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ICOSR 2007—Tracking the Schizophrenia Gene Suspects

Editor's Note: On Sunday, 1 April 2007, at the International Congress on Schizophrenia Research in Colorado Springs, Joel Kleinman, of the Clinical Brain Disorders Branch at NIMH, chaired the session “Pathways to Schizophrenia: Susceptibility genes and their signaling cascades.” Young Investigator travel awardee Eric Epping of the University of Iowa was there to report for you.

6 June 2007. The presentations during the session focused on several important schizophrenia candidate genes and the effect of variation in these genes on pathways that may impact the biology of schizophrenia.

Results from a detailed analysis of the dysbindin gene were presented by Brien Riley from Virginia Commonwealth University. Previous work has shown that this gene, which is found on chromosome 6p, contains a haplotype associated with a high risk for developing schizophrenia. An analysis of the dysbindin haplotype and a region on chromosome 8 with evidence of linkage to schizophrenia was performed to determine if these haplotypes may be epistatic; however, no linkage was found to the chromosome 8 region in families who had the high-risk dysbindin haplotype. A comparative analysis of human and primate alleles was performed to characterize variants that may have arisen during the course of evolution. In the group’s Irish sample, a total of 26 variants were found in dysbindin that were specific to the Irish population high-risk haplotype, with one variant producing a non-synonymous coding change. However, this variant was not associated with schizophrenia. As dysbindin and Akt, a seronine/threonine kinase involved in neuronal growth and survival, may be involved in the same cellular pathways, expression of 21 genes in the Akt cell survival pathway were measured to compare expression according to dysbindin genotype from samples in the Stanley brain collection. No differences in gene expression were found by dysbindin genotype. Further work will include assessment of dysbindin transcript variants, allele-specific expression, and expression in other cell types.

Joel Kleinman discussed work to date on catechol-O-methyl transferase (COMT) in his talk entitled “Complex genetics in the human brain: lessons from COMT.” He provided a thorough summary of previous COMT work from multiple labs that have focused on the differential effects of the COMT Val/Met polymorphism, with the Val form producing increasing COMT-mediated dopamine breakdown. Kleinman cited results indicating that other alleles in COMT itself and other genes likely interact genetically with the Val/Met variant. One example was the work from Nackley and colleagues which identified differences in COMT mRNA stability according to haplotype (Nackley et al., 2006), of which the Val/Met polymorphism is only one variant contained in COMT haplotypes. Nicodemus and colleagues from the NIMH group identified epistasis between COMT and several genes, including GAD1, DAOA, DISC1, GRM3, and RGS4 (Nicodemus et al., 2007). While Lipska and colleagues found no differences in RGS4 expression in postmortem samples when comparing cases with controls, differences in RGS4 expression were found by COMT genotype in both the dorsolateral prefrontal cortex and in the hippocampus (Lipska et al., 2006). Caspi and colleagues also identified a gene-environment interaction between COMT genotype and marijuana use before age 15, with COMT Val homozygotes who used cannabis having the highest risk of developing psychosis by age 26 (Caspi et al., 2005). Kleinman also presented data showing a relationship between COMT and cannabinoid receptor CNR1 expression, with differential decreases in CNR1 expression by COMT genotype as individuals age. It is hypothesized that these differences in COMT-associated CNR1 expression influence susceptibility to psychosis secondary to cannabis use. Based on the results presented, Kleinman’s conclusion was, “Don’t close down the [COMT] mine just yet.”

Amanda Law from the University of Oxford, and in collaboration with researchers at NIMH, presented data on the function of the NRG1/ErbB4 pathway in schizophrenia. Neuregulin 1 (NRG1) was previously identified by genetic linkage and association studies with schizophrenia, and its various isoforms are involved in neuronal growth, synaptic plasticity, and cell migration. ErbB4 is a tyrosine kinase membrane receptor activated by NRG1, and it is also a schizophrenia susceptibility gene. Results from experiments on NRG1 expression in postmortem human hippocampus samples were presented, with specific SNPs at the 5’ end of the gene associated with schizophrenia having effects on NRG1 expression. One SNP and the previously identified high-risk haplotype were found to be associated with increased Type IV NRG1 expression in the human brain in schizophrenia. Type I expression was also increased in subjects with schizophrenia, but increased expression in both control and schizophrenia groups was associated with a SNP in the high-risk haplotype and specifically with type IV (Law et al., 2006; see SRF related news story). Type IV NRG1 expression was also examined in fetal brain tissue and a range of other human tissues. Full-length Type IV was cloned and characterized in the adult human brain, and novel variants in the NRG1 gene were identified during human brain development. Data on ErbB4 expression was also presented, with increased expression found in the dorsolateral prefrontal cortex, as well as association of schizophrenia risk variants with alteration in ErbB4 expression (Law et al., 2007). In order to determine if peripheral blood B lymphoblasts could be used as a model for genetic studies in the NRG1 pathway, ErbB4 expression was examined. Increased expression of the ErbB4 variant found to be altered in the brain in schizophrenia, and in relation to genetic risk in the gene, was replicated in B lymphoblasts. Individuals homozygous for the schizophrenia risk alleles had significantly higher levels of ErbB4. NRG1 expression was not found in this cell type. Downstream factors in this pathway were investigated, with one phosphatidylinositol-3 kinase (PI3K) complex found to have increased expression in B lymphoblasts associated with the ErbB4 high-risk SNP and disease. The same expression differences were found in the hippocampus. As the NRG1 pathway induces signaling processes via the production of the molecule PIP3, the effect of NRG1 on PIP3 levels in schizophrenia was measured in B lymphoblasts. No differences between subjects with schizophrenia or controls were found; however, significant differences were observed in relation to genetic risk variants. Law suggested that alterations in the NRG1/ErbB4 signaling pathway in relation to genetic risk for schizophrenia may mediate their effects by the PI3K pathway.

Barbara Lipska, also from NIMH, presented data from her group’s work on characterization of gene expression in the DISC1 pathway. DISC1 (disrupted in schizophrenia) was identified in a region on chromosome 1 associated with not only schizophrenia, but also depression and bipolar affective disorder when a translocation with this region occurs with chromosome 11. Other genetic associations with DISC1 and schizophrenia, cognitive deficits, as well as altered hippocampal structure and function have been reported. The DISC1 protein is involved in cell division, neuronal migration, neurite outgrowth, and synapse formation. It specifically functions as a scaffold protein (Camargo et al., 2007), and proven or putative interactions with over 100 different proteins have been identified. LIS1, FEZ1, and NUDEL are of particular interest, and their expression was examined in the studies presented (see also Lipska et al., 2006a; Lipska et al, 2006b). Expression of DISC1 in the human brain was measured from postmortem tissues obtained from a range of ages at the time of death (including fetal tissues), with the highest expression of DISC1 found during infancy. LIS1, FEZ1, and NUDEL were found to have significantly increased expression during the second trimester of gestation, but this was not seen with DISC1. The location of DISC1 expression in the brain was also studied, with expression highest in the superior temporal gyrus (STG), but also higher than average in the neocortex, amygdala, hippocampus, entorhinal cortex, and gray matter of the dorsolateral prefrontal cortex (DLPFC). Differential expression of FEZ1, LIS1, and NUDEL was also identified, with FEZ1 expression lower than average in the amygdala, DLPFC, and STG, but higher in the neocortex (which is also where LIS1 expression was found to be higher), but it was lower than average in the amygdala and hippocampus. NUDEL expression was highest in the DLPFC, STG, and neocortex.

DISC1 mRNA expression in controls and schizophrenia patients was found to be similar, and mRNA levels did not vary by DISC1 genotype. However, increased DISC1 protein was found in hippocampal regions of schizophrenia patients by Western blotting, and high-risk alleles were found to be associated with increased protein levels. DISC1 is alternatively spliced into different isoforms, with a novel transcript identified in hippocampal tissue that was expressed in association with schizophrenia risk alleles. The presence of high-risk DISC1 alleles was also associated with decreased FEZ1, NUDEL, and LIS1 expression in the hippocampus and DLPFC of schizophrenia patients. A high-risk allele identified in LIS1 was also found to be associated with differential expression of LIS1. This work has shown variable expression of genes in the DISC1 pathway in schizophrenia in brain regions implicated in the pathophysiology of the disorder, and this expression is modulated by alleles that confer increased risk of developing the disorder.—Eric Epping.

Comments on Related News


Related News: Polymorphisms and Schizophrenia—The Ups and Downs of Neuregulin Expression

Comment by:  William Carpenter, SRF Advisor (Disclosure)
Submitted 22 April 2006
Posted 22 April 2006
  I recommend the Primary Papers

Related News: Polymorphisms and Schizophrenia—The Ups and Downs of Neuregulin Expression

Comment by:  Stephan Heckers, SRF Advisor
Submitted 29 April 2006
Posted 29 April 2006
  I recommend the Primary Papers

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.

View all comments by Stephan Heckers

Related News: Polymorphisms and Schizophrenia—The Ups and Downs of Neuregulin Expression

Comment by:  Bryan Roth, SRF Advisor
Submitted 5 May 2006
Posted 5 May 2006
  I recommend the Primary Papers

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.

View all comments by Bryan Roth

Related News: Polymorphisms and Schizophrenia—The Ups and Downs of Neuregulin Expression

Comment by:  Patricia Estani
Submitted 9 June 2007
Posted 10 June 2007
  I recommend the Primary Papers

Related News: Meta-analysis Supports Case for Cannabis in Etiology of Psychosis

Comment by:  Jim van Os
Submitted 8 August 2007
Posted 8 August 2007

This excellent review confirms the previous meta-analysis by Henquet et al. (2005) and as such does not add anything new. The importance lies in the UK context: previously the Lancet has been mostly skeptical with regard to this issue. The fact that the leading UK medical journal now also allows these findings to see daylight is a significant event and helps stimulate further funding for the effort that several groups worldwide have started working on over the last five years: the search for the mechanism explaining the link.

View all comments by Jim van Os

Related News: Meta-analysis Supports Case for Cannabis in Etiology of Psychosis

Comment by:  John McGrath, SRF Advisor
Submitted 9 August 2007
Posted 10 August 2007
  I recommend the Primary Papers

It is reassuring to see that the results of the latest meta-analysis (Moore et al., 2007) are consistent with previous meta-analyses, and that the various meta-analyses are broadly consistent with the now much-tortured primary data. Despite the meta-analysis fatigue, the results are too important to ignore.

When thinking about the impact of cannabis on schizophrenia frequency measures, it is important to remember that cannabis use may translate to an increase in the prevalence of active psychosis via two mechanisms. The data suggest that as the prevalence of cannabis use increases in a population, the incidence of schizophrenia should also increase (Hickman et al., 2007). Furthermore, in those with established schizophrenia, cannabis use is associated with poorer outcomes (i.e., reduced remission rates). Thus, from a modeling perspective, increased cannabis use could lead to an increase in the prevalence of active psychosis via two mechanisms (i.e., increased “inflow” and decreased “outflow”) (McGrath and Saha, 2007).

The prevalence of active psychosis in the community may be “under the influence” of cannabis from more than one perspective.

References:

Moore THM, Zammit S, Lingford-Hughes A, Barnes TRE, Jones PB, Burke M, Lewis G. Cannabis use and risk of psychotic or affective mental health outcomes: A systematic review. Lancet. 2007 July 28; 370:319-328. Abstract

McGrath J, Saha S. Thought experiments on the incidence and prevalence of schizophrenia “under the influence” of cannabis. Addictions 2007 Apr;102(4):514-5. Abstract

Hickman M, Vickerman P, Macleod J, Kirkbride J, Jones PB. Cannabis and schizophrenia: model projections of the impact of the rise in cannabis use on historical and future trends in schizophrenia in England and Wales. Addiction. 2007 Apr;102(4):597-606. Abstract

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Related News: Meta-analysis Supports Case for Cannabis in Etiology of Psychosis

Comment by:  Dana MarchEzra Susser (SRF Advisor)
Submitted 20 August 2007
Posted 20 August 2007

The recent meta-analysis in the Lancet (Moore et al., 2007) regarding cannabis use and psychotic or affective mental health outcomes is, indeed, a necessary contribution. It is the first systematic review restricted to longitudinal studies of cannabis use and mental health outcomes. For this addition to the contours of the literature, Zammit and colleagues are to be commended.

We may be more optimistic than the authors, however, about the potential for future longitudinal studies to shed further light on the question of causality, and perhaps more cautious about the present state of the evidence. Given the public health and policy implications, we propose a concerted effort to complete observational studies that are designed to rule out the main alternative explanations for the association (e.g., genetic or social factors that independently influence both cannabis use and psychosis). The Swedish conscript study (Zammit et al., 2002) is a fine example of one such study. We should also be considering natural experiments and designs based on instrumental variables enabled by in order to complement this work. For instance, we might capitalize on situations created by policy changes that affect the availability—and therefore use—of cannabis in order to examine the impact on the development of psychosis. Whether at the individual or the population level, both creativity and rigor are required.

References:

Moore TH, Zammit S, Lingford-Hughes A, Barnes TR, Jones PB, Burke M, Lewis G. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. Lancet. 2007 Jul 28;370(9584): 319-28. Abstract

Zammit S, Allebeck P, Andreasson S, Lundberg I, Lewis G. Self reported cannabis use as a risk factor for schizophrenia in Swedish conscripts of 1969: historical cohort study. BMJ. 2002 Nov 23;325 (7374):1199. Abstract

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View all comments by Ezra Susser

Related News: Meta-analysis Supports Case for Cannabis in Etiology of Psychosis

Comment by:  Amresh Shrivastava
Submitted 20 October 2007
Posted 24 October 2007

Current interest in cannabis and the onset of psychosis is laudable. The Lancet paper no doubt establishes a causal link based upon what has been known in the literature (Raphael et al., 2005; Roberts et al., 2007; Rey et al., 2004; Wittchen et al., 2007). The authors need to be congratulated for taking extreme care to incorporate most of the studies and also for making conclusions with a sense of skepticism. That is where further questions arise.

1. Cannabis is used only in certain cultures and known to be involved in a maximum 50 percent of cases of psychosis, schizophrenia, and schizophreniform psychosis (Gregg et al., 2007). In that sense, are there two different phenotypes of schizophrenia, a) where exposure to cannabis is necessarily a factor and b) where a different set of potentiating or precipitating factors work, not cannabis?

2. Even if we focus only on the first possibility, there are few unanswered questions such as, what are the concurrent clinical conditions along with cannabis abuse? Do these patients have cognitive dysfunction? Is that reflective of broader brain mechanism changes?

3. There seems to be no reliable biological explanation as to why exposure to cannabis should precipitate psychosis. Cannabis is one of the most commonly used illicit drugs. Its active compound “cannabidols” has 64 active isomers, each having differing effects on health and behavior. There is strong support for a link between cannabis and development—exacerbation of psychosis as well as other mental health conditions (e.g., anxiety, depression). Further research is needed to determine the underlying neurochemical processes and their possible contributions to etiology, as well as the social factors that contribute to the increasing use of cannabis by young people.

4. There is a theory that preexisting cognitive dysfunction is a core feature of schizophrenia. Accepting this, there are no studies to show “causal relationship” between cannabis and cognitive dysfunction.

The current levels of information and understanding, though collected over last 25-30 years of research, are far from adequate to establish any direct relationship except “mere association.” It is hoped that more precise biological, imaging, and neuropsychological studies would be able to throw fresh light on this important area of research.

Acute cannabis administration can induce memory impairments, sometimes persisting months following abstinence. There is no evidence that residual effects on cognition remain after years of abstinence. The scarce literature on neuroimaging, mainly done in non-psychotic populations, shows little evidence that cannabis has effects on brain anatomy. Acute effects of cannabis include increases of cerebral blood flow, whereas long-term effects of cannabis include attenuation of cerebral blood flow. In animals Δ9-tetrahydrocannabinol enhances dopaminergic neurotransmission in brain regions known to be implicated in psychosis. Studies in humans show that genetic vulnerability may add to increased risk of developing psychosis and cognitive impairments following cannabis consumption. Δ9-tetrahydrocannabinol induces psychotic-like states and memory impairments in healthy volunteers (Linszen et al., 2007).

On the basis of six studies, it is concluded that there was insufficient evidence to prove conclusively that long-term cannabis use causes or does not cause residual abnormalities. The results of several reviews were also inconclusive as to whether cannabis use during adolescence may have a lasting effect on cognitive functioning and brain structure. However, it could not rule out that a) certain cognitive and cerebral abnormalities existed in patients before cannabis use began and b) that patients were suffering from subacute effects of cannabis (Weeda et al., 2006).

Continued cannabis use by persons with schizophrenia predicts a small increase in psychotic symptom severity but not vice versa (Degenhardt et al., 2007). Currently, there is a lot of interest in cannabis use as a risk factor for the development of schizophrenia. Cognitive dysfunction associated with long-term or heavy cannabis use is similar in many respects to the cognitive endophenotypes that have been proposed as vulnerability markers of schizophrenia. In this situation, we need to examine the similarities between these in the context of the neurobiology underlying cognitive dysfunction, particularly implicating the endogenous cannabinoid system, which plays a significant role in attention, learning, and memory, and in general, inhibitory regulatory mechanisms in the brain. Closer examination of the cognitive deficits associated with specific parameters of cannabis use and interactions with neurodevelopmental stages and neural substrates will better inform our understanding of the nature of the association between cannabis use and psychosis. The theoretical and clinical significance of further research in this field is enhancing our understanding of underlying pathophysiology and improving the provision of treatments for substance use and mental illness (Solowij et al., 2007). Many studies now show a robust and consistent association between cannabis consumption and the ulterior development of psychosis. Furthermore, our better understanding of cannabis biology allows the proposal of a plausible hypothetical model, based notably on possible interactions between cannabis and dopaminergic neurotransmission (Jockers-Scherubl, 2006). Do they suffer from other disorders, which are underlying or may be causal or comorbid, and do these comorbid conditions also have neurocognitive changes, e.g., psychosis, ADHD, LD, Tourette disorder, and other movement disorders, or depression? Is there an interrelationship among these factors to cause abuse and degree of cannabis consumption?

References:

Raphael B, Wooding S, Stevens G, Connor J. Comorbidity: cannabis and complexity. J Psychiatr Pract. 2005 May; 11(3): 161-7.

Roberts RE, Roberts CR, Xing Y. Comorbidity of substance use disorders and other psychiatric disorders among adolescents: Evidence from an epidemiologic survey. Drug Alcohol Depend. 2007 Apr;88 Suppl 1:S4-13. Epub 2007 Feb 1. Abstract

Rey JM, Martin A, Krabman P. Is the party over? Cannabis and juvenile psychiatric disorder: the past 10 years. J Am Acad Child Adolesc Psychiatry. 2004 Oct; 43(10): 1194-205. Abstract

Wittchen HU, Frohlich C, Behrendt S. Cannabis use and cannabis use disorders and their relationship to mental disorders: A 10-year prospective-longitudinal community study in adolescents. Drug Alcohol Depend. 2007 Apr;88 Suppl 1:S60-70. Epub 2007 Jan 25. Abstract

Gregg L, Barrowclough C, Haddock G. Reasons for increased substance use in psychosis. Clin Psychol Rev. 2007 May;27(4):494-510. Epub 2007 Jan 19. Abstract

Linszen D, van Amelsvoort T. Cannabis and psychosis: an update on course and biological plausible mechanisms. Curr Opin Psychiatry. 2007 Mar; 20(2): 116-20. Abstract

Weeda MR, Peters BD, De Haan L, Linszen DH. Residual neuropsychological, structural and functional brain abnormalities after long-term cannabis use] Tijdschr Psychiatr. 2006; 48(3): 185-93. Abstract

Degenhardt L, Tennant C, Gilmour S, Schofield D, Nash L, Hall W,McKay D. The temporal dynamics of relationships between cannabis, psychosis and depression among young adults with psychotic disorders: findings from a 10-month prospective study. Psychol Med. 2007 Feb 9; 1-8.

Solowij N, Michie PT. Cannabis and cognitive dysfunction: parallels with endophenotypes of schizophrenia? J Psychiatry Neurosci. 2007 Jan; 32(1): 30-52. Abstract

Jockers-Scherubl MC. [Schizophrenia and cannabis consumption: epidemiology and clinical symptoms] Prax Kinderpsychol Kinderpsychiatr. 2006; 55(7): 533-43. Abstract

Curtis L, Rey-Bellet P, Merlo MC. [Cannabis and psychosis] Rev Med Suisse. 2006 Sep 20; 2(79): 2099-100, 2102-3.

Costentin J. [Neurobiology of cannabis--recent data enlightening driving disturbances] Ann Pharm Fr. 2006 May; 64(3): 148-59. Abstract

View all comments by Amresh Shrivastava