PHARMACOGENOMICS: ALCOHOL AND OTHER

DRUGS Dr GURSHARAN KALSI

gursharan.kalsi@kcl.ac.uk

LEARNING AIMS

Overview of genetics of complex disorders, particularly

addictive disorder

Main pharmacotherapies available for treatment, using

alcohol dependence as example

Genetic variants and their effect on treatment and recovery

Relative efficacy of pharmacotherapies and tailored

treatment for addictions

Pharmacogenomics and addiction

The genetic variation underlying an individual’s

response to drugs

Genetics may affect all aspects of drug

experience, i.e. initiation, motivation, craving,

inability to withdraw and clinical treatment

Nature or nurture

Large twin and family studies were set up in the 50s

Family studies show that children of addicts are 8 times more likely

to develop addiction

In twin studies, the idea is that identical twins share same genes

and family environment whereas non-identical or fraternal twins only

share family environment

In a study looking at 861 pairs of identical twins and 653 fraternal

twins showed that genetic factors contributed 50-60% of the risk of

being alcoholic; other large twin samples showed a similar figures

The effect of genetic and environmental effects across age in 1796 male-male twin pairs

A= caffeineB= cigarettes smokedC= alcoholic drinksD= cannabis

Kendler et al, 2008

Epidemiology

Family, twin and adoption studies support existence of genetic

influences on risk for alcohol dependence (AD) and other drug

dependence

Heritability estimates show 40-60% of liability is due to genetic

factors

Lifetime risk for AD is approx 10-15% for men and 3-5% for women

Age of onset is probably mid-20s to approx 40 yrs of age

The earlier the onset, the greater is the severity and the higher the

probability of a pre-existing independent psychiatric condition

Basics of Genetics

The genome The genome is the hereditary material in an

organism Comprises of the coding and non-coding

sequences Human genome

○ Nuclear genome○ Mitochondrial genome

Size of the human genome is ~ 3 – 3.2Gb and ~20,000 – 25,000 genes

Chromosomes

22 pairs of chromosomes plus sex chromosomes Banding pattern – differences in GC contents

Slide courtesy of Dr Katherine Tansey

The gene

The basic unit of genome is the gene

Gene is the functional unit

Molecular unit of heredity

Double-helical structure

Expression involves transcribing into RNA

Regulatory sequences control the process

Genes can have more than one promoter

microRNAs at 3’ UTR now seen to be important as

epigenetic factors

Structure of the gene

Genetic polymorphisms

Single Nucleotide Polymorphisms (SNPs) A single base-pair has been altered Silent / missense / nonsense

Sequence Repeats (Insertions/deletions) A small segment of DNA (i.e. 2-,3-,4-..1000 base-pairs) is

repeated Variable tandem repeats / micro- / mini-satellites

Positional VariantsChanges > 1000bp inversions / translocations

Types of DNA markers RFLP (Restriction Fragment Length Polymorphism) VNTR (Variable Number Tandem Repeat) SSR (Simple Sequence Repeat) SNP (Single Nucleotide Polymorphism) STR (Short Tandem Repeat)

Single base change at one site along a sequence

cagTcgaor

cagCcga

The possible genotypes for this SNP are TT, TC or CC

Genes and the addiction cycle

• Initiation (impulsivity)• Tolerance• Inability to withdraw• Craving / relapse• Compulsive use• Response to treatment

Genes and drug metabolism

Genetic basis of alcohol metabolism

Alcohol metabolism is catalysed by alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH)

Metabolism is primarily in the liver

Edenberg et al, 2007

Alcohol dehydrogenase, ADH

Association with variants of ADH and ALDH shown to be the strongest

and most widely replicated

Individuals with certain variants suffer reduced risk of alcohol dependence

Presence or absence of variants influence consumption e.g. ADH1B*2

and ADH1B*3 possess high enzyme activity and rapidly metabolize

alcohol to acetaldehyde; accumulated acetaldehyde produces “flushing”

ADH1B*2 found in higher frequency in East Asians; it acts as a protective

variant

In Jewish populations, variant has moderate frequency and prevents

binge drinking

Aldehyde dehydrogenase, ALDH

ALDH2 is primarily responsible for metabolism of acetaldehyde to acetate

ALDH2 deficiency common in parts of Asia ALDH2*1 encodes the active subunit; ALDH2*2 encodes an

essentially inactive subunit Individuals with one or two copies of ALDH2*2 are

deficient in oxidation of acetaldehyde and suffer from adverse reactions to alcohol, including severe facial flushing, nausea, headache and tachycardia

Disulfiram, acts in similar way; it interferes with ALDH and leads to increased acetaldehyde levels, causing same symptoms

Macgregor et al, 2009

Disulfram for alcohol dependence Treatment is based on aversive therapy and has been

in use for over 60 years Stimulates the toxic affects of acetaldehyde

accumulation Disulfram works through inhibiting alcohol

dehydrogenase Induces flushing, sweating, headaches, nausea and

vomiting Also used to treat cocaine dependence In some individuals, it produces psychosis

Genes and nicotine metabolism Nicotine is broken down by a number of liver enzymes, cytochrome

A6 (CYP2A6) and cytochrome B6 (CYP2B6) CYPA6 variants have been associated with higher adult smoking

cessation rates Individuals with CYPA6*2 variant showed reduced function of the

enzyme and reduced smoking

Chenoweth et al, 2013

Ray et al, 2009

Neurochemical basis of addiction

Neural pathways in addiction

Dopamine Glutamate Opioid GABA Serotonin Cannabinoid CRH/HPA

Spanagel R, 2008.

Dopamine (DA) DRD2: 20 year controversial history

focus on Taq1A RFLP; located in ANKK1 A1 allele produces lower DRD2 receptor density in striatumA1 allele associated with alcohol dependence, heroin use, smoking and

cocaine dependence

DRD4 VNTR shown to be associated with craving for addictive substances

DAT1 cocaine dependence and risk for cocaine-induced paranoia; smoking

behaviours

COMT: variation may impact DA levels in PFC, thus affecting social cognition, affect and reward processes high activity 1947G allele associated with risk for methamphetamine

abuse

10 DA family genes in US/Irish study

5 receptors D1-like: DRD1, DRD5D2-like: DRD2-DRD4

2 transporters Vesicular Monoamine Transporter 2

(VMAT2)Dopamine Active Transporter 1

(DAT1) 3 enzymes

Tyrosine Hydroxylase (TH)Dopa Decarboxylase (DDC)Catechol-O-methyl transferase

(COMT) Kreek et al., 2002

Irish Affected Sib Pair Study of Alcohol Dependence (IASPSAD)

Ethnically and genetically homogeneous sample collected in Ireland

Severely affected individuals; clinical sample 575 independent cases and 530 controls Evaluated by structured SSAGA interview Diagnosed using DSM-IV

Results Six SNPs (single nucleotide polymorphisms)

in 5 genes were associated with alcohol dependence or related trait

2 SNPs were associated with disinhibitory symptom count

1 SNP was associated with general disinhibitory factor score

IONOTROPIC METABOTROPIC

Glutamate receptors

Glutamate receptor genes

Crucial role in withdrawal symptoms Gp 1 mGluRs couple to diverse intracellular

signaling transduction pathways Homer family of proteins regulate cellular

localization and function of group 1 mGluR receptorsHomer knockout mice avoid drinking in large quantitiesDo not increase dopamine and extracellular glutamate

mGluR1 increases NMDA receptor trafficking

Glutamate study in IASPSAD

Genes GRM1, GRIN1A, GRIN2B, GRIK1 tested

on addiction array

Association analysis for alcohol dependence

Haplotype analysis

gene x gene interaction

Haplotype association with alcohol dependence in IASPSAD

G X G INTERACTION in IASPSAD

GENE SNP GENE SNP P-VALUE

GRM1 rs9403765

GRIN2B rs12829455

0.000583

GRM1 rs2268666

GRIN2A rs1071504

0.0003489

GRIN2B rs2193511

GRIN2A rs17208905

0.0001232

GRIN2B rs10845848

GRIK1 rs6516926

0.0007265

Opioid system

Critical to euphoric and compulsive behavior of opiates

May be just as, or perhaps more, critical than dopamine

pathway in modulating reward

Regulated through a family of receptors: µ, δ, κ and non-

opioid FQ receptor

Belong to the larger G-protein coupled receptor family

Interact with DA and GABA systems to reinforce hedonic

effects of heroin

µ (mu) opioid receptor: variants

Primary site of action for opioid peptides

Large scale sequencing identified 43 variants

Results of genetic studies

The functional polymorphism A118G

The most prevalent polymorphism (A to G) Produces amino acid change at a putative N-

glycosylation site : Asparagine (Asn) to Aspartic acid (Asp)

Peptide binding studies show A118G variant do not alter binding affinity to opioid peptides

The variant does bind β-endorphin more tightly than the common allele

Used as an active side for many pharmaceutical treatments for drug dependence

Kalsi et al, 2009

Summary of candidate gene studies 100s of genes encoding for receptors,

neurotransmitters and transporters in several different systems have been tested

Results have been generally inconclusive Sample sizes have been small Most studies test diagnostic criteria rather

than narrower, more specific phenotype Results from drug metabolism genes have

been robust

Neurotransmitters and clinical treatments

Naltrexone

Agonists/antagonists for opioid, GABA, serotonin and DA

systems used as pharmaceutical compounds

Naltrexone is an antagonist for the µ-opioid receptor

It is a commonly used treatment for alcohol dependence

The variant A118G affects treatment response; alcohol

dependent individuals with G allele showed lower

relapse rate, reduced drinking and increased abstinence

Sturgess et al, 2011

Pharmacogenetic studies in alcohol dependence

Methadone

Was synthesized for analgesia and targets the opioid system; is an

agonist for the µ-opioid receptor, just like heroin, but has a longer

half-life

Prevents craving, withdrawal and is designed to block the receptor

The polymorphism A118G influences response

Highly variable individual response, influenced by cytochrome P

genes

Response may also be affected by DRD2 polymorphisms; common

allele of 957C>T (CC genotype) higher in non-responders

Sturgess et al, 2011

Acamprosate

Mostly used to treat alcohol-dependence

Acts as antagonist for NMDA receptors and

agonist for GABA receptors

Helps to ameliorate withdrawal symptoms and

affects processes related to reward

Works best when complemented by CBT

Summary

1. Current treatments have limited efficacy

2. Highly variable individual response

3. Need to have better understanding of

mechanisms and hence treatments

Mechanisms....

Rodd et al, 2007

Wang et al, 2011

And finally...

Genetics has revealed heterogeneity of drug dependence and recovery

There is limited efficacy of most treatments (yes, again probably due to genetics)

A systems approach will help to resolve mechanisms

But there is more...epigenetics could play a role

And the environment...

Useful references

1. Sturgess JE, George TP et al (2011) Pharmacogenetics of alcohol, nicotine and drug addiction treatments. Addiction Biol; 16, 357-376

2. Khokhar JY, Ferguson CS et al (2010) Pharmacogenetics of drug dependence: role of gene variations in susceptibility and treatment. Ann Rev Pharmacol Toxicol; 50, 39-61

3. Agrawal A, Verweij KJH et al (2012) The genetics of addiction – a translational perspective. Transl Psychiatry, July 17

4. Arias AJ and Sewell RA (2012) Pharmacogenetically driven treatments for alcoholism: are we there yet? CNS Drugs; 26(6), 461-476

5. Wang J, Yuan W and Li MD (2011) Genes and pathways co-associated with the exposure to multiple drugs of abuse, including alcohol, amphetamine/methamphetamine, cocaine, marijuana, morphine and/or nicotine: a review of proteomics analyses. Mol Neurobiol; 44, 269-286

6. Kalsi G, Prescott CA et al (2009) Unravelling the molecular mechanisms of alcohol dependence. Trends in Genet; 25(1), 49-55