Question

Human papillomavirus (HPV) is the etiological agent for multiple forms of cancer, including cervical, oropharyngeal, penile and anal cancers. Of these, HPV-mediated cervical cancer is the most common. Primary HPV infection occurs in undifferentiated columnar cells of the cervical basal epithelium, the endocervix. Conversely, virions egress exclusively from terminally differentiated cells comprising the outer layer of the cervical epithelium, the ectocervix. Thus, the virus relies on proliferation and subsequent differentiation of endodermal cells up the cervical epithelial wall for the production of new virions. To ensure this process occurs, HPV encodes two proteins, E6 & E7, to inhibit apoptotic pathways and promote cellular proliferation, respectively. E6 binds to and inactivates p53 while E7 binds to and inactivates retinoblastoma protein. In over 80% of HPV-mediated cervical carcinomas, the viral genome integrates into the host genome. Ultimately, this causes unregulated production of oncoproteins E6 & E7 and cancer arises as a result of this overexpression.

d) What would be the result of p53 inactivation with respect to the cell cycle?

e) P53 not only has a role in the DNA damage response but also in the intrinsic apoptotic pathway. Explain how p53 can promote activation of the intrinsic apoptotic pathway.

Answer 1

d. p53 gene is associated with cell cycle regulation. The product of gene acts as transcriptional activator which regulates the expression of other genes (description is provided in the image attached below) lthat control the cell cycle. P53 protein induces the productionof cyclin kinase inhibitor (CKI) which inactivates the Cdk-Cyclin complex which arrests the cell at G1 check point allowing the cell to repair the damaged DNA.

e. Role of p53 in activation of intrinsic apoptotic pathway:

Damage of DNA

p53 tumor suppressor gene

Puma, Noxa

Bcl-2, Bcl-XL, Mcl-1

Bax, Bak(Bid (from Caspase 8, 10) and Bad also upregulates the action of Bax and Bak)

Mitochondria acivates Smac/ DIABLO which inturn activates IAP

Cytochrome C, Apaf-1, Caspase 9

Caspase 3,6,7 (also activates Caspase 8, 10 which also causes Apoptosis)

Apoptosis

P53 activates the apoptosis by intrinsic pathway by inducing the Puma and Noxa which further activates the Bax and Bak that increase the permeability of outer mitochondrial membrane causing cytochrome C to efflux outside. and bind to Apaf1 that activates the apoptosome procaspase 9. Procaspase 9 cleaves and activates the caspases 3.6,7 that results in apoptosis