imprinting in the news colon cancer- 3 rd most deadly cancer in america loss of imprinting (loi) of...

Imprinting in the news

• Colon Cancer- 3rd most deadly cancer in America

Loss of Imprinting (LOI) of the Igf2 gene correlates with colon

cancer

•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

Result= Maternal Igf2 gene is turned on

Potential blood test to screen for colon cancer

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

http://www.pwsa-uk.demon.co.uk/images/alex.htm

• 1 in 25,000 live births

• mostly sporatic

• 80 % have deletion at 15q11-q13– Specifically mutation of UBE3A gene

Angelman Syndrome

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

Normal

–Angelman Syndrome- maternal chromosome deletion

Or 2 paternal chromosomes

Or an imprinting

defect

Definitions• 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

Functional studies- Chimeras

+ Implant into

pseudopregnant female 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

• 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

Only 1 X chromosome

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?


• 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 Dnmt12. Active process via specific demethylation


• 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



Dnmt-/- mice-

Many imprinted genes (e.g. H19) reactivated

..but, Igf2 and Igf2r are silenced

Mechanism- Methylation interferes with transcription factor binding

Problems with model-

1. Promoters of silent Igf2 and Igf2r alleles are unmethylated2. One gene, Mash2, is unaffected by loss of methylation



Mechanism- Promoters compete

for a single enhancer

Problem with model-

Both H19 and Igf2 are expressed if H19 gene replaced with luciferase

Igf2 H19



Epigenetic marker binds to unmethylated DNA

Mechanism-Methylation serves two

purposes1. Inactivate a gene (e.g.

H19)2. Prevent binding of

epigenetic marker so that Igf2 is activated

Igf2 H19

Epigenetic insulator prevents enhancer from “talking to” Igf2

Evidence in support: if delete insulator element- both Igf2 and H19 expressed

Evidence for chromatin boundary mechanism

Deletion of ICR- both genes expressedIdentify protein (called CTCF) that binds ICRCTCF cannot bind methylated DNA

Thorvaldsen and Bartolmei, Science 288:2145, 2000



Mechanism- Antisense transcription of unmethylated chromosome blocks sense strand transcription

Mechanism- Antisense RNA blocks sense strand transcription

Normal expression

patterns

= ON


Normal expression

patterns

= ON


In Prader-Willi deletion, maternal and paternal copies are silent

In Angelman, many genes are activated, but UBE3A is silenced


Proposed mechanism of SNPRN imprinting

In Prader Wille, the Switch Initiator Site (SNSIS) is mutated in paternal chromosome, such that it can’t bind BD RNA

Imprintor gene

RNA (BD RNA)

SNSIS SNRPN gene

Female allele

Imprintor gene

RNA (BD RNA)

SNSIS SNRPN gene

Male allele

Dnmt

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imprinting in the news colon cancer- 3 rd most deadly cancer in america loss of imprinting (loi) of...

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