Genetic Predisposition to Schizophrenia Associated With Increased Use of

Cannabis RA Power KJH Verweij M Zuhair GW Montgomery AK Henders AC Heath PAF Madden SE Medland NR Wray NG Martin

Molecular Psychiatry 19, 1201-1204 (November 2014) 1

Overview• Introduction• Cannabis and Schizophrenia

• Related studies

• ArticleAimMaterial and MethodsResultsDiscussionLimitations 2

Molecular Psychiatry • “Molecular Psychiatry publishes work aimed at elucidating biological

mechanisms underlying psychiatric disorders and their treatment. The emphasis is on studies at the interface of pre-clinical and clinical research, including studies at the cellular, molecular, integrative, clinical, imaging and psychopharmacology levels”

• Impact Factor: 15.147*

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• *2013 Journal Citation Report (Thomson Reuters, 2014)

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Introduction – Cannabis & Schizophrenia• Case control studies have consistently demonstrated higher rates

of cannabis use among patients with schizophrenia as compared with controls.

• A meta-analysis of prospective cohort studies found similarly high rates of broadly defined psychosis outcome (psychotic disorder or psychotic symptoms and assoc disability) in people who had at some time used cannabis compared with those who had never used – Pooled Odds Ratio = 1.41

• Evidence to date suggests some specificity of effect for schizophrenia and non-affective psychosis

Comprehensive Textbook of Psychiatry, 9th Ed Kaplan & Sadock

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Introduction – Cannabis & Schizophrenia

• The association with prior cannabis use and subsequent psychosis is a consistent finding across studies

• The effect is attenuated but remains statistically significant even after adjusting for – Other drug abuse – Known risk factors for schizophrenia – Socio-demographic variables

Comprehensive Textbook of Psychiatry, 9th Ed Kaplan & Sadock

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• There is evidence that a specific functional polymorphism in catechol-O-methyl transferase (COMT) gene moderates the effects of adolescent cannabis use – Indls homozygous for COMT

valine allele may be more likely to exhibit psychotic symptoms / develop schizophreniform disorder in adult life after cannabis use

– Indls homozygous for COMT methionine allele do not show this adverse effect

Comprehensive Textbook of Psychiatry, 9th Ed Kaplan & Sadock

Cannabis and Schizophrenia

• Cannabis was associated with an increased risk of developing schizophrenia in a dose dependent fashionZammit S, Allebeck P, Andreasson S, Lundberg I, Lewis G. Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study. BMJ 2002; 325: 1199

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Cannabis and Schizophrenia

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• “Despite a clear association between the two, the possibility of reverse causation has not been entirely excluded– Is psychosis a risk factor for cannabis use? • Those on a psychotic spectrum more likely to experiment with

drugs? Spencer C, Castle D, Michie PT. Motivations that maintain substance use among individuals with psychotic disorders. Schizophr Bull 2002; 28: 233–247

Power RA et al (2014) Genetic Predisposition to Schizophrenia Associated With Increased Use of Cannabis Molecular Psychiatry

Related studies - 1Cannabis use predicts future psychotic symptoms, and

vice versa• AIM : • To determine if cannabis use is a risk factor for future psychotic

symptoms, and vice versa, in adolescents and young adults from the general population

• METHOD• 14-year follow-up study of 1580 initially 4–16-year-olds who were

drawn randomly from the Dutch general population• Life-time cannabis use and psychotic symptoms, assessed with the

Composite International Diagnostic Interview (CIDI)• Ferdinand RF, et al. (2005) Cannabis use predicts future psychotic symptoms, and vice versa Addiction, 100, 612–618

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Related studies - 1 (contd…)• Results

– Cannabis use, in individuals who did not have psychotic symptoms before they began using cannabis, predicted future psychotic symptoms (hazard ratio = 2.81; 95% confidence interval = 1.79–4.43)

– However, psychotic symptoms in those who had never used cannabis before the onset of psychotic symptoms also predicted future cannabis use (hazard ratio = 1.70; 95% confidence interval = 1.13–2.57)

• Conclusion • The results imply either – a common vulnerability with varying order of onset – a bi-directional causal relationship between cannabis use and

psychosisFerdinand RF, et al. (2005) Cannabis use predicts future psychotic symptoms, and vice versa Addiction, 100, 612–618 10

Related studies - 2Tests of causal linkages between cannabis use

and psychotic symptoms• AIM : To examine possible causal linkages between cannabis use and

psychosis using data gathered over the course of a 25-year longitudinal study

• METHOD• A 25-year longitudinal study of the health, development and

adjustment of a birth cohort of 1265 New Zealand children– Data on cannabis use and psychotic symptoms were available on at least one

occasion from 18, 21 and 25 years– Structural equation models permitted evaluation of reciprocal relationships

between cannabis use and psychosis and using these models to provide a guide to probable patterns of causation

• Fergusson DM et al (2005), Tests of causal linkages between cannabis use and psychotic symptoms Addiction,100, 354–366 11

Related studies - 2 (contd…)• Results

– For both models, the effect of psychotic symptoms on cannabis use was negative and, for Model 2, statistically non-significant.

– it was unlikely that the development of psychotic symptoms led to increased use of cannabis and that, if anything, the development of these symptoms may have inhibited rather than encouraged cannabis use.

– Daily users of cannabis had rates of psychotic symptoms that were between 1.6 and 1.8 times higher (P<0.001) than non-users of cannabis

• Conclusion – the association between cannabis use and psychotic symptoms is unlikely

to be due to confounding factors– the direction of causality is from cannabis use to psychotic symptoms

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• Schizophrenia is a highly heritable condition, with upto 80% of the variance explained by additive genetic effects

• Sullivan PF, Kendler KS, Neale MC. Schizophrenia as a complex trait—Evidence from a meta-analysis of twin studies. Arch Gen Psychiatry 2003; 60: 1187–1192.

• Cannabis use itself has been reported to be heritable (although no genetic risk variants have been identified) Agrawal A, Lynskey MT. The genetic epidemiology of cannabis use, abuse and dependence. Addiction 2006; 101: 801–812.

Genetic Risk for Schizophrenia → Association with cannabis use

• Such an association would suggest that those genetically predisposed to schizophrenia use cannabis more frequently

• Cannabis use itself is not associated with the COMT valine or methionine allele, suggesting an underlying gene–environmental interaction (rather than a gene–environment correlation) is operating to increase risk

• Comprehensive Textbook of Psychiatry, 9th Ed Kaplan & Sadock

• “This would mean that the association between schizophrenia and cannabis use is not simply one of an environmental risk factor, but rather involves gene–environment correlation, as individuals choose and shape their own environment based on their own innate preferences”

• Power RA et al (2014) Genetic Predisposition to Schizophrenia Associated With Increased Use of Cannabis Molecular Psychiatry

Related studies - 3 Genome-wide Association Analysis Identifies

13 New Risk Loci for Schizophrenia

• AIM : To perform a Genome Wide Association Study (GWAS) to identify associations between SNPs/Loci and Schizophrenia

• METHOD– GWAS performed on 13833 cases and 18310 controls

• Ripke S, O'Dushlaine C, Chambert K, Moran JL, Kahler AK, Akterin S et al. Genomewide association analysis identifies 13 new risk loci for schizophrenia. Nat Genet 2013; 45: 1150–1159. 16

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Related studies - 3• First unit for reporting effect is Odds Ratio – Odds of Disease in indls having the allele – Odds of Disease in indls not having the allele

↓• Chi-squared Test gives p-value of a certain significance

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• Pa = Freq of Allele a occurring at a locus • Pb = Freq of Allele b occurring at a locus • Pab = Freq of a and b occurring together in the same gamete • If Pa x Pb ≠ Pab there is a non-random association between a & b

and there is Linkage Disequilibrium in between these alleles • Coeff of LD = Dab = Pab – (Pa x Pb) • Many GWAS findings implicate an extended region containing

multiple SNPs with significant association– These are not independent associations but result because of high LD between

associated SNPs– These associations are summarized in terms of the index SNP with the

strongest association and other SNPs in high LD with the index SNP 20

• Correlation between a pair of Loci • Value = r Freq = r2

r2 = D2 / (p1p2 x q1q2)• If r2 = 0 there is complete Linkage Equilibrium • If r2 = 1 there is complete Linkage Disequilibrium • R2=0.25, high degree of LE

• Polygenic Schizophrenia Risk Scores – Number of Schizophrenia risk alleles weighted by Logistic Regression– PLINK program – (P = 0.0001, 0.001, 0.01. 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 and 1.0)

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Related studies - 3 (contd…)• Results– Identified 22 loci associated at genome-wide significance; 13 of

these are new, and 1 was previously implicated in bipolar disorder. – Examination of candidate genes at these loci suggests the

involvement of neuronal calcium signaling. – Estimate that 8,300 independent, mostly common SNPs (95%

credible interval of 6,300–10,200 SNPs) contribute to risk for schizophrenia and that these collectively account for at least 32% of the variance in liability.

• Conclusion – Common genetic variation has an important role in the etiology of

schizophrenia, and larger studies will allow more detailed understanding of this disorder

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Genetic Predisposition to Schizophrenia Associated With Increased Use of

Cannabis RA Power KJH Verweij M Zuhair GW Montgomery AK Henders AC Heath PAF Madden SE Medland NR Wray NG Martin

Molecular Psychiatry 19, 1201-1204 (November 2014) 23

IntroductionAIM

To discern the direction of causation between cannabis use and schizophrenia

by Studying the association between cannabis use and

Polygenic Risk for Schizophrenia

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Materials & methods – Subjects & Clinical Assessment

• Both groups participated in telephonic interviews • Based on Semi-Structured Assessment of the Genetics of

Alcoholism

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Sample 1 Sample 2

Number 6265 9688

Age 23 to 39 yrs (mean = 29.9) 18 to 91 yrs (mean 46.3)

Years 1996 to 2000 2001 to 2005

Born betn 1964 to 1971 1895 to 19641964 to 1971

• Collaborative Study on the Genetics of Alcoholism (COGA) funded by the NIAAA

• Objectives– Characterize the familial transmission of alcoholism and related phenotypes – Localize susceptibility genes for alcoholism using genetic linkage.

• 600 nuclear families were recruited– All available biologically related individuals were personally interviewed. – A subset of families were selected for more intensive study using standard

biological markers, and neurophysiological and neuropsychological protocols

Polydiagnosis Comorbidity Phenotype

ADS based on numerous diagnostic systems

Depression ASPD

Age of onset, Comorbidity Drinking patterns

Severity of disorder

Materials & methods – Subjects & Clinical Assessment• 14 087 individuals, of whom 7172 were genotyped

– (1) did you ever use marijuana?– (2) how old were you the very first time you tried marijuana (not counting the

times you took it as prescribed)? – (3) how many times in your life have you used marijuana (do not count times

when you used a drug prescribed for you and took the prescribed dose)

• Ever use was measured on a dichotomous scale (ever versus never)• Age at initiation and quantity of use were open questions

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Checking For Pruning for relatedness

Ancestry outliersHardy–Weinberg equilibrium

Mendelian errorsCall rate

Genotypic missingness (>5%)Individual missingness (>5%)

Minor allele frequency

Removing one individual from each pair with relatedness >0.05, as determined from genetic data

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Materials & methods – Subjects & Clinical Assessment• Final sample comprised 2082 ‘unrelated’ indls

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Results• Participant Characteristics – Males showed a higher rate of use than females • 53.5% vs 43.9% (P<0.001)

• Polygenic Risk Scores for Schizophrenia – Positive associations for • ever versus never use of cannabis across all P-value thresholds

– Strongest association for those SNPs with P-values of 0.01 or below in the original schizophrenia GWAS

– Positive association for • analysis of quantity of cannabis use for 9 of the 10 SNP cutoffs

– top association seen for those SNPs with P 0.05 for schizophrenia (R⩽ 2

= 0.85%, P = 0.003)

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Results• Secondary analysis – Mean polygenic risk score for within 990 twin pairs (608 dizygotic

and 382 monozygotic) where data on cannabis use of both twins was available; ordinal regression to predict whether cannabis was being used by • neither (n = 272)• one (n = 273)• or both twins (n = 445)

• Twin pairs where – both reported using cannabis had the greatest burden of

schizophrenia risk alleles– pairs with only one user were found to have an intermediate level– lowest burden was found in pairs where neither twin reported use

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Results

– Positive association of polygenic risk score with ever versus never use of cannabis across all P-value thresholds

– Positive association for analysis of quantity of cannabis use for 9 of the 10 SNP cutoffs

– Twin pairs where • both reported using cannabis had the greatest burden of

schizophrenia risk alleles• pairs with only one user were found to have an intermediate

level• lowest burden was found in pairs where neither twin reported

use34

Discussion • Results suggest that indls with an increased genetic

predisposition to schizophrenia are both – More likely to use cannabis – More likely to use greater quantities of cannabis

• This does NOT refute a causal relationship between use of cannabis and risk of Schizophrenia

• Establishes that part of the association betn cannabis and schizophrenia may be due to a causal relationship in the opposite direction

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Discussion • Variance in cannabis use explained by Schizophrenia

Polygenic Risk Score is small – Consistent with other cross-phenotype analyses • Autism, MDD, BPAD

• Further research is reqd to see whether the illustrated genetic overlap is specific to cannabis or present across illicit drug use / addiction phenotypes

• The possibility that the association may be bidirectional in causation has been supported – Risks of cannabis are being overestimated??

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Limitations

• Schizophrenia GWAS sample is likely to include more cannabis users among cases than controls – ?? An excess of SNPs associated with cannabis use may

mistakenly have been identified as Schizophrenia risk alleles

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Limitations

High Polygenic Risk Score for Schizophrenia ↓↓

??????????↓↓

Cannabis use

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Checklist for assessing the quality of quantitative studies

Criteria Yes(2) Partial(1) No(0) N/A

1 Question/ objective sufficiently described? √

2 Study design evident & appropriate? √

3 Method of subject/comparision group selection or source of information/ input variable describe and appropriate?

√

4 Subject ( and comparison group, if applicable) characteristics sufficiently described?

√

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Yes (2) Partial (1) No (0) NA

5 If interventional and random allocation was possible, was it described? √

6 If interventional and blinding of investigators was possible, was it reported? √

7 If interventional and blinding of subjects was possible, was it reported? √

8 Outcome and (if applicable) exposure measures well-defined and robust to measurement/misclassification bias? Means of assessment reported

√

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Yes (2) Partial (1) No (0) NA

9 Sample size appropriate? √10 Analytic methods

described/justified and appropriate?

√

11 Controlled for confounding? √12 Results reported in sufficient

detail? √13 Conclusions supported by the

results? √

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Take Home Message • A specific set of SNPs confer a significantly

heightened risk for Schizophrenia • There is no refuting the forward association

between cannabis use and Schizophrenia • This study has provided evidence that a heightened

genetic risk for Schizophrenia is associated with higher incidence of cannabis use

• This suggests a reciprocal relationship between Schizophrenia risk and cannabis

• We might have overrated the risk of psychosis assigned to cannabis

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References • Comprehensive Textbook of Psychiatry, 9th Ed Kaplan & Sadock

• Zammit S, Allebeck P, Andreasson S, Lundberg I, Lewis G. Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study. BMJ 2002; 325: 1199

• Fergusson DM et al (2005), Tests of causal linkages between cannabis use and psychotic symptoms Addiction,100, 354–366

• Ripke S, O'Dushlaine C, Chambert K, Moran JL, Kahler AK, Akterin S et al. Genomewide association analysis identifies 13 new risk loci for schizophrenia. Nat Genet 2013; 45: 1150–1159.

Agrawal A, Lynskey MT. The genetic epidemiology of cannabis use, abuse and dependence. Addiction 2006; 101: 801–812.

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Thank you

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