Papillomaviruses induce persistent epithelial lesions, known as papillomas. Genital papillomavirus infection is widespread and associated with the development of cervical cancer. The viral E2 proteins regulate viral transcription, replication and episomal genome maintenance.
Our aim i s to elucidate the mechanisms by which the E2 proteins control the viral life cycle. We have shown that papillomavirus genomes and the E2 transactivator protein interact with cellular mitotic chromosomes in dividing cells. This ensures that viral genomes are properly segregated to daughter cells and are retained within the nucleus. We have elucidated regions of the E2 transactivation domain that are important for chromosomal association and shown that this correlates with binding to the recently identified cellular chromosomal tethering protein, Brd4 (ref 2). Furthermore, we show that E2 does not passively interact with Brd4, but rather greatly increases the affinity of Brd4 with chromatin. Therefore, chromosomal tethering is an active, not passive, process (ref 5). We have shown that E2 cannot segregate plasmids with E2-binding sites in Sacchromyces cerevisiae but that segregation can be rescued by expression of the Brd4 gene (ref 3) We have also identified distinct regions of E2 important for replication, E1 interaction and transcriptional repression (ref 1). We have expressed a series of E2 proteins from many different human and animal papillomaviruses and have shown that most of them have the ability to associate with cellular mitotic chromosomes. In most cervical cancers, papillomavirus DNA is found integrated into cellular chromosomes instead of replicating episomally. This integration disrupts the E1 and/or E2 genes and this has led to the hypothesis that disruption of these regulatory functions is a critical step in malignant progression. To study the mechanism of this progression we have developed a system to immortalize primary human keratinocyte cells containing hybrid papillomavirus genomes that are maintained episomally in the absence of E1 and E2 gene functions. We have used this system to determine the E2 regulatory functions responsible for HPV oncogene expression.
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