Lec 16: Viruses and Cancer Flashcards Preview

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Flashcards in Lec 16: Viruses and Cancer Deck (18)
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1
Q

Cells in malignant tumors can

A

invade nearby tissues and spread to other parts of the body.

2
Q

All cancer starts from: (4)

A
  • a single normal cell
  • tumorigenesis including mutation
  • activation of oncogenes
  • inactivation of tumor suppressor genes.
3
Q

Tumor initiation is thought to be the result of

A

of a genetic alteration leading to abnormal proliferation of a single cell.

4
Q

oncogenic virus =

A

a virus that is able to cause cancer

5
Q

Evidence that a virus is oncogenic:

A
  1. ) regular presence in the tumour cells of virus DNA.
  2. ) virus DNA is integrated into a cell chromosomes.
  3. ) In many cases one or more of the virus genes are expressed in the tumour cell and virus proteins can be detected.
6
Q

Viruses cause cancer:

%’s

A

~20% of cancers in female

~8% of cancers in male

7
Q

How do viruses cause cancer? (3)

A
  1. ) Most virus-induced cancers develop after a long period of persistent infection with an oncogenic virus.
  2. ) Some persistent infections are latent for much of the time, with only small numbers of virus genes expressed.
  3. ) Others, including HBV and HCV infections, are productive. Both of the viruses are able to evolve rapidly, which allows them to keep one step ahead of acquired immune responses.
8
Q

Although many humans are persistently infected by…

Example:

A

…viruses that are potentially oncogenic, only small percentages develop virus-linked cancers.

(Ex: ~3% of women persistently infected with one of the high-risk strains of HPV (human Papillomavirus) develop cervical carcinoma.)

9
Q

Does virus infection alone cause cancer?

A

no, other factors: environment, host genetics and immunodeficiency (AIDS patients are much more likely to develop Kaposi’s sarcoma).

10
Q

4 ways viruses cause cancer:

A
  1. ) deliberate interference with control of the cell cycle
  2. ) accidental activation of cell genes that promote cell proliferation (binding to cell proteins that aren’t supposed to be activated)
  3. ) retroviral oncogenes (ex: RSV –> had oncogenes from previous host cells during replication that accidentally incorporated host oncogene into new host gene)
  4. ) immune defense damage
11
Q

(Deliberate interference with control of the cell cycle)

Several proteins produced by oncogenic viruses can…

A

…interact with p53, pRb and other proteins that control cell growth and division, increasing the probability of a cell being pushed into repeated cycles of division.

12
Q

p53, pRb function:

A

control cell growth and division

13
Q

action of p53:

A

damaged DNA –> p53 –> p21 (cell cycle arrest) OR puma/noxa (apoptosis)

14
Q

Papillomavirus-linked cancers: (4)

A
  1. ) Keratinocytes, the host cells of papillomaviruses, normally stop dividing as they differentiate,
  2. ) But a papillomavirus needs much of the DNA-replicating machinery of the host cell,
  3. ) So the virus induces the cell into the S phase.
  4. ) The cell then undergoes cycles of cell division, but occasionally the division results in chromosomes unchecked and leads to cancer. E.g. HPV promotes S phase in HeLa cells.
15
Q

(Accidental activation of cell genes)

A virus protein might bind to…

A

…cell proteins (not intended targets) and push a cell towards a cancerous state by activating a cell gene that is switched off, or by enhancing the rate of transcription of a gene that is being expressed at a low level.
(ex: Tax protein of HTLV-1 (retrovirus), which affects many gene expression)

16
Q

(Retroviral oncogenes)

Some retroviruses have the ability to…

When a proto-oncogene is…

A

… cause cancer because of the presence in the viral genome of an oncogene.

…mutated or aberrantly expressed it becomes an oncogene.
(ex: The src gene encodes a protein kinase; overphosphorylation of the enzyme’s substrates is a key process in the development of these tumors)

17
Q

(Retroviral oncogenes)

A single gene (src, encodes a protein tyrosine kinase) of RSV is found to be…

A

…responsible for the ability of RSV to induce tumors in birds and transform fibroblasts in culture.

18
Q

(Damage to immune defenses)

Interactions between cell proteins and proteins produced by oncogenic viruses can lead to…

A

…breakdown of immune defenses that may allow the development of a cancer.
(ex: Papillomavirus proteins can prevent apoptosis of virus-infected cells—-leading to cancer)