genetics of alcoholism part ii
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Genetics of Alcoholism Part II. Ian Gizer University of Missouri-Columbia Columbia, MO, USA [email protected]. Definitions. Chromosomes – threadlike structures on which individual genes are located. Karyotype of normal human male. Chromosome #9. - PowerPoint PPT PresentationTRANSCRIPT
Genetics of Alcoholism
Part II
Ian GizerUniversity of Missouri-Columbia
Columbia, MO, [email protected]
Definitions• Chromosomes – threadlike
structures on which individual genes are located
Karyotype of normalhuman male
• Locus (location) and allele (alternative form)
• Centromere, short (p) and long (q) arms
Chromosome #9
ABO locus
p
q
Centromere
(9q34.1)
Definitions• Chromatin: genetic material
contained in chromosomes – DNA & proteins (histones and nonhistones)
• Euchromatin – less condensed/light bands; coding DNA
• Heterochromatin – compacted/dark bands, usually noncoding DNA
Chr 21
UM Bauer
Definitions• DNA: Deoxy ribonucleic acid• Purine and pyrimidine bases• Purines: Cytosine, Thymine• Pyrimidines: Adenine,
Guanine • Double stranded (each strand
has full information content)• Strands are held together by
(hydrogen) bonds that form between the nucleotide bases of the DNA molecule
Adenine (A) <====> Thymine (T)
Guanine (G) <====> Cytosine (C)
Definitions• Gene: A sequence of DNA (a locus
on a chromosome) that is involved in (“codes for”) the synthesis of a functional polypeptide (proteins consist of one or more polypeptides, which are strings of amino acids).
Gene Structure
EXON – EX-pressed or coding DNA that is converted into proteinINTRON – IN-active or noncoding DNA that is not converted to protein
Definitions• Transcription: One of the two DNA
strands is transcribed to a single-stranded nucleic acid called ribonucleic acid (RNA) RNA has the same bases as DNA except uracil (U) substitutes for thymine (T).
• Translation: Conversion of the basic informational unit of 3 nucleotide bases (called a codon) into a single amino acid.
Example
TTT TCCAAA AGG
UUU UCC
Transcription
Phenylalanine Serine
Translation
Non-transcribed DNA strand
Transcribed DNA strand
mRNA
Amino Acid
Genetic Variation• 95% - 98% of human DNA does not
code directly for protein. • An estimated 99.8% - 99.9% of our
DNA is common.• But then .1% of 3,000,000,000 = 3
million differences!• We are interested in these
variations and the transmission and co-aggregation of these variations with AUDs.
Two major types• Microsatellite/short tandem repeat
(STR): a stretch of DNA that is sequentially repeated a variable number of times. • Can cause disease (e.g.
CAG repeat expansion causes Huntington’s disease;
• Can also be benign variation;
• Assume it is close to a disease contributing gene;
Single Nucleotide Polymorphism
• SNPs are single base pair changes that occur as natural variation in the human genome. They can code for protein change (non-synonymous) or not.
Two major methods for identifying genes associated
with AUDs
• Linkage
• Association
Linkage Analysis
AA (BB) Aa (Bb)
AA (BB) AA (BB) AA (BB)Aa (Bb)
Aa (Bb) Aa (Bb) AA (BB)AA (BB)
LINKAGE• Basic idea is identity-by-descent
(IBD) or how often does an affected pair of relatives share the same ancestral DNA. If more often than expected by chance, then somewhere near this shared DNA is a gene that contributes to affection status.
• Need related individuals where multiple relatives are affected.
• Identifies large stretches of DNA.
Linkage Analysis: The BasicsIBD – An Illustration
A. One allele IBS and one allele IBD.B. One allele IBS and zero alleles IBD.
C. Two alleles IBS and at least one allele IBD.
IBD Sharing in pairs affected for disorder
Sib 1
Sib 2
4/16 = 1/4 sibs share BOTH parental alleles IBD = 2
8/16 = 1/2 sibs share ONE parental allele IBD = 1
4/16 = 1/4 sibs share NO parental alleles IBD = 0
AC AD BC BD
AC
BCBD
AD
A/B C/D
AC AD BC BDAC 2 1 1 0AD 1 2 0 1BC 1 0 2 1BD 0 1 1 2
Sib 1 Sib 2
Sib 1
Sib
2
H(0): IBD (0) = 25%; IBD (1) = 50%; IBD (2) = 25%H(A): IBD (0) < 25%; IBD (1) > 50%; IBD (2) > 25%H(A) is evidence for linkage.
LINKAGE via IBD
Linkage studies of AUDs• Most prominent is Collaborative
Study of the Genetics of Alcoholism (COGA).
• Has identified many important genetic regions using STRs and SNPs.
COGA strategy
0
0.5
1
1.5
2
2.5
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160cM
Lod
Sco
res
Wave 1
Wave 2
Combined
1. Ascertain multiplex alcoholic families
2. Linkage analyses to identify chromosomal regions
3. Association analyses to identify specific genes
allele-sharing among affecteds within a family
Gene A Gene B Gene C
Polydiagnostic interviewElectrophysiological data262 Families, 2282 individuals
LOD score
Williams et al., 1999
LOD = Likelihood of Odds;
LOD of 3.0 means it is 1000 times more likely than expected by chance that there is linkage.
Log101000 = 3
Higher the LOD, more likely genes are nearby
Irish affected sib pair study
Prescott et al., 2006
Problems with Linkage• Methodological
problems;• Need BIG sets
of families;• Home in on a
big chunk of DNA – possibility of hundreds of genes!!!
1 cM (centiMorgan) is approximately equal to 1 Megabase or 1000000 bp!!!!Genes may be anywhere in the 50cM region
Cardon & Bell, 2001 Nat Rev Genet
Association Analysis
Association• Family Based(transmission
disequilibrium test)
• How often is the risk allele transmitted to an affected child from a parent who is heterozygous (A/a) for the SNP?
A/a a/a A/a aa a/a A/a
A/a A/a a/a A/a
Association• Case/Control Design
• Is the prevalence of the risk allele greater in affected versus unaffected people?
a/a a/a
A/a a/a
A/a A/a
A/a A/a
Which Genes should I look at?
1. Genes in a linkage region
2. Genes that metabolize alcohol (candidates)
3. All genes
Genes in the linkage region
GABRA2: gamma-amino butyric acid receptor A, subunit 2 gene
• GABA & Alcohol (Buck, 1996; Grobin et al., 1998) – motor incoordination– anxiolytic effects – sedation– ethanol preference– withdrawal signs– tolerance & dependence
• GABAA receptor agonists tend to potentiate the behavioral effects of alcohol, while GABAA receptor antagonists attenuate these effects
GABAmajor inhibitory neurotransmitter of the central nervous
system
GABRA2 and AUDs
Edenberg et al., 2004
Region contains:• GABRG1• GABRA2• GABRA4• GABRB1
Many replications…• Many studies now show an
association between SNPs in GABRA2 and AUDs.
• SNPs are also associated with drug dependence, nicotine dependence, conduct problems and antisocial personality disorder – likely to be general vulnerability to thrill seeking.
• Replicated in family-based and case-control studies.
Genes that metabolize alcohol
-1
-0.5
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 50 100 150 200
chromosome 4 position (cM)
LOD
Symptom CountAlcohol Dependence
ADH cluster (1a,1b,1c,4,5,6,7)
Flushing Response• Dysphoric effects that occur w/i 15
minutes of drinking:– Heart palpitation– Facial reddening– Nausea, dizziness
• There are large ethnic group differences in rate of flushing – metabolic not cultural
Pathway of Alcohol Metabolism
Alcohol Acetaldehyde Acetate
ADH ALDH
ALDH2 Deficiency• ADH1B*2, ADH1C*1 code for
protein subunits that have greater enzymatic activity, suggesting faster conversion to acetaldehyde
• ALDH2*2 – inactive enzyme, can’t break down acetaldehyde– Causes facial flushing, nausea
ADH1B(2)*2 faster to acetaldehydeADH1C(3)*1 faster to acetaldehydeALDH2*2 slower breakdown acetaldehyde
PROTECTIVE EFFECTS
ADH2*2 less common in alcoholics
ADH3*1 less common in alcoholics
ALDH2*2 less common in alcoholics
Wall et al. (2001)
Wall et al. (2001)
Edenberg et al., 2006
MacGregor et al., 2009
0
1
2
3-Log
10p
ADH5 ADH4 ADH6 ADH1A
ADH1B
ADH1C ADH7
Alcohol Dependence
Withdrawal
Severity
rs3762894
rs2066702
Gizer et al., 2011
Examine ALL genes• Called GWAS: Genomewide
association study;• Saturate the genome with a million
SNPs and then test association with each SNP.
• Maybe find something new!
Treutlin et al., 2009
Bierut et al., 2010
Problems with association studies
1. Population stratification (only when using unrelateds) –when an association between a SNP and AUDs is due to ethnic variation in that SNP.
2. P-values need to be adjusted for testing many markers (e.g. 0.05/#markers tested).
3. Replication in other samples.4. What does the gene/SNP do in the
etiology of AUDs?
ENDOPHENOTYPES• Inherited mediators;• Associated with, but not a
consequence of, alcoholism; • Transmitted in families of alcoholics;• Present when disorder is not in active
phase;• Heritable;
• Examples: EEG, P300, Subjective response to alcohol.
Irv Gottesman
Why study EEG for AUDs?• EEG (Electro-encephal0grams) of waves
suggest that certain EEG activity is associated with risk for AUDs;
• EEG is heritable;• In families with AUDs, unaffected
relatives of AUD individuals have distinct EEG patterns;
• EEG pattern is not modified when an individual goes into recovery;
• EEG is an ENDOPHENOTYPE for AUDs
EEG readings
EEG Waves• Alpha waves : major rhythm seen in normal
relaxed adults - it is present during most of life especially beyond the thirteenth year when it dominates the resting tracing.
• Beta activity : dominant rhythm in patients who are alert or anxious or who have their eyes open.
• Theta activity abnormal in awake adults but is perfectly normal in children upto 13 years and in sleep.
• Delta activity : quite normal and is the dominant rhythm in infants up to one year and in stages 3 and 4 of sleep.
Ref: http://www.brown.edu/Departments/Clinical_Neurosciences/louis/eegfreq.html
EEG HeritabilitiesDelta (1.5-3.5 Hz) 76%Theta (4-7.5 Hz) 89%Alpha (8-12.5 Hz) 89%Beta (13-25 Hz) 86%
Van Beijsterveldt et al., 1996
Frequency band
Mean h2
Increased BETA Log Power in Alcoholics (F3-C3)
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
BETA1 BETA2 BETA3p-values : 0.004 0.007 0.004
log
pow
er
CONTROL (n= 257) ALCOHOLICS (n=271)
Rangaswamy et al., 2002
*Significant for all beta bands, particularly Beta 1 for males, and Beta 2 and Beta 3 for femalesHR=high risk; LR=low risk
Increased BETA Power in offspring of alcoholics
Rangaswamy et al., 2004
Beta 1 Beta 2 Beta 3
P300• Event-related potential (ERP)• P300 /oddball task• Subject attends to rarer of two cues• Rarer the event = larger the
amplitude• Reflects context/memory updating
whereby current model of environment is updated with incoming info.
Rangaswamy & Porjesz: P300 amplitude is reduced in alcoholics
Carlson et al., 2004
Discordant stable: One twin has AUD, other does not;Newly discordant: One twin develops AUD, other does not;
20
25
30
Discordant stable Newly Discordant Unaffected
P300
am
plitu
de
Alc
No Alc
Heritable across all levels of alcohol use
Perlman et al., 2009
Sensitivity to Alcohol: SRE• Self-
rating of the effects of alcohol (Schuckit et al, 1997)
Twin Study (Heath, et al.1999)
Behavioral Sensitivity(Schuckit, 1984)
Family historypositive
Family historynegative
Schuckit et al., 1994
Problems with Endophenotypes
• Not specific (e.g. P300 amplitude reduction is also associated with schizophrenia);
• Links between endophenotype and phenotype maybe unknown;
• Underlying genetic architecture may not be any less complex;
• Requires special equipment/lab and subject consent;
Genetic Strategies with Animals
• Forward Genetic Approaches (phenotype-driven)– Inbred strains– Selectively bred strains– Mutagenesis
• Reverse Genetic Approaches (genotype-driven)– Transgenics– Knockouts
QTL mapping
Importance of the Mouse Genome
• Mouse genome (Nature, December 5, 2002):– 2.5Gb– ~27,000 – 30,500 genes
• Relationship to human genome:– ~99% of mouse genes have counterparts
(orthologs) in human– ~96% of human genes have orthologs in
mouse– Conservation of some non-coding regions– Synteny – stretches of DNA that are the same
in mouse and human
Alcohol Preference• % of times in 14-day period animal
selects 10% ethanol solution vs. tap water (both a sweetened with saccharin)
• Marked differences between strains, 0-80%
Selection for Alcohol Preference
Li et al., 1993
Sleep Time: Loss of Righting Reflex (LORR)
Markel et al., 1997
Behavioral Examples –NPY
(Theile et al. Nature, 1998)
• Neurotransmitter known to be a potent stimulator of appetite
• Relevance to alcohol:– QTL studies of rat preference map to NPY
region– Inbred strain comparisons
• Knock-out (loss-of-function) – increased ETOH consumption & decreased sleep time
• Transgenic (gain-of-function) – decreased consumption and increased sleep time
Why we are not animals…• Animals self administer alcohol and
drugs – so do we – but, often, there is a social context for alcohol use in humans.
• The motivational model of alcohol use is strongly linked to continued drinking.
• Environmental modified.• Rather complex to study in animals.
Drinking motives (Cooper et al.)• Drinking motives (How often do you drink to …?)
stem from a motivational model of alcohol use – we drink to achieve a certain socio-cognitive outcome (e.g. drink to reduce stress and/or drink to fit in with friends);
• Motives have both valence (positive/negative) and source (internal/external).
• Motives are moderately heritable (Prescott et al., 2004; Agrawal et al., 2008).
• They share genetic influences with alcohol consumption (Prescott et al., 2004) – they moderate the genetic links between personality and alcohol consumption (Littlefield et al., in prep).
M. Lynne CooperAndrew Littlefield
Why do we DRINK?
Kuntsche et al., 2005, Clin Psych Rev
WHY DO WE DRINK?• Coping motives
– How often do you drink to forget your worries?
• Enhancement Motives– How often do you drink because you
like the feeling?• Social Motives
– How often do you drink to be sociable?• Conformity Motives
– How often do you drink so you won’t be left out?
Prescott et al., 2004
PART II• Genetic regions have been identified
for alcoholism: chromosomes 2,4,5,7• Genes: GABRA2, ADH cluster• GWAS largely unsuccessful• Endophenotypes replicate results
with AUDs but tend to be generalizable to externalizing behaviors.
• Animal studies lack context of drinking.
What next for the genetics of alcoholism?