open Reading Frames 62/71

Varicella Zoster Virus (VZV) causes Herpes Zoster (HZ), a common debilitating and complicated disease affecting up to a third of unvaccinated populations. Novel antiviral treatments for VZV reactivation and HZ are still in need. Here, we evaluated the potential of targeting the replicating and reactivating VZV genome using Clustered Regularly Interspaced Short Palindromic Repeat-Cas9 nucleases (CRISPR/Cas9) delivered by adeno-associated virus (AAV) vectors. After AAV serotype and guide RNA (gRNA) optimization, we report that a single treatment with AAV2-expressing Staphylococcus aureus CRISPR/Cas9 (saCas9) with gRNA to the duplicated and essential VZV genes ORF62/71 (AAV2-62gRsaCas9) greatly reduced VZV progeny yield and cell-to-cell spread in representative epithelial cells and in lytically infected human embryonic stem cell (hESC)-derived neurons. In contrast, AAV2-62gRsaCas9 did not reduce the replication of a recombinant virus mutated in the ORF62 targeted sequence, establishing that antiviral effects were a consequence of VZV-genome targeting. Delivery to latently infected and reactivation-induced neuron cultures also greatly reduced infectious-virus production. These results demonstrate the potential of AAV-delivered genome editors to limit VZV productive replication in epithelial cells, infected human neurons, and upon reactivation. The approach could be developed into a strategy for the treatment of VZV disease and virus spread in HZ.

CRISPR-Cas9 Expressed in Stably Transduced Cell Lines Promotes Recombination and Selects for Herpes Simplex Virus Recombinants

Recombinant herpes simplex virus strains can be constructed by several methods, including homologous recombination, bacterial artificial chromosome manipulation, and yeast genetic methods. Homologous recombination may have the advantage of introducing fewer genetic alterations in the viral genome, but the low level of recombinants can make this method more time consuming if there is no screen or selection. In this study we used complementing cell lines that express Cas9 and guide RNAs targeting the parental virus to rapidly generate recombinant viruses. Analysis of the progeny viruses indicated that CRISPR-Cas9 both promoted recombination to increase recombinant viruses and selected against parental viruses in the transfection progeny viruses. This approach can also be used to enrich for recombinants made by any of the current methods.

Varicella Zoster Virus Neuronal Latency and Reactivation Modeled in Vitro

Latency and reactivation in neurons are critical aspects of VZV pathogenesis that have historically been difficult to investigate. Viral genomes are retained in many human ganglia after the primary infection, varicella; and about one-third of the naturally infected VZV seropositive population reactivates latent virus, which most often clinically manifests as herpes zoster (HZ or Shingles). HZ is frequently complicated by acute and chronic debilitating pain for which there remains a need for more effective treatment options. Understanding of the latent state is likely to be essential in the design of strategies to reduce reactivation. Experimentally addressing VZV latency has been difficult because of the strict human species specificity of VZV and the fact that until recently, experimental reactivation had not been achieved. We do not yet know the neuron subtypes that harbor latent genomes, whether all can potentially reactivate, what the drivers of VZV reactivation are, and how immunity interplays with the latent state to control reactivation. However, recent advances have enabled a picture of VZV latency to start to emerge. The first is the ability to detect the latent viral genome and its expression in human ganglionic tissues with extraordinary sensitivity. The second, the subject of this chapter, is the development of in vitro human neuron systems permitting the modeling of latent states that can be experimentally reactivated. This review will summarize recent advances of in vitro models of neuronal VZV latency and reactivation, the limitations of the current systems, and discuss outstanding questions and future directions regarding these processes using these and yet to be developed models. Results obtained from the in vitro models to date will also be discussed in light of the recent data gleaned from studies of VZV latency and gene expression learned from human cadaver ganglia, especially the discovery of VZV latency transcripts that seem to parallel the long-studied latency-associated transcripts of other neurotropic alphaherpesviruses.

Development of Genome Editing Approaches against Herpes Simplex Virus Infections

Herpes simplex virus 1 (HSV-1) is a herpesvirus that may cause cold sores or keratitis in healthy or immunocompetent individuals, but can lead to severe and potentially life-threatening complications in immune-immature individuals, such as neonates or immune-compromised patients. Like all other herpesviruses, HSV-1 can engage in lytic infection as well as establish latent infection. Current anti-HSV-1 therapies effectively block viral replication and infection. However, they have little effect on viral latency and cannot completely eliminate viral infection. These issues, along with the emergence of drug-resistant viral strains, pose a need to develop new compounds and novel strategies for the treatment of HSV-1 infection. Genome editing methods represent a promising approach against viral infection by modifying or destroying the genetic material of human viruses. These editing methods include homing endonucleases (HE) and the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR associated protein (Cas) RNA-guided nuclease system. Recent studies have showed that both HE and CRISPR/Cas systems are effective in inhibiting HSV-1 infection in cultured cells in vitro and in mice in vivo. This review, which focuses on recently published progress, suggests that genome editing approaches could be used for eliminating HSV-1 latent and lytic infection and for treating HSV-1 associated diseases.

CRISPR-Cas9 gene editing of hepatitis B virus in chronically infected humanized mice

Chronic hepatitis B virus (HBV) infection is a major public health problem. New treatment approaches are needed because current treatments do not target covalently closed circular DNA (cccDNA), the template for HBV replication, and rarely clear the virus. We harnessed adeno-associated virus (AAV) vectors and CRISPR-Staphylococcus aureus (Sa)Cas9 to edit the HBV genome in liver-humanized FRG mice chronically infected with HBV and receiving entecavir. Gene editing was detected in livers of five of eight HBV-specific AAV-SaCas9-treated mice, but not control mice, and mice with detectable HBV gene editing showed higher levels of SaCas9 delivery to HBV⁺ human hepatocytes than those without gene editing. HBV-specific AAV-SaCas9 therapy significantly improved survival of human hepatocytes, showed a trend toward decreasing total liver HBV DNA and cccDNA, and was well tolerated. This work provides evidence for the feasibility and safety of in vivo gene editing for chronic HBV infections, and it suggests that with further optimization, this approach may offer a plausible way to treat or even cure chronic HBV infections.

Gene editing and elimination of latent herpes simplex virus in vivo

We evaluate gene editing of HSV in a well-established mouse model, using adeno-associated virus (AAV)-delivered meganucleases, as a potentially curative approach to treat latent HSV infection. Here we show that AAV-delivered meganucleases, but not CRISPR/Cas9, mediate highly efficient gene editing of HSV, eliminating over 90% of latent virus from superior cervical ganglia. Single-cell RNA sequencing demonstrates that both HSV and individual AAV serotypes are non-randomly distributed among neuronal subsets in ganglia, implying that improved delivery to all neuronal subsets may lead to even more complete elimination of HSV. As predicted, delivery of meganucleases using a triple AAV serotype combination results in the greatest decrease in ganglionic HSV loads. The levels of HSV elimination observed in these studies, if translated to humans, would likely significantly reduce HSV reactivation, shedding, and lesions. Further optimization of meganuclease delivery and activity is likely possible, and may offer a pathway to a cure for HSV infection.

Single AAV-Mediated CRISPR-SaCas9 Inhibits HSV-1 Replication by Editing ICP4 in Trigeminal Ganglion Neurons

Herpes simplex keratitis (HSK) is the most common cause of corneal blindness in developed nations, caused by primary or recurrent herpes simplex virus 1 (HSV-1) infection of the cornea. Latent infection of HSV-1, especially in the trigeminal ganglion (TG), causes recurrence of HSV-1 infection. As antiviral treatment is not effective on latent HSV-1, to test the possibility of inhibiting HSV-1 by SpCas9 (Streptococcus pyogenes Cas9) or SaCas9 (Staphylococcus aureus Cas9), ICP0 and ICP4, two important genes required for HSV-1 replication and reactivation, were chosen as targets. In Vero cells, SpCas9 and SaCas9 targeting ICP0 or ICP4 can effectively inhibit the proliferation of HSV-1 without affecting cell viability. No significant guide RNA (gRNA)-dependent off-targets were observed in the human genome by digenome sequencing and deep sequencing verification. Adeno-associated virus 1 (AAV1)-mediated delivery of SaCas9 inhibits HSV-1 replication by targeting ICP4 in mouse primary TG neuronal cells. SpCas9 and SaCas9 are able to inhibit HSV-1 infection in Vero cells and mouse TG neuronal cultures with high efficiency and good biosafety. AAV1-mediated delivery of SaCas9 shows great potential in treating HSK and inhibiting HSV-1 in TG neurons. Further investigations may be needed to test the inhibition of latent infections, which may result in the development of novel methods for treating viral diseases.

Biologic interactions between HSV-2 and HIV-1 and possible implications for HSV vaccine development

Development of a safe and effective vaccine against herpes simplex virus type 2 (HSV-2) has the potential to limit the global burden of HSV-2 infection and disease, including genital ulcer disease and neonatal herpes, and is a global sexual and reproductive health priority. Another important potential benefit of an HSV-2 vaccine would be to decrease HIV infections, as HSV-2 increases the risk of HIV-1 acquisition several-fold. Acute and chronic HSV-2 infection creates ulcerations and draws dendritic cells and activated CD4+ T cells into genital mucosa. These cells are targets for HIV entry and replication. Prophylactic HSV-2 vaccines (to prevent infection) and therapeutic vaccines (to modify or treat existing infections) are currently under development. By preventing or modifying infection, an effective HSV-2 vaccine could limit HSV-associated genital mucosal inflammation and thus HIV risk. However, a vaccine might have competing effects on HIV risk depending on its mechanism of action and cell populations generated in the genital mucosa. In this article, we review biologic interactions between HSV-2 and HIV-1, consider HSV-2 vaccine development in the context of HIV risk, and discuss implications and research needs for future HSV vaccine development.

CRISPR/Cas9 and Genome Editing for Viral Disease-Is Resistance Futile?

Chronic viral infections remain a major public health issue affecting millions of people worldwide. Highly active antiviral treatments have significantly improved prognosis and infection-related morbidity and mortality but have failed to eliminate persistent viral forms. Therefore, new strategies to either eradicate or control these viral reservoirs are paramount to allow patients to stop antiretroviral therapy and realize a cure. Viral genome disruption based on gene editing by programmable endonucleases is one promising curative gene therapy approach. Recent findings on RNA-guided human immunodeficiency virus 1 (HIV-1) genome cleavage by Cas9 and other gene-editing enzymes in latently infected cells have shown high levels of site-specific genome disruption and potent inhibition of virus replication. However, HIV-1 can readily develop resistance to genome editing at a single antiviral target site. Current data suggest that cellular repair associated with DNA double-strand breaks can accelerate the emergence of resistance. On the other hand, a combination antiviral target strategy can exploit the same repair mechanism to functionally cure HIV-1 infection in vitro while avoiding the development of resistance. This perspective summarizes recent findings on the biology of resistance to genome editing and discusses the significance of viral genetic diversity on the application of gene editing strategies toward cure.

Attitudes and Willingness to Assume Risk of Experimental Therapy to Eradicate Genital Herpes Simplex Virus Infection

BACKGROUND

Current treatment of genital herpes is focused on ameliorating signs and symptoms but is not curative. However, as potential herpes simplex virus (HSV) cure approaches are tested in the laboratory, we aimed to assess the interest in such studies by persons with genital herpes and the willingness to assume risks associated with experimental therapy.

METHODS

We constructed an anonymous online questionnaire that was posted on websites that provide information regarding genital herpes. The questions collected demographic and clinical information on adults who self-reported as having genital herpes, and assessed attitudes toward and willingness to participate in HSV cure clinical research.

RESULTS

Seven hundred eleven participants provided sufficient responses to be included in the analysis. Sixty-six percent were women; the median age was 37 years, and the median time since genital HSV diagnosis was 4.7 years. The willingness to participate in trials increased from 59.0% in phase 1 to 68.5% in phase 2, and 81.2% in phase 3 trials, and 40% reported willingness to participate even in the absence of immediate, personal benefits. The most desirable outcome was the elimination of risk for transmission to sex partner or neonate. The mean perceived severity of receiving a diagnosis of genital HSV-2 was 4.2 on a scale of 1 to 5.

CONCLUSIONS

Despite suppressive therapy available, persons with genital herpes are interested in participating in clinical research aimed at curing HSV, especially in more advanced stages of development.

Applications of Alternative Nucleases in the Age of CRISPR/Cas9

Breakthroughs in the development of programmable site-specific nucleases, including zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), meganucleases (MNs), and most recently, the clustered regularly interspaced short palindromic repeats (CRISPR) associated proteins (including Cas9) have greatly enabled and accelerated genome editing. By targeting double-strand breaks to user-defined locations, the rates of DNA repair events are greatly enhanced relative to un-catalyzed events at the same sites. However, the underlying biology of each genome-editing nuclease influences the targeting potential, the spectrum of off-target cleavages, the ease-of-use, and the types of recombination events at targeted double-strand breaks. No single genome-editing nuclease is optimized for all possible applications. Here, we focus on the diversity of nuclease domains available for genome editing, highlighting biochemical properties and the potential applications that are best suited to each domain.

Genome editing technologies to fight infectious diseases

INTRODUCTION

Genome editing by programmable nucleases represents a promising tool that could be exploited to develop new therapeutic strategies to fight infectious diseases. These nucleases, such as zinc-finger nucleases, transcription activator-like effector nucleases, clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated protein 9 (Cas9) and homing endonucleases, are molecular scissors that can be targeted at predetermined loci in order to modify the genome sequence of an organism. Areas covered: By perturbing genomic DNA at predetermined loci, programmable nucleases can be used as antiviral and antimicrobial treatment. This approach includes targeting of essential viral genes or viral sequences able, once mutated, to inhibit viral replication; repurposing of CRISPR-Cas9 system for lethal self-targeting of bacteria; targeting antibiotic-resistance and virulence genes in bacteria, fungi, and parasites; engineering arthropod vectors to prevent vector-borne infections. Expert commentary: While progress has been done in demonstrating the feasibility of using genome editing as antimicrobial strategy, there are still many hurdles to overcome, such as the risk of off-target mutations, the raising of escape mutants, and the inefficiency of delivery methods, before translating results from preclinical studies into clinical applications.

Double deficiency of Trex2 and DNase1L2 nucleases leads to accumulation of DNA in lingual cornifying keratinocytes without activating inflammatory responses

The cornification of keratinocytes on the surface of skin and oral epithelia is associated with the degradation of nuclear DNA. The endonuclease DNase1L2 and the exonuclease Trex2 are expressed specifically in cornifying keratinocytes. Deletion of DNase1L2 causes retention of nuclear DNA in the tongue epithelium but not in the skin. Here we report that lack of Trex2 results in the accumulation of DNA fragments in the cytoplasm of cornifying lingual keratinocytes and co-deletion of DNase1L2 and Trex2 causes massive accumulation of DNA fragments throughout the cornified layers of the tongue epithelium. By contrast, cornification-associated DNA breakdown was not compromised in the epidermis. Aberrant retention of DNA in the tongue epithelium was associated neither with enhanced expression of DNA-driven response genes, such as Ifnb, Irf7 and Cxcl10, nor with inflammation. Of note, the expression of Tlr9, Aim2 and Tmem173, key DNA sensor genes, was markedly lower in keratinocytes and keratinocyte-built tissues than in macrophages and immune tissues, and DNA-driven response genes were not induced by introduction of DNA in keratinocytes. Altogether, our results indicate that DNase1L2 and Trex2 cooperate in the breakdown and degradation of DNA during cornification of lingual keratinocytes and aberrant DNA retention is tolerated in the oral epithelium.

In vivo dynamics of AAV-mediated gene delivery to sensory neurons of the trigeminal ganglia

The ability to genetically manipulate trigeminal ganglion (TG) neurons would be useful in the study of the craniofacial nervous system and latent alphaherpesvirus infections. We investigated adeno-associated virus (AAV) vectors for gene delivery to the TG after intradermal whiskerpad delivery in mice. We demonstrated that AAV vectors of serotypes 1, 7, 8, and 9 trafficked from the whiskerpad into TG neurons and expressed transgenes within cell bodies and axons of sensory neurons in all three branches of the TG. Gene expression was highest with AAV1, and steadily increased over time up to day 28. Both constitutive and neuronal-specific promoters were able to drive transgene expression in TG neurons. Levels of vector genomes in the TG increased with input dose, and multiple transgenes could be co-delivered to TG neurons by separate AAV vectors. In conclusion, AAV1 vectors are suitable for gene delivery to TG sensory neurons following intradermal whiskerpad injection.

CRISPR/Cas9, a powerful tool to target human herpesviruses

Over 90% of the adult population is infected with one or multiple herpesviruses. These viruses are characterized by their ability to establish latency, where the host is unable to clear the invader from infected cells resulting in a lifelong infection. Herpesviruses cause a wide variety of (recurrent) diseases such as cold sores, shingles, congenital defects and several malignancies. Although the productive phase of a herpesvirus infection can often be efficiently limited by nucleoside analogs, these drugs are ineffective during a latent herpesvirus infection and are therefore unable to clear herpesviruses from the human host. Advances in genome engineering using clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 facilitates virus research and may hold potential to treat or cure previously incurable herpesvirus infections by directly targeting these viruses within infected cells. Here, we review recent applications of the CRISPR/Cas9 system for herpesviral research and discuss the therapeutic potential of the system to treat, or even cure, productive and latent herpesviral infections.

Disruption or Excision of Provirus as an Approach to HIV Cure

An effective approach to HIV cure will almost certainly require a combination of strategies, including some means of reducing the latent HIV reservoir. Because the integrated HIV provirus represents the major source of viral persistence and reactivation, one attractive approach is the direct targeting of provirus for disruption or excision using targeted endonucleases, such as CRISPR/Cas9, zinc finger nucleases, TAL effector nucleases, or meganucleases (homing endonucleases). This article highlights some of the challenges for successful endonuclease therapy for HIV, including optimization of enzyme activity and specificity, the possible emergence of viral resistance, and most importantly, efficient in vivo delivery of the enzymes to a sufficient portion of the latent reservoir.

Multifaceted role of TREX2 in the skin defense against UV-induced skin carcinogenesis

TREX2 is a 3′-DNA exonuclease specifically expressed in keratinocytes. Here, we investigated the relevance and mechanisms of TREX2 in ultraviolet (UV)-induced skin carcinogenesis. TREX2 expression was up-regulated by chronic UV exposure whereas it was de-regulated or lost in human squamous cell carcinomas (SCCs). Moreover, we identified SNPs in the TREX2 gene that were more frequent in patients with head and neck SCCs than in healthy individuals. In mice, TREX2 deficiency led to enhanced susceptibility to UVB-induced skin carcinogenesis which was preceded by aberrant DNA damage removal and degradation as well as reduced inflammation. Specifically, TREX2 loss diminished the up-regulation of IL12 and IFNγ, key cytokines related to DNA repair and antitumor immunity. In UV-treated keratinocytes, TREX2 promoted DNA repair and passage to late apoptotic stages. Notably, TREX2 was recruited to low-density nuclear chromatin and micronuclei, where it interacted with phosphorylated H2AX histone, which is a critical player in both DNA repair and cell death. Altogether, our data provide new insights in the molecular mechanisms of TREX2 activity and establish cell autonomous and non-cell autonomous functions of TREX2 in the UVB-induced skin response.

Activation of the DNA Damage Response Is a Conserved Function of HIV-1 and HIV-2 Vpr That Is Independent of SLX4 Recruitment

There has been extraordinary progress in understanding the roles of lentiviral accessory proteins in antagonizing host antiviral defense proteins. However, the precise primary function of the accessory gene Vpr remains elusive. Here we suggest that engagement with the DNA damage response is an important function of primate lentiviral Vpr proteins because of its conserved function among diverse lentiviral lineages. In contrast, we show that, for HIV-1, HIV-2, and related Vpr isolates and orthologs, there is a lack of correlation between DNA damage response activation and interaction with the host SLX4 protein complex of structure specific endonucleases; some Vpr proteins are able to interact with SLX4, but the majority are not. Using the clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9 method to knock out SLX4, we formally showed that HIV-1 and HIV-2 Vpr orthologs can still activate the DNA damage response and cell cycle arrest in the absence of SLX4. Together, our data suggest that activation of the DNA damage response, but not SLX4 interaction, is conserved and therefore indicative of an important function of Vpr. Our data also indicate that Vpr activates the DNA damage response through an SLX4-independent mechanism that remains uncharacterized.

HIV-1 and HIV-2 belong to a family of viruses called lentiviruses that infect at least 40 primate species, including humans. Lentiviruses have been circulating in primates for at least 5 million years. In order to better fight HIV, we must understand the viral and host factors necessary for infection, adaptation, and transmission of these viruses. Using the natural variation of HIV-1, HIV-2, and related lentiviruses, we have investigated the role of the DNA damage response in the viral life cycle. We have found that the ability of lentiviruses to activate the DNA damage response is largely conserved. However, we also found that the SLX4 host factor is not required for this activation, as was previously proposed. This indicates that the DNA damage response is an important player in the viral life cycle, and yet the mechanism(s) by which HIV-1, HIV-2, and other primate lentiviruses engage the DNA damage response is still unknown.

In vivo disruption of latent HSV by designer endonuclease therapy

A large portion of the global population carries latent herpes simplex virus (HSV), which can periodically reactivate, resulting in asymptomatic shedding or formation of ulcerative lesions. Current anti-HSV drugs do not eliminate latent virus from sensory neurons where HSV resides, and therefore do not eliminate the risk of transmission or recurrent disease. Here, we report the ability of HSV-specific endonucleases to induce mutations of essential HSV genes both in cultured neurons and in latently infected mice. In neurons, viral genomes are susceptible to endonuclease-mediated mutagenesis, regardless of the time of treatment after HSV infection, suggesting that both HSV lytic and latent forms can be targeted. Mutagenesis frequency after endonuclease exposure can be increased nearly 2-fold by treatment with a histone deacetylase (HDAC) inhibitor. Using a mouse model of latent HSV infection, we demonstrate that a targeted endonuclease can be delivered to viral latency sites via an adeno-associated virus (AAV) vector, where it is able to induce mutation of latent HSV genomes. These data provide the first proof-of-principle to our knowledge for the use of a targeted endonuclease as an antiviral agent to treat an established latent viral infection in vivo.

Genome editing and the next generation of antiviral therapy

Engineered endonucleases such as homing endonucleases (HEs), zinc finger nucleases (ZFNs), Tal-effector nucleases (TALENS) and the RNA-guided engineered nucleases (RGENs or CRISPR/Cas9) can target specific DNA sequences for cleavage, and are proving to be valuable tools for gene editing. Recently engineered endonucleases have shown great promise as therapeutics for the treatment of genetic disease and infectious pathogens. In this review, we discuss recent efforts to use the HE, ZFN, TALEN and CRISPR/Cas9 gene-editing platforms as antiviral therapeutics. We also discuss the obstacles facing gene-editing antiviral therapeutics as they are tested in animal models of disease and transition towards human application.

CRISPR/Cas9-Mediated Genome Editing of Herpesviruses Limits Productive and Latent Infections

Herpesviruses infect the majority of the human population and can cause significant morbidity and mortality. Herpes simplex virus (HSV) type 1 causes cold sores and herpes simplex keratitis, whereas HSV-2 is responsible for genital herpes. Human cytomegalovirus (HCMV) is the most common viral cause of congenital defects and is responsible for serious disease in immuno-compromised individuals. Epstein-Barr virus (EBV) is associated with infectious mononucleosis and a broad range of malignancies, including Burkitt’s lymphoma, nasopharyngeal carcinoma, Hodgkin’s disease, and post-transplant lymphomas. Herpesviruses persist in their host for life by establishing a latent infection that is interrupted by periodic reactivation events during which replication occurs. Current antiviral drug treatments target the clinical manifestations of this productive stage, but they are ineffective at eliminating these viruses from the infected host. Here, we set out to combat both productive and latent herpesvirus infections by exploiting the CRISPR/Cas9 system to target viral genetic elements important for virus fitness. We show effective abrogation of HCMV and HSV-1 replication by targeting gRNAs to essential viral genes. Simultaneous targeting of HSV-1 with multiple gRNAs completely abolished the production of infectious particles from human cells. Using the same approach, EBV can be almost completely cleared from latently infected EBV-transformed human tumor cells. Our studies indicate that the CRISPR/Cas9 system can be effectively targeted to herpesvirus genomes as a potent prophylactic and therapeutic anti-viral strategy that may be used to impair viral replication and clear latent virus infection.

Herpesviruses are large DNA viruses that are carried by almost 100% of the adult human population. Herpesviruses include several important human pathogens, such as herpes simplex viruses (HSV) type 1 and 2 (causing cold sores and genital herpes, respectively), human cytomegalovirus (HCMV; the most common viral cause of congenital defects, and responsible for serious disease in immuno-compromised individuals), and Epstein-Barr virus (EBV; associated with infectious mononucleosis and a wide range of malignancies). Current antiviral drug treatments are not effective in clearing herpesviruses from infected individuals. Therefore, there is a need for alternative strategies to combat these pathogenic viruses and prevent or cure herpesvirus-associated diseases. Here, we have assessed whether a direct attack of herpesvirus genomes within virus-infected cells can inactivate these viruses. For this, we have made use of the recently developed CRISPR/Cas9 genome-engineering system to target and alter specific regions within the genome of these viruses. By targeting sites in the genomes of three different herpesviruses (HSV-1, HCMV, and EBV), we show complete inhibition of viral replication and in some cases even eradication of the viral genomes from infected cells. The findings presented in this study open new avenues for the development of therapeutic strategies to combat pathogenic human herpesviruses using novel genome-engineering technologies.

Pharmacodynamics of anti-HIV gene therapy using viral vectors and targeted endonucleases

OBJECTIVES

A promising curative approach for HIV is to use designer endonucleases that bind and cleave specific target sequences within latent genomes, resulting in mutations that render the virus replication incompetent. We developed a mathematical model to describe the expression and activity of endonucleases delivered to HIV-infected cells using engineered viral vectors in order to guide dose selection and predict therapeutic outcomes.

METHODS

We developed a mechanistic model that predicts the number of transgene copies expressed at a given dose in individual target cells from fluorescence of a reporter gene. We fitted the model to flow cytometry datasets to determine the optimal vector serotype, promoter and dose required to achieve maximum expression.

RESULTS

We showed that our model provides a more accurate measure of transduction efficiency compared with gating-based methods, which underestimate the percentage of cells expressing reporter genes. We identified that gene expression follows a sigmoid dose-response relationship and that the level of gene expression saturation depends on vector serotype and promoter. We also demonstrated that significant bottlenecks exist at the level of viral uptake and gene expression: only ∼1 in 220 added vectors enter a cell and, of these, depending on the dose and promoter used, between 1 in 15 and 1 in 1500 express transgene.

CONCLUSIONS

Our model provides a quantitative method of dose selection and optimization that can be readily applied to a wide range of other gene therapy applications. Reducing bottlenecks in delivery will be key to reducing the number of doses required for a functional cure.

Genome-editing Technologies for Gene and Cell Therapy

Gene therapy has historically been defined as the addition of new genes to human cells. However, the recent advent of genome-editing technologies has enabled a new paradigm in which the sequence of the human genome can be precisely manipulated to achieve a therapeutic effect. This includes the correction of mutations that cause disease, the addition of therapeutic genes to specific sites in the genome, and the removal of deleterious genes or genome sequences. This review presents the mechanisms of different genome-editing strategies and describes each of the common nuclease-based platforms, including zinc finger nucleases, transcription activator-like effector nucleases (TALENs), meganucleases, and the CRISPR/Cas9 system. We then summarize the progress made in applying genome editing to various areas of gene and cell therapy, including antiviral strategies, immunotherapies, and the treatment of monogenic hereditary disorders. The current challenges and future prospects for genome editing as a transformative technology for gene and cell therapy are also discussed.

Progress With Developing Use of Gene Editing To Cure Chronic Infection With Hepatitis B Virus

Chronic infection with hepatitis B virus (HBV) occurs in approximately 6% of the world's population. Carriers of the virus are at risk for life-threatening complications, and developing curative treatment remains a priority. The main shortcoming of licensed therapies is that they do not affect viral covalently closed circular DNA (cccDNA), a stable intermediate of replication. Harnessing gene editing to mutate cccDNA provides the means to inactivate HBV gene expression permanently. Reports have described use of engineered zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR) with CRISPR-associated (Cas) nucleases. Although inhibition of viral replication has been demonstrated, reliably detecting mutations in cccDNA has been difficult. Also, the dearth of murine models that mimic cccDNA formation has hampered analysis in vivo. To reach a stage of clinical use, efficient delivery of the editors to HBV-infected hepatocytes and limiting unintended off-target effects will be important. Investigating therapeutic efficacy in combination with other treatment strategies, such as immunotherapies, may be useful to augment antiviral effects. Advancing gene editing as a mode of treating HBV infection is now at an interesting stage and significant progress is likely to be made in the immediate future.

Digital detection of endonuclease mediated gene disruption in the HIV provirus

Genome editing by designer nucleases is a rapidly evolving technology utilized in a highly diverse set of research fields. Among all fields, the T7 endonuclease mismatch cleavage assay, or Surveyor assay, is the most commonly used tool to assess genomic editing by designer nucleases. This assay, while relatively easy to perform, provides only a semi-quantitative measure of mutation efficiency that lacks sensitivity and accuracy. We demonstrate a simple droplet digital PCR assay that quickly quantitates a range of indel mutations with detection as low as 0.02% mutant in a wild type background and precision (≤6%CV) and accuracy superior to either mismatch cleavage assay or clonal sequencing when compared to next-generation sequencing. The precision and simplicity of this assay will facilitate comparison of gene editing approaches and their optimization, accelerating progress in this rapidly-moving field.

Herpes simplex virus type-1: replication, latency, reactivation and its antiviral targets

Infection by herpes simplex virus type-1 (HSV-1) causes several diseases, ranging from cutaneous, oral and genital infections to fatal encephalitis. Despite the availability of antiviral therapies on the market, their efficacies are incomplete, and new cases of resistant strains arise, mainly in the immunocompromised, but also recently documented in immunocompetent patients. Over the last decades a lot has been discovered about the molecular basis of infection which has been of great benefit to the investigation of new anti-HSV-1 molecules. In this review we summarize replication, latency and reactivation highlighting potential antiviral targets and new molecules described in the past several years in the literature.

Detection of treatment-resistant infectious HIV after genome-directed antiviral endonuclease therapy

Incurable chronic viral infections are a major cause of morbidity and mortality worldwide. One potential approach to cure persistent viral infections is via the use of targeted endonucleases. Nevertheless, a potential concern for endonuclease-based antiviral therapies is the emergence of treatment resistance. Here we detect for the first time an endonuclease-resistant infectious virus that is found with high frequency after antiviral endonuclease therapy. While testing the activity of HIV pol-specific zinc finger nucleases (ZFNs) alone or in combination with three prime repair exonuclease 2 (Trex2), we identified a treatment-resistant and infectious mutant virus that was derived from a ZFN-mediated disruption of reverse transcriptase (RT). Although gene disruption of HIV protease, RT and integrase could inhibit viral replication, a chance single amino acid insertion within the thumb domain of RT produced a virus that could actively replicate. The endonuclease-resistant virus could replicate in primary CD4(+) T cells, but remained susceptible to treatment with antiretroviral RT inhibitors. When secondary ZFN-derived mutations were introduced into the mutant virus's RT or integrase domains, replication could be abolished. Our observations suggest that caution should be exercised during endonuclease-based antiviral therapies; however, combination endonuclease therapies may prevent the emergence of resistance.

Progress and prospects of engineered sequence-specific DNA modulating technologies for the management of liver diseases

Liver diseases are one of the leading causes of mortality in the world. The hepatic illnesses, which include inherited metabolic disorders, hemophilias and viral hepatitides, are complex and currently difficult to treat. The maturation of gene therapy has heralded new avenues for developing effective intervention for these diseases. DNA modification using gene therapy is now possible and available technology may be exploited to achieve long term therapeutic benefit. The ability to edit DNA sequences specifically is of paramount importance to advance gene therapy for application to liver diseases. Recent development of technologies that allow for this has resulted in rapid advancement of gene therapy to treat several chronic illnesses. Improvements in application of derivatives of zinc finger proteins (ZFPs), transcription activator-like effectors (TALEs), homing endonucleases (HEs) and clustered regularly interspaced palindromic repeats (CRISPR) and CRISPR associated (Cas) systems have been particularly important. These sequence-specific technologies may be used to modify genes permanently and also to alter gene transcription for therapeutic purposes. This review describes progress in development of ZFPs, TALEs, HEs and CRISPR/Cas for application to treating liver diseases.

Bacterial CRISPR/Cas DNA endonucleases: A revolutionary technology that could dramatically impact viral research and treatment

CRISPR/Cas systems mediate bacterial adaptive immune responses that evolved to protect bacteria from bacteriophage and other horizontally transmitted genetic elements. Several CRISPR/Cas systems exist but the simplest variant, referred to as Type II, has a single effector DNA endonuclease, called Cas9, which is guided to its viral DNA target by two small RNAs, the crRNA and the tracrRNA. Initial efforts to adapt the CRISPR/Cas system for DNA editing in mammalian cells, which focused on the Cas9 protein from Streptococcus pyogenes (Spy), demonstrated that Spy Cas9 can be directed to DNA targets in mammalian cells by tracrRNA:crRNA fusion transcripts called single guide RNAs (sgRNA). Upon binding, Cas9 induces DNA cleavage leading to mutagenesis as a result of error prone non-homologous end joining (NHEJ). Recently, the Spy Cas9 system has been adapted for high throughput screening of genes in human cells for their relevance to a particular phenotype and, more generally, for the targeted inactivation of specific genes, in cell lines and in vivo in a number of model organisms. The latter aim seems likely to be greatly enhanced by the recent development of Cas9 proteins from bacterial species such as Neisseria meningitidis and Staphyloccus aureus that are small enough to be expressed using adeno-associated (AAV)-based vectors that can be readily prepared at very high titers. The evolving Cas9-based DNA editing systems therefore appear likely to not only impact virology by allowing researchers to screen for human genes that affect the replication of pathogenic human viruses of all types but also to derive clonal human cell lines that lack individual gene products that either facilitate or restrict viral replication. Moreover, high titer AAV-based vectors offer the possibility of directly targeting DNA viruses that infect discrete sites in the human body, such as herpes simplex virus and hepatitis B virus, with the hope that the entire population of viral DNA genomes might be destroyed. In conclusion, we believe that the continued rapid evolution of CRISPR/Cas technology will soon have a major, possibly revolutionary, impact on the field of virology.

RNA-guided endonuclease provides a therapeutic strategy to cure latent herpesviridae infection

Latent viral infection is a persistent cause of human disease. Although standard antiviral therapies can suppress active viral replication, no existing treatment can effectively eradicate latent infection and therefore a cure is lacking for many prevalent viral diseases. The prokaryotic immune system clustered regularly interspaced short palindromic repeat (CRISPR)/Cas evolved as a natural response to phage infections, and we demonstrate here that the CRISPR/Cas9 system can be adapted for antiviral treatment in human cells by specifically targeting the genomes of latent viral infections. Patient-derived cells from a Burkitt's lymphoma with latent Epstein-Barr virus infection showed dramatic proliferation arrest and a concomitant decrease in viral load after exposure to a CRISPR/Cas9 vector targeted to the viral genome.

AAV-mediated delivery of zinc finger nucleases targeting hepatitis B virus inhibits active replication

Despite an existing effective vaccine, hepatitis B virus (HBV) remains a major public health concern. There are effective suppressive therapies for HBV, but they remain expensive and inaccessible to many, and not all patients respond well. Furthermore, HBV can persist as genomic covalently closed circular DNA (cccDNA) that remains in hepatocytes even during otherwise effective therapy and facilitates rebound in patients after treatment has stopped. Therefore, the need for an effective treatment that targets active and persistent HBV infections remains. As a novel approach to treat HBV, we have targeted the HBV genome for disruption to prevent viral reactivation and replication. We generated 3 zinc finger nucleases (ZFNs) that target sequences within the HBV polymerase, core and X genes. Upon the formation of ZFN-induced DNA double strand breaks (DSB), imprecise repair by non-homologous end joining leads to mutations that inactivate HBV genes. We delivered HBV-specific ZFNs using self-complementary adeno-associated virus (scAAV) vectors and tested their anti-HBV activity in HepAD38 cells. HBV-ZFNs efficiently disrupted HBV target sites by inducing site-specific mutations. Cytotoxicity was seen with one of the ZFNs. scAAV-mediated delivery of a ZFN targeting HBV polymerase resulted in complete inhibition of HBV DNA replication and production of infectious HBV virions in HepAD38 cells. This effect was sustained for at least 2 weeks following only a single treatment. Furthermore, high specificity was observed for all ZFNs, as negligible off-target cleavage was seen via high-throughput sequencing of 7 closely matched potential off-target sites. These results show that HBV-targeted ZFNs can efficiently inhibit active HBV replication and suppress the cellular template for HBV persistence, making them promising candidates for eradication therapy.

AAV-mediated delivery of zinc finger nucleases targeting hepatitis B virus inhibits active replication

Despite an existing effective vaccine, hepatitis B virus (HBV) remains a major public health concern. There are effective suppressive therapies for HBV, but they remain expensive and inaccessible to many, and not all patients respond well. Furthermore, HBV can persist as genomic covalently closed circular DNA (cccDNA) that remains in hepatocytes even during otherwise effective therapy and facilitates rebound in patients after treatment has stopped. Therefore, the need for an effective treatment that targets active and persistent HBV infections remains. As a novel approach to treat HBV, we have targeted the HBV genome for disruption to prevent viral reactivation and replication. We generated 3 zinc finger nucleases (ZFNs) that target sequences within the HBV polymerase, core and X genes. Upon the formation of ZFN-induced DNA double strand breaks (DSB), imprecise repair by non-homologous end joining leads to mutations that inactivate HBV genes. We delivered HBV-specific ZFNs using self-complementary adeno-associated virus (scAAV) vectors and tested their anti-HBV activity in HepAD38 cells. HBV-ZFNs efficiently disrupted HBV target sites by inducing site-specific mutations. Cytotoxicity was seen with one of the ZFNs. scAAV-mediated delivery of a ZFN targeting HBV polymerase resulted in complete inhibition of HBV DNA replication and production of infectious HBV virions in HepAD38 cells. This effect was sustained for at least 2 weeks following only a single treatment. Furthermore, high specificity was observed for all ZFNs, as negligible off-target cleavage was seen via high-throughput sequencing of 7 closely matched potential off-target sites. These results show that HBV-targeted ZFNs can efficiently inhibit active HBV replication and suppress the cellular template for HBV persistence, making them promising candidates for eradication therapy.

DNA cleavage enzymes for treatment of persistent viral infections: recent advances and the pathway forward

Treatment for most persistent viral infections consists of palliative drug options rather than curative approaches. This is often because long-lasting viral DNA in infected cells is not affected by current antivirals, providing a source for viral persistence and reactivation. Targeting latent viral DNA itself could therefore provide a basis for novel curative strategies. DNA cleavage enzymes can be used to induce targeted mutagenesis of specific genes, including those of exogenous viruses. Although initial in vitro and even in vivo studies have been carried out using DNA cleavage enzymes targeting various viruses, many questions still remain concerning the feasibility of these strategies as they transition into preclinical research. Here, we review the most recent findings on DNA cleavage enzymes for human viral infections, consider the most relevant animal models for several human viral infections, and address issues regarding safety and enzyme delivery. Results from well-designed in vivo studies will ideally provide answers to the most urgent remaining questions, and allow continued progress toward clinical application.