T6 - DNA Repair and Cancer Flashcards
describe the general outline of the different phases of the eukaryotic cell cycle, including G0
- G1 metabolic changes prepare the cell for division
- S phase DNA synthesis replicates the genetic material, and each chromosome now consists of two sister chromatids
- G2 phase metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis
- M phase a nuclear division (mitosis), followed by a cell division (cytokinesis)
- G0 phase a resting or quiescent phase when the cell is not growing or dividing (some cells enter this phase for varying time periods)
G1, S and G2 are collectively known as interphase
describe the variety of exogenous and endogenous factors that can cause DNA damage
DNA can be damaged by lots of agents throughout normal course of life. Usually sensed by the cell and repaired using mechanisms. If repair mechanisms are faulty or damage too big, can’t restore DNA to healthy state… can lead to mutations and expression of disease
free radicals cause DNA damage… these are produced by…
endogenous sources
- metabolism
- inflammation
exogenous sources
- UV light
- air pollution
- ionizing radiation eg radon gas from ground + medical procedures
- smoking
other exogenous sources include
- mutagenic chemicals
- alkylating agents ★
- anti-cancer drugs
★ alkylating agents are compounds that work by adding alkyl gorup to guanine base of DNA molecule… prevents strands of DNA helix bonding as they should
types of DNA damage
- single strand damage
- double strand damage ★
- mismatches (ie G=T)
- deamination (converts cytosine to uracil)
- apurinic site (blocks polymerases)
- pyrimidine dimer (two adjacent pyrimidines are connected on single strand)
- intercalating agent (substance that inserts itself into helix and binds to DNA)
- interstrand crosslink
- bulky adduct (where base is covalently bonded to chemical, which can interfere with transcription, replication and induce mutations)
- replication errors (more detail on another card)
★ much more difficult to repair as other strand can’t be used as template
what is replication stress
- inefficient replication that leads to replication fork slowing, stalling and/or breakage
- eg unrepaired DNA lesions are physical barriers to replication fork progression
- other causes of replication stress include: fragile sites (or oncogene induced stress), ribonucleotide incorporation, limiting nucleotides and DNA secondary structure
- eg ribonucleotide misincorporation can stall the polymerases, therefore slowing DNA replication at the replication fork
repetitive DNA sequences can lead to….
fork slippage
- this is where the wrong number of repetitive sequences are copied
- DNA has trinucleotide repeats (ie large numbers of one stretch of base pair)
- DNA polymerase struggles to accurately copy these
- may incorporate extra base pairs or loose some
- eg newly synthesised or template strand ‘loops out’
- means that the new strand being replicated might have extra or less nucleotides than they should
- fork slippage can lead to trinucleotide expansion, Huntington’s, spinocerebellar ataxia and Fragile X syndrome
Huntington’s: causes and symptoms
- can be caused by replication errors
- have GAG polyglutamine repeats naturally in DNA
- repeats more likely to be miscopied
- replication errors (eg fork slippage) can cause number of polyglutamine repeats to increase and pass threshold
- threshold passed ⇢ Huntington’s triggered in patient
- causes mutated Huntingtin protein
- mutant protein misfolds and aggregates in basal ganglia neurons
- leads to neural degeneration
- causes degeneration of the basal ganglia
- basal ganglia needed to control movement
- progressive, late onset disease (around 30-50 y/o)
- usually fatal after 20 years
- genetic causes, has genetic counselling
- concentration difficulties, depression, mood swings
- stumbling, clumsiness and involuntary jerking
- problems speaking + breathing
https://www.mayoclinic.org/diseases-conditions/huntingtons-disease/symptoms-causes/syc-20356117
explain and describe the DNA damage response
- in a normal situation, the DNA damage can be detected and the body has endogenous response to cope with this
- damage activates signal that is detected by sensor
- activates downstream pathways
- transducers activate effector molecule to repair damage
leads to…
- cell cycle transitions ie stop replication, so damaged genes can no longer be replicated
- apoptosis ie programmed cell death if the damage is too severe
- novel transcription aka de novo transcription of damaged gene… basically leads to new mutation and new variant
- DNA repair in most cases, ie DNA exonuclease cuts out mismatched base pair and allows DNA to polymerase to go back + rectify the error
what are the four different outcomes of mutation
mutation can be caused by endogenous or exogenous factors… in most cases mutations are repaired by body’s normal DNA repair mechanism
- DNA repair
- Senescence (ie cell aging)
- Proliferation (ie rapid reproduction)
- Apoptosis (cell death)
what are the four different outcomes of mutation
mutation can be caused by endogenous or exogenous factors… in most cases mutations are repaired by body’s normal DNA repair mechanism
- DNA repair
- Senescence (ie cell aging)
- Proliferation (ie rapid reproduction)
- Apoptosis (cell death)
what are the types of DNA repair
just list - detail of them on seperate cards
single strand repair
- base excision repair
- nucleotide excision repair
- mismatch repair
follows similar pathway…
detection of mutation or lesion → excision → repair by polymerase → joined by ligase
one side is intact so can be used as a template
double strand repair
- non homologous end joining
- homology-directed repair
- holliday junction resolution (don’t need to know)
base excision repair
- type of single strand base repair
- deamination converts a cytosine into a uracil
- the uracil is detected, leaving a base-less nucloetide
- the base-less nucleotide is removed (excised)
- leaves a small hole in the DNA backbone
- the hole is filled with the right nuceotide by DNA polymerase
- the gap is sealed by ligase
nucleotide excision repair
- type of single strand break repair
- similar to base excision repair, but a larger number of base pairs are removed and then replaced
- eg UV radiation produces a thymine dimer
- once the dimer has been detected, the surrounding DNA is opened to form a bubble
- enzymes cut the damaged region out of the bubble
- a DNA polymerase replaces the excised DNA
- ligase seals the backbone
nucleotide excision repair
- type of single strand break repair
- similar to base excision repair, but a larger number of base pairs are removed and then replaced
- eg UV radiation produces a thymine dimer
- once the dimer has been detected, the surrounding DNA is opened to form a bubble
- enzymes cut the damaged region out of the bubble
- a DNA polymerase replaces the excised DNA
- ligase seals the backbone
mismatch repair
- type of single stranded break repair
- a mismatch is detected in newly-synthesised DNA
- the new DNA strand is cut
- the mispaired nucleotide and its neighbours are removed
- the missing patch is replaced with correct nucleotides by a DNA polymerase
- DNA ligase seals the gap in the DNA backbone
non-homologous end joining
- a type of double strand break repair
- broken ends of each site bound by protein complex
- complex of other proteins binds
- trims off excess base pairs around breakage (restriction)
- DNA ligase repairs break
- repaired piece of DNA
homology directed repair
- type of double strand DNA break repair
- the DNA either side of the break is resected by a protein complex
- the complex then permits a heteroduplex to form (made by pairing up with the other copy found in the sister chromatid)
- this combines the two broken pieces of DNA and the unaffected homologous double strand DNA sequence found on the paired chromosome
- broken DNA uses other copy of gene as a template to repair broken DNA
- displacement loop moves along DNA, making a complementary strand to the template
- newly synthesised DNA is captured by original template
- DNA polymerase and ligase repair the break.
not needed but… holliday junction has the same idea of using the template strand from the genome to repair broken DNA
lynch syndrome
- caused by defective mismatch repair
- aka hereditary nonpolyposis colorectal cancer
- autosomal dominant inheritance
- high risk of colorectal, endometrial, gastric and ovarian cancer
- treated by surgery in many cases
- mutations in mismatch repair genes can cause Lynch
- accumulation of these mutations cause a normal cell to hyper-proliferate, then develop into adenoma → carcinoma and then metastisize
what is intertumour and intratumour heterogeneity
intertumour
heterogeneity between different tumours of the same type in different patients, and the difference between cells in a primary and second tumour
intratumour
lots of different cell types in a single tumour, meaning they are not all genetically identical
this means that cancers can evolve over time, which has an impact on the way we treat them with chemotherapy
clonal expansion and clinical resistance
chemotherapy is used to treat cancer cells = the use of chemicals to try kill cancer cells without affecting healthy cells of the body
differential sensitivity
- some cells in tumour will be killed by chemotherapy
- some cells in tumour will be resistant to chemotherapy
- resistant cells will expand over time
- leads to chemotherapy-resistant tumour
chemotherapy-induced mutagenesis
- chemotherapy itself can cause mutations in cancer cells
- these cells can be resistant to chemotherapy
- resistant cells expand over time
- can no longer use this chemotherapy to treat the tumour
synthetic lethality strategies
- can also use heterogeneity to create new cancer drugs
- in a normal body cell, often see genetic redundancy
- have more than one gene that undertakes the same function ie gene A + B
- if gene B has cancerous mutation, should target gene B
- gene A is healthy and can take gene B’s place
- cells can still carry out normal function
- leads to cell survival, cancer cell death
an example would be PARP inhibitors in breast cancer
- common gene mutated in breast cancer is BRCA 1/2
- PARP inhibitors able to target some types of BRCA 1/2 but not others
- in a non cancer cell, have multiple copies of BRCA 1/2 and treating with PARP inhibitor will not kill the cell
- therefore healthy cells survive
ataxia telangiectasia
- mutations occur in ATM gene
- leads to faliure of homology directed repair
- autosomal recessive inheritance pattern
- patients are sensitive to UV damage
- currently no treatments
- rare, neurodegenerative disease that causes severe disability
- damage to cerebellum leads to difficulty with movement and co-ordination
- weakened immune system
- DNA repair is disrupted, therefore heightened cancer risk
ATM gene = ataxia telangiectasia mutated gene
