CANCER; Lecture 10, 11 and 12 - DNA damage and repair, Cancer as a disease - Colorectal and Leukaemia Flashcards
1
Q
What can damage DNA?
A
- 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
2
Q
What are the types of DNA damage that can occur?
A
3
Q
What are the 2 phases of metabolism of drugs by CYP450?
A
4
Q
What are polycyclic aromatic hydrocarbons?
A
Common environmental pollutants, formed from combustion of fossil fuels and formed from combustion of tobacco
5
Q
How is Benzo[a]pyrene metabolised?
A
- 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)
6
Q
How is aflatoxin B1 epoxidated?
A
Targets the liver 1rily because it is activated by P450
7
Q
How is aflatoxin B1 epoxidated?
A
Targets the liver primarily because it is activated by P450
8
Q
How does UV radiation cause cancer?
A
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
9
Q
How does ionising radiation cause cancer?
A
x
10
Q
How does ionising radiation cause cancer?
A
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
11
Q
How does P53 deal with cellular stress?
A
- 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
12
Q
How does P53 deal with cellular stress?
A
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
13
Q
What are the 2 excision repair pathways?
A
- 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
14
Q
How many times does the cell endogenously become damaged and then repair?
A
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
15
Q
What is the fate of carcinogen-DNA damage?
A
- 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
16
Q
How do we test for DNA damage?
A
17
Q
What is the Ames test?
A
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
18
Q
How do you detect DNA damage in mammalian cells?
A
Look at chromosomes themselves -> chemicals can cause double strand breaks which can lead to fragmentation of chromosome
19
Q
What is the process of in vitro micronucleus assay observation?
A
20
Q
What is the bone marrow micronucleus assay in bone marrow used for?
A
- 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
21
Q
What causes single strand breaks?
A
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
22
Q
What causes base hydroxylations of DNA?
A
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.
23
Q
What causes Abasic sites of DNA?
A
Abasic sites -> entire DNA base has been removed so sugar backbone is maintained and during replication missing base will cause problems.
24
Q
How do double strand breaks occur?
A
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
25
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
26
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
27
What is the function of the colon?
Extraction of water from faeces (electrolyte balance), faecal reservoir and bacterial digestion for vitamins (K and B)
28
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
29
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
30
What are the protective mechanisms present in the colon to eliminate genetically defective cells?
Natural loss, DNA monitors and repair enzymes
31
What is a polyp?
Any projection from mucosal surface into a hollow viscus -\> may be hyperplastic, neoplastic, inflammatory, hamartomatous
32
What is an adenoma?
Benign neoplasm of mucosal epithelial cells
33
What are the types of colonic polyps?
Metaplastic/hyperplastic, adenomas, Juvenile, Peutz Jeghers, lipomas
34
What are hyperplastic polyps?
Very common -\> \<0.5cm, forming 90% of all colonic polyps and no malignant potential. 15% have K-Ras mutations
35
What are the types of colonic adenomas?
Tubular (90%), tubulovillous (10%), villous, (flat, serrated) NB: more villous = worse
36
What is the shape of polyps?
Pedunculated adenomas on a stalk;
Sessile adenomas are flat and raised -\> both can be tubular, tubulovillous or villous
37
What is the microstructure of tubular adenomas?
Columnar cells with nuclear enlargement, elongation, multi-layering and loss of polarity; increased proliferative activity; reduced differentiation; complexity/disorganisation of architecture
38
What is the microstructure of villous adenomas?
Mucinous cells with nuclear enargement, elongation, multilayering ad loss of polarity; exophytic; rarely may have hypersectretory function and result in excess mucus discharge and hypokalaemia
39
What is dysplasia?
Abnormal growth of cells with some cancer features
40
What is APC and FAP?
Increased numbers of polyps can be caused by conditions such as FAP -\> 5q21 gene mutation, with site of mutation determining clinical variants (classic, attenuated, Gardner, Turcot)
41
How do you treat FAP/APC?
Prophylactic colectomy
42
What is the incidence of colonic adenomas?
25% of adults have adenomas by 50, with 5% ebcoming cancers if left -\> larger polyps have higher risks than small ones; cancer stays curable for about 2 years
43
How does the cancer progress from adenoma to carcinoma?
Most colorectal cancers arise from carcinoma -\> with residual adenomas in about 10-30% of colorectal cancers; adenomas preceding cancer by 15y -\> endoscopic removal of polyps decreases incidence of subsequent colorectal cancer
44
Which genes are damaged in the adenoma-carcinoma sequence?
* APC;
* K-Ras,
* Smads,
* p53,
* telomerase activation
45
How does microsatellite instability cause the pathway from adenoma to carcinoma?
Some microsatellites (repeat sequences prone to misalignment) are in coding sequences of genes which inhibit growth/apoptosis (TGFbR11). Mis-match repair genes (MSH2, MLH1) are recessive genes requiring 2 hits - HNPCC has germline mutation of these genes
46
What are the 3 genetic pathways in colorectal cancer?
Adenoma-carcinoma sequence; microsatellite instability (mis-match repair genes); genetic predisposition (FAP - inactivation of APC; HNPCC - microsatellite instability)
47
What are the different incidence rates of colon cancer in countries and onset age range?
35,000/y in UK; 50-80yrs range; high in USA, eastern Europe and Australia; low in Japan, Mexico and Africa
48
What are the dietary factors that can increase colon cancer incidence?
High fat, low fibre, high red meat, refined carbs
49
What is dangerous in food that can increase colon cancer incidence?
Food contains various bioactive chemicals, carcinogens, also Anticancer agents -\> heat can modify chemicals further and bacteria can modify food residues
50
What happens when meat is cooked at high temperatures?
Heterocyclic amines, causes oxidation of PhIP when cooked at high temps -\> N-OH-PhIP + deoxyguanosine which leads to mutagenesis
51
What are the dietary deficiencies of patients with colon cancer?
Folates (important co-enzymes for nucleotide synthesis and DNA methylation); MTHFR (deficiency leading to disruption in DNA synthesis causing DNA instability = mutation; decreased methionine synthesis leads to genomic hypomethylation and focal hypermethylation which can have gene activating and gene silencing effects)
52
What is the clinical presentation of colorectal cancer?
!!Change in bowel habits, per rectal bleeding, unexplained iron deficiency anaemia!!, per rectal mucus, bloating, cramps, weight loss, fatigue -\> rationalisation of symptoms with piles, IBS
53
What are the macroscopic features of colorectal carcinoma?
Small carcinomas may be present within larger polypoid adenomas (pedunculated or sessile)
54
What are the anticancer food elements?
Vitamin C + E = ROS scavengers; isothiocyanates (cruciferous veg); polyphenols (green tea, fruit juice) = activate MAPK, regulates phase 2 detox enzymes and reduce DNA oxidation. Garlic associated apoptosis (ajoene and allicin); green tea (EGCG-induced telomerase activity)
55
What is the distribution of cancer in the colon?
Caecum/asc. = 22%; transverse = 11%; desc. = 6%; rectosigmoid = 55%
56
What are the microscopic structures of carcinomas?
Almost all adenocarcinomas grade 1-3, mucinous carcinomas, signet ring cell and neuroendocrine (v. rare)
57
How do you grade adenocarcinomas?
Proportion of gland differentiation relative to solid areas or nests and cords of cells without lumina -\> 10% well diff; 70% moderately diff and 20% poorly diff
58
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
59
What are some clinical features that affect prognosis?
Improves: diagnosis of asymptomatic patients, rectal bleeding presenting as symptom. Tumour location = colon better than rectum and left better than right. Diminishes prognosis = Age \<30, preop serum CEA high, distant metastases (Very diminished)
60
What are some pathological features that affect prognosis?
Improved: local inflammation and immunologic reaction. Depth of bowel wall penetration (increased = poor prognosis); number of regional LN involved (1-4 better than \>4 nodes); degree of differentiation (Well is better than poor). Diminished prognosis: Mucinous or signet ring cell, venous invasion, lymphatic invasion, perineural invasion
61
What are the treatment options for colorectal cancer?
x
62
When do you screen for high risk colon cancer?
Previous adenoma 1st Degree relative affected by colorectal cancer before the age of 45 2 affected first degree relatives evidence of dominant familial cancer trait including colorectal, uterine, and other cancers UC and Crohn’s disease Hereditable cancer families (include other sites)
63
What is the definition of screening?
The practice of investigating apparently healthy individuals with the object of detecting unrecognised disease or people with an exceptionally high risk of developing disease, and of intervening in ways that will prevent the occurrence of disease or improve the prognosis when it develops.
64
What are the criteria or a screening programme?
Importance of disease and natural history of disease should be known to ID where screening can occur and to enable effects of any intervention to be assessed. Should be simple and acceptable to patient, sensitive and selective, screening popn have equal access and cost effective
65
What is the NHS screen for colon cancer?
Look for faecal occult blood, and from 55y onwards they send an FOB kit; if blood then endoscopy performed -\> 50-60y then sigmoidoscopy, older then full colonoscopy -\> look for adenomas that can be removed but in some people they find cancer
66
What is leukaemia?
Bone marrow disease, which can have an increase in white cells. Cancer of the blood -\> most common in men and women aged 15-24; main cause of cancer death in people aged 1-34y
67
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
68
Which white cells are involved in leukaemia?
69
How is leukaemia different from other cancers?
Don't have solid tumours; leukaemic cells replacing normal bone marrow cells and circulating freely in the blood stream; haemopoietic and lymphoid cells behave differently from other body cells (normal haemPO stem cells circulate in blood and both stem cells and cells derived can enter tissues; normal lymphoid stem cells recirculate between tissues and blood
70
What is benign and malignant in leukaemia?
Benign are called chronic (go on for long time), malignant are called acute (aggressive and if not treated quickly then patient dies quite rapidly -\> can also be myeloid (granulocytic, monocytic, erythroid, megakaryocytic) or lymphoid (B/T cells) depending on cell of origin
71
What are 4 types of leukaemia?
Acute lymphoblastic leukaemia (ALL) Acute myeloid leukaemia (AML) Chronic lymphocytic leukaemia (CLL) Chronic myeloid leukaemia (CML) NB: lymphocytic = problem with mature cells, lymphoblastic = problem with primitive cells
72
Why does leukaemia occur?
Series of mutation in a single stem cell; some from IDable oncogenic influences, other random errors -\> Mutation in a known proto-oncogene Creation of a novel gene, e.g. a chimaeric or fusion gene Dysregulation of a gene when translocation brings it under the influence of the promoter or enhancer of another gene. / Loss of function of tumour suppressor gene can also contribute (deletion or mutation); tendency to increase chromosomal breaks = increase risk of leukaemia
73
Which inherited or constitutional abnormalities can contribute to leukaemogenesis?
Down’s syndrome Chromosomal fragility syndromes Defects in DNA repair Inherited defects of tumour-suppressor genes
74
What is the difference between acute and chronic myeloid leukaemia?
Whereas in AML there is a failure of production of end cells, in CML there is increased production of end cells. AML = cells continue to prolif but don't mature so build up of immature cells occurs (blast cells) in bone marrow which will spread into the blood; failure of production of normal functioning end cells (neutrophils, monocytes, erythrocytes, platelets). CML = cell kinetics and function not as seriously affected, but cell becomes independent of external signals and alterations in interaction with stroma and there is reduced apoptosis. AML = responsible mutation usually affect TF so transcription of multiple genes is affected, cell behaviour profoundly disturbed (product of oncogene prevents normal function of protein);//CML = responsible mutations usually affect gene encoding a protein in signalling pathway between cell surface receptor and nucleus (protein may be membrane receptor or cytoplasmic protein)
75
What is the difference between acute and chronic lymphoid leukaemias?
ALL = increase in lymphoblasts and don't mature into B/T cells, replacing bone marrow and lead to failure. CLL = Leukaemic cells are mature but abnormal as all belong to single clone
76
What are the clinical features of Acute lymphoblastic leukaemia?
Caused by accumulation of abnormal cells: Bone pain, Hepatomegaly Splenomegaly Lymphadenopathy Thymic enlargement Testicular enlargement Cuased by crowding out of normal cells: Fatigue, lethargy, pallor, breathlessness (caused by anaemia) Fever and other features of infection (caused by neutropenia) Bruising, petechiae, bleeding (caused by thrombocytopenia)
77
What are the haem features of Acute lymphoblastic leukaemia?
Leucocytosis with lymphoblasts in the blood Anaemia (normocytic, normochromic) Neutropenia Thrombocytopenia Replacement of normal bone marrow cells by lymphoblasts
78
How would you investigate acute lymphoblastic leukaemia?
Blood count and film Check of liver and renal function and uric acid Bone marrow aspirate Cytogenetic/molecular analysis Chest X-ray
79
Why do we need to carry out cytogenetic and molecular genetic analysis?
Managing the individual patient because it gives us information about prognosis; Avances knowledge of leukaemia because it has permitted the discovery of leukaemogenic mechanisms Detected by 2 fluorescent probes using FISH -\> TKIs used to target product of BCR-ABL1 fusion gene in CML which also occurs in cases of ALL in elderly; Hyperdiploidy in ALL is a good prognosis; translocation indicated poor prognosis
80
What are the leukaemogenic mechanisms of ALL?
Formation of a fusion gene (translcation) Dysregulation of a proto-oncogene by juxtaposition of it to the promoter of another gene, e.g. a T-cell receptor gene Point mutation in a proto-oncogene
81
How can we treat ALL?
Supportive Red cells Platelets Antibiotics (if fever) Systemic chemotherapy; Intrathecal (as they can spread into CNS) chemotherapy
82
How does leukaemia cause disease characteristics?
Abnormal cells accumulate leading to: Leucocytosis, bone pain (if leukaemia is acute), hepatomegaly, splenomegaly lymphadenopathy (if lymphoid), thymic enlargement (if T lymphoid), skin infiltration. Metabolic effects of leukaemic cell proliferation: hyperuricaemia and renal failure, weight loss, low grade fever and sweating. Crowding out of normal cells leading to lack of end cells (esp bad when acute leukaemia), anaemia, neutropenia, throbocytopenia. Loss of normal immune function due to loss of normal T/B cell function (CLL feature)
83
What occurs in the mouth in acute myeloid leukaemia?
Hypertrophy of gum caused by infiltration of leukaemic cells and monocytes; haemorrhage due to thrombocytopenia -\> Acute leukaemia cells of monocyte lineage may migrate to tissues that have inflammation, shown in gums (v. likely)
84
What is the epidemiology of ALL?
May be due to exposure to common pathogen, largely disease of children; evidence relates to family size and socioeconomic class; some leukaemias in children result from irradiation in utero, exposure to certain chemicals in utero
