Genetics of malignancy

Question 1

G1-S checkpoint

Answer 1

Cell cycle is tightly regulated
Extrinsic growth signals required
Cancer cells circumvent normal regulation

Question 2

Identification of oncogenes

Answer 2

Chemical carcinogens added to mouse fibroblasts
DNA from the transformed cells was isolated and transfected back into normal mouse fibroblasts
Cells grow in unregulated manner and form tumours when injected into mice

Question 3

Mechanisms for oncogene activation

Answer 3

Deletion/point mutation in sequence leading to hyperactive protein (ras)
Regulatory mutation leading to overproduced normal protein
Gene amplification leading to overproduced protein (HER2)
Rearrangement moves regulatory sequence causing overproduced protein (myc) or fusion producing hyperactive protein (bcr-abl)

Question 4

Types of oncogenes

Answer 4

Growth factors
Growth factor receptors
G proteins
Intracellular serine/threonine kinases
Intracellular tyrosine kinases
Transcription factors
Negative apoptosis regulators

Question 5

Growth factors as oncogenes

Answer 5

Autocrine signalling
TGFalpha is a ligand of the epidermal growth factor receptor (EGFR)
Both receptor and ligand produced by lung, prostate and breast cancers

Question 6

Growth factor receptors as oncogenes

Answer 6

Receptor tyrosine kinases
Mutations in structure or overexpression lead to ligand-independent firing
HER2 oncogene overexpressed in breast cancer. Receptor is NRG/EGF

Question 7

G proteins

Answer 7

Ras is monomeric small GTPase
Activated by GTP binding (triggered by GEF)
Inactivated by GTP hydrolysis (induced by GAP)
RTK signal firing activates GEF which catalyses exchange of GDP for GTP (activation)
Ras activates cell growth signalling

Question 8

Ras mutations

Answer 8

Very common in cancer
KRAS most common overall and is primarily in pancreatic cancer
Codon 12 mutated in most tumours (hotspot mutations in oncogenes)
Mutations decrease GTPase activity so locked in GTP-bound (active form)

Question 9

Intracellular tyrosine kinases as oncogenes

Answer 9

bcr-abl oncogene results from translocation between chromosome 9 and 22
Encodes Bcr-Abl fusion protein (constitutively active tyrosine kinase)
Found in 95% of chronic myelogenous leukaemia (CML)

Question 10

Transcription factors as oncogenes

Answer 10

Myc TF regulates expression of genes involved in promoting cell proliferation/survival

Myc locus amplified in leukaemias and carcinomas
Myc expression deregulated in Burkitt’s lymphoma via chromosomal translocation

Question 11

Intracellular serine/threonine kinases as oncogenes

Answer 11

Downstream of RAS

B-raf V600E and V600k mutations frequently seen in melanoma