Our research interest is in understanding how high-risk HPV leads to cancer. We want to study the way in which HPV activates telomerase. This is a critical step in oncogenic progression in many cancers and is universally found in HPV-associated cancers, and the way in which HPV hijacks normally-expressed cellular proteins to disrupt growth and differentiation.
Human papillomavirus (HPV) is a double stranded DNA virus, with a genome that is approximately eight kilobases in length. There are over 200 genotypes of HPV that have been identified, and more than 30 types infect the mucosa and can be sexually transmitted. HPV is the most common sexually transmitted infection, with more than 75% of adults having evidence of a current or prior infection with HPV,5and with a third of adolescents getting infected with HPV after being sexually active for at least two years.1HPV requires a full thickness epidermis to complete its life cycle and produce infectious virions. HPV genes are named E for early, and L for late, based on their expression patterns in a normal viral life cycle. The E1 and E2 proteins are involved in HPV DNA replication and expression. The L1 and L2 proteins form the capsomers for HPV. The E6 and E7 proteins drive normal epithelial cells to continue to divide as they leave the basal layers of the epidermis and differentiate.
HPV that infects the mucosa can be further divided as high-risk or low-risk, based on their association with cancer. E6 and E7 genes in these high-risk HPVs are considered to be oncogenes; they are universally expressed in all HPV-associated cancers. The expression of high-risk HPV E6 and E7 immortalizes epithelial cells grown in culture. We are interested in the role of HPV E6 in this cellular immortalization step and its drive to disrupt the normal balance of cellular growth and differentiation.
Telomeric DNA is repetitive DNA that caps the ends of linear chromosomes. With each cellular DNA replication, approximately 200 nucleotides of DNA are lost. Thus, the length of telomeric DNA marks the age of a cell; the older a cell is, the shorter its telomeres. Once the telomeric DNA of cell becomes critically shortened, the cell goes through either senescence or apoptosis (the "Hayflick Limit").2All stem cells express telomerase, a ribonucleoprotein that can extend the repetitive DNA at the ends of chromosomes, to avoid this limit. Telomerase is comprised of several subunits, and in humans it includes the protein dyskerin, the RNA template TERC, and the catalytic subunit hTERT. The expression of hTERT is rate limiting for the enzyme; with more hTERT expression, there is more telomerase activity in cells. Cancer cells need to avoid senescence or apoptosis signals as a part of oncogenic progression; thus, the majority of cancers activate telomerase.
High-risk HPV E6 activates telomerase through transcriptional activation of the hTERT promoter. We have found that HPV E6 also increases hTERT expression through post-transcriptional regulation.3,4HPV E6 interacts with several endogenous proteins in epithelial cells, and we are studying the way in which hTERT mRNA is regulated by these endogenous proteins, including NFX1-123 and cytoplasmic poly (A) binding proteins. More broadly, we are interested in the normal role of NFX1-123 and cytoplasmic poly (A) binding proteins in RNA regulation and other RNAs that HPV E6 affects in epithelial cells. All of these studies will shine light on in the normal HPV viral life cycle, on cancer progression with or without HPV infection, and finally could be utilized as a marker of disease progression in patients.
Differentiation and Cell Growth
HPV requires full-thickness epithelium or mucosa to complete its life cycle. It co-opts the normal host cells' pattern of differentiation to trigger its viral genome amplification and late gene expression; however, HPV also drives differentiating host cells to continue to divide through its viral genes E6 and E7.
We are interested in the manner by which HPV uncouples cellular differentiation from cell cycle arrest in its host cell during an HPV infection. This "differentiating while dividing" phenotype would be ideal for a long-lived and productive HPV infection, and persistent high-risk HPV infections are the number one risk factor for cancer development. We received a grant from the National Institutes of Health to study this dysregulation mechanistically and to conduct model studies of cervical cancer disease development and progression.
Please contact Rachel Katzenellenbogen to discuss the laboratory's current projects and learn more about our areas of focus.
- Forhan, S. E., S. L. Gottlieb, M. R. Sternberg, F. Xu, S. D. Datta, G. M. McQuillan, S. M. Berman, and L. E. Markowitz. 2009. Prevalence of sexually transmitted infections among female adolescents aged 14 to 19 in the United States. Pediatrics. 124:1505-1512.
- Hayflick, L. 1965. The limited in vitro lifetime of human diploid cell strains. Exp.Cell Res. 37:614-636.
- Katzenellenbogen, R. A., E. M. Egelkrout, P. Vliet-Gregg, L. C. Gewin, P. R. Gafken, and D. A. Galloway. 2007. NFX1-123 and Poly (A) Binding Proteins Synergistically Augment Activation of Telomerase in Human Papillomavirus Type 16E6 Expressing Cells. J.Virol. 81:3786-3796. doi:10.1128/JVI.02007-06.
- Katzenellenbogen, R. A., P. Vliet-Gregg, M. Xu, and D. A. Galloway. 2009. NFX1-123 increases hTERT expression and telomerase activity posttranscriptionally in human papillomavirus type 16 E6 keratinocytes. J.Virol. 83:6446-6456.
- Koutsky, L. A. and N. Kiviat. 1999. Genital human papillomavirus, p. 347-359. In: K. K. Holmes, P. F. Sparling, P.-A. Mardh, S. Lemon, W. Stamm, P. Piot, and JN. Wasserheit (eds.), Sexually Transmitted Diseases. Third ed. McGraw Hill, San Francisco.