How do HPV oncoproteins cause cancer? Solving the perplexing case of TIP60

SMCX and components of the TIP60 complex contribute to E2 regulation of the HPV E6/E7 promoter

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The high-risk human papillomaviruses (HPVs) cause cervical cancer, other anogenital cancers and approximately 20% of head and neck cancers. These cancers are driven by two viral oncoproteins, E6 and E7, that target tumor suppressor pathways. Progression to cancer involves the dysregulated expression of E6 and E7, which is often due to the loss of viral E2 expression. E2 is a multifunctional protein and one of its functions is repression of the promoter that drives E6 and E7 transcription. Initial models from the 1980s and 1990s suggested that E2 binding to its cognate sites in the promoter blocked binding of transcriptional activators. However E2 mutant studies indicated that E2 repression was more likely mediated by the recruitment of specific cellular factors to the promoter.

Several years ago, we carried out an unbiased whole genome siRNA screen to identify cellular proteins involved in E2 mediated repression. In the screen, we validated a role for Brd4 in E2 repression and identified a number of other proteins that functioned independently to repress the E6/E7 promoter, among which were additional chromatin modifiers and readers. This study extended our findings and focused on SMCX, a histone demethylase, and EP400, a component of the TIP60 histone acetyltransferase complex.

We had hypothesized that E2 might recruit SMCX to the promoter for E6 and E7 in order to demethylate histone H3 at lysine 4 (H3K4), thus creating a transcriptionally repressive signal. This hypothesis was supported by observing higher levels of demethylated H3K4 at the promoter only in the presence of E2. Our hypothesis was further supported when the level of demethylated H3K4 decreased when either E2 or SMCX were knocked down.

In our E2 repression screen we identified EP400 and several other components of the TIP60 histone acetyltransferase complex but not TIP60 per se. This was somewhat perplexing. However the initial screen had used a pool of four siRNA duplexes against TIP60, and we found by testing the individual siRNAs that indeed depletion of TIP60 relieves E2-mediated repression. This is an example of a false negative that can be observed with pools of siRNAs. Thus we validated a role for TIP60 itself and established that E2 can complex TIP60 as a component of the histone acetyltransferase complex. E2 alters the chromatin microenvironment using specific cellular proteins to repress transcription of E6 and E7.

Two mechanistic models for E2-mediated transcription of the HPV E6 and E7 oncogenes. (A) E2 binding to the promoter region blocks the binding of required transcriptional factors. (B) E2 recruits chromatin repressor factors to the promoter region thus creating a transcriptionally repressive signature.
Two mechanistic models for E2-mediated transcription of the HPV E6 and E7 oncogenes. (A) E2 binding to the promoter region blocks the binding of required transcriptional factors. (B) E2 recruits chromatin repressor factors to the promoter region thus creating a transcriptionally repressive signature.

Introducing the authors

Howley2Howley1

(1) Jennifer A. Smith, Peter M. Howley and Elizabeth A. White (Harvard Medical School)
(2) Friederike S. Haberstroh (currently a graduate student at the Max Planck Institute of Biochemistry, Munich)

About the research

SMCX and components of the TIP60 complex contribute to E2 regulation of the HPV E6/E7 promoter
Virology, Volumes 468–470, November 2014, Pages 311–321
Jennifer A. Smith, Friederike S. Haberstroh, Elizabeth A. White, David M. Livingston, James A. DeCaprio, Peter M. Howley

Read the full article on ScienceDirect.

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