20.06.09 Cancer pathways

Question 1

List main cancer pathways

Answer 1

• MAPK (ERK, JNK, P38, ERK5 pathways)
• WNT
• PI3K/AKT
• TGFβ
• Although due to complex cross talk between pathways, they cannot be considered in isolation

Question 2

Review of MAPK pathway

Answer 2

• MAPK (Mitogen-activated protein kinase)
• Links extracellular signals to fundamental cellular processes (proliferation, differentiation, migration, apoptosis)
• 4 MAPK pathways. ERK, JNK, P38, ERK5

Question 3

Review of ERK pathway

Answer 3

• Extracellular signal-regulated kinase (ERK) MAPK pathway.
• Deregulated in 30% of cancers.
• Raf phosphorylates MEK, which phosphorylates ERK.
• ERK activation promotes upregulated expression of EGFR ligands (TGFα)
• Creates an positive feedback loop which is critical for tumourigenesis

Question 4

Common mutations in EGFR/Ras/Raf/ MEK/ ERK pathway

Answer 4

• EGFR: exon 19 mutations account for 90% of non-small cell lung cancer.
• RAS (HRAS, KRAS, NRAS): most commonly mutated oncogenes in human cancer. Lead to constitutively active proteins.
• RAF (BRAF). V600E most common (90%)

Question 5

Drug therapies targeting EGFR/Ras/Raf/ MEK/ ERK pathway

Answer 5

• RAF inhibitor= vemurafenib and MEK inhibitor= trametinib, used for BRAF mut malignant melanoma.
• KRAS-wt= Cetuximab for colorectal cancer
• EGFR- mut= Gefitinib.

Question 6

What are the stress activated MAPK pathways

Answer 6

• JNK family of kinases respond to cytokines, UV radiation, DNA damage reagents. Activates c-Jun and phosphorylates p53 (leading to apoptosis)
• p38 MAPK are activated by environmental stress and activates p53 (apoptosis).

Question 7

What are WNTs

Answer 7

• A Family of 19 secreted glycoproteins
• Involved in the regulation of cellular process such as self-renewal, proliferation, differentiation, survival, migration

Question 8

What are the two WNT pathways

Answer 8

• CTNNB1 dependent (canonical)

- CTNNB1 independent (non-canonical)

Question 9

Review of canonical WNT signalling

Answer 9

• Activated by the binding of a Wnt-protein ligand to a Frizzled family receptor, which passes the biological signal to the Dishevelled protein inside the cell.
• causes an accumulation of β-catenin in the cytoplasm and its eventual translocation into the nucleus to act as a transcriptional coactivator of transcription factors that belong to the TCF/LEF family.
• In the absence of WNT, a destruction complex containing APC, GSK3B and AXIN1 target CTNNB1 for ubiquitylation and thus proteosomal degradation.

Question 10

Examples of somatic mutations in WNT pathways

Answer 10

• APC: seen in 39% of cancers of large intestine
• CTNNB1: seen in 42% soft tissue cancers.
• AXIN1: seen in 38% biliary tract cancers.

Question 11

Effect of somatic mutations on WNT pathway

Answer 11

-Inactivate APC, AXIN1 or WTX, leading to hyperactivation of signalling pathway (accumulation of β-catenin).

Question 12

Example of cross communication between WNT and EGFR/Ras/Raf/Mek/ERK pathways

Answer 12

• β-catenin can bind to EGFR

- EGFR can activate β-catenin-dependent signalling through PI3K

Question 13

Therapies targeting WNT pathway

Answer 13

• Direct targeting of WNT signalling has been difficult due to lack of pathway-specific targets and redundancy of many pathway components.
• Examples: small molecules, blocking antibodies, peptides
• Combination therapies, where inhibiting canonical WNT pathway sensitizes cells to chemotherapeutics.

Question 14

What is the PI3K/AKT pathway

Answer 14

-Downstream of a receptor tyrosine kinase and G-protein coupled receptors, leading to activation of serine/threonine kinase AKT and down stream effector pathways.

Question 15

Key components of PI3K/AKT pathway

Answer 15

• PI3K (phosphatidyl 3-kinases), lipid kinase heterodimer composed of a regulatory and catalytic subunit. PIK3CA gene encodes the catalytic subunit.
• AKT1, serine/threonine kinases that acts downstream of PI3K.
• PTEN. Negative regulator of PI3K pathway (often downregulated in cancer)
• mTOR (mammalian target of rapamycin), serine/threonine kinase that is involved in regulating cell growth and proliferation by monitoring nutrient availability and cellular energy levels. Influenced by activity of TCS2/1 complex

Question 16

Review of PI3K/AKT pathway in cancer

Answer 16

• Aberrant signalling affects cell proliferation, survival, motility and angiogenesis.
• Somatic PIK3CA mutations are seen in 32% of CRC
• Inactivating somatic PTEN mutations in many cancer types. Can be identified via immunohistochemistry.
• Activating somatic mutations in AKT1 in 1-6% CRCs and other cancer types.
• Often mutations are mutually exclusive (i.e. AKT1 and PTEN or AKT1 and PI3K are mutually exclusive)
• Activating mTOR mutations lead to hyperactivation of downstream pro-survival signalling pathways.

Question 17

Examples of cancer therapies that target PI3K/AKT pathway

Answer 17

• PI3K inhibitors (pan-PI3K or isoform specific).
• PTEN loss makes colorectal cancers less sensitive to anti-EGFR antibodies, so patients would not benefit from cetuximab.
• mTOR inhibitors.
• SAR24509 and BEZ235 are dual PI3KmTOR inhibitors

Question 18

Other disease pathologies caused by PI3K/AKT pathway

Answer 18

• Diabetes, heart conditions, hamartoma syndromes.

- e.g. PTEN hamartoma tumour syndrome

Question 19

What is the TGFβ pathway

Answer 19

• Regulates differentiation, migration and cell death during normal development.
• Operates through two tyrosine kinase receptors TβRI and TβRII

Question 20

What two receptors are involved in the TGFβ pathway

Answer 20

• TβRII is constitutively active

- TβRI is activated after ligand binding

Question 21

What happens in TGFβ pathway

Answer 21

• Binding of TGFβ to receptor leads to release of inhibitory FKBP12.
• Activated TβRI receptor phosphorylates and activates SMAD2 and SMAD3 (R-SMADs), which in turn binds SMAD4 to form SMAD4/R-SMAD complex.
• SMAD4/R-SMAD complex translocates to the nucleus to activate gene expression.

Question 22

What happens in non-canonical TGFβ signalling

Answer 22

-Activates MAPK pathway, PI3K/AKT pathway and Rho-like GTPase signalling pathways.

Question 23

What are the components of TGFβ pathway

Answer 23

• TGFβ: 30 different members. TGFβ , activins, BMPs (bone morphogenetic proteins), GDFs (growth and differentiation factors), AMH (anti-Müllerian hormone)
• SMAD4: one of 8 SMAD proteins. Signal transduction protein that is a central mediator for downstream signalling pathways.

Question 24

Involvement of TGFβ pathway in cancer

Answer 24

• High TGFβ levels correlate with increased angiogenesis
• 35% of metastatic CRCs have mutations in components of TGFβ pathway
• SMAD4 loss is seen in 50% of pancreatic mutations. Reduced SMAD4 protein expression on immunohistochemistry is associated with worse overall survival.
• Inactivating SMAD2 mutations seen in 6% of CRCs.

Question 25

Therapies targeting the TGFβ pathway

Answer 25

• Identifying low SMAD4 to identify patients with early recurrence
• Antisense oligonculeotides to prevent TGFβ synthesis. e.g. trabedersen.
• Ligand traps: TGFβ-neutralising monoclonal antibodies, soluble receptors. e.g. fresolimumab (pan anti-TGFβ antibody)
• TGFβ receptor kinase inhibitor to prevent signal transduction. e.g. galunisertib.

Question 26

Other disease pathologies associated with TGFβ pathway

Answer 26

• Germline SMAD4 mutations cause syndromes with cancer predisposition. e.g. Juveline polyposis syndrome and HHT (hereditary hemorrhagic telangiectasia) syndrome.
• TGFβ is a cytokine involved in immunity, cancer, asthma, lung fibrosis, heart disease, fibrosis, diabetes, HHT, marfan syndrome etc.