Lecture 6 - Infectious agents Flashcards
Early work on viruses and cancer
1908 - Ellerman and Bang identify infectious leukaemia in chickens
1911 - Francis Peyton Rous shows viruses can cause sarcomas in chickens by Rous sarcoma virus
1933 - Rabbit papillomavirus identified
1936 - Mouse mammary tumour virus discovered (MMTV)
1950s - Mouse leukaemia and polyoma viruses identified
1962 - Adenovirus and SV40 shown to induce tumours in rodents
Rous experiment
- Chicken with sarcoma in breast muscle
- Remove sarcoma and break up into small chunks of tissue
- Grind up sarcoma with sand
- Collect filtrate that passed through fine-pore filter
- Inject filtrate into young chicken
- Observe sarcoma in injected chicken
Rous Sarcoma virus
- RNA Retrovirus
- env encodes surface glycoprotein envelope
- gag encodes core proteins that comprise the coat
- pol encodes reverse transcriptase that plays essential role in life cycle
Retrovirus lifecycle
- Virus enters cell and sheds envelope
- Reverse transcriptase makes DNA/RNA then DNA/DNA helix
- DNA copy integrates into host chromosome (forms provirus)
- Integrated DNA is transcribed to form many RNA copies
- RNA is translated into capsid proteins, envelope proteins, and reverse transcriptase
- Assembly of new virus
Experiemnt for aberrant growth by RSV
Aberrant growth
Temin (1950) experiment:
1. Cell in cell monolayer infected with Rous Sarcoma Virus in petri dish
- Transformation of cell
- Focus forms as cluster begins to form
Cancerous phenotype and active viral proteins
Normal morphology at 37 degrees
Infected with ts RSV mutant
Transformed morphology
Temperature increases to 41 degrees - cancerous phenotype forms
Temperature decreases back to 37 degrees - cancerous phenotype lost
src oncogene in cancer
src gene in RSV is avian and required for cancer
Proposed model of source of v-src
ALV virion -> infection and reverse transcriptase -> dsDNA provirus
dsDNA provirus + host cell chromosomal DNA with c-src -> integration -> Combined DNA which is later transcribed to form v-src RNA which is packaged into RSV virions
How does c-Src activate
- Inactive stage:
- Y527 is phosphorylated
- SH2 is bound to pY527
- SH3 is bound to polyproline type II helix
- Activation folded to block kinase activity - Intermediate stage:
- Src activator binds SH2/SH3
- Disrupts intramolecular interactions to free SH2/SH3
- Y527 dephosphorylated
- Activation loop more accessible - Active stage
- Y416 phosphorylated
- Kinase adopts open conformation
- Catalytic cleft accessible
- Src activated
Avian leukosis virus
- Slowly transforming retrovirus
- Provirus inserts randomly into host genome
- Very occasionally (1 in 10,000,000) inserts upstream of c-myc, leading to transcription of myc mRNAs and translation that causes uncontrolled proliferation
- Induction of cancer is a rare event via insertional mutagenesis
What increases the risk of viral cancer
Immunosuppression
Kaposi’s sarcoma was identified in 1800s but rare until the 1980s
- Cluster of cases in gay men in New York was reported in 1981 - associated with HIV/AIDS
- KSHV agent not identified till 1994
- KS 800x more likely in people with HIV
cervical cancer stats
Cervical cancer - 4th most common cancer in women worldwide, 9th most common in UK but 2nd most common in young women
660k new cases and 250k deaths in 2022
94% of deaths are in Low/middle income countries
HPV virus
dsDNA virus
Many subtypes, most innocuous
Show tissue tropism
HPV causes over 95% of cervical cancers
14 HPV subtypes linked to cancer
High risk HPVs 16 and 18 cause >2/3 of cervical cancer
HPV-16
7904bp
E6 -> E7 -> E1 -> E2 -> E4 -> E5 -> L2 -> L1
E - Early genes (viral replication)
L - Late genes - capsid proteins
Normal tissue without HPV infection
Intact mature squamous layer
Intact squamous later
intact para-basal (non-dividing) keratinocytes
Antigen detecting cells present
Transit amplifying cells and epithelial basement membrane
Infected tissue with HPV
HPV infects basal epithelial cells
Viral episomes in nucleus
E1 and E2 expressed in basal cells
E6 and E7 then begin to express in transit and suprabasal cells
E4 expressed in suprabasal cells
L1 and L2 expressed in mature squamous epithelial cells
E7 and RB
E7 binds RB instead of E2F
RB degraded by proteasome
Activates E2F target genes -> cell cycle entry proliferation
E6 and p53
E6 binds p53 and E6AP
p53 ubiquitinated and degraded by proteasome
Name the tumour promoting effects of E6 and E7
E6 - derepresses expression of hTERT (a component of telomerase)
E6 stimulates expression of VEGF (an angiogenic factor)
E6 and E7 stimulate genomic instability
E6/E7 in carcinogenesis
E7 inhibits pRB/E2F -> Abberant proliferation
E6 inhibits p53 -> Extended proliferation
E6 upregulates expression of hTERT, inhibiting telomere erosion -> indefinite life span
E6 and E7 cause genomic instability -> Cellular transformation
Other infections linked to cancer
Southeast Asian Liver Fluke - liver cancer
Bilharzia - bladder cancer
Helicobacter pylori - stomach cancer
Helicobacter pylori
- Causes ~90% stomach cancers
Causes chronic inflammation:
Chronic superficial gastritis -> Atrophic gastritis -> Dysplasia -> Carcinoma
CagA virluence factor
- Has pleiotrophic effects - can disrupt cytoskeleton, cell polarity and cell junctions - mitogenic and pro-inflammatory
- Allows H. pylori to bind integrins on basolateral surface
- Type IV secretion systems injection peptidoglycan and CagA into cell
Stimulates inflammation
Others types of virsues that cause cancer
Epstein-Barr (dsDNA herpesvirus) - Burkitt’s lymphoma and nasopharyngeal carcinoma
Hepatitis B (ssDNA/dsDNA hepadenovirus) - Hepatocellular carcinoma
Human T-lymphotrophic Virus I - + strand, ssRNA retrovirus - Adult T cell leukaemia
