Epigenetic Control of Cytomegalovirus Latency and Reactivation

Abstract

Cytomegalovirus (CMV) gene expression is repressed in latency due to heterochromatinization of viral genomes. In murine CMV (MCMV) latently infected mice, viral genomes are bound to histones with heterochromatic modifications, to enzymes that mediate these modifications, and to adaptor proteins that may recruit co-repressor complexes. Kinetic analyses of repressor binding show that these repressors are recruited at the earliest time of infection, suggesting that latency may be the default state. Kidney transplantation leads to epigenetic reprogramming of latent viral chromatin and reactivation of immediate early gene expression. Inflammatory signaling pathways, which activate transcription factors that regulate the major immediate early promoter (MIEP), likely mediate the switch in viral chromatin.

Transplant-induced reactivation of murine cytomegalovirus immediate early gene expression is associated with recruitment of NF-κB and AP-1 to the major immediate early promoter

“…IE-3 is the MCMV functional homologue of HCMV IE-2, which is the major transactivator of early gene expression (Angulo et al, 2000;Keil et al, 1987aKeil et al, , 1987bMartínez et al, 2010;Messerle et al, 1992). We previously showed that allogeneic transplantation induced reactivation of IE gene expression 48 h after transplant, and that RNA polymerase phosphorylated at Ser2 (Pol II pS2) was recruited to both the MIEP and the IE-3 coding region (Hummel et al, 2001;Liu et al, 2013). Here, we show that IE-3 RNA expression was detectable by reverse transcriptase-qPCR at 48 h, but not at 3 or 24 h post-transplant ( Fig.…”

“…CMV genomes are heterochromatinized and the IE genes, which are required for lytic replication, are transcriptionally silent in latent infection (Grzimek et al, 2001;Hummel et al, 2001;Kurz et al, 1997Kurz et al, , 1999Liu et al, 2008Liu et al, , 2010Seckert et al, 2013;Simon et al, 2005). Activation of IE gene expression, through recruitment of transcription factors that control MIEP activity and remodelling of viral chromatin, is therefore likely required for reactivation of the virus.…”

“…Chromatin remodelling complexes are recruited to the MIEP upon reactivation of IE gene expression MCMV genomes are highly enriched in histones in latently infected mice, suggesting that the nucleosomes are in a compacted configuration, which is closed to transcription (Liu et al, 2008). Many inducible genes require remodelling of the chromatin by the SWI/SNF complex in order to activate gene expression (Ramirez-Carrozzi et al,…”

“…However, due to the species specificity of HCMV, it has not been possible to test this hypothesis in the context of organ transplantation. Murine cytomegalovirus (MCMV) is very similar to HCMV in many important ways, including: (i) ability to establish latency and to reactivate; (ii) hierarchical regulation of viral gene expression; (iii) structure, function and organization of the major immediate early (IE) genes (Keil et al, 1987a, b;Meier & Stinski, 2013;Stenberg et al, 1984); (iv) repression of IE gene expression during latency by heterochromatinization of viral genomes (Grzimek et al, 2001;Hummel & Abecassis, 2002;Hummel et al, 2001;Kurz et al, 1999;Liu et al, 2008;Reeves & Sinclair, 2013;Reeves et al, 2005;Seckert et al, 2013); (v) the presence of similar regulatory elements (e.g. NF-k B, AP-1, Sp1 and CREB/ATF binding sites) in the viral IE enhancers Meier & Stinski, 2013); and (vi) (re)activation of the MIEP and/or IE transcription in response to inflammatory mediators or allogeneic stimulation (Döcke et al, 1994;Fietze et al, 1994;Huang et al, 2012;Hummel & Abecassis, 2002;Hummel et al, 2001;Kew et al, 2014;Lee et al, 2004;Liu et al, 2013;O'Connor & Murphy, 2012;Prösch et al, 1995;Reeves & Compton, 2011;Simon et al, 2005;Stein et al, 1993).…”

“…We previously developed a renal transplant model for reactivation of MCMV IE gene expression (Hummel et al, 2001). In this model, transplantation of latently infected kidneys into allogeneic recipients induced IE gene expression and epigenetic reprogramming of the MIEP within 48 h (Hummel et al, 2001;Liu et al, 2013). In addition, allogeneic transplantation induced expression of a lacZ reporter transgene under the control of the HCMV MIEP (MIEP-lacZ) (Hummel et al, 2001).…”