Interspecies comparisons of functional genetic variations and their implications in neuropsychiatry

Modelling Autistic Features in Mice Using Quantitative Genetic Approaches

Animal studies provide a unique opportunity to study the consequences of genetic variants at the behavioural level. Human studies have identified hundreds of risk genes for autism spectrum disorder (ASD) that can lead to understanding on how genetic variation contributes to individual differences in social interaction and stereotyped behaviour in people with ASD. To develop rational therapeutic interventions, systematic animal model studies are needed to understand the relationships between genetic variation, pathogenic processes and the expression of autistic behaviours. Genetic and non-genetic animal model strategies are here reviewed in their propensity to study the underpinnings of behavioural trait variation. We conclude that an integration of reverse and forward genetic approaches may be essential to unravel the neurobiological mechanisms underlying ASD.

Altered Disrupted-in-Schizophrenia-1 Function Affects the Development of Cortical Parvalbumin Interneurons by an Indirect Mechanism

Disrupted-in-Schizophrenia-1 (DISC1) gene has been linked to schizophrenia and related major mental illness. Mouse Disc1 has been implicated in brain development, mainly in the proliferation, differentiation, lamination, neurite outgrowth and synapse formation and maintenance of cortical excitatory neurons. Here, the effects of two loss-of-function point mutations in the mouse Disc1 sequence (Q31L and L100P) on cortical inhibitory interneurons were investigated. None of the mutations affected the overall number of interneurons. However, the 100P, but not the 31L, mutation resulted in a significant decrease in the numbers of interneurons expressing parvalbumin mRNA and protein across the sensory cortex. To investigate role of Disc1 in regulation of parvalbumin expression, mouse wild-type Disc-1 or the 100P mutant form were electroporated in utero into cortical excitatory neurons. Overexpression of wild-type Disc1 in these cells caused increased densities of parvalbumin-expressing interneurons in the electroporated area and in areas connected with it, whereas expression of Disc1-100P did not. We conclude that the 100P mutation prevents expression of parvalbumin by a normally sized cohort of interneurons and that altering Disc1 function in cortical excitatory neurons indirectly affects parvalbumin expression by cortical interneurons, perhaps as a result of altered functional input from the excitatory neurons.

Heterospecific SNP diversity in humans and rhesus macaque (Macaca mulatta)

BACKGROUND

Conservation of single nucleotide polymorphisms (SNPs) between human and other primates (i.e., heterospecific SNPs) in candidate genes can be used to assess the utility of those organisms as models for human biomedical research.

METHODS

A total of 59,691 heterospecific SNPs in 22 rhesus macaques and 20 humans were analyzed for human trait associations and 4207 heterospecific SNPs biallelic in both taxa were compared for genetic variation.

RESULTS

Variation comparisons at the 4207 SNPs showed that humans were more genetically diverse than rhesus macaques with observed and expected heterozygosities of 0.337 and 0.323 vs. 0.119 and 0.102, and minor allele frequencies of 0.239 and 0.063, respectively. In total, 431 of the 59,691 heterospecific SNPs are reportedly associated with human-specific traits.

CONCLUSION

While comparisons between human and rhesus macaque genomes are plausible, functional studies of heterospecific SNPs are necessary to determine whether rhesus macaque alleles are associated with the same phenotypes as their corresponding human alleles.

Analyzing gene expression data in mice with the Neuro Behavior Ontology

We have applied the Neuro Behavior Ontology (NBO), an ontology for the annotation of behavioral gene functions and behavioral phenotypes, to the annotation of more than 1,000 genes in the mouse that are known to play a role in behavior. These annotations can be explored by researchers interested in genes involved in particular behaviors and used computationally to provide insights into the behavioral phenotypes resulting from differences in gene expression. We developed the OntoFUNC tool and have applied it to enrichment analyses over the NBO to provide high-level behavioral interpretations of gene expression datasets. The resulting increase in the number of gene annotations facilitates the identification of behavioral or neurologic processes by assisting the formulation of hypotheses about the relationships between gene, processes, and phenotypic manifestations resulting from behavioral observations.

Cross-species behavioural genetics: A starting point for unravelling the neurobiology of human psychiatric disorders

Identifying the genetic and neurobiological mechanisms underlying certain behavioural traits is an important strategy to understand the aetiology of various psychiatric disorders and to find potential new treatment possibilities. It has proven a great challenge to develop paradigms that allow translational research for behavioural phenotypes that are relevant for disorders across the psychiatric spectrum. Recently, there has been increasing attention for studies that implement rodent behavioural paradigms in the home cage to assess the association between genetic backgrounds and behavioural traits. The application of interspecies genetics to unravel these traits has revealed novel insights in the genetic mechanisms that are encoding phenotypes relevant to biological processes underlying psychiatric disorders. By means of two examples, namely the stress-induced hyperthermia paradigm and the home cage environment, this review aims to show that by using individual genetic variations with phenotypes obtained from mice and across categories of neuropsychiatric disorders, novel insights in the neurobiological trajectory of psychiatric disorders can be obtained.

Advances in multidisciplinary and cross-species approaches to examine the neurobiology of psychiatric disorders

Current approaches to dissect the molecular neurobiology of complex neuropsychiatric disorders such as schizophrenia and major depression have been rightly criticized for failing to provide benefits to patients. Improving the translational potential of our efforts will require the development and refinement of better disease models that consider a wide variety of contributing factors, such as genetic variation, gene-by-environment interactions, endophenotype or intermediate phenotype assessment, cross species analysis, sex differences, and developmental stages. During a targeted expert meeting of the European College of Neuropsychopharmacology (ECNP) in Istanbul, we addressed the opportunities and pitfalls of current translational animal models of psychiatric disorders and agreed on a series of core guidelines and recommendations that we believe will help guiding further research in this area.

Contribution of nonprimate animal models in understanding the etiology of schizophrenia

Schizophrenia is a severe psychiatric disorder that is characterized by positive and negative symptoms and cognitive impairments. The etiology of the disorder is complex, and it is thought to follow a multifactorial threshold model of inheritance with genetic and neurodevelop mental contributions to risk. Human studies are particularly useful in capturing the richness of the phenotype, but they are often limited to the use of correlational approaches. By assessing behavioural abnormalities in both humans and rodents, nonprimate animal models of schizophrenia provide unique insight into the etiology and mechanisms of the disorder. This review discusses the phenomenology and etiology of schizophrenia and the contribution of current nonprimate animal models with an emphasis on how research with models of neuro transmitter dysregulation, environmental risk factors, neurodevelopmental disruption and genetic risk factors can complement the literature on schizophrenia in humans.

Animal models for depression and the mismatch hypothesis of disease

Early life stress is one of the best characterized risk factors for psychiatric disorders, including depression, and many animal models have therefore studied the long-term physiological and behavioural consequences of early life stress. In most approaches a very deterministic view of adverse experiences early in life prevails, linking these events inevitably with later pathology. By summarizing literature on early life programming and adaptive phenotypic plasticity the current review proposes that early life challenges may induce changes that prepare an individual for life in a more hostile environment and are therefore predominantly beneficial. Adult diseases as depression might thus not be promoted by early life adversity per se, but by a mismatch of the programmed and the later actual environment in combination with a more vulnerable or resilient genetic predisposition. The present review further discusses the ability of currently available animal models for depression to investigate this novel hypothesis. Finally, a number of criteria and research strategies are outlined that would be necessary to address the mismatch hypothesis of depression.

Chromosomal mapping of excessive physical activity in mice in response to a restricted feeding schedule

Excessive physical activity plays an important role in the progression of anorexia nervosa (AN) by accelerating weight loss during dietary restriction. To search for mechanisms underlying this trait, a panel of mouse chromosome substitution strains derived from C57BL/6J and A/J strains was exposed to a scheduled feeding paradigm and to voluntary running wheel (RW) access. Here, we showed that A/J chromosomes 4, 12 and 13 contribute to the development of a disrupted RW activity in response to daily restricted feeding. This pattern is characterized by intense RW activity during the habitual rest phase and leads to accelerated body weight loss. Regions on mouse chromosomes 4, 12 and 13 display homology with regions on human chromosomes linked with anxiety and obsessionality in AN cohorts. Therefore, our data open new roads for interspecies genetic studies of AN and for unraveling novel mechanisms and potential effective treatment strategies for these neurobehavioral traits.

Compulsivity in mouse strains homologous with chromosomes 7p and 15q linked to obsessive-compulsive disorder

Obsessive-compulsive disorder (OCD) is a severe anxiety disorder characterized by obsessions and compulsions. The core symptom of OCD is compulsivity, the inability to stop thinking or acting when you want to, despite being aware of the uselessness of the content or the adverse consequences. To initiate a systematic search for genetic mechanisms underlying the pathophysiology of compulsivity, a panel of chromosome substitution (CS) strains, derived from mice that suppress (C57BL/6J strain) or maintain (A/J strain) high levels of repetitive wheel running during 2 hr of daily limited food access, was screened for this compulsive behavior. Following the genetic screen, we found linkage between compulsive wheel running and mouse chromosomes 2, 6, and 7 that show overlap with recently identified human linkage regions for OCD on chromosomes 7p and 15q. In the overlapping (human/mouse) genomic region, the CRH receptor 2 (CRHR2) gene was tested in a human case-control study. An initial exploration in OCD cases versus controls failed to detect an association between four-candidate CRH2R SNP's within this homologous linkage region and OCD. Genetic fine mapping of compulsivity in mice provides new opportunities to reveal mechanisms underlying this significant psychiatric trait.

Interspecies trait genetics reveals association of Adcy8 with mouse avoidance behavior and a human mood disorder

BACKGROUND

Identifying susceptibility genes for endophenotypes by studying analogous behaviors across species is an important strategy for understanding the pathophysiology underlying psychiatric disorders. This approach provides novel biological pathways plus validated animal models critical for selective drug development. One such endophenotype is avoidance behavior.

METHODS

In the present study, novel automated registration methods for longitudinal behavioral assessment in home cages are used to screen a panel of recently generated mouse chromosome substitution strains that are very powerful in quantitative trait loci (QTL) detection of complex traits. In this way, we identified chromosomes regulating avoidance behavior (increased sheltering preference) independent of motor activity levels (horizontal distance moved). Genetic information from the mouse QTL-interval was integrated with that from the homologous human linkage region for a mood disorder.

RESULTS

We genetically mapped a QTL for avoidance behavior on mouse chromosome 15, homologous with a human genome region (8q24) linked to bipolar disorder. Integrating the syntenic mouse QTL-interval with genotypes of 1868 BPD cases versus 14,311 control subjects revealed two associated genes (ADCY8 and KCNQ3). Adenylyl cyclase 8 (Adcy8) was differentially expressed in specific brain regions of mouse strains that differ in avoidance behavior levels. Finally, we showed that chronic infusion of the human mood stabilizer carbamazepine (that acts via adenylyl cyclase activity) significantly reduced mouse avoidance behavior, providing a further link between human mood disorders and this mouse home cage behavior.

CONCLUSIONS

Our data suggest that Adcy8 might encode a translational behavioral endophenotype of bipolar disorder.

Interspecies genetics of eating disorder traits

Family and twin studies have indicated that genetic factors play a role in the development of eating disorders, such as anorexia and bulimia nervosa, but novel views and tools may enhance the identification of neurobiological mechanisms underlying these conditions. Here we propose an integrative genetic approach to reveal novel biological substrates of eating disorder traits analogous in mouse and human. For example, comparable to behavioral hyperactivity that is observed in 40-80% of anorexia nervosa patients, inbred strains of mice with different genetic backgrounds are differentially susceptible to develop behavioral hyperactivity when food restricted. In addition, a list of characteristics that are relevant to eating disorders and approaches to their measurement in humans together with potential analogous rodent models has been generated. Interspecies genetics of neurobehavioral characteristics of eating disorders has the potential to open new roads to identify and functionally test genetic pathways that influence neurocircuits relevant for these heterogeneous psychiatric disorders.