31-326 LCCC, CB #7295
The work in our laboratory focuses on the pathogenesis of human papillomaviruses (HPV); small DNA viruses that exhibit epithelial tropism. Of the over 100 types of HPV identified, fifteen of these are categorized as high-risk and are considered the causative agents of cervical cancer. High-risk HPVs are also associated with cancers of the anus, oropharynx and esophagus, identifying HPV as a risk factor for multiple human cancers. The life cycle of HPV is dependent on cellular factors and epithelial differentiation. Differentiation triggers the productive phase of the life cycle, which includes viral genome amplification, late gene expression and virion production. Paradoxically, these events occur in differentiated cells that normally would have exited the cell cycle. To ensure virion production, HPV proteins re-program the cellular DNA synthesis machinery upon differentiation, pushing cells into S phase to allow for viral replication. The ability of the HPV oncoproteins E6 and E7 to target critical regulators of cell cycle progression results in the bypass of checkpoints that would normally eliminate abnormal cells. This results in the accumulation of genetic alterations that eventually lead to transformation and cancer development. However, the mechanisms by which the differentiation-dependent phase of the viral life cycle is regulated are unclear. My lab is interested in defining signaling pathways modulated by HPV that promote the productive phase of the life cycle, in turn contributing to viral pathogenesis and possibly transformation.
We have identified caspase activation and the DNA damage response as two novel mechanisms by which HPV proteins regulate viral replication in differentiating cells. High-risk HPV31 induces the activation of caspases-3, -7 and -9 at low levels upon differentiation, which occurs in the absence of cell death, but in the presence an anti-apoptotic response. We found that caspase activation is necessary for viral genome amplification through cleavage of the viral replication protein E1. Both E6 and E7 can induce caspase activation upon differentiation, but the mechanism by which this occurs is unclear. In more recent studies, we have found that HPV positive cells exhibit activation of an ATM-dependent DNA damage response throughout the viral life cycle, in both undifferentiated and differentiating cells. Inhibition of the ATM kinase, as well as its downstream target Chk2, blocks viral genome amplification upon differentiating, suggesting an important role for DNA damage signaling in the viral life cycle. In addition, inhibition of Chk2 diminishes caspase activation, indicating a link between the DNA damage response, caspase activation and viral genome amplification. We are interested in further defining the contributions of DNA damage signaling and caspase activation to the viral life cycle. Specific projects related to these findings are as follows:
- How Does HPV Induce Caspase Activation in Differentiating Cells?
The pattern of caspase activation in differentiating HPV positive cells suggests that HPV proteins stimulate the intrinsic, or mitochondrial-mediated, apoptotic pathway. Although activation of a DNA damage response appears to be important in promoting caspase activation upon differentiation, the mechanism by which this occurs is unclear. In response to DNA damage, Chk2 can activate the intrinsic apoptotic pathway in a p53-dependent or p53-independent manner through phosphorylation and stabilization of the E2F1 transcription factor. E2F1-mediated transcription of p53 or its homolog p73 results in increased transcription of pro-apoptotic genes, leading to alteration of mitochondrial integrity, cytochrome c release and caspase activation. Since both E6 and E7 can activate Chk2, we are interested in determining if modulation of Chk2 activity and E2F1 phosphorylation in differentiating cells leads to caspase activation through alteration of mitochondrial integrity. Similarly to Chk2, the DNA repair proteins Nbs1 and Brca1 have also been reported to have role in DNA damage-induced apoptosis, and may cooperate with Chk2 in mediating caspase activation in HPV positive cells. Identifying how HPV modulates DNA damage and apoptotic signaling pathways will be important in understanding the productive phase of the life cycle.
- What are the Roles of DNA Repair Proteins in the Viral Life Cycle?
DNA repair signaling ensures the fidelity of replication through a temporary halt in cell cycle progression, with ATM responding primarily to double-strand DNA breaks. Treatment with an ATM inhibitor blocks HPV genome amplification, suggesting that activation of ATM pathway members is necessary for productive replication. Multiple targets of ATM are activated in HPV-positive cells, including Chk2, as well as the repair proteins Nbs1 and Brca1, but it is unclear which of these proteins is important in facilitating productive replication. However, given the considerable cross-talk between ATM pathways, it is likely that several of these repair factors play a role in viral replication, and we are interested in defining the individual contributions of these proteins to the viral life cycle. In addition to caspase activation, components of ATM signaling may affect viral replication through the activation of S- or G2- checkpoints, which for many viruses has been shown to be essential for productive replication. In addition, we have recently found that repair proteins localize to sites of viral genome amplification, suggesting that the DNA damage response may be necessary to ensure the fidelity of viral replication. Further understanding of how HPV proteins activate and utilize DNA repair proteins to promote late viral events will provide great insight into the mechanisms of HPV pathogenesis.
Fradet-Turcotte A, Bergeron-Labrecque F, Moody CA, Lehoux M, Laimins LA, Archambault J. 2011. Nuclear accumulation of the papillomavirus E1 helicase blocks S-phase progression and triggers an ATM-dependent DNA damage response. J Virol. Jul 6. [Epub ahead of print].
Knight GL, Pugh AG, Yates E, Bell I, Wilson R, Moody CA, Laimins LA, Roberts S. 2011. A Cyclin-Binding Motif in Human Papillomavirus Type 18 (HPV18) E1^E4 is Necessary for Association with CDK-Cyclin Complexes, but is Not Required for Differentiation-Dependent Viral Genome Amplification or L1 Caspid Protein Expression. Virology. 2011 Mar 30;412(1):196-210.
Fradet-Turcotte A., Moody C., Laimins L.A., and J. Archambault. 2010. Nuclear Export of Human Papillomavirus type 31 E1 is Regulated by Cdk2 Phosphorylation and Required for Viral Genome Maintenance. J Virol. 84(22):11747-60.
Moody, C.A. and L.A. Laimins. 2010. HPV Oncoproteins: Pathways to Transformation. Nature Reviews Cancer. 10(8):550-60.
Moody, C.A. and L.A. Laimins. 2009. Human Papillomaviruses Activate the ATM DNA Damage Pathway for Viral Genome Amplification Upon Differentiation. PLoS Pathogens. 5(10), p.e1000605.
Moody, C.A., and L.A. Laimins. 2009. The Life Cycle of Human Papillomaviruses. In: DNA Tumor Viruses, Damania B and Pipas J (ed). Springer Press, New York, New York. Pgs 75-104.
Côté-Martin, A., Moody, C.A., Fradet-Turcotte, A., D’Abramo, C., Lehoux, M., Joubert, S., Poirier, G.G, Coulombe, B., Laimins, L.A., and J. Archambault. 2008. The Human Papillomavirus E1 Helicase Interacts with the WD Repeat Protein p80 to Promote Maintenance of the Viral Genome in Keratinocytes. Journal of Virology. 83, 1271-1283.
Moody C.A., Fradet-Turcotte, A., Archambault, J., Laimins, L. 2007. Human Papillomaviruses Activate Caspases Upon Epithelial Differentiation to Induce Viral Genome Amplification. Proceedings of the National Academies of Sciences. 104, 19541-19546.
Moody, C.A., R.S. Scott, C-A Nathan, L.S. Young, J.W. Sixbey. 2005. Modulation of the Cell Growth Regulator mTOR by Epstein-Barr Virus-Encoded LMP2A. Journal of Virology. 79, 5499-5506.
Scott, R.S., C.A. Moody, and J.W. Sixbey. 2005. Epstein-Barr Virus and Oral Malignancies. In: Robertson E (ed). Epstein-Barr virus: Pathogenesis, Molecular Biology, and Infection. Horizon Scientific Press/Caister Academic Press, Norfolk, U.K.