The two latest additions to the burgeoning DISC1 literature provide additional support for a role of this gene in cognitive function and schizophrenia, and suggest that more comprehensive studies will be useful as we move to a greater understanding of its role in CNS function. Koike et al. (2006) found that a relatively common mouse strain has a naturally occurring mutation in DISC1 resulting in a truncated form of the protein, similar in size (exon 7 vs. exon 8 disruptions) to that observed in the members of the Scottish pedigree in which the translocation was first detected. C57/BL/6J mice, into which mutant alleles were transferred, displayed significant impairments on a spatial working memory task similar to one used in humans (Lencz et al., 2003). These data are similar to those observed by our group (Burdick et al., 2005) and others (  Read more

The two latest additions to the burgeoning DISC1 literature provide additional support for a role of this gene in cognitive function and schizophrenia, and suggest that more comprehensive studies will be useful as we move to a greater understanding of its role in CNS function. Koike et al. (2006) found that a relatively common mouse strain has a naturally occurring mutation in DISC1 resulting in a truncated form of the protein, similar in size (exon 7 vs. exon 8 disruptions) to that observed in the members of the Scottish pedigree in which the translocation was first detected. C57/BL/6J mice, into which mutant alleles were transferred, displayed significant impairments on a spatial working memory task similar to one used in humans (Lencz et al., 2003). These data are similar to those observed by our group (Burdick et al., 2005) and others (Callicott et al., 2005; Hennah et al., 2005; Cannon et al., 2005), although no study to date has utilized the same neurocognitive tasks. Lipska et al. (2006) report that genes and proteins (NUDEL, FEZ1) known to interact with DISC1 are also aberrant in schizophrenia postmortem tissue, with some evidence that DISC1 risk polymorphisms also influence expression across the pathway.

Taken together, these two papers suggest that the assessment of genes involved in the DISC1 pathway may be worthwhile in the evaluation of working memory function. To date, most studies have focused on risk alleles within DISC1, with little attention paid to the critical interacting genes. Studies are now underway assessing the relationship between FEZ1 and NUDEL and risk for schizophrenia in a number of populations, as well as studies examining their role in neurocognitive and neuroimaging parameters. Clearly, as the Lipska paper indicates, studies that attempt to assess multiple genes in this pathway will be critical, although the common concern of power in assessing gene-gene interactions, especially across multiple genes, may be a limitation. Moreover, these studies indicate that interaction studies will need to consider additional phenotypes other than diagnosis, and perhaps “purer” tasks of neurocognitive function may be worthwhile, as suggested by Koike et al. Finally, both of these papers underscore the fact that the next wave of genetic studies of schizophrenia will encompass the use of multiple probes, whether with neurocognitive assessments, postmortem analyses, or animal models of disease, amongst others, to fully validate the relationships between putative risk genes and the pathophysiology of schizophrenia and related disorders.

References:

Burdick KE, Hodgkinson CA, Szeszko PR, Lencz T, Ekholm JM, Kane JM, Goldman D, Malhotra AK. DISC1 and neurocognitive function in schizophrenia. Neuroreport 2005; 16(12): 1399-1402. Abstract

Callicott JH, Straub RE, Pezawas L, Egan MF, Mattay VS, Hariri AR, Verchinski BA, Meyer-Lindenberg A, Balkissoon R, Kolachana B, Goldberg TE, Weinberger DR. Variation in DISC1 affects hippocampal structure and function and increases risk for schizophrenia. Proc Natl Acad Sci U S A. 2005 Jun 14;102(24):8627-32. Epub 2005 Jun 6. Abstract

Cannon TD, Hennah W, van Erp TG, Thompson PM, Lonnqvist J, Huttunen M, Gasperoni T, Tuulio-Henriksson A, Pirkola T, Toga AW, Kaprio J, Mazziotta J, Peltonen L. Association of DISC1/TRAX haplotypes with schizophrenia, reduced prefrontal gray matter, and impaired short- and long-term memory. Arch Gen Psychiatry, 2005; 62(11):1205-1213. Abstract

Hennah W, Tuulio-Henriksson A, Paunio T, Ekelund J, Varilo T, Partonen T, Cannon TD, Lonnquist J, Peltonen L. A haplotype within the DISC1 gene is associated with visual memory functions in families with high density of schizophrenia. Mol Psychiatry 2005; 10(12):1097-1103. Abstract

Lencz T, Bilder RM, Turkel E, Goldman RS, Robinson D, Kane JM, Lieberman JA. Impairments in perceptual competency and maintenance on a visual delayed match-to-sample test in first episode schizophrenia. Arch Gen Psychiatry 2003; 60(3):238-243. Abstract

In their recent paper, Koike et al. provide new evidence in support of a genetic determinant of working memory function in the vicinity of the mouse DISC1 gene. They report their discovery of a naturally occurring DISC1 deletion variant in the 129S6/SvEv mouse strain that leads to reduced protein expression and that provides a potentially very important new tool for analyzing the cellular and behavioral phenotypes associated with DISC1 insufficiency. Given the strong evidence of a relationship between a cytogenetic abnormality that leads to DISC1 truncation in humans and major mental illness (Millar et al., 2000), this murine model stands to greatly serve our understanding of the molecular and cellular determinants of poor cognition in schizophrenia and bipolar disorder.

The authors are parsimonious in reminding us of the substantial limitations of models such as this. Specifically, the current approach does not allow...  Read more

In their recent paper, Koike et al. provide new evidence in support of a genetic determinant of working memory function in the vicinity of the mouse DISC1 gene. They report their discovery of a naturally occurring DISC1 deletion variant in the 129S6/SvEv mouse strain that leads to reduced protein expression and that provides a potentially very important new tool for analyzing the cellular and behavioral phenotypes associated with DISC1 insufficiency. Given the strong evidence of a relationship between a cytogenetic abnormality that leads to DISC1 truncation in humans and major mental illness (Millar et al., 2000), this murine model stands to greatly serve our understanding of the molecular and cellular determinants of poor cognition in schizophrenia and bipolar disorder.

The authors are parsimonious in reminding us of the substantial limitations of models such as this. Specifically, the current approach does not allow for a clear statement that the DISC1 gene itself modulates the traits of interest. The DISC1 deletion variant may simply be in linkage disequilibrium with the actual phenotype-determining gene, and/or variation in DISC1 may influence cognition in a manner that is modified by a nearby genetic region. For example, Cannon et al. recently showed that a 4-SNP haplotype spanning DISC1 and an adjacent gene, translin-associated factor X (TRAX) is more predictive of anatomical and cognitive indices of reduced prefrontal cortical and hippocampal function than are any known haplotypes spanning DISC1 only. Clearly, additional consideration of the genetic environment in which DISC1 lies is necessary, and discovery of flanking regions that contain modifiers of the actions of DISC1, and vice versa, would be extremely interesting.

The greatest impact of the paper by Koike et al. is hinged on the fact that mice carrying one or two copies of the deletion variant exhibit poor choice accuracy in a delayed non-match to position task. Specifically, mutant DISC1 mice made fewer correct choices than did wild-type littermate C57 mice. Because a procedure such as this is necessarily psychologically complex, performance failure is hardly prima facie evidence for impairments of spatial working memory, or for prefrontal cortical dysfunction, in general. Nevertheless, the data are remarkable in establishing a phenotypic bridge between species and in laying the foundation for more sophisticated behavioral studies that will narrow in on the psychological constructs and neural systems affected by variation in this genetic region. Through facilitating a greater understanding of the cognitive phenotypes associated with DISC1 variation, the model should open doors to understanding key phenotypic aspects of schizophrenia and bipolar disorder.

References:

Koike H, Arguello PA, Kvajo M, Karayiorgou M, Gogos JA. Disc1 is mutated in the 129S6/SvEv strain and modulates working memory in mice. Proc Natl Acad Sci U S A. 2006 Feb 16; [Epub ahead of print] Abstract

Millar JK, Wilson-Annan JC, Anderson S, Christie S, Taylor MS, Semple CA, Devon RS, Clair DM, Muir WJ, Blackwood DH, Porteous DJ. Disruption of two novel genes by a translocation co-segregating with schizophrenia. Hum Mol Genet. 2000 May 22;9(9):1415-23. Abstract

Cannon TD, Hennah W, van Erp TG, Thompson PM, Lonnqvist J, Huttunen M, Gasperoni T, Tuulio-Henriksson A, Pirkola T, Toga AW, Kaprio J, Mazziotta J, Peltonen L. Association of DISC1/TRAX haplotypes with schizophrenia, reduced prefrontal gray matter, and impaired short- and long-term memory. Arch Gen Psychiatry. 2005 Nov;62(11):1205-13. Abstract

Disrupted In Schizophrenia 1 was first identified as a genetic susceptibility factor in schizophrenia because it is disrupted by a translocation between chromosomes 1 and 11 in a large Scottish family with a high loading of schizophrenia and related mental illness. Since then, numerous genetic studies have implicated DISC1 as a risk factor in psychiatric illness in several populations. Given the limitations on studies using brain tissue from patients, an obvious next step was to engineer knockout mice, but these have been slow in coming. As a first step toward this, Kioke and colleagues now report an unexpected naturally occurring genetic variant in the 129/SvEv mouse strain.

Kioke et al. report that the 129/SvEv mouse strain carries a 25 bp deletion in DISC1 exon 6, and that this results in a shift of open reading frame and introduction of a premature stop codon. Several embryonal stem cell lines have been isolated for the 129 strain, favoring it for gene targeting studies. However, this strain has a number of well-established behavioral characteristics (  Read more

Disrupted In Schizophrenia 1 was first identified as a genetic susceptibility factor in schizophrenia because it is disrupted by a translocation between chromosomes 1 and 11 in a large Scottish family with a high loading of schizophrenia and related mental illness. Since then, numerous genetic studies have implicated DISC1 as a risk factor in psychiatric illness in several populations. Given the limitations on studies using brain tissue from patients, an obvious next step was to engineer knockout mice, but these have been slow in coming. As a first step toward this, Kioke and colleagues now report an unexpected naturally occurring genetic variant in the 129/SvEv mouse strain.

Kioke et al. report that the 129/SvEv mouse strain carries a 25 bp deletion in DISC1 exon 6, and that this results in a shift of open reading frame and introduction of a premature stop codon. Several embryonal stem cell lines have been isolated for the 129 strain, favoring it for gene targeting studies. However, this strain has a number of well-established behavioral characteristics (http://www.informatics.jax.org/external/festing/mouse/docs/129.shtml). Therefore, to assign any phenotype specifically to the DISC1 deletion variant, the 129 DISC1 variant had to be transferred to a C57BL/6J background, with its own, rather different but equally characteristic behavior (http://www.informatics.jax.org/external/festing/mouse/docs/C57BL.shtml). There were no detectable gross morphological alterations in the prefrontal cortex, cortex, and hippocampus on transferring the 129 DISC1 locus onto the C57BL/6J background. However, the mutation did result in working memory deficits, consistent with several reports linking DISC1 to cognition.

It is difficult to know what phenotype to expect from a mouse model for schizophrenia, but in humans it is widely believed that mutations confer only a susceptibility to developing illness. Many susceptible individuals function apparently normally, although subtle neurological endophenotypes are detectable. In individuals who do go on to develop schizophrenia, cognitive deficits are a major characteristic. These mild cognitive deficits in mice with loss of DISC1 function are therefore close to what we might predict.

The molecular mechanism by which DISC1 confers susceptibility to psychiatric illness is the subject of some debate. Sawa and colleagues have suggested that a mutant truncated protein resulting from the t(1;11) is responsible for the psychiatric disorders in the Scottish family. Millar and colleagues, however, report that there is no evidence for such a hypothetical protein in t(1;11) cell lines, but rather that the levels of DISC1 transcript and protein are reduced, consistent with a haploinsufficiency model. Identification of the deletion in mice may shed further light on this debate, since while the mutation does not affect DISC1 transcript levels, no mutant truncated protein is detectable, even though such a protein might theoretically be produced as a result of the premature stop codon. Moreover, both homozygotes and heterozygotes have cognitive impairment, demonstrating that DISC1 haploinsufficiency is sufficient to affect central nervous system function.

In this initial study, Kioke and colleagues have left many questions unanswered. In particular, the behavioral studies are limited to one working memory task and one test of locomotion. Ideally, a whole battery of behavioral and cognitive tests should be performed. Since 129/SvEv mice reportedly have impaired hippocampal function, high levels of anxiety-like behavior and altered NMDA receptor-related activity, it will be interesting to discover which, if any, of these phenotypes also co-segregate with the 129 DISC1 variant. It is also interesting to note that the 129 strain is effectively a null for full-length DISC1, but with no gross alteration in brain morphology. This has to be reconciled with the observed effect of transient RNAi mediated down-regulated expression in utero (Kamiya et al., 2005) and the possible, but still anecdotal observation of embryonic lethality in experimental DISC1 knockouts.

This is outstanding work reporting DISC1 genetically engineered mice. Thus far, one type of DISC1 mutant mouse has been reported, by Gogos and colleagues (Koike et al., 2006).

There are two remarkable points in this work. First, of most importance, John Roder and Steve Clapcote have been very successful in using mice with ENU-induced mutations for their questions. Due to the complexity of the DISC1 gene and isoforms, several groups, including ours, have tried but not succeeded in generating knockout mice. However, Roder and Clapcote found alternative mice that could be used in testing our main hypothesis. I believe that the majority of the success in this work is on this particular point. Indeed, to explore animal models for other susceptibility genes for major mental illnesses, this approach should be considered.

Second, it is very interesting that different mutations in the same gene display different types of phenotypes. I appreciate the excellence in the extensive behavioral assays in this work.

Although we need...  Read more

This is outstanding work reporting DISC1 genetically engineered mice. Thus far, one type of DISC1 mutant mouse has been reported, by Gogos and colleagues (Koike et al., 2006).

There are two remarkable points in this work. First, of most importance, John Roder and Steve Clapcote have been very successful in using mice with ENU-induced mutations for their questions. Due to the complexity of the DISC1 gene and isoforms, several groups, including ours, have tried but not succeeded in generating knockout mice. However, Roder and Clapcote found alternative mice that could be used in testing our main hypothesis. I believe that the majority of the success in this work is on this particular point. Indeed, to explore animal models for other susceptibility genes for major mental illnesses, this approach should be considered.

Second, it is very interesting that different mutations in the same gene display different types of phenotypes. I appreciate the excellence in the extensive behavioral assays in this work.

Although we need to wait for any molecular and mechanistic analyses of these mice in the future, this work provides us outstanding methodologies in studying major mental conditions. I anticipate that four to five papers will come out in this year that report various types of DISC1 genetically engineered mice. Neutral comparison of all the DISC1 mice from different groups will provide important insights for DISC1 and its role in major mental conditions.

This paper demonstrates that mutations in DISC1 can alter mouse behavior, brain structure, and biochemistry, consistent with the idea that DISC1 is related to major psychiatric disorders. This is already an important result. But more strikingly, the authors’ interpretation is that one mutation (L100P) causes a phenotype similar to schizophrenia, while the other mutation (Q31L) results in a phenotype similar to affective disorder.

There are a number of caveats that need to be considered. No patients with equivalent mutations have been identified. The behavioral tests have only a hypothesized or empiric relevance to behavior in the human illnesses. DISC1 itself, while a very strong candidate gene, is still not fully validated, and the best evidence for its role in schizophrenia still arises from the single large pedigree in Scotland.

Despite these caveats, I believe this paper is potentially a major advance. The authors’ interpretations are provocative, and could have far-reaching implications for understanding of the biological bases of psychiatric diseases. The...  Read more

This paper demonstrates that mutations in DISC1 can alter mouse behavior, brain structure, and biochemistry, consistent with the idea that DISC1 is related to major psychiatric disorders. This is already an important result. But more strikingly, the authors’ interpretation is that one mutation (L100P) causes a phenotype similar to schizophrenia, while the other mutation (Q31L) results in a phenotype similar to affective disorder.

There are a number of caveats that need to be considered. No patients with equivalent mutations have been identified. The behavioral tests have only a hypothesized or empiric relevance to behavior in the human illnesses. DISC1 itself, while a very strong candidate gene, is still not fully validated, and the best evidence for its role in schizophrenia still arises from the single large pedigree in Scotland.

Despite these caveats, I believe this paper is potentially a major advance. The authors’ interpretations are provocative, and could have far-reaching implications for understanding of the biological bases of psychiatric diseases. The models provide strong support for further study of DISC1. DISC1 has numerous very interesting interacting proteins and thus may provide an entry into pathogenic pathways for psychiatric diseases. We have suggested that interactors at the centrosome, involved with neuronal development, may be especially relevant to schizophrenia, while interactors at the synapse, or related to signal transduction, may be especially relevant to affective disorder (Ross et al., 2006). The beginnings of an allelic series of DISC1 mutations will presage more detailed genotype-phenotype studies in a variety of mouse models, with potential relevance to both schizophrenia and affective disorder.

Mutant Mice Bring Further Excitement to the DISC1-PDE4 Arena
DISC1 continues to ride a wave of optimism as we look for real breakthroughs in the molecular events underlying major psychiatric disorders including schizophrenia, bipolar, and depression. In 2005, its fortunes became entwined with those of the phosphodiesterase PDE4B as they were shown to functionally and physically interact (Millar et al., 2005). Evidence linking PDE4B to depression has been known for some time, but in the wake of the DISC1 finding, its link to schizophrenia has hardened (Siuciak et al., 2007; Menniti et al., 2006; Pickard et al., 2007).

The Roder and Porteous labs have come together to produce a fantastic paper describing two ENU mutant mice lines with specific mutations in the N-terminus of DISC1. Luck was on their side as the mutations seem to have a direct impact on the interaction with the PDE4B. Furthermore, the two lines look to have...  Read more

Mutant Mice Bring Further Excitement to the DISC1-PDE4 Arena
DISC1 continues to ride a wave of optimism as we look for real breakthroughs in the molecular events underlying major psychiatric disorders including schizophrenia, bipolar, and depression. In 2005, its fortunes became entwined with those of the phosphodiesterase PDE4B as they were shown to functionally and physically interact (Millar et al., 2005). Evidence linking PDE4B to depression has been known for some time, but in the wake of the DISC1 finding, its link to schizophrenia has hardened (Siuciak et al., 2007; Menniti et al., 2006; Pickard et al., 2007).

The Roder and Porteous labs have come together to produce a fantastic paper describing two ENU mutant mice lines with specific mutations in the N-terminus of DISC1. Luck was on their side as the mutations seem to have a direct impact on the interaction with the PDE4B. Furthermore, the two lines look to have distinct phenotypes—one a little schizophrenic, the other depressive. It is known from the clinical and genetic data that DISC1 is associated with schizophrenia, bipolar, and MDD, so this mouse dichotomy is very intriguing.

The mutant line Q31L is claimed to have a “depressive-like” phenotype. This comes from behavioral experiments including a range of assays looking at depressive-like behaviors where this strain had severe deficits, treatable with the dual serotonin-noradrenaline reuptake inhibitor (SNRI) bupropion, commonly prescribed for depression. Together these findings could just as easily be linked to the negative symptoms of schizophrenia. Furthermore, Q31L also shows modest deficits in two sensory processing paradigms (latent inhibition and pre-pulse inhibition), for which antipsychotics had no impact, and a working memory deficit, so this strain has characteristics of all the three key domains of schizophrenia. The pharmacology gets more interesting when these animals are dosed with rolipram (PDE4 inhibitor, raises cAMP levels) and look to be resistant to its effects. At the protein level, while it effects no changes in absolute levels of DISC1 and PDE4B, it leads to a 50 percent reduction in PDE4 activity. This information connects together nicely with the rolipram resistance, and thus the authors suggest that elevated cAMP might explain the behaviors observed, but they unfortunately do not show any cAMP levels in these animals. The paper also reports a decreased binding of the mutant form of DISC1 with PDE4B in overexpressed systems; coupled with the decreased PDE activity, this is in slight contradiction to the original Millar paper (Millar et al., 2005), but as the authors explain, the complexity of the DISC1-PDE4 molecular partnership could easily explain this. From my perspective, the lack of data to date on DISC1-PDE4 brain complexes is a major weak point of this story—this needs to be addressed as we move forward. This will also allow us to understand better the role of different DISC1 isoforms.

L100P is the “schizophrenic” brother of Q31P and has severe deficits in two sensory processing paradigms (latent inhibition and pre-pulse inhibition) which is reversed by typical and atypical antipsychotic and rolipram. Rolipram is able to modulate the behavior as PDE4 activity levels are at a wild-type level. Again, it shows decreased levels of DISC1-PDE4 binding.

Together, these two lines, along with the Gogos mice and a further bank of DISC1 mice which we should expect to see in the next year, puts the field in a position where we are now able to start to dissect out the clearly complex biological functions of DISC1. But as I indicated earlier, we need more information on relevant DISC1 isoforms. We know from the DISC1 interactome that there are many exciting partnerships to develop, but we may not have the fortune of an ENU screen to pull out mice with specific effects on an interaction. The differences in the behavior and pharmacology of these two strains is striking. In combination with the impact on PDE4-DISC1 binding and PDE4 activity, it highlights how much still needs to be understood for this interaction alone. More immediately, the mice show clearly that specific DISC1 mutations may give rise to specific clinical end-points and open up DISC1 pharmacogenomics as a real possibility.

The paper by Walsh et al. is an important addition to the expanding literature on copy number variations in the human genome and their potential role in causing neuropsychiatric disorders. It is clear that copy number variations are important aspects of human genetic variation and that deletions and duplications in diverse genes throughout the genome are likely to affect the function of these genes and possibly the development and function of the human brain. So-called private variations, such as those described in this paper, i.e., changes in the genome found in only a single individual, as all of these variations are, are difficult to establish as pathogenic factors, because it is hard to know how much they contribute to the complex problem of human behavioral variation in a single individual. If the change is private, i.e., only in one case and not enriched in cases as a group, as are common genetic polymorphisms such as SNPs, how much they account for case status is very difficult to prove.

An assumption implicit in this paper is that these private variations may be...  Read more

The paper by Walsh et al. is an important addition to the expanding literature on copy number variations in the human genome and their potential role in causing neuropsychiatric disorders. It is clear that copy number variations are important aspects of human genetic variation and that deletions and duplications in diverse genes throughout the genome are likely to affect the function of these genes and possibly the development and function of the human brain. So-called private variations, such as those described in this paper, i.e., changes in the genome found in only a single individual, as all of these variations are, are difficult to establish as pathogenic factors, because it is hard to know how much they contribute to the complex problem of human behavioral variation in a single individual. If the change is private, i.e., only in one case and not enriched in cases as a group, as are common genetic polymorphisms such as SNPs, how much they account for case status is very difficult to prove.

An assumption implicit in this paper is that these private variations may be major factors in the case status of the individuals who have them. The data of this paper suggest, however, this is actually not the case, at least for the childhood onset cases. Here’s why: mentioned in the paper is a statement that only two of the CNVs in the childhood cases are de novo, i.e., spontaneous and not inherited (and one of these is on the Y chromosome, making its functional implications obscure). If most of the CNVs are inherited, they can’t be causing illness per se as major effect players because they are coming from well parents.

Also, if you add up all CNVs in transmitted and non-transmitted chromosomes of the parents, it’s something like 31 gene-based CNVs in 154 parents (i.e., 20 percent of the parents have a gene-based deletion or duplication in the very illness-related pathways that are highlighted in the cases), which is at least as high a frequency as in the adult-onset schizophrenia sample in this study…and three times the frequency as found in the adult controls. This is not to say that such variants might not represent susceptibility genetic factors, or show variable penetrance between individuals, like other polymorphisms, and contribute to the complex genetic risk architecture, like other genetic variations that have been more consistently associated with schizophrenia. However, the CNV literature has tended to seek a more major effect connotation to the findings.

As new technologies are applied to understanding the etiology and pathophysiology of schizophrenia, considering the clinical features of the cases studied and the implications of the findings is of value. The conclusion of the Walsh et al. paper, “these results suggest that schizophrenia can be caused by rare mutations….“ is worth considering carefully.

What evidence is needed to link an observation in the laboratory or clinic to cause? Recent recommendations for the content of papers in epidemiology (von Elm et al., 2008) remind us of the suggestions of A.V. Hill (Hill, 1965). To discern the implications of a finding, or association, for causality, Hill suggests assessment of the following:

1. Strength of the association: this is not the observed p-value, but a measure of the magnitude of the association. In the Walsh et al. study, the primary outcome measure, structural variants duplicating or deleting genes was observed in 15 percent of cases, and 5 percent of controls. But...  Read more

As new technologies are applied to understanding the etiology and pathophysiology of schizophrenia, considering the clinical features of the cases studied and the implications of the findings is of value. The conclusion of the Walsh et al. paper, “these results suggest that schizophrenia can be caused by rare mutations….“ is worth considering carefully.

What evidence is needed to link an observation in the laboratory or clinic to cause? Recent recommendations for the content of papers in epidemiology (von Elm et al., 2008) remind us of the suggestions of A.V. Hill (Hill, 1965). To discern the implications of a finding, or association, for causality, Hill suggests assessment of the following:

1. Strength of the association: this is not the observed p-value, but a measure of the magnitude of the association. In the Walsh et al. study, the primary outcome measure, structural variants duplicating or deleting genes was observed in 15 percent of cases, and 5 percent of controls. But what is the association with? The diagnostic entity of schizophrenia, or some risk factor for the illness? Of interest, and noted in the Supporting Online Material, these variants were present in 7/15 (47 percent) of the cases with presumed IQ <80, but only 15/135 (11 percent) of the cases with IQ >80. Are the structural variants more strongly associated with mental retardation (within schizophrenia 47 percent vs. 11 percent) than with diagnosis (11 percent vs. 5 percent of controls, assuming normal IQ)? This is of particular interest in the context of the speculation in the paper concerning the importance of genes putatively involved with brain development in the etiology of schizophrenia.

2. Consistency of results in the literature across studies and research groups: there are now several papers examining copy number variation in schizophrenia, including a report from our group (Wilson et al., 2006). The authors of the present paper state that each variant observed was unique, and so consistency of the specific findings could be argued to be irrelevant, if the model is of novel mutations (more on models below). Undoubtedly, future meta-analyses and accumulating databases help determine if there is anything consistent in the findings, other than a higher frequency of any abnormalities in cases rather than controls.

3. Specificity of the findings to the illness in question: this was not addressed experimentally in the paper. However, the findings of more abnormalities in the putative low IQ cases, and the similarity of the findings to reports in autism and mental retardation, suggest that this criterion for supporting causality is unlikely to be met.

4. Temporality: the abnormalities should precede the illness. If DNA from terminally differentiated neurons harbors the same variants as DNA from constantly renewed populations of lymphocytes, then clearly this condition is met. While it seems highly likely that this is the case, it is worthwhile considering the possibility that DNA structure may vary between tissue types, or between cell populations. Even within human brain there is some evidence for chromosomal heterogeneity (Rehen et al., 2005).

5. Biological gradient: presence of a “dose-response” curve strengthens the likelihood of a causal relationship. This condition is not met within cases: only 1/115 appeared to have more than one variant. However, in the presumably more severe childhood onset form of schizophrenia, four individuals carried multiple variants, and the observation of a higher prevalence of variants overall. Still, the question of what the observations of CNV are associated with is relevant, since one of the inclusion/exclusion criteria for COS allowed IQ 65-80, and it is uncertain how many of these cases had some degree of intellectual deficit.

6. Plausibility: biological likelihood—quite difficult to satisfy as a criterion, in the context of the limits of knowledge concerning the mechanisms of illness of schizophrenia.

7. Coherence of the observation with known facts about the illness: the genetic basis of schizophrenia is quite well studied, and there is no dearth of theories concerning genetic architecture. However, a coherent model remains lacking. As examples, the suggestion is made that the observations concerning inherited CNVs in the COS cases are linked with a severe family history in this type of illness. This appears inconsistent with a high penetrance model for CNVs as suggested in the opening of the paper (presuming the parents in COS families are unaffected, as would seem likely). Elsewhere, CNVs are proposed by the authors to be related to de novo events, and an interaction with an environmental modifier, folate (and exposure to famine), is posited (McClellan et al., 2006). A model of the effects of CNVs, which generates falsifiable hypotheses is needed.

8. Experiment: the ability to intervene clinically to modify the effects of CNVs disrupting genes seems many years away.

9. Analogy: the novelty of the CNV findings is both intriguing, but limiting in understanding the likelihood of causal relationships.

The intersection of clinical realities and novel laboratory technologies will fuel the need for better translational research in schizophrenia for many, many more years.

References:

von Elm E, Altman DG, Egger M, Pocock SJ, Gøtzsche PC, Vandenbroucke JP. Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) statement: guidelines for reporting observational studies. J Clin Epidemiol. 2008 Apr 1;61(4):344-349. Abstract

HILL AB. THE ENVIRONMENT AND DISEASE: ASSOCIATION OR CAUSATION? Proc R Soc Med. 1965 May 1;58():295-300. Abstract

Wilson GM, Flibotte S, Chopra V, Melnyk BL, Honer WG, Holt RA. DNA copy-number analysis in bipolar disorder and schizophrenia reveals aberrations in genes involved in glutamate signaling. Hum Mol Genet. 2006 Mar 1;15(5):743-9. Abstract

Rehen SK, Yung YC, McCreight MP, Kaushal D, Yang AH, Almeida BSV, Kingsbury MA, Cabral KMS, McConnell MJ, Anliker B, Fontanoz M, Chun J: Constitutional aneuploidy in the normal human brain. J Neurosci 2005; 25:2176-2180. Abstract

McClellan JM, ESusser E, King M-C: Maternal famine, de novo mutations, and schizophrenia. JAMA 2006; 296:582-584. Abstract

The new study by Walsh et al. (2008), as well as recent data from other groups working in schizophrenia, autism, and mental retardation, make a strong case for including copy number variants as an important source of risk for neurodevelopmental phenotypes. These findings raise several intriguing new questions for future research, including: the degree of causality/penetrance that can be attributed to individual CNVs; diagnostic specificity; and recency of their origins. While these questions are difficult to address in the context of private mutations, one potential source of additional information is the examination of common, recurrent CNVs, which have not yet been systematically studied as potential risk factors for schizophrenia.

Still, the association of rare CNVs with schizophrenia provides additional evidence that genetic transmission patterns may be a complex hybrid of common, low-penetrant alleles and rare, highly penetrant variants. In diseases ranging from Parkinson's to colon cancer, the literature demonstrates that rare penetrant loci are...  Read more

The new study by Walsh et al. (2008), as well as recent data from other groups working in schizophrenia, autism, and mental retardation, make a strong case for including copy number variants as an important source of risk for neurodevelopmental phenotypes. These findings raise several intriguing new questions for future research, including: the degree of causality/penetrance that can be attributed to individual CNVs; diagnostic specificity; and recency of their origins. While these questions are difficult to address in the context of private mutations, one potential source of additional information is the examination of common, recurrent CNVs, which have not yet been systematically studied as potential risk factors for schizophrenia.

Still, the association of rare CNVs with schizophrenia provides additional evidence that genetic transmission patterns may be a complex hybrid of common, low-penetrant alleles and rare, highly penetrant variants. In diseases ranging from Parkinson's to colon cancer, the literature demonstrates that rare penetrant loci are frequently embedded within an otherwise complex disease. Perhaps the most well-known example involves mutations in amyloid precursor protein and the presenilins in Alzheimer’s disease (AD). Although extremely rare, accounting for <1 percent of all cases of AD, identification of these autosomal dominant subtypes greatly enhanced understanding of pathophysiology. Similarly, a study of consanguineous families in Iran has very recently identified a rare autosomal recessive form of mental retardation (MR) caused by glutamate receptor (GRIK2) mutations, thereby opening new avenues of research (Motazacker et al., 2007). In schizophrenia, we have recently employed a novel, case-control approach to homozygosity mapping (Lencz et al., 2007), resulting in several candidate loci that may harbor highly penetrant recessive variants. Taken together, these results suggest that a diversity of methodological approaches will be needed to parse genetic heterogeneity in schizophrenia.

References:

Motazacker MM, Rost BR, Hucho T, Garshasbi M, Kahrizi K, Ullmann R, Abedini SS, Nieh SE, Amini SH, Goswami C, Tzschach A, Jensen LR, Schmitz D, Ropers HH, Najmabadi H, Kuss AW. (2007) A defect in the ionotropic glutamate receptor 6 gene (GRIK2) is associated with autosomal recessive mental retardation. Am J Hum Genet. 81(4):792-8. Abstract

Lencz T, Lambert C, DeRosse P, Burdick KE, Morgan TV, Kane JM, Kucherlapati R,Malhotra AK (2007). Runs of homozygosity reveal highly penetrant recessive loci in schizophrenia. Proc Natl Acad Sci U S A. 104(50):19942-7. Abstract

In my mind, the study of CNVs in autism (and likely soon in schizophrenia/bipolar disorder, which are a little behind) is likely to put biological meat on the bones of illness etiology and finally lay to rest the annoyingly persistent taunts that genetics hasn’t delivered on its promises for psychiatric illness.

I don’t think it’s necessary at the moment to wring our hands at any inconsistencies between the Walsh et al. and previous studies of CNV in schizophrenia (e.g., Kirov et al., 2008). There are a number of factors which I think are going to influence the frequency, type, and identity of CNVs found in any given study.

1. CNVs are going to be found at the rare/penetrant/familial end of the disease allele spectrum—in direct contrast to the common risk variants which are the targets of recent GWAS studies. In the short term, we are likely to see a large number of different CNVs identified. The nature of this spectrum, however, is that there will be more common pathological CNVs which should be replicated sooner—NRXN1, APBA2 (Kirov et al., 2008), CNTNAP2...  Read more

In my mind, the study of CNVs in autism (and likely soon in schizophrenia/bipolar disorder, which are a little behind) is likely to put biological meat on the bones of illness etiology and finally lay to rest the annoyingly persistent taunts that genetics hasn’t delivered on its promises for psychiatric illness.

I don’t think it’s necessary at the moment to wring our hands at any inconsistencies between the Walsh et al. and previous studies of CNV in schizophrenia (e.g., Kirov et al., 2008). There are a number of factors which I think are going to influence the frequency, type, and identity of CNVs found in any given study.

1. CNVs are going to be found at the rare/penetrant/familial end of the disease allele spectrum—in direct contrast to the common risk variants which are the targets of recent GWAS studies. In the short term, we are likely to see a large number of different CNVs identified. The nature of this spectrum, however, is that there will be more common pathological CNVs which should be replicated sooner—NRXN1, APBA2 (Kirov et al., 2008), CNTNAP2 (Friedman et al., 2008)—and may be among some of these “low hanging fruit.” For the rarer CNVs, proving a pathological role is going to be a real headache. Large studies or meta-analyses are never going to yield significant p-values for rare CNVs which, nevertheless, may be the chief causes of illness for those few individuals who carry them. Showing clear segregation with illness in families is likely to be the only means to judge their role. However, we must not expect a pure cause-and-effect role for all CNVs: even in the Scottish t(1;11) family disrupting the DISC1 gene, there are several instances of healthy carriers.

2. Sample selection is also likely to be critical. In the Kirov paper, samples were chosen to represent sporadic and family history-positive cases equally. In the Walsh paper, samples were taken either from hospital patients (the majority) or a cohort of childhood onset schizophrenia. Detailed evidence for family history on a case-by-case basis was not given but appeared far stronger in the childhood onset cases. CNVs appeared to be more prevalent, and as expected, more familial, in the latter cohort. A greater frequency was also observed in the Kirov study familial subset.

3. Inclusion criteria are likely to be important—particularly in the more sporadic cases without family history. This is because CNVs found in this group may be commoner and less penetrant—they will be more frequent in cases than in controls but not exclusively found in cases. Any strategy, such as that used in the Kirov paper, which discounts a CNV based on its presence—even singly—in the control group is likely to bias against this class.

4. Technical issues. Certainly, the coverage/sensitivity of the method of choice for the “event discovery” stage is going to influence the minimum size of CNV detectable. However, a more detailed coverage often comes with a greater false-positive rate. Technique choice may also have more general issues. In both of the papers, the primary detection method is based on hybridization of case and pooled control genomes prior to detection on a chip. Thus, a more continuously distributed output may result—and the extra round of hybridization might bias against certain sequences. More direct primary approaches such as Illumina arrays or a second-hand analysis of SNP genotyping arrays may provide a more discrete copy number output, but these, too, can suffer from interpretational issues.

The other major implication of these and other CNV studies is the observation that certain genes “ignore” traditional disease boundaries. For example, NRXN1 CNVs have now been identified in autism and schizophrenia, and CNTNAP2 translocations/CNVs have been described in autism, Gilles de la Tourette syndrome, and schizophrenia/epilepsy. This mirrors the observation of common haplotypes altering risk across the schizophrenia-bipolar divide in numerous association studies. It might be the case that these more promiscuous genes are likely to be involved in more fundamental CNS processes or developmental stages—with the precise phenotypic outcome being defined by other variants or environment. The presence of mental retardation comorbid with psychiatric diagnoses in a number of CNV studies suggests that this might be the case. I look forward to the Venn diagrams of the future which show us the shared neuropsychiatric and disease-specific genes/gene alleles. It will also be interesting to see if the large deletions/duplications involving numerous genes give rise to more severe, familial, and diagnostically more defined syndromes or, alternatively, a more diffuse phenotype. Certainly, it has not been easy to dissect out individual gene contributions to phenotype in VCFS or the minimal region in Down syndrome.

References:

Friedman JI, Vrijenhoek T, Markx S, Janssen IM, van der Vliet WA, Faas BH, Knoers NV, Cahn W, Kahn RS, Edelmann L, Davis KL, Silverman JM, Brunner HG, van Kessel AG, Wijmenga C, Ophoff RA, Veltman JA. CNTNAP2 gene dosage variation is associated with schizophrenia and epilepsy. Mol Psychiatry. 2008 Mar 1;13(3):261-6. Abstract

We agree with the comments of Weinberger, Lencz and Malhotra, and Pickard, and the question raised by Honer about the extent to which the association may be more to mental retardation than schizophrenia. These new studies of copy number variation represent important advances, but need to be interpreted carefully.

We are now getting two different kinds of data on schizophrenia, which can be seen as two opposite poles. The first is from association studies with common variants, in which large numbers of people are required to see significance, and the strengths of the associations are quite modest. These kinds of vulnerability factors would presumably contribute a very modest increase in risk, and many taken together would cause the disease. By contrast, the “private” mutations, as identified by the Sebat study, could potentially be completely causative, but because they are present in only single individuals or very small numbers of individuals, it is difficult to be certain of causality. Furthermore, since some of them in the early-onset schizophrenia patients were...  Read more

We agree with the comments of Weinberger, Lencz and Malhotra, and Pickard, and the question raised by Honer about the extent to which the association may be more to mental retardation than schizophrenia. These new studies of copy number variation represent important advances, but need to be interpreted carefully.

We are now getting two different kinds of data on schizophrenia, which can be seen as two opposite poles. The first is from association studies with common variants, in which large numbers of people are required to see significance, and the strengths of the associations are quite modest. These kinds of vulnerability factors would presumably contribute a very modest increase in risk, and many taken together would cause the disease. By contrast, the “private” mutations, as identified by the Sebat study, could potentially be completely causative, but because they are present in only single individuals or very small numbers of individuals, it is difficult to be certain of causality. Furthermore, since some of them in the early-onset schizophrenia patients were present in unaffected parents, one might have to assume the contribution of a common variant vulnerability (from the other parent) as well.

If a substantial number of the private structural mutations are causal, then one might expect to have seen multiple small Mendelian families segregating a structural variant. The situation would then be reminiscent of the autosomal dominant spinocerebellar ataxis, in which mutations (currently about 30 identified loci) in multiple different genes result in similar clinical syndromes. The existence of many small Mendelian families would be less likely if either 1) structural variants that cause schizophrenia nearly always abolish fertility, or 2) some of the SVs detected by Walsh et al. are risk factors, but are usually not sufficient to cause disease. The latter seems more likely.

We think these two poles highlight the continued importance of segregation studies, as have been used for the DISC1 translocation. In order to validate these very rare “private” copy number variations, we believe that it would be important to look for sequence variations in the same genes in large numbers of schizophrenia and control subjects, and ideally to do so in family studies.

One very exciting result of the new copy number studies is the implication of whole pathways rather than just single genes. This highlights the importance of a better understanding of pathogenesis. The study of candidate pathways should help facilitate better pathogenic understanding, which should result in better biomarkers and potentially improve classification and treatment. In genetic studies, development of pathway analysis will be fruitful. Convergent evidence can come from studies of pathogenesis in cell and animal models, but this will need to be interpreted with caution, as it is possible to make a plausible story for so many different pathways (Ross et al., 2006). The genetic evidence will remain critical.

References:

Ross CA, Margolis RL, Reading SA, Pletnikov M, Coyle JT. Neurobiology of schizophrenia. Neuron. 2006 Oct 5;52(1):139-53. Abstract

The idea that a proportion of schizophrenia is associated with rare chromosomal abnormalities has been around for some time, but it has been difficult to be sure whether such events are pathogenic given that most are rare. Two instances where a pathogenic role seems likely are first, the balanced ch1:11 translocation that breaks DISC1, where pathogenesis seems likely due to co-segregation with disease in a large family, and second, deletion of chromosome 22q11, which is sufficiently common for rates of psychosis to be compared with that in the general population. This association came to light because of the recognizable physical phenotype associated with deletion of 22q11, and the field has been waiting for the availability of genome-wide detection methods that would allow the identification of other sub-microscopic chromosomal abnormalities that might be involved, but whose presence is not predicted by non-psychiatric syndromal features. This technology is now upon us in the form of various microarray-based methods, and we can expect a slew of studies addressing this...  Read more

The idea that a proportion of schizophrenia is associated with rare chromosomal abnormalities has been around for some time, but it has been difficult to be sure whether such events are pathogenic given that most are rare. Two instances where a pathogenic role seems likely are first, the balanced ch1:11 translocation that breaks DISC1, where pathogenesis seems likely due to co-segregation with disease in a large family, and second, deletion of chromosome 22q11, which is sufficiently common for rates of psychosis to be compared with that in the general population. This association came to light because of the recognizable physical phenotype associated with deletion of 22q11, and the field has been waiting for the availability of genome-wide detection methods that would allow the identification of other sub-microscopic chromosomal abnormalities that might be involved, but whose presence is not predicted by non-psychiatric syndromal features. This technology is now upon us in the form of various microarray-based methods, and we can expect a slew of studies addressing this hypothesis in the coming months.

Structural chromosomal abnormalities can take a variety of forms, in particular, deletions, duplication, inversions, and translocations. Generally speaking, these can disrupt gene function by, in the case of deletions, insertions and unbalanced translocations, altering the copy number of individual genes. These are sometimes called copy number variations (CNVs). Structural chromosomal abnormalities can also disrupt a gene sequence, and such disruptions include premature truncation, internal deletion, gene fusion, or disruption of regulatory or promoter elements.

It is, however, worth pointing out that structural chromosomal variation in the genome is common—it has been estimated that any two individuals on average differ in copy number by a total of around 6 Mb, and that the frequency of individual duplications or deletions can range from common through rare to unique, much in the same way as other DNA variation. Also similar to other DNA variation, many structural variants, indeed almost certainly most, may have no phenotypic effects (and this includes those that span genes), while others may be disastrous for fetal viability. Walsh and colleagues have focused upon rare structural variants, and by rare they mean events that might be specific to single cases or families. For this reason, they specifically targeted CNVs that had not previously been described in the published literature or in the Database of Genomic Variants. The reasonable assumption was made that this would enrich for CNVs that are highly penetrant for the disorder. Indeed, Walsh et al. favor the hypothesis that genetic susceptibility to schizophrenia is conferred not by relatively common disease alleles but by a large number of individually rare alleles of high penetrance, including structural variants. As we have argued elsewhere (Craddock et al., 2007), it seems entirely plausible that schizophrenia reflects a spectrum of alleles of varying effect sizes including common alleles of small effect and rare alleles of larger effect, but data from genetic epidemiology do not support the hypothesis that the majority of the disorder reflects rare alleles of large effect.

Walsh et al. found that individuals with schizophrenia were >threefold more likely than controls to harbor rare CNVs that impacted on genes, but in contrast, found no significant difference in the proportions of cases and controls carrying rare mutations that did not impact upon genes. They also found a similar excess of rare structural variants that deleted or duplicated one or more genes in an independent series of cases and controls, using a cohort with childhood onset schizophrenia (COS).

The results of the Walsh study are important, and clearly suggest a role for structural variation in the etiology of schizophrenia. There are, however, a number of caveats and issues to consider. First, it would be unwise on the basis of that study to speculate on the likely contribution of rare variants to schizophrenia as a whole. It is likely correct that, due to selection pressures, highly penetrant alleles for disorders (like schizophrenia) that impair reproductive fitness are more likely to be of low frequency than they are to be common, but this does not imply that the converse is true. That is, one cannot assume that the penetrance of low frequency alleles is more likely to be high than low. Thus, and as pointed out by Walsh et al., it is not possible to know which or how many of the unique events observed in their study are individually pathogenic. Whether individual loci contribute to pathogenesis (and their penetrances) is, as we have seen, hard to establish. Estimating penetrance by association will require accurate measurement of frequencies in case and control populations, which for rare alleles, will have to be very large. Alternatively, more biased estimates of penetrance can be estimated from the degree of co-segregation with disease in highly multiplex pedigrees, but these are themselves fairly rare in schizophrenia, and pedigrees segregating any given rare CNV obviously even more so.

As Weinberger notes, the case for high penetrance (at the level of being sufficient to cause the disorder) is also undermined by their data from COS, where the majority of variants were inherited from unaffected parents. This accords well with the observation that 22q11DS, whilst conferring a high risk of schizophrenia, is still only associated with psychosis in ~30 percent of cases. It also accords well with the relative rarity of pedigrees segregating schizophrenia in a clearly Mendelian fashion, though the association of CNVs with severe illness of early onset might be expected to reduce the probability of transmission.

Third, there are questions about the generality of the findings. Cases in the case control series were ascertained in a way that enriched for severity and chronicity. Perhaps more importantly, the CNVs were greatly overrepresented in people with low IQ. Thus, one-third of all the potentially pathogenic CNVs in the case control series were seen in the tenth of the sample with IQ less than 80. The association between structural variants and low IQ is well known, as is the association between low IQ and psychotic symptoms, and it seems plausible to assume that forms of schizophrenia accompanied by mental retardation (MR) are likely to be enriched for this type of pathogenesis. The question that arises is whether the CNVs in such cases act simply by influencing IQ, which in turn has a non-specific effect on increasing risk of schizophrenia, or whether there are specific CNVs for MR plus schizophrenia, and some which may indeed increase risk of schizophrenia independent of IQ. In the case of 22q11 deletion, risk of schizophrenia does not seem to be dependent on risk of MR, but more work is needed to establish that this applies more generally.

Another reason to caution about the generality of the effect is that Walsh et al. found that cases with onset of psychotic symptoms at age 18 or younger were particularly enriched for CNVs, being greater than fourfold more likely than controls to harbor such variants. There did remain an excess of CNVs in cases with adult onset, supporting a more general contribution, although it should be noted that even in this group with severe disorder, this excess was not statistically significant (Fisher’s exact test, p = 0.17, 2-tailed, our calculation). The issue of age of onset clearly impacts upon assessing the overall contribution CNVs may make upon psychosis, since onset before 18, while not rare, is also not typical. A particular contribution of CNVs to early onset also appears supported by the second series studied, which had COS. However, this is a particularly unusual form of schizophrenia which is already known to have high rates of chromosomal abnormalities. Future studies of more typical samples will doubtless bear upon these issues.

Even allowing for the fact that many more CNVs may be detected as resolution of the methodology improves, the above considerations suggest it is premature to conclude a substantial proportion of cases of schizophrenia can be attributed to rare, highly penetrant CNVs. Nevertheless, even if it turns out that only a small fraction of the disorder is attributable to CNVs, as seen for other rare contributors to the disorder (e.g., DISC1 translocation), such uncommon events offer enormous opportunities for advancing our knowledge of schizophrenia pathogenesis.

References:

Craddock N, O'Donovan MC, Owen MJ. Phenotypic and genetic complexity of psychosis. Invited commentary on ... Schizophrenia: a common disease caused by multiple rare alleles.Br J Psychiatry. 2007 90:200-3. Abstract

Walsh et al. claim that rare and severe chromosomal structural variants (SVs) (i.e., not described in the literature or in the specialized databases as of November 2007) are highly penetrant events each explaining a few, if not singular, cases of schizophrenia.

However, their definition of rareness is questionable. Indeed, it is unclear why SVs that are rare (<1 percent) but previously described should be omitted from their analysis. In addition, contrary to their own definition of rareness, the authors included in the COS sample several SVs that have been previously mentioned in the literature (e.g. “115 kb deletion on chromosome 2p16.3 disrupting NRXN1”). Furthermore, some of these SVs (entire Y chromosome duplication) are certainly not rare (by the authors’ definition), nor highly penetrant with regard to psychosis (Price et al., 1967). Finally, as their definition of rareness depends on a specific date, the results of this study will change over time.

As to the assessment of...  Read more

Walsh et al. claim that rare and severe chromosomal structural variants (SVs) (i.e., not described in the literature or in the specialized databases as of November 2007) are highly penetrant events each explaining a few, if not singular, cases of schizophrenia.

However, their definition of rareness is questionable. Indeed, it is unclear why SVs that are rare (<1 percent) but previously described should be omitted from their analysis. In addition, contrary to their own definition of rareness, the authors included in the COS sample several SVs that have been previously mentioned in the literature (e.g. “115 kb deletion on chromosome 2p16.3 disrupting NRXN1”). Furthermore, some of these SVs (entire Y chromosome duplication) are certainly not rare (by the authors’ definition), nor highly penetrant with regard to psychosis (Price et al., 1967). Finally, as their definition of rareness depends on a specific date, the results of this study will change over time.

As to the assessment of severity, it can equally be concluded from table 2 and using their statistical approach that "patients with schizophrenia are significantly more likely to harbor rare structural variants (6/150) that do not disrupt any gene compared to controls(2/268) (p = 0.03)", thus contradicting their claim. In fact, had they used criteria in the literature (Lee et al., 2007; (Brewer et al., 1999) (i.e., deletion SVs are more likely than duplications to be pathogenic) and appropriate statistical contrasts, deletions are significantly (p = 0.02) less frequent in patients (5/23) than in controls (9/13) who have SVs. In addition, the assumption of high penetrance is questionable given the high level (13 percent) of non-transmitted SVs in parents of COS patients. Is the rate of psychosis proportionately high in the parents? From the data presented, we know that only 2/27 SVs in COS patients are de novo and that “some” SVs are transmitted. Adding this undetermined number of transmitted SVs to the reported non-transmitted SVs will lead to an even larger proportion of parents carrying SVs. Disclosing the inheritance status of SVs in COS patients along with information on diagnoses in parents from this “rigorously characterised cohort,” represents a major criterion for assessing the risk associated with these SVs.

Consequently, it appears that the argument of rareness is rather idiosyncratic and contains inconsistencies, and the one of severity is very open to interpretation. Most importantly, it should be emphasized that amalgamated gene effects at the population level do not allow one to conclude that any single SV actually contributes to schizophrenia in an individual. Thus it is unclear how this study of grouped events differs from the thousands of controversial and underpowered association studies of single genes.

References:

Price WH, Whatmore PB. Behaviour Disorders and Pattern of Crime among XYY males Identified at a Maximum Security Hospital. Brit Med J 1967;1:533-6.

Lee C, Iafrate AJ, Brothman AR. Copy number variations and clinical cytogenetic diagnosis of constitutional disorders. Nat Genet 2007 July;39(7 Suppl):S48-S54.

Brewer C, Holloway S, Zawalnyski P, Schinzel A, FitzPatrick D. A chromosomal duplication map of malformations: regions of suspected haplo- and triplolethality--and tolerance of segmental aneuploidy--in humans. Am J Hum Genet 1999 June;64(6):1702-8.

The study of Seshadri, Sawa, and colleagues presents novel evidence of a potential biological link between two lead schizophrenia susceptibility genes, NRG1 and DISC1. The principal finding of the study is that NRG1 (EGFβ) regulates expression of a specific isoform of DISC1, mediated via ErbB2/3 but not ErbB4. The influence of NRG1 on expression of the DISC1 isoform was confirmed in a variety of in-vitro and in-vivo models. Specifically, the authors report (using Western blotting with the DISC1 antibodies: D27 and mExon3), that treatment with NRG1 (and NRG2), but not NRG3, increases levels of DISC1 immunoreactivity at 130 kDa in immature and mature rat primary neuron cultures. Interestingly, NRG1 (or NRG2) had no effect on expression of the previously reported full-length DISC1 immunoreactive bands of 100-105 kDa. Convincingly, reduction of the 130 kDa DISC1 band was observed in BACE1 -/- and NRG1 +/- mice, both of which have reduced NRG1 signaling. Taken together, these findings suggest that NRG1 signaling regulates expression of a unique 130 kDa DISC1 protein.

This...  Read more

The study of Seshadri, Sawa, and colleagues presents novel evidence of a potential biological link between two lead schizophrenia susceptibility genes, NRG1 and DISC1. The principal finding of the study is that NRG1 (EGFβ) regulates expression of a specific isoform of DISC1, mediated via ErbB2/3 but not ErbB4. The influence of NRG1 on expression of the DISC1 isoform was confirmed in a variety of in-vitro and in-vivo models. Specifically, the authors report (using Western blotting with the DISC1 antibodies: D27 and mExon3), that treatment with NRG1 (and NRG2), but not NRG3, increases levels of DISC1 immunoreactivity at 130 kDa in immature and mature rat primary neuron cultures. Interestingly, NRG1 (or NRG2) had no effect on expression of the previously reported full-length DISC1 immunoreactive bands of 100-105 kDa. Convincingly, reduction of the 130 kDa DISC1 band was observed in BACE1 -/- and NRG1 +/- mice, both of which have reduced NRG1 signaling. Taken together, these findings suggest that NRG1 signaling regulates expression of a unique 130 kDa DISC1 protein.

This is an important and thoughtful paper, but there are some details that raise questions about the interpretation of the results. Interestingly, two previous studies that characterized the D27 (and mExon3) antibody in mouse brain (Schurov et al., 2004; Ishizuka et al., 2007) failed to report the 130 kDa band described here. Ishizuka et al. reported that immunoprecipitation with the mExon3 antibody followed by detection with the D27 antibody recognized two primary signals (100 and 105 kDa), thought to correspond to full-length DISC1. In contrast, in the present study the authors report that immunoprecipitation of neuronal lysates using mExon3, followed by Western blotting with D27, consistently identifies an additional 130 kDa band (Fig. S2B), which is also present in the P0 mouse cortex (Fig 3C). Whilst it is not clear what accounts for these apparent differences in signal detection of the 130 kDa band using the same antibodies, factors such as species specificity (rat vs. mouse), tissue type, and developmental stage are likely relevant. Such factors are important considerations for future work. Similarly, it will be crucial to determine whether the 130 kDa band is present in human brain and how it relates to risk for schizophrenia. Of final note, the authors performed extensive experimentation in an attempt to confirm the identity of the 130 kDa band (including successful knockdown by a previously characterized RNAi to DISC1), but interestingly they fail to identify any DISC1 sequence in the 130 kDa signal using mass spectrometry (see discussion). In light of this, it is paramount that future studies determine exactly what the 130 kDa proposed DISC1 band represents (i.e., a novel splice isoform, post-transcriptionally modified protein, etc.), given that NRG1’s effects are specifically related to this variant.

In conclusion, this study provides intriguing evidence of a potential molecular link between NRG1 and DISC1, but at present, the interpretation of the results rests on an immunoblot band of unknown identify.

References:

Schurov IL, Handford EJ, Brandon NJ, Whiting PJ. Expression of disrupted in schizophrenia 1 (DISC1) protein in the adult and developing mouse brain indicates its role in neurodevelopment. Mol Psychiatry . 2004 Dec 1 ; 9(12):1100-10. Abstract

Ishizuka K, Chen J, Taya S, Li W, Millar JK, Xu Y, Clapcote SJ, Hookway C, Morita M, Kamiya A, Tomoda T, Lipska BK, Roder JC, Pletnikov M, Porteous D, Silva AJ, Cannon TD, Kaibuchi K, Brandon NJ, Weinberger DR, Sawa A. Evidence that many of the DISC1 isoforms in C57BL/6J mice are also expressed in 129S6/SvEv mice. Mol Psychiatry . 2007 Oct ; 12(10):897-9. Abstract

This paper raises an interesting issue. It is unclear how an immuno band that has no DISC1 sequences can result from "alternative splicing or post-translational modification." Could someone provide a mechanistic account, at the molecular level, of how this may be possible? To support that this band is DISC1, at least some DISC1 sequence should have been detected. This issue could be related to the non-specific cross-reactivity of many DISC1 antibodies (see Kvajo et al., 2008 for a discussion) and now also raises the possibility of off-target effects of DISC1 RNAi.

Resolving these issues will be paramount for making meaningful insights into how variations in DISC1 contribute to psychotic disorders.

References:

Kvajo M, McKellar H, Arguello PA, Drew LJ, Moore H, MacDermott AB, Karayiorgou M, Gogos JA. A mutation in mouse Disc1 that models a schizophrenia risk allele leads to specific alterations in neuronal architecture and cognition. Proc Natl Acad Sci U S A. 2008 May 13;105(19):7076-81. Abstract

View all comments by Alexander Arguello

We are very glad to see Dr. Law’s thoughtful and very supportive comments on the work by Seshadri et al. We share the recognition, as we pointed out in the discussion of the paper, that identification of 130 kDa signal at the molecular level is an important future question. To confirm the authenticity of immunoreactivity, we tested if the 130 kDa signal is immunoprecipitated and immunoblotted by different DISC1 antibodies. Similar immunoreactive approaches have been used earlier to distinguish DISC1 isoforms, including a 71 kDa isoform in association with PDE4 (Millar et al., 2005; Chubb et al., 2008). Knockout mice deficient in DISC1 that we have recently generated (unpublished) were used for evaluating the specificity of several antibodies against DISC1 (Schurov et al., 2004; Ishizuka et al., 2007; Duan et al., 2007;   Read more

We are very glad to see Dr. Law’s thoughtful and very supportive comments on the work by Seshadri et al. We share the recognition, as we pointed out in the discussion of the paper, that identification of 130 kDa signal at the molecular level is an important future question. To confirm the authenticity of immunoreactivity, we tested if the 130 kDa signal is immunoprecipitated and immunoblotted by different DISC1 antibodies. Similar immunoreactive approaches have been used earlier to distinguish DISC1 isoforms, including a 71 kDa isoform in association with PDE4 (Millar et al., 2005; Chubb et al., 2008). Knockout mice deficient in DISC1 that we have recently generated (unpublished) were used for evaluating the specificity of several antibodies against DISC1 (Schurov et al., 2004; Ishizuka et al., 2007; Duan et al., 2007; Koike et al., 2006). Loss of this immunoreactivity by authentic shRNAs further supports this idea. The sequences of shRNAs are the same as those used in the study by Mao et al. (Mao et al., 2009) to demonstrate that DISC1 may be involved in progenitor cell proliferation.

Of note, mass spectrometry cannot be an ultimate confirmation, because with this technique it is hard to distinguish the signals from two adjacent or overlapped bands in Western blots of 1D gels, one of which is real and the other not. Therefore, regardless of our initial mass spectrometry analysis (even if one finds sequences of the target protein), validation with both immunoprecipitation and RNAi is required to draw a conclusion on the identity of 130 kDa signal. In the study by Seshadri et al., these two ways of validation were successfully made.

Furthermore, whether or not this 130 kDa isoform is also expressed in humans is a critical question. It is also very important to consider context-dependent expression of unique isoforms of genetic susceptibility factors. This unique form (130 kDa) is likely to be in that category; thus, as Dr. Law suggested, comparative analysis is very useful. Further analysis of the genesis, function, and processing of various DISC1 isoforms in the brain will be a worthy pursuit in the context of schizophrenia.

References:

Millar JK, Pickard BS, Mackie S, James R, Christie S, Buchanan SR, Malloy MP, Chubb JE, Huston E, Baillie GS, Thomson PA, Hill EV, Brandon NJ, Rain JC, Camargo LM, Whiting PJ, Houslay MD, Blackwood DH, Muir WJ, Porteous DJ. DISC1 and PDE4B are interacting genetic factors in schizophrenia that regulate cAMP signaling. Science. 2005 Nov 18 ; 310(5751):1187-91. Abstract

Chubb JE, Bradshaw NJ, Soares DC, Porteous DJ, Millar JK. The DISC locus in psychiatric illness. Mol Psychiatry. 2008 Jan 1 ; 13(1):36-64. Abstract

Schurov IL, Handford EJ, Brandon NJ, Whiting PJ. Expression of disrupted in schizophrenia 1 (DISC1) protein in the adult and developing mouse brain indicates its role in neurodevelopment. Mol Psychiatry. 2004 Dec 1 ; 9(12):1100-10. Abstract

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