Resistance to methylation de novo of the human cytomegalovirus immediate early enhancer in a model for virus latency and reactivation in vitro

Regulation of the MIE Locus During HCMV Latency and Reactivation

Human cytomegalovirus (HCMV) is a ubiquitous herpesviral pathogen that results in life-long infection. HCMV maintains a latent or quiescent infection in hematopoietic cells, which is broadly defined by transcriptional silencing and the absence of de novo virion production. However, upon cell differentiation coupled with immune dysfunction, the virus can reactivate, which leads to lytic replication in a variety of cell and tissue types. One of the mechanisms controlling the balance between latency and reactivation/lytic replication is the regulation of the major immediate-early (MIE) locus. This enhancer/promoter region is complex, and it is regulated by chromatinization and associated factors, as well as a variety of transcription factors. Herein, we discuss these factors and how they influence the MIE locus, which ultimately impacts the phase of HCMV infection.

Bright and Early: Inhibiting Human Cytomegalovirus by Targeting Major Immediate-Early Gene Expression or Protein Function

The human cytomegalovirus (HCMV), one of eight human herpesviruses, establishes lifelong latent infections in most people worldwide. Primary or reactivated HCMV infections cause severe disease in immunosuppressed patients and congenital defects in children. There is no vaccine for HCMV, and the currently approved antivirals come with major limitations. Most approved HCMV antivirals target late molecular processes in the viral replication cycle including DNA replication and packaging. “Bright and early” events in HCMV infection have not been exploited for systemic prevention or treatment of disease. Initiation of HCMV replication depends on transcription from the viral major immediate-early (IE) gene. Alternative transcripts produced from this gene give rise to the IE1 and IE2 families of viral proteins, which localize to the host cell nucleus. The IE1 and IE2 proteins are believed to control all subsequent early and late events in HCMV replication, including reactivation from latency, in part by antagonizing intrinsic and innate immune responses. Here we provide an update on the regulation of major IE gene expression and the functions of IE1 and IE2 proteins. We will relate this insight to experimental approaches that target IE gene expression or protein function via molecular gene silencing and editing or small chemical inhibitors.

Genome sequence divergences and functional variations in human cytomegalovirus strains

Genome sequences of numerous and wide-ranging species have been completed, but genome-wide sequence variation patterns linked to biological functions are just starting to be investigated. Here, by comparatively analyzing the genome variation patterns of human cytomegalovirus (HCMV) genomes, we revealed large sequence divergences and functional variations existing in HCMV genomes. They are divergent in genome-size, inversion, orientation and coding potential, even within conserved genes, including nucleotide polymorphism, DNA strand composition asymmetry, and evolutionary rate variation in conserved genes. These divergences in conserved genes are linked to HCMV biology. Codon usage variation of conserved genes located in the negative DNA strand is significantly different between HCMV strains, and this variation associates with virion production and virulence factor, suggesting that the negative DNA strand primarily contributes to virion production and virulence factor in HCMV. In addition, we also revealed that genes functioning for entry and egress are the most adaptable, and that those for transcription and replication are the most conserved in HCMV genomes. The conserved-transcription system is generally controlled by a genome-wide motif GCGC revealed in this study by Chaos map analysis. Our findings demonstrated that genome sequences of HCMV are generally divergent and these divergences directly reflect viral biology.

Epigenetics: regulation through repression

Epigenetics is the study of heritable changes in gene expression that occur without a change in DNA sequence. Epigenetic phenomena have major economic and medical relevance, and several, such as imprinting and paramutation, violate Mendelian principles. Recent discoveries link the recognition of nucleic acid sequence homology to the targeting of DNA methylation, chromosome remodeling, and RNA turnover. Although epigenetic mechanisms help to protect cells from parasitic elements, this defense can complicate the genetic manipulation of plants and animals. Essential for normal development, epigenetic controls become misdirected in cancer cells and other human disease syndromes.

Inactivation of the very strong HCMV immediate early promoter by DNA CpG methylation in vitro

The influence of DNA methylation in vitro on the activity of the very strong human cytomegalovirus (HCMV) major immediate early (IE) modulator/enhancer/promoter region was investigated by transient transfection experiments of premonocytic HL-60 cells. While sequence-specific methylation of the major IE enhancer and/or modulator with the cytosine methyl-transferases FnuDII, HhaI and HaeIII had no significant effect, the promoter activity was completely repressed by methylation of the cytosine in 5'-CpG sites with the Spiroplasma methyltransferase SssI. Addition of TNF-alpha or PMA which are strong stimulators of HCMV major IE enhancer/promoter activity in premonocytic HL-60 cells had no effect on repression. Inactivation of the IE enhancer/promoter via methylation by M.SssI could be partially alleviated by co-transfection with an excess of untranscribable highly methylated DNA. These results indicate that a methyl-CpG binding factor is involved as mediator in the inhibitory effect of HCMV enhancer/promoter methylation. Taken together, the HCMV major IE enhancer/ promoter has been shown to be susceptible to transcriptional inactivation by methylation of the cytosines in CpG dinucleotides, a process that is proposed to play a modulatory role in viral latency.

Detection of latent human cytomegalovirus in organ tissue and the correlation with serological status

The presence of human cytomegalovirus (HCMV) genome in spleen tissue was studied by using DNA hybridization techniques in seropositive and seronegative organ donors without clinical or laboratory confirmed HCMV infection. The serum samples of these patients were screened by latex agglutination test (LA) and enzyme linked immuno sorbent assay (ELISA) for the presence of HCMV antibodies, and confirmed by immunoblotting technique (IB). For the detection of HCMV sequences in spleen tissue dot blot DNA hybridization (DBH) using probes derived from immediate-early and late regions (ES and BH fragment respectively) of the HCMV genome were used. Samples positive in DBH were further tested by in situ DNA hybridization (ISH) using the ES probe. The number of spleen tissue specimens positive for HCMV nucleic acids indicated that HCMV may be present in human beings, even without serological evidence.

Gene expression from heterologous promoters in a replication-defective avian retrovirus vector in quail cells

Avian retrovirus vectors, with potential for use in avian transformation, were constructed to evaluate the relative efficiency of promoters placed internal to the viral long terminal repeats (LTR). The vectors are replication-defective reticuloendotheliosis plasmids that contain the neomycin phosphotransferase gene under control of the 5' LTR and an internal promoter that directs expression of the chloramphenicol acetyltransferase gene. The internal promoters were the SV40 early, the mouse metallothionein I, and the human cytomegalovirus immediate early (HCMV-IE) promoters. Under transient conditions in QT6 cells, the HCMV-IE promoter construct was by far the strongest. However, expression dropped greatly from the HCMV-IE promoter after integration into the quail cell genome. Evidence suggests that the HCMV-IE promoter is selectively suppressed by methylation after stable transfection but not after infection.