Genetic Mechanisms in Angelman Syndrome

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Genetic Mechanisms in Angelman Syndrome

• Jill Clayton-Smith
• St.Marys Hospital
• Manchester

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Angelman Syndrome

• Severe learning disability
• Ataxic, jerky movements
• Seizure disorder
• Absent speech
• Characteristic facial features
• Happy, sociable affect

Harry Angelman 1965
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1. Angelman Syndrome is due to a deletion of
15q11-13 in 65 of cases
Kaplan et al., Magenis et al., 1987
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Deletions are detected more easily by FISH
analysis
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Principle of FISH
A T C etc
DNA sequence of 15q11-13
q11-13
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T A G
T A G
Gene probe, a piece od DNA which matches The
sequence in 15q11-13 and will stick to it Probe
is made to fluoresce
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Principle of FISH
Normal 15. Probe sticks to 15q11-13
and shines
Deleted 15. Probe doesnt stick No
signal
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15q11-13 deletions tend to be of a similar size
4 Mb
bp bp bp
duplicon repeat sequences of DNA
bp common breakpoints
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Chromosome 15q11-13 is an imprinted region
Genetic information from the maternal and
paternal chromosome 15q11-13 regions is expressed
differently
The parental imprint is set during formation
of the egg and the sperm
mat pat
An intact maternal and paternal chromosome
15q11-13 are needed for normal development
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In Angelman syndrome the deletion arises on the
maternally inherited chromosome
m p
A deletion arising on the paternal chromosome
gives rise to the Prader-Willi syndrome
m p
Almost all deletions arise de novo
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Maternal and Paternal 15q11-13show Differential
Methylation
methylation analysis
N AS PWS
CH3
m p
M P
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Some deletions arise due to a parental
chromosome rearrangement
AS
PWS
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Parental chromosome rearrangement
inv(15)
inv(15) inv(15)
AS del 15q11-13
PWS
del 15q11-13
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2. Angelman Syndrome May Arise as a Result of
Paternal Uniparental Disomy
Two paternal chromosome 15s No maternal 15.
Accounts for 3 AS
p p
trisomy rescue
mitotic error
meiotic error
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3. Angelman Syndrome may arise due to failure to
reset the parental imprint during gametogenesis
Imprinting defect
m p
Intact chromosomes are inherited from both
parents but the maternal chromosome has a
paternal imprint
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The Imprinting Centre Controls Setting of the
Parental Imprint
4kb
IC
bp bp bp
duplicon repeat sequences of DNA
bp common breakpoints
imprinting centre (IC)
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Imprinting Defects

• Mutation within the imprinting centre
• No mutation identified. Failure of imprint to be
  reset is a chance (stochastic) event
• Mosaic imprinting defect

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4. Angelman Syndrome may arise due to a mutation
within the UBE3A gene
4kb
IC UBE3A
bp bp bp
UBE3A gene
duplicon repeat sequences of DNA
bp common breakpoints
imprinting centre (IC)
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UBE3A mutation
Point mutation within the UBE3A gene leads to
lack of normal expression of gene
m p
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UBE3A gene

• Responsible for producing a protein called a
  ubiquitin protein ligase which is involved in
  protein degradation within the brain
• Only the maternal copy is active in brain
• The gene works in specific regions of the brain
  involved in learning, memory and movement

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Angelman Syndrome Mouse Model-Impaired
contextual learning and memory
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UBE3A gene
1 2 3 4 5 6 7 8 9
10 11 13 14 15 16
truncating
missense
HECT domain
splice site
polymorphism
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UBE3A mutation may be familial
AS
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UBE3A mutations may be familial due to the mother
carrying the fault in her ovaries, but not blood
AS AS
This is called GONADAL MOSAICISM
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UBE3A mutations

• Found in 80 familial AS and 20 sporadic AS
• Not present in mother in 75 sporadic cases
• Mother carries mutation in 20 cases
• Significant number of mothers are gonadal mosaics
  so pre-natal diagnosis recommended

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5. Clinical features of AS with no genetic
abnormality

• Not AS. Consider other diagnoses.
• Mutations not detectable by current techniques
• Another genetic mechanism
• Another transcript of UBE3A
• Another gene
• Mosaicism

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Summary
recurrence risk
I De novo deletion 75 low
Familial rearrangment 1 can be
high II Uniparental disomy 3
low III Imprinting defect 2 low
(no IC mut/de novo)
50(parental mut) IV
UBE3A mutation 5-10 low (de novo)

high(familial/gon.mosaic) V Clinical
diagnosis alone 10-12 12
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deletion of maternally inherited 15
paternal uniparental disomy
imprinting defect UBE3A mutation on
maternal 15 no detectable abnormality

methylation test
ve
P M
ve
P P
ve
P M
-ve
P M
-ve
P M
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Algorithm for Genetic Testing
Clinical suspicion of AS
methylation analysis
ve
-ve
FISH
typical AS
clinical review
no del
screen UBE3A
del15q
not AS
RFLP analysis
no mut.
UBE3A mutation
no UPD
UPD
keep on review
Imprinting defect
Screen for IC mutation
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Lessons We Have Learned

• Not everyone with AS has the typical features
• The behaviour is the most consistent feature. If
  its typical, keep looking at the AS gene/region
• There is a lot of overlap in ability between
  del/UPD/UBE3A/Imprintor
• Mosaics are likely to be more difficult to pick
  up
• UBE3A mutations are not that uncommon gt50
  detected in our lab so far

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Questions?