Upstream promoter elements of the herpes simplex virus type 1 glycoprotein H gene

Adaptation of transgene mRNA translation boosts the anticancer efficacy of oncolytic HSV1

BACKGROUND

Transgenes deliver therapeutic payloads to improve oncolytic virus immunotherapy. Transgenes encoded within oncolytic viruses are designed to be highly transcribed, but protein synthesis is often negatively affected by viral infection, compromising the amount of therapeutic protein expressed. Studying the oncolytic herpes simplex virus-1 (HSV1), we found standard transgene mRNAs to be suboptimally translated in infected cells.

METHODS

Using RNA-Seq reads, we determined the transcription start sites and 5'leaders of HSV1 genes and uncovered the US11 5'leader to confer superior activity in translation reporter assays. We then incorporated this 5'leader into GM-CSF expression cassette in oncolytic HSV1 and compared the translationally adapted oncolytic virus with the conventional, leaderless, virus in vitro and in mice.

RESULTS

Inclusion of the US11 5'leader in the GM-CSF transgene incorporated into HSV1 boosted translation in vitro and in vivo. Importantly, treatment with US11 5'leader-GM-CSF oncolytic HSV1 showed superior antitumor immune activity and improved survival in a syngeneic mouse model of colorectal cancer as compared with leaderless-GM-CSF HSV1.

CONCLUSIONS

Our study demonstrates the therapeutic value of identifying and integrating platform-specific cis-acting sequences that confer increased protein synthesis on transgene expression.

The initiator element in a herpes simplex virus type 1 late-gene promoter enhances activation by ICP4, resulting in abundant late-gene expression

The start site regions of late genes of herpes simplex virus type 1 are similar to the eukaryotic initiator sequence (Inr), have been shown to affect the levels of expression, and may also play a role in transcription activation by the viral activator ICP4. A series of linker-scanning mutations spanning the start site of transcription and several downstream mutations in the true late gC promoter were analyzed in reconstituted in vitro transcription reactions with and without ICP4, as well as in the context of the viral genome during infection. The nucleotide contacts previously found to be important for Inr function were also found to be important for optimal induction by ICP4. While the Inr had a substantial effect on the accumulation of gC RNA during infection, no other sequence downstream of the TATA box to +124 had a significant effect on levels of expression during infection. Therefore, these studies suggest that TATA box and the Inr are the only cis-acting elements required to achieve optimal expression of gC, and that the high levels of late-gene transcription may be largely due to the induction by ICP4, functioning through the Inr element.

Regulation of herpes simplex virus gene expression

Expression of the more than 80 individual genes of herpes simplex virus 1 (HSV-1) takes place in a tightly regulated sequential manner that was first described over 20 years ago. Investigations since that time have focused on understanding the mechanisms that regulate this orderly and efficient expression of viral genes. This review examines recent findings that have shed light on how this process is regulated during productive infection of the cell. Although the story is still not complete, several aspects of HSV gene expression are now clearer as a result of these findings. In particular, several new functions have recently been ascribed to some of the known viral regulatory proteins. The results indicate that the viral gene expression is regulated through transcriptional as well as post-transcriptional mechanisms. In addition, it has become increasingly clear that the virus has evolved specific functions to interact with the host cell in order to divert and redirect critical host functions for its own needs. Understanding the interactions of HSV and the host cell during infection will be essential for a complete understanding of how viral gene expression is regulated. Future challenges in the field will be to develop a complete understanding of the mechanisms that temporally regulate virus gene expression, and to identify and characterize the relevant interactions between the virus and the distinctive cell types normally infected by the virus.

On the control of late gene expression in Kaposi's sarcoma-associated herpesvirus (human herpesvirus-8)

Herpesvirus late genes require viral DNA replication for maximal expression. Although late gene expression appears to require DNA replication in cis in alphaherpesviruses, studies in Epstein-Barr virus (EBV) suggest that this cis-requirement might not pertain to the gammaherpesviruses. Based on these findings, a system was created to investigate the elements required for the regulation of Kaposi's sarcoma-associated herpesvirus (KSHV; human herpesvirus-8) late gene expression. The transcript of a classic late gene encoding the viral assembly protein was characterized and reporter genes driven by the assembly protein promoter region were constructed. Unlike the EBV case, expression of a reporter gene under the control of the assembly protein promoter did not display authentic regulation when removed from the context of the viral genome. Although reporter expression rose in cells displaying lytic replication, this expression was not diminished by specific inhibitors of viral DNA synthesis. Minimal core promoters were similarly unable to reproduce late gene regulation. These results suggest that proper KSHV late gene expression is likely to be dependent upon virus lytic replication in cis and indicate that the regulation of KSHV late genes more closely resembles that observed in herpes simplex virus than that described for EBV.

The exchange of cognate TATA boxes results in a corresponding change in the strength of two HSV-1 early promoters

Previous analysis of two Herpes Simplex Virus Type-1 (HSV-1) promoters controlling expression of mRNA encoding early genes (U(L)37 and U(L)50) showed that the U(L)50 (dUTPase) promoter is at least 6-fold stronger both in its normal genomic location and in the non-essential gC locus. In the present report we demonstrate that the TATA element of either promoter is the major determinant of promoter strength. When the U(L)37 TATA element (CGTATAAC) was mutated with two base changes to the U(L)50 TATA sequence (CATAAAAC) in recombinant viruses, the activity of the U(L)37 promoter was increased to that of the U(L)50 promoter. Conversely, when the U(L)50 TATA element was changed to that of the U(L)37 promoter, U(L)50 promoter activity was reduced to the level of the U(L)37 promoter. In addition, we investigated the spacing of the TATA box with respect to upstream promoter elements. We found that re-positioning the U(L)37 TATA box to a location equivalent to that of the U(L)50 promoter relative to the transcript start site; i.e. three bases upstream of its cognate location, significantly diminished activity. Substitution of the U(L)50 TATA box at the new position could only partially restore promoter activity. Thus, we also conclude that the spacing of TATA elements vis-à-vis upstream promoter elements is also a critical determinant of promoter strength.

Multiple regulatory effects of varicella-zoster virus (VZV) gL on trafficking patterns and fusogenic properties of VZV gH

Varicella-zoster virus (VZV) is an extremely cell-associated alphaherpesvirus; VZV infection is spread almost exclusively via cell membrane fusion. The envelope glycoprotein H (gH) is highly conserved among the herpesviruses. A virus-encoded chaperone, glycoprotein L (gL), associates with gH, and the gH:gL complex is required for gH maturation and membrane expression. We recently demonstrated that in the VZV system, the gH:gL complex facilitated cell membrane fusion and extensive polykaryon formation in transfected cells (K. M. Duus, C. Hatfield, and C. Grose, Virology 210:429-440, 1995). To further define the functions of the unusual VZV gL chaperone protein, we have performed a series of mutagenesis experiments with both gH and gL and analyzed the mutants by laser scanning confocal microscopy in a transfection-based fusion assay. We established the fact that immature gH exited the endoplasmic reticulum (ER) when coexpressed with either gE or gI and appeared on the cell surface in a patch pattern. A similar effect was observed on the cell surface with gH with a cytoplasmic tail mutagenized to closely resemble the vaccinia virus hemagglutinin cytoplasmic tail. Site-directed mutagenesis of the five gL cysteine residues demonstrated that four of five cysteines participated in the gL chaperone function required for proper maturation of gH. On the other hand, the same gL mutants facilitated transport of immature gH to the cell surface, where patching occurred. Studies of gL processing demonstrated that maturation did not require transport beyond the medial-Golgi; furthermore, gL was not detected in the outer cell membrane, nor was it secreted into the medium. Colocalization studies with 3,3'-dihexyloxa-cabocyanine iodide and N-(e-7-nitrobenz-2-oxa-1,3-diazol-4-yl-aminocaproyl)-D-erythro-sphingosine confirmed that gL was found primarily in the ER and cis/medial-Golgi when expressed alone. When all of these data were considered, they suggested a posttranslational gH:gL regulation model whereby the gL chaperone modulated gH expression via retrograde flow from the Golgi to the ER. In this schema, mature gL returns to the ER, where it escorts immature gH from the ER to the Golgi; thereafter, mature gH is transported from the trans-Golgi to the outer cell membrane, where it acts as a major fusogen.

Herpes simplex virus recombination vectors designed to allow insertion of modified promoters into transcriptionally "neutral" segments of the viral genome

The use of recombinant viruses has been essential in investigation of the biology of herpes simplex virus (HSV). In this communication we describe a number of viral recombination vectors that we have generated for use in promoter structure/function analysis within the context of the HSV-1 genome. We have utilized two regions of the HSV genome that contain genes nonessential for replication in cultured cells--the glycoprotein C (gC or UL44) locus in the UL of the genome and the area encompassing the promoter and 5' portion of the latency associated transcript (LAT) within the RL factual influence on promoters due to the site of insertion. Two different kinetic promoters were analyzed, those controlling expression of the gamma UL 38 and the beta dUTPase genes, in both loci. All constructs tested displayed reporter gene mRNA expression with expected kinetics, and we conclude that there are no neighboring cryptic promoter elements that could interfere with expression studies using the vectors described.

Transcriptional activation of the herpes simplex virus type 1 UL38 promoter conferred by the cis-acting downstream activation sequence is mediated by a cellular transcription factor

The herpes simplex virus (HSV) type 1 strict late (gamma) UL38 promoter contains three cis-acting transcriptional elements: a TATA box, a specific initiator element, and the downstream activation sequence (DAS). DAS is located between positions +20 and +33 within the 5' untranslated leader region and strongly influences transcript levels during productive infection. In this communication, we further characterize DAS and investigate its mechanism of action. DAS function has a strict spacing requirement, and DAS contains an essential 6-bp core element. A similarly positioned element from the gamma gC gene (UL44) has partial DAS function within the UL38 promoter context, and the promoter controlling expression of the gamma US11 transcript contains an identically located element with functional and sequence similarity to UL38 DAS. These data suggest that downstream elements are a common feature of many HSV gamma promoters. Results with recombinant viruses containing modifications of the TATA box or initiator element of the UL38 promoter suggest that DAS functions to increase transcription initiation and not the efficiency of transcription elongation. In vitro transcription assays using uninfected HeLa nuclear extracts show that, as in productive infection with recombinant viruses, the deletion of DAS from the UL38 promoter dramatically decreases RNA expression. Finally, electrophoretic mobility shift assays and UV cross-linking experiments show that DAS DNA forms a specific, stable complex with a cellular protein (the DAS-binding factor) of approximately 35 kDa. These data strongly suggest that the interaction of cellular DAS-binding factor with DAS is required for efficient expression of UL38 and other HSV late genes.

The herpes simplex virus type 1 major capsid protein (VP5-UL19) promoter contains two cis-acting elements influencing late expression

The herpes simplex virus type 1 (HSV-1) major capsid protein VP5 gene (UL19) is expressed with beta gamma (gamma 1 [leaky late]) kinetics. We have previously described the construction of recombinant HSV-1 in which the VP5 promoter was engineered to control the expression of the bacterial beta-galactosidase gene as a reporter (C.-J. Huang, S. A. Goodart, M. K. Rice, J. F. Guzowski, and E. K. Wagner, J. Virol. 67:5109-5116, 1993). Here we describe further mutational analysis in recombinant viruses. We have precisely defined the boundaries of the VP5 promoter and identified two regions important for both the level and the kinetics of expression. The 5' boundary was located at -48 relative to the initiation site of transcription by analyzing a series of nested deletions in the upstream sequence, and although a number of cis-acting sites influencing transient expression have been identified upstream of this point, these sites have no role in promoter activity during productive infection. Deletion of an Sp1-binding site located between -48 and the TATA box at -30 greatly reduced VP5 promoter activity late but not early after infection. A cis-acting element whose sequence resembles the human immunodeficiency virus type 1 initiator was located between -2 and +10 in the VP5 sequence by characterizing a series of deletions and site-directed block mutations downstream the TATA box. This element defines the 3' limit of the VP5 promoter, and like the upstream element, disruption of this element also inhibited promoter activity late in the productive cycle.

Herpes simplex virus thymidine kinase and specific stages of latency in murine trigeminal ganglia

From marker rescue, sequencing, transcript, and latency analyses of the thymidine kinase-negative herpes simplex virus mutant dlsactk and studies using the thymidine kinase inhibitor Ro 31-5140, we infer that the virus-encoded thymidine kinase is required in murine trigeminal ganglia for acute replication and lytic gene expression, for increasing the numbers of cells expressing latency-associated transcripts, and for reactivation from latent infection.

Mutational analysis of the herpes simplex virus type 1 strict late UL38 promoter/leader reveals two regions critical in transcriptional regulation

The unusual TATA homology TTTAAA at -31 relative to the transcriptional start site of the herpes simplex virus type 1 (HSV-1) strict late (gamma) UL38 gene defines the 5' extent of this promoter in recombinant virus. We have further analyzed this promoter by generating recombinant viruses containing nested deletions 3' of the transcriptional start site and with recombinant viruses containing specific promoter/leader alterations. A recombinant virus containing the UL38 promoter/leader from -50 to +9 expressed reporter gene enzyme levels at approximately 10% of those from a recombinant containing the full viral promoter/leader (-50 to +99). The accumulation of reporter gene mRNA in infections with the -50 to +9 recombinant was still regulated with gamma kinetics. Further removal of UL38 leader sequences resulted in a nearly complete loss of expression. Analysis of promoter chimera recombinant viruses has shown that sequences downstream of the TATA box and spanning the transcriptional start site of the UL38 promoter are functionally distinct from those of either the beta UL37 gene or the beta gamma VP16 (UL48) gene; thus, we conclude that sequences from -31 to +9 of the UL38 gene constitute a core gamma promoter. Further deletional and substitutional analyses have also demonstrated the presence of a 14-bp element (the downstream activation sequence) located between +20 to +33 in the nontranslated leader region which is required for full levels of transcription.

Substitution of a TATA box from a herpes simplex virus late gene in the viral thymidine kinase promoter alters ICP4 inducibility but not temporal expression

The role of cis-acting promoter elements associated with herpes simplex virus type 1 (HSV-1) early and late genes was evaluated during productive infection with regard to activation of gene expression by the HSV-1 transactivator ICP4 and control of temporal regulation. A set of recombinant viruses was constructed such that expression of an HSV-1 early gene, thymidine kinase (tk), was placed under the control of either the tk TATA box or the TATA box from the late gene, glycoprotein C (gC), in the presence or absence of the upstream Sp1 and CCAAT sites normally found in the tk promoter. The presence of Sp1 sites in the promoter or replacement of the tk TATA box with the gC TATA box resulted in a decreased activation of tk mRNA expression by ICP4. Substitution of the A + T-rich region from the gC TATA box in the context of the remainder of the surrounding tk sequences resulted in a promoter that bound recombinant TATA-binding protein (TBP) better at lower concentrations than the wild-type tk promoter did. These results indicate that tk promoters that are better able to utilize TBP are less responsive to ICP4 activation and suggest that activation by ICP4 involves the general transcription factors that interact with TBP or TBP itself. Additionally, all of the viruses expressed tk at early times postinfection, indicating that cis-acting promoter elements that control the level of expression of HSV-1 early and late genes do not determine temporal regulation.

Effect of genomic location on expression of beta-galactosidase mRNA controlled by the herpes simplex virus type 1 UL38 promoter

To examine the effect of genomic location on the details of expression of selected herpes simplex virus promoters, we have constructed recombination vectors for placing such promoters controlling the beta-galactosidase reporter gene into two regions of the viral genome lacking any nearby promoter or regulatory elements. The first vector generates the promoter-beta-galactosidase reporter gene inverted within the locus of the gC (UL44) translational reading frame; the second replaces the LAT promoter and the first 600 bases of the primary transcript in both copies of the RL region. These locations were chosen to obviate any possible influence of upstream but noncontiguous heterologous or homologous DNA sequence elements upon promoter activity. When the reporter gene controlled by the strict late (gamma) UL38 promoter was placed in the gC location, it was significantly less active than in its normal location; in contrast, promoter activity was comparable to wild-type values when the promoter was recombined into the RL region. The low level of activity in the gC location could be partially alleviated by the incorporation of additional DNA sequences upstream of the UL38 promoter. Despite the effect of genomic location upon the level of expression, the kinetics of expression in either location mirrors the wild-type UL38 strict late kinetics of expression. Finally, we used deletional analysis to demonstrate that no more than 29 bases of DNA sequence 5' of the mRNA cap site are required for promoter activity in either location; this result is consistent with earlier results of transient-expression assays and indicates that the UL38 promoter shares general features with other strict late (gamma) herpes simplex virus promoters.

Mutational analysis of two herpes simplex virus type 1 late promoters

To investigate the cis-acting sequence elements that are involved in the regulation of herpes simplex virus type 1 late gene expression, linker-scanning mutations were constructed in the promoters of the glycoprotein C and glycoprotein H genes. Each promoter mutation was inserted upstream of the Escherichia coli lacZ gene in a recombinant virus, and the relative activities of beta-galactosidase expressed from individual recombinant viruses were compared. This analysis identified three sequence elements in each promoter: a TATA element, an element that overlapped the start of transcription, and an element downstream from the start of transcription. Primer extension analysis confirmed these results and showed that mutations in either the TATA element or the initiation sequence could eliminate normal transcription initiation. Analysis of expression from hybrid promoters revealed that the TATA and the initiation elements were interchangeable, at least when correctly aligned, and that the initiation element plays a pivotal role in determining the actual site of transcription initiation.