PAPILLOMA VIRUSES

1. PAPILLOMA VIRUSES

2. Papilloma Viruses • Characteristics • dsDNA viruses (circular) • Genone 8 kbp • Genome associated with cellular histones • Naked capsid • Widespread in humans and other animals • They cause ‘warts’ • Some strains 16,18 and 31 associated with high risk of cervical cancer • Over 100 types of HVP found so far

4. Papilloma Viruses • Different types infect different anatomical sites • HPV-1 target feet • HPV-2, 4, 7 hands • Virus targets mucosa (oral, nasal, genital) OR skin • Transmission of genital tract HPV thru sexual contact • Transmission of skin HPV thru touching, or contaminated surface • HPV that infect genital tract mucosal can cause • cervical warts referred to as ‘condylomas’ (benign rarely progress to cancer) • Strains 16,18 and 31 associated with high risk of cervical cancer such as Squamous Cell Carcinoma • Referred to as Oncogenic HPVs • Genital tract infections are transient in many cases cause no symptoms • Lack of symptoms allows for easier spreading

5. HPV 16 • HPV 16 is of interest due to oncogenic properties • 6 Early genes (E1, E2, E4, E5, E6 and E7) • 2 Late genes (L1 and L2) • Genome has multiple ORFs • Variable Splicing of mRNA transcripts yields variety of mRNAs with multiple ORFs • Virus targets mucosa (oral, nasal, genital) OR skin • Transmission of genital tract HPV thru sexual contact • Transmission of skin HPV thru touching, or contaminated surface

6. Infectious Cycle • Infection requires entry to NON-DIFFERENTIATED basal cells • Virus binds to heparin and 6-integrin • Virions are taken in by endocytosis • Genome ends up inside nucleus • Stays as circular DNA • Does not incorporate itself into host genome • Replication of genome occurs generating 50-100 copies • Every cell division genome is duplicated and split equally between parent and daughter cell • This type of division is referred to as ‘plasmid replication’ • When Basal Cells become Keratinocytes burst of viral replication occurs • This burst is called vegetative replication • During this same period L1 and L2 genes are expressed producing capsids • Release of assembled virions occurs during cell death

7. HPV 16, 31 Genome Structure • In undifferentiated basal cells Pearly transcribes all six E genes • Transcript terminates at Poly(A)early • Poly(A)early immediately after E genes resulting in no expression of Late genes • mRNAs generated have multiple ORFs, unclear how ribosomes translate them

8. HPV 16, 31 Genome Structure • Late promoter (Plate) active in Keratinocytes • When E1 and E2 levels increase a shift occurs towards vegetative DNA replication • Transcripts from Plate terminate to Poly(A)late Giving rise to L1 and L2 • Alternative splicing is responsible for the shift in E1 and E2 and ultimately increase in L1 and L2 • L1 and L2 encode for capsid proteins which leads to an increase in virion production • Splicing of Plate-Poly(A)late transcript results in removal of E genes and expression of only L1 and L2 genes

9. Functions of HPV Proteins • E1 is a DNA helicase, binds replication origin and initiates DNA replication. By unwinding genome cellular DNA polymerases can replicate viral genome • E2 enhances binding of E1 onto replication origin • E6 enhances degradation of p53 protein which is a major cell cycle control protein • E7 binds Rb protein increasing cell cycling • L1 major capsid protein • Splicing of Plate-Poly(A)late transcript results in removal of E genes and expression of only L1 and L2 genes • Some cellular transcription factors known to bind regulatory sequences of PV genome are: NF-1, SP1 and AP1

10. Viral Replication Surges in Keratinocytes • Replication surge is unusual given the fact that keratinocytes are terminally differentiated and do not carry any significant DNA replication • E7 is responsible for this peculiarity, binds Rb resulting in release of E2F transcription factor • E2F then participates in expression of a variety of genes involved in cell cycle • E7 binds Rb protein increasing cell cycling, essentially convincing keratinocytes to divide • Rb is a major tumor suppressor protein. Many retinoblastoma cancers are due to mutated Rb

11. E6 Eliminates p53 • p53 normally induces apoptosis of cells that should not be dividing such as Keratinocytes • p53 neutralization is achieved by ubiquitination which leads to proteasome degradation • similar to Rb, p53 mutations are observed in a variety of cancers

12. p53 Function • p53 normally is in low levels in the cytosol and inactive • In stress conditions: • increase in phosphorylation occurs making it more stable • Increase in p53 expression also occurs • p53 activation can stop cell cycling at G1 for repair • OR cause apoptosis • In many cancers p53 is mutated, in HPV is NORMAL!! • No reason to mutate, simply eliminate by ubiquitination

13. Cervical Cancer Cell Lines Induced by HPV • Surprisingly do not produce HPV virions! • In these cell lines genome is inserted into host genome • Normal HPV infected cells do not incorporate HPV genome into their own genome • E6 and E7 are the only genes being expressed in HPV related cancers • No surprise E6 and E7 target p53 and Rb respectively • Viral integration into host cellular genome has no advantage • Expression of E7 allows neutralization of Rb, release of E2F and expression of cell cycling genes • Normally p53 would eliminate them but p53 itself is eliminated through E6

14. Cervical Cancer Diagnosis and Prevention • Pap test has been the main diagnostic tool for cervical cancer for decades. • The discoverer is George Papanicolaou • Significantly reduces mortality rates of cervical cancer • Cervical warts are pre-malignant tumors and often removed surgically or freeze killed with liquid nitrogen • HVP vaccines are now available that raise immunity against L1 protein of HPV strains 16, 18 (Guardasil includes L1 from stains 6, 11, 16 and 18) • L2 is considered in vaccination trials, could provide immunity against wider range of HPV types • Vaccination is recommended at young age before encounter with virus i.e ages 11-12