Nature and nurture in neuropsychiatric genetics: where do we stand?

2010·0.96 MB·English

by  Dick, Danielle M.

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Nature and nurture in neuropsychiatric
genetics: where do we stand?
Danielle M. Dick, PhD; Brien Riley, PhD; Kenneth S. Kendler, MD
O
ur knowledge of psychiatric and substance-use
genetics comes from two key fields of research, both
dynamic areas in rapid change. First, genetic epidemiol-
ogy asks whether there is risk in excess of the population
baseline in the relatives of cases, and, if so, whether the
excess risk is attributable to the genetic factors or the
environments they share. Beyond simply estimating her-
itability, genetic epidemiology has evolved to address
more sophisticated questions, such as whether liability
genes have the same effects across the lifespan, how they
Both genetic and nongenetic risk factors, as well as inter- may influence multiple disorders, and how they might
actions and correlations between them, are thought to interact with environmental risks.
contribute to the etiology of psychiatric and behavioral Genetic epidemiology of psychiatric and behavioral phe-
phenotypes. Genetic epidemiology consistently supports notypes has consistently demonstrated that: i) genetic
the involvement of genes in liability. Molecular genetic risk factors are, in aggregate, important etiological com-
studies have been less successful in identifying liability ponents; ii) they cannot completely account for observed
genes, but recent progress suggests that a number of spe- risk, meaning these phenotypes are multifactorial traits,
cific genes contributing to risk have been identified. with important nongenetic (or environmental) con-
Collectively, the results are complex and inconsistent, with tributing factors; and iii) the risk alleles appear to be of
a single common DNA variant in any gene influencing small effect size and to occur in a large number of genes.
risk across human populations. Few specific genetic vari- Psychiatric and behavioral phenotypes are influenced by
ants influencing risk have been unambiguously identified. a large number of risk factors that individually are within
Contemporary approaches, however, hold great promise the range of normal human variation and produce mod-
to further elucidate liability genes and variants, as well as est individual increases in risk.
their potential inter-relationships with each other and The initial goal of the second major research area, mole-
with the environment. We will review the fields of cular genetics, is to identify genes which influence these
genetic epidemiology and molecular genetics, providing
examples from the literature to illustrate the key concepts Author affiliations: Virginia Institute of Psychiatric and Behavioral Genetics;
Department of Psychiatry; Department of Human and Molecular Genetics,
emerging from this work.
Virginia Commonwealth University School of Medicine, Richmond, VA, USA
© 2010, LLS SAS Dialogues Clin Neurosci.2010;12:7-23. (Danielle M. Dick*, Brien Riley*, Kenneth S. Kendler); Department of Psychology,
Virginia Commonwealth University, Richmond, Virginia, USA (Danielle M. Dick*)
Keywords: genetics; gene-environment interaction; alcohol dependence; twin *These authors contributed equally to the review
study; schizophrenia
Address for correspondence:Kenneth S. Kendler MD, Dept Psychiatry MCV, PO
Box 980126, Richmond VA 23298, USA
(e-mail: [email protected])
Copyright © 2010 LLS SAS. All rights reserved 7 www.dialogues-cns.org
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phenotypes and to identify the specific risk variants within tribute to individual differences in risk, a statistic com-
them. There are substantial differences in DNA sequences monly referred to as the heritability of the trait. These
between individuals, and gene identification methods test study designs have been applied to virtually all psychi-
whether specific alleles at these variable positions are atric disorders and to a number of related traits, yielding
more common in affected than in unaffected individuals, compelling evidence that genetic influences play a criti-
most commonly with linkage studies (in families) and cal role in virtually all psychiatric outcomes. There is
association studies (primarily in case/controls, but also in considerable variability in the magnitude of genetic
numerous other designs). We will discuss the underlying influence across different disorders. On the high end are
causes of these two genetic phenomena, the methods for disorders such as schizophrenia, bipolar disorder, and
detecting them, and the limitations of each. autism, which yield heritability estimates of the order of
The second goal of molecular genetics is to identify spe- 80% or higher. Alcohol and other drug dependence
cific risk alleles and to use functional studies to elucidate shows moderate heritability, in the range of 50% to 60%.
how a gene functions normally, how the risk allele alters On the lower end of the spectrum, though still showing
normal function, and how these alterations contribute to significant evidence of genetic influence, are anxiety and
disease. The aim of this work is to explain the aggregate depressive disorders, as well as eating disorders, which
genetic risks observed through the effects of risk alleles yield heritability estimates of ~30% to 40%. So, while
on gene expression, protein structure and function, there is variability in the magnitude of importance of
and/or biological processes. This area remains largely genetic effects, it is widely accepted that a significant
unsuccessful to date for complex traits generally. genetic component plays a role in virtually all psychiatric
In this review we focus on the basic methods of genetic traits. It is a sign of the paradigm shift that has taken
epidemiology and molecular genetics, and provide exam- place in psychiatry that heritability estimates are no
ples, across a variety of psychiatric and substance use dis- longer considered controversial, since the original stud-
orders, of questions currently being addressed. In con- ies finding evidence for genetic effects represented
trast to this first section on genetic epidemiology, the strong challenges to predominant views favoring envi-
sections on molecular genetics focus narrowly on schiz- ronmental theories on the causation of most psychiatric
ophrenia, where there is a much longer history of mole- conditions, ranging from schizophrenia to autism to alco-
cular genetic studies, because we judged that emphasiz- hol dependence—disorders that are all now widely rec-
ing a single disorder would provide a more coherent ognized as having genetic components.
example of ongoing research progress and challenges. While demonstration of heritability played an important
role in altering fundamental assumptions about the eti-
Basic genetic epidemiology ology of psychiatric disorders, if not understood in their
proper context, heritability estimates can also have a
The most fundamental question addressed by psychiatric number of unfortunate side effects. Firstly, the heritabil-
genetic epidemiology is whether a particular trait or dis- ity statistic created a dichotomy of genetic versus envi-
order shows evidence for genetic influence. Both twin ronmental influence—nature versus nurture. How much
and adoption studies provide methods to address this is genetic? How much is environmental? This is, as we
question and tease apart the degree to which genetic and hope to show, a somewhat arbitrary distinction. Genetic
environmental influences are important on a given out- predispositions by necessity are expressed in the context
come. Twin studies accomplish this by comparisons of of the organism’s environment, and the environment can
the similarity of monozygotic twins (MZs; who share differentially affect individuals based on their unique
100% of their genetic variation), with dizygotic twins genetic makeup. Further, many environments are not
(DZs; who share on average just 50% of their genetic simply “imposed” on an individual; rather, individuals
variation). Adoption studies compare similarity among play an active role in selecting and shaping their envi-
adopted-apart biological relatives, who share genetic ronments. Accordingly, it is generally more informative
variation, but not their environments, and adoptive rel- to elucidate pathways of risk and show how genetic and
atives, who share their environment, but not their environmental influences come together in this process,
genetic makeup. Through these comparisons, we can rather than trying to divide influence into that which is
quantify the degree to which genetic influences con- genetic and that which is environmental. Secondly,
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demonstration of heritability led to the idea that there However, individuals play an active role in selecting
were genes “for” a given disorder. More complex mod- their friends, and multiple genetically informative sam-
els that have examined genetic influences across multi- ples have now demonstrated that a genetic predisposi-
ple different conditions suggest that the Diagnostic and tion toward substance use is associated with the selec-
Statistical Manual of Mental Disorders (DSM) structure tion of other friends who use substances.2-4Interestingly,
of psychiatric diagnoses often does not map onto the there is evidence that genetic effects on peer-group
underlying genetic architecture of psychiatric traits. deviance show a strong and steady increase across devel-
Genetic influences appear to be shared across many psy- opment,5 suggesting that as individuals get older and
chiatric conditions, and likely operate through mediat- have increasing opportunities to select and create their
ing characteristics that alter risk for a number of differ- own social environment, genetic factors assume increas-
ent outcomes. Finally, static heritability estimates fail to ing importance. Another area where gene-environment
capture the dynamic nature of genetic and environmen- correlation is known to play a significant role is in the
tal influences on psychiatric outcome. Heritability esti- risk pathways associated with depression. Stressful life
mates are specific to the population under study. Lost in events have been consistently associated with the man-
heritability estimates are potential differences across ifestation of depression. However, there is evidence for
environmental conditions, across populations or gender, genetic influence on the occurrence of stressful life
and across ages. Accordingly, genetic epidemiology has events,6,7indicating that an individual’s predisposition
undergone an evolution in the kinds of questions being plays a role in the likelihood that they will experience
addressed. No longer is the question simply “Are genetic difficulties that are then associated with risk for depres-
influences important on Trait X?” or even “How impor- sive episodes. For example, research has shown that a
tant are genetic influences on Trait X?”. Rather, the genetic liability to major depression increases the risk
focus has shifted to addressing the complexities raised for a range of stressful life events, particularly those
here, using the paradigm we have called advanced reflecting interpersonal and romantic difficulties.8These
genetic epidemiology. represent only a couple of areas where individuals are
known to play an active role in shaping environmental
Advanced genetic epidemiology factors that are associated with subsequent risk for psy-
chiatric problems.
Moving beyond genes versus environment: Another way that genetic and environmental influences
gene-environment interaction and correlation are linked is via gene-environment interaction or, as we
might prefer, genetic control of sensitivity to the envi-
Parsing genetic and environmental influences into sep- ronment. In these situations, genetic influences may vary
arate sources represents a necessary oversimplification, in importance as a function of environmental conditions
as for most traits we know about, genetic and environ- and/or that the environment differs in importance as a
mental influences are inexorably intertwined. Most mea- function of an individual’s genetic predisposition (these
sures of the environment show some degree of genetic two conceptualizations of gene-environment interaction
influence, illustrating the active role that individuals play are indistinguishable statistically). Heritability estimates
in selecting and creating their social worlds.1 To the essentially average across environments; accordingly, if
extent that these choices are impacted upon by an indi- there is reason to believe that the importance of genetic
vidual’s genetically influenced temperaments and behav- effects might vary as a function of the environment, this
ioral characteristics, an individual’s environment is not information can be incorporated into the twin model to
purely exogenous, but rather, in some sense, is in part an test for significant differences in heritability as a func-
extension and reflection of the individual’s genotype. tion of the environment. Substance use provides one
This concept is called gene-environment correlation or, area where gene-environment interaction effects have
perhaps more descriptively, genetic control of exposure been found to be particularly important. Environments
to the environment. It is likely an important process in that exert more social control and present less opportu-
the risk associated with several psychiatric outcomes. For nity to engage in substance use consistently show
example, there is considerable evidence for peer reduced evidence for the importance of genetic effects.
deviance being associated with adolescent substance use. In this sense, the environment is essentially constraining
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the expression of a predisposition toward substance Another area of investigation examines whether there
use/problems. This has been demonstrated with respect are differences in the importance of genetic and envi-
to enhanced parental monitoring in adolescents,9a more ronmental factors at different stages of the disorder. For
religious upbringing,10and enhanced community stabil- example, the development of substance dependence is
ity,11among other factors. One nice example of this can necessarily preceded by several stages, including the ini-
be found in an analysis of the heritability of adolescent tiation of the substance, the progression to regular use,
smoking across the United States using data from the and the subsequent development of problems, whether
National Longitudinal Study of Adolescent Health. they be psychological, social, and/or physiological. Twin
Genetic influences on daily smoking were lower in states studies can investigate the degree to which each of these
with relatively high taxes on cigarettes and in those with steps in the pathway of risk is influenced by genetic
greater controls on vending machines and cigarette and/or environmental factors, and the extent to which
advertising, again suggesting the importance of social the same or different genetic/environmental factors
control mechanisms in moderating the importance of impact different stages. For example, data from two pop-
genetic influences on substance use.12 ulation-based, longitudinal Finnish twin studies found
that shared environmental factors played a large role in
Delineating phenotypic boundaries of genetic risk initiation of alcohol use, and a more moderate role on
frequency of use, and it was largely the same influences
The rationale of the basic twin design can be expanded acting across these stages of use. However, there was no
to examine the extent to which genetic and environ- significant evidence of shared environmental influences
mental factors contribute to the co-occurrence of psy- on alcohol problems in early adulthood. Problems were
chiatric conditions. Comorbidity among psychiatric dis- largely influenced by genetic factors that overlapped
orders is common, and multivariate twin studies have with genetic influences on frequency of use.17In a study
helped address the etiological mechanisms that con- from Virginia in male twins, similar results were found
tribute to these observed epidemiological patterns. A for alcohol, cannabis, and nicotine.18In the early years of
fascinating result to emerge from these studies is that adolescence, shared environmental influences were
psychiatric conditions with distinct clinical presentations responsible for nearly all twin resemblance for levels of
(eg, major depression and anxiety) are not necessarily intake of these psychoactive substances. However, as
distinct genetically. For example, a study of major individuals aged, the impact of shared environment
depression and generalized anxiety disorder found a decreased and that of genetic factors increased.
genetic correlation of 1.0, suggesting that the same Finally, there is known to be tremendous heterogeneity
genetic influences impact depression and anxiety, but among individuals with psychiatric conditions. Twin stud-
differences in environmental experiences contribute to ies can provide insight into whether clinical hetero-
the manifestation of different outcomes.13An expanded geneity may reflect differences in etiological risk factors.
study that examined the genetic and environmental For example, alcohol dependence with comorbid drug
architecture across seven common psychiatric and sub- dependence has been found to be a particularly herita-
stance-use disorders found that genetic influences load ble form of the disorder,19,20and twin studies have sug-
broadly onto two factors that map onto internalizing dis- gested a genetic influence on typical versus atypical
orders (depression, anxiety disorders), and externalizing forms of major depression.21
disorders (alcohol and other drug dependence, child-
hood conduct problems, and adult antisocial behavior).14 Changing genetic influence across development
These findings indicate that while distinguishing these
disorders as “separate conditions” in the DSM may be Another active area of research is the clarification of
useful for clinical purposes, these categories do not nec- how genetic and environmental influences may change
essarily reflect differences in biological etiology. These across development. A recent meta-analysis examined
findings, along with similar results from phenotypic published studies with at least two heritability time
analyses (eg, refs 15,16) have led some to suggest a reor- points across adolescence and young adulthood for eight
ganization of the “metastructure” of psychiatric disor- different behavioral domains. These analyses revealed
ders in DSM-V. significant cross-time heritability increases for external-
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izing behaviors, anxiety symptoms, depressive symptoms, the extent to which differentgenes and/or environments
IQ, and social attitudes, and nonsignificant increases for may be important. Large-scale twin studies have sug-
alcohol consumption and nicotine initiation. The only gested, for example, that the genetic risk factors for both
domain that showed no evidence of heritability changes depression26and alcohol dependence,27while correlated,
across time was attention-deficit/hyperactivity disorder.22 are not entirely the same for males and females. Results
Similarly, in a large study of >11 000 pairs of twins from from two large twin studies in the US and Sweden agree
four countries, the heritability of general cognitive abil- that the genetic influences of major depression are mod-
ity was found to increase significantly and linearly from estly stronger in women than in men.26,28
41% in childhood (9 years) to 55% in adolescence (12
years) and to 66% in young adulthood (17 years).23The Do we still need twin studies in the era of
robust finding of increases in the importance of genetic gene finding?
influences across development likely reflects, in part,
active gene-environment correlation, as individuals As advances in molecular genetics and statistical analy-
increasingly select and create their own experiences sis have made it possible to conduct large-scale projects
based on their genetic propensities. aimed at identifying the specific genes involved in sus-
In addition to changes in the relative magnitude of ceptibility to psychiatric outcome (detailed in the next
importance of genetic and environmental influences, sections), some have raised questions about the contin-
another dynamic change is that different genes may be uing utility of genetic epidemiology. The argument is that
acting at different time points. This is nicely illustrated heritability has now been established, which provides the
in recent analyses of alcohol use problems, as assessed foundation and justification for moving beyond twin
at five time points from ages 19 to 28 in the Dutch Twin studies, on to large-scale gene identification projects.
Registry (Kendler et al, in preparation). Kendler and However, as detailed in this paper, most twin studies are
colleagues found strong innovation and attenuation of no longer conducted simply to test for the presence of
genetic factors across this age range—indicating that genetic effects; rather, they focus on the more complex
some genetic influences on alcohol problems that were kinds of questions summarized above. These analyses
evident at age 19 declined in importance across time, are not only informative about the nature of etiological
while new genetic influences became important starting pathways of risk, but they can also be used to guide gene
at ages 21 and 23. Thus, although the overall heritability identification efforts and to further our understanding
of alcohol problems remained fairly stable, it appeared of the risk associated with specific genes as they are
that different genetic factors were important at different identified.
timepoints. In analyses in the TCHAD Swedish study Currently, gene-finding efforts for psychiatric disorders
which followed twins from ages 9 to 20 across four waves (and other common, complex medical conditions) have
of assessment, large changes were seen in the genetic met with limited success. Findings from genetic epi-
risk factors for fears and phobias24and for symptoms of demiology can be used to inform the phenotypes used in
anxiety and depression,25with particularly pronounced gene-finding studies. For example, based on the twin lit-
evidence for genetic innovation at puberty. These analy- erature (reviewed above) suggesting that much of the
ses suggest that genetic influences of many psychiatric predisposition to alcohol dependence is via a broad
and substance use disorders are likely to be develop- externalizing factor, externalizing factor scores were cre-
mentally dynamic. ated in the Collaborative Study on the Genetics of
Alcoholism (COGA) sample, comprised of symptoms of
Sex differences alcohol and other drug dependence, and childhood and
adult antisocial behavior, as well as the personality traits
Sex differences in the prevalence of psychiatric disor- of novelty-seeking and sensation-seeking, which also
ders, and in risk and protective factors associated with index general behavioral disinhibition. This latent exter-
psychiatric outcomes, are widespread in epidemiology. nalizing factor score was then used in both linkage and
Twin studies allow us to investigate the extent to which association analyses, with results compared with analyz-
there are differences in the relative importance of ing separately the individual symptoms of each of the
genetic and environmental influences on outcome, and psychiatric disorders that went into the creation of the
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general externalizing score.29The results demonstrated with identified genes will be critical in eventually trans-
that this broader externalizing phenotype was useful in lating this information into improved prevention and
both linkage and association analyses, suggesting that intervention programs.
creating phenotypes grounded in the twin literature can
aid in identifying susceptibility genes. Twin data has also
been used to aid in genetic association studies in the area Gene identification methods
of internalizing disorders. Using data from the Virginia
Adult Twin Study of Psychiatric and Substance Use The field of psychiatric genetics has used two different
Disorders, multivariate structural equation modeling was methods to attempt to identify individual risk genes:
used to identify common genetic risk factors for major linkage and association. These are fundamentally differ-
depression, generalized anxiety disorder, panic disorder, ent approaches with different study designs applied, until
agoraphobia, social phobia, and neuroticism. Cases and recently, to very different research questions. It is impor-
controls were then identified for genetic association tant to understand both in order to understand why
studies based on scoring at the extremes of the genetic association approaches have become the norm in follow-
factor extracted from the twin analysis, with the subse- up studies of linkage regions as well as the primary cur-
quent association analyses yielding evidence for associ- rent approach in genome-wide studies.
ation with the gene GAD1.30
Another area where genetic epidemiology intersects DNA polymorphisms
with gene identification efforts is in the characterization
of risk associated with identified genes. Most major Humans are ~99.9% identical at the nucleotide level on
gene identification efforts for psychiatric disorders cur- average. Molecular genetic studies depend critically on
rently focus on adult psychiatric outcomes. As we iden- the remaining 0.1% (~3 million nucleotides) where vari-
tify genes that are reliably associated with these disor- ation occurs between individuals, collectively known as
ders, one of the next interesting research challenges will genetic polymorphisms or markers. Linkage studies gen-
be to study how risk associated with these genes unfolds erally use short tandem repeat polymorphisms (STRs).
across development and in conjunction with the envi- STR alleles are differing numbers of a repeating unit of
ronment. Here, findings from genetic epidemiology can nucleotides and have specific sequence lengths and mol-
again be useful in developing hypotheses to test the risk ecular weights as a result, allowing them to be separated
associated with specific genes. For example, based on and identified. STRs are very common and tend to be
the twin literature suggesting that adult alcohol depen- extremely polymorphic (ie, to have many alleles—where
dence and childhood externalizing symptoms overlap in an allele is one of the possible variants that exist in a
large part due to a shared genetic predisposition,31genes population at a particular genetic locus) and therefore
that were originally identified as associated with adult to have high heterozygosity (the proportion of individ-
alcohol dependence (eg, GABRA2,32CHRM233) have uals who have two different alleles at the marker locus).
been tested for association with externalizing behavior This high heterozygosity is important for linkage analy-
in younger samples of children and adolescents. These ses, which require a unique allele at each position on
studies suggest that children carrying the genetic vari- each homologous chromosome to be informative.
ants associated with alcohol problems later in life dis- In contrast, single nucleotide polymorphisms (SNPs) are
play elevated rates of conduct problems earlier in devel- changes of a single base or insertion/deletion variation
opment, before any association with alcohol up to a few nucleotides in size. SNPs generally have only
dependence has manifested.34-36Further, based on the two alleles, and have lower heterozygosity and lower
twin literatures suggesting that genetic influences on information content. Association studies tend to use
externalizing behaviors are moderated by parental SNPs as the marker of choice, because alleles of these
monitoring9 and peer deviance,37,38 further analyses markers evolve more slowly than those of STRs and pre-
demonstrated that the associations between these genes serve more of the evolutionary relationships on which
and externalizing behavior were stronger under condi- genetic association is based. SNPs can also be used for
tions of lower parental monitoring and higher peer linkage, but about ten times as many SNPs as STRs are
deviance. Characterizing the risk pathways associated required to capture the linkage information.
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Linkage seen at scales of thousands to tens of thousands of base
pairs, because the number of recombinations present in
In marker genotype data from families, new combina- the evolutionary history of a population is large, mean-
tions of alleles at a series of markers on individual chro- ing that the physical distances between loci in LD must
mosomes are observed in each generation. This recom- be correspondingly small if recombination is to occur
bination of alleles is observed because there is at least rarely (if ever) between them.
one physical exchange of material (or crossover) LD occurs because a new allele always arises on a spe-
between each homologous chromosome pair in every cific background chromosome (and its existing haplo-
meiosis (Figure 1). Recombination between loci on dif- type of marker alleles), and will, until separated by
ferent chromosomes (because of independent assort- recombination, only exist in conjunction with the other
ment of homologous chromosome pairs) or far apart on alleles present on that background. Over time, the orig-
the same chromosome (because of crossover at meiosis) inal LD (and thus the genetic association) between more
is observed 50% of the time. Linkage is observed distant loci decays as a result of recombination events,
between loci in close proximity on a chromosome while the rarity of recombination between nearby loci
because their alleles are separated by crossover less than preserves the original LD and association. Association
50% of the time. can also be detected spuriously, eg, if observed differ-
Mendelian diseases are caused by mutations in a single ences in allele frequency are due to population differ-
gene at a single chromosomal location, so disease phe- ences rather than to true association between marker
notypes can be treated as marker alleles in linkage and phenotype. Association approaches are also sub-
analysis. Because these illnesses are rare, for a dominant stantially reduced in power in the presence of allelic het-
disorder, the rare risk allele must segregate from one erogeneity (the existence of more than one risk allele at
parent (often affected or with family history) into a locus), while this phenomenon has no effect on the
affected offspring, or arise as an even rarer de novo detection of linkage.
mutation. By following the segregation of marker alle-
les from the affected lineage into offspring, linkage Challenges associated with gene identification in psy-
between markers and phenotypes can be observed when chiatric and substance-use disorders
affected offspring inherit a particular set of marker alle-
les (and thus a specific parental chromosomal segment) A number of features of psychiatric and behavioral phe-
compared with their unaffected relatives. notypes contribute to an overall reduction in study
power. Association is more powerful, generally for
Association detecting genes of small effect,39but the specific features
of psychiatric and behavioral phenotypes also reduce the
While linkage occurs in families, association is a popu- power of association studies.
lation-based phenomenon. Genetic association studies
test whether specific alleles at variable sites are more First, psychiatric phenotypes are almost certainly influ-
common in individuals affected by a disease (cases) than enced by multiple common alleles of small effect in many
individuals not affected by the disease (controls). This genes. Both linkage and association study designs are
association between allele and phenotype can occur for more powerful for alleles of large effect size, and are
two reasons. Either the allele being studied directly influ- much less powerful when examining highly polygenic
ences risk for the disorder or, more commonly, the allele phenotypes. Replication studies are hampered by the
is in linkage disequilibrium (LD) with the disease-pre- need for sample sizes larger than the discovery sample
disposing allele. Linkage disequilibrium means that spe- (in order to maintain power) and stochastic sampling
cific alleles at two nearby loci tend to occur together in variation, the expected variation in the extent to which
an entire population. Linkage, (the cosegregation of a any specific risk factor is present (and association
chromosome region and a disease observed in families), detectable) in any particular sample.
occurs at scales of tens of millions of base pairs because
of the limited number of recombinations observed in Second, interactions between genes (GxG) or between
each generation of a family. Association (and LD) are genes and environmental variables (GxE) seem necessary
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to account for observed risks, but we rely heavily on ana- 8p22-p21, 6p24-p22, and 1q32-42. Two additional regions
lytic approaches that assess single genes. In a few cases, with little support in the primary literature, 2p11.1-q21.1
genes with known molecular interactions with the can- and 3p25.3-p22.1, were among the most significant in a
didates have also generated replicated association. meta-analysis of schizophrenia genome scans. A number
Environmental risk factors remain largely unknown and of other regions (including 5q22-q31 and 15q13-q14)
are difficult or very expensive to test in many samples. have less strong summary evidence but also overlap with
evidence from more recent GWAS and structural varia-
Third, these phenotypes are common, so the liability alle- tion studies.
les seem likely to be common, although increased rates of
rare deletions and duplications (structural or copy num- Chromosome 22q, the VCFS microdeletion,
ber variants) in cases have been observed multiple times and COMT
and suggest that rare variation may also contribute to
risk in a proportion of cases. The common risk variants Chromosome 22q has been widely studied using many
are expected to occur with relatively high frequency in different designs. Primary linkage signals were observed
the general population, reducing contrast between in a few samples but have generally been widely repli-
affected and unaffected individuals and reducing power. cated. However, the cosegregation of a known
The impact of individual rare structural variants in the microdeletion in the region with a phenotype in which
subset of cases where they are observed is harder to psychosis is a common feature added significantly to
assess currently, but the observation of an aggregate interest in this region. Velo-cardio-facial syndrome
increase appears robust, further increasing the apparent (VCFS) is caused by two overlapping, recurrent dele-
etiological complexity. tions at 22q11. Historically, about 10% of VCFS patients
were thought to present with a psychotic phenotype, but
Fourth, the expected frequency of risk alleles and the more recent studies suggest much higher rates of 25%
clinical variability in presentation, course, and outcome to 29%.42,43Conversely, preliminary results suggest that
suggest that the etiology of individual cases may be het- about 2% of adult onset and 6% of childhood onset
erogeneous, derived from different specific genes or alle- schizophrenic patients have microdeletions in this
les between individuals. Allelic heterogeneity substan- region, in excess of the estimated general population fre-
tially reduces the power of association designs. quency of such deletions of 0.025%.44 Interest in this
region has been further increased recently by studies
Fifth, diagnostic boundaries are difficult to draw, and the assessing structural variation (see below). The gene for
best phenotype to study is a complex choice. It is criti- catechol-O-methyl transferase (COMT), involved in the
cally important to consider this last point and the phe- degradation of catecholamines, maps to this region; the
notypes that yield the strongest evidence in some detail. enzyme is functionally polymorphic with a variable
amino acid, Val158Met, affecting activity. Although
An example: schizophrenia widely studied, the results from genetic studies of
gene identification COMTare inconclusive as reviewed recently.45
Through 2004, 25 complete or nearly complete genome Chromosome 8p22-p21, NRG1, and ERBB4
scans for schizophrenia (in which about 400 individual
genetic markers are genotyped at regular intervals over Studies of pedigrees from numerous different ethnic
the entire human genome) were published (for review backgrounds have detected linkage to schizophrenia on
see refs 40,41). None provided evidence for genes of 8p, as did a statistically robust meta-analysis.46Although
major effect. Some linkage regions were replicated in numerous samples support a locus on 8p, comparison
these studies, and a number of promising genes emerged between individual studies is consistent with the pres-
from sequential linkage and association studies and mul- ence of multiple susceptibility genes, a feature of a num-
tiple replication reports. We focus here on those regions ber of linkage regions. Almost certainly the most impor-
with the best replication record and with evidence tant result on 8p so far is the widely replicated
emerging from other contemporary studies: 22q12-q13, association with the neuregulin 1 (NRG1) gene in fami-
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Nature and nurture in neuropsychiatric genetics - Dick et al Dialogues in Clinical Neuroscience - Vol 12 .No. 1 .2010
lies and case/controls from Iceland.47NRG1is a large genome coverage (only ~6%) and lack of replication
gene with multiple transcripts yielding distinct protein reduced the apparent importance of these findings. The
molecules. It is expressed at central nervous system first strong evidence for linkage of schizophrenia to the
synapses and is involved in the expression and activation 6p region came from studies of Irish families with a high
of neurotransmitter (including glutamate) receptors. density of disease.80 This study was also important
Initial replication studies48,49detected association on hap- because it addressed the question of diagnostic bound-
lotypes identical or closely related to those identified in aries in some detail. Evidence for linkage was modest
the Icelandic cases; 13 additional studies in multiple pop- under a narrow diagnostic model, increased substantially
ulations reported association with more variation in as the diagnostic definition broadened to include psy-
associated alleles or haplotypes,50-62while nine studies did chosis spectrum disorders, and fell when the definition
not.63-71A meta-analysis of studies of NRG1supported was broadened further to include nonspectrum disor-
involvement of the gene in schizophrenia liability, but ders, in keeping with observed risks in relatives for these
did not provide evidence supporting association of the traits. Multiple independent studies of this region of 6p
most prominent marker in the original studies.72In a pat- observed evidence for linkage, as did a multicenter col-
tern observed for a number of the best supported schiz- laborative study81and a robust meta-analysis.46
ophrenia genes, several studies have also shown associ- The dystrobrevin binding protein 1 or dysbindin
ation between NRG1and bipolar disorder.62,73,74 (DTNBP1) gene was first reported to be associated in
ErbB4, encoded by the ERBB4gene, is a receptor for the same Irish families.82,83Many studies support associ-
NRG1 and has important roles in neurodevelopment ation in DTNBP1in samples from diverse ethnic back-
and the modulation of NMDA receptor functioning. grounds although the markers, alleles and haplotypes
Both activation of ErbB4 and suppression of NMDA associated vary significantly from study to study: 13 stud-
receptor activation by NRG1 are increased in the pre- ies of 15 independent samples reported significant pos-
frontal cortex in individuals with schizophrenia com- itive association with schizophrenia (most consistently
pared with controls.75 This functional relationship with common alleles and the highest frequency common
prompted genetic study of ERBB4, which demonstrated allele haplotype),70,82-93while 14 studies of 18 independent
association in ERBB4and evidence of interaction with samples did not.61,63,85,94-104A further four studies have also
NRG1.59,76-78Associated alleles in ERBB4 alter splice- provided positive evidence for association of DTNBP1
variant expression79and both NRG1 and ErbB4 protein with bipolar disorder.105-108 Although the function of
are increased in the brain in schizophrenia. These results DTNBP1 in brain is unknown, both RNA109 and pro-
may be of particular importance as there is a biologically tein110expression is reduced in cases.
plausible mechanism for gene x gene interactions, and
even if the interaction is not confirmed, both genes Chromosome 1q and DISC1
impact the glutamatergic system (supporting the widely
held view that part of the complexity may be explained Interest in chromosome 1 in schizophrenia began with
by effects at the level of the pathway or system). reports of a balanced 1:11 translocation segregating with
Important tests of both interaction and system effects serious mental illness in a large pedigree from
unbiased by candidate selection will be undertaken in Scotland.111The chromosome 1 breakpoint lies at 1q42.1,
the current GWAS datasets. and the breakpoint directly disrupts a novel gene,
Disrupted in Schizophrenia 1 (DISC1).112There are now
Chromosome 6p24-p22, DTNBP1, and the nine positive reports of association of DISC1with schiz-
HLA region ophrenia74,113-120and 2 of association with positive symp-
toms121,122 suggesting that this gene influences schizo-
Chromosome 6 has a long history in genetic studies of phrenia liability in the general population, as well as in
schizophrenia with major shifts in the apparent impor- the family with the chromosomal anomaly. Other rare
tance of particular results. Early linkage studies observed variants in this gene besides the breakpoint have also
evidence of linkage in human leukocyte antigen (HLA) been reported to be associated with schizophrenia123,124
genes in the major histocompatibility complex (MHC) and association has been reported for additional psy-
region on chromosome 6p21.3-22.1, but the limited chiatric diagnoses, reviewed in ref 125, and for bipolar
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S t a t e o f t h e a r t
disorder.126A smaller number of negative reports have study.131One replication attempt supported association
also been published.103,127-130 in IL3R.155The third, using the CATIE156 sample (738
cases, 733 controls), did not detect any genome-wide sig-
Other chromosomal regions and genes nificant results in its primary analysis.157The fourth, using
a multistage design of discovery (479 cases, 2937 con-
Two additional chromosome regions, 5q22-q31, where trols) and targeted replication (6666 cases, 9897 controls)
association was recently reported in the interleukin-3 samples, identified one genome-wide significant SNP in
(IL3) gene131and 15q13-q14, where evidence for linkage the zinc-finger protein transcription factor ZNF804A
of an evoked potential abnormality common in gene,158but only in the meta-analysis including the orig-
patients132 was supported by five additional studies inal sample. One independent replication attempt sup-
reporting linkage of schizophrenia to the same narrow ported the association of ZNF804A, and showed that
region,133-137show some overlap with the results of cur- expression was increased from the associated haplo-
rent studies discussed below. Other high-profile candi- type.159
date genes such as PRODH2on 22q138and PPP3CCon Three substantially larger GWAS of schizophrenia were
8p139have not replicated well. One exception is AKT1,140 published in 2009, in the SGENE+ sample160(multiple
which has similar numbers of positive141-145 and nega- European sites, 2663 cases/13498 controls), the
tive61,103,146-149replications. International Schizophrenia Consortium (ISC) sample161
(multiple European sites, 3322 cases/3587 controls) and
Genome-wide association studies the Molecular Genetics of Schizophrenia (MGS) sam-
ple162 (multiple US sites, European ancestry: 2681
By assaying 500 000 to 1 000 000 DNA variants in a sin- cases/2653 controls; African ancestry: 1286 cases/973
gle experiment, GWAS provide unbiased genome-wide controls), analyzed both separately and together. The
coverage, avoiding selection of candidate genes. They use one region of the genome with significant overlap in sig-
an association framework for analysis, avoiding the nals from the 3 studies was the MHC region on chro-
weaknesses of linkage in complex traits. They impose mosome 6p21.3-p22.1, site of some of the earliest genetic
stringent criteria due to the number of tests performed evidence in schizophrenia discussed above. The
(typically around P<5 x 10-8 for genome-wide signifi- SGENE+ sample detected significant association with
cance). They hold enormous potential to move beyond several markers spanning the MHC region, as well as
the identification of single genes (which may show small signals upstream of the neurogranin (NRGN) gene on
effects and be difficult to detect individually) toward the 11q24.2 and in intron four of the transcription factor 4
simultaneous identification of multiple genes through (TCF4) gene on 18q21.2. The ISC sample detected asso-
their interactions or involvement in systems. ciation in ~450 SNPs spanning the MHC region and the
Seven GWAS of schizophrenia have been published to myosin XVIIIB (MYO18B) gene on 22q and supported
date, four of which were small and underpowered. The ZNF804A. The MGS sample did not detect any individ-
first (320 cases, 325 controls) was of limited density as it ual genome-wide significant signals, but detected signals
genotyped only 25 000 SNPs in 14 000 known genes, and in the range of 10-5-10-7in the CENTG2gene (reported
did not detect any association that reached genome-wide deleted in autism cases163) on chromosome 2q37.2 and
significance150; nominal association was reported in the JARID2(the gene adjacent to DTNBP1) in European-
plexin A2 (PLXNA2) gene. Only one of four samples ancestry subjects, and in ERBB4and NRG1in African-
tested in three independent studies replicates the asso- American subjects.
ciation.151-153The second (extremely underpowered with Meta-analysis of data from all European-ancestry MGS,
178 cases, 144 controls) identified one genome-wide sig- ISC and SGENE samples detected genome-wide signif-
nificant association in the X/Y pseudoautosomal region icant association signals for 7 SNPs spanning 209 Kb of
(a homologous region of the sex chromosomes where the MHC region. LD is high between the 7 SNPs and
recombination can occur), near the interleukin 3 recep- extends over a region of 1.5 Mb on chromosome 6p22.1,
tor (IL3R) gene.154 Cytokines have been suggested as making it difficult to determine if the signal is driven by
possible candidates previously and IL3(in the 5q link- one or many genes. The genic content of this region is not
age region) was associated with schizophrenia in one limited to histocompatibility loci, and also includes genes
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