Week 11: Cancer Flashcards
1
Q
What is the role of Hedgehog signalling in development?
A
- Hh signalling regulates the morphogenesis of tissues and organs
- The Hh signalling pathway has 3 ligands:
1. Sonic hedegehog (Shh): expressed in many tissues- the primary ligand
2. Indian hedgehog (Ihh): bone
3. Desert hedgehog (Dhh): male sexual development - Each ligand activates the same pathway after binding to the receptor
- Roles of Shh signalling in development are:
1. Limb development
2. Neural tube differentiation
3. Facial morphogenesis
4. Hair and feather development
5. Forming the midline of the body - Shh is a morphogen
2
Q
What are the two major receptors of hedgehog ligands?
A
- Patched (Ptch) receptor:
- A 12-pass membrane receptor
- Represses the Hh pathway (when unbound by ligand)
- A tumour suppressor gene - Smoothened (Smo) receptor:
- A 7 pass membrane receptor
- Activates the Hh pathway
- A protooncogene
3
Q
How does Hh signalling work when a ligand is bound?
A
- When Shh binds to the patched receptor
- Ptch stops repressing smoothened receptor which undergoes a conformational change and activates
- Activated Smo regulated the activity of a family of TFs known as glioma-associaated TFs (Gli)
- Activation of Gli by Smo allows them to translocate to the nucleus and transcribe a variety of Hh target genes, including:
- Gli1 (when levels of Gli1 are high, there is activation of the Hh pathway)
- Ptch (enables negative feedback of Hh pathway)
- Cyclin D (promotes cellular proliferation)
- VEGF (promotes angiogenesis) - When ligand is bound SUFU is also inactive so the Gli 1-3 TFs are not converted to GliR
4
Q
How does Hh signalling work when no ligand is bound?
A
- When no Shh ligand is bound to Patched, Ptch is able to repress the function of smoothened
- Inactivation of Smo results is the complete inactivation of the downstream signalling pathway
- SUFU is active when no ligand is bound and contributes to the processing of Gli (1-3) into a repressor of transcription (GliR)
5
Q
How does Hh signalling change the distribution of receptors on the cilia?
A
- Cilia are microtubule based protrusions of the plasma membrane
- Cilia form at G1 and resorb prior to mitosis
- There is dynamic movement of signalling molecules at the cilia with the intraflagellar transport of Ptch, Smo and Gli
- Membrane bound receptors also move into the cilia from the somatic plasma membrane (also through interaction with IFT proteins)
- In the absence of Shh ligand:
- Ptch is localied to the primary ciliium membrane and repressed Smo which is outside the cilium membrane on the somatic plasma membrane
- In the presence of Shh ligand, The Shh ligand binds the Ptch receptor which prevents the inhibition of Smo
- This causes Ptch to leave the cilium and Smo to enter the cilia membrane from the somatic plasma membrane
6
Q
What are the positive regulators of Shh signalling?
A
- Ligand binding (Shh)
- Smoothened receptor
- Gli activator
7
Q
What are the negative regulators of Shh signalling?
A
- Patched receptor activity when unbound
- Suppressor of fused (SUFU)
- Gli repressor
8
Q
What are the 3 types of Hh signalling dependent cancers?
A
Type 1:
- Loss of function of Ptch1 or gain of function of Smo
- Ligand independent
Type 2:
- Autocrine model
- All tumour cells and cancer stem cells respond to the Hh ligand produced by the cancer cells
- Ligand dependent
Type 3:
- Paracrine model
- Tumour cells secrete Hh ligand and stromal cells respond and produce growth factors etc. so support tumour growth such as VEGF
- Ligand dependent
9
Q
What is medulloblastoma?
A
- The most common malignant brain tumour in children (median age 5 years)
- Cancer of the cerebellum
- Often disseminates throughout the CNS early in disease
- There are 4 subtypes of medulloblastoma:
1. Wnt
2. SHH
3. Group 3
4. Group 4 - Shh medulloblastoma is derived from granule cell precursors in the cerebellum
10
Q
Describe the development of the cerebellum and how it relates to the cell of origin of Shh medulloblastoma:
A
- Granule cell precursors (GCPs) undergo massive cell proliferation early in post natal life
- Purkinje cells secrete Shh ligand which stimulates the proliferation of the GCPs in the external granular layer
- GCPs from the external granular layer migrate as they differentiate into neurons and then settle in the Internal granular layer
- The Granule neurons in the internal granular layer are the only neuronal cell type in the cerebellum
- Granule neurons are the cell of origin of Shh medulloblastomas
- Ligand dependent medulloblastomas are associated with an increase in Purkinje cell secretion of Shh or hypersensitivity to the Shh
- Ligand independent medulloblastomas are associated with mutations in Ptch of Smo receptors of GPCs which results in the hyperactiviton of the Hh signalling pathway in these cells (resulting in increased proliferation of GPCs in the EPL and reduced migration and differentiation into neurons)
11
Q
What genetic events are common in the development of medulloblastoma?
A
- Loss of function mutations in Ptch1 (TSG)
- Gain of function mutations in Smo (oncogene)
- Loss of function mutations in SUFU (TSG)
- Gain of function mutations in Gli (which keep is contiuitively active) (Oncogene)
12
Q
Explain the molecular basis of cancer predisposition syndrome:
A
- Some people inherit one mutated allele of a tumour suppressor gene so they are heterozygotes
- Over months to years, the second allele of the tumour suppressor gene mutates resulting in a loss of heterozygosity
- This can lead to the development of cancer
13
Q
How does ligand-dependent Hedgehog signalling cause cancer?
A
Hh-dependent tumours may be ligand dependent and driven by:
- Overexpression of the ligand Shh
- This causes hyperactivation of the Hh signalling pathway
14
Q
How does ligand-independent Hedgehog signalling cause cancer?
A
- Loss of one Patched allele (heterozygous): reduces repression of smoothened
- Most common - Activating mutations in Smoothened: Smo is no longer inhibited by Patched so there is increased signalling
- Most common - Loss of SUFU of inactivating mutations in SUFU: Promotes Gli activation and Hh target gene transcription
- Unusual - Overexpression of GliA: renders GliA constiuitively active and thus promotes Hh signal transduction and target gene transcription
- Moderately common
15
Q
What genetic changes are often observed in Medulloblastoma?
A
- Heterozygous loss of Ptch causes medulloblastoma
- Oncogenic gain of function mutation of SmoM2 resulting in it consituitively being active and expressed on primary cilium
- More aggressive mutation
16
Q
What are the treatment options of medulloblastoma?
A
Traditionally:
- Aggressive surgery, radiotherapy and chemotherapy
- Has a 50% 5 year cure rate
- Treatment often results in significant endocrinological issues however
New treatment options:
- Targeted therapeutics such as inhibitors of the Hh pathway
E.g. inhibitors of Smo, inhibitors of Gli and inhibitors of microtubule assembly (to prevent the formation of primary cilia)
- As Hh signalling has a critical role in many developmental processes- use of Hh signalling inhibitors in children is a concern
- There has been success with trialled Smo inhibitors however relapse of disese tends to occur due to drug resistant mutations
17
Q
Describe traditional chemotherapy:
A
- Not selective for cancer cells over fast-normal dividing cells e.g. crypt cells in GI tract
- All of the mechanisms essentially target cell division and thus
- Traditional mechanisms of action of chemotherapeutic drugs are:
1. Antimetabolites: - Block enzyme required for DNA synthesis or induce DNA damage
2. Alkylating agents: - Bind via an alkyl group to DNA resulting in crosslinking which causes DNA strand breaking and apoptosis
3. Antimicrotuble agents: - Either stabilise or prevent the assembly of microtubules which blocks cell division
4. Topoisomerase inhibitors: - Block DNA unwinding and thus blocks DNA replication and transcription
5. Cytotoxic antibiotics: - Intercalate and generate free radicals that damage DNA
18
Q
What are the best targets of targeted cancer therapies?
A
- Protein kinases are molecules that transfer a phosphate group from ATP to a serine, theronine or tyrosine residue on a substrate protein
- Many oncogenes are deregulated protein kinases that have increased activity due to gene translocation, amplification, mutation or overexpression
- The reaction involves the transient binding of protein kinase enzymes to both the ATP and the substrate protein
E.g. Bcr-Abl translocation resulting in CML
E.g. erbB2/HER2 amplification in breast cancer
E.g. EGFR in NSCLC
19
Q
Describe the structure of protein kinases and how it changes upon activation:
A
- Kinase domain
- Active site that binds ATP
- In inactive kinases, the activation loops blocks the ATP and substate binding cleft
- Upon kinase receptor activation e.g. by binding of a ligand) the tyrosine phosphorylation of the activation loops leads to a major conformational change
- This allows ATP and substrate access to the binding clefts and allows phosphorylation of the substrate protein to occur
20
Q
What types of protein kinase inhibitors are there?
A
Type 1:
- Binds to the ATP pocket when it is in active conformation (DFG in)
- ATP competitive
Type 2:
- Binds to the ATP pocket when it is inactive (GFP out)
- Locks the active site into the inactive conformation so it can not be activated
Type 3:
- Bind outside of the ATP binding pocket
- Harder to develop but potentially more selective
