epigenetics: impact of dna methylation
TRANSCRIPT
Max Planck Institute for Molecular Genetics
Epigenetics:
Impact of DNA methylation
Christine Steinhoff
Max Planck Institute for Molecular Genetics
Berlin, Germany
Max Planck Institute for Molecular Genetics
• Introduction and Definitions
• Historical aspects
• Overview: Impact of DNA methylation on cellular regulation
•Accepted issues
•Controversial issues
• The DNA methylation enzymes
• Interaction with „the histone epigenetics component“
• Overview: Experimental methods
• Human epigenomics projects
Outline
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsEpigenetics
You can inherit something beyond the DNA sequence.
That‘s where the real excitement in genetics is now (2003)
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsTwo Components of Epigenetics
DNA MethylationMost times: Addition of methyl marks leads to repression of gene activity
Histone modificationCombination of molecules attached to histones alters DNA activity in different ways
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsTwo Components of Epigenetics
DNA Methylation• Established during embryonic development
• Stable through multiple cell divisions (cellular memory)
• Differs between tissues, developmental stages
• Normally associated with gene silencing
• Methylation state-sensitive transcription factor binding
• up to 70% of CG dinucleotides methylated in human somatic cell
Max Planck Institute for Molecular Genetics
Introduction and Definitions:Chemistry
Figure 1.4b Genomes 3 (© Garland Science 2007)
RNA
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsChemistry
5‘ Methylcytosine
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsDeamination
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsDeamination
SAM
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsDefinitions
5‘CpG3‘
phosphate
„CG/GC are either completely methylated or completely unmethylated“(Bird 1978)
5‘ 3‘
5‘3‘
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsDefinitions
Gardiner-Garden, M. & Frommer, M. (1987)
> 200-bp stretch DNA G+C content > 50% observed CpG/expected CpG > 0.6
Takai D, Jones PA (2002)
> 500-bp stretch DNAG+C content >= 55% observed CpG/expected CpG > 0.65
CpG Islands - Definition
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsDefinitions
• in mammalian genomes typically 300-3,000 base pairs
• mammals: in/near approximately 40% of gene promoters
• humans: 56% genes associated with CpG Islands
• mouse: 47% genes associated with CpG Islands
• normally not methylated
CpG Islands some properties
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsWhere?
yes
nono
nono
nono
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no no
no no
no no no
no
yes
yes yes
yes yes yes yes
Max Planck Institute for Molecular Genetics
Historical AspectsWhat is Epigenetics
“… the branch of biology which studies the causal interactions between genes and their products, which bring the phenotype into being”
“… Epigenesis … development of individual organic form from unformed ”
“… The study of mitotically and/or meiotically heritable changes in genes function that cannot be explained by changes in DNA sequence” Current „working definition“
Epi: greek: above, over, on, upon, besides
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Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationOverview
Regulatory Involvement of DNA methylation
• Repeat elements• Imprinted Genes• X Chromosome Inactivation• „Diseases“
DNA methylation specific patterns
• Tissue• Development• Cancer
Impact of DNA methylation on cellular regulationAccepted Issues: Repeat elements
Max Planck Institute for Molecular Genetics
Genomes 3 (© Garland Science 2007)
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationAccepted Issues: Repeat elements
LINE 1 Element
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationAccepted Issues: Repeat elements
Methylation of LINE-1 promoter
• densely methylated in normal somatic tissues
• contained in inactive chromatin
• reactivation by hypomethylation (cancer, autoimmune diseases)
• demethylated -> chromosomal instability
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationAccepted Issues: Imprinting
• Genes that are expressed in a parent-of-origin specific manner
• in diploid cells
• demonstrated in insects, mammals and flowering plants
• Not necessarily imprinted in all developmental stages
• Not necessarily imprinted in all tissues.
• in human and mouse probably 100-600 genes
• in mammals so far all genes DNA methylation involvement
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationAccepted Issues: Imprinting
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationAccepted Issues: X Chromosome Inactivation
Kinetics of random X chromosome inactivation
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationAccepted Issues: Diseases
Example: RETT Syndrome
• Neurodevelopmental disorder
• Initial development normal
• Autism
• Learning disability
• X linked mental retardation
• Unusual stereotyped hand movements
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationAccepted Issues: Diseases
Example:
Prader Willi /Angelman Syndrome
• 1/10,000 births
• developmental delay
• PWS: obsessive compulsive behavior
• AS: „happy disposition“
• 70% deletions: 15q11-q13
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationControversial Issues: Tissue Specificity
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationControversial Issues: Developmental Specificity
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationControversial Issues
Arguments against regulatory impact of DNA methylation
• CpG Islands in expressing and nonexpressing tissues are unmethylated
• Demethylation can be interpretated as consequence of gene activation
• Loss of methylation as consequence of TF binding
• Developmental and tissue specific genes with methylation profile are not CpG
Island genes and thus few (random?) CpG sites
• Knock out defects are „too late“
• Microarray experiments only identify few genes with high fold changes
• conserved developmental processes also in non-methylation organisms
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationControversial Issues
a) Methylatedb) Non expressing in some tissue
Transfer to tissue, where it is normally expressed
Arguments against regulatory impact of DNA methylationProposed experiment
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The enzymesMammals
DNMT 1
DNMT 2
DNMT 3a
DNMT 3b
DNMT 3L
DNMT=DNA methyltransferase
<60
Max Planck Institute for Molecular Genetics
The enzymes
Max Planck Institute for Molecular Genetics
The enzymes
Max Planck Institute for Molecular Genetics
The enzymes
Max Planck Institute for Molecular Genetics
The enzymesDNMT1 = Maintenance methylation
• Preferred substrate: hemimethylated DNA
• Inactivation leads to genomewide loss of
CpG methylation
• Lethal E8.5
• X chromosome activation
• Activation of silent retrotransposons
• Abnormal imprinting expression
Max Planck Institute for Molecular Genetics
The enzymesDNMT3a = De novo methylation
• no preference for hemimethylated DNA
• Postnatal lethal 4-8 weeks
• Male sterility
• Methylation imprint fails in germ cells
Max Planck Institute for Molecular Genetics
The enzymesDNMT3b = De novo methylation
• no preference for hemimethylated DNA
• ko: Demethylation of satellite DNA
• Lethal at E14.5
• Vascular and liver defects
Max Planck Institute for Molecular Genetics
The enzymesDNMT3L = De novo methylation
• Establishment of imprinting patterns
Mouse
Max Planck Institute for Molecular Genetics
The enzymesDNMT2 = weak DNA methylation activity
• The most strongly conserved, most widely
distributed
• no change in CpG methylation
• no obvious developmental phenotypes
• unusual case: DNMT by sequence and
structure, BUT no evidence of such a function in
genetic and biochemical tests lacking
• methylates small RNAs (tRNA)
Max Planck Institute for Molecular Genetics
Interaction with histones
Meissner et al 2008
Max Planck Institute for Molecular Genetics
Interaction with histones
Max Planck Institute for Molecular Genetics
Experimental methodsBisulfite sequencing
Max Planck Institute for Molecular Genetics
Experimental methodsBisulfite sequencing
PCRSequencing
Max Planck Institute for Molecular Genetics
Experimental methodsHighthroughput
Weber et al, 2005, 2007 Nature meDIP
Sample + EcoR1
MspI (not meth specific)
Bisulfite treatment
Small DNA Pieces
C->U, Cm->C
C‘CGG cut
Fill in ends and extend
sequence
C C G G AAAAAAAAAAAAG G C C TTTTTTTTTTTTT
High throughput sequencing
Experimental methodsHighthroughput
Max Planck Institute for Molecular Genetics
Human epigenomics projects
Eckhardt et al. (2006), Rakyan et al. (2004)
HEP Project Consortium
Barski et al. (2007)National Heart, Lung and Blood Institute of the NIH
Mikkelsen et al. (2007)Broad Institute of MIT and Harvard
HEROIC Project Consortium (2005)HEROIC Project Consortium
ENCODE Project Consortium (2007, 2004)
ENCODE Project Consortium
Alliance for Human Epigenomics and Disease (2007); Jones and Martienssen (2005)
AHEAD Task Force
ReferenceInitiator
10/2007
Max Planck Institute for Molecular Genetics
Human epigenomics projects
Eckhardt et al. (2006), Rakyan et al. (2004)
HEP Project Consortium
ReferenceInitiator
• Differential methylation ~ evolutionary conservation
• 17% differentially methylated in 5‘UTR:
only 1/3 anticorrelation with transcription
• CpG density inversely correlated with methylation
Max Planck Institute for Molecular Genetics
Human epigenomics projects
Max Planck Institute for Molecular Genetics
Human epigenomics projects
ENCODE Project Consortium (2007, 2004)
ENCODE Project Consortium
ReferenceInitiator
Max Planck Institute for Molecular Genetics
Thanks
Thank you for your attention
Max Planck Institute for Molecular Genetics
Introduction and DefinitionsWhere?
C
mC
U
T
Lecture: Introduction to DNA methylation and its impact on gene regulation V
Evolution CpG -> TpGBisulfite Sequencing
Spontaneous deamination followed
by substitution
C
mC
T
C
C
Max Planck Institute for Molecular Genetics
Historical AspectsWhat is Epigenetics, Conrad Waddington
Max Planck Institute for Molecular Genetics
Impact of DNA methylation on cellular regulationAccepted Issues: Diseases
Max Planck Institute for Molecular Genetics
The enzymes