Cancer and cell signalling

Question 1

stadia in cancer

Answer 1

1. normal
2. dysplasia –> growth advantage
3. carcinoma in situ –> ability to spread but not yet through basal lamina
4. malignant carcinoma –> invaded through basal lamina
5. metastasizing

Question 2

carcinogens

Answer 2

• chemical: genotoxins like smoke and non-genotoxins like hormones
• radiation: UV from sun and ionizing radiation
• microbial: oncogeneic viruses (HPV - produce protein that inhibit Rb) or bacteria that cause long lasting inflammation (helicobacter pylori)
• all chemicals/radiation attack nucleotides on distinct sites (guanine is alkylated on C6 and 7 for example)

Question 3

DNA adduct –> mutataion

Answer 3

• normal base pairing C : G (3) and A : T (2)
  1. when G is alkylated on C6 a binding site is lost –> can be repaired by BER/NER
  2. polymerases wil make a O-alkyl-G : T/A pair when replicating
  3. this is not yet a mutation just a mismatch
  4. only when the affected cell devides again will the O-alkyl-G be replaced by an A to form a A : T match which is a mutation

Question 4

DNA repair - general

Answer 4

1. damage (DSB, replication block, etc.)
2. sensors (yH2AX, ATM, ATR)
3. mediators
4. Chks
5. effectors (p52, p16, etc.)
6. repair, senescence or apoptosis

Question 5

nucleotide excision repair

Answer 5

• Exogenous DNA damage – chemical, UV
• Bulky lesions that block DNA replication are detected (helix distortions)
• Incision on both sides of lesion
• Path of DNA is excised and repaired by repolymerization and ligation
• Eukaryotes have a complex system of proteins while E. coli only has 4 enzymes UvrABCD
• Can be coupled to transcription

Question 6

base excision repair

Answer 6

• Endogenous DNA damage – ROS, depurination/amination
• Lesions do not block polymerases they are very small
• Oxidative lesions: 8-oxo-G (GC  AT)
• Deoxyuracil: insertion of U or deamination of CU
• Spontaneous depurination (only sugar no base, a-basic site)
• Long and short-patch:
  
  • long patch: glycosylase has no lyase activity (sugar flap must be removed by a
    lyase separately)
  • short patch: glycosylase has lyase activity

Question 7

homologous recombination

Answer 7

• Radiation causes DSB
• A complementary chromosome is used only happens in S phase and G2 phase since there is n2 at these time points
  1. DSB
  2. sticky ends are created by exonucleases (ATM, RAD51, BRCA1/2, and more, initiate this step)
  3. one sticky end finds homologous strand and invaginates it (RAD proteins)
  4. this strand is elongated and eventually dissociates
  5. the sticky end of the other strand binds to the end of the newly elongated strand
  6. gap is filled in by polymerase and ligase

Question 8

non-homologous end joining

Answer 8

• Last resort

- Just putting the strands together with risk of mutation or loss of nucleotides

Question 9

DNA damage tolerance

Answer 9

upon a replication block multiple events can occur:

• damage avoidance –> damage repair or or polymerase switches template
• the less favorable translesion synthesis –> y-family polymerase will be used to just fill in any nucleotide (often A) just to make sure replication or transcription can move on

The choice depends on the ubiquitination of PCNA (part of polymerase-alfa)
Poly-ubiquitin –> damage avoidance
ubiquitin mutations lead to translesion synthesis and thus to missmatch mutations

Question 10

signalling via oxadative DNA damage

Answer 10

• nuclear: genes that need to be expressed during oxadative stress have many Gs in their promotor since these are easily damaged to 8-oxo-guanine –> serves as signal
• OGG1 (glycosylase) binds to 8-oxo-guanine and recruits transcription machinery to start expressing these genes
• OGG1 may also open op the chromatin in this process
• extranuclear: OGG1 cuts out 8-oxo-G which goes into the cytosol where it binds to another OGG1 –> this complex can activate GTPases (turn GDP into GTP) which causes cell signalling (resulting often in inflammation)

Question 11

signalling via DNA aducts - methylation

Answer 11

• BER can cut out 3-methyl-adenine which goes to cytosol and atcs as a growth inhibitor
• inhibits PI3K

Question 12

BRCA1/2

Answer 12

• Both BRCA1 and BRCA2 have been implicated recently in transcriptional regulation and DNA repair (associate with Rad51 and polymerases)
• The evidence for BRCA function in DNA repair suggests that they may not be direct tumour suppressors after all but may be involved in the maintenance of genome integrity –> stability genes
• The loss of genome integrity, however, would lead to DNA damage and activation of the normal checks on cell division involving, in particular, the p53 pathway.
  • Thus it seems likely that loss of the p53 checkpoint will be a prerequisite for growth of BRCA1-/- or BRCA2-/- cells.