Genomic studies of schizophrenia: mapping madness?

1
Genomic studies of schizophrenia mapping madness?

• Mike Owen, Department of Psychological Medicine,
  Wales College of Medicine, Cardiff University, UK.

2
Genetic epidemiology of schizophrenia

• Familial, ?s 10
• Individual differences in liability are largely
  genetic
• Heritability 0.6-0.9
• Non-genetic factors also important
• Multi-locus model consistently supported by
  analysis of family data
• Genes with population ?s gt 3 unlikely
• Number of susceptibility loci, degree of risk
  conferred by each and degree of interaction all
  unknown

3
Finding genes for schizophrenia

• Linkage
• Chromosomal abnormalities
• Association
• Convergent genomics

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Genome wide significant
Multiple positives
Published linkage data 2006
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Cardiff schizophrenia sib-pair genome screen
VCFS
CNP
351 microsatellite markers (10.3 cM) in 354
affected sib-pairs 179 (UK), 134 (Sweden), 41
(USA)
Williams et al. Am J Hum Genet 2003
6
Linkage studies of schizophrenia 2006

• Data consistent with observed recurrence risks in
  relatives
• Meta-analysis suggests some consensus
• Many underpowered studies on uncertain value

• Larger studies needed for more genes, stronger
  evidence, interactions and better location
• 800-1000 ASPs optimal
• A number of groups are now seeking genes in
  currently positive regions

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Positional genetics and schizophrenia

• Positional Candidates
• NRG1, 8p 1
• DTNBP1, 6p 2
• G72 (DAOA), 13q 3
• Multiple positive studies as well as negative
• No clear pattern of associated alleles or
  haplotypes
• Risk variants not identified

1. Stefansson et al. 2002 2. Straub et al. 2002
3. Chumakov et al. 2002
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Positional candidate genes for schizophrenia
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NRG1 association with schizophrenia
Tosato, Dazzan and Collier, Schiz. Bull. 2005.
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DTNBP1 complex pattern of association findings
in schizophrenia
Williams, ODonovan and Owen, Schiz Bull, 2005,
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DTNBP1 and schizophrenia

• Multiple studies suggest that variation in DTNBP
  is associated with schizophrenia (11/14
  positive).
• Protection may be mediated by IQ.
• No individual SNPs or haplotypes have
  consistently been implicated in susceptibility.
• No systematic study aimed at detecting all common
  genetic variation.
• More studies needed to identify risk variants.
• Reduced expression of message and protein in
  schizophrenia cause or compensation?
• Cis-acting elements regulate DTNBP expression.
• Can we relate specific haplotypes to gene
  expression?

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DTNBP1 risk haplotype is associated with reduced
expression

• Allele ratios at SNP rs1047631, stratified by
  heterozygosity for the defined schizophrenia risk
  haplotype.
• Schizophrenia risk haplotype tags one or more
  cis-acting variants that result in a relative
  reduction in DTNBP1 mRNA expression in human
  cerebral cortex.
• Further analyses suggest that risk haplotypes
  identified in other Caucasian samples also index
  lower DTNBP1 expression.
• Ties risk haplotypes to altered function
• Suggests explanation for some of the
  discrepancies between studies

Bray et al, Human Molecular Genetics, 2005.
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Chromosomal abnormalities and schizophrenia
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VCFS and Schizophrenia

• 25 of adults with VCFS develop schizophrenia
  (Murphy et al 2000)
• Does a gene on 22q11 predispose to schizophrenia
  in the general population?
• Some claims (COMT, PRODH, ZDHHC8, ARVCF) but none
  convincing

15
DISC1 and schizophrenia (Porteous, Millar,
Blackwood, St Clair et al)

• Possibility of position effect
• Evidence for haplotype association with SZ
• Associations with visual and working memory
  deficits
• t (1,16) disrupts PDE4B (binding partner of
  DISC1) in family with psychosis (Millar et al
  2005)
• DISC1 complex protein associated with numerous
  cytoskeletal proteins involved in centrosomal and
  microtubule function, and with cell migration,
  neurite outgrowth, and membrane trafficking of
  receptors and possibly mitochondrial function.

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Candidate genes for schizophrenia

• NRG1 and DTNBP1 multiple positive studies.
• DAOA(G72) some positives for both schizophrenia
  and bipolar disorder
• DISC1 highly promising.
• Effect sizes small-moderate (OR 1.3-2)
• RGS4 some positives but support weakening.
• COMT, PRODH, ZDHHC8, PPP3CC, CAPON, CHRNA7, TRAR4
  and others not yet convincing.
• Cannot be explained by variants with manifest
  functional consequences.
• Presumably effects on expression, splicing etc.
• Remain cautious until risk variants identified.
• Inconsistencies between studies.
• Lack of power
• Genotyping error
• Stratification
• Incomplete genetic information from indirect
  studies
• NB dysbindin
• Susceptibility variants on different
  backgrounds??
• Allelic heterogeneity/complexity
• Heterogeneity differences in case definition and
  ascertainment

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Emil Kraepelin, 1896, splits psychosis.

• crystallized dementia praecox and
  manic-depressive illness from an amorphous mass
  of madness (Brockington Leff, 1979).
• Organic
• Functional
• Dementia Praecox
• Manic-depressive insanity

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Deconstructing Psychosis Challenges to the
Kraepelinian Dichotomy.

• No point of rarity
• Risk factors in common (Murray et al 2004)
• Familial co-occurrence of SZP, SA and BP
• Cardno et al twin study
• Overlapping linkage regions
• 13q, 22q, 6q
• New genetic studies confirm this and suggest
  association with clinical syndromes.

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Studying candidate genes across the Kraepelinian
divide Dysbindin.

• No association between BP and the Cardiff
  haplotype in DTNBP1.
• Suggestive evidence for association with BP with
  predominant psychosis.

Upward trend p 0.014
Raybould et al, Biological Psychiatry, 57
696-701, 2005.
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Studying candidate genes across the Kraepelinian
divide Neuregulin.

• NRG1 HAPICE confers risk to illness with both
  schizophrenia and mood features.
• Effect size of NRG1 HAPICE increases with
  preponderance of mood-incongruent psychotic
  symptoms (sign test p0.002).

Green et al, Archives of General Psychiatry, 2005.
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Studying candidate genes across the Kraepelinian
divide G72 (DAOA).

• Some positive replications of G72 but no clear
  associated alleles or haplotypes.
• Independent support for association with Bipolar
  disorder in several studies.
• G72 probably strongest candidate gene for BP.
• Is this a gene for both disorders?

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DAOA (G72) in schizophrenia and bipolar disorder

• Significant whole gene association in BP (n706,
  p0.045) but not SZ (n709) vs controls (n1416).
  
• Significant whole gene association in Mood (n
  1153, p0.0086) and in schizophrenia-mood (n112,
  p0.02) but not psychosis (n818).
• DAOA is probably a susceptibility locus for mood
  disorder rather than schizophrenia per se.
• Extent to which association seen in schizophrenia
  depends upon clinical characteristics of sample.

(Williams et al Archives of General Psychiatry,
2006).
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DISC1 is associated with broad psychosis and mood
phenotype.

• t(111) segregates with Sz, BP and UP.
• Linkage to SA
• Hamshere et al 2005.
• Evidence for haplotype association
• Hennah et al 2003
• Hodgkinson et al 2004
• Schizophrenia, SA and BP
• Thomson et al 2005
• SZ and BP.

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Using genetics to dissect psychosis
Craddock, ODonovan and Owen, Schizophrenia
Bulletin, 2006.
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Do genetic findings in psychosis point to a
common mechanism?

• The genes most clearly implicated (NRG1, DTNBP1,
  G72) all code for proteins that potentially
  impact, directly or indirectly, on the function
  of glutamate synapses.
• Harrison and Owen, Lancet, 2003.
• But caution required!
• proteins implicated poorly understood
• multiple processes implicated for NRG1 and DTNBP1

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White matter abnormalities in schizophrenia (Mt.
Sinai Conte Center)

• Imaging studies
• Defects of connectivity
• DTI
• Multiple gene expression studies in postmortem
  schizophrenia brains have found significant
  reduction of expression of myelin and
  oligodendrocyte related genes (e.g. Hakak et al.,
  2001)
• Quantitative anatomical analyses have
  demonstrated decreased oligodendrocyte numbers in
  prefrontal cortex (Hof et al 2002, 2003).
• Cause or effect?

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• CNP is marker for oligodendrocytes
• Message and protein show reduced brain expression
  in schizophrenia
• Located at 17q21.2
• rs2070106 is associated with CNP expression
  (P0.001).
• the lower-expressing allele was significantly
  associated with schizophrenia (P.04) in a
  case-control sample.
• All affected individuals in a linked pedigree
  were homozygous for the lower-expression allele
  (P.03).

Archives of General Psychiatry (2006) 63 18-24.
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Oligodendrocyte Lineage Transcription Factor 2
(OLIG2) a master regulator of all stages of
oligodendrocyte lineage.

• Basic helixloophelix transcription factor
  central to oligodendrocyte development
• Down-regulated in schizophrenia (Tchakev et al,
  2003 Katsel et al, 2005 Iwamoto et al, 2005)
• Centrality in OL allows for a primary change
  responsible for several others (parsimony)

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OLIG2 associated with schizophrenia non
redundant (r2lt0.9) positives (submitted).
Meff 9 (all 16 pooled SNPs) gene
corrected p 0.0009 Experiment-wide corrections

p 0.013 (primary 14 genes) p 0.038 (all
44 genes)
CONSERVATIVE
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Conclusions

• Some highly promising findings (NRG1, DTNBP1,
  G72/DAOA, DISC1)
• Need to establish risk nucleotides/mechanisms
• And this might not be easy without functional
  readout (endophenotypic, animal, cellular)
• Needs more detailed studies, collaboration,
  re-sequencing
• Expect allelic heterogeneity, effects of
  ascertainment
• Refining the associated phenotype by iteration
  (Symptoms, Course and outcome, Cognitive
  function, Imaging). NB samples.
• Curation of association data (its epidemiology)
• Quality standards
• Meta-analysis
• Need to support genetic associations with biology
• DTNBP1 and NRG1 expression studies

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Future

• There are more significant linkages to account
  for.
• Few if any exhaustive fine mapping studies
• WGAs
• Depend on CDCV hypothesis
• Need very large samples
• Sample characteristics will be crucial
• Data handling and statistical challenges are huge
• WTCCC
• CNV analysis
• Candidate pathway analysis (GxG)
• Inclusion of E in genetic studies (GxE)
• WGS (CDRV)
• Understanding gene regulation (identifying
  regulatory sequences, miRNA, chromatin effects
  etc)

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Acknowledgements

• N Williams
• N Norton
• H Williams
• N Bray
• A Preece
• J Wilkinson
• S Dwyer
• Elaine Green
• Rachel Raybould
• Detelina Grozeva
• T Peirce
• B Glaser
• L Carroll

• M ODonovan
• N Craddock
• G Kirov
• I Jones
• M Hamshere, P Holmans. S Macgregor, V Moskvina,