Human cytomegalovirus UL145 gene is highly conserved among clinical strains

Human cytomegalovirus (HCMV), a ubiquitous human pathogen, is the leading cause of birth defects in newborns. A region (referred to as UL/b') present in the Toledo strain of HCMV and low-passage clinical isolates) contains 22 additional genes,which are absent in the highly passaged laboratory strain AD169. One of these genes,UL145 open reading frame (ORF), is located between the highly variable genes UL144 and UL146. To assess the structure of the UL145 gene,the UL145 ORF was amplified by PCR and sequenced from 16 low-passage clinical isolates and 15 non-passage strains from suspected congenitally infected infants. Nine UL145 sequences previously published in the GenBank were used for sequence comparison. The identities of the gene and the similarities of its putative protein among all strains were 95.9 -100% and 96.6-100%, respectively. The post-translational modification motifs of the UL145 putative protein in clinical strains were conserved,comprising the protein kinase C phosphorylation motif (PKC)and casein kinase II phosphorylation site (CK-II). We conclude that the structure of the UL145 gene and its putative protein are relatively conserved among clinical strains, irrespective of whether the strains come from patients with different manifestations, from different areas of the world, or were passaged or not in human embryonic lung fibroblast (HELF) cells.     

Human cytomegalovirus <Emphasis Type="Italic">UL145</Emphasis> gene is highly conserved among clinical strains

Human cytomegalovirus (HCMV), a ubiquitous human pathogen, is the leading cause of birth defects in newborns. A region (referred to as UL/b′) present in the Toledo strain of HCMV and low-passage clinical isolates) contains 22 additional genes, which are absent in the highly passaged laboratory strain AD169. One of these genes, UL145 open reading frame (ORF), is located between the highly variable genes UL144 and UL146. To assess the structure of the UL145 gene, the UL145 ORF was amplified by PCR and sequenced from 16 low-passage clinical isolates and 15 non-passage strains from suspected congenitally infected infants. Nine UL145 sequences previously published in the GenBank were used for sequence comparison. The identities of the gene and the similarities of its putative protein among all strains were 95.9–100% and 96.6–100%, respectively. The post-translational modification motifs of the UL145 putative protein in clinical strains were conserved, comprising the protein kinase C phosphorylation motif (PKC) and casein kinase II phosphorylation site (CK-II). We conclude that the structure of the UL145 gene and its putative protein are relatively conserved among clinical strains, irrespective of whether the strains come from patients with different manifestations, from different areas of the world, or were passaged or not in human embryonic lung fibroblast (HELF) cells.     

Human cytomegalovirus encodes an MHC class I-like molecule (UL142) that functions to inhibit NK cell lysis

Clinical and low passage strains of human CMV (HCMV) encode an additional MHC class I-related molecule UL142, in addition to the previously described UL18. The UL142 open reading frame is encoded within the ULb' region which is missing from a number of common high passage laboratory strains. Cells expressing UL142 following transfection, and fibroblasts infected with a recombinant adenovirus-expressing UL142, were used to screen both polyclonal NK cells and NK cell clones, in a completely autologous system. Analysis of 100 NK cell clones derived from five donors, revealed 23 clones that were inhibited by fibroblasts expressing UL142 alone. Small-interfering RNA-mediated knockdown of UL142 mRNA expression in HCMV-infected cells resulted in increased sensitivity to lysis. From these data we conclude that UL142 is a novel HCMV-encoded MHC class I-related molecule which inhibits NK cell killing in a clonally dependent manner.     

Role of a mutation in human cytomegalovirus gene UL104 in resistance to benzimidazole ribonucleosides

The benzimidazole D-ribonucleosides TCRB and BDCRB are potent and selective inhibitors of human cytomegalovirus (HCMV) replication. Two HCMV strains resistant to these compounds were selected and had resistance mutations in genes UL89 and UL56. Proteins encoded by these two genes are the two subunits of the HCMV "terminase" and are necessary for cleavage and packaging of viral genomic DNA, a process inhibited by TCRB and BDCRB. We now report that both strains also have a previously unidentified mutation in UL104, the HCMV portal protein. This mutation, which results in L21F substitution, was introduced into the genome of wild-type HCMV by utilizing a recently cloned genome of HCMV as a bacterial artificial chromosome. The virus with this mutation alone was not resistant to BDCRB, suggesting that this site is not involved in binding benzimidazole nucleosides. As in previous proposals for mutations in UL104 of murine cytomegalovirus and HCMV strains resistant to BAY 38-4766, we hypothesize that this mutation could compensate for conformational changes in mutant UL89 and UL56 proteins, since the HCMV terminase is likely to interact with the portal protein during cleavage and packaging of genomic DNA.     

The Gene Product of Human Cytomegalovirus Open Reading Frame UL56 Binds the pac Motif and Has Specific Nuclease Activity

Using the cis-acting human cytomegalovirus (HCMV) packaging elements (pac 1 and pac 2) as DNA probes, specific DNA-protein complexes were detected by electrophoretic mobility shift assay (EMSA) in both HCMV-infected cell nuclear extracts and recombinant baculovirus-infected cell extracts containing the HCMV p130 (pUL56) protein. DNA-binding proteins, which were common in uninfected and infected cell extracts, were also detected. Mutational analysis showed that only the AT-rich core sequences in these cis-acting motifs, 5′-TAAAAA-3′ (pac 1) and 5′-TTTTAT-3′ (pac 2), were required for specific DNA-protein complex formation. The specificity of the DNA-protein complexes was confirmed by EMSA competition. Furthermore, a specific endonuclease activity was found to be associated with lysates of baculovirus-infected cells expressing recombinant p130 (rp130). This nuclease activity was time dependent, related to the amount of rp130 in the assay, and ATP independent. Nuclease activity remained associated with rp130 after partial purification by sucrose gradient centrifugation, suggesting that this activity is a property of HCMV p130. We propose a possible involvement of p130 in HCMV DNA packaging.Human cytomegalovirus (HCMV), one of eight human herpesviruses, can cause serious illness in neonates as well as in immunocompromised adults (2). For example, transplant and AIDS patients may develop life-threatening diseases as a consequence of primary infection or reactivation of latent infection. Present therapeutic approaches are limited, and new strategies that may result from a better understanding of the molecular events involved in viral maturation are needed.The HCMV virion consists of an envelope, an amorphous tegument, and an icosahedral nucleocapsid, which is assembled in the nuclei of infected cells. The precise molecular events of HCMV capsid assembly and subsequent DNA packaging are not well understood. It is generally accepted that viral DNA is packaged into a procapsid consisting of major capsid protein (UL86), minor capsid protein (UL85), minor capsid protein-binding protein (UL46), smallest capsid protein (UL47/48), assembly protein (UL80.5), and proteinase precursor protein (UL80a) (8). The assembly protein is removed during DNA insertion. It is unclear how the concatenated viral DNA contacts empty capsids and is cleaved and packaged into the capsid.Recent studies with herpes simplex virus type 1 (HSV-1) mutants that were temperature sensitive suggest that cleavage of the concatenated DNA does not occur in the absence of packaging (1). One possible model would be the involvement of cleavage packaging protein(s) which could facilitate incorporation of DNA into the procapsid by attaching to a specific motif within the viral genome. With HSV-1, the UL36 gene product (ICP1) and a smaller protein (possibly encoded by UL37) are part of a complex that recognizes the HSV-specific a sequence and are required for cleavage and packaging of viral DNA from concatemers (6, 7). In addition, the HSV-1 ICP 18.5 (UL28) gene product and the pseudorabies virus (PrV) homolog (16) were also reported to play an important role in DNA packaging (1, 14). Addison et al. (1) demonstrated that empty capsids were observed under conditions nonpermissive for the expression of the HSV-1 ICP 18.5 gene product. The HSV-1 ICP 18.5 mutants failed to cleave concatenated viral DNA in noncomplementing cells, suggesting that cleavage and packaging require ICP 18.5. Similar results were reported by Mettenleiter et al. (14) for PrV mutant protein. These observations suggest that the HSV-1 UL36, UL37, and UL28 gene products are involved in cleavage and packaging of concatenated viral DNA.In a recent study, we identified and partially characterized the gene product of HCMV UL56 (4). The HCMV UL56 gene product of 130 kDa is the homolog of the HSV-1 UL28 gene product. It is therefore postulated that UL56 possesses properties comparable to those of HSV-1 UL28, implying an involvement in cleavage and packaging of DNA. The HCMV genomic a sequence is a short sequence located at both termini of the genome and repeated in an inverted orientation at the L-S junction. The a sequence plays a key role in replication as a cis-acting signal for cleavage and packaging of progeny viral DNA and circularization of the viral genome. The HCMV a sequence contains two conserved motifs, pac 1 and pac 2, which are required for cleavage and packaging of the viral DNA (18). Both sequence motifs are located on one side of the cleavage site. The pac 1 and pac 2 motifs have an AT-rich core flanked by a GC-rich sequence. During the initial step of viral DNA packaging, a capsid-associated protein may bind to the pac sequences and may be involved in cleavage of the viral DNA concatemer.In this study, electrophoretic mobility shift assays (EMSAs) were performed with DNA probes spanning the region of these cis-acting elements. These studies demonstrate that specific proteins from HCMV-infected nuclear extracts or baculovirus-UL56-infected cell extracts bind to the pac motifs. Using affinity-purified monospecific antibodies, we show that p130 is present in specific DNA-protein complexes containing the pac motifs of the viral genome. Furthermore, evidence is presented for a sequence-specific endonuclease activity of recombinant HCMV p130, using circular plasmid DNA bearing the a sequence as a substrate.     

Molecular analysis of the human cytomegalovirus strains isolated from infected infants.

Polymerase chain reaction (PCR) techniques were developed to facilitate the study of the molecular epidemiology of human cytomegalovirus (HCMV). In the present study analysis of HCMV DNA was applied for the determination of the reinfection frequency and genotypes of HCMV strains isolated from infected infants, treated with ganciclovir and non-treated. Urines from 92 infants, aged 1 to 5 months, were investigated. Isolates were analysed by PCR method using primers for a-seq and glycoprotein B (gB) HCMV genes. PCR products of gB gene were digested with RsaI and HinfI endonucleases (PCR-RFLP). A-seq gene amplified products were visualized on agarose gels and analysed by densitometry. Genotyping based on hypervariable a-seq region in comparison with restriction analysis of gB gene fragment allowed better differentiation and discrimination of particular HCMV strains. Analysis of the a-sequence PCR products allowed to distinguish 9 profile groups. The patterns obtained consisted of fragments with different size (100 bp to 350 bp), suggesting considerable diversity of HCMV strains. A-sequence analysis revealed that 5 (15.6%) of treated children and 14 (20.7%) of those non-treated, excreted virus of stable genotype. Twenty one (65.6%) of treated and 32 (52.5%) of non-treated children excreted HCMV with a-sequence product of different size, suggesting that in these cases reinfection was caused by genetically distinct strains. Results suggest that reinfection is more frequent in children treated with ganciclovir.     

Dominance of virus over host factors in cross-species activation of human cytomegalovirus early gene expression

Human cytomegalovirus (HCMV) exhibits a highly restricted host range. In this study, we sought to examine the relative significance of host and viral factors in activating early gene expression of the HCMV UL54 (DNA polymerase) promoter in murine cells. Appropriate activation of the UL54 promoter at early times is essential for viral DNA replication. To study how the HCMV UL54 promoter is activated in murine cells, a transgenesis system based on yeast artificial chromosomes (YACs) was established for HCMV. A 178-kb YAC, containing a subgenomic fragment of HCMV encompassing the majority of the unique long (UL) region, was constructed by homologous recombination in yeast. This HCMV YAC backbone is defective for viral growth and lacks the major immediate-early (IE) gene region, thus permitting the analysis of essential cis-acting sequences when complemented in trans. To quantitatively measure the level of gene expression, we generated HCMV YACs containing a luciferase reporter gene inserted downstream of either the UL54 promoter or, as a control for late gene expression, the UL86 promoter, which directs expression of the major capsid protein. To determine the early gene activation pathway, point mutations were introduced into the inverted repeat 1 (IR1) element of the UL54 promoter of the HCMV YAC. In the transgenesis experiments, HCMV YACs and derivatives generated in yeast were introduced into NIH 3T3 murine cells by polyethylene glycol-mediated fusion. We found that infection of YAC, but not plasmid, transgenic lines with HCMV was sufficient to fully recapitulate the UL54 expression program at early times of infection, indicating the importance of remote regulatory elements in influencing regulation of the UL54 promoter. Moreover, YACs containing a mutant IR1 in the UL54 promoter led to reduced ( approximately 30-fold) reporter gene expression levels, indicating that HCMV major IE gene activation of the UL54 promoter is fully permissive in murine cells. In comparison with HCMV, infection of YAC transgenic NIH 3T3 lines with murine cytomegalovirus (MCMV) resulted in lower (more than one order of magnitude) efficiency in activating UL54 early gene expression. MCMV is therefore not able to fully activate HCMV early gene expression, indicating the significance of virus over host determinants in the cross-species activation of key early gene promoters. Finally, these studies show that YAC transgenesis can be a useful tool in functional analysis of viral proteins and control of gene expression for large viral genomes.     

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 ΔUL52 mutant as well as a complementing cell line. We found that replication of viral DNA was not impaired in noncomplementing cells infected with the ΔUL52 virus, but viral concatemers remained uncleaved. Since the subnuclear localization of the known cleavage-packaging proteins pUL56, pUL89, and pUL104 was unchanged in ΔUL52-infected fibroblasts, pUL52 does not seem to act via these proteins. Electron microscopy studies revealed only B capsids in the nuclei of ΔUL52-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.     

Human cytomegalovirus UL97 D605E polymorphism has a high prevalence in immunocompetent Japanese infants and children

There is no existing data on UL97 mutation in human cytomegalovirus (HCMV) isolates obtained from individuals who have never been exposed to ganciclovir (GCV). UL97 codons 439 to 645 from 61 CMV isolates from 61 immunocompetent Japanese infants and children were sequenced directly. No known GCV resistance mutations were found, meaning that the UL97 mutation had resulted from the use of GCV. On the other hand, a mutation at codon 605 (D to E) was frequently identified (56/61: 91.8%). This could be a genetic marker for HCMV in East Asian counties, because of its low prevalence in the strains of HCMV circulating in Western countries.     

The non-ELR CXC chemokine encoded by human cytomegalovirus UL146 genotype 5 contains a C-terminal β-hairpin and induces neutrophil migration as a selective CXCR2 agonist

Human cytomegalovirus (HCMV) is a major pathogen in immunocompromised patients. The UL146 gene exists as 14 diverse genotypes among clinical isolates, which encode 14 different CXC chemokines. One genotype (vCXCL1_GT1) is a known agonist for CXCR1 and CXCR2, while two others (vCXCL1_GT5 and vCXCL1_GT6) lack the ELR motif considered crucial for CXCR1 and CXCR2 binding, thus suggesting another receptor targeting profile. To determine the receptor target for vCXCL1_GT5, the chemokine was probed in a G protein signaling assay on all 18 classical human chemokine receptors, where CXCR2 was the only receptor being activated. In addition, vCXCL1_GT5 recruited β-arrestin in a BRET-based assay and induced migration in a chemotaxis assay through CXCR2, but not CXCR1. In contrast, vCXCL1_GT1 stimulated G protein signaling, recruited β-arrestin and induced migration through both CXCR1 and CXCR2. Both vCXCL1_GT1 and vCXCL1_GT5 induced equally potent and efficacious migration of neutrophils, and ELR vCXCL1_GT4 and non-ELR vCXCL1_GT6 activated only CXCR2. In contrast to most human chemokines, the 14 UL146 genotypes have remarkably long C-termini. Comparative modeling using Rosetta showed that each genotype could adopt the classic chemokine core structure, and predicted that the extended C-terminal tail of several genotypes (including vCXCL1_GT1, vCXCL1_GT4, vCXCL1_GT5, and vCXCL1_GT6) forms a novel β-hairpin not found in human chemokines. Secondary NMR shift and TALOS+ analysis of vCXCL1_GT1 supported the existence of two stable β-strands. C-terminal deletion of vCXCL1_GT1 resulted in a non-functional protein and in a shift to solvent exposure for tryptophan residues likely due to destabilization of the chemokine fold. The results demonstrate that non-ELR chemokines can activate CXCR2 and suggest that the UL146 chemokines have unique C-terminal structures that stabilize the chemokine fold. Increased knowledge of the structure and interaction partners of the chemokine variants encoded by UL146 is key to understanding why circulating HCMV strains sustain 14 stable genotypes.     

Characterization of Drug Resistance-Associated Mutations in the Human Cytomegalovirus DNA Polymerase Gene by Using Recombinant Mutant Viruses Generated from Overlapping DNA Fragments

A number of specific point mutations in the human cytomegalovirus (HCMV) DNA polymerase (UL54) gene have been tentatively associated with decreased susceptibility to antiviral agents and consequently with clinical failure. To precisely determine the roles of UL54 mutations in HCMV drug resistance, recombinant UL54 mutant viruses were generated by using cotransfection of nine overlapping HCMV DNA fragments into permissive fibroblasts, and their drug susceptibility profiles were determined. Amino acid substitutions located in UL54 conserved region IV (N408D, F412C, and F412V), region V (A987G), and δ-region C (L501I, K513E, P522S, and L545S) conferred various levels of resistance to cidofovir and ganciclovir. Mutations in region II (T700A and V715M) and region VI (V781I) were associated with resistance to foscarnet and adefovir. The region II mutations also conferred moderate resistance to lobucavir. In contrast to mutations in other UL54 conserved regions, those residing specifically in region III (L802M, K805Q, and T821I) were associated with various drug susceptibility profiles. Mutations located outside the known UL54 conserved regions (S676G and V759M) did not confer any significant changes in HCMV drug susceptibility. Predominantly an additive effect of multiple UL54 mutations with respect to the final drug resistance phenotype was demonstrated. Finally, the influence of selected UL54 mutations on the susceptibility of viral DNA replication to antiviral drugs was characterized by using a transient-transfection-plus-infection assay. Results of this work exemplify specific roles of the UL54 conserved regions in the development of HCMV drug resistance and may help guide optimization of HCMV therapy.     

The cytomegaloviral protein pUL138 acts as potentiator of tumor necrosis factor (TNF) receptor 1 surface density to enhance ULb'-encoded modulation of TNF-α signaling

Human cytomegalovirus is a ubiquitous herpesvirus that establishes lifelong latent infection. Changes in immune homeostasis induce the reactivation of lytic infection, which is mostly inapparent in healthy individuals but often causes overt disease in immunocompromised hosts. Based on discrepant tumor necrosis factor receptor 1 surface disposition between human cytomegalovirus AD169 variants differing in the ULb' region, we identified the latency-associated gene product pUL138, which also is expressed during productive infection, as a selective potentiator of tumor necrosis factor receptor 1, one of the key receptors of innate immunity. Ectopically expressed pUL138 coprecipitated with tumor necrosis factor receptor 1, extended the protein half-life, and enhanced its signaling responses, thus leading to tumor necrosis factor receptor 1 hyperresponsiveness. Conversely, the targeted deletion of UL138 from the human cytomegaloviral genome strongly reduced tumor necrosis factor receptor 1 surface densities of infected cells. Remarkably, the comparison of UL138 deficiency to ULb' deficiency revealed the presence of further positive modulators of tumor necrosis factor alpha signal transduction encoded within the human cytomegalovirus ULb' region, identifying this region as a hub for multilayered tumor necrosis factor alpha signaling regulation.