Construction of a cytomegalovirus-based amplicon: a vector with a unique transfer capacity

Construction and Characterization of a High-Capacity Replication-Competent Murine Cytomegalovirus Vector for Gene Delivery

We investigated the basic characteristics of a new murine cytomegalovirus (MCMV) vector platform. Using BAC technology, we engineered replication-competent recombinant MCMVs with deletions of up to 26% of the wild-type genome. To this end, we targeted five gene blocks (m01-m17, m106-m109, m129-m141, m144-m158, and m159-m170). BACs featuring deletions from 18% to 26% of the wild-type genome exhibited delayed virus reconstitution, while smaller deletions (up to 16%) demonstrated reconstitution kinetics similar to those of the wild type. Utilizing an innovative methodology, we introduced large genomic DNA segments, up to 35 kbp, along with reporter genes into a newly designed vector with a potential cloning capacity of 46 kbp (Q4). Surprisingly, the insertion of diverse foreign DNAs alleviated the delayed plaque formation phenotype of Q4, and these large inserts remained stable through serial in vitro passages. With reporter-gene-expressing recombinant MCMVs, we successfully transduced not only mouse cell lines but also non-rodent mammalian cells, including those of human, monkey, bovine, and bat origin. Remarkably, even non-mammalian cell lines derived from chickens exhibited successful transduction.

New intranasal and injectable gene therapy for healthy life extension

SignificanceUsing CMV as a gene therapy vector we illustrated that CMV can be used therapeutically as a monthly inhaled or intraperitoneally delivered treatment for aging-associated decline. Exogenous telomerase reverse transcriptase or follistatin genes were safely and effectively delivered in a murine model. This treatment significantly improved biomarkers associated with healthy aging, and the mouse lifespan was increased up to 41% without an increased risk of cancer. The impact of this research on an aging population cannot be understated as the global aging-related noncommunicable disease burden quickly rises.

Developing extrachromosomal gene expression vector technologies: an overview

Extrachromosomal, or episomal, vectors offer a number of advantages for therapeutic and scientific applications compared to integrating vectors. Extrachromosomal vectors persist in the nucleus without the requirement to integrate into the host genome, hence avoiding the recent concerns surrounding the genotoxic effects of vector integration. By avoiding integration, episomal vectors avoid vector rearrangement, which can occur at integration, and also avoid any effect of surrounding DNA activity on transgene expression ("position effect"). Extrachromosomal vectors offer a very high transgene capacity, allowing either the incorporation of large promoter and regulatory elements into an expression cassette, or the use of complete genomic loci of up to 100 kb or larger as transgenes. Whole genomic loci transgenes offer an elegant means to express genes under physiological and developmental-stage regulation, to express multiple transcript variants from a single locus, and to express multiple genes from a single tract of genomic DNA. The combined advantages of episomal vectors of prolonged transgene persistence in the absence of vector integration, avoiding silencing by flanking heterochromatin, and high capacity, facilitating delivery and expression of genomic DNA transgenes, will be reviewed here and potential therapeutic and scientific uses outlined.

Prospects for the use of artificial chromosomes and minichromosome-like episomes in gene therapy

Artificial chromosomes and minichromosome-like episomes are large DNA molecules capable of containing whole genomic loci, and be maintained as nonintegrating, replicating molecules in proliferating human somatic cells. Authentic human artificial chromosomes are very difficult to engineer because of the difficulties associated with centromere structure, so they are not widely used for gene-therapy applications. However, OriP/EBNA1-based episomes, which they lack true centromeres, can be maintained stably in dividing cells as they bind to mitotic chromosomes and segregate into daughter cells. These episomes are more easily engineered than true human artificial chromosomes and can carry entire genes along with all their regulatory sequences. Thus, these constructs may facilitate the long-term persistence and physiological regulation of the expression of therapeutic genes, which is crucial for some gene therapy applications. In particular, they are promising vectors for gene therapy in inherited diseases that are caused by recessive mutations, for example haemophilia A and Friedreich's ataxia. Interestingly, the episome carrying the frataxin gene (deficient in Friedreich's ataxia) has been demonstrated to rescue the susceptibility to oxidative stress which is typical of fibroblasts from Friedreich's ataxia patients. This provides evidence of their potential to treat genetic diseases linked to recessive mutations through gene therapy.

The impact of genome length on replication and genome stability of the herpesvirus guinea pig cytomegalovirus

The impact of genome length on replication and genome stability was assessed for guinea pig cytomegalovirus (GPCMV), a member of the Herpesviridae. The 233-kb genome could be decreased by 15.1 kb without discernable impact on viral replication efficiency in vitro. Viruses with genomes under-length by up to 31 kb replicated with decreased efficiencies but this appeared to arise from the loss of augmenting viral genes rather than decreased genome length. Two deletions that were non-lethal on their own were lethal when combined, suggesting that the resulting 40.1 kb under-length genome fell below a minimum packageable size. Genomes over-length by 8.8 kb gave rise to spontaneous deletions just to the right of the major immediate early locus, the same region that undergoes deletions during fibroblast passage of human and rhesus cytomegaloviruses. These results suggest that genome integrity should be confirmed for herpesvirus mutants in which genome length is increased even modestly.

Advances in high-capacity extrachromosomal vector technology: episomal maintenance, vector delivery, and transgene expression

Recent developments in extrachromosomal vector technology have offered new ways of designing safer, physiologically regulated vectors for gene therapy. Extrachromosomal, or episomal, persistence in the nucleus of transduced cells offers a safer alternative to integrating vectors which have become the subject of safety concerns following serious adverse events in recent clinical trials. Extrachromosomal vectors do not cause physical disruption in the host genome, making these vectors safe and suitable tools for several gene therapy targets, including stem cells. Moreover, the high insert capacity of extrachromosomal vectors allows expression of a therapeutic transgene from the context of its genomic DNA sequence, providing an elegant way to express normal splice variants and achieve physiologically regulated levels of expression. Here, we describe past and recent advances in the development of several different extrachromosomal systems, discuss their retention mechanisms, and evaluate their use as expression vectors to deliver and express genomic DNA loci. We also discuss a variety of delivery systems, viral and nonviral, which have been used to deliver episomal vectors to target cells in vitro and in vivo. Finally, we explore the potential for the delivery and expression of extrachromosomal transgenes in stem cells. The long-term persistence of extrachromosomal vectors combined with the potential for stem cell proliferation and differentiation into a wide range of cell types offers an exciting prospect for therapeutic interventions.

The essential human cytomegalovirus gene UL52 is required for cleavage-packaging of the viral genome

Replication of human cytomegalovirus (HCMV) produces large DNA concatemers of head-to-tail-linked viral genomes that upon packaging into capsids are cut into unit-length genomes. The mechanisms underlying cleavage-packaging and the subsequent steps prior to nuclear egress of DNA-filled capsids are incompletely understood. The hitherto uncharacterized product of the essential HCMV UL52 gene was proposed to participate in these processes. To investigate the function of pUL52, we constructed a DeltaUL52 mutant as well as a complementing cell line. We found that replication of viral DNA was not impaired in noncomplementing cells infected with the DeltaUL52 virus, but viral concatemers remained uncleaved. Since the subnuclear localization of the known cleavage-packaging proteins pUL56, pUL89, and pUL104 was unchanged in DeltaUL52-infected fibroblasts, pUL52 does not seem to act via these proteins. Electron microscopy studies revealed only B capsids in the nuclei of DeltaUL52-infected cells, indicating that the mutant virus has a defect in encapsidation of viral DNA. Generation of recombinant HCMV genomes encoding epitope-tagged pUL52 versions showed that only the N-terminally tagged pUL52 supported viral growth, suggesting that the C terminus is crucial for its function. pUL52 was expressed as a 75-kDa protein with true late kinetics. It localized preferentially to the nuclei of infected cells and was found to enclose the replication compartments. Taken together, our results demonstrate an essential role for pUL52 in cleavage-packaging of HCMV DNA. Given its unique subnuclear localization, the function of pUL52 might be distinct from that of other cleavage-packaging proteins.

Targeted deletion of regions rich in immune-evasive genes from the cytomegalovirus genome as a novel vaccine strategy

Human cytomegalovirus (CMV), a ubiquitous human pathogen, is a leading cause of congenital infections and represents a serious health risk for the immunosuppressed patient. A vaccine against CMV is currently not available. CMV is characterized by its large genome and by multiple genes modulating the immunity of the host, which cluster predominantly at genome termini. Here, we tested whether the deletion of gene blocks rich in immunomodulatory genes could be used as a novel concept in the generation of immunogenic but avirulent, herpesvirus vaccines. To generate an experimental CMV vaccine, we selectively deleted 32 genes from the mouse cytomegalovirus (MCMV) genome. The resulting mutant grew to titers similar to that of wild-type MCMV in vitro. In vivo, the mutant was 10,000-fold attenuated and well tolerated, even by highly susceptible mice deficient for B, T, and NK cells or for the interferon type I receptor. Equally relevant for safety concerns, immune suppression did not lead to the mutant's reactivation from latency. Immunization with the replication-competent mutant, but not with inactivated virus, resulted in protective immunity, which increased over time. Vaccination induced MCMV-specific antibodies and a strong T-cell response. We propose that a targeted and rational approach can improve future herpesvirus vaccines and vaccine vectors.

Analysis of human cytomegalovirus oriLyt sequence requirements in the context of the viral genome

During the lytic phase of infection, replication of herpesvirus genomes initiates at the lytic origin of replication, oriLyt. Many herpesviruses harbor more than one lytic origin, but so far, only one oriLyt has been identified for human cytomegalovirus (HCMV). Evidence for the existence of additional lytic origins of HCMV has remained elusive. On the basis of transient replication assays with cloned viral fragments, HCMV oriLyt was described as a core region of 1.5 kbp (minimal oriLyt) flanked by auxiliary sequences required for maximal replication activity (complete oriLyt). It remained unclear whether minimal oriLyt alone can drive the replication of HCMV in the absence of its accessory regions. To investigate the sequence requirements of oriLyt in the context of the viral genome, mutant genomes were constructed lacking either minimal or complete oriLyt. These genomes were not infectious, suggesting that HCMV contains only one lytic origin of replication. Either minimal or complete oriLyt was then ectopically reinserted into the oriLyt-depleted genomes. Only the mutant genomes carrying complete oriLyt led to infectious progeny. Remarkably, inversion of the 1.5-kbp core origin relative to its flanking regions resulted in a replication-defective genome. Mutant genomes carrying minimal oriLyt plus the left flanking region gave rise to minifoci, but genomes harboring minimal oriLyt together with the right flanking region were noninfectious. We conclude that the previously defined minimal lytic origin is not sufficient to drive replication of the HCMV genome. Rather, our results underline the importance of the accessory regions and their correct arrangement for the function of HCMV oriLyt.

Human cytomegalovirus plasmid-based amplicon vector system for gene therapy

We have constructed and evaluated the utility of a helper-dependent virus vector system that is derived from Human Cytomegalovirus (HCMV). This vector is based on the herpes simplex virus (HSV) amplicon system and contains the HCMV orthologs of the two cis-acting functions required for replication and packaging of HSV genomes, the complex HCMV viral DNA replication origin (oriLyt), and the cleavage packaging signal (the a sequence). The HCMV amplicon vector replicated independently and was packaged into infectious virions in the presence of helper virus. This vector is capable of delivering and expressing foreign genes in infected cells including progenitor cells such as human CD34+ cells. Packaged defective viral genomes were passaged serially in fibroblasts and could be detected at passage 3; however, the copy number appeared to diminish upon serial passage. The HCMV amplicon offers an alternative vector strategy useful for gene(s) delivery to cells of the hematopoietic lineage.