Schizophrenia

Chapter 10

Psychopathology

• Sooner or later, half the population suffers a serious run-in– Reactive or endogenous

• Very expensive

• Historically, known to run in families

Schizophrenia

• Lifetime risk ~1%

• Long-term thought disorder– Cognitive, emotional, motivational

• Chronic and acute forms

• Late adolescence to early adult onset

• Earlier onset, the worse the prognosis

• Very expensive (10% of homelessness)

Schizophrenia

• Disorganized thinking, hallucinations, delusions, blunted affect, poverty of speech, lack of motivation

• Subtypes– Paranoid, Disorganized, Catatonic,

Undifferentiated

Family Risk

• Runs in families• As r-value increases, so does probability of having

schizophrenia• But not strictly hereditary

– Cousins, 4%– Siblings, 9%– Dizygotic twins, 17%– Monozygotic twins, 48%– Parents of a schizophrenic, 6%– Offspring of schizophrenic parent, 13%

Diversity

• Risk is heritable, but subtypes (e.g., catatonic vs. paranoid) are not– So, is this one disorder or several?

• More severe forms are more heritable• Type-I (hallucinations) have better prognosis;

type-II (withdrawal, blunted affect) is more severe and more heritable

• Some symptom dimensions (e.g., disorganization) more heritable than others

Diathesis-Stress Model

• Individual may have genes for schizophrenia, but only phenotypically express them after particular stressful life event

• Genes create predisposition

Identification of Genes

• Very active area of research

• Numerous candidates genes proposed

• Highly polygenic

• So far, primary candidate genes proposed on chromosomes 2, 5, 6, 8, 13, 22

• Results very difficult to interpret, though

NOTCH4

• On chromosome 6p (also containing some HLAs)• Member of a type 1 transmembrane protein family• Receptor for membrane bound ligands• Roles in variety of developmental processes by

controlling cell fate decisions, including post-mitotic differentiation of cortical neurons

• Evolutionarily conserved intercellular signaling pathway (e.g., homolog to human gene in Drosophila)

NOTCH4

• Wei & Hemmings (2000)• Association of schizophrenia with 4 SNPs in the

NOTCH4 gene• Highly significant association results for 3 of the

SNPs in a sample of 80 British parent-offspring trios

Follow-up Studies• Only two with Caucasians (Prasad et al., 2004;

Skol et al., 2003) showed weak to modest associations for 2 of the SNP markers, and

• Six studies with Caucasians (e.g., Anttila et al., 2003; Luo et al. 2004) found no associations;

• Two studies of African samples found associations (Luo et al. 2004; Skol et al. 2003)

• But, these studies focused only on a small portion of the gene (5´ promoter region and first exon); additional sites throughout gene need study

Liu et al. (2007)

• Scanned entire genomic region of NOTCH4 gene using 14 SNPs, 7 of which were validated (minor allele frequency over 10%)

• Used relatively large family sample of schizophrenia (218 families with at least two affected siblings)

• Asian sample

Results

• Evidence of association with distal genomic region of NOTCH4

• Failed to find association with 4 SNPs from 5´ region

• Weak to modest association found for a few other previously reported SNPs throughout the gene

Why Such Poor Replication

• Clinical and genetic heterogeneity of schiz.• Ethnicity

– Caucasian and African samples show some evidence for proximal region of NOTCH4

– Asian samples don’t, but show evidence for distal region of NOTCH4

• Could be two or more disease-underlying variants at NOTCH4 locus (see Zhang et al. 2004)

SNPs and Ethnicity

• Each SNP has its own genetic ancestral heritage

• Frequencies of each variant may differ between ethnic populations

• Remember, synonymous vs. non-synonymous SNPs

COMT

• Catechol-O-methyl transferase• Enzyme degrading dopamine, epinephrine,

norepinephrine• A non-synonymous SNP (substituting valine for

methionine) affects cognitive tasks (set shifting, set inhibition, abstract thought) by reducing dopamine at four times the regular rate

• Neurons with mutation need higher levels of activation to release normal levels of dopamine post-synaptically

Interesting Interaction

• Caspi et al. (2004)

• Cannabis use linked to twofold increase in late onset schizophrenia

• Majority of young users do not develop any psychosis

• Suggests some individuals vulnerable to its effect

• Gene-environment interaction

Candidate Gene

• COMT gene on chromosome 22q11; region implicated in genome scans for schizophrenia

• Dopaminergic function disturbances implicated in schizophrenia (e.g., Kapur 2003)

• Valine (V) for methionine (M) substitution• Genotypes: M/M (“wildtype”) has lowest

COMT activity, V/V highest, M/V intermediate (co-dominant alleles)

Outcome

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6 20

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M/M V/M V/VCOMT genotype

No adolescent cannabis use

Adolescent cannabis use

And so…

• Evidence that adolescent, but not adult, cannabis use is associated with schizophrenia through V-M polymorphism

• May be limited to a sensitive period of brain development

• Study is not identifying a major cause of schizophrenia

• Supports COMT V-M functional polymorphism as having a role in psychosis, but perhaps only in context of exposure to environmental pathogens

Estrogen

• Sex differences in schizophrenia• Affected females have better course of disease

than males• Interestingly, women show a second peak-of-onset

post-menopause (not seen in men)• Clinical observations of affected women show

increased symptoms when estrogen levels are low (pre-menstrual, post-partum, post-menopause) and remission of symptoms when estrogen high (e.g., pregnancy)

Martorell et al. (2008)

• Study to evaluate hypothesis that estrogen receptor genes ESR1 and ESR2 are involved in schizophrenia onset

• Also genes APOE and COMT, both regulated by estrogen receptors could also be involved

Method

• Analyzed SNPs– 26 in ESR1, 14 in ESR2, 7 in APOE, 12 in

COMT

• Allele frequencies evaluated in 585 schizophrenics and 615 controls

• Both male and female subjects, but grouped and sex-separated analysis performed

Findings• Analysis failed to find any significant associations

between each of the candidate genes and the diagnosis of schizophrenia

• A little unexpected, as various earlier animal models indicate role of estrogen genes in dopamine function– Also, Shifman et al. (2002) found association between

3 COMT SNPs and schizophrenia in Ashkenazi Jews; ethnic variability?

• Worth considering previous study’s findings that exposure to environmental toxins may be relevant factor

Zinkstok et al. (2008)

• Used MRIs to examine gray and white matter density and volume differences between genotype groups for COMT and another gene, PRODH

• 51 schizophrenic patients scanned and genotyped• Hypothesize COMT and PRODH polymorphisms

may result in structural and functional brain abnormalities

Findings

• A SNP in the promoter region of COMT is associated with a gray matter increase in the right superior temporal gurus

• Two nonsynonymous SNPs in PRODH gene associated with reductions in white matter density reductions in frontal lobes

• Interestingly, evidence for COMT and PRODH epistasis– Only patients with a COMT valine allele and one or two

mutated PRODH alleles showed increased white matter density in left inferior frontal lobes

So Where Are We?

• Lots of suggestions for candidate genes and SNP effects

• Inconsistent findings across many studies• Very complicated• Individual effects by genes most likely small and

highly variable due to variability of genotype, environmental interactions, epistasis

• So far, results not terribly conclusive• Is there another approach?

Endophenotype• Psychiatric concept; biomarker

– Associated with illness– Heritable– Stable over time– Within families, endophenotype and illness co-segregate– Found in relatives of affected individual at higher rate than

in general population

• Divide behavioural symptoms into more stable phenotypes with clear genetic connection

• Overt symptom is a psychosis; underlying phenotype are endophenotype. e.g., sensory gating and decline in working memory (the endophenotypes)

Braff, Schork & Gottesman (2007)

• Argue for endophenotype strategy to understand genetic architecture of schizophrenia

• The ultimate endophenotypes are perturbed levels of specific proteins or gene expression

• But this level of analysis not yet possible for schizophrenia

• So, use neurophysiological and neurocognitive measures– Ones that reflect more elementary aspects of the

biology than clinical features themselves

Gur et al. (2007)

• Examines computerized neurocognitive measures as candidate endophenotypic markers

• Multiplex multigenerational families– Beneficial because power to detect genes for

quantitative traits through linkage analysis increases with family size (better than sibpairs)

– 349 European Americans from 35 multiplex multigenerational families (58 patients, 291 relatives) and 154 psychiatrically healthy European Americans (control comparator group)

Approach

• Heterogeneity of schizophrenia at phenotypic and genotypic levels

• Endophenotypes genetically simpler than disease endpoints; an advantage (big one!)

• Can measure such quantitative parameters in family members where clinical diagnosis may be absent or difficult to gain

• If neurocognitive deficits are associated with genetic liability, they should increase with relation to affected individuals

Measures

• Diagnostic assessment• Computerized neurocognitive “scan”

– Abstraction and mental flexibility– Attention– Verbal memory– Face memory– Spatial memory– Spatial processing– Sensorimotor dexterity– Emotional processing

0.5

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Accuracy

Speed

***

**

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*** *** ******

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Comparison subjects (N=154) Relatives of schizophrenic patient (N=291) Schizophrenia patients (N=58)

Abstraction/ flexibility

Attention Verbalmemory

Facememory

Spatialmemory

Spatialprocessing

Sensorimotor Emotionidentification

*, **, ** p<0.5, p<0.01, p<0.001, respectively, vs. comparison subjects

***

Interpretation• Schizophrenics show deficits on various

neurocognitive tasks• Relatives (both 1st and 2nd degree) without

schizophrenia also show impairments in specific tasks– Extended family tested; can analyze based on shared genes,

but won’t have shared environment– Advantage of the multigenerational design. – Parent-sib and co-sib studies can’t separate out shared

environment from certain genetic effects

• Supports neurocognitive measures as endophenotypic markers of vulnerability to schizophrenia

• Target endophenotypic markers for gene identification

Hall et al. (2007)

• Event-related potentials as candidate endophenotypes– P300 (reduced amplitude and prolonged latency)– P50 (reduction of inhibitory response)– Mismatch negativity (reduced amplitude)

• Objectives– Estimate heritabilities for range of event-related potential

indices– Quantify strength of relationship of each index with

schizophrenia– Examine genetic and environmental overlap with the

illness

ERPs• Reliably measured using electroencephalography (EEG)• Deflections in voltage in brain activity after a stimulus

presentation• Reflect “higher” cognitive processes

– Memory, expectation, attention, etc.

• Positive (P), negative (N)– N400: negative voltage deflection 400ms after stimulus

– P300: positive deflection, 300ms delay (this one responds to unexpected/novel/cognitively salient stimuli across senory/perceptual domains)

Twin Design

• 16 MZ twins concordant for schizophrenia

• 9 MZ twins discordant for schizophrenia

• 45 normal MZ twins

• 32 normal DZ twins

ResultsC

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MismatchNegativityAmplitude

P300Amplitude

P300Latency

P50Suppression Ratio

In MZ twins concordant or discordant for schizophrenia (N = 50) and healthy comparison twins (N=154)

Breakdown

Mismatch P300 P300 P50Negativity Amplitude Latency Suppression

Type of Correlation Amplitude Ratio

Phenotypic correlation 0.31 -0.35 0.35 0.40

Due to additive genetic factors 0.24 -0.35 0.18 0.40

Due to shared environmental factors 0.01 0.05 0.13 0.00

Due to unique environmental factors 0.06 -0.05 0.03 -0.01

P50

• Attractive ERP endophenotype– Little environmental variance (except measurement

error)– Highest phenotypic correlation with schizophrenia– Almost all the correlation explained by genetic factors

• Measure of sensory inhibition in brain; individual’s ability to filter out repetitive stimuli to avoid information overload

• Linked to alpha-7 nicotinic receptor gene (CHRNA7)