The role of heterochromatin markers in lytic human herpesvirus egress and maturation

Abstract

The human herpesviruses are large, double-stranded DNA viruses known for their persistent latent phase. Within the herpesvirus family there are 3 subfamilies; alphaherpesviruses, classified as such because of their more rapid replication and latency in neurons; betaherpesviruses, known for slow replication cycles and latency in myeloid cells; and gammaherpesviruses, oncogenic viruses with a more variable replication cycle and latency in lymphocytes. Many of these human herpesviruses cause human disease ranging from sores and mild "cold-like" symptoms to encephalitis, congenital defects, and cancers. All herpesviruses replicate in the host nucleus, assemble capsids then egress out of the nucleus, mature through acquisition of tegument proteins and secondary envelopment, and finally bud out of the host cell. The virus must contend with components of the host nucleus to replicate, including the host chromatin. Chromatin consists of DNA, RNA, and proteins that maintain the DNA structure. Host chromatin is organized into units called nucleosomes that consist of approximately 147bp of DNA wrapped around an octamer of histone proteins. The structural and transcriptional control of the chromatin is tightly regulated and closely linked. Transcriptionally active, open chromatin is referred to as euchromatin, while tightly packed, transcriptionally repressed chromatin is referred to as heterochromatin. Chromatin state is controlled by incorporation of histone variants and post-translational modification of histone tails, also called histone marks. Trimethylation of histone 3 at the 9th and 27th lysine residue (H3K9me3 and H3K27me3 respectively) and histone variant macroH2A are common markers of heterochromatin. Over the course of lytic herpesvirus infection, host chromatin undergoes extensive gross remodeling as chromatin is marginalized to the nuclear periphery to accommodate viral replication and later to provide structural support during nuclear egress. However, the specific role of heterochromatin in these lytic infections remain unknown. In this thesis, I will show that host heterochromatin markers have unique roles in lytic herpesvirus infection. To this end, I will first show the heterochromatin marker macroH2A1 supports the formation of heterochromatin channels required for the alphaherpesvirus, Herpes Simplex Virus-1 (HSV-1), to efficiently reach the nuclear membrane and egress. Next, I will show that histone post-translational modification , H3K27me3, directs capsid docking on the nuclear membrane and subsequent Us3 phosphorylation for efficient nuclear budding of HSV-1 virions. Finally, I will show that macroH2A1 plays a dramatically different role in HCMV lytic replication, by regulating expression of dormant host genes required for maturation, while H3K27me3 reduction had minimal effect on HCMV progeny production. This thesis work expands our understanding of chromatin control of cellular processes, especially during cellular stress, and the biology of lytic replication of two important human pathogens.