The latency-associated transcript promoter of pseudorabies virus directs neuron-specific expression in trigeminal ganglia of transgenic mice

Engineered compact pan-neuronal promoter from Alphaherpesvirus LAP2 enhances target gene expression in the mouse brain and reduces tropism in the liver

Small promoters capable of driving potent neuron-restricted gene expression are required to support successful brain circuitry and clinical gene therapy studies. However, converting large promoters into functional MiniPromoters, which can be used in vectors with limited capacity, remains challenging. In this study, we describe the generation of a novel version of alphaherpesvirus latency-associated promoter 2 (LAP2), which facilitates precise transgene expression exclusively in the neurons of the mouse brain while minimizing undesired targeting in peripheral tissues. Additionally, we aimed to create a compact neural promoter to facilitate packaging of larger transgenes. Our results revealed that MiniLAP2 (278 bp) drives potent transgene expression in all neurons in the mouse brain, with little to no expression in glial cells. In contrast to the native promoter, MiniLAP2 reduced tropism in the spinal cord and liver. No expression was detected in the kidney or skeletal muscle. In summary, we developed a minimal pan-neuronal promoter that drives specific and robust transgene expression in the mouse brain when delivered intravenously via AAV-PHP.eB vector. The use of this novel MiniPromoter may broaden the range of deliverable therapeutics and improve their safety and efficacy by minimizing the potential for off-target effects.

Persistent transgene expression in peripheral tissues one year post intravenous and intramuscular administration of AAV vectors containing the alphaherpesvirus latency-associated promoter 2

Significant progress has been made in enhancing recombinant adeno-associated virus (rAAV) for clinical investigation. Despite its versatility as a gene delivery platform, the inherent packaging constraint of 4.7 kb imposes restrictions on the range of diseases it can address. In this context, we present findings of an exceptionally compact and long-term promoter that facilitates the expression of larger genes compared to conventional promoters. This compact promoter originated from the genome of the alphaherpesvirus pseudorabies virus, latency-associated promoter 2 (LAP2, 404 bp). Promoter driving an mCherry reporter was packaged into single strand (ss) AAV8 and AAV9 vectors and injected into adult C57BL/6 mice at a dose of 5 × 1011 vg/mouse by single intravenous or intramuscular administration. An ssAAV8 and ssAAV9 vector with elongation factor-1α promoter (EF1α, 1264 bp) was injected side-by-side for comparison. After 400 days, we sacrificed the mice and examined mCherry expression in liver, kidney, heart, lung, spleen, pancreas, skeletal muscle, and brain. We found that LAP2 exhibited robust transgene expression across a wide range of cells and tissues comparable to the larger EF1α, which is currently recognized as a rather potent and ubiquitous promoter. The AAV8-LAP2 and AAV9-LAP2 constructs displayed strong transduction and transcription in liver, kidney, and skeletal muscle on both route of administration. However, no expression was detected in the heart, lung, spleen, pancreas, and brain. The outcomes of our investigation propose the viability of LAP2 for gene therapy applications demanding the expression of large or multiple therapeutic genes following a single viralvector administration.

Local and systemic administration of AAV vectors with alphaherpesvirus latency-associated promoter 2 drives potent transgene expression in mouse liver, kidney, and skeletal muscle

Adeno-associated virus (AAV) has great potential as a source of treatments for conditions that might respond to potent and ubiquitous transgene expression. However, among its drawbacks, the genetic "payload" of AAV vectors is limited to <4.9 kb and some commonly used gene promoters are sizeable and susceptible to transcriptional silencing. We recently described a short (404 bp), potent, and persistent promoter obtained from the genome of pseudorabies virus (PrV) called alphaherpesvirus latency-associated promoter 2 (LAP2). Here, we evaluated the biodistribution and potency of transgene expression in mouse peripheral tissues in response to local and systemic administration of AAV8-LAP2 and AAV9-LAP2. We found that administration of these vectors resulted in levels of transgene expression that were similar to the larger EF1α promoter. LAP2 drives potent transgene expression in mouse liver and kidney when administered systemically and in skeletal muscle in response to intramuscular delivery. Notably, in skeletal muscle, administration of vectors with LAP2 and EF1α promoters resulted in preferential transduction of myofibers type 2. A direct side-by-side comparison between LAP2 and the EF1α promoter revealed that, despite its smaller size, LAP2 was equally potent to the EF1α promoter and resulted in widespread gene expression after IV and IM administration of AAV8 or AAV9 vectors. Collectively, these findings suggest that constructs that include LAP2 may have the capacity to deliver large therapeutically effective payloads in support of future gene therapy protocols.

The Role of Latency-Associated Transcripts in the Latent Infection of Pseudorabies Virus

Pseudorabies virus (PRV) can cause neurological, respiratory, and reproductive diseases in pigs and establish lifelong latent infection in the peripheral nervous system (PNS). Latent infection is a typical feature of PRV, which brings great difficulties to the prevention, control, and eradication of pseudorabies. The integral mechanism of latent infection is still unclear. Latency-associated transcripts (LAT) gene is the only transcriptional region during latent infection of PRV which plays the key role in regulating viral latent infection and inhibiting apoptosis. Here, we review the characteristics of PRV latent infection and the transcriptional characteristics of the LAT gene. We also analyzed the function of non-coding RNA (ncRNA) produced by the LAT gene and its importance in latent infection. Furthermore, we provided possible strategies to solve the problem of latent infection of virulent PRV strains in the host. In short, the detailed mechanism of PRV latent infection needs to be further studied and elucidated.

Small Alphaherpesvirus Latency-Associated Promoters Drive Efficient and Long-Term Transgene Expression in the CNS

Recombinant adeno-associated viruses (rAAVs) are used as gene therapy vectors to treat central nervous system (CNS) diseases. Despite their safety and broad tropism, important issues need to be corrected such as the limited payload capacity and the lack of small gene promoters providing long-term, pan-neuronal transgene expression in the CNS. Commonly used gene promoters are relatively large and can be repressed a few months after CNS transduction, risking the long-term performance of single-dose gene therapy applications. We used a whole-CNS screening approach based on systemic delivery of AAV-PHP.eB, iDisco+ tissue-clearing and light-sheet microscopy to identify three small latency-associated promoters (LAPs) from the herpesvirus pseudorabies virus (PRV). These promoters are LAP1 (404 bp), LAP2 (498 bp), and LAP1_2 (880 bp). They drive chronic transcription of the virus-encoded latency-associated transcript (LAT) during productive and latent phases of PRV infection. We observed stable, pan-neuronal transgene transcription and translation from AAV-LAPs in the CNS for 6 months post AAV transduction. In several CNS areas, the number of cells expressing the transgene was higher for LAP2 than the large conventional EF1α promoter (1,264 bp). Our data suggest that the LAPs are suitable candidates for viral vector-based CNS gene therapies requiring chronic transgene expression after one-time viral-vector administration.

Molecular biology of pseudorabies virus: impact on neurovirology and veterinary medicine

Pseudorabies virus (PRV) is a herpesvirus of swine, a member of the Alphaherpesvirinae subfamily, and the etiological agent of Aujeszky's disease. This review describes the contributions of PRV research to herpesvirus biology, neurobiology, and viral pathogenesis by focusing on (i) the molecular biology of PRV, (ii) model systems to study PRV pathogenesis and neurovirulence, (iii) PRV transsynaptic tracing of neuronal circuits, and (iv) veterinary aspects of pseudorabies disease. The structure of the enveloped infectious particle, the content of the viral DNA genome, and a step-by-step overview of the viral replication cycle are presented. PRV infection is initiated by binding to cellular receptors to allow penetration into the cell. After reaching the nucleus, the viral genome directs a regulated gene expression cascade that culminates with viral DNA replication and production of new virion constituents. Finally, progeny virions self-assemble and exit the host cells. Animal models and neuronal culture systems developed for the study of PRV pathogenesis and neurovirulence are discussed. PRV serves asa self-perpetuating transsynaptic tracer of neuronal circuitry, and we detail the original studies of PRV circuitry mapping, the biology underlying this application, and the development of the next generation of tracer viruses. The basic veterinary aspects of pseudorabies management and disease in swine are discussed. PRV infection progresses from acute infection of the respiratory epithelium to latent infection in the peripheral nervous system. Sporadic reactivation from latency can transmit PRV to new hosts. The successful management of PRV disease has relied on vaccination, prevention, and testing.