Tumour precursors, Carcinogenesis and genetic changes

Question 1

What are the most common cancers for males and females?

Answer 1

Female; breast, lung, bowel

Male; prostate, lung, bowel

Question 2

How long does cancer take to develop?

Answer 2

Different cancers have different mutation rates, with defective DNA repair, UV and smoking associated with the highest levels of mutation
Cancer does not develop suddenly (multiple steps). Changes in the epithelium that precede invasion and metastasis

Question 3

What is the evidence of precursors?

Answer 3

Changes in architecture of epithelium

Cytological morphology of the cells that precede invasion and metastasis

Question 4

What are the precancerous stages of cervical cancer described as?

Answer 4

(and the vulva, anus, skin & bronchus) intra-epithelial neoplasms or intra epithelial lesions (also called severe dysplasia or carcinoma in situ for high grades).

Question 5

What are the precancerous stages called/ what are they?

Answer 5

Abnormal neoplastic cells not invaded (benign)
Epithelial side of basement membrane- intra-epithelial
- Cervical Intra-epithelial Neoplasia (CIN – UK & Europe)
- Squamous Intra-epithelial Lesions (SIL – in USA) Spectrum: low grade to high grade

Question 6

What are the cytological features of neoplasia?

Answer 6

(Intra-epithelial neoplasms have a proliferating epithelium)

• abnormal nuclei (pleomorphism & hyperchromasia)
• abnormal mitosis
• loss of nuclear polarity- now vertically orientated
• loss of differentiation.

Question 7

What tests are done for cervical cancer screening?

Answer 7

Pap smear, Liquid Based Cytology

Question 8

What is Ductal Carcinoma In Situ (DCIS)?

Answer 8

Excess numbers of neoplastic epithelial cells larger than normal with a range of nuclear cytological abnormalities build up within enlarged ducts or groups of small ducts, causing them to dilate (breast)

Question 9

What is an adenoma (intestine)?

Answer 9

Intra-epithelial phase preceding adenocarcinoma of colon/ rectum- 90%, also known as polyps
dysplastic glandular epithelium and this is graded into low grade or high grade dysplasia
Invasive cancers develop from adenoma; “adenoma-carcinoma” sequence

Question 10

How do neoplasms occur?

Answer 10

result of the escape of tumour cells from the normal homeostatic mechanisms that control the maintenance of organ and tissue architecture and function
-corruption or breakdown of them results from loss or errors in key controls and the downstream effects from this

Question 11

How do mutations lead to neoplasms?

Answer 11

• The “errors” are direct damage to DNA – mutations
• The targets for mutations are genes controlling proliferation and genomic stability
• The transition from a normal growth controlled cell through to a malignant cancer cell requires several mutations

Question 12

What are Carcinogens/oncogens?

Answer 12

agents which induce cancer in man or animals

Question 13

What is Carcinogenesis/Oncogenesis?

Answer 13

the process of cancer induction

Question 14

What are the classes of carcinogens?

Answer 14

Chemical - synthetic or naturally occurring molecules
Physical- UV or ionising radiation
Biological- Viruses, bacteria, parasites

Question 15

How do we study cancer progression?

Answer 15

• Animal models of carcinogenesis
• In vitro carcinogenesis - transformation
• Replicative Senescence, Immortalisation & Telomeres
• Inherited cancers in humans
• Molecular genetic analysis of cancers and their precursor lesions

Question 16

What concepts of carcinogenic activity was defined by animal models in the early 1900’s?

Answer 16

(a) Dose response- linear relationship between the amount of carcinogen in single dose and number of tumours which develop
(b) Latent period- time lag between administration of carcinogen and appearance of macroscopic tumours.
Length is dose-dependent – high reduce, low extend
(c) Threshold dose- below which no tumours form Secondary non-carcinogenic growth-promoting stimulus (e.g. wounding, chemicals such as Phorbol Esters)- subthreshold dose of carcinogen, then tumours develop.
(d) Initiation and Promotion

Question 17

What is initiation?

Answer 17

the alteration of a normal cell to a potentially cancerous cell
Carcinogens cause this and it is irreversible
Carcinogens are mutagens.

Question 18

What is promotion?

Answer 18

a process which permits the clonal amplification of the initiated cell.
Promoters are NOT carcinogens, they induce proliferation (“fix the mutation”- make mutation permanent). A benign neoplasm (e.g. papilloma) forms.

Question 19

What is progression?

Answer 19

acquisition of further mutations within the neoplastic clone drive progression to a malignant neoplasm.

Question 20

Describe in-vitro carcinogenesis/ the transformation assay

Answer 20

Monolayer of growth controlled cells treated with carcinogen, cellular transformation pile up in focuses, treated with serum, can grow in suspension, acquired features of neoplasm- behaviour and phenotype

Question 21

What is replicative senescence of primary diploid cells?

Answer 21

Cells can only undergo a defined number of cell divisions in tissue culture - the Hayflick number
Cell cycle arrest- held in G0- die by apoptosis

Question 22

How do cells become immortal?

Answer 22

Escape senescence at low frequency
Rare and spontaneous in long-lived animals
The viral oncogenes of DNA tumour viruses eg SV40 T, adenovirus E1A and E1B, HPV 16 E6 and E7, can immortalize primary human cells (chemical carcinogens rarely do so)

Question 23

What are telomeres?

Answer 23

Repetitive sequences (TTAGGG) at ends of chromosomes, Form loops to protect chromosome ends 
(not appear as DNA d/s breaks – causing end-end fusions) 3 prime overhang invades into double strand

Question 24

What are the problems with semiconservative replication?

Answer 24

Never fully replicates entire chromosome, gene shortening, last okazaki fragment not synthesised

Question 25

How to telomeres replicate?

Answer 25

Clock mechanism- Expression of telomerase enzyme, binds to repetitive sequence allows enzyme to fully replicate, uses Telomerase RNA and primer

Question 26

Describe the Telomere hypothesis of replicative senescence

Answer 26

Telomerase OFF- somatic cells, telomere length reduced

Telomerase ON- germ and stem cells, length maintained

Question 27

Describe the progression of telomeres in cancer

Answer 27

Telomere length decreases until too short (crisis)- induction of telomerase 90% active in cancers- immortalisation

Question 28

How does telomerase affect the hierarchy of cells?

Answer 28

•Stem cells express telomerase
•Transit-amplifying progenitor cells have a programmed
decline in replicative capacity (no telomerase)
•replicative senescence is one of the strict controls
that minimise the chances of cells escaping mechanisms that restrict proliferation

Question 29

Can cancer be inherited?

Answer 29

In rare instances tumour mutations can be inherited in the germ line giving rise to an inherited predisposition to a particular cancer

Question 30

What is retinoblastoma?

Answer 30

cancer of retinoblasts with a peak incidence at 3-4 years of age
both sporadic (no family history) and inherited

Question 31

What is the Knudson 2 step mutational process?

Answer 31

Inherited: (1) pre-zygotic mutation, (2) post-zygotic mutation (bilateral)
Sporadic: (1) & (2) both steps post-zygotic (acquired)- unilateral- in same cell
mutations affect the 2 alleles of the gene RB1, loss of heterozygosity

Question 32

Examples of syndromes in which the heterozygotes express the tumour phenotype

Answer 32

Retinoblastoma- RB1- Cell cycle checkpoint control
Familial adenomatous polyposis coli - APC- Signal transduction
Li Fraumeni- p53 or TP53- Cell cycle control/DNA damage
Hereditary non polyposis colon cancer/ Lynch- MLH1,MSH2- DNA Mismatch repair
Familial breast and ovarian cancer- BRCA-1, BRCA-2, DNA repair – d/s break
Basal cell naevus - Ptch - Signal transduction

Question 33

Examples of syndromes in which the homozygotes express the tumour phenotype

Answer 33

Ataxia telangiectasia- AT- Checkpoint control/ DNA repair
Blooms- BI- DNA Helicase
Fanconi’s anaemia- FA- DNA repair (crosslink repair)
Xeroderma pigmentosum- XP- DNA repair (Excision)

Question 34

What do studies of inherited cancer tell us?

Answer 34

Cancer is a genetic disease – the initiating event
is a somatic mutation in a single cell
More than one mutation is necessary for progression
Maintaining error free DNA is crucial
Controls restricting cellular lifespan must be overcome
for tumour progression
Cancer is a multi stage process

Question 35

What genes are targeted for mutation in cancer?

Answer 35

• Genes that control: Cell proliferation, Cell death, Signalling to other cells from other cells to the matrix from the matrix
• Genes that maintain tissue organisation and architecture: Cell movement, Cell adhesion
• Genes that maintain genetic stability / genomic integrity
Tumour Suppressor Genes, Oncogenes