Schizophrenia Research Forum - A Catalyst for Creative Thinking

Live Discussion: The DISC1 Pathway in Major Mental Illness: Clinical, Genetic and Biological Evidence—Current Status and Future Prospects


David Porteous

Ishizuka et al. 2006

Porteous et al. 2006

The past year has seen a flurry of activity—on both the experimental and speculative fronts—surrounding disrupted-in-schizophrenia 1 (DISC1). To help make some sense of the recent studies and reviews, David Porteous of the University of Edinburgh helped us convene a virtual “roundtable” on the topic on January 23, 2007. If you missed the discussion, we invite you to read the background text below, not to mention the suggested articles (at left), comments left before the chat, and especially, the chat transcript.

Any comments you contribute to this discussion will appear in our What's New section and in our newsletter (See also the SRF news stories mentioned in the text below, as well as an SRF meeting report on DISC1 presentations at the Neuroscience 2006 meeting).

Special Update: Following the posting of the transcript, we asked Akira Sawa of Johns Hopkins University, who was unable to join us for the live chat, to serve as a discussant. He obliged by reviewing some of the interesting discussion, as well as mentioning questions that were not brought up. Please read his review.

Our thanks to Biological Psychiatry and Elsevier for granting open access to these two papers:

Ishizuka K, Paek M, Kamiya A, Sawa A. A review of Disrupted-In-Schizophrenia-1 (DISC1): neurodevelopment, cognition, and mental conditions. Biol Psychiatry. 2006 Jun 15;59(12):1189-97. Abstract

View comments by:
Judith GaultPosted 23 January 2007

Porteous DJ, Thomson P, Brandon NJ, Millar JK. The genetics and biology of DISC1--an emerging role in psychosis and cognition. Biol Psychiatry. 2006 Jul 15;60(2):123-31. Abstract

View Transcript of Live Discussion — Posted 25 February 2007

View Comments By:
Akira Sawa — Posted 16 January 2007
Mikhail Pletnikov — Posted 19 January 2007
Nick Brandon — Posted 19 January 2007
Christopher Ross — Posted 20 January 2007
Chris Carter — Posted 20 January 2007
David J. Porteous — Posted 23 January 2007
Katherine E. Burdick — Posted 23 January 2007
John Roder — Posted 23 January 2007
Akira Sawa — Posted 16 March 2007


Background Text
By David Porteous

The purpose of this forum is to discuss recent clinical, genetic, and biochemical evidence linking DISC1 and DISC1 interactors to susceptibility to major mental illness (see recent reviews by Ishikura et al. (2006), Biological Psychiatry, 59, 1189-1197; Porteous et al. (2006); and Porteous and Millar (2006)).

DISC1 was discovered as a novel gene at the breakpoint on chromosome 1q42 in a single Scottish family in which a balanced translocation between chromosomes 1 and 11 co-segregates with major mental illness, diagnosed as schizophrenia, bipolar disorder, or recurrent major depression. Recently, a number of independent genetic studies in various populations have implicated the DISC1 locus by linkage or association for schizophrenia, schizoaffective disorder, bipolar disorder, and major depression. Other studies of clinical and normal variation in measures of cognition and brain structure and function have also implicated a role for DISC1.

In parallel, our knowledge and understanding of DISC1 has increased (see SRF related news story). DISC1 is widely expressed, but developmentally regulated, with high levels of brain expression during prenatal neurogenesis and in the adult hippocampus. Downregulation of Disc1 expression by RNAi in the mouse neonate has been reported to result in abnormal neuronal migration and arborization. The 129 strain of mouse has a mutant form of Disc1, and this is associated with a deficit in working memory (see SRF related news story).

There is now a large body of direct evidence to show that DISC1 acts as a “hub” or scaffold protein, interacting with multiple protein partners, several of which are already known to play important roles in neurodevelopment, neurotransmission and signaling, the cytoskeleton, and centrosomal function.

The purpose of this forum is to review the current data, identify gaps or ambiguities in the current evidence, consider some unanswered questions of immediate relevance to the field, and discuss how these might best be addressed by prospective studies.

Some provisional questions:

1. What are the nature and effects of DISC1 mutations?
2. How is DISC1 expression regulated?
3. What are the consequences of aberrant DISC1 expression at the cellular and developmental level?
4. How, when, and where is DISC1 expression altered in the brains of subjects with mental illness?
5. What is the role of DISC1 in determining brain development?
6. What is the mechanism for DISC1 regulating cognitive processes, learning, and memory?
7. Can mouse models of the DISC1 pathway provide developmental and mechanistic insights not possible in patient studies? 8. Does DISC1 identify a target pathway for rational drug development in the treatment of psychosis and mood disorder?


Transcript

Attendees/Participants
Alexander Arguello, Columbia University, New York
Nick Brandon, Wyeth Neuroscience-Princeton
Katherine Burdick, Zucker Hillside Hospital/AECOM
Miguel Camargo (via e-mail), Merck & Co.
Joe Callicott, National Institute of Mental Health, Bethesda, Maryland
Chris Carter, unaffiliated
Thomas Comery, Discovery Neuroscience, Wyeth Research, Princeton
Tom Fagan, Schizophrenia Research Forum
Jinbo Fan, Broad Institute, Cambridge, Massachusetts
Sydney Gary, Cold Spring Harbor Laboratory, New York
Hakon Heimer, Schizophrenia Research Forum
Barbara Lipska, NIMH, Bethesda, Maryland
Heather McKellar, Columbia University, New York
Huân Ngô, Yale University, New Haven, Connecticut
Misha Pletnikov, Johns Hopkins University, Baltimore, Maryland
David Porteous, University of Edinburgh, Scotland, UK
John Roder, Samuel Lunenfeld Research Institute, Toronto, Canada
Christopher Ross, Johns Hopkins University, Baltimore, Maryland
Ezra Susser, Columbia University, New York
Marquis Vawter, University of California, Irvine, California
Ryan Westphal, Bristol Myers Squibb Neuroscience, Wallingford, Connecticut

Note: The transcript has been edited for clarity and accuracy.


David Porteous
Hello, there. I have just come in from a long meeting, but ready to switch from gene therapy to schizophrenia and DISC1!

Chris Carter
Hello, everyone.

Hakon Heimer
Hello, Chris, and thanks for preparing the interaction figure. I bet you spent some late nights on that!

Chris Carter
Just a few, but well oiled.

Hakon Heimer
Hello, Tom and Misha.

Mikhail Pletnikov
Hello!

Tom Fagan
Hello, everyone.

Kate Burdick
Hi, Hakon and all.

Nicholas Brandon
Hi. It's Nick B here. Miguel Camargo sends his apologies that he is unable to join. He became a dad on Saturday!

Mikhail Pletnikov
Nick, Send him our congrats!

Hakon Heimer
I would like to introduce and thank our chat leader, David Porteous. In the informal spirit of this event, and because I think most people here are at least acquaintances, I won't go into any length about his biography and work, except to say that David is Head of the Medical Genetics Section at the University of Edinburgh, and that it was the work by this group that opened this promising footpaths into what one might call the Swamp of Schizophrenia Etiology. I'll now turn the floor over to David.

David Porteous
And as possibly the worst typist in the world, I will apologize now for my efforts on the keyboard, but thank you all for joining the DISC1 chat.

Tom Fagan
Don't worry, David, we don't care about typos. They will be edited out later....

David Porteous
Shall we get started then? First of all, great to see so many people in the chat room, and thanks for all the early comments. Could we start with a discussion about genetic evidence, heterogeneity and clinical phenotype? Christopher Ross had some comments on this, as did others. Thanks to all of those who have already posted several interesting comments summarized as: what insights can be gained for systems biology as to what DISC1 does? Can we predict phenotype from DISC1 allelotype? What are the strengths and weaknesses of modeling DISC1 function in the mouse? Recent DISC1 papers from Hennah, Taya, and Shinoda have also sparked comments. NB: these papers are nicely summarized by Tom Fagan on the Forum website. I note that there are comments that are based upon the assumption of a truncated protein being produced in the t(1;11) family. I have to remind you that there is no such evidence, but also agree that a dominant negative phenotype is possible, but by other mechanisms.

Let me say a couple of things. First, although there is now lots of evidence for DISC1 linkage and association, there is no proven mutation other than the t(1;11) translocation. Second, there are missense mutations that may be causative, but are low penetrant. And third, there is bound to be allelic heterogeneity. Any disagreement?

Mikhail Pletnikov
Dr. Porteous, I agree. It may be even more complicated because we are still uncertain about the translocation mutation, meaning if there is expression of mutant protein, and so on.

Christopher Ross
David, I agree. Barbara, I think your findings of decreases of DISC1 interactors are very interesting (see SRF related news story). Is there any sense that some patients have decreases in one set and other patients have decreases in another set? That might lead to subtyping. But it would probably require a very large sample.

Barbara Lipska
That’s not possible to test, even in our large cohort, but lymphoblast data may clarify some of the changes.

David Porteous
Kate, Barbara, and anyone from Helsinki, the data on DISC1 genetic “interaction” or “conditioned” effects on NDE1 and NDEL1 are provocative. Can we say anything more about this or the clinical correlations?

Kate Burdick
In our dataset the DISC Ser704Cys is not a risk locus, but it is the primary site of interaction with NDEL1 (haplotype); this may speak to some of the differential findings at this site.

Barbara Lipska
These "interacting" genes may also confer susceptibility on their own. We are in the process of testing this.

David Porteous
Misha, true, but following Barbara's point, what we have is good evidence for 50 percent expression in the t(1;11) lymphoblast cells (likewise for the phosphodiesterase 4 (PDE4) translocation for PDE4 activity) so I think this is one mechanism, and I would predict that downregulation of DISC1 (all isoforms) would be pathogenic.

Nick Brandon
Barbara, did you look at NDE1?

Kate Burdick
In our data NDEL1 does, in fact, confer risk on its own, but we have evidence of epistasis—in that the risk is only present in DISC1 704 Ser homozygotes.

Marquis Vawter
Barbara and Akira, are the same mRNA isoforms present in brain and lymphoblasts/lymphocytes?

Barbara Lipska
Mark Vawter, we're now trying to solve the puzzle of isoforms. There are some novel ones, and we cannot assay some already reported.

David Porteous
Marquis, yes, in our hands all DISC1 isoforms are detectable in LBCs, but we can’t conclude from that that there regulation is the same as in the brain.

Barbara Lipska
We are looking at NDE1. It appears weak so far.

Marquis Vawter
David and Barbara, I am not a DISC1er (yet); how many splice variants are there? Has anyone used the exon arrays with any luck on lymphocytes/brain for this question?

Barbara Lipska
In addition to L and Lv, we see at least four shorter ones (not fully confirmed yet)—all in brain.

David Porteous
DISC1, I think then, is the first case in the field where we can talk about epistasis. Can anyone think of another good example?

Barbara Lipska
COMT.

David Porteous
COMT episatic with? What is the current status?

Kate Burdick
Consistent with what Barbara is saying, we have similar data to theirs suggesting possible epistasis between COMT and DTNBP1. This COMT-DTNBP1 interaction is with cognition only—not disease.

Chris Carter
Regarding epistasis: see Nicodemus et al., 2007).

Barbara Lipska
David, see Nicodemus et al. from our group; it just came out.

Marquis Vawter
Barbara, thanks for the update on the isoforms.

David Porteous
DTNBP1 is linked by Miguel Camargo in his Y2H screens to DISC1 (see Camargo et al., 2007).

Barbara Lipska
David, and DTNBP1 links to microtubules.

David Porteous
So the systems biology approach is starting to give some insights and plausible new lines of enquiry.

Nick Brandon
The link in Miguel's work is through the plakin family.

Barbara Lipska
David, too many to handle!

Huan Ngo
David, epistasis is a very broad stroke. Can you folks specify more of what you are suggesting, evidence, and more importantly, suggestions about possible mechanisms?

Barbara Lipska
Huan, the paper above dealt only with statistical evidence.

David Porteous
Huan, you are correct, hence my parenthesis. It is going to be difficult to prove directly in human studies. Perhaps in the mouse it will be easier?

John Roder
Hi, David. Yes, if we had a good mouse model one could breed the two together and look for epistasis.

David Porteous
John, it is an important future route I think.

Mikhail Pletnikov
John, a few mouse models are coming soon, and we can test those questions using them.

Nick Brandon
John, what's a good mouse model?

Mikhail Pletnikov
Nick, I think it all depends on what question(s) you have.

John Roder
Yes, Mikhail, I believe so.

Christopher Ross
The question of mouse models raises again the issue, what does loss of DISC1 function look like? Has anyone reported a knockout? Whatever mouse models one has, I think a question would be to look at whether expression of DISC1 or its interactors would be decreased?

Mikhail Pletnikov
So we are coming to the point when we should be talking about a systems approach to this molecule to understand it. Does anyone have a good example from other candidate genes or mutations that we can draw on for some help? Chris, how about Huntington’s disease?

Christopher Ross
I would suggest that subtyping could come from the broad classes of interactors: alterations in proteins involved with cortical development might cause negative symptom-type schizophrenia; alterations in interactors involved with neuromodulation might cause a more affective phenotype. I will have to think about the huntingtin question.

David Porteous
Chris Carter has done the same sort of thing schizophrenia as he did for Alzheimer's disease. (Ed note: see Chris Carter comment).

David Porteous
I want to hear from anyone who has or knows of a DISC1 variant that acts like the t(1;11) as a dominant trait with reduced penetrance (you don't need to tell us what you have found, just if you have found something!).

Barbara Lipska
I would love to know what is "abnormal" about either mRNA or protein in schizophrenia or carriers of risk alleles! We found some novel (unconfirmed yet) isoforms whose expression appears associated with some high-risk polymorphisms (but again in subpopulations...), though they are not ready for prime time yet.

Marquis Vawter
It seems that the postmortem work that Barbara at NIMH is working on with isoforms would be a great place to start for developing mouse models based on human isoforms.

David Porteous
Chris, Nick, Misha, like clinical studies, the model is only as good as the phenotyping, but I think we can approach this as a top-down and bottom-up problem, that is, when we have a mouse that we think shows a phenotype of schizophrenia or bipolar affective disorder, is there evidence that the DISC1 pathway is perturbed, and if the DISC1 pathway is perturbed genetically, does the mouse have a schizophrenia or bipolar phenotype? A cautionary note would be to make sure we make clinically relevant mouse mutations.

Christopher Ross
I agree. Barbara, do you have some hypotheses for us that we could translate to mice?

Mikhail Pletnikov
Chris, I agree, but the mouse system can react differently, though I believe we still can address other pathways.

John Roder
Chris, with mice DISC1 might be loss or gain of function.

Christopher Ross
David, ah! And besides the translocation, which is hard to model, what is a clinically relevant mutation?

Hakon Heimer
David, all, what phenotypes are most attractive for testing future DISC1 mice? Working memory, prepulse inhibition—any others?

Barbara Lipska
Hakon, depending on your question, but it is complex, see our paper on modeling schizophrenia.

John Roder
Hakon, I believe working memory, sensory motor gating, depression, latent inhibition, and others…

Mikhail Pletnikov
I think that requiring a mouse model to mimic schizophrenia or bipolar disorder is too much. How about some endophenotypic features? These may be easily modeled in mice or in brain slices.

David Porteous
Chris, that must be the next stage of genetic studies—to go beyond association to biological tests for functional evidence and by large-scale genomic resequencing.

Christopher Ross
David, yes, I agree, but speaking as a mouse modeler now (with Misha), I think a key question is what mutations to make. Knockouts?

Mikhail Pletnikov
I agree with Barbara; different models will answer different questions. So, maybe more is better?

Hakon Heimer
David, given that the translocation t(1;11) co-segregated several major mental illnesses (seven schizophrenia, 10 major depression, one bipolar), must we think in terms of studying a gene that should really be called "disrupted in schizophrenia, major depression ..." and if so, does that complicate the picture enormously?

Barbara Lipska
Just remember that if we agree that schizophrenia is a polygenic disorder, by definition it will not be possible to model everything with one gene manipulation.

Tom Fagan
All, how about that spontaneous mutation in mice that truncates DISC1? How does that fit into the picture? Is it a useful model?

Mikhail Pletnikov
Tom, 129 DISC1 mutation does not look so simple as it may have in the beginning, but is a useful system, of course.

David Porteous
It’s a pity Jill Morris has left because she has started up a DISC1 program in zebra fish. What about other models to understand the basic biology of DISC1?

Christopher Ross
Yes I would like to hear more from Jill. I think zebra fish could be great. They don’t really have a cortex, but they do have a forebrain. What can one learn from animals much lower on the evolutionary scale? Genetic interactions?

David Porteous
Christopher Ross, and zebra fish can be used to look at peripheral nerves par excellence

John Roder
Hi, David. We have started a study in C. elegans as well.

Barbara Lipska
All, are we sure that this is "truncation," however defined?

David Porteous
Barbara, let’s give the genetic architecture question an airing. Studies designed to detect polygenic risk factors missed DISC1. It seems to me very unclear at this stage what proportion of the genetic liability for schizophrenia or bipolar is accountable by common variants and what proportion by rarer variants.

John Roder
Hi, Tom. Yes, I believe not only those exact DISC1 changes found in humans, but many others in this gene would be useful in establishing an allelic series.

Christopher Ross
I agree with John; phenotypes from a series of different mutants could be very interesting.

David Porteous
John, I agree. An allelic series will be key to this and may help sort out the genotype-phenotype question. Everyone, now that we have better and better information on the sites of protein interaction, we can speculate that some variants might have very specific effects (affecting, for example, splicing) and others have more general effects.

Mikhail Pletnikov
To all, I would like to find out more about non-neuronal DISC1? Does anyone have new data on this? Thanks.

David Porteous
Misha, it is high in the heart, positive in most tissues, including lymphoblast cells.

Mikhail Pletnikov
I guess I meant its expression in astrocytes, oligodendrocytes, microglia.

Barbara Lipska
We have done co-localization confocal studies showing very clearly that there is a lot of DISC1 in glial fibrillary acidic protein (GFAP)-positive cells.

Tom Fagan
Mikhail, do you think non-neuronal DISC1 may be as important as neuronal?

Mikhail Pletnikov
Barbara and Tom, yes. It is an interesting finding since it will help us to see how DISC1 can help us address gene-environment interactions in mouse and other models.

Marquis Vawter
The idea that DISC1 is mitochondrially localized (from David's work) has interesting implications for systems affected. However, briefly looking at the Camargo paper, I didn't recall mitochondrial proteins being found in the interactome.

David Porteous
Marquis, yes, a specific isoform 71 kDa is mitochondrial. Points up the strengths and weaknesses of a yeast two-hybrid screen—with the emphasis on “yeast”—that is, not human.

Nick Brandon
On interactome and mitochondrial proteins: It’s true we do not find too many mitochondrial proteins. I'd need to check with Miguel, but I think there were some weak interactors that may not have made the final edits.

Miguel Camargo
(via e-mail) I don't think the lack of mitochondrial proteins in the DISC1 interactome suggests a weakness in Y2H but rather a deficit in protein annotation. Annotation is incomplete for many proteins and we cannot assume at the moment that we know the localization of all the DISC1 interactors. DISC1 in itself can be found in different cellular compartments.

Marquis Vawter
David, the 71 kDa protein. Which mRNA is it: l, lv, s, es?

David Porteous
Marquis, I wish we knew for sure—it’s not trivial. Several groups are working on this. Barbara's for sure. Also Akira's and us (a bit).

Barbara Lipska
Yes.

Marquis Vawter
Barbara and David, we are screening some dorsolateral prefrontal cortex tissue right now for mtDNA gene expression, so I thought maybe a few key DISC1 mRNAs would be of interest. Any suggestions?

David Porteous
Can I ask for comments on the DISC1 pathway as a target for drug development?

Christopher Ross
David, well, PDE is an enzyme....

Barbara Lipska
As is Nudel.

David Porteous
Chris, and so is NUDEL (EORPA).

Christopher Ross
David, there are also drugs that act on microtubules.

David Porteous
All true. And PDEs are general targets, so we might get something back for efforts in other fields, but anti-PDE novel chemical entities often have side effects. Nick, any comments?

John Roder
Hi, David. One dream would be to isolate modifier genes in a genetic screen and use those as a new target for drug screens.

Christopher Ross
John, yes, that would be a good potential source for druggable targets. Any candidates?

John Roder
Not yet, Chris.

Nick Brandon
Drug targets to date: PDE4 is clearly an attractive target, and it’s clear that the Millar paper has influenced thinking in industry. NDEL1/EOPA is also very attractive but not validated. Beyond that, the interactome does open up other possible pathways to think about. We need to do the validation, though.

Chris Carter
Both DISC1 and GABAB receptors bind to ATF4. Could you use GABAB agonists/antagonists to study this area?

David Porteous
Nick, from an industry perspective, what will be most persuasive—genetic evidence, biological evidence, available mouse model, or others—for selecting targets?

Nick Brandon
David, all of the above! Ryan and others from industry, what do you think?

John Roder
Hi, Nicholas. What criteria are required for validation?

Ryan Westphal
I agree with Nick that the more validation information that connects a novel target to disease, the better.

Nick Brandon
John, very general, but you need to have some level of confidence that modulating your target is going to have a beneficial effect. As many of these proteins are novel, you need to understand their biology and do the experiments to understand the consequences of effecting them—mouse models, knockdown, tool compounds, etc.

David Porteous
We are 50 minutes into the discussion. Can I ask for burning questions not already raised, and can I pose one? What are the intermediate phenotypes we should be looking at?

Barbara Lipska
David, cognition.

Joseph Callicott
Our general experience with single candidate genes has been that we've seen larger effect sizes for functional and structural imaging phenotypes.

Kate Burdick
David, I am biased as a neuropsychologist, but the cognition data are fairly consistent now—early processing right up through verbal memory. An interesting thought is to also keep the intermediate phenotypes in mind when targeting drugs—they may be more amenable to treatment than the larger diagnostic phenotype.

David Porteous
Kate, I agree completely—thanks for that. It is a really important area that I think may be at the heart of the DISC1 effect.

Hakon Heimer
David, all, did we touch on whether there any studies in the works to look at associations between DISC1 genotypes against clinical phenotypes—deficit versus paranoid, etc.?

David Porteous
Hakon, you anticipated my question. Here, we are looking at brain imaging (as are Zucker and NIMH and others). The Finns and Ty Canon have good memory task data. We are also looking in non-schizophrenia or bipolar cohorts, for example, the Scottish Mental Study—cognitive aging.

John Roder
Yes, David, we need many more clinical endophenotypes, some of which will likely associate with gene x and others with gene y, and ideally not on drugs.

Joseph Callicott
David and Kate, I think one potential advantage of cognitive data over imaging is in the realm of subtyping. It has been more difficult studying large enough samples to subdivide.

Kate Burdick
Joseph, agreed. Our cognition dataset offers the power, but we, too, find larger effect sizes with imaging parameters.

David Porteous
Kate and Joe, and we can take cognitive tests beyond the clinically affected into first-degree relatives and the general population, the premise being that normal variation in DISC1 is likely to have an effect with clinical disorder.

Joseph Callicott
We've certainly taken that approach in the general population as a means of validating these SNPs of unknown functional impact, but our experience in first-degree relatives has been less satisfying, perhaps because these individuals have other genes or environmental factors that prevent fulminant illness.

Hakon Heimer
We have about 5 minutes in the official time, though the room is open for all those who want to "rush the speaker" or continue discussing. All will be included in the transcript.

David Porteous
Has anyone defined the DISC1 promoter?

Nick Brandon
Following on from David's last question—has anyone looked at the promoters of the interactors to relate back to Barbara's work?

David Porteous
With all the interaction data and the evidence for Barbara and others that these interactors are affected, perhaps we can subvert the usual criticism of mouse models (can't self-report on psychosis) and use biomarkers to validate?

Huan Ngo
David, Nicholas, more importantly, is there any evidence of methylation status and epigenetic control of the promoter, which would tie back to earlier discussion of environmental links and possibly epistasis?

David Porteous
[Post hoc reply] Nothing specific, but we have someone looking at this.

Hakon Heimer
David, all, are there any more kindreds identified with the translocation, or are there more family members in the original family?

David Porteous
[Post hoc reply] The translocation family has been under study semi-continuously for 30 years. Blackwood and colleagues (Blackwood et al., 2001) gave an update on the original publication of St. Clair and colleagues (St. Clair et al., 1990). It would be very unlikely to find other subjects with the same translocation that were not directly related. This is in all likelihood a relatively recent mutation. I recall a study presented as a poster at the WCPG in Quebec which looked in a Taiwanese cohort for evidence of the translocation, but as I would have predicted, found none.

Hakon Heimer
Our official time is up, so I'll thank David for preparing the discussion text and (in Irv Gottesman's terms) playing speed chess with multiple grand masters.

Mikhail Pletnikov
Thank you everyone and the organizers. It was a very interesting first-time experience!

Nicholas Brandon
Some interesting strands to follow up on. Cheers!

David Porteous
Congratulations to Miguel on becoming a dad! Hope all is well.

Hakon Heimer
Thanks to all the rest of you, and please stay in the room as long as you like

Kate Burdick
Very interesting discussion; thanks!!

David Porteous
All, I think Hakon is calling time up. It has been great to have such a lively discussion. Thanks, everyone.

Comments on Online Discussion
Comment by:  Akira Sawa, SRF Advisor
Submitted 16 January 2007
Posted 16 January 2007

I have been impressed by the recent paper in Molecular Psychiatry (Camargo et al., 2006), exploring molecular networks involving DISC1.

Exploration of in vivo models is very important (having several different types of DISC1 animal models will be important), provided that the underlying mechanisms of genetic manipulations in mice are clarified in cell biology first.

View all comments by Akira SawaComment by:  Mikhail Pletnikov
Submitted 19 January 2007
Posted 19 January 2007

I sometimes cannot help thinking that without DISC1 we would still have been hunting for additional subtypes of 5-HT or DA receptors to provide more targets for fine tuning of neuronal functioning. It seems that DISC1 and a few other genetic risk factors, e.g., neuregulin, are finally pulling the research in mental illnesses away from the simplistic classical psychopharmacology toward the realm of cellular neurobiology. The recent publications by several groups have clearly demonstrated this point. However, a nagging feeling is emerging that the picture is going to have to be even more complicated than just 7 or 8 subtypes of 5-HT receptors. We are going to have to think in the terms of systems analysis. The entire biology and entire interactome of the neuron has to be considered to understand the role(s) of DISC1. In this context, I would like to comment on two questions put forward by Dr Porteous.

I would be inclined to be a bit more cautious with “the mechanism for DISC1 regulating cognitive processes, learning, and memory" (question 6). DISC1 is unlikely to regulate those complex psychological entities. What we perhaps should be talking about here is how the intracellular mechanisms of DISC1 interactions with its multiple partners can be mechanistically translated into inter-cellular interactions, eventually resulting in psycho-physiological processes.

Concurring with Dr Porteous’ question #7 and Dr Sawa’s comments, I would also like to emphasize that mouse models will likely provide important insights into the biology of DISC1 and related proteins. However, by pursuing the previous approaches, animal models are going to demonstrate “more of the same”, i.e., different molecular pathways usually converge into the same behavioral abnormalities, saying little if anything about specificity and relevance of multiple phenotypic changes to certain psychiatric disorders. Instead, a different approach appears more fruitful. As expressed by Gould and Gottesman in their commentary in Genes, Brain and Behavior (2005), “the current standard of 6+ rodent phenotypes to make a high-impact paper is questionable given the nature of the genetics of these (psychiatric, MP) disorders. Thus, it is conceivable that an endophenotype approach to genetic studies, when applied to preclinical studies in rodents, will not run the gamut of fulfilling multiple, diverse animal models of bipolar disorder; instead it may singularly affect one biological process that will only be present in tests of that single arena of focus (endophenotype) in laboratory animals. These may have limited (or no) face validity vis-à-vis symptom-based models.” (Ed. Note: see full text of Gould and Gottesman, 2005 at SRF Live Discussion) So, I think it is in this context that mouse models may be able to provide answers that cannot be addressed in patients.

View all comments by Mikhail PletnikovComment by:  Nick Brandon (Disclosure)
Submitted 19 January 2007
Posted 19 January 2007

It's an exciting time for research around DISC1. There is real momentum in the field which is culminating in some really beautiful cell biology studies as seen by the recent manuscripts in the Journal of Neuroscience (see SRF news story) and going back further to the Kamiya paper in 2005 in Nature Cell Biology (see SRF news story). As Akira has commented already, the exploration of animal models is crucial to the field. We must be careful in this regard though—The relevance of the mutations or pertubations must be considered very carefully. DISC1 has really opened up some very attractive areas of cell biology to be explored for schizophrenia and I look forward to a discussion on how we develop these further.

View all comments by Nick BrandonComment by:  Christopher Ross
Submitted 20 January 2007
Posted 20 January 2007

DISC1’s increasingly extensive interactome and rich cell biology, combined with the rapidly developing genetics (e.g., comment above by Brandon and links within, and the studies of Craddock’s group—e.g., Hamshere et al., 2005) suggest that DISC1 has the potential to be a kind of “Rosetta stone” for psychiatric genetics—helping us both to uncover pathogenic pathways and also to reconceptualize phenotypes.

Some hypotheses: pathways involving proteins (e.g., NudEL or PCM1) at the centrosome or motor proteins involved in nucleokinesis may lead to developmental phenotypes most like deficit schizophrenia; pathways involving proteins potentially regulating synaptic plasticity may lead to phenotypes most like schizophrenia and schizoaffective disorder; and pathways with proteins (e.g., PDE4B) involved in neurotransmission and neuromodulation may lead to phenotypes most like affective disorder (see also Ross et al., 2006). While very speculative, perhaps hypotheses like these could help drive interactions between cell biological and genotype-phenotype studies?

References:

Hamshere ML, Bennett P, Williams N, Segurado R, Cardno A, Norton N, Lambert D, Williams H, Kirov G, Corvin A, Holmans P, Jones L, Jones I, Gill M, O'Donovan MC, Owen MJ, Craddock N. Genomewide linkage scan in schizoaffective disorder: significant evidence for linkage at 1q42 close to DISC1, and suggestive evidence at 22q11 and 19p13. Arch Gen Psychiatry. 2005 Oct;62(10):1081-8. Abstract.

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

View all comments by Christopher RossComment by:  Chris Carter
Submitted 20 January 2007
Posted 20 January 2007

DISC1 as a Hub Gene

Camargo and colleagues, in their DISC1 and dysbindin interactome study, have emphasised the convergence of schizophrenia risk genes on common biological processes(Camargo et al., 2007) (see also SRF news story).

I thought it might be interesting, for this roundtable, to take this a step further and see whether the DISC1 binding partners interact with other schizophrenia or bipolar susceptibility candidates. This is not a fully referenced or in depth analysis and is taken primarily from the interaction section of the ENTREZ gene data for each DISC1 binding partner.

DISC1 binds directly to six other genes implicated in schizophrenia or bipolar disorder (CIT, DPYSL2, FEZ1, MLC1, NDE1, PDE4B) and can be linked in two steps to many other susceptibility candidates, related in particular to glutamate and growth factor signalling, axonal transport and other processes, including those controlled by the transcription factors ATF4 and ATF5.

The results are illustrated in a figure posted at Polygenic Pathways (click DISC1 ROUNDTABLE). The individual genes should be clickable in some browsers, leading to ENTREZ gene data. [Ed Note: works with Internet Explorer, but not Mozilla Firefox.]

DISC1 cannot be in all places at once. Are there any signals (NMDA, growth factors, etc) that result in its translocation from one compartment to another?

References:
Camargo L. M., Collura V., Rain J. C., Mizuguchi K., Hermjakob H., Kerrien S., Bonnert T. P., Whiting P. J., and Brandon N. J. (2007) Disrupted in Schizophrenia 1 Interactome: evidence for the close connectivity of risk genes and a potential synaptic basis for schizophrenia. Mol Psychiatry 12, 74-86. Abstract)

View all comments by Chris CarterComment by:  David J. Porteous, SRF Advisor
Submitted 23 January 2007
Posted 23 January 2007

Several of you have posted interesting comments which I hope we can pick up on later today. A brief summary of some of the key points would be: what can we learn about DISC1 function from a systems approach; what might be the strengths and possible limitations of mouse models; is there allelic heterogeneity at the DISC1 locus and how might we predict that this would be reflected in the clinical phenotype? Recent papers by Hennah et al, Carmargo et al, Taya et al and Shinoda el al have raised interest. Read Tom Fagan's excellent summary on the Forum website. Go to Carter's web site for his view of the DISC network. Talk to you later.

View all comments by David J. PorteousComment by:  Katherine E. Burdick
Submitted 23 January 2007
Posted 23 January 2007

There has been a growing interest in the DISC1 gene, as convergent evidence suggests that it acts to influence a number of clinical traits including susceptibility to a range of psychiatric illnesses, cognition, brain morphology, and positive symptoms. These broad-ranging effects suggest that its action may be mediated by multiple loci within the gene via differential effects on expression, in addition to complex interactions among DISC1 and its multiple confirmed binding partners (Porteous et al. 2006).

Several recent studies have begun to elucidate the functional biology of the DISC1 protein and its binding partners, particularly with regard to NUDEL: a) The enzyme activity of NUDEL is inhibited by DISC1 (Hayashi et al. 2005) b) NUDEL expression is reduced in hippocampus of patients with schizophrenia and DISC1 risk alleles predict reduced expression of NUDEL (Lipska et al. 2006); c) The intact interaction between DISC1 and NUDEL is necessary for neurite outgrowth (Kamiya et al. 2006) and other key neurodevelopmental processes (Taya et al. 2007) that are known to be abnormal in patients with schizophrenia; and finally, d) recent linkage data have identified a critical interaction between a DISC1 risk haplotype and the NDEL1 paralogue, NDE1, and risk for schizophrenia in a Finnish sample (Hennah et al. 2006).

Given this widespread interest in DISC1 and its binding partners, our group has recently investigated the effect of NDEL1 on susceptibility to schizophrenia and its impact upon the key clinical phenotype of neurocognitive function (data under review). We report preliminary evidence that variation within NDEL1 modestly influences the risk for SZ and affects cognition in SZ patients and healthy controls, on the same cognitive measures that were previously associated with DISC1 in our cohort (Burdick et al. 2005). Perhaps most interestingly, the effect of NDEL1 on risk for SZ appears to be reliant on a background of DISC1 704 SerSer genotype, representing early evidence of an epistatic gene-gene interaction between NDEL1 and DISC1. As these data are preliminary, additional studies will be necessary to replicate these results and extend this approach to investigate interactions among other candidate genes within the DISC1 pathway.

References:
Burdick, K.E., Hodgkinson, C.A., Szeszko, P.R., Lencz, T., Ekholm, J.M., Kane, J.M., Goldman, D., Malhotra, A.K. ( 2005) DISC1 and neurocognitive function in schizophrenia. Neuroreport, 16(12), 1399-402. Hayashi, M.A., Portaro, F.C., Bastos, M.F., Guerreiro, J.R., Oliveira, V., Gorrao, S.S., Tambourgi, D.V., Sant'Anna, O.A., Whiting, P.J., Camargo, L.M., et al. (2005) Inhibition of NUDEL (nuclear distribution element-like)-oligopeptidase activity by disrupted-in-schizophrenia 1. Proc. Natl. Acad. Sci. U S A, 102(10), 3828-33. Hennah, W., Tomppo, L., Hiekkalinna, T., Palo, O.M., Kilpinen, H., Ekelund, J., Tuulio-Henriksson, A., Silander, K., Partonen, T., et al. (2006) Families with the Risk Allele of DISC1 Reveal a Link Between Schizophrenia and Another Component of the Same Molecular Pathway, NDE1. Hum. Mol. Genet., Epub ahead of print. Hodgkinson, C.A., Goldman, D., Jaeger, J., Persaud, S., Kane, J.M., Lipsky, R.H., Malhotra, A.K. (2004) Disrupted in schizophrenia 1 (DISC1): association with schizophrenia, schizoaffective disorder, and bipolar disorder. Am. J. Hum. Genet., 75(5), 862-72. Kamiya, A., Tomoda, T., Chang, J., Takaki, M., Zhan, C., Morita, M., Cascio, M.B., Elashvili, S., Koizumi, H., Takanezawa, Y., et al. (2006) DISC1-NDEL1/NUDEL protein interaction, an essential component for neurite outgrowth, is modulated by genetic variations of DISC1. Hum. Mol. Genet., 15(22), 3313-23. Lipska, B.K., Peters, T., Hyde, T.M., Halim, N., Horowitz, C., Mitkus, S., Weickert, C.S., Matsumoto, M., Sawa, A., Straub, R.E., et al. (2006) Expression of DISC1 binding partners is reduced in schizophrenia and associated with DISC1 SNPs. Hum. Mol. Genet., 15(8), 1245-58. Porteous, D.J., Thomson, P., Brandon, N.J., Millar, J.K. (2006) The genetics and biology of DISC1--an emerging role in psychosis and cognition. Biol. Psychiatry, 60(2), 123-31. Taya, S., Shinoda, T., Tsuboi, D., Asaki, J., Nagai, K., Hikita, T., Kuroda, S., Kuroda, K., Shimizu, M., Hirotsune, S., et al. (2007) DISC1 regulates the transport of the NUDEL/LIS1/14-3-3epsilon complex through kinesin-1. J. Neurosci., 27(1), 15-26.

View all comments by Katherine E. BurdickComment by:  John Roder
Submitted 23 January 2007
Posted 23 January 2007

Being newcomers to the field, we reviewed it a few years ago and decided that DISC1 was the best candidate gene for schizophrenia and/or mood disorders, based on the co-segregation of these diseases with a chromosomal translocation disrupting DISC1 in a large Scottish family. Reciprocal translocations have led to great strides in the cancer area. For example, translocations sometimes activate human oncogenes leading to tumors in humans. Conventional association studies in people do not have the resolving power to yield single genes. Also the DISC1 association held up, not only in the original Scottish family, but also in other populations throughout the world. Although there are many other candidate genes, we found the evidence for DISC1 to be overwhelming (see featured reviews). The only missing data would be to show DISC1 perturbations in mice caused a partially overlapping phenotype to humans.

We set up assays of behavioural endophenotypes for some shared “traits.” We were able to establish assays for sensorimotor gating (pre-pulse inhibition of the acoustic startle), attention deficits (latent inhibition), anxiety (elevated plus-maze), depression (Porsolt swim test, sucrose preference, social interaction), working memory (T-maze), and motor activity (open field). To validate these tasks, we tested mice with known genetic or pharmacological perturbations and used antipsychotics or antidepressants to reverse these effects. A forward genetic screen in our lab of ENU mutant mice screened first in the PPI task failed to confirm any mutant loci by chromosomal mapping. Therefore, we began a reverse genetic screen of a mutant archive (at that time, n=2,000) at RIKEN, Japan. Two interesting mutants in DISC1 were identified and are currently being characterized at the endophenotypic level (by two talented PDFs in my lab, Steve Clapcote and Tatiana Lipina). The results will, hopefully, fulfill one of Koch’s postulates established for disease causation.

A valid animal model would allow drug screens not possible in humans. Modifier genes (enhancers or suppressors) would also suggest other targets for DISC1 regulation. In addition, mouse models would address the issue of haploinsufficiency vs. dominant/negative models of disease. Furthermore, mice, unlike humans, can be assayed for a very large number of phenotypes. For example, data from the Gogos group shows absence of some, but not all, DISC11 isoforms in the 129 strain leads to a deficit in working memory. Other labs have made truncated DISC1 expressed ectopically in transgenic mice (Silva, Sawa). Reduction of DISC1 early in development using RNAi leads to developmental perturbations (Sawa Lab).

DISC1 is involved in interaction with many proteins in the cell (Brandon Lab), and the functional role of DISC1 could be understood from discovering protein-protein interactions. For instance, the role of DISC1 in schizophrenia/depression could be better understand due to A DISC1 interacting protein, PDE4B, which is involved in depression and cognition. It is reassuring that a second gene product (PDE4B) that interacts with DISC1 was recently identified in the Porteous/Miller lab again in unequivocal patients with reciprocal translocation, although the sample size (n=2) was low.

View all comments by John RoderComment by:  Akira Sawa, SRF Advisor
Submitted 16 March 2007
Posted 16 March 2007

On behalf of all the investigators who believe DISC1 is a promising lead to understanding the molecular pathology of major mental illnesses, I would like to say thank you very much to Hakon Heimer and David Porteous for planning and organizing this roundtable discussion. Here, I would like to review the DISC1 research based on the fruitful discussions in the roundtable. The following subjects will be covered in this overview:

1. Clinical Subtypes and Biological Pathways Involving DISC1
2. DISC1 Isoforms
3. DISC1 Animal Models
4. DISC1: Potential Target for Therapeutic Strategies?

Of note, many basic neuroscientists (not scientists primarily working on psychiatric disorders), especially those who are interested in synaptic plasticity and brain development, are currently interested in DISC1. This trend was not well covered in the roundtable discussion (see, e.g., the work by Kozo Kaibuchi's group, discussed in Tom Fagan's SRF article).

1. Clinical Subtypes and Biological Pathways Involving DISC1
Studies of the original Scottish family, as well as many association studies in several independent populations, consistently suggest that genetic variations in the DISC1 gene impact both schizophrenia and mood disorders in humans. Since clinical diagnoses are defined without any consideration in their etiologies, this is not so surprising. Probably DISC1 plays a key role in one, or more than one, endophenotype(s), which, in turn, contribute to the pathophysiology of both schizophrenia and mood disorders.

Therefore, it would be crucial to examine genetic and biological interactions of DISC1 with other factors, such as COMT, NDEL1, and NDE1, in the light of clinical subtyping, in order to understand how DISC1 plays a role in schizophrenia and mood disorders. I found that very productive ideas/comments were exchanged over this subject in the roundtable discussion. It is conceivable that different genetic variations in DISC1 combined with risks in different sets of DISC1 interactors may determine distinct clinical outcomes (see also Sawa and Snyder, 2005).

2. DISC1 Isoforms
The DISC1 protein is multifunctional with many isoforms. Molecular diversity of DISC1, due to alternative splicing and post-translational modifications, such as phosphorylation, seems much more complex than we expected. Even at the mRNA level, it is obvious that there are a greater number of spliced variants than published before. There seems to be more coding exons than 13 exons already reported, for example.

Peripheral tissues, such as lymphoblasts, may be useful to address this question, but we should be very careful about tissue specificity. As far as I am aware, DISC1 isoforms are substantially different in brains and peripheral tissues. Therefore, it sounds very exciting that Barbara Lipska “found some novel isoforms whose expression appears associated with some high-risk polymorphisms” in brains. We may also need to pay attention to age-dependent differences in brain isoforms; that is, DISC1 isoforms during neurodevelopment when this molecule has a crucial role may not be the same as those in adult brains.

Although it is laborious and time-consuming to identify full sets of DISC1 isoforms, this information is necessary for us to interpret data in gene expression profiling and genotyping linked to clinical subtyping. Furthermore, without this information, we may not be able to properly generate genetically engineered mice.

3. DISC1 Animal Models
At this moment, at least five groups have generated DISC1 genetically engineered mice: (i) Gogos’s lab at Columbia published a type of DISC1 mouse that displays deficits in working memory (Koike et al., 2006 and SRF related news story); (ii) Pletnikov’s lab at Johns Hopkins has reported transgenic mice expressing a C-terminal truncated DISC1 in an inducible manner (information from meeting abstracts—see SRF related news story); (iii) Ty Cannon and Alcino Silva’s group at UCLA has generated a couple of types of transgenic mice expressing different portions of DISC1 (information from meeting abstracts); (iv) John Roder at Toronto said that he had succeeded in generating/obtaining novel types of DISC1 mice; and (v) my lab has also generated two types of transgenic mice expressing a dominant-negative DISC1 protein under two different types of promoters. In addition, in collaboration with Japanese investigators, my lab has also characterized mice with in utero injection of RNAi to DISC1. If my understanding is correct, several groups have tried to generate DISC1 knockout mice, but none of them have been successful yet, due to the complexity of exon usage of the DISC1 gene.

At present, it is very difficult to say which model is more or less useful. As a couple of roundtable participants mentioned, each model will provide different types of valuable information by setting appropriate hypotheses to be tested. It would be very important for us to appreciate the information from all the DISC1 mice in a neutral manner and compare both similarities and differences among the models. In my opinion, the first step we need to focus on is to understand the basic role for DISC1 in cognition, emotion, and other brain functions in vivo. As described above, DISC1 is a gene for neither schizophrenia nor mood disorders. Instead, DISC1 is a gene for one or more than one endophenotype(s) that contribute to the pathophysiology of both schizophrenia and mood disorders in humans. I support Mikhail Pletnikov’s opinion that “requiring a mouse model to mimic schizophrenia or bipolar disorder is too much.” Instead, as he also mentioned, we need to seriously consider endophenotypic features that are translatable between humans and rodents.

In addition to mouse models, Jill Morris has studied the zebrafish model of DISC1. This model, as the roundtable participants also expressed, is very promising in testing both loss- and gain-of-function models. John Roder mentioned that his lab is studying C. elegans models of DISC1. My group has generated and is characterizing fly models. But we may not be able to generate loss-of-function models in fly and C. elegans, because no DISC1 ortholog exists in these species. I appreciate that John Roder clearly pointed out the key advantage of these models, which is that they will allow us to “isolate modifier genes in a genetic screen and use those as a new target for drug screens.”

4. DISC1: Potential Target for Therapeutic Strategies?
There are several types of molecules that make easy drug targets, such as receptors, ion channels, GPCR, and enzymes. In comparison, DISC1 may not be a straightforward target for drug development. Because DISC1 modulates functions of enzymes, such as PDE4 and NDEL1 (EOPPA) at least in vitro, we may have a chance to explore therapeutic strategies by studying protein interactions of DISC1 with PDE4 and NDEL1. As described, identification of modifiers will help explore drug targets. It is true that, as a DISC1er, I am always interested in attempts to find therapeutic strategies directly aimed at DISC1, including approaches pioneered by Nick Brandon and many others. However, frankly speaking, I feel that DISC1 is more valuable for understanding the key disease pathways. Once the disease pathways are understood more clearly in association with corresponding endophenotypes, I predict that we will have many novel therapeutic targets for major mental conditions.

Conclusions
I am certain that the roundtable discussion was extremely productive. I regret that due to the conflict in my schedule I missed the chance to participate in the real-time discussion. I would like to propose that this roundtable discussion be held annually.

View all comments by Akira Sawa