One of the most interesting as well as challenging observations has been the degree of phenotypic variability associated with individual CNVs, i.e. the “expressivity” of the genotype. Virtually every CNV allele that is associated with a psychiatric disorder is present at a low frequency in populations of healthy controls, and virtually every CNV is also associated with a wide variety of other neuropsychiatric or neurodevelopmental conditions including bipolar disorder, seizure disorder, intellectual disability, attention deficit hyperactivity disorder (ADHD) etc (Cooper et al., 2011
; Elia et al., 2011
; Girirajan and Eichler, 2010
; Sahoo et al., 2011
; Williams et al., 2011
). Several examples of variable expressivity of CNV genotypes are described in
Some well characterized examples of variable expressivity are the clinical phenotypes associated with rearrangements at two loci, 1q21.1 (Class I/1 Mb) (Brunetti-Pierri et al., 2008
; Mefford et al., 2008
) and 16p11.2 (Class I/600 kb) (Bijlsma et al., 2009
; Fernandez et al., 2010
; Jacquemont et al., 2011
; Shinawi et al., 2010
). The clinical phenotypes associated with a single allele are diverse and include pediatric neurodevelopmental disorders and adult psychiatric conditions. Psychiatric diagnoses of individuals carrying identical microduplications of 1q21.1 include autism or schizophrenia (). Likewise microduplications of 16p11.2 are associated with autism, schizophrenia or bipolar disorder (McCarthy et al., 2009
; Weiss et al., 2008
). Both can also be carried by apparently asymptomatic individuals. Thus, even the rare subtype of a disorder (as defined by a CNV genotype) is complex.
Phenotypic variability can be attributed to other aspects of nature and nurture. Undoubtedly, the phenotypic expression of rare high-penetrance alleles is modulated by other genetic factors, including rare variants, as well as common (polygenic) variation (Purcell et al., 2009
) or epigenetic regulation (Hirasawa and Feil, 2010
). Indeed, evidence from CNV studies supports an oligogenic model where multiple rare variants contribute to genetic risk (Girirajan et al., 2010
). Another model has been proposed that attributes phenotypic variability to a combination of locus heterogeneity and pleiotropic effects of the individual alleles (State and Levitt, 2011
How exactly does CNV genotype relate to psychiatric phenotype? One possibility worth considering is that CNVs may not be at all specific in their effects. It has been postulated that CNVs linked to ASD are primarily associated with intellectual disability rather than with aspects of social cognition (Skuse, 2007
). According to this theory, the CNV confers risk simply because clinically recognizable psychiatric conditions are more likely to arise among individuals with low intelligence. Indeed a number of large deletions are strongly associated with intellectual disability or developmental delay (). However, not all genetic findings are consistent with this model. Intellectual disability is itself a highly variable trait, and does not appear to be a primary characteristic for a number of disease-associated CNVs. Some CNV alleles have no association with intellectual disability (e.g. 17p12/HNPP) or a relatively weak one compared with the association with psychiatric phenotypes (e.g., microduplications of 1q21.1 and 16p11.2), see . In addition, a recent study of de novo CNVs in ASD has found that de novo CNVs are not a strong predictor of low intelligence quotient (Sanders et al., 2011
). These observations suggest that the degree of risk conferred for a psychiatric disorder is related to specific genes within the CNV region and how changes in gene dosage influence neurodevelopment.
For some of the more well-characterized genomic disorders, a relationship between CNV genotype and clinical phenotype is beginning to emerge (Brunetti-Pierri et al., 2008
; McCarthy et al., 2009
). For instance, reciprocal rearrangements of 1q21.1 and 16p11.2 influence neuropsychiatric traits, susceptibility to epilepsy and head size in humans. Furthermore, deletions and duplications of each region have contrasting effects on head size and psychiatric features (McCarthy et al., 2009
) (). While the underlying molecular, cellular, neuroanatomical mechanisms are still unclear, these results suggest that the psychiatric features associated with a mutation might relate to specific effects of the mutation on brain growth.
Behavioral abnormalities associated with CNVs have been confirmed in animal models (Horev et al., 2011
; Nakatani et al., 2009
; Peca et al., 2011
; Tabuchi et al., 2007
; Tamada et al., 2010
). Mice with a paternal duplication of 15q11-13 display poor social interaction, behavioral inflexibility, abnormal ultrasonic vocalizations, and correlates of anxiety (Nakatani et al., 2009
). Mice with reciprocal deletions and duplication of 16p11.2 have contrasting effects on mobility, grooming and repetitive behaviors (Horev et al., 2011
). Mice lacking neurexin-1α display a decrease in pre-pulse inhibition, an increase in grooming behaviors, impairment in nest-building activity, and an improvement in motor learning (Etherton et al., 2009
). Mice lacking Contactin-associated protein 2 (Cntnap2) display deficits in social interaction and communication, hyperactivity, and seizures (Penagarikano et al., 2011
). These observations confirm some effects of CNV genotype on behavior; however determining the genes responsible for specific behavioral phenotypes in mouse and relating this to human phenotypes will be a challenge.
Compared to behavior, neuroanatomical features are more analogous between model organisms and human, and the neuroanatomical effects of CNVs might be as well. For example reciprocal deletion and duplication of 16p11.2 result in similar brain structural alterations in human and mouse, the deletion associated with brain overgrowth and the duplication associated with reduced brain volume (Horev et al., 2011
; McCarthy et al., 2009
; Shinawi et al., 2010
), and structural alterations appear to be widely distributed across multiple brain regions. A recent study has shown that over expression of human genes from the 16p11.2 CNV region in zebrafish influences brain size (Nicholas Katsanis, http://www.schizophreniaforum.org/new/detail.asp?id=1694
), consistent with the observations in human and mouse.
Relating CNV genotype to neurobiology
Specific abnormalities at the cellular level have also been linked to CNVs. Mice lacking neurexin-1α have defects in synaptic calcium channel function and neurotransmitter release (Missler et al., 2003
). Mice lacking Shank3 have defects in striatal synapses and cortico-striatal circuits (Peca et al., 2011
). Mice lacking Cntnap2 exhibit neuronal migration abnormalities, reduced number of interneurons, and abnormal neuronal network activity (Penagarikano et al., 2011
). Furthermore, temporal lobe sections from human subjects lacking Cntnap2 display abnormal patterns of neuronal migration (Strauss et al., 2006
Characterization of cellular phenotypes in humans is becoming tractable with the use of induced pluripotent stem cell (iPSC) technology (Dolmetsch and Geschwind, 2011
). Human-derived iPSCs, which can be differentiated into a variety of neuronal cell types, offer great promise in understanding of innate cellular and molecular defects that contribute to the initiation and progression of neuropsychiatric disorders. Unlike genetically-engineered model systems, neuronal cell cultures derived from patients captures the complete set of risk alleles present in the patient germline and the genetic diversity of the patient population.
As a proof-of-principle, several recent studies have now shown that hiPSC-derived neurons from patients with psychiatric disorders exhibit significant aberrations in neuronal connectivity, synapse maturation, and synaptic function compared with those of healthy controls (Brennand et al., 2011
; Cheung et al., 2011
; Marchetto et al., 2010
; Pasca et al., 2011
). Brennand et al (2011)
studied hiPSC-derived neurons from four schizophrenia patients with unknown disease etiologies. Schizophrenia-hiPSC-derived neurons had significantly reduced neuronal connectivity, reduced neurite outgrowth, reduced dendritic levels of PSD95, and altered gene expression profiles. Defects in neuronal connectivity and gene expression were ameliorated following treatment with the dopamine receptor antagonist loxapine. These early studies provide clues into the neurobiological processes that underlie schizophrenia, but without information on the genetic contributors in these patients, a clear mechanistic understanding is lacking.
hiPSC-models of monogenic disorders have begun to facilitate a mechanistic understanding of how genes contribute to disease. Pasca et al (2011)
showed that human mutations in the Timothy Syndrome gene Cav1.2 influence calcium signaling and the differentiation of cortical neurons, and the observed defects on calcium (Ca2+) signaling were reversible with the L-type calcium channel blocker roscovitine. Marchetto et al (Marchetto et al., 2010
) showed that cultured neurons derived from humans with mutations in the Rett Syndrome gene MeCP2 had fewer synapses, reduced spine density, smaller soma size, and exhibited a reduction in the intracellular calcium response and decrease in the frequency and amplitude of spontaneous excitatory and inhibitory postsynaptic currents.