Proto-oncogenes & tumor suppressor genes Viral factors of oncogenesis

Question 1

The cell cycle

Answer 1

G1= an active cell w/ normal biological functions
S= synthesis phase, DNA-replication = duplicating whole genome.
G2= production of enzymes, membrane components & cell organelles
M= mitosis, celldivision
G0= cells with no more proliferation potential

Question 2

Apoptosis

Answer 2

programmed cell death

• Regulated process
• Only the given cell is involved
• Requires energy
• Catalyzed by caspase enzymes
• Cell organelles are fragmented into apoptotic bodies •Physiological, essential process
• No inflammation
• Apoptosis vs. necrosis!

Question 3

Regulation of growth, replication & transcription: tyrosine kinase dependent pathways

Answer 3

Ras/ MAP kinase cascade
•1.Receptor Sos=>Ras=>Raf=>MEK (MAPKK)=>Erk (MAP kinase) => transcription!
•2.MAPK-cascade=> Ras=>PI-3-kinase => PKB/Akt

Ras= GTP-as protein
Sos=guanine nucleotide exchange factor
Erk= EC signal-regulated kinase
Raf, Mek, Erk= protein kinase phosphorylation cascade

Question 4

(Proto-)oncogenes & tumor suppressor genes–definitions

Answer 4

•Proto-oncogenes:
–Involved in the signalling pathways of cell division, cell growth: stimulatory action
–Mutation or increased expression transformed to oncogenes oncogenesis (tumorigenic transformation)
•Tumor suppressor genes:
–Main functions:
•Inhibit Cell division +DNA replication
•Stimulate cell differentiation
•Induce apoptosis
–Mutation or decreased expression=> inactivation =>lack of protective action against cancer

Question 5

(Proto-)oncogenes

Answer 5

•Growth factors: e.g. PDGF, EGF, IGF-1, VEGF…
•Growth factor receptors: tyrosin kinase type receptors! –e.g. EGF receptor= EGFR (HER2)
•Protein kinases: e.g. MAP kinase cascade- Ras, Raf
•Regulators of cell cycle: cyclins
-Taxol/paclitaxel: Bcl-2 inhibition
-Mammary gland, ovary-, cervix, GI tumors
•Anti-apoptotic proteins: e.g. Bcl family
•Transcription factors: e.g. NF kappaB

Question 6

Effects of NF-kappaB

Answer 6

• Inactive in normal cells, active in tumor cells!
• Proto-oncogene effect
• Stimulation of cell division
• Inhibition of apoptosis
• Stimulation of angiogenesis(VEGF, IL-8)
• Stimulation of metastases
• Other diseases: emergence of asthma, diabetes mellitus, arteriosclerosis!

Question 7

Effects of NF-kappaB

Answer 7

• Inactive in normal cells, active in tumor cells!
• Proto-oncogene effect
• Stimulation of cell division
• Inhibition of apoptosis
• Stimulation of angiogenesis(VEGF, IL-8)
• Stimulation of metastases
• Other diseases: emergence of asthma, diabetes mellitus, arteriosclerosis!

Question 8

Tumor suppressor genes

Answer 8

•Cell surface molecules
–TGFβ: stimulating CDK inhibitors (cyclin/ cyclin-dependent kinase inhibitors=> cell cycle arrest)
–p16 gene: inactivation melanoma, pancreas, kidney, lung tumors
–p21 gene
•Cytoplasmic signalling molecules: inhibiting the activation of proto-oncogenes
–GAP: GTPase-activating proteins activating the GTPase function of Ras
•Transcription factors
–p53 gene: regulation of cell cycle(CDK inhibition!) and apoptosis
–Rb(Retinoblastoma) gene: dephosphorylated protein=> binding & inactivation of E2F transcription factors
•DNS repair genes

Question 9

The function of p53 protein

Answer 9

• The most important tumor suppressor!! (human tumors: >50% with p53 mutation)
• DNA damage=>p53 protein detaches from MDM-2 & gets phosphorylated=> binds to p21 gene promoter=> p21 protein production inhibition of cyclin/cyclin-dependent kinase complex =>cell cycle arrest, no proliferation+ apoptosis induction
• p53 mutation =>no protective action=> oncogenesis

Question 10

Role of p53 in cancer therapy

Answer 10

•„Traditional” chemo-/radiotherapy: DNA damage activation of p53
•Removal of mutantp53: Adenoviruses(E1Bdeleted) replicating only in p53-mutant tumor cells
•Insertion of wild-typep53:
–Gene therapy w/ Retrovirus vector (lung tumors)
–Gene therapy w/ Adenovirus vector(Ad-p53) (ovarian tumors) –Gendicine (Advexin)
•Activating p53 signalling

Question 11

The function of GAP

Answer 11

regulates Ras (a GTPase) activity

Question 12

Viral oncogenesis–Introduction

Answer 12

• 20% of tumors: infectious background
• Historical significance
• Discovery of oncogenes and tumor suppressor genes
• Therapy and prevention: –Vectors –Vaccines

Question 13

Viral oncogenesis–Mechanisms

Answer 13

•Infection with an oncovirus=> transformation of the host cell=> tumor cell
•Consequences of a viral tumor transformation:
–Uncontrolled cell division
–Invasion of the EC matrix
–De-differentiation
–Evading apoptosis

Question 14

Viral oncogenesis–Rous sarcoma

Answer 14

• Peyton Rous–1911: RSV = Rous Sarcoma Virus ~1960: investigating the mechanism
• Src gene: first known viral oncogene tyrosine kinase enzyme
• 1966: Nobel prize

Question 15

Viral oncogenesis–Mechanisms–RNA viruses

Answer 15

–Direct oncogenesis: „acute tumor viruses”
•Insertion of a viral oncogene
•A copy of the virus particle is presented in each tumor cell => viral antigen expression =>immune suppression, vaccines!
•E.g. Rous Sarcoma Virus

–Indirect oncogenesis: cis-activating tumor viruses •Promoter insertion=> activating the cellular protooncogenes: cis-activation
•Mutations, chronic inflammation
•The virus particle is not presented in all tumor cells („hit and run”)
•E.g. HCV = Hepatitis C Virus, FeLV= Feline Leukemia Virus

–Indirectoncogenesis: trans-activatingtumor viruses •tax gene => Taxprotein: activating cellular protooncogenes & stimulating virus replication: trans activation
•E.g. BLV = Bovine Leukemia Virus, HTLV-1 = Human TcellLymphotropVirus1

Question 16

Important human oncoviruses

Answer 16

• Epstein-Barr virus(EBV) –Burkitt-lymphoma; hepatitis B Virus (HBV) –hepatocellular carcinoma
• HPV –cervical cancer
• Hepatitis C Virus(HCV) – hepatocellular carcinoma

Question 17

Important oncoviruses in veterinary praxis

Answer 17

•FeLV= FelineLeukemiaVirus
–FeLV-B: oncovirus(cis-activation)! => leukosis, lymphoid leukaemia
–FeLV-C: severe anaemia(non-regenerative, normochrome)
–FeLV-T: T-cell depletion immune-suppression
–Cytology, blood smear(leukocytosis, pancytopenia)
–Importance of vaccination(outdoor cats!)

•BLV = Bovine Leukemia Virus
–Trans-activating oncovirus (Taxprotein!): integrated into the genome of B-lymphocytes
–Patho-mechanism similar to HTLV-1
–Mostly benign cases =>certain animals: B cell leukemia= enzootic bovine leukosis

Question 18

Viral oncogenesis–role of cofactors

Answer 18

•EBV:
–Burkitt-lymphoma only in malaria-endemic areas
–Nasopharyngeal carcinoma: Southern China
•HBV: liver tumor in combination with aflatoxin
•HPV:UV light=>skin cancer
•Role of immunosuppression!! (HIV, FIV: damaged cellular immunity)

Question 19

Viral oncogenesis–prevention with vaccines

Answer 19

• Effective subunit vaccines(recombinant technology) •Human oncology: HPV, HBV vaccines
• Veterinary praxis: FeLV, BLV vaccines