A Genetic Deconstruction of Neurocognitive Traits in Schizophrenia and Bipolar Disorder

Background: Impairments in cognitive functions are common in patients suffering from psychiatric disorders, such as schizophrenia and bipolar disorder. Cognitive traits have been proposed as useful for understanding the biological and genetic mechanisms implicated in cognitive function in healthy individuals and in the dysfunction observed in psychiatric disorders. Methods: Sets of genes associated with a range of cognitive functions often impaired in schizophrenia and bipolar disorder were generated from a genome-wide association study (GWAS) on a sample comprising 670 healthy Norwegian adults who were phenotyped for a broad battery of cognitive tests. These gene sets were then tested for enrichment of association in GWASs of schizophrenia and bipolar disorder. The GWAS data was derived from three independent single-centre schizophrenia samples, three independent single-centre bipolar disorder samples, and the multi-centre schizophrenia and bipolar disorder samples from the Psychiatric Genomics Consortium. Results: The strongest enrichments were observed for visuospatial attention and verbal abilities sets in bipolar disorder. Delayed verbal memory was also enriched in one sample of bipolar disorder. For schizophrenia, the strongest evidence of enrichment was observed for the sets of genes associated with performance in a colour-word interference test and for sets associated with memory learning slope. Conclusions: Our results are consistent with the increasing evidence that cognitive functions share genetic factors with schizophrenia and bipolar disorder. Our data provides evidence that genetic studies using polygenic and pleiotropic models can be used to link specific cognitive functions with psychiatric disorders

Gene expression in the rat brain: High similarity but unique differences between frontomedial-, temporal- and occipital cortex

<p>Abstract</p> <p>Background</p> <p>The six-layered neocortex of the mammalian brain may appear largely homologous, but is in reality a modular structure of anatomically and functionally distinct areas. However, global gene expression seems to be almost identical across the cerebral cortex and only a few genes have so far been reported to show regional enrichment in specific cortical areas.</p> <p>Results</p> <p>In the present study on adult rat brain, we have corroborated the strikingly similar gene expression among cortical areas. However, differential expression analysis has allowed for the identification of 30, 24 and 11 genes enriched in frontomedial -, temporal- or occipital cortex, respectively. A large proportion of these 65 genes appear to be involved in signal transduction, including the ion channel <it>Fxyd6</it>, the neuropeptide <it>Grp </it>and the nuclear receptor <it>Rorb</it>. We also find that the majority of these genes display increased expression levels around birth and show distinct preferences for certain cortical layers and cell types in rodents.</p> <p>Conclusions</p> <p>Since specific patterns of expression often are linked to equally specialised biological functions, we propose that these cortex sub-region enriched genes are important for proper functioning of the cortical regions in question.</p

Gene-Based Analysis of Regionally Enriched Cortical Genes in GWAS Data Sets of Cognitive Traits and Psychiatric Disorders

Background: Despite its estimated high heritability, the genetic architecture leading to differences in cognitive performance remains poorly understood. Different cortical regions play important roles in normal cognitive functioning and impairment. Recently, we reported on sets of regionally enriched genes in three different cortical areas (frontomedial, temporal and occipital cortices) of the adult rat brain. It has been suggested that genes preferentially, or specifically, expressed in one region or organ reflect functional specialisation. Employing a gene-based approach to the analysis, we used the regionally enriched cortical genes to mine a genome-wide association study (GWAS) of the Norwegian Cognitive NeuroGenetics (NCNG) sample of healthy adults for association to nine psychometric tests measures. In addition, we explored GWAS data sets for the serious psychiatric disorders schizophrenia (SCZ) (n = 3 samples) and bipolar affective disorder (BP) (n = 3 samples), to which cognitive impairment is linked. Principal Findings: At the single gene level, the temporal cortex enriched gene RAR-related orphan receptor B (RORB) showed the strongest overall association, namely to a test of verbal intelligence (Vocabulary, P = 7.7E-04). We also applied gene set enrichment analysis (GSEA) to test the candidate genes, as gene sets, for enrichment of association signal in the NCNG GWAS and in GWASs of BP and of SCZ. We found that genes differentially expressed in the temporal cortex showed a significant enrichment of association signal in a test measure of non-verbal intelligence (Reasoning) in the NCNG sample. Conclusion: Our gene-based approach suggests that RORB could be involved in verbal intelligence differences, while the genes enriched in the temporal cortex might be important to intellectual functions as measured by a test of reasoning in the healthy population. These findings warrant further replication in independent samples on cognitive traits

LOC689986, a unique gene showing specific expression in restricted areas of the rodent neocortex

Background The neocortex is a highly specialised and complex brain structure, involved in numerous tasks, ranging from processing and interpretation of somatosensory information, to control of motor functions. The normal function linked to distinct neocortical areas might involve control of highly specific gene expression, and in order to identify such regionally enriched genes, we previously analysed the global gene expression in three different cortical regions (frontomedial, temporal and occipital cortex) from the adult rat brain. We identified distinct sets of differentially expressed genes. One of these genes, namely the hypothetical protein LOC689986 (LOC689986), was of particular interest, due to an almost exclusive expression in the temporal cortex. Results Detailed analysis of LOC689986 in the adult rat brain confirmed the expression in confined areas of parieto-temporal cortex, and revealed highly specific expression in layer 4 of the somatosensory cortex, with sharp borders towards the neighbouring motor cortex. In addition, LOC689986 was found to be translated in vivo, and was detected in the somatosensory cortex and in the Purkinje cells of the cerebellar cortex. The protein was present in neuronal dendrites and also in astrocyte cells. Finally, this unique gene is apparently specific for, and highly conserved in, the vertebrate lineage. Conclusions In this study, we have partially characterised the highly conserved LOC689986 gene, which is specific to the vertebrate linage. The gene displays a distinct pattern of expression in layer 4 of the somatosensory cortex, and areas of the parieto-temporal cortex in rodents

Behavior of <i>Csmd1</i> KO and WT mice to acoustic stimuli and tail suspension.

<p>(A) Startle responses of <i>Csmd1</i> KO mice in response to acoustic stimuli in the range of 80–120 dB, as compared to WT mice. The startle baseline was similar in male and female WT mice. A marginal increase in startle responses could be observed for higher acoustic stimuli in both genders of <i>Csmd1</i> KO mice, reaching statistical significance when analyzing all mice together (genotype-group interaction <i>P</i>-value<0.05). (B) There was no difference in amplitude response between KO and WT mice in the degree of inhibition after pre-pulse inhibition. (C) Tail suspension test demonstrated longer accumulated time immobile for KO mice as compared WT mice (P-value<0.05). Borderline statistical significance was observed for male <i>Csmd1</i> KO mice as compared to WT mice (P-value = 0.1).</p

Behavior of <i>Csmd1</i> KO and WT mice in the open field arena.

<p>(A) Total time spent in the center of the arena is shown for KO and WT mice, respectively. (B) Sequential time bin analysis demonstrates that WT mice adapt to the test arena after the first time bin, while KO mice avoid the center throughout the test period. (C and D) Comparison of total path length and sequential bin analysis of path length demonstrate no statistically significant difference between KO and WT mice. Abbreviations: n.s., not significant; asterisk, statistically significant <i>P</i>-value<0.05; s, seconds.</p

<i>Csmd1</i> RNA and protein expression in <i>Csmd1</i> knock-out and wild-type mice.

<p>(A) Schematic representation of the KO-strategy. A 1 kb genomic region (white lines) of exon1/intron1 was replaced with a selection cassette (grey box). (B) Expression of <i>Csmd1</i> mRNA measured by QPCR in an adult mouse tissue panel. <i>Csmd1</i> is predominantly expressed in brain tissues as compared to peripheral tissues. The highest expression level was identified in areas of the cortex. (C) Depletion of <i>Csmd1</i> mRNA in the cortex was documented by two exon-exon specific QPCR assays. Transcription of exon 1–2 was depleted, while about 20% residual expression could be observed when amplifying exon 32–33. KO mice lacked a protein band of expected size (389 KDa, arrow), as demonstrated by immunoblotting. Signals of lower molecular weight are indicated (a and b). (D) Mapping of RNA-seq reads to the <i>Csmd1</i> locus. RNA sequencing of cortex is shown for 4wild-type (green) and 4 <i>Csmd1</i> KO (red) mice (transcript scale: 0–150 reads). Coverage signals of modified nucleosomes (H3K4me3, H3K4me1 and H3K27Ac) and polymerase-2 binding profiles are shown for the mouse cortex. The 1 kb deleted sequence of <i>Csmd1</i> is highlighted in yellow (upper panel) and blue (lower panel). No RNA reads were mapped to the deleted genomic region in the KO mice. Abbreviations: Cx, cortex; VCx, visual cortex; FCx, frontal cortex; Hipp, hippocampus; Hyp, hypothalamus; Ob, olfactory bulb; Cer, cerebellum; Visc. Fat, visceral fat.</p

Behavior of <i>Csmd1</i> KO and WT mice in the elevated plus maze.

<p>(A) Analysis of total time spent on open arms demonstrate significant less time for KO mice (N = 13) as compared to WT mice (N = 8). (B) Compiled tracks from all mice show that <i>Csmd1</i> KO mice avoid entering open arms, as opposed to WT mice traveling over the entire test arena. Abbreviations: asterisk, statistically significant (<i>P</i>-value<0.05); s, seconds.</p