autism ppt1

Epigenetics of Autism Spectrum Disorder

Chinyelu Mozie

University of Texas at Dallas

March 25, 2009

What is Epigenetics

Weinhold:

“any process that alters gene activity without changing the DNA sequence, and leads to modifications that can be transmitted to daughter cells” (p. 163).

What is epigenetics• Means above the genome• Born with epigenome

– All cells have same genes

– Tells our cells what sort of cells they should be-cell differentiation

• skin, heart, hair, liver, etc

• Regulating gene function– Affect the way we produce proteins

• Can be transgenerational– Contains heritable epigenetic information

• Interaction with gene and environment

What is epigenetics

• Does not only occur in utero but throughout life span

• Conrad Waddington credited for term epigenetics in 1942– Increase of usage of word in 1990’s

Epigenetic Processes

• Gene silencing

• X-chromosome inactivation

• Imprinting

Mechanisms

• DNA Methylation– addition of methyl

groups to DNA at CpG sites

• Chromatin/Histone modifications– Addition and removal

of acetyl groups from DNA

http://www.universe-review.ca/F11-monocell.htm

Epigenetic Mechanisms

Some Background Information

• Deoxyribonucleic acid• Watson and Crick in

1953• Hold genetic

instructions on we look and behave

• Four nucleotides bases– Guanine

– Adenine

– Cytosine

– Thyminehttp://instruct.westvalley.edu/svensson/CellsandGenes/

DNA-structure

Some Background Information (cont.)

• DNA organized into chromosome– chromatin

• Genes located on chromosome

http://images1.clinicaltools.com/images/gene

Chromosome Structure

Background Information• Central Dogma

– DNA RNA protein

• Transcription– DNA to RNA

• Translation– RNA to protein

Translation/transcription process

DNA Methylation• Addition of methyl groups

to CpG sites– remains unmethylated at

CpG islands

• Most common method in gene silencing

• Methylated gene– Inactive– Prevents transcription

factors from binding to promoter

• Non methylated gene– active– Transcription factors able

to bind to promoter

Chromatin/Histone modifications• Addition and removal of acetyl groups from

histones.

• Addition of acetyl groups (acetylation) decondense chromatin, uncoiling the DNA strand which causes :– gene to become accessible to transcription factors,

exposes promoter of gene.– transcribed gene-expression of gene

• Removal of acetyl groups (deacetylation) cause chromatin to condense, tight coiling of DNA. – gene becomes inaccessible for transcription factors.– silent gene

Epigenetic Mechanisms for Repressing Transcription

Environ Health Perspect. 2006 March; 114(3): A160–A167

Role in Environment

• Driven by multiple factors in environment:

– Single nutrients

• ex: maternal diet in rats, exposure to parents, grandparents during famine

– Toxins- pesticides, smoking, drinking

• ex: cancer

– Behavior

• ex: grooming of rat pups

Single nutrients• Study by Dr. Jirtle (2006)

concerning epigenetic changes by dietary supplements

• Epigenetic regulation of agouti gene

• pregnant mothers diet– Vitamin B12, folic acid,

choline and betaine.– brown coat color in offspring– inherited by next generation

• pregnant mother diet– BPA– yellow coat color in offspring

A pup of a different color.

Supplementation of maternal diet with genistein and other compounds induced alterations in DNA methylation that were reflected in offspring coat color changes. Environ Health Perspect. 2006 March; 114(3): A160–A167.

Famine• First proof of environmental effect being inherited

in humans• Research by Marcus Pembrey and Olov

Bygren(2008) concerning famine exposure• Effect of famine was different in grandmother and

grandfather– Grandmother susceptible while still in the womb

– Grandfather affected in late childhood

Famine• Women who experienced famine earlier in

pregnancy had paternal granddaughters die earlier in life– Famine affected genetic makeup within X chromosome

– IGF2 gene different in siblings not exposed

• Men who experienced famine at age 10 had paternal grandsons that lived longer than those whose grandfathers didn’t experience famine– diabetes

• You are what your parents and grandparents ate

Cancer• P-53: tumor suppressor

gene– Excessive

deacetylation/methylation silences gene

– Results in uncontrollable cell growth

• Epigenetic Therapy– Reactivating gene by

removing methyl tags

Behavior• Study by Dr. Moshe Szyf (2004) showed that grooming, nursing,

and licking of a rat pup can affect long-term behavior of offspring (p. 164).– Rats pups receiving high nurturing

• methyl groups removed by nurturing signals, activated GR receptor gene-calm adults

– Rats pups receiving low nurturing• methyl groups remain attached to DNA, inactivated (gene silenced)

GR receptor gene-anxious adults

• Hippocampus in pups– Production of GR protein (glucocorticoid receptor)– Cortisol (stress hormone) binds to these receptors

• Affects more than just GR gene

Behavior• Epigenetic difference in high and low nurtured

rats.

• Can be reversed– Place pups with licking mother

– Drugs that add and remove methyl tags

Twin Studies• Provide evidence of epigenetic influences in

humans• Good way in determining heritability of disorders

– MZ - same DNA, same sex

– DZ - different DNA, different sex

• Muhle et al. (2004) concordance rate– High concordance in MZ-60-90%

– Low concordance in DZ- 0-10%

• Fraga et al. (2005) showed epigenetic differences arise in older twins– Ex: cancer

Connection between Epigenetics and Autism

• ASD comprises of a complex group of behaviorally related disorders that are genetic in origin

• No single gene found– Multiple genes contribute to autism

• Risch et al. (1999) found 2-15 genes contributing to susceptibility

– Cannot be traced to single mutation or gene

• Involvement of epigenetic regulatory mechanisms

Connection between Epigenetics and Autism (cont.)

• Can use other disorders that share same clinical features to map out genetic and epigenetic components

• Overlapping phenotypes

– Rett Syndrome

– Angelman’s syndrome and Prader-Willi syndrome

– Fragile X syndrome

Chromosomes/Genes

• 46 chromosomes (23 pairs)

• 45-46: sex chromosomes

• XX-woman• XY-man• Phenotype

– Observable manifestation of trait

• Genotype– Genetic constitution

Rett Syndrome• Neurological disorder

classified as a PDD by the DSM-IV

• Symptoms– cognitive impairments– problems with

socialization– no verbal skills

• X-linked neurodevelopmental disorder

• Caused by mutations in the MECP2 gene on Xq25

n-Rett Syndrome

Mother Father

XX XⁿY

Children

XXⁿ XY

Fragile X syndrome

• Genetic disorder caused by mutation of the FMR1 gene on the X chromosome

• Some symptoms meet the diagnostic criteria for autism

• X-linked dominant condition

-girls: mild MR

-boys: severe MR

Mother Father

X Xª XY

X Xª Xª Y

Angelman Syndrome/Prader-Willi Syndrome

• Both produced by same genetic mutation– AS: deletion of maternal

copy, maternal mutation of UBE3A gene or UPD of chromosome 15q11-q13

– Severe mental retardation, happy demeanor, non-verbal

– PWS: deletion of paternal copy or UPD of chromosome 15q11-q13

– Language, motor and developmental delays, excessive weight gain

• Result from imprinting

http://www.ucl.ac.uk/~ucbhjow/bmsi/bmsi_4.html

AS/PWS Diagram

Methyl-CpG binding domain (MBD).

• MBD1-gene– Mbd1: protein

• MECP2-gene– MeCP2: protein

• Multiple rules in regulating gene expression in neurons

• Capable of binding specifically to methylated DNA

Fig. 1. The overlapping disorders, phenotypes and genotypes regulated by MECP2/MeCP2 through epigenetic mechanisms. AS, Angelman's syndrome; RTT, Rett syndrome.

Study• Study by Allan et al. (2008)• Mice lacking key regulatory protein Mbd1

show autism-like symptoms• Exhibit core deficits

– reduced social interaction

– learning deficits

– anxiety

– defective sensory motor gating

– depression

– abnormal serotonin activity

Study (cont.)• Methyl-CpG binding proteins (MBDs) are central

components of DNA methylation-mediated epigenetic gene regulation.

• Role in transcriptional repression• Mutation found in MBD1 gene in autistic patient

and direct relatives• Analyzing Mbd1-/-

– Understand role of Mbd1 in adult neurogenesis and learning

– Understand importance of epigenetic regulation in mammalian brain development and cognitive functions.

Subjects• Mbd1-/- knockout (KO) mice

– Mutant

– No gross abnormality/ mild reduction of forebrain weight

– Nearly normal lifespan

• Mbd1 (WT)– Most common phenotype population

• Sex– male and female

• Age– 2-3 months

Methods• Social interaction

– Place novel toy in tent for first 5min then mouse in for last 5min

• Sensorimotor gating– PPI(Prepulse inhibition)– prepulse stimulus (auditory tone)– startle stimulus (air puff)

• 120 and 300ms interstimulus interval– access how well they are able to inhibit stimulus

• Learning deficits– Cued for fear conditioning and contextual fear conditioning

• foot shocks to train mice• tested 24hrs later using auditory tone (cued test) and spatial context

(context test)– learning and memory accessed by how long they freeze

t

Methods (cont.)

• Anxiety– Light-dark preference test– Elevated plus-maze

• Open arms

• Susceptibility to depression– Learned helplessness test

• foot shocks– Forced swim test

• Serotonin receptor– Wet dog shake observed in behavioral experiments

• indicator of increased serotonin activity

Results• Social Interaction

– Mbd1-/-

• Have normal motor activity and exploratory behavior

• Reduced interest in social interaction

• No gender differences

Figure 1: Mbd1-/- mice have impaired social interaction

Allan, A. M. et al. Hum. Mol. Genet. 2008 17:2047-2057; doi:10.1093/hmg/ddn102

Copyright restrictions may apply.

Results (cont.)

• Sensorimotor Gating– Mbd1-/-

• Impaired sensorimotor gating


Figure 2: Mbd1-/- mice have impaired sensorimotor gating as assessed by prepulse inhibition (PPI)


Results (cont.)• Learning Deficits

– Mbd1-/-

• Exhibited deficits in fear-conditioning test-assessed amygdala-mediated learning.

• Deficits in contextual fear conditioning test-assessed both hippocampus and amygdala mediated learning.

Figure 3: Mbd1-/-mice exhibited deficits in fear-conditioning learning tests



Results (cont.)• Anxiety

– Mbd1-/-

• Increased anxiety

• Spent less time in well-lit room

• Spent less time in open arms and more in open arms

– General locomotion same between genotypes (data not shown)



Figure 4: Mbd1-/- mice exhibited increased anxiety

Results (cont.)• Susceptibility to

depression– Mbd1-/-

• Enhanced susceptibility to depressive behaviors

• Longer latency in escaping shocks

• Greater number of failed escapes

• Exhibited more non-escape-directed behaviors and fewer escape-directed behaviors

Figure 5: Mbd1-/- mice exhibited enhanced susceptibility to depressive behaviors



Other Findings• Loss of Mbd1 leads to elevated expression of

Htr2c receptor.– Increased binding of serotonin to receptor

– Receptors in hippocampus and medial frontal cortex

• Htr2c does not function properly in Mbd1-/-– Low affinity of serotonin to receptor

• Dysregulation of Htr2c– Abnormal serotonin system linked to autism

Stahl SM. Mechanism of action of serotonin selective reuptake inhibitors: serotonin receptors and pathways mediate therapeutic effects and side effects. J Affect Disord. 1998;51:215-235



Mbd1 directly regulates the expression of serotonin receptor Htr2c

Current Research• Dr. Walter Kaufmann at Kennedy Krieger Institute

and Dr. Andy Feinberg at Johns Hopkins University– compared brain scans of identical twins discordant

for autism

– found that hippocampus smaller in twin with severe autism

– Hypothesis: since genome the same, one has epigenetic change

– Searching for methyl marks in DNA

Future Studies• Recent attention of epigenetics in ASD• Autism Speaks: $3.6 million to investigate

environmental risk factors for autism• Dr. Yong-hui Jiang at Baylor College of Medicine

– how folic acid supplementation affects epigenetic modulation of SHANK3 protein expression

• Dr. Robert Plomin at Institute of Psychiatry in London– Looking at epigenetic markers in twins who do not

share dianosis of autism.

Future Studies (cont.)• Dr. Emile Rissman at University of Virginia

– BPA (Bisphenol A)

• Dr. Bruce Hammock at University of California at Davis– Vitamin D levels

• lack of Vitamin D could influence brain development and function

• It’s too early to speculate on treatment for epigenetic factors, but mouse models should help evaluate treatment

References

1. Allan, A.M., Liang, W., Luo,Y., Pak, C., Li, X., Szulwach, K.E., Chen, D., Jin, P., Zhao, (2008). The loss of methyl-CpG binding protein 1 leads to autism-like behavioral deficits. Human Molecular Genetics, 17(13), 2047-2057.

2. Fraga, M.F., Ballestar, E., Paz, M.F., Ropero, S., Setien, F., Ballestar, M.L.., et al. (2005). Epigenetic differences arise during the lifetime of monozygotic twins. PNAS, 102(30), 10604-10609.

3. Jirtle, R.L., Dolinoy, D.C., Weidman, J.R., Waterland, R.A. (2006). Maternal genistein alters coat color and protects mouse offspring from obesity by modifying fetal epigenome. Environmental Health Perspectives, 114(4), 567-572.

4. Lewis, M., Lopez-Rangel, E. (2006). HotSpots: Loud and clear evidence for gene silencing by epigenetic mechanisms in autism spectrum and related neurodevelopmental disorders.

Clinical Genetics, 69, 21-25.

5. Muhle, R., Trentacoste, S.V., Rapin, I. (2004). The genetics of autism, Pediatrics, 113(5), 472-486.

6. Pembrey, M.E, Bygren, L.O., Kaati, G. (2008). Sex-specific male-line transgenerational responses in humans. Human Genetics, 14, 159-166.

7. Risch, N., Spiker, D., Lotspeich, L., Nouri, N., Hinds, D., Hallmayer, J., et al. (1999). A geomic screen of autism: evidence for a multilocus etiology. Human Genetics, 65, 493-507.

8. Szyf, M., Weaver, I.C.G., Cervoni, N., Champagne, F.A., D’Alessio, A.C., Sharma, S., et al. (2004). Epigenetic programming by maternal behavior. Nature Neuroscience, 7(8), 847-854.

9. Weinhold, B. (2006). Epigenetics: The science of change. Environmental Health Perspective, 114(3), 160-167.

autism ppt1

Documents

gr gene

promoter of gene

gene function

gene activity

silent gene

dna http

methylated gene inactive

transcribed geneexpression