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 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.

Tom Fagan mentions that calcineurin regulates phosphorylations elicited by both glutamatergic and dopaminergic signaling. The activity of D-amino acid oxidase is increased in schizophrenia, and this also affects signaling through both these pathways. Is there any clinical benefit with the use of sodium benzoate which inhibits DAO activity?

He also mentions that EGR3 can be regulated by the activity of neuregulin. Interestingly, Roberts et al. suggest that BDNF, which is decreased in first-episode psychosis (Buckley et al., 2007), induces synthesis of EGR3 to regulate activity of GABRA4. Ma and colleagues (Ma et al., 2005) conclude that GABRA4 is involved in the etiology of autism and it has also has been implicated in nicotine dependence (Saccone et al.,...  Read more

Tom Fagan mentions that calcineurin regulates phosphorylations elicited by both glutamatergic and dopaminergic signaling. The activity of D-amino acid oxidase is increased in schizophrenia, and this also affects signaling through both these pathways. Is there any clinical benefit with the use of sodium benzoate which inhibits DAO activity?

He also mentions that EGR3 can be regulated by the activity of neuregulin. Interestingly, Roberts et al. suggest that BDNF, which is decreased in first-episode psychosis (Buckley et al., 2007), induces synthesis of EGR3 to regulate activity of GABRA4. Ma and colleagues (Ma et al., 2005) conclude that GABRA4 is involved in the etiology of autism and it has also has been implicated in nicotine dependence (Saccone et al., 2007).

Glorioso and colleagues (Glorioso et al., 2006) report changes in genes encoding early-immediate genes such as EGR1 and EGR2 and RGS4 which is involved in cellular signaling and has been implicated in schizophrenia following BDNF gene ablation. Several studies report that zinc increases BDNF expression. Does this give support to the zinc deficiency theory of schizophrenia?

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