What’s in a name? Genetic overlap between major psychiatric disorders

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Mirrored from http://wiringthebrain.blogspot.com

The criteria used to assign patients to specific psychiatric disease categories are set out in the Diagnostic and Statistical Manual of Mental Disorders, published by the American Psychiatric Association. (There is also a World Health Organisation equivalent, the International Classification of Disease). Every so often, these criteria are revised to reflect new research and changing concepts of disease. The APA has just released a draft of preliminary revisions to the current diagnostic criteria (available at http://www.dsm5.org/) as part of the preparations for the fifth release (DSM-5), due out in 2013.

The specific diagnosis given to any patient who shows up with a spectrum of symptoms has major implications not only for their clinical treatment but also for insurance, education, employment and many other aspects of their lives. Given the authority and influence of this tome for clinical practice as well as research purposes, it is timely to consider how genetics is, or is not, informing the diagnostic criteria.

One example where this question is especially relevant is to debates that have been going on for over a century about the underlying validity of the distinction between schizophrenia and bipolar disorder (and the various names each has gone under over that time). DSM-IV currently defines these disorders in such a way that they are effectively mutually exclusive, though they share many individual symptoms. Certain combinations of these symptoms co-occur often enough to warrant a label for the syndrome. One could argue that the distinction between these two categories is quite valid and clinically useful, in terms of prognosis and treatment. (Though see “Madness Explained: Psychosis and Human Nature” by Richard P. Bentall for a contrary viewpoint). On the other hand, the definitions have been explicitly designed in order to separate them at either end of a spectrum, with the intervening spaces filled in with a variety of other categories, such as schizotypal personality disorder or schizoaffective disorder. While the ends of the spectrum might be quite different, each move along it might be more dimensional than categorical.

Recent genetic findings are much more congruent with a dimensional distinction than a categorical one and suggest substantial shared etiology between these disorders. Surprisingly, they also demonstrate overlapping risk for other major psychiatric disorders such as attention deficit hyperactivity disorder, autism, epilepsy and mental retardation (or intellectual disability).

First, very large-scale epidemiological studies have confirmed that psychiatric disorders tend to co-occur generally within families. Thus, if a person has a first-degree relative with schizophrenia, while their risk of schizophrenia is ten times higher than the average person’s, their risk of bipolar disorder or autism or epilepsy is also dramatically increased. If these disorders are caused by single mutations (and the field seems to be slowly moving to acceptance of this model), then the implication is that any such mutation may result in a spectrum of symptoms in one individual, which leads to a diagnosis of disorder A, and in a different spectrum in another individual, which leads to a diagnosis of disorder B.

This model is borne out by the analysis of the effects of mutations in a growing number of specific loci. These include mutations in single genes such as CNTNAP2, NRXN1, and DISC1, as well as co-called copy number variants (CNVs) – deletions or duplications of chunks of a chromosome that may contain more than one gene – including recurrent CNVs at 1q21.1, 15q11.2, 15q13.3, 16p11.2, 22q11.2 and about thirty others. In all these cases, such mutations are associated with increased risk not to just one disorder but to many.

These findings raise some crucially important questions: why can so many mutations result in the same disorder (or spectrum of symptoms)? And, conversely, why can one mutation result in quite different spectra of symptoms in different individuals? The mapping from genotype to phenotype is not direct, at least if phenotype is defined by clinical category. Clearly, genetic interactions with other mutations in each person’s genome are likely to have an important influence. Just as clearly, these cannot be the sole determinants of the eventual phenotype as monozygotic twins are often discordant for psychiatric disorders. While environment and experience may influence phenotype, it is also likely that intrinsic developmental variation plays an important part. All of these factors are likely to contribute to an individual’s developmental trajectory and eventual phenotypic end-point.

These findings obviously raise some doubts about the clinical validity of the diagnostic categories. Perhaps they are overly and arbitrarily defined constructs of limited usefulness that actually hamper medical practice and research. Alternatively, they may describe truly distinct phenotypic end-points reflecting real differences in current pathophysiology, despite the similar genotypic starting point. For example, one patient with psychosis may share more in common with another patient with psychosis (and a different genetic etiology) than they do with another carrier of the same mutation who does not have psychosis.

The draft recommendations for DSM-5 include the welcome addition of dimensional criteria that can be applied across diagnoses to give a quantitative measure of specific symptoms. Genetics and neuroscience will continue to offer greater insights into pathogenic and pathophysiological mechanisms over the next few years – it will be interesting to see how these can be incorporated into the final version of DSM-5 and what their influence will be on clinical practice.

LICHTENSTEIN, P., YIP, B., BJORK, C., PAWITAN, Y., CANNON, T., SULLIVAN, P., & HULTMAN, C. (2009). Common genetic determinants of schizophrenia and bipolar disorder in Swedish families: a population-based study The Lancet, 373 (9659), 234-239 DOI: 10.1016/S0140-6736(09)60072-6

Rzhetsky, A., Wajngurt, D., Park, N., & Zheng, T. (2007). Probing genetic overlap among complex human phenotypes Proceedings of the National Academy of Sciences, 104 (28), 11694-11699 DOI: 10.1073/pnas.0704820104

Sebat J, Levy DL, & McCarthy SE (2009). Rare structural variants in schizophrenia: one disorder, multiple mutations; one mutation, multiple disorders. Trends in genetics : TIG, 25 (12), 528-35 PMID: 19883952

9 Comments

  1. i’m lovin’ these posts recently. kepp ‘em comin’!!

  2. YIP, B., BJORK, C., PAWITAN, Y., CANNON, T., SULLIVAN, P., & HULTMAN, C. (2009). Common genetic determinants of schizophrenia and bipolar disorder in Swedish families: a population-based study The Lancet

  3. One don’t need the insight of George Price or Isaac Newton to think those with classic signs of a schizophrenia spectrum disorder such as intense interest in finding hidden meanings in the bible – like Price and Newton – share genetic etiology as well as phenotypic end-point with others similarly afflicted

    If bipolar is subsumed into schizophrenia spectrum that is hardly evidence for the idea that any one of large number of possible mutations are resonsible for what is still a restricted range of symptoms.

    This post almost seems to be adopting an ‘anti psychiatry’ viewpoint but these well established rubrics are far better supported than the mutations hypothesis.

  4. Interesting post. Is it possible that, to some extent, whether or not someone has a mental illness is more genetic, but what form the mental illness will take is more environmental?

  5. The DSM-IV is a most peculiar book. I’ve just glanced at it but it seems to be an incredibly elaborate, Byzantine set of pigeonholes more oriented toward bureaucratic procedure and insurance schedules than to an actual understanding of the diseases or their treatment.

  6. Thank you all for your comments. The argument that I and many others are making is that the reification of specific diagnostic categories may not reflect a true biological distinction between them – at least they seem to share substantial genetic etiology, as well as significant overlap in clinical symptoms. The model to explain this is that mutations in any of a large number of genes can lead to disturbed neurodevelopment, which can be expressed in different individuals in different ways, sometimes resulting in a spectrum of symptoms that leads to a specific diagnosis. The evidence that psychiatric disorders are caused by genetic variants is now extremely strong and growing as more and more such mutations are identified. This is not to say that the environment can not have an influence and may, as John suggests, impact the ultimate phenotypic expression of particular mutations. As I mention in the post, there is also a counter-argument which says that even though different disorders may result from the same mutations, they may still represent distinct phenotypic endpoints that are clinically useful. Exactly what factors determine how genotype is mapped to phenotype is now a major challenge for the field, one which must involve a developmental perspective.

  7. It seems like it would be a useful study to look at the genes of people who respond to different psychiatric drugs. How are people whose, say, depression is alleviated by Prozac different genetically from people whose depression didn’t respond to Prozac but was then alleviated by Wellbutrin?

    Somewhat similarly, it’s common for a psychiatrist to arrive at a diagnosis of depression, then try different anti-depression drugs: e.g., start with Prozac, and if that doesn’t work, try Wellbutrin, then try something else. (Or maybe the depression goes away by itself — it’s hard to tell [although not always impossible]).

    Or, sometimes one drugs works for a variety of diagnoses: Prozac sometimes works for depression, sometimes for OCD, sometimes for whatever.

  8. “These include mutations in single genes as co-called copy number variants (CNVs) – deletions or duplications of chunks of a chromosome that may contain more than one gene – including recurrent CNVs at 1q21.1, 15q11.2, 15q13.3, 16p11.2, 22q11.2 and about thirty others. In all these cases, such mutations are associated with increased risk not to just one disorder but to many.”

    Crespi recently used copy number variation to test models of mental illness like subsumed, separate, overlapping and found that that autism and schizophrenia are over aganst one another: at 4 places in the genome deletions pre-dispose to one, while duplications pre-dispose to the other. To me that means they are not the same thing at all.

  9. I would second what Sailer said, and remind the author that bipolars and schizophrenics respond to very different drugs– lithium and antidepressants for the former, and antipsychotic medication for the latter. That suggests that they affect different neural substrates. I would also argue that there are significant symptomatic differences between the two, rather than significant overlap.

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