Two papers published online recently in Nature Genetics report results from the largest studies to date searching for genetic variants associated with schizophrenia and bipolar disorder. The findings are important. For schizophrenia, five new genetic loci are reported, including a variant in the microRNA gene, mir-137, which has previously been implicated in neurodevelopment – and two previously reported loci are confirmed. One of the confirmed loci is within the major histocompatibility complex (MHC), consistent with an immune component in schizophrenia risk. For bipolar disorder, the new report finds a locus in ODZ4 and, importantly, replicates a previous finding in the calcium channel gene, CACNA1C.
Psychiatric genetics has had its critics. Many early findings of candidate genes were not replicated. Genome-wide association studies, which look for associations in an unbiased search across the entire genome, failed to find anything significant in several early studies. And many critics wondered why there should be such an effort on genetics when the discovery of causative genes for “simple” genetic diseases, like Huntington’s disease, had not led to therapeutic breakthroughs. Even those who were sympathetic to genetics as a necessary step for developing a pathophysiology of mental illness, have been concerned about the heterogeneity of the diagnostic categories.
These new studies should allay some of these concerns. It’s important to realize that genome-wide association studies, which scan over a million common variants across the entire genome, identify risk factors, not causes of disease. In diabetes and inflammatory bowel disease, we now recognize scores of such risk factors, each contributing a small amount of risk, but in aggregate explaining a substantial share of the heritability of these disorders. For schizophrenia and bipolar disorder, which are syndromes with high heritability, genome-wide association studies have identified far fewer common risk variants Was this because there were few common variants contributing to mental disorders? Or were the studies not large enough?
The answer seems to be the size of the earlier studies. These new studies -- with 17,836 cases and 33,859 controls for schizophrenia and 11.974 cases and 51,792 controls for bipolar disorder -- are much larger than previous studies and finally we are seeing genetic risk factors emerge. The identified common variants are not causes of schizophrenia or bipolar disorder, but they suggest pathways and they may also reveal treatment targets. In diabetes, all of the current medications target pathways identified in genomic studies. There are many additional genetic findings which could indicate new medication targets. In schizophrenia and bipolar disorder, where we need new medication targets, none of the variants identify the presumed targets of current anti-psychotic drugs or mood stabilizers. Whether CACNA1C or mir-137 will lead to new therapies is now an empirical question.
But just as important as the results of these studies and the new pathways they reveal, these projects represent a new paradigm for the science of mental illness. Both studies are the products of the psychiatric genomics consortium (PGC), collaboration across more than 200 scientists in 65 institutions and 19 countries. These scientists and more than 100,000 research subjects have agreed to share data, so that we finally have the statistical power to scan across the entire genome, picking up changes in sequence which may increase risk. This broad sharing has been essential to progress in genetics, especially for common diseases where scores of variants, each contributing very slightly to risk, appear to interact in complex ways.
The PGC is also indicative of the reorientation of the psychiatric genomics community toward team science. In the team science approach, an entire community of investigators rallies around the idea of combining and leveraging intellectual and other resources (including data and samples) to answer a pressing scientific problem. Team Science as exemplified in the PGC not only leverages resources, but also creates a large platform from which new and cutting-edge science can be derived to move the entire field ahead.
What’s next? In addition to these subtle changes identified by these new papers on schizophrenia and bipolar disorder, much of the focus in psychiatric genetics has been the discovery of rare, structural changes in the chromosomes, which are far more common in mental disorders. With large numbers of patients and DNA samples, the next step will be putting these findings of rare and common variants together, finally getting the major pieces of the jigsaw puzzle we call mental disorders. From these and other studies, it already appears unlikely that the genetics will align with DSM disorders. Genetics may give us a different map of these disorders, based on neurodevelopmental processes. And, with luck, genetics can point us towards the next generation of treatments.
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