Molecular Biology of Cancer 1: How do cancers arise?

How do cancer cells arise even with DNA repair mechanisms?

Genomic DNA is constantly under attack, cells have repair mechanisms but if these fail, DNA damage can accumulate

• Cells with damaged DNA can undergo apoptotic cell death, but if it fails it can lead to the development of a cancer cell

Both _________ and _________ agents can cause mutations to genomic DNA.

exogenous (within cell), exogenous (outside of the cell)

What are some endogenous causes of DNA damage?

• Errors in replication (1 in 10^7)

  • Introduced by polymerases, in most cases are repaired

• Reactive oxygen species (ROS)

  • By products of metabolism can modify nucleotides + bases

What are some exogenous causes of DNA damage?

• Ultraviolet [UV 200-300nm] radiation from the sun

• Other radiation - x-rays and gamma rays

• Mutagenic chemicals e.g., DNA intercalating agents

• Cancer chemotherapy and radiotherapy

• Kill cells by inducing DNA damage, normal cells overcome (Repair), cancer cells cannot

Different DNA damaging agents can give rise to different forms of damage (Or types of mutations), what are some possible forms?

2 Principle Outcomes:

1. Block replication machinery, so genome is not replicated how it should be

   1. Double strand break

   2. Single strand breaks

   3. Pyrimidine dimers- two adjacent pyrimidine nucleotides become joined

   4. Altered bases (8 oxo guanine lesion)

2. Alter the DNA sequence, which will produce abnormally coded protein

   1. Insertions

   2. Deletions

   3. Mismatched bases

**DIfferent DNA damaging agents can cause different types of damage which causes different DNA lesions.

Several viruses are able to induce cancers in animals such as?

Some transmissible cancers exist, what are these caused by?

Transmissible cancer are caused by living cancer cells that are transmitted from animal to animal and are distinct from cancers caused by infectious agents (viruses)

• No infectious agent - are the tumor cells themselves

What are some examples of transmissible cancers?

Canine Transmissable Veneral tumour

• Causes tumours on the external genitalia

• Mainly transmitted by coitus

• Biting, licking can also transfer tumours to other body sites

• Genetic studies indicate it arose in somatic cells of a dog thousands of year ago- ancient in origin ~11,000 years old

Devil Faced Tumour Disease in Tasmanian devils DFTD

• Characterised by tumours on the face neck and mouth

• Animals die within a few months

• Responsible for widespread decline in devil population; Poses a threat to the survival of species

Cancer is primarily a _____ disease that is _____ in nature.

genetic, clonal

How does cancer usually start?

Usually starts when a single cell acquires a series of mutations, usually one after another, that collectively change a once-normal cell into a cancerous cell that divides uncontrollably

Genetic changes can be as subtle as _____ _____ or as _____ _______ _____.

point mutations, gross chromosomal losses

How many mutations are required for cancer to normally develop?

3 - 7 mutations, this is known as the multi-hit hypothesis

What is the multi-hit hypothesis?

The multi-hit hypothesis of cancer proposes that cancer development typically requires a series of genetic mutations (or "hits") to accumulate in a cell, leading to uncontrolled growth.

• Cancers require step-wise accumulation of mutations which occur overtime

How does age correlate with cancer?

• Cancer incidence increases with age in humans and animals, mutations accumulate over time

What are tumour suppressor genes?

Genes that act to inhibit the cell cycle, blocking proliferation and tumour development

• Apply “brakes” to the cell cycle

Why are mutations to TSG often termed "loss of function”?

• Abolish the inhibitory function = loss of function mutation

What are some examples of TSGs?

• pRb

• p53

• BRCA 1/2

What is one of the most common genetic alterations detected in cancers?

TSG frequently mutated

Describe the role of the p53 tumour suppressor in response to DNA damage.

• DNA damage occurs

• Then… kinase enzymes (ATM and / or ATR ) are activated

• Kinase activity causes phosphorylation of p53, this stabilizes the otherwise unstable p53 protein

• p53 then functions to trigger cell cycle arrest or apoptosis

  • Cell cycle arrest is activated by blocking CDK-cyclin function in G1, allows for DNA repair

  • Apoptosis is triggered by blocking Bcl-2, a key trigger for inducing apoptosis

What happens if the p53 protein function is lost, and what is it’s prevalence in cancer?

• >80% of cancers (humans, animals) have mutated p53 protein hence no functional p53

• If p53 function is lost then the cell is unable to trigger a halt of the cell cycle or apoptosis

• Cell with mutation can then proceed through the cell cycle

• Mutations will be copied to daughter cells; leading to genomic instability meaning more mutations can be acquired

Are there any other ways that the p53 function can be lost (other than cancer)?

• Note: p53 function can also be 'lost' via non mutational mechanisms

• Some viruses e.g. Human Papillomaviruses (16/18) that cause cervical cancer in humans can degrade p53 protein

Describe the role of the pRb tumour suppressor in response to DNA damage.

• pRb gene encodes the retinoblastoma protein (pRb)

• Normal pRb protein prevents G1/S phase progression; a critical control of cell proliferation

What happens if pRB becomes mutated?

• Mutated pRB is stimulatory (remains phosphorylated); control on cell cycle is lost; signal for cell to go forward into S phase (DNA synthesis) is always “on”

• When pRb is phosphorylated by CDK/cyclin, E2F dissociates and triggers S phase transition from G1

What are proto-oncogenes?

• What happens / what are they termed if they become activated/mutation?

• Proto-oncogenes are normal genes that function to regulate cell growth; Mutation of a proto-oncogene results in an oncogene

• Oncogenes promote cell proliferation- 'gas' pedals, hence mutations in oncogenes are termed 'gain of function' mutations

  • (Proto-oncogenes are the same gene but not activated, once activated becomes a oncogene…)

What three mechanisms activate oncogenes?

Oncogenes are activated by 3 mechanisms

1.) Translocation - places the gene under control of a strong gene promoter (regulator)

• Breaks in two chromosomes, which join up in the incorrect location.

• Results in excessive gene expression.

2.) Gene amplification- numerous copies of the gene results in more protein produced

3.) Point mutation- produces a protein with enhanced/increased function

What are some examples of translocation and what role does it play in cancer pathogenesis?

Ex. Burkitts Lymphoma

• Translocation involves immunoglobulin (IgH) gene (chromosome 14) and c-myc proto-oncogene locus (chromosome 8)

• Translocation places c-myc under the control of a new strong promoter (IgH promoter), resulting in higher expression levels of c- myc

• Downstream of a strong promoter, which transcripts + translates, producing excessive proteins

Ex. Chronic Myelogenous Leukaemia

• Abl proto-oncogene inserts into the bcr gene, two genes fuse, the altered abl genes functions improperly, resulting in CML

What is gene amplification and what role does it play in cancer pathogenesis?

Gene amplification increases the number of copies of gene through repeated duplications which results in increased protein levels

• Normal cells have a designated number of receptors, if they undergo amplification, number of receptors increase → Excessive protein prod.

What is point mutation and what role does it play in cancer pathogenesis?

Point mutation/deletion due to mutation results in expression of mutant oncoprotein with an enhanced function often called 'hyperactive'

• Oncogenes mutations are 'gain of function' mutations (in this instance stronger activity)