Lecture 13: Oncogenes 2

LOS: Tumor supressor genes existence to explain the cancer phenotype, lessons from retinoblastoma, mechanisms of actions of tumour suppressor genes

How were oncogenes discovered?

Through viral oncogenes

Describe what happened when viral oncogenes were added in vitro

dominant negative, normal cells to cancer cells

Describe the inheritance of cancer

dominant in viruses, recessive in human

How were cancer inheritance patterns determined in human

fused normal cell and cancer cell to create heterokaryon, fusing agent was sendai or PEG, the hybrid was unable to form tutors

What was concluded from the heterokaryon experiment?

cancer is recessive in normal cells, there are genes which constrain proliferation (tumour suppressor genes)

What were the arguments for and against TSG existance?

For: easier to lose TSG by mutation than activating an oncogene Against: two copies of each allele therefore likelihood of losing both is low

What is a retinoblastoma

a tumour of the retina arising from embryonic cells failing to differentiate

What are the two types of retinoblastoma

sporadic, familial

Breifly outline Knudson’s 1 hit/2 hits hypothesis for familial and sporadic retinoblastoma

in familial one hits inherited only needs one mutation for tumour to occur, sporadic needs two

What is a proposition for how two copies of the gene can be lost given the very low probability

mitotic recombination results in cell lacking any functional Rb

What are TSGs two mechanisms of action?

Direct suppression in response to growth inbibs, Inhibition in response to metabolic imbalance/ DNA damage

What is p53?

A key TSG

What is NF1 and what does it do?

A RasGAP (GTPase activating protein) negatively feedbacks the Ras pathway

What happens in neurofibromiosis

LOH of NF1, Ras signalling proceeds

How long does NF1 normally take to kick in?

60-90 mins

What happened to Akt when NF1 is lost? (neurofibromatosis)

Increased levels, increased proliferation

What happened to mTOR when NF1 was inhibited?

More mTOR, apoptosis inhibited

Does NF1 increase or decrease proliferation?

decrease

What was found in NF1 null cells?

more colonies in anchorage independent conditions

What was found when an mTOR inhibitor was added to NF1 null cells?

colony number reduced

How are the majority of colon cancers inherited?

Sporadically

What gene is responsible for colon cancer?

APC

Loss of APC is the first step in colon cancer progression, what is the second?

oncogeneic mutations of RAS or loss of p53

How are stem cells regulated in healthy colonic crypts?

stem cells at bottom, most move up and die within 4 days (therefore safe from mutations)

How do mutations in colonic crypts drive cancer?

mutations blocking out-migration of cells from crypt

What molecule is involved in out-migration and what signalling pathway is this molecule controlled by? (Von Hippen-Lindau syndrome)

b-catenin via Wnt signalling

Describe normal APC action

negatively controls levels of b-cat in the cytosol, not expressed in cells at the bottom of the pit allowing b-cat to accumulate and move into the nucleus, as cells move upwards APC expression increase

What happens when APC is inactivated

b-catenin cystoillic accumulation and nuclear translation resulting in translation of growth promoting genes such as myc

What is myc?

A growth promoting gene

Describe what was seen when APC was knocked out in mice

larger crypts and more nuclear b-catenin

What does pVHL do?

modulate the hypoxic response

Briefly outline von Hippen-lindau syndrome

hereditary predisposition to variety of tumours (eye, lung, pancreas, kidney…), germ line mutations in tumour suppression VHL (codes for pVHL)

What is pVHLs primary function?

promoting destruction of HIF-1a transcription factor

What happens in the pVHL pathway in normoxia?

HIF-1a is marked by oxygen, pVHL attaches and results in HIF-1a degradation

What happens to HIP-1a in hypoxia?

isn’t marked by oxygen, isn’t degraded by pVHL, TF for genes to survive hypoxia develop new BV

What happens to VHL in tumours

pVHL undetectable resulting in constitutive activation of HIF1 TFS which promote genes to stimulate proliferation