|PML nuclear bodies are foci in mammalian cell nuclei that have PML proteins as the main structural component. Six nuclear isoforms of PML proteins, differing in their C-terminal regions, interact with each other to form nuclear bodies. Many proteins transiently or stably associate with these foci, which are required for several cellular processes including DNA repair, p53 activation (by acetylation) and apoptosis. In addition, PML nuclear bodies are part of the host innate immune defense that inhibits the lytic infection of DNA viruses. As a result, DNA viruses such as herpesviruses encode proteins that disrupt PML nuclear bodies thereby facilitating viral infection. We discovered that EBNA1 induces the loss of all PML isoforms in nasopharyngeal and gastric carcinoma cells (EBV-induced cancers), thereby inhibiting p53 activation, DNA repair and apoptosis, and providing a mechanism by which EBNA1 could contribute to EBV-induced tumours. The effect of EBNA1 on PML was also confirmed in primary gastric tumour samples, where EBV-positive tumours were found to have significantly less PML than EBV-negative tumours. In addition, the ability of EBNA1 to induce PML loss was shown to facilitate lytic infection, since the PML bodies suppress EBV lytic infection. We also found that EBNA1 must bind USP7 to induce PML loss. This was the first indication that USP7 is a PML regulator and this finding was subsequently confirmed in cells lacking EBV or EBNA1.
Despite their great importance as tumor suppressors, there is suprisingly little known about which cellular proteins regulate the turnover of PML proteins. We have recently conducted a screen of all cellular ubiquitin pathway proteins for their effects on PML nuclear bodies, leading to the identification of several potential regulators that are being further studied. In addition we have previously conducted a screen of over 200 herpesvirus proteins (from HSV-1, CMV and EBV) for those that disrupt PML nuclear bodies, resulting in the identification of several uncharacterized proteins that induce the loss or reorganization of PML bodies. Determining the mechanisms by which all of these proteins affect PML bodies will provide considerable insight into PML regulation.
Finally, another interesting question is how the six different isoforms of PML contribute to the various functions and regulation of PML nuclear bodies. To study this question, we have built cell lines that express only one PML isoform (for each of PML I through VI) and have already used them to identify PML IV as the major isoform mediating regulation by EBNA1 and USP7. These cell lines will be valuable tools for dissecting the roles of the PML isoforms.
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Salsman, J., Zimmerman, N., Chen, T., Domagala, M. and Frappier, L. 2008 Genome-wide Screening of Herpes Simplex virus, Cytomegalovirus and Epstein-Barr virus Proteins for Subcellular Localization and Alteration of PML Nuclear Bodies. PLoS Pathogens 4(7): e1000100. doi:10.1371/journal.ppat.1000100