Download Imprinting

Imprinting in the news
Potential blood test to screen for colon cancer
• Colon Cancer- 3rd most deadly cancer in America
Loss of Imprinting
(LOI) of the Igf2 gene
correlates with colon
cancer
Result= Maternal Igf2 gene
is turned on
•With family history- 5X more likely to show LOI
•Polyps detected- 3X more likely to show LOI
•Personal history- 22X more likely to show LOI
Source- AP News, March 14, 2003
Genomic Imprinting
• Definition- Differential expression of
two parental alleles
– Only occurs in eutherians (placental,
nonmarsupial) mammals
– Not in other vertebrates
• Of 20-some identified genes, most are
involved in
1. Fetal growth
• Igf2, IgF2r, H19, Grb1
2. Brain development
• Prader-Willi syndrome (PS), Angelman syndromes (AS),
Peg1/Mest
Categories of imprinted genes
1. Fetal growth genes- Insulin-like growth
factor-like II (IGF2) response pathway
•
•
•
•
•
•
•
IGF2
Igf2r
Grb10
H19
Gnas
Rasgrf
Mash2
– Why?- Embryo develops in a parasite-like
relationship with mother.
Categories of imprinted genes
2. Brain development– Prader Willi Syndrome- paternal
chromosome deletion
– Angelman Syndrome- maternal
chromosome deletion
– Why?- Any ideas?
Prader-Willi Syndrome
•
•
•
•
•
1 in 15,000 live births
mostly sporatic
deletion at 15 q11-q13
diagnosis at 2 years
obesity, short, small hands/feet, unusual
facial features, mild mental retardation
• compulsive overeaters
A typical Prader-Willi patient
Prader Willi Syndrome- Due to
paternal chromosome deletion
Angelman Syndrome
• 1 in 25,000 live births
• mostly sporatic
• 80 % have deletion at 15q11-q13
– Specifically mutation of UBE3A gene
Angelman Syndrome
•Speech impairment
•None or minimal use of words
•Receptive and non-verbal communication skills higher than
verbal ones
•Movement or balance disorder, usually ataxia of gait
•Behavioral uniqueness: any combination of frequent
laughter/smiling; apparent happy demeanor
•easily excitable personality, often with hand flapping
movements; hypermotoric behavior; short attention span
Angelman Syndrome
–Angelman Syndrome- maternal chromosome deletion
Or an
imprinting
defect
Normal
Or 2 paternal chromosomes
Functional studies- Chimeras
+
Definitions
Implant into
pseudopregnant
female
• Androgenotes- two paternal genomes
• Gynogenotes- two maternal genomes
• Both of these (andro- and gyno-genotes)
result in an imbalance (increase or
decrease) of expression of imprinted
genes
– Result is developmental abnormalities
chimera
Functional studies- Chimeras
• Androgenetic- oversized, small brains
• Gynogenetic- growth retarded, large
brains (cortex)
– Conclude- imprinted genes contribute to
rapid expansion of the cortex
Turner syndrome
Only 1 X chromosome
• 1/2000; females only
• short stature, failure to mature sexually
• Often learning difficulties, skeletal abnormalities,
hearing loss, liver dysfunction, heart and kidney
abnormalities, infertility, and thyroid dysfunction
• Females with single X chromosome
– If X from father- better verbal and social skills
than if X from mother
– Conclude- Some imprinted genes on X
escape inactivation only if from father
Parent Offspring Conflict
Hypothesis (Haig hypothesis)
• Conflict between male and female over
allocation of maternal resources to offspring
• Dad uses imprinting to direct all resources to
immediate offspring (not future litters)
• Mom uses imprinting to allocate resources to
multiple litters
– Thus, predict paternally expressed genes
would promote growth, maternally expressed
genes should slow it down
– Prediction mostly hold true
• Example- Igf2 (paternally expressed)-if defective=40%
reduction in growth
Parent Offspring Conflict
Hypothesis (Haig hypothesis)
Example – The Igf2 gene and its receptor Igf2r
• Igf2 (paternally expressed)-if defective=40%
reduction in growth
• Igf2r (Igf2 receptor)- if defective=increase growth
• Igf2-/Igf2r- = normal
Another test- Ask if imprinting fails to occur in a monogomous
species
The Beach mouse is entirely monogomous
….but imprinting still occurs, contrary to model
Is the imprint erased during embryogenesis?
Is the imprint erased during embryogenesis?
• Observation-Aparthenogenetic
embryos generated from from immature
embryos proceed to late developmental
stage that from using mature embryos• Answer- Probably
Two general mechanisms proposed:
1. Passive process via direct methylation of Dnmt1
2. Active process via specific demethylation
Is the imprint erased during embryogenesis?
• Evidence in support
– Biallelic expression of imprinted genes
occurs in primordial germ cells
– Igf2r imprinting control region (ICR) is
methylated in E8 embryos, but
unmethylated E12.5 embryos
– Dnmt1 is at high levels during Igf2r
maternal imprint
• Answer- Probably
How are imprinted genes silenced?
S. Tilghman, Cell 96:185
How are imprinted genes silenced?
Dnmt-/- miceMany imprinted genes
(e.g. H19) reactivated
..but, Igf2 and Igf2r are
silenced
Mechanism- Methylation interferes with transcription factor binding
Problems with model1. Promoters of silent Igf2 and Igf2r alleles are unmethylated
2. One gene, Mash2, is unaffected by loss of methylation
S. Tilghman, Cell 96:185
How are imprinted genes silenced?
MechanismPromoters compete
for a single
enhancer
Igf2
H19
Problem with modelBoth H19 and Igf2 are expressed if H19 gene replaced
with luciferase
S. Tilghman, Cell 96:185
How are imprinted genes silenced?
Epigenetic marker binds to
unmethylated DNA
Igf2
H19
MechanismMethylation serves two
purposes
1. Inactivate a gene (e.g.
H19)
2. Prevent binding of
epigenetic marker so
that Igf2 is activated
Epigenetic insulator prevents enhancer from “talking to” Igf2
Evidence in support: if delete insulator element- both Igf2 and H19 expressed
S. Tilghman, Cell 96:185
Evidence for chromatin boundary mechanism
Deletion of ICR- both genes expressed
Identify protein (called CTCF) that binds ICR
CTCF cannot bind methylated DNA
Thorvaldsen and Bartolmei, Science 288:2145, 2000
How are imprinted genes silenced?
Mechanism- Antisense
transcription of
unmethylated
chromosome blocks
sense strand transcription
Mechanism- Antisense
RNA blocks sense strand
transcription
S. Tilghman, Cell 96:185
Prader-Willi Syndrome
Normal
expression
patterns
= ON
Prader-Willi Syndrome
Normal
expression
patterns
= ON
In Prader-Willi deletion,
maternal and paternal
copies are silent
In Angelman, many
genes are activated, but
UBE3A is silenced
Prader-Willi Syndrome
Proposed mechanism of SNPRN imprinting
Female allele
Imprintor gene
SNSIS
1
3
2
RNA
(BD RNA)
SNRPN gene
Dnmt
Male allele
Imprintor gene
SNSIS
SNRPN gene
RNA
(BD RNA)
In Prader Wille, the Switch Initiator Site (SNSIS) is mutated in
paternal chromosome, such that it can’t bind BD RNA