CANCER; Lecture 10, 11 and 12 - DNA damage and repair, Cancer as a disease - Colorectal and Leukaemia Flashcards
What can damage DNA?
- Can lead to mutation which can lead to cancer.
- Diet is strongly associated with cancer (40-45%), herbicides and pesticides have quite a small proportion of cancer burdens, medical treatments, such as RT, can also damage DNA and increase risk of cancer.
- Some things are endogenous (mitochondria produce reactive oxygen species) which have the ability to damage DNA -> age causes accumulation which can lead to neoplastic phenotype of cell
What are the types of DNA damage that can occur?
What are the 2 phases of metabolism of drugs by CYP450?
What are polycyclic aromatic hydrocarbons?
Common environmental pollutants, formed from combustion of fossil fuels and formed from combustion of tobacco
How is Benzo[a]pyrene metabolised?
- Substrate for CYP450 which oxidises it to form Benzo[a]pyrene-7,8-oxide which is reactive and wants to find electrons ->
- defence in body is epoxide hydroxylase cleaves the three membered strained ring of oxide to form dihydrodiol which is not toxic ->
- BUT it is also a substrate for CYP450, which then converts to oxide which is very reactive and forms DNA adducts (high electron density)
How is aflatoxin B1 epoxidated?
Targets the liver 1rily because it is activated by P450
How is aflatoxin B1 epoxidated?
Targets the liver primarily because it is activated by P450
How does UV radiation cause cancer?
Pyrimidine dimers can be formed, where if 2 are next to each other on the DNA and under presence of UV they can covalently link together -> SKIN cancer
How does ionising radiation cause cancer?
x
How does ionising radiation cause cancer?
Super oxide radical is very powerful molecule of O2 with extra electron so its very reactive. Hydroxyl radical -> even more reactive than super oxide radical
How does P53 deal with cellular stress?
- Tumour suppressor gene ->
- tied up with MDM2 which keeps it inactive ->
- mild physiological stress = p53 orchestrating transcriptional series of events and activates proteins that help repair the problem.
- SEVERE stress can make p53 activate apoptotic pathway by interacting with apoptosis proteins
How does P53 deal with cellular stress?
Tumour suppressor gene -> tied up with MDM2 which keeps it inactive -> mild physiological stress = p53 orchestrating transcriptional series of events and activates proteins that help repair the problem. SEVERE stress can make p53 activate apoptotic pathway by interacting with apoptosis proteins
What are the 2 excision repair pathways?
- Base: DNA glycosylate hydrolyses between sugar and DNA base, then AP-endonuclease splits DNA strand so there is gap in backbone; DNA polymerase fills missing base and DNA ligase then seals DNA to form intact DNA.
- Nucleotide -> Endonuclease makes two cuts in the DNA on either side of the site of damage
- These patches can be long (100-200 nucleotides) or short (~10-20 nucleotides) Helicase will then remove this patch, leaving the double stranded DNA with a patch missing DNA Polymerase then replaces the bases that have been removed using the complementary strand as a template DNA Ligase then joins the DNA up again
- This process is energy-demanding and requires a lot of proteins
How many times does the cell endogenously become damaged and then repair?
We have spare capacity to deal with both endo/exogenous damage;
if damage poorly repaired then greater risk of carcinogenesis;
single strand breaks are easiest to deal with
What is the fate of carcinogen-DNA damage?
- If DNA damage is excessive then cells will commit apoptosis ->
- but most problems occur between excessive and small amounts of damage ->
- could lead to incorrect repair/altered primary sequence, which if undergoes DNA replication and cell division = fixed damage in daughter cells;
- can lead to transcriptional and translational problems leading to formation of aberrant proteins/carcinogenesis
How do we test for DNA damage?
What is the Ames test?
Put the chemical in a prep of rat liver enzymes (with P450), so it is converted into active form; then genetically engineered bacteria (to not produce histidine which is required for growth) are introduced to chemical. If its a mutagen then the bacterial colonies will grow, as it will damage their DNA and mutate it so they can proliferate without histidine. The more DNA damaging the chemical, the more colonies will grow and survive on the agar
How do you detect DNA damage in mammalian cells?
Look at chromosomes themselves -> chemicals can cause double strand breaks which can lead to fragmentation of chromosome
What is the process of in vitro micronucleus assay observation?
What is the bone marrow micronucleus assay in bone marrow used for?
- Using pluripotent nature of bone marrow in producing blood cells ->
- animals treated with chemical and bone marrow cells/peripheral erythrocytes examined for micronuclei ->
- RBC can remove nucleus during development but not small fragments of DNA ->
- DNA damage = micronuclei in RBC
What causes single strand breaks?
Very common and useful, physiological enzyme that break it (topoisomerase involved in relaxing and unwinding DNA, by chopping strand of DNA, allowing strand to unwind and we can gain access to DNA as strand is reannealed
What causes base hydroxylations of DNA?
Base hydroxylations: oxidative reaction occurring on one of DNA bases and can cause problems; could mean DNA has to get repaired and during repair process can become mutated.
What causes Abasic sites of DNA?
Abasic sites -> entire DNA base has been removed so sugar backbone is maintained and during replication missing base will cause problems.
How do double strand breaks occur?
Tendency for the 2 DNA strands to drift apart when they break, which is intolerable for cell -> DNA repair mechanisms exist but sometimes they can go wrong and introduce DNA damage
What are DNA adducts and alkylation?
Caused by chemicals, due to chemicals being metabolically activated into electrophiles which are attracted to e- rich DNA, forming a covalent bond with DNA, which during replication can be a problem as DNA polymerase can’t recognise the base
What is the epidemiology of colorectal cancer?
Major cancer in developed coountries -> 4th most common overall, 2nd leading cause of cancer death -> environmental and genetic factors important. Adenocarcinomas mainly
What is the function of the colon?
Extraction of water from faeces (electrolyte balance), faecal reservoir and bacterial digestion for vitamins (K and B)
How is the colon organised normally?
Mucosa is folded, but smooth, with thick muscle layer -> cells divide in crypts and are shunted to the top of the villus, where they are shed
How quickly is the colonic epithelium shed and how are the crypts organised?
2-5million cells die per minute in the colon, making the cells vulnerable -> APC gene product reduces the risk of istakes during replication, with mutation causing cell proliferation
What are the protective mechanisms present in the colon to eliminate genetically defective cells?
Natural loss, DNA monitors and repair enzymes
What is a polyp?
Any projection from mucosal surface into a hollow viscus -> may be hyperplastic, neoplastic, inflammatory, hamartomatous
What is an adenoma?
Benign neoplasm of mucosal epithelial cells
What are the types of colonic polyps?
Metaplastic/hyperplastic, adenomas, Juvenile, Peutz Jeghers, lipomas
What are hyperplastic polyps?
Very common -> <0.5cm, forming 90% of all colonic polyps and no malignant potential. 15% have K-Ras mutations
What is the shape of polyps?
Pedunculated adenomas on a stalk;
Sessile adenomas are flat and raised -> both can be tubular, tubulovillous or villous
Which genes are damaged in the adenoma-carcinoma sequence?
- APC;
- K-Ras,
- Smads,
- p53,
- telomerase activation
What is Dukes classification?
Apical lymph node is the highest lymph node that has been removed, which if positive indicates that cancer could have spread further in lymphatics
What are the treatment options for colorectal cancer?
x
What causes leukaemia?
Results form series of mutations in a single lymphoid or myeloid stem cell;
Mutations lead the progeny of cell to show abnormalities in proliferation, differentiation or cell survival leading to steady expansion of leukaemic clone
Which white cells are involved in leukaemia?
