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Q. What is schizophrenia? 
Schizophrenia is a debilitating psychiatric disorder that afflicts approximately 1 percent of people worldwide. It is characterized by hallucinations, emotional withdrawal, and a decline in cognitive function. Symptoms most frequently begin to manifest in patients when they are teenagers and young adults. The disorder was first described more than 130 years ago, and prior to this study, its molecular roots were unknown.

Q. Who conducted the study?
The study was led by Steven McCarroll, director of genetics for the Stanley Center for Psychiatric Research at ӳ����ý and an associate professor at Harvard Medical School (HMS). Major contributors included first author Aswin Sekar, an M.D./Ph.D. student at HMS; Beth Stevens, a neuroscientist and assistant professor of neurology at Boston Children’s Hospital and institute member at the ӳ����ý; and Michael Carroll, a professor at HMS and researcher at Children’s.

Q. What was the motivation behind the study?
Since the first description of schizophrenia over a century ago, the disorder has been a virtual black box whose molecular causes are poorly understood. Part of the reason for this is that psychiatric diseases are very hard to study in the lab, due to limitations of animal and cellular models and lack of appropriate human tissues to study. Medications for schizophrenia treat only one of the disorder’s symptoms (psychosis) and not the more debilitating cognitive impairments, in part because the underlying molecular mechanisms have not been identified and therefore, cannot be targeted or inhibited.

Over the past five years, large-scale genomic studies of schizophrenia involving the collection of more than 100,000 DNA samples from around the globe have revealed more than 100 genetic risk factors for the disorder, including one very strong, yet complex, signal on chromosome 6 that scientists couldn’t unravel. The researchers on this study thought that if they could identify the gene or genes, and in turn the biological pathways, that underlie the signal, it could shed light on what goes awry in the disorder and where potential treatments could be targeted.

Q. What are the highlights of the study?
Steve McCarroll and Aswin Sekar created a novel analytical playbook that they used to explore complex structural variation (i.e., many different forms that vary in length and composition) in the gene for the immune molecule known as complement component 4 (C4). They used a new molecular technique known as digital droplet PCR and analyzed inheritance patterns to characterize more than a dozen structural forms of C4 in hundreds of human DNA samples and relate the four most common of those forms to patterns of genetic markers in the genome. They next examined genetic markers and C4 gene activity in nearly 700 post-mortem brain samples, creating genetic predictors of gene expression (i.e., patterns of single letter differences, or SNPs, that correlated with C4 gene activity). They found that the C4 gene structure (DNA) could predict the C4 gene activity (RNA) in each person’s brain – and used this information to infer C4 gene activity from genome data for 65,000 people with and without schizophrenia. This data revealed a striking correlation: patients who had particular structural forms of the C4 gene showed higher expression of that gene and had, on average, a higher risk of developing schizophrenia. This pattern is evidence that structural variation in the C4 gene, at least in part, underlies the complex signal for risk in chromosome 6.

To answer the question of what C4 was doing in the brain, Beth Stevens, an expert on the role of the complement system in brain development, led experiments in the lab that showed that C4 is expressed by human neurons and found at synapses. Using mice with differing numbers of copies of the C4 gene developed by Mike Carroll (who had long studied C4 for its role in immune disease), the researchers demonstrated that mice lacking the C4 gene exhibit impairments in the pruning of synapses (connections) between brain cells during development, direct evidence that C4 plays a key role in this process in the developing brain.

Q. What is C4?
Complement component 4 (C4) is an immune molecule that, outside of the central nervous system, helps to contain infection. Prior to this study, it was known that C4 is present in the brain, but there were few clues as to its function in development or disease. The findings from this Nature paper show that C4 is a clear risk factor for schizophrenia, it is expressed by human neurons and found at synapses, and it mediates the pruning of synapses that normally takes place during development, a function distinct from C4’s role in the immune system.

Q. What is synaptic pruning?
Synaptic pruning is the normal developmental process by which synapses, the connections between brain cells, are selectively removed as the brain develops. The process is active during adolescence and continues through early adulthood to help shape the adult brain. Work from the labs of Beth Stevens and others has revealed the role of complement proteins in synaptic pruning in the brain, and this study reveals that C4 plays a key role in this process in the developing brain by tagging synapses for removal by other cells. In particular, C4 instructs another complement component — C3, an “eat-me” signal spotted at synapses in Stevens’ earlier work — to deposit onto synapses. The synapses are then eliminated or “pruned” by the brain’s immune cells.

Q. How does this study change the view of schizophrenia pathogenesis?
The results support the hypothesis that excessive or prolonged synaptic pruning could lead to symptoms of schizophrenia. The findings may help explain the longstanding mystery of why brains from people with schizophrenia tend to have a thinner cerebral cortex with fewer synapses than unaffected individuals do. The work may also help to explain why the onset of schizophrenia symptoms tends to occur in late adolescence: the human brain normally undergoes widespread synapse pruning during adolescence, especially in the cerebral cortex (the brain’s outer layer, responsible for many aspects of cognition). Excessive synaptic pruning during adolescence and early adulthood, due to increased complement (C4) activity, could lead to the cognitive symptoms seen in schizophrenia.

Q. Why is this a landmark study?
These results represent the strongest evidence to date for a molecular mechanism underlying schizophrenia, a disorder in which the biological underpinnings were almost completely unknown. This study establishes the first clear connection between genetics, the biology of brain development, and mental illness. The findings support a hypothesis that may explain the age of symptom onset and cortical thinning seen in many patients. By shedding light on the biological roots of the disease, the study may reinvigorate the field of psychiatric genetics by providing the first molecular toehold for researchers to explore the roots of schizophrenia. Further study into the role of C4, other members of the complement pathway, and the process of synaptic pruning may shed more light on schizophrenia and other psychiatric or neurodegenerative disorders and potentially reveal new options for diagnosing or treating these debilitating conditions.

Q. Is there therapeutic or diagnostic potential in these findings?
The findings suggest that C4 or another component of the pruning pathway could one day be a therapeutic target, but much more work is necessary before a therapy based on this work could reach the clinic. C4, or a related molecule, may also hold diagnostic potential as a biomarker for schizophrenia, which could potentially enable early intervention to prevent excessive synaptic pruning and lessen the disorder’s cognitive symptoms. C4 has been well studied for its role in the immune system, so researchers working on these fronts can tap into a wealth of existing knowledge to identify possible therapeutic avenues.

Q. How was the study funded?
This study was funded by the National Institutes of Health and the Stanley Center for Psychiatric Research at the ӳ����ý, which aims to reduce the burden of serious mental illness through research.

Q. Where can I find the study?
Sekar A, et al. Schizophrenia risk from complex variation of complement component 4. Nature. .