Understanding Schizophrenia 2: What causes schizophrenia?

The Royal Bethlem psychiatric hospitalThe previous post in this series described the symptoms of schizophrenia. Here we turn to the causes of schizophrenia. There has been major progress in this area over the last twenty years. A number of factors have been identified which carry a risk for schizophrenia. Some of these factors are genetic, others impact during the course of life.

Usually schizophrenia emerges in late adolescence or early adulthood, as the intellect, personality and neural networks are being sculpted. The population risk is slightly less than 1%, with a slight excess of male sufferers (1.4:1). Males also tend to show a more severe pattern of illness, with more impoverishment of the personality and psychological decline.

Risk Factor Categories

The risk factors for schizophrenia can be grouped into several categories (Figure 1). The perinatal category includes hypoxic and nutritional insults to the developing brain in-utero. The second category includes being brought up in city environment, particularly for immigrants. The third category includes drugs of abuse, specifically strong cannabinoid CB1 receptor agonists. Finally, there is the genetic category, which can be subdivided into single nucleotide polymorphisms (SNPs) and copy number variants (CNVs).

Figure 1. Risk factors for schizophrenia.

Figure 1. Risk factors for schizophrenia.

Genetic risk factors

It has long been recognized that schizophrenia runs in families (Figure 2), but until the last decade attempts to identify specific genes floundered. Technological advances have revolutionized the field. It is now feasible to screen an individual’s DNA at every base pair (A, T, C, G) in every chromosome. Variants (say the substitution of an A for a T) are called single nucleotide polymorphisms (SNPs) when they occur in at least 5% of the overall population.

In the technique known as GWAS (genome wide association study) tens of thousands of patients are compared against tens of thousands of controls. So far, 145 SNPs have been shown to confer risk for schizophrenia (Figure 3). Each SNP on its own carries a very small risk, but they are common in the population, and their effects are additive. Two SNPs of considerable interest are the gene for a calcium channel (CaV1.2) and the gene for a protein called complement C4a.

Figure 3. Single nucleotide polymorphisms which confer a risk for schizophrenia.

Figure 3. Single nucleotide polymorphisms which confer a risk for schizophrenia.

The second major breakthrough in schizophrenia genetics are copy number variants (CNVs). Copy number variants are deletions or duplications of a long stretch of DNA, typically incorporating half a dozen genes or so. So far eight CNVs which confer a risk for schizophrenia have been identified. Each of these CNVs carry a very high risk. A CNV of considerable interest is NRXN1 (Figure 4).The NRXN1 protein forms a physical bridge which stabilses synaptic connections in the brain. The NRXN1 story provides strong evidence for a long-held theory that the pathology of schizophrenia stems from abnormal connectivity within neural networks.

Figure 4. Copy number variants associated with schizophrenia.

Figure 4. Copy number variants which carry a risk for schizophrenia.

We can recap. A number of factors confer risk for the development of schizophrenia. These can be categorized into several categories – perinatal, environmental, cannabinoid CB1 drugs, and genes. The gene category includes SNPs (such as, complement C4a, the calcium channel CaV1.2) and CNVs (such as NRXN1). In the next post in this series we will look at the neurobiology of these components and cannabinoid CB1 drugs.

Further posts in this series:

What exactly is schizophrenia?

 Future posts in this series:

What happens to the nervous system in schizophrenia?

The prognosis of schizophrenia.

How is schizophrenia treated?

Psychiatric genetics: CNVs are revealing the neuroscience of schizophrenia

Long stretches of DNA, from 1000 to several million base pairs, can be deleted or duplicated within a chromosome. These changes are known as copy number variants (CNVs). It is now recognised that eight CNVs are associated with schizophrenia. For a person carrying a CNV, their risk of developing schizophrenia is increased by 3-58 times compared to the general population. It is also known that approximately 2.5-5% of people suffering from schizophrenia will carry at least one of these CNVs.

A new paper by Danish researchers serves as an excellent introduction to this rapidly developing and fundamental topic. The authors detail the characteristics of each of the eight CNVs.

The effect of CNVs: Neurexin

The majority of the CNVs harbor multiple genes. The exception is deletion of a stretch of DNA which harbors a single gene coding for a protein called neurexin (NRXN)1.

The molecular biology of glutamate synapses.

Synapses are held together by cell adhesion molecules, one of which is neurexin (NRXN1).
Deletion in the gene for neurexin is a risk factor for schizophrenia and autism.

Neurexin is a component which holds the two sides of a synapse together. When people talk about connectivity in the nervous system, they are talking about components like neurexin. Healthy information processing in the brain depends on healthy stable connections between neurons.

The effect of CNVs: The other seven

Researchers are beginning to understand how the the seven other CNVs might operate to confer risk for schizophrenia, and indeed many other psychiatric syndromes. A property known as pleitropy means that a genetic change actually confers risk for a range of disorders. The CNVs associated with schizophrenia are also risk factors for autism, ADHD, epilepsy and intellectual disability. Indeed, all eight schizophrenia CNVs confer risk for autism spectrum disorders (Table 1).

Copy number variants (CNVs) which confer risk for schizophrenia and other neuropsychiatric syndromes

Copy number variants (CNVs) which confer risk for schizophrenia and other neuropsychiatric syndromes.

Using animal models, researchers can decipher how the CNVs alter brain and behviour since DNA stretches are conserved across mammalian species. Animal models of five of the eight schizophrenia CNVs are now available. Those CNVs have been shown to cause deficits in cognition, social behavior, information processing and synaptic plasticity.

Further developments in this area are bound to reveal much more detail about how altered neuronal dynamics give rise to schizophrenia and other major psychiatric syndromes.

Psychiatric illness ‘explained’: Disorders of CNS Connectivity

The power of the nervous system:

network-of-cortical-neurons

The astonishing power of the nervous system does not reside in a single neuron. (That said, an advanced supercomputer is required for the task of modelling the processing power of even a single neuron).

Nervous tissue is immensely powerful because of the rich connectivity between neurons. A 1mm voxel of cerebral cortex (a standard fMRI unit), contains ~300 million synaptic connections and ~50 thousand neurons [ref].  Scaled up to the whole human brain, there are estimated to be several hundred trillion synaptic connections within a total pool of ~100 billion neurons. Neuronal networks are the foundation of, perception, movement, thinking, memory and the personality.

Network learning

A crucial property of neuronal networks is that they learn from experience. Experience may stem from the external world (sensation) or the inner world. Learning is achieved by adjusting the strength of the connections between neurons. New connections can form, and weak connections wither away – essentially a process of re-wiring. Taking up a musical instrument or a new language, for example, constitutes a major re-wiring exercise, although higher, more mysterious faculties – such as selfhood, agency and individual identity – are already wired-up in infancy, and remain a foundation throughout life, except if threatened by the most severe psychiatric disorders.

Alzheimer’s disease is the prototypical example of a network illness. Progressive       shrivelling of the network mirrors the decline of the faculties, from initial problems with memory right up to the disintegration of selfhood.

Network health

Network health is vital for mental health. The stabilisation of essential connections, the formation of new connections and the controlled elimination of redundant connections involves many components.

  • There are components which span the gap between nerve terminals and dendritic spines to ensure that connections remain tightly bound [link].
  • There are signalling pathways which control the dynamic, flexible actin scaffold which give terminals and spines their anatomical structure.
  • There is, ready-to-hand, protein-synthesis machinery for making additional spines as learning proceeds.
  • Finally, and most recently explored, there are mechanisms for ‘clearing up’ the debris when connections are no longer required. Such components (microglia, complement proteins) are much more familiar in their role as immune cells and immune signals, but their role extends beyond inflammation. Microglia and complement are now recognised as key components in the wiring of the brain as it learns and develops.

Major psychiatric illness

dendritic spine

Where those components involved in the function and structure of synaptic connections are defective, psychiatric illness can result. Mutations in the components which bind the nerve terminal and dendritic spine are a cause of autism. The cause of many learning disability cases, hitherto unknown, are mutations in proteins which control the actin scaffold. The psychiatric manifestations of Fragile X syndrome (intellectual deficits / autistic features / hyperactivity) result from abnormal protein synthesis in dendritic spines and subsequent abnormal local wiring.

dendritic-spine

Microglia & complement proteins

pink-eatme-cake-topperThe latest components to receive attention, as pertains to psychiatric illness are the microglia and their signalling pathways, specifically complement proteins.

Complement proteins function as a tag, essentially an ‘eat-me’ signal, on synapses destined for elimination. The tag is recognised by the phagocytic microglia which engulf and clear the redundant synaptic elements [link].

Although the role of immune components in psychiatric illness has become a hot topic, many researchers are still accustomed to regard microglia and complement in the context of inflammation rather than CNS re-wiring. Both major depression and schizophrenia, have been linked with abnormal immune components, but neither disorder is inflammatory in the same sense as encephalitis or meningitis. The main histological finding in schizophrenia is decreased connectivity between neurons, not inflamed nervous tissue. Similarly, an anatomical correlate of depression is impoverished connectivity in the hippocampus, not inflammation.

A major development in Alzheimer’s research has been the recognition of up-regulated complement proteins and microglial phagocytosis commensurate with the loss of neuronal connections. The crucial observation is that such changes occur prior to amyloid deposition and tangle formation [link]. Alzheimer’s appears to be a disorder of runaway synaptic loss. Drug discovery efforts are aimed at blocking complement protein receptors to protect synapses [link].

Schizophrenia has been associated with changes in the genes coding for a specific complement protein (C4A). Knockout of the C4A gene in an animal model causes a marked alteration in the pruning of synaptic connections in later life [link]. Schizophrenia, albeit to a far less extent than Alzheimer’s, is regarded as a disorder of impoverished connectivity, (whereas Autism is associated with increased dendritic spines and increased connectivity) [link].

Hold on –  what about the ‘dominant’ wet-ware hypotheses?

hoovers

An older generation of psychiatric researchers may ask where dopamine [link]] and perhaps glutamate [link] fit into a model of psychiatric illness in which abnormal connectivity between neurons appears to carry robust explanatory power. Earlier models posited that an excess or deficiency of neurotransmitter or receptors lay at the root of major depression and schizophrenia. Such models stemmed from the relatively primitive knowledge of the synapse available at the time (circa 1965-1975). Then, the hot topics in neuroscience were; the nature of neurotransmitter release (Sir Bernard Katz, UCL) and the ‘visualisation’ of receptors (Solomon Snyder, John Hopkins).

The answer (to the question of how glutamate and dopamine are accommodated) is fairly straightforward: Glutamate (finally admitted to the neurotransmitter club circa 1983-87) is the fast neurotransmitter between nerve terminals and dendritic spines, throughout nervous tissue. Dopamine determines the strength of the connection between the glutamate terminal and the dendritic spine within specific CNS structures. Dopamine functions as a teaching signal; adjusting connectivity and promoting learning in higher centres.

Frontier psychiatry

hippocampus

The obvious strategy of searching for molecules which can impact on connectivity is well underway.

That said, existing psychiatric treatments, such as antidepressants, lithium and dopamine antipsychotics have an impact upon connectivity to the extent that structural changes can already be detected, albeit in a population of patients rather than the individual, with routine MRI scans. Drugs impact upon plasticity: Drugs impact upon CNS structure.

A more basic question goes back to the very roots of modern psychiatry. The question is whether, for some, the neuronal networks are destined to be unwell from the outset (endogenous psychiatric illness), or if, for others, adverse experiences during development cause the network to wire-up pathologically (exogenous psychiatric illness). Then again, there is the third position, in which the choreography between the neuronal hardware and the external environment determines who will succumb to psychiatric syndromes. Whatever the proximal cause(s), endogenous or exogenous, major psychiatric illness appears to stem from abnormal connectivity within neuronal networks.

Next-generation, evidence-based healthcare management

barnsleyfernHealthcare delivery is at last beginning to adopt mathematical and engineering principles [link]. There is an awareness, especially in the USA, that the leadership of an inherently chaotic system requires a professional mathematical sensibility, infinitely more sophisticated than the tired mix of power-point, polished-comportment, faux-bonhomie, affected-positivity and sloganeering.

Complex systems are made up of many components. A decision which impacts upon one component in a healthcare system may affect other components, and indeed the whole system, in a way which was not predicted at the outset. Quite often a decision aimed at conservation can actually end up leaking resource as unforeseen consequences on the whole system emerge.

Mathematical modelling of complex systems, such as a healthcare trust, can provide insight into how the change in one component affects other parts of the system. With a deeper analysis of the system, decisions can be taken with less uncertainty over downstream consequences.

Healthcare delivery services and their administrative and managerial supports are recognised as complex adaptive systems. Adaption signifies that the system learns from feedback and moves, overtime, ever closer towards an optimal configuration. Systems differ in their adaptiveness, however. Some evolve, almost effortlessly, towards an optimal configuration, but others appear to move chaotically from one state to another. The perception is that healthcare delivery in the UK typifies the latter.

Mathematical modelling is the ideal method for guiding healthcare delivery systems towards an optimal configuration. It is suggested that the optimal configuration of a healthcare system is aligned with the principle of utilitarianism;

Maximal well-being for a maximal number, at minimal cost.

Trendy Psychiatric Research: A need to sanitise hubris and bad faith?

An article in the Times by Dorothy Bishop explores some of the problems in biomedical research which arise from the obsession with high-impact journals and expensive grants.

monopoly boardHer critique is especially apt in the case of the physical basis of mental illness, in which researchers seeking fame and fortune must master the storytelling arts of simplicity, metaphor and metonymy. Those seeking H-impact & lucre must stay “on message” and above all, never stray into the chaos of imperfect methods and noisy data.

 

http://www.timeshighereducation.co.uk/comment/opinion/the-big-grants-the-big-papers-are-we-missing-something/2017894.article#pq=M87JTT

Bishop concludes with a warning, that the relentless focus on publishing in prestigious journals encourages…

1. Over-claiming the significance of research findings.

2. Leaving important, but contradictory results unpublished.

Hubris is the orientation of the former, bad faith the foundation of the latter.

“…what changes everything is the fact that in bad faith it is from myself that I am hiding the truth“. http://www.philosophymagazine.com/others/MO_Sartre_BadFaith.html