Histone loaders CAF1 and HIRA restrict Epstein-Barr virus B-cell lytic reactivation

Abstract

19Epstein-Barr virus (EBV) infects 95% of adults worldwide and causes infectious mononucleosis. 20 EBV is associated with endemic Burkitt lymphoma, Hodgkin lymphoma, post-transplant 21 lymphomas, nasopharyngeal and gastric carcinomas. In these cancers and in most infected B-cells, 22 Suggestive of an early role in establishment of latency, EBV strongly upregulated CAF1 32 expression in newly infected primary human B-cells prior to the first mitosis, and histone 3.1 and 33 3.3 were loaded on the EBV genome by this timepoint. Knockout of CAF1 subunit CHAF1B 34 impaired establishment of latency in newly EBV-infected Burkitt cells. A non-redundant latency 35 maintenance role was also identified for the DNA synthesis-independent histone 3.3 loader HIRA. 36Since EBV latency also requires histone chaperones ATRX and DAXX, EBV coopts multiple host 37 histone pathways to maintain latency, and these are potential targets for lytic induction therapeutic 38 approaches. 39 40 IMPORTANCE 41Epstein-Barr virus (EBV) was discovered as the first human tumor virus in endemic Burkitt 42 lymphoma, the most common childhood cancer in sub-Saharan Africa. In Burkitt lymphoma and 43 in 200,000 EBV-associated cancers per year, epigenetic mechanisms maintain viral latency, where 44 lytic cycle factors are silenced. This property complicated EBV's discovery and facilitates tumor 45 immunoevasion. DNA methylation and chromatin-based mechanisms contribute to lytic gene 46 silencing. Here, we identify histone chaperones CAF1 and HIRA, which have key roles in host 47 DNA replication-dependent and replication independent pathways, respectively, are each 48 important for EBV latency. EBV strongly upregulates CAF1 in newly infected B-cells, where viral 49 genomes acquire histone 3.1 and 3.3 variants prior to the first mitosis. Since histone chaperones 50 ATRX and DAXX also function in maintenance of EBV latency, our results suggest that EBV 51 coopts multiple histone pathways to reprogram viral genomes and highlights targets for lytic 52 induction therapeutic strategies. 53 54The gamma-herpesvirus Epstein-Barr virus (EBV) persistently infects nearly 95% of adults 59 worldwide (1). EBV is the etiological agent of infectious mononucleosis and is also causally-60 associated with multiple human cancers, including endemic Burkitt lymphoma (eBL), Hodgkin 61 lymphoma, post-transplant lymphoproliferative disease, HIV-associated lymphomas, 62 nasopharyngeal carcinoma and gastric carcinoma (2). Tumor cells contain multiple copies of 63 chromatinized, non-integrated, double-stranded DNA EBV genomes, where incompletely defined 64 epigenetic pathways maintain a state of viral latency and in which most cells do not produce 65 infectious virus. 66 EBV initiates lifelong infection by translocating across the tonsillar epithelium to colonize the 67 B-cell compartment (3, 4). Virion deliver unchromatinized, encapsidated, linear EBV genomes to 68 newly infected cells, which traffic to the nucleus. Upon nuclear entry, incoming genomes are 69 circularized by host D...

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