PAPILLOMA VIRUSES Papilloma Viruses Characteristics ds DNA viruses

PAPILLOMA VIRUSES

Papilloma Viruses • – – Characteristics ds. DNA 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

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

HPV 16 • HPV 16 is of interest due to oncogenic properties • • 6 Early genes (E 1, E 2, E 4, E 5, E 6 and E 7) 2 Late genes (L 1 and L 2) Genome has multiple ORFs Variable Splicing of m. RNA transcripts yields variety of m. RNAs 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

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 L 1 and L 2 genes are expressed producing capsids – Release of assembled virions occurs during cell death

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

HPV 16, 31 Genome Structure Ø Late promoter (Plate) active in Keratinocytes Ø When E 1 and E 2 levels increase a shift occurs towards vegetative DNA replication Ø Transcripts from Plate terminate to Poly(A)late Giving rise to L 1 and L 2 ØAlternative splicing is responsible for the shift in E 1 and E 2 and ultimately increase in L 1 and L 2 ØL 1 and L 2 encode for capsid proteins which leads to an increase in virion production ØSplicing of Plate-Poly(A)late transcript results in removal of E genes and expression of only L 1 and L 2 genes

Functions of HPV Proteins Ø E 1 is a DNA helicase, binds replication origin and initiates DNA replication. By unwinding genome cellular DNA polymerases can replicate viral genome Ø E 2 enhances binding of E 1 onto replication origin Ø E 6 enhances degradation of p 53 protein which is a major cell cycle control protein ØE 7 binds Rb protein increasing cell cycling ØL 1 major capsid protein ØSplicing of Plate-Poly(A)late transcript results in removal of E genes and expression of only L 1 and L 2 genes ØSome cellular transcription factors known to bind regulatory sequences of PV genome are: NF-1, SP 1 and AP 1

Viral Replication Surges in Keratinocytes Ø Replication surge is unusual given the fact that keratinocytes are terminally differentiated and do not carry any significant DNA replication Ø E 7 is responsible for this peculiarity, binds Rb resulting in release of E 2 F transcription factor ØE 2 F then participates in expression of a variety of genes involved in cell cycle ØE 7 binds Rb protein increasing cell cycling, essentially convincing keratinocytes to divide ØRb is a major tumor suppressor protein. Many retinoblastoma cancers are due to mutated Rb

E 6 Eliminates p 53 Ø p 53 normally induces apoptosis of cells that should not be dividing such as Keratinocytes Ø p 53 neutralization is achieved by ubiquitination which leads to proteasome degradation Ø similar to Rb, p 53 mutations are observed in a variety of cancers

p 53 Function Ø p 53 normally is in low levels in the cytosol and inactive Ø In stress conditions: • increase in phosphorylation occurs making it more stable • Increase in p 53 expression also occurs Ø p 53 activation can stop cell cycling at G 1 for repair ØOR cause apoptosis ØIn many cancers p 53 is mutated, in HPV is NORMAL!! ØNo reason to mutate, simply eliminate by ubiquitination

Cervical Cancer Cell Lines Induced by HPV Ø Surprisingly do not produce HPV virions! Ø In these cell lines genome is inserted into host genome ØNormal HPV infected cells do not incorporate HPV genome into their own genome ØE 6 and E 7 are the only genes being expressed in HPV related cancers ØNo surprise E 6 and E 7 target p 53 and Rb respectively ØViral integration into host cellular genome has no advantage Ø Expression of E 7 allows neutralization of Rb, release of E 2 F and expression of cell cycling genes ØNormally p 53 would eliminate them but p 53 itself is eliminated through E 6

Cervical Cancer Diagnosis and Prevention Ø Pap test has been the main diagnostic tool for cervical cancer for decades. Ø The discoverer is George Papanicolaou ØSignificantly reduces mortality rates of cervical cancer ØCervical warts are pre-malignant tumors and often removed surgically or freeze killed with liquid nitrogen ØHVP vaccines are now available that raise immunity against L 1 protein of HPV strains 16, 18 (Guardasil includes L 1 from stains 6, 11, 16 and 18) ØL 2 is considered in vaccination trials, could provide immunity against wider range of HPV types ØVaccination is recommended at young age before encounter with virus i. e ages 11 -12