8. Cancer 2: Developmental Biology Shh Flashcards
What was observed in sheep?
What caused this?
- Normal adult sheep were giving birth to cyclic lambs
- Due to adult sheep eating corn lilies, and the compound that caused abnormalities was cyclopamine
What was discovered by Nusslein-Volhard and Wieschaus mutagenesis screen in Drosophila?
- Identified mutation in a gene that caused embryo to appear curled up and spiky - gene called Hedgehog
- Hedgehog is conserved from invertebrates to vertebrates and the vertebrate homolog was cloned and characterised - Sonic Hedgehog
What is the Hh/Shh signalling pathway?
- Hh membrane receptor is Patched (Ptc)
- In absence of Hh, Ptc represses membrane protein Smoothened (Smo) - Hh pathway not activated
- Binding of Hh to Ptc alleviates Ptc repression of Smo - Hh pathway activated and GliA upregulates transcription of target genes
What did researchers show about Shh -/- mice?
Is this the same in humans?
- Shh -/- mice embryos have cyclopia and holoprosencephaly
- Mutations in Shh/Shh pathway correlate with cyclopia and holoprosencephaly in humans
What was the first instance 2 and 2 were put together?
- Cyclopamine leads to cyclopia
- Loss of Shh/Shh signalling leads to cyclopia
Hypothesis: Cyclopamine antagonises Shh or component in Shh signalling pathway
How does cyclopamine inhibit Shh signalling?
Cyclopamine inhibits Smoothened therefore prevents activation of Shh signalling
What did studies in animal models reveal about Shh signalling?
- Shh signalling is important in the development of many tissues/organs (e.g. Brain, Muscle, Pancreas)
- Shh signalling is usually downregulated at many end stages of development
E.g. in limb bud
- In early stages - Shh expressed in ZPA
- When limb and digits formed - Shh is downregulated
What was found about target genes of Shh signalling?
- Although initially identified as a patterning morphogen, genes that are directly activated by Shh signalling (CmyC, Cyclin D1) promote proliferation, suppress apoptosis and promote stem cell self-renewal
- This explains why Shh is expressed in early proliferating tissues but then downregulated in late differentiated tissues
What was found about Shh and cancer?
- Dysregulation of Shh signalling is involved in many cancers
Predict to see cancers developing in:
- Places where Shh is involved in development and then downregulated
- Places were Shh is held at a particular level in careful balance e.g. stem cell niche
Describe the skin stem cell niche.
Skin cells are continually generated through life from skin stem cells
Skin stem cells located in a niche:
- At distal end - stem cells
- At proximal end - differentiating/ed cells
Shh is expressed in stem cell niche and its levels are carefully regulated by Wnt and BMP to achieve the correct balance of stem cells, proliferating progenitors and differentiating cells
Proliferating cells in deep dermis express Shh signalling pathway components (i.e. respond to Shh)
How can Basal Cell Carcinomas form?
- Activating Smoothened mutations cause constitutive activation of Shh signalling pathway leading to abundant GliA, this leads to increased proliferation and BCC develops in deep dermis
- Loss of Patched function - normally Ptc represses Smo to suppress Shh signalling, if both copies of Ptc are mutated this suppression is lost and causes constitutive activation of Shh signalling pathway leading to abundant GliA, this leads to increased proliferation and BCC develops in deep dermis
What was the second instance 2 and 2 were put together?
- Constitutive activation of Shh signalling causes increased proliferation
- Cyclopamine inhibits Shh signalling
Hypothesis: Failure to repress Shh signalling leads to increased proliferation but can be reversed by cyclopamine
What was found about cancers and cyclopamine?
- Oncogenic mutations in Smo or Ptc can be reversed by cyclopamine
- Cyclopamine acts as a competitive antagonist of Smo to block activation of Shh signalling and Gli1 remains inactive
- This prevents expression of Shh target genes (CmyC, Cyclin D1) that promote proliferation and tumorigenesis
Describe the pre-clinical development of Vismodegib.
- 2003 pharma company Curis files patent for “Methods of inhibiting unwanted cell proliferation using Hh antagonists”
- 2005 Genentech collaborate with Curis for pre-clinical development of compounds inhibiting Hh pathway
- Identify Vismodegib (Erivedge) as a substance acting as a cyclopamine-competitive antagonist of Smo (with less dramatic side effects)
Describe the clinical development of Vismodegib.
- 2007 phase 1 and 2 trials began in adults with locally advanced or metastatic BCC that was refractory to standard therapy
- 2011 submitted Vismodegib as an FDA-approved drug and was approved as a major advance in treatment
- 2012 Vismodegib became first licensed treatment of advanced BCC (gave dramatic benefits)
