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BS3018 Genes and Development > Hedgehog signalling - part 2 > Flashcards

Hedgehog signalling - part 2 Flashcards

1
Q

What phenotype is predominantly produced from a reduction in sonic hedgehog function?

A

Holoprosencephaly (HPE)

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2
Q

What happens to an individual with HPE?

A

They have incomplete development at the midline of the brain and face

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3
Q

Is the phenotype the same for all patients affected by HPE?

A

No, the severity and phenotype varies

Can be cyclopia or just a single incisor

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4
Q

What is the frequency of HPE in live births?

A

1:10,000 births

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5
Q

What is the frequency of HPE in miscarried foetuses?

A

1:250 miscarries

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6
Q

Describe the rare familial dominant form of HPE

A

Several loci
One includes shh
Mutations identified

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7
Q

What happens if you knockout the mouse sonic hedgehog gene?

A

The mouse has cyclopia

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8
Q

Where is environmental HPE an endemic?

A

In western USA during the 50’s in cattle and sheep

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9
Q

What was the environmental HPE endemic traced back to?

A

Corn lilly

Teratogens identified - steroidal alkaloids

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10
Q

What do corn lilly teratogens resemble?

A

Cholesterol

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11
Q

What do corn lilly teratogens induce in chick embryos and what phenotype does this produce?

A 
Severe HPE (cyclopia)
Arrest of midline development causing the failure of forebrain division and the absence of two telencephalon vesicles; and causes the optic field to not divide
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12
Q

What happens in a chick embryo without sonic hedgehog?

A

No midline forms and the eye field does not divide

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13
Q

Treating the chick embryo with cyclopamine phenocopies what?

A

The shh mutant

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14
Q

Where does cyclopamine block the induction of gene expression?

A

In the neural tube

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15
Q

Does cyclopamine interfere with cholesterol modification and how is this known?

A

No, sonic hedgehog processing in cultured cells is unaffected by cyclopamine

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16
Q

Does cyclopamine inhibit sonic hedgehog signalling and how is this known?

A

Yes, sonic hedgehog normally induces gene expression in explants of neural tube in vitro, but this induction is blocked when cyclopamine is added to the culture medium

17
Q

What are isl1/2 and HNF-3?

A

Two genes that are switched on in response to sonic hedgehog signalling

18
Q

What is the phenotype of a mouse knockout mutant of sonic hedgehog?

A

Cyclopia

19
Q

What is the phenotype of a human heterozygous mutant of sonic hedgehog?

A

Variable effects:

Undivided brain, cyclopia, cleft palate, nasal defects, dentition

20
Q

What do the phenotypes of human heterozygous mutants of sonic hedgehog suggest about the gene?

A

It acts during multiple stages of midline craniofacial development
This is confirmed through experimental manipulation by implanting alpha-shh secreting cells in chick embryos at different stages

21
Q

What is the most common teratogenic birth defect?

A

Foetal alcohol syndrome

22
Q

Describe the phenotype that an individual with foetal alcohol syndrome might show

A 
Small head size
Reduced philtrum
Low nose bridge
Brain can be much smaller
Mental retardation
HPE
23
Q

Describe the variety of effects that a mouse model with foetal alcohol syndrome might show

A

Neural crest cell migration - premature differentiation into facial bones
Induces neuronal apoptosis in forebrain
Disrupts cell adhesion (L1)
Affects shh expression

24
Q

Does ethanol affect sonic hedgehog?

A

Yes

25
Q

What happens to the expression of sonic hedgehog when exposed to ethanol in utero?

A

Reduced in anterior prechordal mesendoderm

26
Q

What happens to the production of sonic hedgehog protein when exposed to ethanol in vitro?

A

Reduced and increased apoptosis

27
Q

What might the inhibition of sonic hedgehog activity in PME cells affect?

A

Specification of midline facial structures

28
Q

Give an example of cyclops in popular culture

A

Ulysses blinding polyphemus

29
Q

Is cholesterol needed for hedgehog activity?

A

Yes

BS3018 Genes and Development (4 decks)

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