It is really a fascinating article which is a step towards understanding the molecular mechanisms underlying phenotypes of schizophrenia. Relating genotypes to phenotypes is really necessary for untangling the puzzle of a complex disorder. However, when a regulatory SNP interferes with normal binding of a transcription factor, is it understood that the transcription factor should play a role in brain and therefore in the molecular pathology of schizophrenia? Is there any direct role for involvement of serum response factor (SRF) in brain development or any neurological process?

View all comments by Ali Mohamad Shariaty

In response to Ali Mohamad Shariaty’s comment: Serum response factor (SRF) plays a key role in regulating the transcription of a number of genes involved in brain development. Genetic manipulation of SRF has revealed a direct role for it as a regulator of cortical and hippocampal function (e.g., Etkin et al., 2006) influencing both learning and memory. At the cellular level SRF has been shown to regulate dendritic morphology and neuronal migration. Therefore, SRF is indeed an important neurodevelopmental molecule, mediated via its regulation of genes, such as NRG1. Genetic variations that are predicted to interfere with SRF binding (such as the SNP characterized in our study) may affect critical aspects of brain development and function that contribute to schizophrenia. Since SRF regulates the expression of a number of genes, beyond that of NRG1, its involvement in schizophrenia is likely mediated “indirectly” via its effects on the regulation of genes associated with the disorder.

References:

Etkin A, Alarcón JM, Weisberg SP, Touzani K, Huang YY, Nordheim A, Kandel ER. A role in learning for SRF: deletion in the adult forebrain disrupts LTD and the formation of an immediate memory of a novel context. Neuron. 2006 Apr 6;50(1):127-43. Abstract

View all comments by Amanda Jayne Law

The gene Neuregulin 1 (NRG1) on chromosome 8p has been identified as one of the risk genes for schizophrenia. It is unclear how the DNA sequence variation linked to schizophrenia leads to abnormalities of mRNA expression. This would be important to know, in order to understand the downstream effects of the neuregulin gene on neuronal functioning in schizophrenia.

Law and colleagues explored this question in post-mortem specimens of the hippocampus of control subjects and patients with schizophrenia. This elegant study of the expression of four types of NRG1 mRNA (types I-IV) is exactly what we need to translate findings from the field of human genetics into the field of schizophrenia neuropathology. The findings are complex and cannot be translated easily into a model of neuregulin dysfunction in schizophrenia. I would like to highlight two findings.

First, the level of NRG1 type I mRNA expression was increased in the hippocampus of schizophrenia patients. This confirms an earlier study of NRG1 mRNA expression in schizophrenia. It remains to be seen how this change in...  Read more

The gene Neuregulin 1 (NRG1) on chromosome 8p has been identified as one of the risk genes for schizophrenia. It is unclear how the DNA sequence variation linked to schizophrenia leads to abnormalities of mRNA expression. This would be important to know, in order to understand the downstream effects of the neuregulin gene on neuronal functioning in schizophrenia.

Law and colleagues explored this question in post-mortem specimens of the hippocampus of control subjects and patients with schizophrenia. This elegant study of the expression of four types of NRG1 mRNA (types I-IV) is exactly what we need to translate findings from the field of human genetics into the field of schizophrenia neuropathology. The findings are complex and cannot be translated easily into a model of neuregulin dysfunction in schizophrenia. I would like to highlight two findings.

First, the level of NRG1 type I mRNA expression was increased in the hippocampus of schizophrenia patients. This confirms an earlier study of NRG1 mRNA expression in schizophrenia. It remains to be seen how this change in NRG1 type I mRNA expression relates to the finer details of neuregulin dysfunction in schizophrenia.

Second, one single nucleotide polymorphism (SNP8NRG243177) of the risk haplotype linked to schizophrenia in earlier studies predicts NRG1 type IV mRNA expression. The SNP determines a binding site for transcription factors, providing clues for how DNA sequence variation may lead, via modulation of mRNA expression, to neuronal dysfunction in schizophrenia. It is exciting to see that we can now test specific hypotheses of molecular mechanisms in the brains of patients who have suffered from schizophrenia. The study by Law et al. is an encouraging step in the right direction.

I think this is a very interesting and potentially significant paper. It is important to point out, however, that it deals with changes in mRNA abundance rather than alterations in neuregulin protein expression. No measures of isoform protein expression were performed, and it is conceivable that neuregulin isoform protein expression could be increased, decreased, or not changed. A second point is that although statistically significant changes in mRNA were measured, they are modest.

Finally, although multiple comparisons were performed, the authors chose not to perform Bonferroni corrections, noting in the primary paper that, "Correction for random effects, such as Bonferroni correction, would be an excessively conservative approach, particularly given that we have restricted our primary analyses to planned comparisons (based on strong prior clinical association and physical location of the SNPs) of four SNPs and a single haplotype comprised of these SNPs. Because the SNPs are in moderate LD, the degree of independence between markers is low and, therefore, correcting for...  Read more

I think this is a very interesting and potentially significant paper. It is important to point out, however, that it deals with changes in mRNA abundance rather than alterations in neuregulin protein expression. No measures of isoform protein expression were performed, and it is conceivable that neuregulin isoform protein expression could be increased, decreased, or not changed. A second point is that although statistically significant changes in mRNA were measured, they are modest.

Finally, although multiple comparisons were performed, the authors chose not to perform Bonferroni corrections, noting in the primary paper that, "Correction for random effects, such as Bonferroni correction, would be an excessively conservative approach, particularly given that we have restricted our primary analyses to planned comparisons (based on strong prior clinical association and physical location of the SNPs) of four SNPs and a single haplotype comprised of these SNPs. Because the SNPs are in moderate LD, the degree of independence between markers is low and, therefore, correcting for multiple testing would result in a high type II error rate. The prior probability and the predictable association between the deCODE haplotype and expression of NRG1 isoforms (especially type IV, which is its immediate physical neighbor) combined with the LD between SNPs in this haplotype makes statistical correction for these comparisons inappropriate. Nevertheless, our finding regarding type IV expression and the deCODE haplotype and SNP8NRG243177 requires independent replication."

It will thus be important to determine if these changes in neuregulin mRNA isoform abundance are mirrored by significant changes in neuregulin isoform protein expression and if the findings can be independently replicated with other cohorts.

Convergent evidence supporting the role of a schizophrenia-associated polymorphic variant in the NRG1 gene (SNP8NRG1243177) with the regulation of cortical function and the development of psychosis
The study of Hall and colleagues describes association of a schizophrenia-related polymorphism in the NRG1 gene promoter (SNP8NRG1243177) with cortical and cognitive dysfunction and the emergence of psychotic symptoms in young individuals at high genetic risk for developing schizophrenia. We have previously demonstrated that the same polymorphism (SNP8NRG1243177) and a 22kb risk haplotype, including this SNP, predicts transcription levels of a novel isoform of the NRG1 gene (Type IV) in the brain of patients with schizophrenia (Law et al., 2006; see SRF related news story). The SNP resides in the NRG1 promoter region for the novel E187 exon (Type IV) and our investigations indicate that the SNP is central to a regulatory...  Read more

Convergent evidence supporting the role of a schizophrenia-associated polymorphic variant in the NRG1 gene (SNP8NRG1243177) with the regulation of cortical function and the development of psychosis
The study of Hall and colleagues describes association of a schizophrenia-related polymorphism in the NRG1 gene promoter (SNP8NRG1243177) with cortical and cognitive dysfunction and the emergence of psychotic symptoms in young individuals at high genetic risk for developing schizophrenia. We have previously demonstrated that the same polymorphism (SNP8NRG1243177) and a 22kb risk haplotype, including this SNP, predicts transcription levels of a novel isoform of the NRG1 gene (Type IV) in the brain of patients with schizophrenia (Law et al., 2006; see SRF related news story). The SNP resides in the NRG1 promoter region for the novel E187 exon (Type IV) and our investigations indicate that the SNP is central to a regulatory transcription factor binding domain. We previously suggested that a potential molecular mechanism behind the clinical association of NRG1 with schizophrenia (at least in the 5’ region of the gene) involves altered transcriptional regulation of the gene, which modifies to a small degree and in an isoform-specific fashion, the efficiency of NRG1 signaling effects on neural development and plasticity. We predicted that such effects may translate into altered adult brain function.

With this in mind, the study of Hall and colleagues provides a remarkable level of functional convergence suggesting a potential link between a molecular phenotype related to genetic risk at this loci (i.e., increased transcriptional regulation of the novel Type IV isoform, Law et al., 2006) and abnormal cortical development, function, and the subsequent manifestation of psychotic symptoms.

The major objective of the study was to determine the relationship between previously identified genetic variants in the 5’ region of NRG1 (Stefansson et al., 2002; see also Harrison and Law, 2006) with aspects of the schizophrenia phenotype (including decreased IQ, altered cortical function, and psychosis) in individuals who are at high risk of developing the disorder. Subjects were followed throughout the course of the study or until they developed schizophrenia. It is noteworthy that the incidence rate of developing schizophrenia was highest in subjects homozygous for the risk (T) allele at NRG1243177 (25 percent). Conversely, the occurrence of schizophrenia in non-risk C/C individuals was lower (15 percent), but still present, demonstrating the complex heterogeneous nature of the disease.

The study was performed on a modest sample of 79 high-risk individuals, 63 of whom fMRI data was available for. Firstly, brain activation patterns were determined by fMRI whilst individuals were performing the Hayling sentence completion task. Subjects who were homozygous for the risk T allele (T/T) at SNP8NRG1243177 exhibited decreased activation of Brodmann area 9 and the right temporo-occipital junction (Brodmann areas 39 and 19) when the activation during the task was compared to the resting state. However, unlike the medial prefrontal cortex, the difference in activation of the right temporal-occipital junction derived from the fact that T/T individuals had a “higher” resting activity compared to C/C individuals (as stated by the authors). Based on this observation, it is difficult to interpret which phenotype, in terms of cortical activation in this region, genetic risk at the allele is associated with—that is, is the risk variant associated with an overactive right temporo-occipital cortex at rest, or with decreased ability to further activate the region during demand?

In the supplementary notes, the authors address this issue, stating that a failure to deactivate the temporal cortex during rest may suggest that frontotemporal activity is disrupted in individuals homozygous for the T allele at SNP8NRG1243177. Furthermore, based on our studies, it would be important to see if genetic risk at SNP8NRG1243177 predicts hippocampal activation during a task that activates this area, allowing one to link the molecular changes in the hippocampus in schizophrenia, related to genetic risk at this SNP, to an outcome measure of brain function. Conversely, it would also be of use to determine whether NRG1 Type IV expression is altered in the brain areas implicated by Hall and colleagues.

Importantly, the study also shows that the genotype effects at SNP8NRG1243177 on cortical function are not related to medication status (all subjects were medication-free). Secondly, Hall and colleagues investigated the effects of the SNP8NRG1243177 risk allele on the development of psychotic symptoms in high-risk individuals. In a remarkable observation, 100 percent of individuals who had the risk T/T genotype developed psychotic symptoms, compared to less than 50 percent of C/C individuals, although the small sample size must be kept in mind. One interesting observation that is not readily apparent in the study is the fact that of the 12 T/T individuals who developed psychotic symptoms, only three of those (25 percent) developed schizophrenia before the end of the study. This may be due to the fact that others later went on to develop the disorder or that they developed other complex mental illnesses which include psychosis, such as bipolar disorder. (This is not clear from the study.) The association of genetic risk in the NRG1 gene and psychotic symptom development is consistent with the fact that genetic risk at NRG1 has been linked to psychosis in other brain diseases such as bipolar disorder and Alzheimer’s disease (see Harrison and Law, 2006). Finally, and perhaps most compelling, there is the observation that genetic risk at SNP8NRG1243177 is related to decreased IQ (measured by NART) in high-risk individuals.

Overall, the study of Hall and colleagues provides novel evidence that genetic variation in the NRG1 promoter, in particular a genetic variant that predicts altered expression of the NRG1 gene in the brain in schizophrenia (Law et al., 2006), is associated with abnormalities in cortical function and cognition and contributes to psychotic symptoms in individuals at high risk of developing the disease.

The readers might find our results (now in press) interesting in the context of the brilliant work by Law and colleaguesLaw et al (2006)and now Hall and colleagues. We examined the potential impact of 18 single nucleotide polymorphisms (SNPs) within the DTNBP1, NRG1, DAOA/G32 and DAAO genes, on cognition and self-rated schizotypy, in a representative population of 2,243 young male military conscripts. Single SNP and haplotype associations were evaluated. The risk allele of functional SNP8NRG243177 was associated with reduced spatial working memory capacity.

This is of particular interest since it has recently been reported that SNP8NRG243177 is a functional polymorphism, the risk allele (T) predicting higher levels of type IV NRG1 mRNA expression (Law et al., 2006), and associated with lower prefrontal (and temporal) activation and development of psychotic symptoms in high risk individuals for schizophrenia (  Read more

The readers might find our results (now in press) interesting in the context of the brilliant work by Law and colleaguesLaw et al (2006)and now Hall and colleagues. We examined the potential impact of 18 single nucleotide polymorphisms (SNPs) within the DTNBP1, NRG1, DAOA/G32 and DAAO genes, on cognition and self-rated schizotypy, in a representative population of 2,243 young male military conscripts. Single SNP and haplotype associations were evaluated. The risk allele of functional SNP8NRG243177 was associated with reduced spatial working memory capacity.

This is of particular interest since it has recently been reported that SNP8NRG243177 is a functional polymorphism, the risk allele (T) predicting higher levels of type IV NRG1 mRNA expression (Law et al., 2006), and associated with lower prefrontal (and temporal) activation and development of psychotic symptoms in high risk individuals for schizophrenia (Hall et al., 2006). If not a chance finding, our result constitutes the first independent confirmation that functional SNP8NRG243177 impacts aspects of human prefrontal brain function. Since spatial working deficits constitute an effective endophenotype for schizophrenia, this finding also suggests a mechanism by which this NRG1 variant may confer risk for the disorder at an information processing level. In contrast to Hall and colleagues, no association of SNP8NRG243177 with psychotic-like symptoms or IQ was detected in this study.

References:
Nicholas C. Stefanis, Thomas A. Trikalinos, Dimitrios Avramopoulos, Nikos Smyrnis, Ioannis Evdokimidis, Evangelia E. Ntzani, John P. Ioannidis and Costas N. Stefanis. Impact of schizophrenia candidate genes on schizotypy and cognitive endophenotypes at the population level. Biological Psychiatry (in press).

The findings of Savonenko et al. (2008) are an impressive addition to the growing evidence supporting a role for neuregulin-1 (NRG1) in schizophrenia pathology. The authors not only revealed a novel relationship between schizophrenia-like behavior and the loss of BACE1 proteolytic function, but also showed that this association results from disruption of BACE1-mediated NRG1 cleavage. These observations support the notion that aberrant processing of NRG1 may contribute to the development of schizophrenia-like phenotypes, providing a basis for examining other NRG1-cleaving pathways in the context of schizophrenia. Savonenko et al. were thorough in their behavioral assessment of the BACE1 mutant mice, convincingly showing that these animals exhibit schizophrenia-related behaviors that could be exacerbated by psychostimulants and improved by antipsychotic drug treatment.

What remains unclear, however, is the relationship between the NRG1/ErbB4 protein findings in the BACE1 mutant mouse brain and those previously reported in the schizophrenic human brain. For example, the...  Read more

The findings of Savonenko et al. (2008) are an impressive addition to the growing evidence supporting a role for neuregulin-1 (NRG1) in schizophrenia pathology. The authors not only revealed a novel relationship between schizophrenia-like behavior and the loss of BACE1 proteolytic function, but also showed that this association results from disruption of BACE1-mediated NRG1 cleavage. These observations support the notion that aberrant processing of NRG1 may contribute to the development of schizophrenia-like phenotypes, providing a basis for examining other NRG1-cleaving pathways in the context of schizophrenia. Savonenko et al. were thorough in their behavioral assessment of the BACE1 mutant mice, convincingly showing that these animals exhibit schizophrenia-related behaviors that could be exacerbated by psychostimulants and improved by antipsychotic drug treatment.

What remains unclear, however, is the relationship between the NRG1/ErbB4 protein findings in the BACE1 mutant mouse brain and those previously reported in the schizophrenic human brain. For example, the authors reported reductions in ErbB4-PSD95 coupling in the BACE1 mutant mouse, whereas Hahn et al. (2006) demonstrated increased ErbB4-PSD95 interaction in the prefrontal cortices of schizophrenic patients. In addition, our recent investigation found elevated prefrontal cortical levels of both NRG1 C-terminal fragment (ICD) and full-length ErbB4 protein in schizophrenic subjects (Chong et al., 2008), while Savonenko et al. showed decreased NRG1 C-terminal fragment levels with no alterations in ErbB4 protein in the BACE1 mutant mouse cortex. On the other hand, the lack of variations in overall cortical ErbB4 in these mice may correspond to the findings of Hahn et al. (2006) who reported no alterations in prefrontal cortical ErbB4 protein levels in schizophrenic subjects.

These seemingly conflicting results could suggest that any imbalance in cortical NRG1 signaling, whether increased or diminished, may lead to schizophrenia. Indeed, studies have suggested that improper tuning of other cortical signaling systems, particularly those of dopamine, can contribute to cognitive deficits associated with this disease (Vijayraghavan et. al, 2007). Optimal synaptic function may display “inverted-U” shaped response to NRG1-ErbB4 activity as proposed by Role and Talmage (2007). Alternatively, the authors speculated that some of the discrepancies between the findings in the BACE1 mutant mice and those observed in the schizophrenic humans may be due to differences in the duration of NRG1 signaling modification between the animals and the patients, who had a lifetime of mental illness. One way to examine the validity of this suggestion is to look at cortical ErbB4-PSD95 coupling and NRG1/ErbB4 protein levels in the BACE1 mutant mice at different developmental and adult time points. This approach could test whether these animals at later stages in life display alterations in cortical ErbB4-PSD95 interactions and/or in NRG1/ErbB4 protein levels comparable to those seen in schizophrenic subjects of the human studies, which primarily consisted of adults beyond middle age. Also of interest would be to create NRG1 and ErbB4 gain-of-function mutants where the timing of over-expression could be controlled.

Given the significance of NRG1 signaling/cleavage in the BACE1 mutant mouse schizophrenia-like phenotypes, it may also be important to consider pathways leading to changes in ErbB4 C-terminal fragment levels in schizophrenia etiology. A recent paper by Walsh et al. (2008) demonstrated that at least one schizophrenic patient in their study has a gene deletion encompassing the C-terminal intracellular kinase domain of ErbB4, and we have found decreases in ErbB4 C-terminal fragments relative to full-length ErbB4 in the frontal cortex of schizophrenic subjects (Chong et al., 2008). These observations together with those of Savonenko et al. raise interesting questions regarding how molecular alterations in NRG1 signaling and cleavage may impact ErbB4 signaling and cleavage and whether changes in NRG1 and/or ErbB4 could be primary or secondary to the schizophrenia disease process.

In summary, Savonenko et al. have provided a novel avenue to probe NRG1 function and processing in relation to schizophrenia pathology. They have also introduced BACE1 as a potentially important schizophrenia susceptibility molecule that to our knowledge has not been directly investigated in subjects with schizophrenia and may be worth studying in the brain tissues of these patients. In addition, it would be interesting to examine how the schizophrenia-related traits of the BACE1 mutant mice compare with those of other NRG1 mutant mice such as the heterozygous NRG1 transmembrane knock-out mice (Stefansson et al., 2002). Such an investigation could provide insight into whether similar NRG1 signaling deficiencies underlie the schizophrenia-like phenotypes of these animal models.

References:

Hahn CG, Wang HY, Cho DS, Talbot K, Gur RE, Berrettini WH, Bakshi K, Kamins J, Borgmann-Winter KE, Siegel SJ, Gallop RJ, Arnold SE. (2006) Altered neuregulin 1-erbB4 signaling contributes to NMDA receptor hypofunction in schizophrenia. Nat Med. 12:824-8. Abstract

Chong VZ, Thompson M, Beltaifa S, Webster MJ, Law AJ, Weickert CS. (2008) Elevated neuregulin-1 and ErbB4 protein in the prefrontal cortex of schizophrenic patients. Schizophr Res. 100:270-80. Abstract

Vijayraghavan S, Wang M, Birnbaum SG, Williams GV, Arnsten AF. (2007) Inverted-U dopamine D1 receptor actions on prefrontal neurons engaged in working memory. Nat Neurosci. 10:376-84. Abstract

Role LW, Talmage DA (2007) Neurobiology: new order for thought disorders. Nature. 448:263-5. Abstract

Walsh T, McClellan JM, McCarthy SE, Addington AM, Pierce SB, Cooper GM, Nord AS, Kusenda M, Malhotra D, Bhandari A, Stray SM, Rippey CF, Roccanova P, Makarov V, Lakshmi B, Findling RL, Sikich L, Stromberg T, Merriman B, Gogtay N, Butler P, Eckstrand K, Noory L, Gochman P, Long R, Chen Z, Davis S, Baker C, Eichler EE, Meltzer PS, Nelson SF, Singleton AB, Lee MK, Rapoport JL, King MC, Sebat J. (2008) Rare structural variants disrupt multiple genes in neurodevelopmental pathways in schizophrenia. Science. 320:539-43. Abstract

Stefansson H, Sigurdsson E, Steinthorsdottir V, Bjornsdottir S, Sigmundsson T, Ghosh S, Brynjolfsson J, Gunnarsdottir S, Ivarsson O, Chou TT, Hjaltason O, Birgisdottir B, Jonsson H, Gudnadottir VG, Gudmundsdottir E, Bjornsson A, Ingvarsson B, Ingason A, Sigfusson S, Hardardottir H, Harvey RP, Lai D, Zhou M, Brunner D, Mutel V, Gonzalo A, Lemke G, Sainz J, Johannesson G, Andresson T, Gudbjartsson D, Manolescu A, Frigge ML, Gurney ME, Kong A, Gulcher JR, Petursson H, Stefansson K. (2002) Neuregulin 1 and susceptibility to schizophrenia. Am J Hum Genet. 71:877-92. Abstract

In this very important and innovative study, Sestan and colleagues report a transcriptome-wide survey across multiple brain regions of the fetal mid-gestation brain. They show dramatic differences in expressed transcripts, including alternative splice variants, between brain regions, and most surprisingly, between several cortical regions. The authors have undertaken an ambitious task of further characterizing differentially expressed genes by functional clustering and co-expression clustering and comparing the results with genes identified through neurobiological experiments. They have also performed extensive validation using several additional fetal brains. Most interestingly, the authors showed that differentially expressed genes are more frequently associated with human-specific evolution of putative cis-regulatory elements. For this, they have identified genes that are near highly conserved non-coding sequences (CNSs) and found that the genes that are differentially expressed between the regions are more frequently near human-specific accelerated evolution CNSs.

The...  Read more

In this very important and innovative study, Sestan and colleagues report a transcriptome-wide survey across multiple brain regions of the fetal mid-gestation brain. They show dramatic differences in expressed transcripts, including alternative splice variants, between brain regions, and most surprisingly, between several cortical regions. The authors have undertaken an ambitious task of further characterizing differentially expressed genes by functional clustering and co-expression clustering and comparing the results with genes identified through neurobiological experiments. They have also performed extensive validation using several additional fetal brains. Most interestingly, the authors showed that differentially expressed genes are more frequently associated with human-specific evolution of putative cis-regulatory elements. For this, they have identified genes that are near highly conserved non-coding sequences (CNSs) and found that the genes that are differentially expressed between the regions are more frequently near human-specific accelerated evolution CNSs.

The weakness of the study is a very small number of fetal brains (four) and the fact that they range in age from 18 to 23 weeks of gestation. During these several weeks of fetal life, the brain undergoes dramatic developmental changes and expression of many genes, either increases or decreases steeply. Thus, it would be critical to fully characterize these changes across fetal age. It is also crucial to explore genetic influences on fetal gene expression as it appears that in adult brain both gene expression and splicing are strongly genetically regulated. The authors have made an important contribution to our understanding of development of human brain, and further research of this type will generate the data that would help in better understanding of human brain disorders. In particular, genetic-expression effects in human brain across the entire lifespan, including fetal period, may help identify molecular mechanisms whereby candidate genes increase risk for developing the disorder. Using expression levels of transcripts and their splicing characteristics as intermediate phenotypes may yield statistically positive associations and improved understanding of the mechanisms that lead to neurodevelopmental disorders such as autism and schizophrenia, as they are the most proximal phenotypes to the risk alleles.

This outstanding study reinforces how much we still do not understand about human brain development and function! It is just mind-boggling that the mid-fetal human brain expresses more than three quarters of the human genome, and that region-specific splicing appears to be an absolutely critical feature of the developing brain. Interestingly, the structural and functional interhemispheric differences do not appear to be related to gene expression differences in mid-fetal life, but rather, either they develop independently of gene expression patterns, or they are developing at later stages of cortical maturation, perhaps in a postnatal activity-driven pattern.

So, how is this developmental expression machinery related to various neurodevelopmental disorders, such as schizophrenia? Is usage of an "inappropriate" splice variant sufficient to alter the neuronal phenotypic development to a degree that would predispose the brain to developing a disease? Are environmental insults capable of disrupting this finely tuned, region-specific splicing machinery? As this is a likely...  Read more

This outstanding study reinforces how much we still do not understand about human brain development and function! It is just mind-boggling that the mid-fetal human brain expresses more than three quarters of the human genome, and that region-specific splicing appears to be an absolutely critical feature of the developing brain. Interestingly, the structural and functional interhemispheric differences do not appear to be related to gene expression differences in mid-fetal life, but rather, either they develop independently of gene expression patterns, or they are developing at later stages of cortical maturation, perhaps in a postnatal activity-driven pattern.

So, how is this developmental expression machinery related to various neurodevelopmental disorders, such as schizophrenia? Is usage of an "inappropriate" splice variant sufficient to alter the neuronal phenotypic development to a degree that would predispose the brain to developing a disease? Are environmental insults capable of disrupting this finely tuned, region-specific splicing machinery? As this is a likely possibility, we must rethink the existing disease-related gene expression findings in the context of the present study, and accept that our previous gene expression measurements may have been too crude to uncover some of the most meaningful changes that are potentially hallmarks of various brain disorders. Furthermore, as the genes that show the most widespread regional use of splice variants can be essential for proper neuronal migration or connectivity, one can argue that these genes should be the primary targets for evaluation in the various regions of postmortem tissue of diseased individuals.

Finally, there is also a minor, cautionary note arising from this study. The fact that the Affymetrix U133 and the Exon array results showed a correlation of R2 >0.5 is encouraging, but underscores that platform-dependence of the findings remains a significant interpretational challenge. Some platforms will be better suited to identify certain gene expression changes, while others will have a greater power to reveal a different (but also potentially valid!) set of mRNA alterations.

The seminal identification of mutations in DISC1 associated with schizophrenia and other psychiatric disorders raises several obvious questions: what does the DISC1 protein normally do? What are its biochemical and cellular functions, and what processes are affected by its mutation? How do defects in these cellular processes ultimately lead to altered brain function and psychopathology? Which brain systems are affected and how? Similar questions could be asked for the growing number of other genes that have been implicated by the identification of putatively causal mutations, including NRG1, ERBB4, NRXN1, CNTNAP2, and many copy number variants. Finding the points of biochemical or phenotypic convergence for these proteins or mutations may be key to understanding how mutations in so many different genes can lead to a similar clinical phenotype and to suggesting points of common therapeutic intervention.

The papers by Kim et al. and Enomoto et al. add more detail to the complex picture of the biochemical interactions of DISC1 and its diverse cellular functions. The links...  Read more

The seminal identification of mutations in DISC1 associated with schizophrenia and other psychiatric disorders raises several obvious questions: what does the DISC1 protein normally do? What are its biochemical and cellular functions, and what processes are affected by its mutation? How do defects in these cellular processes ultimately lead to altered brain function and psychopathology? Which brain systems are affected and how? Similar questions could be asked for the growing number of other genes that have been implicated by the identification of putatively causal mutations, including NRG1, ERBB4, NRXN1, CNTNAP2, and many copy number variants. Finding the points of biochemical or phenotypic convergence for these proteins or mutations may be key to understanding how mutations in so many different genes can lead to a similar clinical phenotype and to suggesting points of common therapeutic intervention.

The papers by Kim et al. and Enomoto et al. add more detail to the complex picture of the biochemical interactions of DISC1 and its diverse cellular functions. The links with Akt and PTEN signaling are especially interesting, given the previous implication of these proteins in schizophrenia and autism. Akt, in particular, may provide a link between Nrg1/ErbB4 signaling and DISC1 intracellular functions.

These studies also reinforce the importance of DISC1 and its interacting partners in neurodevelopment, specifically in cell migration and axonal extension. In particular, they highlight the roles of these proteins in postnatal hippocampal development and adult hippocampal neurogenesis. They also raise the question of which extracellular signals and receptors regulate these processes through these signalling pathways. The Nrg1/ErbB4 pathway has already been implicated, but there are a multitude of other cell migration and axon guidance cues known to regulate hippocampal development, some of which, for example, semaphorins, signal through the PTEN pathway.

Whether or how disruptions in these developmental processes contribute to psychopathology also remains unclear. It seems likely that the effects of mutations in any of these genes will be highly pleiotropic and have effects in many brain systems. The reported pathology in schizophrenia is not restricted to hippocampus but extends to cortex, thalamus, cerebellum, and many other regions. Similarly, while the cognitive deficits receive a justifiably large amount of attention, given that they may have the most clinical impact, motor and sensory deficits are also a stable and consistent part of the syndrome that must be explained. Pleiotropic effects on prenatal and postnatal development, as well as on adult processes, may actually be the one common thread characterizing the genes so far implicated. These new papers represent the first steps in the kinds of detailed biological studies that will be required to make explanatory links from mutations, through biochemical and cellular functions, to effects on neuronal networks and ultimately psychopathology.

DISC1 disruption by chromosomal translocation cosegregates with several neuropsychiatric disorders, including schizophrenia (Blackwood et al., 2001; Millar et al., 2000). Recent attention has focused on the effects of DISC1 on the structure and function of the dentate gyrus, one of the few brain regions that exhibit neurogenesis throughout life. The downregulation of DISC1 has several deleterious effects on the dentate gyrus, including aberrant neuronal migration (Duan et al., 2007). However, the mechanisms through which DISC1 regulates the structure and function of the dentate gyrus remain unknown. The dentate gyrus and its output to the CA3 area, the mossy fiber, show several abnormalities in schizophrenia and other neuropsychiatric diseases (Kobayashi, 2009). Thus, understanding how a gene associated with neuropsychiatric disease, DISC1, mechanistically impacts the dentate gyrus is an...  Read more

DISC1 disruption by chromosomal translocation cosegregates with several neuropsychiatric disorders, including schizophrenia (Blackwood et al., 2001; Millar et al., 2000). Recent attention has focused on the effects of DISC1 on the structure and function of the dentate gyrus, one of the few brain regions that exhibit neurogenesis throughout life. The downregulation of DISC1 has several deleterious effects on the dentate gyrus, including aberrant neuronal migration (Duan et al., 2007). However, the mechanisms through which DISC1 regulates the structure and function of the dentate gyrus remain unknown. The dentate gyrus and its output to the CA3 area, the mossy fiber, show several abnormalities in schizophrenia and other neuropsychiatric diseases (Kobayashi, 2009). Thus, understanding how a gene associated with neuropsychiatric disease, DISC1, mechanistically impacts the dentate gyrus is an important question with much clinical relevance.

The recent papers by Kim et al. and Enomoto et al. characterize an interaction between DISC1 and girdin (also known as KIAA1212), and reveal how girdin, and the interaction between DISC1 and girdin, impact axon development, dendritic development, and the proper positioning of newborn neurons in the dentate gyrus. Girdin normally stimulates the function of AKT (Anai et al., 2005), and Kim et al. show that DISC1 binds to girdin and inhibits its function. Thus, the loss of DISC1 leaves girdin unopposed, resulting in excessive AKT signaling. Indeed, the developmental defects in neurons lacking DISC1 can be rescued by pharmacologically blocking the activation of an AKT downstream target. However, as shown by Enomoto et al., the loss of girdin produces deleterious effects on neuronal morphology, suggesting that a proper balance of girdin function is crucial.

Collectively, these studies thoroughly characterize the interaction between DISC1 and girdin, and shed much light on the consequences of this interaction on neuronal morphology as well as on the positioning of neurons in the dentate gyrus. The role of girdin in the pathology of neuropsychiatric diseases is unknown, and remains an interesting question for the future. Characterizing the molecules that act up- or downstream of DISC1 remains an important area of investigation and could aid the development of pharmacological interventions in the future. It’s intriguing that DISC1 acting through girdin regulates the activity of AKT as AKT1 was previously identified as a schizophrenia risk gene (Emamian et al., 2004). This suggests a convergence of multiple schizophrenia-associated genes in a shared pathway, and thus it will be important to determine if the DISC1-girdin-AKT1 pathway is particularly vulnerable in neuropsychiatric disorders.

References:

Blackwood DH, Fordyce A, Walker MT, St Clair DM, Porteous DJ, Muir WJ. Schizophrenia and affective disorders--cosegregation with a translocation at chromosome 1q42 that directly disrupts brain-expressed genes: clinical and P300 findings in a family. Am J Hum Genet . 2001 Aug 1 ; 69(2):428-33. Abstract

Millar JK, Christie S, Semple CA, Porteous DJ. Chromosomal location and genomic structure of the human translin-associated factor X gene (TRAX; TSNAX) revealed by intergenic splicing to DISC1, a gene disrupted by a translocation segregating with schizophrenia. Genomics . 2000 Jul 1 ; 67(1):69-77. Abstract

Duan X, Chang JH, Ge S, Faulkner RL, Kim JY, Kitabatake Y, Liu XB, Yang CH, Jordan JD, Ma DK, Liu CY, Ganesan S, Cheng HJ, Ming GL, Lu B, Song H. Disrupted-In-Schizophrenia 1 regulates integration of newly generated neurons in the adult brain. Cell . 2007 Sep 21 ; 130(6):1146-58. Abstract

Kobayashi K. Targeting the hippocampal mossy fiber synapse for the treatment of psychiatric disorders. Mol Neurobiol . 2009 Feb 1 ; 39(1):24-36. Abstract

Anai M, Shojima N, Katagiri H, Ogihara T, Sakoda H, Onishi Y, Ono H, Fujishiro M, Fukushima Y, Horike N, Viana A, Kikuchi M, Noguchi N, Takahashi S, Takata K, Oka Y, Uchijima Y, Kurihara H, Asano T. A novel protein kinase B (PKB)/AKT-binding protein enhances PKB kinase activity and regulates DNA synthesis. J Biol Chem . 2005 May 6 ; 280(18):18525-35. Abstract

Emamian ES, Hall D, Birnbaum MJ, Karayiorgou M, Gogos JA. Convergent evidence for impaired AKT1-GSK3beta signaling in schizophrenia. Nat Genet . 2004 Feb 1 ; 36(2):131-7. Abstract