Date Published: May 15, 2014
Publisher: Public Library of Science
Author(s): Moon Kyoo Jang, Kui Shen, Alison A. McBride, Paul Francis Lambert.
It has long been recognized that oncogenic viruses often integrate close to common fragile sites. The papillomavirus E2 protein, in complex with BRD4, tethers the viral genome to host chromatin to ensure persistent replication. Here, we map these targets to a number of large regions of the human genome and name them Persistent E2 and BRD4-Broad Localized Enrichments of Chromatin or PEB-BLOCs. PEB-BLOCs frequently contain deletions, have increased rates of asynchronous DNA replication, and are associated with many known common fragile sites. Cell specific fragile sites were mapped in human C-33 cervical cells by FANCD2 ChIP-chip, confirming the association with PEB-BLOCs. HPV-infected cells amplify viral DNA in nuclear replication foci and we show that these form adjacent to PEB-BLOCs. We propose that HPV replication, which hijacks host DNA damage responses, occurs adjacent to highly susceptible fragile sites, greatly increasing the chances of integration here, as is found in HPV-associated cancers.
Papillomaviruses are an ancient group of viruses that establish a persistent infection in the host epithelium. To maintain such a long-term infection, the E2 protein from a subset of papillomaviruses binds to the viral genome and tethers it to the host chromosomes –. The bromodomain protein, BRD4, binds to mitotic chromosomes with E2 , , is essential for regulation of viral transcription – and is recruited to early viral replication foci , . BRD4 is a mitotic chromosome-associated protein  that interacts with acetylated histone tails  and is a key regulator of the pTEF-b elongation factor . There has been a recent explosion of data as BRD4 has been implicated in regulation of cell cycle, mitotic memory, transcription of MYC and regulation of viral gene expression –. BRD4 is highly enriched at super-enhancers that maintain expression of oncogenes in tumors  and is a promising therapeutic target for a number of cancers .
We show that HPV1 E2, and the HPV16 E1/E2 protein complex, bind with BRD4 to common fragile sites in the human genome. Like other persistent viruses that form long-term associations with their host, HPVs are masters at hijacking cellular processes. The E2 proteins interact with BRD4 to regulate viral transcription, and associate with host chromatin to partition the viral genome in dividing cells. We demonstrate that this association is not random, and that the virus has taken advantage of very susceptible regions of the host genome that are prone to replication stress. Both oncogenic and non-oncogenic papillomaviruses induce a DDR and both probably replicate adjacent to these susceptible regions. Most likely, both oncogenic and non-oncogenic HPV types have the propensity to become integrated into unstable regions of the genome on rare occasions. However, only oncogenic HPVs could give the cells a selective growth advantage, in turn further increasing genetic instability, and eventually leading to carcinogenic progression.